Material Property Characterization of 

Ultra-High Performance Concrete

PUBLICATION NO. FHWA-HRT-06-103 AUGUST 2006

Research, Development, and Technology

Turner-Fairbank Highway Research Center

6300 Georgetown Pike

McLean, VA 22101-2296
FOREWORD 

Advances in the knowledge and understanding of the behaviors of concrete on the 

microstructural level have led to the development of the next generation of concrete, namely 

ultra-high performance concrete (UHPC). This report characterizes the material behaviors of one 

UHPC in terms of accepted concrete testing methodologies. The Federal Highway 

Administration (FHWA) has been investigating the optimal use of UHPC in highway bridges, 

and this report presents results from the first phase of this research program. Of primary 

importance, the results contained herein provide a starting point for bridge owners interested in 

advancing the state of bridge engineering through the use of extremely high strength and high 

durability concretes. This report presents both what can be achieved today through the use of a 

commercially available concrete as well as the types of advancements that can be achieved by 

reevaluating the traditional components and proportions normally present in cementitious 

structural materials. 

 Gary Henderson 

 Director, Office of Infrastructure 

 Research and Development 

Notice 

This document is disseminated under the sponsorship of the U.S. Department of Transportation 

in the interest of information exchange. The U.S. Government assumes no liability for the use of 

the information contained in this document. This report does not constitute a standard, 

specification, or regulation. 

The U.S. Government does not endorse products or manufacturers. Trademarks or 

manufacturers’ names appear in this report only because they are considered essential to the 

objective of the document. 

Quality Assurance Statement 

The Federal Highway Administration (FHWA) provides high-quality information to serve 

Government, industry, and the public in a manner that promotes public understanding. Standards 

and policies are used to ensure and maximize the quality, objectivity, utility, and integrity of its 

information. FHWA periodically reviews quality issues and adjusts its programs and processes to 

ensure continuous quality improvement. 
Form DOT F 1700.7 (8-72) 

1. Report No.

FHWA-HRT-06-103 

2. Government Accession No. 3. Recipient’s Catalog No.

5. Report Date 

August 2006 

4. Title and Subtitle 

Material Property Characterization of 

Ultra-High Performance Concrete 6. Performing Organization Code

7. Author 

Benjamin A. Graybeal 

8. Performing Organization Report No. 

9. Performing Organization Name and Address 10. Work Unit No. 

PSI, Inc. 

2930 Eskridge Road 

Fairfax, VA 22031 

11. Contract or Grant No.

13. Type of Report and Period Covered 

Final Report, October 2002–December 2005

12. Sponsoring Agency Name and Address 

Office of Infrastructure Research and Development 

Federal Highway Administration 

6300 Georgetown Pike 

McLean, VA 22101-2296 

14. Sponsoring Agency Code 

15. Supplementary Notes 

Additional FHWA Contacts—Joseph Hartmann (Technical Advisor), William Wright (COTR) 

16. Abstract 

In the past decade significant advances have been made in the field of high performance concretes (HPC). The next 

generation of concrete, ultra-high performance concrete (UHPC), exhibits exceptional strength and durability 

characteristics that make it well suited for use in highway bridge structures. This material can exhibit compressive 

strength of 193 megapascals (MPa) (28 kilopounds per square inch (ksi)), tensile strength of 9.0 MPa (1.3 ksi), 

significant tensile toughness, elastic modulus of 52.4 gigapascals (GPa) (7,600 ksi), and minimal long-term creep or 

shrinkage. It can also resist freeze-thaw and scaling conditions with virtually no damage and is nearly impermeable to 

chloride ions. 

This report presents the results from a large suite of material characterization tests that were completed in order to 

quantify the behaviors of a commercially available UHPC. The characteristics of this UHPC under four different 

curing regimes were captured. This study focused on strength-based behaviors (e.g., compressive and tensile strength), 

long-term stability behaviors (e.g., creep and shrinkage), and durability behaviors (e.g., chloride ion penetration and 

freeze-thaw). 

17. Key Words 

UHPC, ultra-high performance concrete, fiberreinforced, durability, material characterization, 

tensile behavior, compressive behavior 

18. Distribution Statement 

No restrictions. This document is available to the public 

through the National Technical Information Service, 

Springfield, VA 22161. 

19. Security Classif. (of this report) 

Unclassified 

20. Security Classif. (of this page) 

Unclassified 

21. No of Pages

186 

22. Price 
ii

SI* (MODERN METRIC) CONVERSION FACTORS 

APPROXIMATE CONVERSIONS TO SI UNITS

Symbol When You Know Multiply By To Find Symbol 

LENGTH 

in inches 25.4 millimeters mm 

ft feet 0.305 meters m 

yd yards 0.914 meters m 

mi miles 1.61 kilometers km 

AREA 

in2 square inches 645.2 square millimeters mm2

ft2 square feet 0.093 square meters m2

yd2 square yard 0.836 square meters m2

ac acres 0.405 hectares ha

mi2 square miles 2.59 square kilometers km2

VOLUME 

fl oz fluid ounces 29.57 milliliters mL 

gal gallons 3.785 liters L 

ft3 cubic feet 0.028 cubic meters m3 

yd3 cubic yards 0.765 cubic meters m3 

NOTE: volumes greater than 1000 L shall be shown in m3

MASS 

oz ounces 28.35 grams g

lb pounds 0.454 kilograms kg

T short tons (2000 lb) 0.907 megagrams (or "metric ton") Mg (or "t") 

TEMPERATURE (exact degrees) o

F Fahrenheit 5 (F-32)/9 Celsius oC 

or (F-32)/1.8 

ILLUMINATION 

fc foot-candles 10.76 lux lx

fl foot-Lamberts 3.426 candela/m2 cd/m2

FORCE and PRESSURE or STRESS 

lbf poundforce 4.45 newtons N 

lbf/in2 poundforce per square inch 6.89 kilopascals kPa 

APPROXIMATE CONVERSIONS FROM SI UNITS 

Symbol When You Know Multiply By To Find Symbol 

LENGTH

mm millimeters 0.039 inches in 

m meters 3.28 feet ft 

m meters 1.09 yards yd 

km kilometers 0.621 miles mi 

AREA mm2

 square millimeters 0.0016 square inches in2 

m2 square meters 10.764 square feet ft2 

m2 square meters 1.195 square yards yd2 

ha hectares 2.47 acres ac

km2 square kilometers 0.386 square miles mi2 

VOLUME 

mL milliliters 0.034 fluid ounces fl oz 

L liters 0.264 gallons gal 

m3 cubic meters 35.314 cubic feet ft3 

m3 cubic meters 1.307 cubic yards yd3 

MASS 

g grams 0.035 ounces oz

kg kilograms 2.202 pounds lb

Mg (or "t") megagrams (or "metric ton") 1.103 short tons (2000 lb) T 

TEMPERATURE (exact degrees) o

C Celsius 1.8C+32 Fahrenheit oF 

ILLUMINATION 

lx lux 0.0929 foot-candles fc 

cd/m2 candela/m2 0.2919 foot-Lamberts fl

FORCE and PRESSURE or STRESS 

N newtons 0.225 poundforce lbf 

kPa kilopascals 0.145 poundforce per square inch lbf/in2

*SI is the symbol for th International System of Units. Approp e riate rounding should be made to comply with Section 4 of ASTM E380. 

(Revised March 2003)
iii 

TABLE OF CONTENTS 

CHAPTER 1. INTRODUCTION.................................................................................................1 

1.1 INTRODUCTION ..............................................................................................................1 

1.2 OBJECTIVE .......................................................................................................................1 

1.3 SUMMARY OF APPROACH ...........................................................................................1 

1.4 OUTLINE OF REPORT.....................................................................................................2 

CHAPTER 2. BACKGROUND AND PREVIOUS WORK ......................................................3 

2.1 UHPC CONSTITUENT MATERIALS .............................................................................3 

2.2 MANUFACTURER-SUPPLIED UHPC MATERIAL PROPERTIES .............................3 

2.3 STEEL FIBER MATERIAL PROPERTIES......................................................................4 

2.4 RELEVANT MATERIAL PROPERTY CHARACTERIZATION STUDIES .................5 

2.4.1 Fiber Orientation Effect on Mechanical Properties ...................................................5 

2.4.2 Permeability of Cracked Concrete.............................................................................6 

2.4.3 Creep and Shrinkage of UHPC..................................................................................6 

2.4.4 Abrasion Resistance of HSC via ASTM C944..........................................................7 

CHAPTER 3. UHPC MATERIAL CHARACTERIZATION ..................................................9 

3.1 RESEARCH PLAN ............................................................................................................9 

3.1.1 Batch and Specimen Nomenclature...........................................................................9 

3.1.2 Test Matrix...............................................................................................................10 

3.2 BATCHING, CASTING, AND CURING OF UHPC......................................................15 

3.3 COMPRESSION TESTING.............................................................................................23 

3.3.1 Strength....................................................................................................................24 

3.3.2 Strength, Modulus of Elasticity, and Strain Capacity With Time ...........................28 

3.3.3 Linearity of UHPC Compressive Response.............................................................37 

3.3.4 Compression Specimen Geometry...........................................................................43 

3.3.5 Demolding Age Effect on Compressive Strength....................................................44 

3.3.6 Long-Term Delayed Steam Effect on Compressive Strength..................................46 

3.3.7 Fiber Effect on Compression Failure.......................................................................46 

3.3.8 Load Rate Effect on Compression Testing Results .................................................49 

3.4 TENSION TESTING........................................................................................................50 

3.4.1 Flexural Prism..........................................................................................................50 

3.4.2 Split Cylinder...........................................................................................................74 

3.4.3 Mortar Briquette.......................................................................................................80 

3.4.4 Direct Tension..........................................................................................................90 

3.5 FRACTURE TESTING....................................................................................................94 

3.6 PENETRATION RESISTANCE TESTING ..................................................................106 

3.7 SHRINKAGE TESTING................................................................................................107 

3.7.1 Long-Term Shrinkage Testing...............................................................................107 

3.7.2 Early Age Shrinkage Testing.................................................................................108 

3.8 CREEP TESTING ..........................................................................................................110 

3.8.1 Long-Term Creep Testing......................................................................................110 

3.8.2 Early Age High-Stress Creep Testing....................................................................112 

3.9 COEFFICIENT OF THERMAL EXPANSION.............................................................116 
iv 

3.10 HEAT OF HYDRATION.............................................................................................117 

3.11 AIR VOID ANALYSIS................................................................................................119 

3.12 STEEL FIBER DISPERSION TESTING ....................................................................119 

3.13 DURABILITY TESTING ............................................................................................121 

3.13.1 Rapid Chloride Ion Penetrability Testing ............................................................122 

3.13.2 Chloride Penetration ............................................................................................123 

3.13.3 Scaling Resistance ...............................................................................................125 

3.13.4 Abrasion Resistance.............................................................................................128 

3.13.5 Freeze-Thaw Resistance ......................................................................................132 

3.13.6 Alkali-Silica Reaction..........................................................................................138 

3.14 SPLIT-CYLINDER TENSION TESTING ON CRACKED CYLINDERS ................140 

CHAPTER 4. DISCUSSION OF RESULTS..........................................................................145 

4.1 TENSILE BEHAVIOR OF UHPC.................................................................................145 

4.1.1 Summary of Experimental Results ........................................................................145 

4.1.2 Summary of Experimental Test Methods ..............................................................148 

4.2 LOCAL AND GLOBAL MECHANICAL FAILURE MODES OF UHPC..................149 

4.3 EFFECT OF CURING PROCEDURE ON UHPC PROPERTIES................................151 

4.4 EARLY AGE STRENGTH GAIN OF UHPC ...............................................................153 

4.5 COMPARISON OF CYLINDER AND CUBE COMPRESSION 

 STRENGTH RESULTS .................................................................................................154 

4.6 SHRINKAGE BEHAVIOR OF UHPC..........................................................................155 

4.7 LONG-TERM STABILITY OF UHPC..........................................................................156 

4.8 MODULUS OF ELASTICITY OF UHPC.....................................................................157 

4.9 COMPRESSIVE STRESS-STRAIN BEHAVIOR OF UHPC.......................................161 

CHAPTER 5. CONCLUSIONS AND FUTURE RESEARCH ............................................167 

5.1 INTRODUCTION ..........................................................................................................167 

5.2 CONCLUSIONS.............................................................................................................167 

5.3 ONGOING AND FUTURE RESEARCH......................................................................170 

REFERENCES...........................................................................................................................171 
v 

LIST OF FIGURES 

Figure 1. Graph. Sample tensile stress-strain response for steel fiber reinforcement......................5 

Figure 2. Photos. Mixing of UHPC. (a) Water addition. (b) HRWA addition. (c) Prepaste 

consistency. (d) Fiber addition. (e) Finished mix. ...................................................................17 

Figure 3. Graph. Mix time as affected by premix age. ..................................................................22 

Figure 4. Graph. Final flow diameter as affected by premix age. .................................................22 

Figure 5. Photos. (a) Grinding and (b) measuring of 76-mm (3-inch) diameter cylinders............24 

Figure 6. Photos. 76-mm (3-inch) diameter cylinders (a) before and (b) after compression 

testing.......................................................................................................................................25 

Figure 7. Graph. Compressive strength and density of control cylinders......................................27 

Figure 8. Graph. Compressive strength and cylinder end planeness of control cylinders.............28 

Figure 9. Photos. Modulus ring attachment (a) before and (b) during testing...............................29 

Figure 10. Graph. Selected stress-strain responses for steam-treated UHPC (N1A).....................30 

Figure 11. Graph. Selected stress-strain responses for steam-treated UHPC (N1AxxA)..............30 

Figure 12. Graph. Selected stress-strain responses for untreated UHPC.......................................31 

Figure 13. Graph. Selected stress-strain responses for tempered steam-treated UHPC. ...............31 

Figure 14. Graph. Selected stress-strain responses for delayed steam-treated UHPC...................32 

Figure 15. Graph. Compressive strength gain from casting up to 8 weeks of age. .......................33 

Figure 16. Graph. Modulus of elasticity gain from casting up to 8 weeks of age. ........................33 

Figure 17. Graph. Strain at peak compressive stress from casting up to 8 weeks of age. .............34 

Figure 18. Graph. Sample untreated stress-strain curve with linearity descriptors. ......................37 

Figure 19. Graph. Secant modulus from casting up to 8 weeks of age..........................................41 

Figure 20. Graph. Ratio of elastic to secant modulus from casting up to 8 weeks of age.............41 

Figure 21. Graph. Compressive stress to strength ratio at 1 percent stress drop from 

linear elastic. ............................................................................................................................42 

Figure 22. Graph. Compressive stress to strength ratio at 5 percent stress drop from 

linear elastic. ............................................................................................................................42 

Figure 23. Photo. Compression cubes and cylinders including (clockwise from upper left) 

102-mm (4-inch), 76-mm (3-inch) overlength, 76-mm (3-inch), and 51-mm (2-inch) 

diameter cylinders and 51-mm (2-inch) and 100-mm (4-inch) cubes. ....................................43 

Figure 24. Photos. (a) Cylinder and (b) cube compression testing................................................44 

Figure 25. Photos. Compression failure of a steam-treated UHPC cylinder containing no fiber 

reinforcement (a) 1/6 second before failure, (b) 1/30 second before failure, (c) at failure, 

and (d) 1/10 second after failure. .............................................................................................48 

Figure 26. Photos. Prism flexural test setup for (a) a 229-mm (8.9-inch) span and 

(b) a 305-mm (11.9-inch) span. ...............................................................................................51 

Figure 27. Graph. Examples of first crack shown on load-deflection response curves.................53 

Figure 28. Graph. ASTM C1018 load-deflection response results for steam-treated 51- by 

51-mm (2- by 2-inch) prisms over a 152-mm (6-inch) span with third-point loading. ...........54 

Figure 29. Graph. ASTM C1018 load-deflection response results for untreated 51- by 

51-mm (2- by 2-inch) prisms over a 152-mm (6-inch) span with third-point loading. ...........54 
vi 

Figure 30. Graph. ASTM C1018 load-deflection response results for tempered steamtreated 51- by 51-mm (2- by 2-inch) prisms over a 152-mm (6-inch) span with 

third-point loading. ..................................................................................................................55 

Figure 31. Graph. ASTM C1018 load-deflection response results for delayed steamtreated 51- by 51-mm (2- by 2-inch) prisms over a 152-mm (6-inch) span with 

third-point loading. ..................................................................................................................55 

Figure 32. Graph. ASTM C1018 load-deflection response results for steam-treated 76- by 

102-mm (3- by 4-inch) prisms over a 305-mm (12-inch) span with third-point loading. .......56 

Figure 33. Graph. ASTM C1018 load-deflection response results for untreated 76- by 102- 

mm (3- by 4-inch) prisms over a 305-mm (12-inch) span with third-point loading................56 

Figure 34. Graph. ASTM C1018 load-deflection response results for tempered steamtreated 76- by 102-mm (3- by 4-inch) prisms over a 305-mm (12-inch) span with 

third-point loading. ..................................................................................................................57 

Figure 35. Graph. ASTM C1018 load-deflection response results for delayed steamtreated 76- by 102-mm (3- by 4-inch) prisms over a 305-mm (12-inch) span with 

third-point loading. ..................................................................................................................57 

Figure 36. Graph. ASTM C1018 load-deflection response results for steam-treated 51- by 

51-mm (2- by 2-inch) prisms over a 229-mm (9-inch) span with third-point loading. ...........58 

Figure 37. Graph. ASTM C1018 load-deflection response results for untreated 51- by 51-mm 

(2- by 2-inch) prisms over a 229-mm (9-inch) span with third-point loading.........................58 

Figure 38. Graph. ASTM C1018 load-deflection response results for tempered steamtreated 51- by 51-mm (2- by 2-inch) prisms over a 229-mm (9-inch) span with third-point 

loading......................................................................................................................................59 

Figure 39. Graph. ASTM C1018 load-deflection response results for delayed steam-treated 

51- by 51-mm (2- by 2-inch) prisms over a 229-mm (9-inch) span with third-point 

loading......................................................................................................................................59 

Figure 40. Graph. ASTM C1018 load-deflection response results for steam-treated 51- by 

51-mm (2- by 2-inch) prisms over a 305-mm (12-inch) span with third-point loading. .........60 

Figure 41. Graph. ASTM C1018 load-deflection response results for untreated 51- by 

51-mm (2- by 2-inch) prisms over a 305-mm (12-inch) span with third-point loading. .........60 

Figure 42. Graph. ASTM C1018 load-deflection response results for tempered steam-treated 

51- by 51-mm (2- by 2-inch) prisms over a 305-mm (12-inch) span with third-point 

loading......................................................................................................................................61 

Figure 43. Graph. ASTM C1018 load-deflection response results for delayed steam-treated 

51- by 51-mm (2- by 2-inch) prisms over a 305-mm (12-inch) span with third-point 

loading......................................................................................................................................61 

Figure 44. Graph. ASTM C1018 load-deflection response results for steam-treated 51- by 

51-mm (2- by 2-inch) prisms over a 381-mm (15-inch) span with 76 mm (3 inches) 

between loads...........................................................................................................................62 

Figure 45. Graph. ASTM C1018 load-deflection response results for untreated 51- by 

51-mm (2- by 2-inch) prisms over a 381-mm (15-inch) span with 76 mm (3 inches) 

between loads...........................................................................................................................62 

Figure 46. Graph. ASTM C1018 load-deflection response results for tempered steam-treated 

51- by 51-mm (2- by 2-inch) prisms over a 381-mm (15-inch) span with 76 mm (3 inches) 

between loads...........................................................................................................................63 
vii 

Figure 47. Graph. ASTM C1018 load-deflection response results for delayed steam-treated 

51- by 51-mm (2- by 2-inch) prisms over a 381-mm (15-inch) span with 76 mm 

(3 inches) between loads..........................................................................................................63 

Figure 48. Equation. Flexural cracking strength of a concrete prism............................................64 

Figure 49. Equation. AFGC correction factor for concrete prism flexural strength......................65 

Figure 50. Equation. Centerline deflection of a simply supported prismatic beam.......................65 

Figure 51. Graph. Ratio of shear to flexural deflection for a third-point loaded prism.................66 

Figure 52. Graph. ASTM C1018 toughness results for steam-treated UHPC prisms. ..................70 

Figure 53. Graph. ASTM C1018 toughness results for untreated UHPC prisms..........................70 

Figure 54. Graph. ASTM C1018 toughness results for tempered steam-treated UHPC prisms. ..71 

Figure 55. Graph. ASTM C1018 toughness results for delayed steam-treated UHPC prisms......71 

Figure 56. Graph. ASTM C1018 residual strength results for steam-treated prisms. ...................72 

Figure 57. Graph. ASTM C1018 residual strength results for untreated prisms. ..........................72 

Figure 58. Graph. ASTM C1018 residual strength results for tempered steam-treated prisms.....73 

Figure 59. Graph. ASTM C1018 residual strength results for delayed steam-treated prisms.......73 

Figure 60. Equation. Tensile stress in an ASTM C496 split-cylinder test. ...................................74 

Figure 61. Photos. Split-cylinder tensile test including (a) standard test setup, (b) lateral 

expansion measuring apparatus, and (c) UHPC cylinder during test. .....................................76 

Figure 62. Graph. Typical response for a UHPC cylinder during the ASTM C496 test...............77 

Figure 63. Chart. Average tensile cracking results from the ASTM C496 test.............................77 

Figure 64. Chart. Average split cylinder peak strength from the ASTM C496 test. .....................78 

Figure 65. Photos. AASHTO T132 setup including (a) test grips and (b) specimen. ...................81 

Figure 66. Graph. Load-displacement response for steam-treated briquettes (28 days). ..............82 

Figure 67. Graph. Load-displacement response for steam-treated briquettes (56 days). ..............82 

Figure 68. Graph. Load-displacement response for steam-treated briquettes (84 days). ..............82 

Figure 69. Graph. Load-displacement response for untreated briquettes (28 days). .....................83 

Figure 70. Graph. Load-displacement response for untreated briquettes (56 days). .....................83 

Figure 71. Graph. Load-displacement response for untreated briquettes (84 days). .....................83 

Figure 72. Graph. Load-displacement for tempered steam-treated briquettes (28 days)...............84 

Figure 73. Graph. Load-displacement for tempered steam-treated briquettes (56 days)...............84 

Figure 74. Graph. Load-displacement for tempered steam-treated briquettes (84 days)...............84 

Figure 75. Graph. Load-displacement for delayed steam-treated briquettes (28 days).................85 

Figure 76. Graph. Load-displacement for delayed steam-treated briquettes (56 days).................85 

Figure 77. Graph. Load-displacement for delayed steam-treated briquettes (84 days).................85 

Figure 78. Chart. Tensile cracking strength of UHPC briquettes..................................................86 

Figure 79. Chart. Postcracking peak strength of UHPC briquettes. ..............................................87 

Figure 80. Chart. Area under the load-displacement response curve after cracking. ....................88 

Figure 81. Chart. Ratio of postcracking to precracking areas under the load-displacement 

curve.........................................................................................................................................89 

Figure 82. Photos. (a) Notched cylinder and (b) testing of an unnotched cylinder. ......................92 

Figure 83. Photo. Test setup for 102- by 51-mm (4- by 2-inch) notched prisms loaded on 

a 406-mm (16-inch) span.........................................................................................................95 

Figure 84. Photo. Resistance foil gage to monitor crack propagation...........................................96 

Figure 85. Photos. Prism M1P00 after (a) 86 mm (3.6 inches) and (b) 98 mm (3.8 inches) 

of crack extension. ...................................................................................................................97 

Figure 86. Photo. Prism M2P03 after 93 mm (3.63 inches) of crack extension............................97 
viii 

Figure 87. Graph. Load-CMOD response for steam-treated prism M1P00. .................................98 

Figure 88. Graph. Load-CMOD response for steam-treated prism M1P01. .................................99 

Figure 89. Graph. Load-CMOD response for steam-treated prism M1P02. ...............................100 

Figure 90. Graph. Load-CMOD response for steam-treated prism M1P03. ...............................101 

Figure 91. Graph. Load-CMOD response for untreated prism M2P00. ......................................102 

Figure 92. Graph. Load-CMOD response for untreated prism M2P01. ......................................103 

Figure 93. Graph. Load-CMOD response for untreated prism M2P02. ......................................104 

Figure 94. Graph. Load-CMOD response for untreated prism M2P03. ......................................105 

Figure 95. Graph. Long-term shrinkage results...........................................................................108 

Figure 96. Equation. Shrinkage as a function of time after casting.............................................108 

Figure 97. Photo. Embeddable vibrating wire gage.....................................................................109 

Figure 98. Graph. Early age shrinkage. .......................................................................................109 

Figure 99. Photos. (a) Creep cylinders in load frame and (b) measurement of creep. ................111 

Figure 100. Graph. Long-term creep results................................................................................113 

Figure 101. Equation. Creep as a function of time after loading.................................................113 

Figure 102. Photo. Short-term creep test setup............................................................................114 

Figure 103. Graph. Early age creep behavior of 55 to 65 MPa (7,975 to 9,425 psi) UHPC. ......115 

Figure 104. Graph. Early age creep behavior of 86 MPa (12,470 psi) UHPC. ...........................115 

Figure 105. Graph. Heat generated in 152-mm (6-inch) diameter cylinders during initial 

curing. ....................................................................................................................................117 

Figure 106. Graph. Heat generated in 152-mm (6-inch) diameter cylinders from casting 

through steaming....................................................................................................................118 

Figure 107. Graph. Heat signature for 152-mm (6-inch) diameter cylinders in a wellinsulated calorimeter..............................................................................................................118 

Figure 108. Graph. Fiber dispersion analysis results for cylinders impacted on an ASTM 

C230 flow table......................................................................................................................121 

Figure 109. Photos. Fiber dispersion analysis photographs for a 645-mm2 (1.3-inch2) area 

in the (a) bottom, (b) lower middle, (c) upper middle, and (d) top of a cast cylinder. ..........122 

Figure 110. Photos. (a) Cylinder and (b) setup for rapid chloride ion penetrability test.............123 

Figure 111. Graph. Average current passed versus time results for three sets of cylinders. .......124 

Figure 112. Graph. Chloride ion content results after 90 days of ponding..................................125 

Figure 113. Photos. (a) Cylinder before and (b) after 90 days of chloride ponding....................125 

Figure 114. Photo. Scaling slab before initiating ASTM C672 testing. ......................................126 

Figure 115. Photo. Scaling slab after ASTM C672 testing..........................................................127 

Figure 116. Photo. Surface deterioration of a vertical surface after 70-plus-145 cycles of 

wetting/drying with a chloride solution in a freezing/thawing environment.........................128 

Figure 117. Photo. ASTM C944 abrasion test setup. ..................................................................129 

Figure 118. Photo. Steel cast surface untreated and steam-treated abrasion specimens after 

8 and 10 minutes of abrading, respectively. ..........................................................................130 

Figure 119. Chart. ASTM C944 weight loss (grams) per abrading.............................................130 

Figure 120. Chart. Average weight loss (grams) per abrading....................................................131 

Figure 121. Chart. Linear best-fit weight loss (grams) per abrading...........................................131 

Figure 122. Photo. Resonant frequency testing of a freeze/thaw prism. .....................................133 

Figure 123. Photos. Freeze-thaw prism (a) before testing and (b) after 564 cycles. ...................133 

Figure 124. Graph. Resonant frequency of freeze/thaw prisms...................................................134 

Figure 125. Graph. Relative dynamic modulus of elasticity of freeze/thaw prisms....................135 
ix 

Figure 126. Graph. Mass change of prisms during freeze-thaw testing. .....................................135 

Figure 127. Graph. Resonant frequency of prisms maintained at room temperature in a 

laboratory environment or in a water bath.............................................................................136 

Figure 128. Graph. Relative dynamic modulus of elasticity of prisms maintained at room 

temperature in a laboratory environment or in a water bath..................................................137 

Figure 129. Graph. Mass change of prisms maintained at room temperature in a laboratory 

environment or in a water bath. .............................................................................................137 

Figure 130. Photo. Length comparator for ASR measurements..................................................139 

Figure 131. Graph. ASTM C1260 alkali-silica reactivity expansion results...............................140 

Figure 132. Photo. Crack in a split cylinder tensile specimen.....................................................142 

Figure 133. Photo. Crack in a split-cylinder tensile specimen under 350x magnification. .........142 

Figure 134. Chart. Split-cylinder peak strength results. ..............................................................144 

Figure 135. Equation. Concrete tensile strength approximation..................................................147 

Figure 136. Graph. Compressive strength gain as a function of time after casting.....................154 

Figure 137. Equation. Compressive strength at any age after casting.........................................154 

Figure 138. Equation. ACI 318 approximation of modulus of elasticity.....................................157 

Figure 139. Equation. ACI 318 approximation of modulus of elasticity including density........158 

Figure 140. Equation. Comité Européen du Beton approximation for modulus of elasticity 

(metric units)..........................................................................................................................158 

Figure 141. Equation. Comité Européen du Beton approximation for modulus of elasticity 

(English units)........................................................................................................................158 

Figure 142. Equation. Kakizaki approximation for modulus of elasticity...................................158 

Figure 143. Equation. ACI 363 approximation for modulus of elasticity. ..................................158 

Figure 144. Equation. Ma approximation for modulus of elasticity............................................158 

Figure 145. Graph. Modulus of elasticity as a function of 28-day compressive strength. ..........159 

Figure 146. Equation. Approximation for UHPC modulus of elasticity (in psi).........................160 

Figure 147. Graph. Modulus of elasticity as a function of compressive strength. ......................160 

Figure 148. Equation. UHPC modulus of elasticity approximation (in psi) for compressive 

strengths up to 131 MPa (19 ksi). ..........................................................................................161 

Figure 149. Graph. Compressive stress-strain behavior compared with linear elastic 

response..................................................................................................................................163 

Figure 150. Graph. Normalized compressive stress-strain results for steam-treated 

UHPC.....................................................................................................................................164 

Figure 151. Graph. Deviation from linear elastic compressive behavior for steam-treated 

UHPC.....................................................................................................................................165 

Figure 152. Equation. Deviation of compressive stress-strain response from linear elastic 

behavior..................................................................................................................................165 

Figure 153. Equation. Compressive stress-strain behavior defined as a function of the 

deviation from linear elastic behavior....................................................................................165 

Figure 154. Graph. Compressive stress-strain response approximations. ...................................166 
x 

LIST OF TABLES 

Table 1. Typical UHPC composition...............................................................................................3 

Table 2. Manufacturer-supplied material characteristics.................................................................4 

Table 3. Chemical composition of steel fibers.................................................................................5 

Table 4. Batching descriptions with associated specimens and curing regimes............................11 

Table 5. Batching descriptions with associated specimens and curing regimes for 

nonstandardized batches. .........................................................................................................14 

Table 6. Batching and casting properties of steam-treated and untreated UHPC..........................19 

Table 7. Batching and casting properties of batches cast to complete the study by addressing 

special issues............................................................................................................................21 

Table 8. Control cylinder compressive strength results from the M and N deliveries. .................26 

Table 9. Control cylinder compressive strength results from the L delivery.................................27 

Table 10. Strength, modulus of elasticity, and strain at peak stress results at various ages 

after casting..............................................................................................................................35 

Table 11. Compressive stress-strain response linearity at various ages after casting....................39 

Table 12. Cylinder and cube compressive strength results............................................................45 

Table 13. Demolding age effect on 28-day compressive strength results. ....................................46 

Table 14. Long-term delayed steam effect on compressive strength.............................................47 

Table 15. Load rate effect on compression testing results.............................................................49 

Table 16. ASTM C1018 strength results. ......................................................................................67 

Table 17. Definition of toughness indices (from ASTM C1018 FIG. X1.1).................................68 

Table 18. ASTM C1018 toughness results. ...................................................................................69 

Table 19. Split tensile strength normalized by 28-day compressive strength................................79 

Table 20. First-crack parameters determined by instantaneous lateral expansion of cylinder 

and aural observations..............................................................................................................80 

Table 21. Fiber influence on postcracking behavior......................................................................90 

Table 22. Direct tension test results...............................................................................................93 

Table 23. Penetration resistance results.......................................................................................106 

Table 24. Long-term shrinkage....................................................................................................107 

Table 25. Early age shrinkage rate...............................................................................................110 

Table 26. Long-term creep results. ..............................................................................................112 

Table 27. Early age creep results. ................................................................................................114 

Table 28. Coefficient of thermal expansion results. ....................................................................116 

Table 29. Air void analysis results...............................................................................................120 

Table 30. Rapid chloride ion penetrability results.......................................................................123 

Table 31. Effect of a water bath on the compressive strength of steam-treated and untreated 

UHPC.....................................................................................................................................138 

Table 32. Timetable for ASTM C1260 specimens. .....................................................................139 

Table 33. ASTM C1260 alkali-silica reactivity expansion results. .............................................140 

Table 34. Crack width and split-cylinder peak strength results for ponded cylinders.................143 

Table 35. UHPC material characterization results for average tensile properties of UHPC. ......146 

Table 36. Average UHPC material properties presented according to curing treatment.............152 

Table 37. Compressive strength and modulus of elasticity results..............................................157 

Table 38. Constants for equation in figure 152............................................................................165 
1 

CHAPTER 1. INTRODUCTION 

1.1 INTRODUCTION 

Ultra-high performance concrete (UHPC) is a new class of concrete that has been developed in 

recent decades. When compared with high performance concrete (HPC), UHPC tends to exhibit 

superior properties such as advanced strength, durability, and long-term stability. 

Many researchers around the world have developed concretes that could be classified as UHPC. 

Although there are differences among types of UHPC, there are also many overall similarities. 

The Association Française de Génie Civil (AFGC) document Ultra High Performance FibreReinforced Concretes—Interim Recommendations indicates that UHPC tends to have the 

following properties: compressive strength that is greater than 150 megapascals (MPa) (21.7 

kilopounds per square inch (ksi)), internal fiber reinforcement to ensure nonbrittle behavior, and 

a high binder content with special aggregates.(1) Furthermore, UHPC tends to have a very low 

water content and can achieve sufficient rheological properties through a combination of 

optimized granular packing and the addition of high-range water reducing admixtures. 

Characterization of the material behaviors of UHPC has progressed to such an extent that the 

full-scale structural use of this concrete is on the horizon. To date, UHPC has been used in the 

construction of two public highway bridges,(2,3) numerous pedestrian bridges,(4,5) and a wide 

variety of other projects.(6,7,8) Research and observations to date indicate that UHPC has the 

potential to expand the use of concrete into new forms that have heretofore been impossible. 

This research program focused on determining the behaviors of UHPC because this information 

is relevant to the highway bridge industry in the United States. Currently, the only UHPC that is 

commercially available in the United States is Ductal®, a product jointly developed by Lafarge, 

Bouygues, and Rhodia. Therefore, Ductal was the UHPC product used in this research program. 

1.2 OBJECTIVE 

The objective of this research is to evaluate the potential use of UHPC in the highway bridge 

infrastructure by characterizing mechanical- and durability-based material behaviors. 

1.3 SUMMARY OF APPROACH 

The research included a significant experimental phase and an associated analytical phase. The 

experimental phase focused on determining the material behaviors of UHPC from the testing of 

over 1,000 individual specimens, with an emphasis toward determining the compressive and 

tensile behaviors, the long-term stability, and the durability of UHPC. Many of the material 

characterization tests were completed according to the American Society for Testing and 

Materials (ASTM) and the American Association of State Highway and Transportation Officials 

(AASHTO) standard test procedures; however, in some instances these tests were modified or 

new tests were devised to accurately capture the relevant behaviors of the concrete. The 

analytical phase of this research combined, analyzed, and elaborated upon the results from the 

experimental phase. This work included defining behaviors that can be anticipated to occur in 
2 

standardized concrete tests, as well as developing predictor equations for relating basic 

properties of UHPC. 

1.4 OUTLINE OF REPORT 

This report is divided into five chapters. Chapters 1 and 2 provide an introduction and relevant 

background information. Chapter 3 presents the results of the material characterization study. 

Chapter 4 presents further analysis and discussion of the research, relating results from various 

aspects of this study to one another and to the practical engineering disposition of this research. 

Finally, chapter 5 presents the conclusions of this research program. 
3 

CHAPTER 2. BACKGROUND AND PREVIOUS WORK 

2.1 UHPC CONSTITUENT MATERIALS 

The UHPC used in this study is a patented product of a major worldwide concrete manufacturer. 

The product is a reactive powder concrete that is marketed under the name Ductal. This product 

has a number of different material compositions depending on the particular application. A 

typical composition is provided in table 1. 

The constituent material proportions were determined, in part, based on an optimization of the 

granular mixture. This method allows for a finely graded and highly homogeneous concrete 

matrix. Fine sand, generally between 150 and 600 micrometers (μm), is dimensionally the largest 

granular material. The next largest particle is cement with an average diameter of approximately 

15 μm. Of similar size is the crushed quartz with an average diameter of 10 μm. The smallest 

particle, the silica fume, has a diameter small enough to fill the interstitial voids between the 

cement and the crushed quartz particles. 

Dimensionally, the largest constituent in the mix is the steel fibers. In this study, the fibers in the 

mix had a diameter of 0.2 millimeters (mm) (0.008 inch) and a length of 12.7 mm (0.5 inch). 

Given the relative sizes of the sand and the fibers, the steel fibers are able to reinforce the 

concrete matrix on the micro level. A further discussion of the properties of the steel fibers is 

provided in section 2.3. 

Table 1. Typical UHPC composition. 

Material Amount (kg/m3

 (lb/yd3)) Percent by Weight 

Portland Cement 712 (1,200) 28.5 

Fine Sand 1020 (1,720) 40.8 

Silica Fume 231 (390) 9.3 

Ground Quartz 211 (355) 8.4 

Superplasticizer 30.7 (51.8) 1.2 

Accelerator 30.0 (50.5) 1.2 

Steel Fibers 156 (263) 6.2 

Water 109 (184) 4.4 

1 kg/m3 = 1.686 lb/yd3

RDM = relative dynamic modulus (see p. 134) 

2.2 MANUFACTURER-SUPPLIED UHPC MATERIAL PROPERTIES 

As previously discussed, the UHPC used in this study is a proprietary product. The manufacturer 

has performed significant material property testing and has reported typical characteristics. Table 

2 provides some of the material properties relevant to using this material in bridge applications. 

In general, these properties have not been verified and are provided here solely for completeness. 
4 

Table 2. Manufacturer-supplied material characteristics. 

Material Characteristic Range 

Compressive Strength (MPa) 180–225

Modulus of Elasticity (GPa) 55–58.5

Flexural Strength (MPa) 40–50

Chloride Ion Diffusion (m2/s) 1.9 x 10-14

Carbonation Penetration Depth (mm) < 0.5

Freeze-Thaw Resistance (RDM) 100%

Salt-Scaling Resistance (kg/m2) < 0.012

Entrapped Air Content 2–4%

Post-Cure Shrinkage (microstrain) 0

Creep Coefficient 0.2–0.5

Density (kg/m3) 2,440–2,550

1 MPa = 145 psi 

1 GPa = 145 ksi 

1 m2

/s = 1,550 inches2/s 

1 kg/m2

 = 0.205 lb/ft2

1 kg/m3

 = 1.69 lb/yd3

2.3 STEEL FIBER MATERIAL PROPERTIES 

The steel fibers used in this test program were straight steel wire fibers manufactured by Bekaert 

Corporation. The fibers have a nominal diameter of 0.2 mm (0.008 inch) and a nominal length of 

12.7 mm (0.5 inch). The chemical composition of the fibers is shown in table 3. A thin brass 

coating is applied to the fibers during the drawing process; therefore, virgin fibers may be goldcolored. This coating disappears during the mixing process and is no longer clearly visible 

during the casting of the UHPC. 

The intended function of these fibers within UHPC requires that the fibers have a very high 

tensile strength. The manufacturer’s specified minimum tensile strength is 2,600 MPa (377 ksi), 

and tension tests are performed as a means of quality control on the fiber production. The stressstrain behavior as recorded during one of these quality control tests is presented in figure 1. The 

results from three quality control tests were used to determine an average yield strength of 

3,160 MPa (458 ksi) as calculated by the 0.2 percent offset method. The average modulus of 

elasticity was 205 gigapascals (GPa) (29,800 ksi), and the average ultimate strength was 

3,270 MPa (474 ksi). These results clearly show that these high-strength steel wires have little 

reserve strength or ductility capacity beyond yield. 
5 

Table 3. Chemical composition of steel fibers. 

Element Composition (percent) 

Carbon 0.69–0.76 

Silicon 0.15–0.30 

Manganese 0.40–0.60 

Phosphorus ≤ 0.025 

Sulfur ≤ 0.025 

Chromium ≤ 0.08 

Aluminum ≤ 0.003 

1 MPa = 145 psi 

1 GPa = 145,000 psi 

Figure 1. Graph. Sample tensile stress-strain response for steel fiber reinforcement. 

2.4 RELEVANT MATERIAL PROPERTY CHARACTERIZATION STUDIES 

2.4.1 Fiber Orientation Effect on Mechanical Properties 

Stiel, Karihaloo, and Fehling have conducted a research program investigating the effect of fiber 

orientation on the mechanical properties of UHPC.(9) These researchers focused on a patented 

UHPC marketed under the name CARDIFRC®. This UHPC is composed of similar constituent 

materials and in similar proportions to the UHPC investigated in the present study. One primary 

difference is that CARDIFRC contains two lengths of steel fibers and a total fiber volumetric 

percentage of 6 percent. 

This research program focused on the effect of UHPC flow direction during casting on the 

compressive and flexural tensile behaviors of the concrete. Fiber reinforcement tends to align 

0

500

1000

1500

2000

2500

3000

3500

0.000 0.005 0.010 0.015 0.020

Strain

Stress (MPa)

Yield Stress = 3150 MPa

Maximum Stress = 3250 MPa

Modulus of Elasticity = 210 GPa
6 

with the direction of flow during casting. This research program investigated the tensile and 

compressive behaviors of UHPC when loaded parallel to and perpendicular to the direction of 

flow during casting. The compression tests were performed on 100-mm (4-inch) cubes. The 

three-point bending flexure tests were performed on 100-mm by 100-mm (4- by 4-inch) prisms 

with a 500-mm (20-inch) length. 

The cube compression tests indicated that preferential fiber alignment has no significant effect 

on either the compressive strength or the modulus of elasticity of UHPC. However, the threepoint flexure tests showed that the peak equivalent flexural strength of the UHPC prisms was 

decreased by a factor of more than three when the fibers were preferentially aligned 

perpendicular to the principal flexural tensile forces. This preferential fiber alignment was 

clearly apparent on failure surfaces of the prisms. These prisms also did not exhibit the 

traditional postcracking toughness behaviors normally associated with UHPC and frequently 

exhibited an abrupt load decrease immediately after first cracking. All of these findings point to 

the importance of understanding the structural loadings that will be carried by a UHPC member 

and following correct placement techniques when casting UHPC members. 

2.4.2 Permeability of Cracked Concrete 

Rapoport et al. investigated the permeability of steel fiber-reinforced concrete as compared to 

normal concrete.(10) The research focused on creating small cracks in 0.5 percent and 1.0 percent 

steel fiber-reinforced concrete, then determining the permeability of the concrete. The two 

primary findings of interest from this study are as follows. First, this study confirmed the 

findings of other researchers that cracks less than 0.1 mm (0.004 inch) wide have little impact on 

the permeability of normal concrete.(11) Second, this study confirmed that steel fiber 

reinforcement reduces the total permeability of a strained section of concrete by changing the 

cracking mechanism from a few large width cracks to many small width cracks. As would be 

expected, the concrete with the higher volume percentage of fiber reinforcement displayed more 

distributed cracking and had a lower permeability. 

2.4.3 Creep and Shrinkage of UHPC 

Recall the very low post-steam treatment creep and shrinkage values presented in table 2. 

Lafarge, the manufacturer and distributor of the UHPC discussed in this report, has performed 

significant research focusing on the creep and shrinkage behaviors of this concrete. Some results 

of this research were presented in Acker wherein the microstructural behaviors leading to creep 

and shrinkage of UHPC, HPC, and normal concrete are discussed.(12) Additional discussion with 

further experimental results is presented in Acker.(13)

Acker argues that creep and shrinkage are closely related behaviors that cannot generally be 

uncoupled and studied separately. He indicates that shrinkage is primarily caused by selfdesiccation of the concrete binder resulting in the irreversible collapse of calcium-silicatehydrate (CSH) sheets. As UHPC contains a very low water-to-cementitious materials ratio, this 

concrete completely self-desiccates between casting and the conclusion of steam treatment. 

Thus, UHPC exhibits no post-treatment shrinkage. 
7 

In regard to creep, Acker restates previous research indicating that the CSH phase is the only 

constituent in UHPC that exhibits creep. Also, he points out that concrete creep tends to be much 

more pronounced when it occurs as the concrete is desiccating. Thus, the collapsed CSH 

microstructure and the lack of internal water both work to reduce UHPC creep.

2.4.4 Abrasion Resistance of HSC via ASTM C944 

Horszczaruk studied the abrasion resistance of high-strength fiber-reinforced concrete using the 

ASTM C944 standard procedure.(14,15) This is the same procedure that was followed in the 

abrasion tests discussed in section 3.13.4 of the present report. Horszczaruk’s study focused on 

83 to 100 MPa (12 ksi to 14.5 ksi) compressive strength concretes containing basalt aggregates 

(2.5 to 12.7 mm (0.1 to 0.5 inch) diameter) and natural river sands (less than 2.5 mm (0.1 inch) 

diameter). The testing followed ASTM C944, except that the duration of test was increased from 

2 to 40 minutes to allow for differentiation between concretes. 

The relevant results from this study include the following. The linear best-fit approximation of 

the concrete mass loss per 2-minute abrading cycle ranged from 0.14 to 0.78 grams (0.005 to 

0.027 ounce). Of the 10 concretes tested, six of them ranged from 0.14 to 0.25 grams (0.005 to 

0.009 ounce). Horszczaruk also indicates that the rate of mass loss was relatively consistent 

throughout the abrading, with no clear increased abrasion resistance during the abrading of the 

smooth exterior face of the concrete.

9 

CHAPTER 3. UHPC MATERIAL CHARACTERIZATION 

3.1 RESEARCH PLAN 

The stated goal of the UHPC material characterization study is to determine the basic behaviors 

of UHPC with the intent of using UHPC in highway bridges. Many material behaviors are 

critical to the successful use of concrete in a highway bridge. These behaviors include strength, 

durability, and long-term stability. Each of these behaviors will be discussed in depth in this 

chapter. 

The curing treatment applied to concrete—which is always important—is even more important 

in the case of UHPC. The UHPC studied in this research program is normally steam treated once 

it has reached sufficient strength to undergo the process. However, steam treatment of UHPC in 

a controlled environment may not always be feasible or even desirable. For this reason, the focus 

of the research, which is discussed in this chapter, is on characterizing UHPC that had been 

treated to one of four curing conditions. The four curing conditions were the standard steam 

treatment, a delayed version, a lower temperature version of the same steam treatment, and an 

untreated regime wherein no steaming was conducted. These curing regimes will be described in 

more detail later in this chapter. 

This chapter describes the results of the material characterization study. The chapter begins by 

introducing specimen nomenclature and test matrix information. Next, the batching, casting, and 

curing procedures and results are presented. Results from individual tests that focused on specific 

aspects of UHPC behavior are discussed in the remainder of this chapter. 

3.1.1 Batch and Specimen Nomenclature 

The material characterization study detailed in this chapter included well over 1,000 individual 

UHPC specimens. A naming scheme was created to allow for easy, unique identification of each 

specimen. The large majority of specimens cast for this study were part of the standardized set of 

batches designed to investigate the behavior of UHPC cured under various curing conditions. 

The other specimens were cast in extraneous batches that focused on specific behaviors of 

UHPC, primarily as related to compression testing. 

For clarity, the nomenclature discussed here will not be mentioned throughout most of this 

chapter. In most instances, the presentation and discussion of results can be completed without 

naming individual specimens. However, in some instances, the naming system has to be used 

because of the large amount of similar specimens and testing procedures. 

The nomenclature used to describe the specimens in the standardized batches is based on a fivedigit alphanumeric identifier. A letter that identifies the premix delivery fills the first digit in the 

identifier. This letter is L, M, or N for the first, second, or third delivery, respectively. The 

second digit in the identifier is filled by an integer that identifies the curing treatment applied to 

the specimen. This number ranges from one to four for the steam-treated, untreated, tempered 

steam-treated, and delayed steam-treated regimes, respectively. The third digit in the identifier is 

reserved for a letter that identifies the tests performed on that batch of specimens. These letters 
10 

and the associated tests will be discussed further in the following section. A two-digit number 

indicating the particular specimen within the batch occupies the fourth and fifth digits. In the few 

cases in which a batch was repeated, the second batch was named identically to the first batch, 

except that the letter A was placed at the end of the identifier for each specimen. 

An example of this alphanumeric identification scheme is as follows. The first three digits in the 

name M1F01 indicate that this specimen was from the second premix delivery that was first 

steam-treated and then subjected to testing within the general durability batches. The “01” in the 

fourth and fifth digits indicates that this specimen was a prism subjected to freeze-thaw testing. 

A simpler naming scheme was used for the extraneous batches. These batches were named 

sequentially based on the premix delivery. For example, the 10th batch cast from the L delivery 

focused on the effect of varying the load rate on compression test results. Thus, the specimens 

from this batch are identified as L10-xx, with the xx being an integer identifier for each of the 

cylinders cast. In general, 30 or more cylinders populated these batches, and the associated 

testing was designed to indicate the effect of some external action on compressive behavior. The 

primary exception to this rule is batch N06, which focused on the early age shrinkage behavior 

of UHPC. 

3.1.2 Test Matrix 

The test matrix devised for the material characterization study was intended to cover a wide 

range of the basic behaviors of UHPC. The types of tests performed can generally be grouped 

into three classes. First, strength tests focused on the compressive and tensile behaviors of UHPC 

at various ages and under various curing conditions. Second, durability tests focused on the 

durability of UHPC under conditions with standardized aggressors. Finally, stability tests 

focused on the long-term ability of UHPC to maintain dimensional stability under various 

loading and environmental conditions. 

Table 4 lists the standardized batches that were used throughout this study. The batch letter listed 

in the first column is the same letter that would reside in the third digit of the alphanumeric 

identifier in a specimen’s name. A simplified description of the batch along with the associated 

testing is included in the next column. The specimens cast for each particular batch are included 

in the third column. Finally, the last two columns include the volume of material that was in an 

individual batch as well as the curing regimes for which each batch was cast. A batch of the size 

indicated was cast for each curing regime listed. Note that this large table continues across three 

pages. 

Table 5 provides similar information for the extraneous batches that were cast. The only 

difference between this table and table 4 is that the batches were not cast for each curing regime 

listed. Because the first column lists individual batches of concrete, the curing regimes listed 

were applied to some of the specimens in each batch. 
11

Table 4. Batching descriptions with associated specimens and curing regimes. 

Batch Batch Description Specimens Cast † Batch Size (m3) Curing 

Test Completed 

A Compressive Strength 0.033 1,

Compressive Strength, Stress-Strain 36 76x152 cylinders – Penetration Resistance 1 152x152 cylinder – 

B Cubes/Cylinders Compression 0.025 

28-day Compressive Strength 6 76x152 cylinders – 

28-day Compressive Strength 3 76x165 cylinders – 28-day Compressive Strength 6 51x102 cylinders – 

28-day Compressive Strength 5 102x203 cylinders – 28-day Compressive Strength 6 51 mm cubes – 

28-day Compressive Strength 5 100 mm cubes – 

C Split Tensile 0.027 1,

Split Tensile 12 102x203 cylinders – 

28-day Compressive Strength 6 76x152 cylinders – 

D Direct Tension 0.028 1,

Direct Tension 12 102x203 cylinders – Mortar Briquette 18 briquettes – 

28-day Compressive Strength 6 76x152 cylinders – 

E Prism Flexure 0.031 1,

Prism Flexure 3 76x102x406 prisms – 

Prism Flexure 8 51x51x279 prisms – 

Prism Flexure 8 51x51x432 prisms – 

28-day Compressive Strength 6 76x152 cylinders – 

P Fracture/Fatigue of Prisms 0.030 

Notched Prism Fracture 4 51x102x457 prisms – 

Flexural Toughness 5 51x51x432 prisms – 

Flexural Fatigue 7 51x51x432 prisms – 

28-day Compressive Strength 6 76x152 cylinders – 
12

Table 4. Batching descriptions with associated specimens and curing regimes (continued

Batch Batch Description Specimens Cast † Batch Size (m3) Curing 

Test Completed 

F Durability (Cl- Pen, FT, etc.) 0.028 1,

Rapid Chloride Penetrability 3 102x76 cylinders – Chloride Penetration 3 102x76 cylinders – Freeze-Thaw 3 76x102x406 prisms – 

Abrasion 3 152x76 cylinders – ASR 6 25x25x279 prisms – 

28-day Compressive Strength 6 76x152 cylinders – 

Q Freeze-Thaw Supplemental 0.031 1,

Frequency Response 4 76x102x406 prisms – 28-day Compressive Strength 6 76x152 cylinders – 

G Scaling Slabs 0.025 1,

Scaling 2 76x356x356 slabs – 

28-day Compressive Strength 6 76x152 cylinders – 

R Split Tensile Crack Corrosion 0.030 

Split Tensile 9 102x203 cylinders – Split Tensile w/ Ponding 6 102x203 cylinders – 

28-day Compressive Strength 6 76x152 cylinders – 

H Creep, Shrinkage, Thermal Expansion 0.030 1,

Creep 5 102x203 cylinders – Shrinkage 3 76x76x279 prisms – 

Thermal Expansion 3 102x203 cylinders – 28-day Compressive Strength 6 76x152 cylinders – 

S Early Age Sustained Compressive Stress 0.027 

Compressive Strength 25 76x152 cylinders – 

28-day Compressive Strength 8 76x152 cylinders – 
13

Table 4. Batching descriptions with associated specimens and curing regimes (continued

Batch Batch Description Specimens Cast † Batch Size (m3) Curing 

Test Completed 

J Air Content, Fiber Dispersion 0.020 

Air Content, Fiber Dispersion 8 102x203 cylinders – 28-day Compressive Strength 6 76x152 cylinders – 

K * Air Content, Fiber Dispersion 0.020 

Air Content, Fiber Dispersion 8 102x203 cylinders – 28-day Compressive Strength 6 76x152 cylinders – 

M Heat of Hydration 0.025 

Heat of Hydration 

3 152x305 cylinders – 

Heat of Hydration 

2 76x152 cylinders – 

28-day Compressive Strength 6 76x152 cylinders – 

N * Heat of Hydration 0.025 

Heat of Hydration 

3 152x305 cylinders – 

Heat of Hydration 

2 76x152 cylinders – 

28-day Compressive Strength 6 76x152 cylinders – 

† Cylinders listed as diameter x height. Prisms listed as depth x width x length in tested configuration. ‡ 1 = Steam, 2 = Untreated, 3 = Tempered Steam, 4 = Delayed Steam * Batch mix design did not contain any accelerator. 

1 m3 = 35.315 ft3
14

Table 5. Batching descriptions with associated specimens and curing regimes for nonstandardize

Batch Batch Description Specimens Cast † Batch Size (m3) Curing 

Test Completed 

L10 Load Rate Effect 0.028 

Compressive Strength 24 76x152 cylinders – 

L12 Fiber Effect on Compression 0.0042 

Compressive Strength 3 76x152 cylinders – 

L21 Long-Term Delayed Steam 0.028 

Compressive Strength 36 76x152 cylinders – 

L22 Long-Term Delayed Steam 0.028 

Compressive Strength 36 76x152 cylinders – 

L23 Demolding Time Effect 0.030 

Compressive Strength 36 76x152 cylinders – 

N06 Early Age Shrinkage 0.0048 

Shrinkage 2 76x76x279 prisms – Compressive Strength 2 76x152 cylinders – 

† Cylinders listed as diameter x height. Prisms listed as depth x width x length. ‡ 1 = Steam, 2 = Untreated 

1 m3 = 35.3 ft3
15

3.2 BATCHING, CASTING, AND CURING OF UHPC 

The first phase of the material characterization study described in this chapter is to determine the 

properties of fresh UHPC. To achieve consistent results throughout the entire study, a series of 

specific, standardized procedures were implemented for the creation of the specimens described 

in the previous section. The casting of these specimens allowed for the mixing, associated 

testing, and observation of over 50 batches of UHPC. 

The UHPC used in this study can be divided into three parts—premix, fibers, and liquids. The 

premix consists of all of the cementitious, aggregate, and filler materials (described in chapter 2). 

The premix was batched and blended by the manufacturer and delivered in bulk to the 

researchers. All of the UHPC testing described in this chapter consists of specimens created from 

one of the three premix deliveries made to the researchers over the course of 18 months. As 

described in the previous section, these deliveries were designated as the L, M, and N premixes. 

For the purpose of this study, all of the premixes are assumed to be identical; however, it is 

realized that manufacturing processes can vary with time, and the final premix product could 

show slight variations. 

The liquids that were mixed with the UHPC included water, accelerator, and a high-range waterreducing admixture (HRWA). The accelerator used in this study was Rheocrete CNI. The 

HRWA was Glenium 3000NS. 

The fibers included in the UHPC were always nondeformed cylindrical steel fibers that were 

12.7 mm (0.5 inch) long and had a 0.2-mm (0.008-inch) diameter. These fibers were included in 

the mix at a concentration of 2 percent by volume. 

The mix proportions used throughout this study included: 

• Premix 2,195 kg/m3 (137.0 lb/ft3) of concrete. 

• Water 109 kg/m3 (6.81 lb/ft3) of concrete. 

• HRWA 30.8 kg/m3 (1.92 lb/ft3) of concrete. 

• Accelerator 30.0 kg/m3 (1.87 lb/ft3) of concrete. 

• Steel Fibers 156 kg/m3 (9.74 lb/ft3) of concrete. 

These mix proportions were followed for all except three batches. Those three batches followed 

a slightly different mix design as required by the material property being investigated. In two 

batches, M1K and M1N, the accelerator was replaced by an additional 28.8 kg/m3

 

(1.80 lb/ft3) of water. In the remaining batch, L12, the fibers were not included in the mix so that 

compression test behavior in nonreinforced UHPC could be studied. 

A 0.057-m3

 (2.0-ft3) capacity pan mixer was used for nearly all of the UHPC mixing. This 1934 

vintage mixer was somewhat underpowered for this application, resulting in extended mix times 

as compared to mixing UHPC in modern mixers. Regardless, this mixer was able to impart 

enough energy into the mix to obtain sufficient rheology for the casting of laboratory specimens. 

A 0.0085-m3

 (0.3-ft3) mixer was used for the two batches that required only a small volume of 

material. 
16

Key points in the mixing procedure are shown in figure 2. The mixing procedure for UHPC 

included the following steps: 

• Weigh all constituent materials. Add half of HRWA to water. 

• Place premix in mixer pan and mix for 2 minutes. 

• Add water (with half of HRWA) to premix slowly over the course of 2 minutes. 

• Wait 1 minute, then add remaining HRWA to premix over the course of 30 seconds. 

• Wait 1 minute, then add accelerator over the span of 1 minute. 

• Continue mixing as the UHPC changes from a dry powder to a thick paste. The time 

for this process will vary. 

• Add fibers to the mix slowly over the course of 2 minutes. 

• After the fibers have been added, continue running mixer for 1 minute to ensure that 

the fibers are well dispersed. 

As soon as mixing was completed, the casting of specimens and the measurement of the 

rheological properties of the UHPC commenced. The rheology of the UHPC was measured via a 

flow table test similar to that described in ASTM C1437.(16) In the test that was implemented in 

this study, the mini slump cone is filled, then removed to allow the concrete to flow outward. 

Once the concrete reaches a steady state, the average diameter is determined by measuring the 

concrete at three locations. Next, the flow table is dropped 20 times in approximately 20 seconds. 

Again, the concrete is allowed to settle, and then its average diameter is recorded. 

The casting of all UHPC specimens used in this material characterization study was completed 

within 20 minutes after the completion of mixing. All specimens were cast on a vibrating table 

and were allowed to remain on the table for approximately 30 seconds after filling. The filling of 

molds was completed via scoops used to move the UHPC from the mixing pan into the mold. In 

prisms specimens for flexure tests, the UHPC was always placed in one end of the mold and 

allowed to flow to the other end to complete the filling. As was discussed in section 2.4.1, the 

tensile properties of UHPC depend on fiber reinforcement orientation, which is a direct result of 

casting procedure. 

After filling, specimens were removed from the vibrating table and were screeded. Although 

screeding is not normally recommended for UHPC and is very difficult to complete on a largescale cast, it was implemented here to make the later preparation of cured specimens easier. 

After screeding, each specimen had its exposed surface covered in plastic to prevent moisture 

loss. The specimens then sat undisturbed until final set had occurred. 

The demolding of the specimens occurred approximately 24 hours after casting. As will be 

discussed later in this chapter, this timetable sometimes resulted in the demolding of specimens 

that were marginally ready and had only very recently achieved set. Demolding of larger 

specimens that would have needed sufficient strength to support their greater self-weight would 

not have been possible in all cases on this timetable. 

As previously mentioned, four curing regimes were implemented to study UHPC characteristics 

under different curing conditions. The standard, manufacturer-recommended curing treatment 

included steaming the UHPC at 90 oC (194 oF) and 95 percent relative humidity (RH) for 48 

hours. 
17

Figure 2. Photos. Mixing of UHPC. (a) Water addition. (b) HRWA addition. (c) Prepaste 

consistency. (d) Fiber addition. (e) Finished mix. 

(b)

(d)

(e)

(c)

(a)
18

In practice, this procedure included 2 hours of increasing steam and 2 hours of decreasing steam, 

leaving 44 total hours of constant steaming at 90 oC (194 oF) and 95 percent RH. This treatment 

was initiated within 4 hours after demolding. This curing condition will henceforth be referred to 

as steam treatment. 

The remaining three regimes include untreated, tempered steam treatment, and delayed steam 

treatment. The untreated regime allowed the specimens to remain in a standard laboratory 

environment from demolding until testing. The tempered steam treatment is very similar to the 

steam treatment, except that the temperature inside the steam chamber was limited to 60 oC 

(140 oF). Finally, the delayed steam treatment is a curing regime wherein the steam treatment 

described above is followed, but it is not initiated until the 15th day after casting. Until the 15th

day, delayed steam specimens are equivalent to untreated specimens. Note that this report also 

uses the term steam-based treatment, which refers to the collection of specimens that underwent 

either steam treatment, tempered steam treatment, or delayed steam treatment. 

Tables 6 and 7 provide information relating to the casting and curing of each of the batches 

included in this material characterization study. Table 6 focuses on the primary sets of specimens 

that were implemented across most curing regimes. These batches were all cast with premix 

from the M or N deliveries. Table 7 provides the results from other batches cast and primarily 

focuses on material cast from the L premix delivery. 

The tables include a listing of the age of the UHPC premix at the time of casting along with the 

total mixing time for each batch. Observation of the mixing and casting procedure throughout all 

the mixes completed in this study indicated that the behavior of the mix changed as the premix 

aged. This qualitative observation is confirmed by the results as shown in figure 3. The total 

mixing time for each mix in the M and N premixes is plotted against the premix age at casting. 

This figure shows a clear trend in which younger premixes could mix within 15 minutes while 

older mixes could take twice as long. Although not verified, it is likely that these increased times 

result from the agglomeration of fine particles within the premix as it ages. These mix times are 

relative and are specifically applicable only to the pan mixer used in this study. 

The rheology measures are also listed in tables 6 and 7. The final values ranged from 

165 to 210 mm (6.5 to 8.25 inches). This wide range is indicative of large differences in 

rheology. The UHPC exhibiting the stiffer results was much more difficult to cast and would 

definitely have been problematic outside of a laboratory setting. Alternatively, a flow measure 

that was near or above 200 mm (8 inches) was consistent with UHPC that was easy to place. 

Figure 4 shows the final rheology values for the M and N premixes. The first castings of the M 

premix were completed during a time period when the laboratory conditions included very low 

humidity. For this reason, the results are shown in two series that indicate whether the humidity 

was low or normal during casting. Very stiff rheology results were obtained for a few mixes just 

before the rectification of the humidity situation. It is expected that the large diameter of the pan 

in the pan mixer combined with the low air moisture content worked to sap a small amount of 

moisture from the UHPC. This moisture loss was sufficient to adversely affect the rheological 

properties. 
19

Table 6. Batching and casting properties of steam-treated and untreated UHPC. 

Batch Batch Description Age at Flow (mm) 

Casting 

Batch 

Size (m3

)

Mixing 

Time (min) Initial Final 

Demolde

(hours) 

N1A Compressive Stren

gth 4 months 0.032 24.0 170 195 22.5 

N1AxxA Compressive Stren

gth 5 months 0.027 28.0 170 190 28.0 

M1B Cubes/C

ylinders Compression 6 weeks 0.025 20.0 150 170 26.0 

N1C S

plit Tensile 2 weeks 0.027 20.0 165 190 22.0 

M1D Direct Tension 9 weeks 0.028 22.0 165 190 28.0 N1E Prism Flexure 4 weeks 0.031 21.0 170 195 23.0 M1F Durabilit

y (Cl- Pen, FT, etc.) 4 weeks 0.028 15.0 165 190 26.0 

M1G Scalin

g Slabs 4 weeks 0.025 14.0 180 190 26.0 

N1H Cree

p and Shrinka

ge 15 weeks 0.030 23.0 170 195 22.5 

M1J Air Content, Fiber Dis

persion 6 weeks 0.020 21.5 140 165 26.0 

M1JxxA Air Content, Fiber Dis

persion 8 weeks 0.020 23.0 160 190 26.0 

M1K † Air Content, Fiber Dis

persion 7 weeks 0.020 20.0 165 190 72.0 

M1M Heat of H

ydration 6 weeks 0.025 21.5 140 170 26.0 

M1MxxA Heat of H

ydration 7 weeks 0.014 23.5 145 170 26.0 

M1MxxB Heat of H

ydration 8 weeks 0.025 20.0 160 185 26.0 

M1N † Heat of H

ydration 7 weeks 0.025 16.5 185 210 72.0 

M1P Fracture/Fati

gue of Prisms 7 weeks 0.030 20.0 160 190 26.0 

M1

Q Freeze-Thaw Supplemental 12 weeks 0.031 20.0 150 180 29.0 

N2A Compressive Stren

gth 8 weeks 0.032 22.0 170 200 20.0 

N2C S

plit Tensile 3 weeks 0.027 21.0 180 210 24.0 

M2D Direct Tension 9 weeks 0.028 21.0 150 180 26.0 N2E Prism Flexure 3 weeks 0.031 20.0 170 195 24.0 M2F Durabilit

y (Cl- Pen, FT, etc.) 5 weeks 0.030 17.5 150 180 26.0 

M2G Scalin

g Slabs 5 weeks 0.025 17.0 160 185 26.0 

N2H Cree

p and Shrinka

ge 8 weeks 0.030 21.5 165 190 22.0 

M2P Fracture/Fati

gue of Prisms 12 weeks 0.030 20.0 150 185 29.0 

M2

Q Freeze-Thaw Supplemental 12 weeks 0.031 19.5 160 190 29.0 

† Batch mix design did not contain any accelerator. 
20

Table 6. Batching and casting properties of tempered steam and delayed steam-treated UHPC (c

Batch Batch Description Age at Flow (mm) 

Casting 

Batch 

Size (m3)

Mixing 

Time (min) Initial Final 

Demo

(hou

N3A Compressive Strength 8 weeks 0.032 20.0 165 190 21 N3C Split Tensile 3 weeks 0.027 21.0 180 200 23 M3D Direct Tension 8 weeks 0.028 20.0 165 190 26 N3E Prism Flexure 5 weeks 0.031 21.0 170 195 25 M3F Durability (Cl- Pen, FT, etc.) 5 weeks 0.030 15.0 160 185 26 

M3G Scaling Slabs 5 weeks 0.025 14.0 165 185 26 N3H Creep and Shrinkage 7 weeks 0.030 20.0 165 195 22 M3Q Freeze-Thaw Supplemental 12 weeks 0.031 19.5 160 190 29 N4A Compressive Strength 2 weeks 0.032 16.0 165 195 24 N4C Split Tensile 2 weeks 0.027 20.0 170 200 23 M4D Direct Tension 7 weeks 0.028 20.0 160 190 26 N4E Prism Flexure 2 weeks 0.031 20.0 170 200 22 M4F Durability (Cl- Pen, FT, etc.) 4 weeks 0.028 16.0 170 195 26 M4G Scaling Slabs 4 weeks 0.025 15.0 165 185 26 N4H Creep and Shrinkage 7 weeks 0.030 21.0 185 210 23 M4Q Freeze-Thaw Supplemental 12 weeks 0.031 20.0 150 180 29 

1 m3 = 35.3 ft3
21

Table 7. Batching and casting properties of batches cast to complete the study by addressing spe

Batch Batch Description Age at Flow (mm) 

Casting 

Batch 

Size (m3)

Mixing 

Time (min) Initial Final 

Demo

(hou

L1B Cubes/Cylinders Compression 16 months 0.025 26 150 180 48 L1F Durability (Cl- Pen, FT, etc.) 11 months 0.025 20 160 180 41 

L1G Scaling Slabs 11 months 0.028 23 160 185 44 L1R Split Tensile Cracked Corrosion 16 months 0.031 22 165 190 48 L2B Cubes/Cylinders Compression 8 months 0.025 34 150 180 114 L2F Durability (Cl- Pen, FT, etc.) 11 months 0.028 17 145 170 50 L2G Scaling Slabs 11 months 0.025 20 145 170 43 L2R Split Tensile Cracked Corrosion 16 months 0.031 24 170 195 47 L2S Early Age Sustained Compressive Stress 26 months 0.028 23 170 195 – 

L03 † Fiber Dispersion 8 months 0.028 19 210 235 139 

L10 Load Rate Effect 10 months 0.028 27 165 185 47 L12 Fiber Effect on Compression 11 months 0.0042 18 185 200 42 L21 Long-Term Delayed Steam 25 months 0.028 22 185 210 44 L22 Long-Term Delayed Steam 25 months 0.028 22 185 200 43 L23 Demolding Time Effect 25 months 0.028 22 165 190 VarioN06 Early Age Shrinkage 5 months 0.0048 23 N/A N/A – 

† Batch mix design did not contain any accelerator. 

 1 m3 = 35.3 ft3
22

Figure 3. Graph. Mix time as affected by premix age. 

1 mm = 0.039 inch 

Figure 4. Graph. Final flow diameter as affected by premix age. 

140

160

180

200

220

0 20 40 60 80 100 120 140 160

Premix Age at Casting (days)

Final Flow Diameter (mm) m

M premix, low humidity

M premix, normal humidity

N premix

0

5

10

15

20

25

30

0 20 40 60 80 100 120 140 160

Premix Age at Casting (days)

Mixing Time (minutes) m

M premix, low humidity

M premix, normal humidity

N premix
23

3.3 COMPRESSION TESTING 

A significant portion of this study focused on the behaviors of molded UHPC cylinders under 

compressive loading. The compressive tests discussed in this section were all completed 

nominally according to the ASTM C39 standard test method for cylinders and the ASTM C109 

standard test method for cubes.(17,18) Throughout the entire material characterization study, nearly 

1,000 compression specimens were tested. Although the load configuration and basic test setup 

remained constant, various properties were investigated through the use of different data 

collection techniques and different specimen geometries. 

The basic compression test on UHPC was the ASTM C39 test on a 76-mm (3-inch) diameter 

cylinder. This test was used as a control test throughout the material characterization study to 

ensure that consistent batching, mixing, and curing of the UHPC had occurred. The 76-mm 

(3-inch) cylinders were cast on a vibrating table in 152-mm (6-inch)-tall plastic molds. The 

concrete was scooped into the molds and was not rodded due to the presence of the fibers. Once 

full, the molds were held on the vibrating table for a few extra seconds before being removed and 

having their top surfaces screeded. After the molds were set, curing treatments were applied 

according to each of the four curing regimes. 

The preparation of the cylinders for testing was somewhat more involved than that normally used 

for cylinder testing. The largest difference is that the end planeness of the cylinders was ensured 

through the use of an end grinder. Figure 5 shows a picture of the end grinding procedure. All 

cylinders that were projected to have strengths above 83 MPa (12 ksi) were subjected to 

grinding. UHPC cylinders with strengths under this level were sulfur-capped, as grinding tends 

to pull out the fibers and create a nonuniform end surface at these lower strengths. 

After grinding, the cylinders were measured to verify end planeness and to determine length, 

diameter, and density. The end planeness was verified through the use of a flat steel plate and a 

dial gage as shown in figure 5. Sliding the cylinder under the dial gage allowed for determination 

of the longest and shortest lengths of the cylinder to an accuracy of 0.025 mm (0.001 inch). The 

out-of-planeness of the cylinder ends could be determined from this information. Each cylinder 

had to exhibit under 1 degree of out-of-planeness or the ends were reground. Next, the length 

was measured at four points around the circumference of the cylinder, and the average was 

calculated. The diameter was then measured at six locations—at the top, middle, and bottom of 

the cylinder on two perpendicular diameters. The average value was then used to calculate the 

area of the cylinder. Finally, the mass of the cylinder was determined, and the density was 

calculated. The entire grinding and measuring procedure was completed after any steam-based 

treatment was applied to the cylinder or at least 2 weeks after casting for the untreated 

specimens, unless the testing timetable required otherwise. In this case, a cylinder was prepared 

for testing within 1 day of performing the test. 

The testing of all cylinders was completed in a Forney 4,450-kilonewton (kN) (1,000-kilopound 

(kip)) capacity compression testing machine. The operator of this hydraulically operated machine 

uses a needle valve to set and control the loading rate. Aside from some preliminary testing that 

will be discussed later, the load rate for all cylinder and cube compression tests was set at 

1.0 MPa/s (150 pounds per square inch per second (psi/s)). This load rate is higher than the load 

rates recommended in ASTM C39 and C109. The increased load rate was necessary, as the high 
24

strengths of UHPC would mean that a UHPC cylinder tested at the ASTM C39 load rate might 

take 15 to 20 minutes to reach failure. A small study, completed to determine if the higher load 

rate would have any adverse effect on results, indicated that the higher load rate was acceptable. 

A further discussion of this set of tests is presented subsequently in this chapter. 

Figure 5. Photos. (a) Grinding and (b) measuring of 76-mm (3-inch) diameter cylinders. 

A 165-mm (6.5-inch) diameter spherical bearing was used as the upper loading platen for all 

cylinder and cube tests. Although this bearing would normally only be used for 102-mm (4-inch) 

diameter cylinders, it was used for all of the tests because of the high stress levels placed on the 

bearing by this high-strength concrete. The lower test platen was always either the machine’s 

lower platen or a machined steel plate resting on the machine’s platen. 

Figure 6 shows a cylinder that is prepared for testing. All specimens were loaded via the 

hydraulically controlled constant load rate from approximately 22 kN (5 kips) through failure. 

The cylinders and cubes exhibited very little decrease in stiffness throughout testing, thus the 

load rate remained relatively constant. Figure 6 also shows a picture of a cylinder after testing. 

Notice that the cylinder remains intact. This is due to the presence of the fibers as will be 

discussed more fully later in this chapter. 

3.3.1 Strength 

The compressive strength results from 44 batches of UHPC are presented in table 8. These 

results are from the control cylinders cast with each batch from the M and N deliveries of UHPC 

premix. Control cylinder results from the L delivery are shown in table 9. All tests were 

performed on the corresponding day that is listed in the table. In general, six cylinders were 

tested for each batch, and the results were averaged to obtain the result shown in the table. 

(a) (b)
25

Figure 6. Photos. 76-mm (3-inch) diameter cylinders (a) before 

and (b) after compression testing. 

Based on these tests, the average 28-day compressive strength of UHPC has been determined. 

Steam-treated UHPC was found to have a compressive strength of 193 MPa (28.0 ksi) with a 

95 percent confidence interval of 191 to 195 MPa (27.7 to 28.3 ksi). Untreated UHPC has a 

compressive strength of 126 MPa (18.3 ksi) with a 95 percent confidence interval of 

123 to 129 MPa (17.9 to 18.7 ksi). Tempered steam-treated UHPC has a compressive strength of 

171 MPa (24.8 ksi) with a 95 percent confidence interval of 168 to 174 MPa (24.3 to 25.3 ksi). 

Finally, delayed steam-treated UHPC has a compressive strength of 171 MPa (24.8 ksi) with a 

95 percent confidence interval of 168 to 174 MPa (24.3 to 25.3 ksi). 

These results, although quite high for concrete, are likely lower than would normally be observed 

with this UHPC. Two factors that have been found to clearly influence the compressive strength 

are the environment that UHPC is kept in before any steam-based treatment and the steaming 

environmental conditions. In this study, the UHPC was demolded as soon as it had sufficient 

strength to maintain its geometric integrity. At this age, the UHPC is still rather permeable and is 

susceptible to moisture loss and resulting lower strength values. This factor will be discussed in 

more depth later in this chapter. Second, the UHPC manufacturer recommends 48 hours of steam 

treatment, whereas in this study the UHPC actually received only 44 hours at the steam treatment 

level with 2 hours of ramping up and down from room temperature. 

(a) (b)
26

Table 8. Control cylinder compressive strength results from the M and N deliveries. 

Batch Curing Batch Description Compressive Strength (MPa) 

Day No. Average St. Dev. 

N1A Steam Compressive Strength 28 6 181 7.0

N1AxxA Steam Compressive Strength 30 6 196 14.3

M1B Steam Cubes/Cylinders Compression 29 6 203 4.2

N1C Steam Split Tensile 28 6 181 12.1

M1D Steam Direct Tension 28 6 168 6.6

N1E Steam Prism Flexure 28 6 189 8.6

M1F Steam Durability (Cl- Pen, FT, etc.) 28 6 202 11.0

M1G Steam Scaling Slabs 28 6 201 10.5

N1H Steam Creep and Shrinkage 28 6 188 9.2

M1J Steam Air Content, Fiber Dispersion 28 6 191 8.5

M1JxxA Steam Air Content, Fiber Dispersion 28 6 177 7.4

M1K Steam Air Content, Fiber Dispersion 28 6 188 9.2

M1M Steam Heat of Hydration 28 6 194 6.9

M1MxxA Steam Heat of Hydration 28 6 191 15.0

M1MxxB Steam Heat of Hydration 28 6 197 7.5

M1N Steam Heat of Hydration 28 6 195 9.4

M1P Steam Fracture/Fatigue of Prisms 28 6 177 17.4

M1Q Steam Freeze-Thaw Supplemental 28 4 143 5.0

N2A Untreated Compressive Strength 28 6 112 5.0

N2C Untreated Split Tensile 28 6 121 4.5

M2D Untreated Direct Tension 28 6 106 3.4

N2E Untreated Prism Flexure 28 6 121 4.8

M2F Untreated Durability (Cl- Pen, FT, etc.) 28 6 131 5.2

M2G Untreated Scaling Slabs 28 6 117 3.3

N2H Untreated Creep and Shrinkage 28 6 114 3.4

M2P Untreated Fracture/Fatigue of Prisms 28 6 107 4.3

M2Q Untreated Freeze-Thaw Supplemental 28 4 105 1.4

N3A T.Steam Compressive Strength 28 6 154 6.3

N3C T.Steam Split Tensile 28 6 168 4.6

M3D T.Steam Direct Tension 28 6 174 5.0

N3E T.Steam Prism Flexure 28 6 173 4.2

M3F T.Steam Durability (Cl- Pen, FT, etc.) 28 6 183 8.3

M3G T.Steam Scaling Slabs 28 6 170 9.4

N3H T.Steam Creep and Shrinkage 28 5 177 2.1

M3Q T.Steam Freeze-Thaw Supplemental 28 5 149 5.0

N4A D.Steam Compressive Strength 30 6 173 7.2

N4C D.Steam Split Tensile 28 6 170 6.1

M4D D.Steam Direct Tension 28 6 158 11.9

N4E D.Steam Prism Flexure 28 5 172 4.3

M4F D.Steam Durability (Cl- Pen, FT, etc.) 28 6 179 7.4

M4G D.Steam Scaling Slabs 28 6 179 7.2

N4H D.Steam Creep and Shrinkage 28 5 168 8.3

M4Q D.Steam Freeze-Thaw Supplemental 28 5 123 6.4

1 MPa = 145 psi 
27

Table 9. Control cylinder compressive strength results from the L delivery. 

Batch Curing Batch Description Compressive Strength (MPa) 

Day No. Average St. Dev. 

L1B Steam Cubes/Cylinders Compression 28 6 210 7.0 

L2B Untreated Cubes/Cylinders Compression 28 8 149 3.8 

L1R Steam Split Tensile Ponding 28 6 206 5.2 

L2R Untreated Split Tensile Ponding 28 6 142 4.1 

1 MPa = 145 psi 

The results from the individual compression tests listed in table 8 are shown in figures 7 and 8 

relative to density and cylinder end planeness, respectively. The density results show that this 

concrete ranges from 2,400 to 2,500 kg/m3 (150 to 156 lb/ft3). An estimate of 2,480 kg/m3 

(155 lb/ft3) is reasonable regardless of the type of curing treatment applied. Also, within each 

curing regime there seems to be a slight increase in compressive strength as the density 

increases. The cylinder end planeness results show that the out-of-planeness of the cylinders had 

little impact on the compressive strength. The spherical bearing used in these tests clearly was 

able to accommodate for these differences in planeness. 

1 MPa = 145 psi 

1 kg/m3 = 1.69 lb/yd3

Figure 7. Graph. Compressive strength and density of control cylinders. 

0

50

100

150

200

250

2350 2400 2450 2500 2550

Density (lb/ft3)

Compressive Strength (MPa)

Steam Treated Untreated

Tempered Steam Treated Delayed Steam Treated

Density (kg/m3)
28

1 MPa = 145 psi 

Figure 8. Graph. Compressive strength and cylinder end planeness of control cylinders. 

3.3.2 Strength, Modulus of Elasticity, and Strain Capacity With Time 

The age-dependent compressive behavior of UHPC was investigated through a series of 

compression tests completed from 1 day to 8 weeks after casting. This testing provided results 

related to strength, modulus of elasticity, and compressive strain capacity both before and after 

the application of any curing treatment. 

These tests were also completed in the 4,450-kN (1,000-kip) Forney test machine following the 

same procedures discussed previously. However, in these tests, a deformation measuring device 

was attached to each cylinder to capture the axial movement. These tests were completed 

nominally in accordance with ASTM C469 with one exception: A number of the allowances 

made by this test method for the collection of data via simplified systems were bypassed in favor 

of more robust methodologies.(19) 

The test setup included the specially designed axial deformation measuring device shown in 

figure 9. The two parallel rings are both rigidly attached to the cylinder with a 51-mm (2-inch) 

gage length between attachment points. The upper ring holds three linear variable displacement 

transducers (LVDTs) whose ends bear on the lower ring. Thus, the axial deformation of the 

cylinder can be measured accurately from initiation of loading through failure. The load and the 

output from the three LVDTs were digitally recorded at approximately 4 hertz (Hz) throughout 

the test. 

The testing of each cylinder was completed in a single constant load application from start to 

failure. This procedure runs counter to the standard procedure presented in ASTM C469 but is 

consistent with both ASTM C39 and the alternate loading procedure presented in ASTM C469. 

It is expected that the normal ASTM C469 procedure was designed to allow for assurance of 

seating by repeatedly loading and unloading the cylinder and to allow for removal of the 

deformation measuring apparatus by stopping the test well before reaching failure. In this test 

program, proper seating of the cylinder could be assured by monitoring the load-deformation 

0

50

100

150

200

250

0.0 0.2 0.4 0.6 0.8 1.0

Degrees Out of Parallel

Compressive Strength (MPa)

Steam Treated Untreated

Tempered Steam Treated Delayed Steam Treated
29

response during the test. Removal of the deformation measuring device was not necessary, as a 

loose-fitting plastic sleeve was sufficient to stop any UHPC chips from damaging the LVDTs 

during cylinder failure. 

As previously mentioned, the preparation of specimens for tests at early ages was completed 

within 1 day prior to the test. Also, cylinders with projected strengths below 83 MPa (12 ksi) 

were sulfur capped instead of ground. 

Figure 9. Photos. Modulus ring attachment (a) before and (b) during testing. 

Selected results from these tests are compiled in figures 10 through 14. These figures show one 

stress-strain curve for each of the ages at which compression responses were collected. Note that 

a replicate (N1AxxA) was completed for the steam-treated portion of the testing as the 

compressive strength results from the initial set (N1A) were somewhat below the anticipated 

value. Unfortunately, the replicate compressive strength results were also somewhat lower than 

anticipated. 

The stress-strain responses obtained from these tests will be the focus of the remainder of this 

section as well as the next section. In a qualitative sense, note the change in the overall shape of 

the curves in figures 10 through 14. At 1, 2, or even 3 days after casting if no steam-based curing 

is applied, the UHPC exhibits a ductile stress-strain response. The rounded curve and the steady 

decrease after peak stress illustrate that the UHPC can sustain some load-carrying capacity 

through large deformations without brittle failure. The overall behavior changed dramatically 

after the steam-based treatment. In this case, the UHPC exhibits more of a brittle response 

wherein the attainment of the peak compressive load results in a rapid, uncontrolled decrease in 

load-carrying capacity. As the curing progresses or as the steam-based treatment becomes more 

acute, the brittle nature of the compressive failure response also becomes more acute. 

 (a) (b)
30

1 MPa = 145 psi 

Figure 10. Graph. Selected stress-strain responses for steam-treated UHPC (N1A). 

1 MPa = 145 psi 

Figure 11. Graph. Selected stress-strain responses for steam-treated UHPC (N1AxxA). 

0

5

10

15

20

25

30

0.000 0.003 0.006 0.009 0.012

Axial Strain

Axial Stress (ksi)

24 hours

3 days

Steam

(N1A) 15 days

28 days

56 days

35

Axial Stress (MPa) 70

105

140

175

210

0

5

10

15

20

25

30

0.000 0.003 0.006 0.009 0.012

Axial Strain

Axial Stress (ksi)

30 hours

5 days

Steam

(N1AxxA)

15 days

55 days 30 days

35

Axial Stress (MPa) 70

105

140

175

210
31

1 MPa = 145 psi 

Figure 12. Graph. Selected stress-strain responses for untreated UHPC. 

1 MPa = 145 psi 

Figure 13. Graph. Selected stress-strain responses for tempered steam-treated UHPC. 

0

35

70

105

140

175

210

0.000 0.003 0.006 0.009 0.012

Axial Strain

Axial Stress (MPa)

48 hours

24 hours

71 hours

7 days

9 days

18 h

14 days

21 hours

28 days 56 days

Untreated

(N2A)

0

5

10

15

20

25

30

0.000 0.003 0.006 0.009 0.012

Axial Strain

Axial Stress (ksi)

24 hours

4 days

14 days

Tempered Steam

(N3A)

28 days

56 days

35

Axial Stress (MPa) 70

105

140

175

210
32

1 MPa = 145 psi 

Figure 14. Graph. Selected stress-strain responses for delayed steam-treated UHPC. 

Figure 15 shows the strength results for five sets of specimens. The results presented in the 

figure are also shown in table 10 along with standard deviation values and the numbers of 

cylinders tested. In a similar fashion, figure 16 shows the modulus of elasticity results for the 

five sets of specimens. The modulus of elasticity was calculated based on the average LVDTbased deformation measurements and the load reading. A best-fit linear approximation of the 

stress-strain results from 10 to 30 percent of the compressive strength of each individual cylinder 

was used. Again, table 10 presents further information relating to these results. 

Additional modulus of elasticity tests were also completed on spare cylinders from various 

batches of UHPC cast for this research program. These tests were completed following the same 

procedures discussed above and were all completed at 1 month of age. In general, between 20 

and 30 cylinders were tested at this age for each curing regime. The overall modulus of elasticity 

results include a stiffness of 52.8 GPa (7,650 ksi) for steam-treated, 42.8 GPa (6,200 ksi) for 

untreated, 51.0 GPa (7,400 ksi) for tempered steam-treated, and 50.3 GPa (7,300 ksi) for delayed 

steam-treated UHPC. 

Finally, the axial strain carried by each cylinder at peak compressive loading was also 

determined based on the load-deformation results from each cylinder. This strain value was 

calculated as the average deformation exhibited by the cylinder over the 51-mm (2-inch) gage 

length of the deformation measuring system. These results are presented in figure 17 and table 

10. The additional modulus of elasticity tests discussed above also provided extra information 

related to strain at peak stress. The overall strain at peak stress results include 0.0041 for steamtreated, 0.0035 for untreated, 0.0035 for tempered steam-treated, and 0.0039 for delayed steamtreated UHPC. 

The results presented thus far in this section illustrate three primary findings. First, after steam or 

delayed steam treatment, the UHPC is stabilized. In terms of strength, stiffness, and strain at 

0

5

10

15

20

25

30

0.000 0.003 0.006 0.009 0.012

Axial Strain

Axial Stress (ksi)

25 hours

42 hours

72 hours

6 days

9 days

14 days

Delayed Steam

21, 28, and 56 days (N4A)

35

Axial Stress (MPa) 70

105

140

175

210
33

peak load, the UHPC shows very little change after a steam-based treatment has been applied. 

Second, untreated UHPC continues to gain strength for at least 8 weeks after casting, but its 

increase in stiffness and decrease in strain at peak load seem to be curtailed at 1 month. Finally, 

UHPC gains strength and stiffness very quickly at early ages, while also becoming much less 

ductile with a large decrease in strain at peak load. Specifically, the compressive strength and 

stiffness after 24 hours are around 10 MPa and 10.3 GPa (1.5 ksi and 1,500 ksi), respectively. 

After 3 days, however, these values have increased to over 69 MPa and 34.5 GPa (10 ksi and 

5,000 ksi), respectively, without any curing treatment. 

1 MPa = 145 psi 

Figure 15. Graph. Compressive strength gain from casting up to 8 weeks of age. 

1 GPa = 145,000 psi 

Figure 16. Graph. Modulus of elasticity gain from casting up to 8 weeks of age. 

0

50

100

150

200

0 10 20 30 40 50 60

Days After Casting

Compressive Strength (MPa)

N1A N1AxxA N2A N3A N4A

0

10

20

30

40

50

60

0 10 20 30 40 50 60

Days After Casting

Modulus of Elasticity (GPa)

N1A N1AxxA N2A N3A N4A
34

Figure 17. Graph. Strain at peak compressive stress from casting up to 8 weeks of age. 

0.000000

0.002000

0.004000

0.006000

0.008000

0.010000

0 10 20 30 40 50 60

Days After Casting

Strain at Peak Stress

N1A N1AxxA N2A N3A N4A
35

Table 10. Strength, modulus of elasticity, and strain at peak stress results at various ages after 

1 MPa = 145 psi 1 GPa = 145,000 psi 

Test Age Compressive Strength (MPa) Modulus of Elasticity (GPa) Strain at Peak 

(days) No. Ave. St. Dev. No. Ave. St. Dev. No. Ave. 

Steam (N1A)  1.0 6 7.6 0.6 6 4.6 1.0 6 0.009020  3 6 169 4.0 6 51.5 1.5 1 0.003580  15 6 179 4.8 6 52.2 1.4 1 0.003970  28 6 180 3.4 6 52.3 1.0 2 0.003740  56 6 186 5.7 5 52.5 1.3 2 0.003900 Steam (N1AxxA)  1.3 5 27 2.5 5 19.4 1.9 5 0.006830  5 5 185 6.3 5 52.4 0.8 4 0.004080  15 6 193 5.2 6 52.5 1.1 4 0.004020  30 6 199 6.7 6 51.4 1.2 4 0.004580  55 6 194 8.3 6 52.5 0.7 4 0.003920 Untreated (N2A)  1.0 3 15 1.4 3 10.5 2.2 3 0.009360  2.0 2 65 0.7 2 28.3 0.5 3 0.005020  3 2 73 0.6 2 36.0 0.3 3 0.003850  7 3 89 0.6 3 39.0 0.8 3 0.003620  9 2 101 4.1 2 37.6 0.8 3 0.003400  14 6 110 4.2 6 41.2 1.2 6 0.003710  28 6 119 3.7 6 41.9 1.1 4 0.003430  57 6 125 4.8 6 42.0 1.0 3 0.003570 
36

1 MPa = 145 psi 1 GPa = 145,000 psi 

Table 10. Strength, modulus of elasticity, and strain at peak stress results at various ages after castin

Test Age Compressive Strength (MPa) Modulus of Elasticity (GPa) Strain at Peak 

(days) No. Ave. St. Dev. No. Ave. St. Dev. No. Ave. 

Tempered Steam (N3A)  1.0 6 14 1.2 6 9.3 1.8 6 0.008380  4 6 139 4.0 4 49.9 0.9 2 0.003100  14 6 146 6.8 6 49.7 0.7 3 0.003260  28 6 147 5.4 6 49.9 1.1 1 0.003150  58 6 157 5.2 6 50.4 0.9 3 0.003550 Delayed Steam (N4A) 

 0.8 2 9.7 0.4 2 7.9 3.6 3 0.007120  14 4 115 2.8 4 42.9 1.7 3 0.003810  21 6 174 4.4 5 50.3 1.0 0 –  30 6 170 4.8 6 50.7 0.7 1 0.003760  56 6 172 4.5 6 49.1 1.5 3 0.003890 
37

3.3.3 Linearity of UHPC Compressive Response 

It is normally assumed that concrete begins to develop internal microcracking and exhibit an 

associated reduction in stiffness at high stresses. The stress level varies depending on the 

composition of the concrete. The linearity of the compressive stress-strain response of UHPC 

was investigated through the elastic modulus testing results that were described in the previous 

section. 

Two methods are used to describe the linearity of the compressive stress-strain response of 

concrete. The first is the secant modulus. The secant modulus, E0, is defined as the compressive 

strength of the concrete divided by the strain at the peak strength. The second method involves 

determining the proportion of the strength at which the observed stress is some percentage below 

the elastic modulus predicted stress. Figure 18 shows an example of both of these linearity 

descriptors in conjunction with an untreated UHPC cylinder tested at 57 days after casting. Both 

of these linearity descriptors relate to the elastic modulus, also known as the tangent modulus, 

which was defined previously as being the linear best-fit approximation of the stress-strain 

response between 10 and 30 percent of the compressive strength. 

1 MPa = 145 psi 

Figure 18. Graph. Sample untreated stress-strain curve with linearity descriptors. 

The linearity to peak strength results, as defined by the secant modulus, are presented in table 11 

and figures 19 and 20. The secant modulus shows very similar qualitative behaviors compared 

with the elastic modulus behaviors described in the previous section. More importantly, the ratio 

0

20

40

60

80

100

120

140

0.000 0.001 0.002 0.003 0.004

Axial Strain

Axial Stress (MPa)

1

E0

E

1

5% Stress Decrease
38

of the tangent to secant modulus values illustrates how the UHPC behavior changes as curing 

progresses. At 1 day after casting this ratio ranges from 5 to 7, but by 3 days it is under 2 even 

without any supplemental curing. The ratio tends to stabilize around 1.1 for the steam-based 

specimens and around 1.2 for the untreated specimens. For reference, this value is normally 

approximately 4 for normal weight 7 MPa (1,000 psi) compressive strength concrete and 

approximately 1.3 for 70 MPa (10 ksi) compressive strength concrete.(20) 

The linearity results—in terms of the stress at which the stress will have dropped some 

percentage from the linear elastic expected value—are presented in table 11 and figures 21 and 

22. The research program focused on stress drops of 1, 3, and 5 percent. The results from the 1 

percent drop are highly dependent on the predefined range for calculation of the elastic modulus 

and as such are of less interest. The results from the 5 percent drop clearly show that UHPC 

exhibits nearly linear behavior up to high stress levels. Figure 22 shows that the cylinders that 

underwent steam-based treatment reach between 80 and 90 percent of their compressive strength 

before diverging 5 percent from linear elastic behavior. The untreated cylinders seem to be 

asymptotically approaching the same type of response by 8 weeks after casting, having reached 

70 percent of their compressive strength before diverging 5 percent from the linear elastic 

behavior. 
39

Table 11. Compressive stress-strain response linearity at various ages after casting. 

1 MPa = 145 psi 

Linearity (1%) Linearity (3%) 

Test Age 

(days) 

Compressive 

Strength 

(MPa) 

Strain at 

Peak 

Stress E / E0 No. Strain Stress (MPa) Strain Stress (MPa) Str

Steam (N1A)  1.0 7.6 0.009020 5.53 6 0.000440 1.9 0.000490 2.1 0.000 3 169 0.003580 1.09 6 0.001540 79 0.002520 126 0.003 15 179 0.003970 1.16 6 0.001470 76 0.002620 132 0.003 28 180 0.003740 1.08 6 0.001570 81 0.002570 130 0.003 56 186 0.003900 1.10 5 0.001610 83 0.002670 137 0.003Steam (N1AxxA)  1.3 27 0.006830 4.90 5 0.000340 6.5 0.000420 7.6 0.000 5 185 0.004080 1.16 5 0.001730 90 0.002790 141 0.003 15 193 0.004020 1.09 6 0.001830 95 0.002890 147 0.003 30 199 0.004580 1.18 5 0.001930 99 0.002960 149 0.003 55 194 0.003920 1.06 6 0.001870 97 0.002960 151 0.003Untreated (N2A)  1.0 15 0.009360 6.49 2 0.000330 3.0 0.000380 3.8 0.000 2.0 65 0.005020 2.20 0 – – – –  3 73 0.003850 1.90 2 0.000590 21 0.000740 26 0.000 7 89 0.003620 1.59 3 0.000680 26 0.000930 35 0.00 9 101 0.003400 1.26 2 0.000920 34 0.001250 46 0.00 14 110 0.003710 1.39 6 0.000860 34 0.001200 48 0.00 28 119 0.003430 1.21 6 0.001040 43 0.001530 62 0.00 57 125 0.003570 1.20 6 0.001180 49 0.001760 72 0.002
40

Table 11. Compressive stress-strain response linearity at various ages after casting (contin

1 MPa = 145 psi 

Linearity (1%) Linearity (3%) 

Test Age 

(days) 

Compressive 

Strength 

(MPa) 

Strain at 

Peak 

Stress E / E0 No. Strain Stress (MPa) Strain Stress (MPa) S

Tempered Steam (N3A)  1.0 14 0.008380 5.34 6 0.000390 3.5 0.000460 4.1 0.0 4 139 0.003100 1.11 4 0.001310 65 0.001940 94 0.0 14 146 0.003260 1.11 6 0.001380 68 0.002110 102 0.0 28 147 0.003150 1.07 6 0.001180 58 0.002070 100 0.0 58 157 0.003550 1.14 6 0.001360 68 0.002150 105 0.0Delayed Steam (N4A)  0.8 9.7 0.007120 5.61 1 0.000480 2.5 0.000550 2.8 0.0 14 115 0.003810 1.42 4 0.000870 37 0.001190 49 0.0 21 174 – – 5 0.001490 74 0.002250 110 0.0 30 170 0.003760 1.13 6 0.001630 82 0.002420 119 0.0 56 172 0.003890 1.11 6 0.001700 83 0.002630 125 0.0
41

1 MPa = 145 psi 

Figure 19. Graph. Secant modulus from casting up to 8 weeks of age. 

1 MPa = 145 psi 

Figure 20. Graph. Ratio of elastic to secant modulus from casting up to 8 weeks of age. 

0

10

20

30

40

50

0 10 20 30 40 50 60 70

Days Since Casting

E0 (MPa)

N1A N1AxxA N2A N3A N4A

0

1

2

3

4

5

6

7

0 10 20 30 40 50 60 70

Days Since Casting

E / E0 

N1A N1AxxA N2A N3A N4A
42

Figure 21. Graph. Compressive stress to strength ratio at 1 percent 

stress drop from linear elastic. 

Figure 22. Graph. Compressive stress to strength ratio at 5 percent 

stress drop from linear elastic. 

0.0

0.2

0.4

0.6

0.8

1.0

0 10 20 30 40 50 60 70

Days After Casting

Stress/Strength at 1% stress 

drop from linear

N1A N1AxxA N2A N3A N4A

0.0

0.2

0.4

0.6

0.8

1.0

0 10 20 30 40 50 60 70

Days After Casting

Stress/Strength at 5% stress 

drop from linear

N1A N1AxxA N2A N3A N4A

0.0

0.2

0.4

0.6

0.8

1.0

0 10 20 30 40 50 60 70

Days After Casting

Stress/Strength at 1% stress 

drop from linear

N1A N1AxxA N2A N3A N4A

0.0

0.2

0.4

0.6

0.8

1.0

0 10 20 30 40 50 60 70

Days After Casting

Stress/Strength at 5% stress 

drop from linear

N1A N1AxxA N2A N3A N4A
43

3.3.4 Compression Specimen Geometry 

In the United States, the cylinder is the standard concrete compression test specimen. 

Conversely, the cube is the standard specimen in parts of Europe. It is generally accepted that 

with normal strength concrete the cube will result in a higher compressive strength due to its 

shorter aspect ratio and the proportionally larger lateral confinement provided by the machine 

platens. 

A series of tests were completed to determine the effect of various specimen geometries on the 

compressive strength of UHPC. Compression tests were completed according to ASTM C39 on 

51-, 76-, and 102-mm (2-, 3-, and 4-inch) diameter cylinders with cast lengths of twice their 

diameters. ASTM C39 compression tests were also performed on overlength 76-mm (3-inch) 

diameter cylinders that had a cast length of 165 mm (6.5 inches). Cube compression tests were 

completed according to ASTM C109 on 51- and 100-mm (2- and 4-inch) cubes. Figure 23 shows 

the range of sizes and geometries of the specimens tested. Figure 24 shows the cylinder and cube 

specimens in the compression testing machine. All specimens were tested in a hydraulically 

actuated 4,450 kN (1,000 kip) Forney test machine with a load rate equivalent to 1.0 MPa/s (150 

psi/s). 

The test program described above was repeated three times. Each time, the set of cylinder and 

cube specimens was cast from a single batch. Two of the batches were steam treated while the 

third batch was untreated. Table 12 provides the results from the compression tests. The 76-mm 

(3-inch) diameter cylinder—used as the control specimen throughout the entire study—was also 

the control specimen for this series of tests. 

Figure 23. Photo. Compression cubes and cylinders including (clockwise from upper left) 

102-mm (4-inch), 76-mm (3-inch) overlength, 76-mm (3-inch), and 51-mm (2-inch) 

diameter cylinders and 51-mm (2-inch) and 100-mm (4-inch) cubes. 
44

Figure 24. Photos. (a) Cylinder and (b) cube compression testing. 

The combined results from the three sets of tests indicate that the compressive strength of UHPC 

can be measured, relatively accurately, through various sizes of cubes and cylinders. Results 

were never more than 8 percent removed from the control cylinder result. The cubes tended to 

exhibit slightly higher strengths on the order of 5 percent. In addition, the 51-mm (2-inch) 

diameter cylinders tended to exhibit slightly lower strengths. The smaller cubes and cylinders 

tended to exhibit larger standard deviations. Conversely, the large specimens tended to exhibit 

smaller standard deviations. These results were expected because heterogeneities in the concrete 

would likely remain in a uniform size range but would be proportionally larger in smaller 

specimens. 

3.3.5 Demolding Age Effect on Compressive Strength 

Throughout this test program, control cylinder compressive strengths were sometimes below the 

anticipated value. As previously mentioned, the mix design, constituent materials, and mixing 

procedure were always the same. For this reason, the deviations in strength were troubling. Two 

observations were made during the early stages of the test program: (1) the set time of the UHPC 

sometimes varied and (2) in general, the older the premix, the longer the set time. Regardless of 

these observations, the cylinders were nearly always demolded at approximately 24 hours after 

casting. This delayed setting sometimes resulted in the cylinders barely having enough strength 

to remain intact throughout the demolding process. 

After the bulk of the test program was complete, an additional batch was added to the test 

program. This batch included two sets of 76-mm (3-inch) diameter cylinders that were demolded 

at various ages after casting. One of the sets was then steam treated while the other was 

untreated. The earliest that any cylinders could be stripped was determined to be 28 hours after 

casting because of the advanced age of the premix by this point in the test program. The other 

cylinders were stripped at 47 and 55 hours after casting. Otherwise, the cylinders in each set 

were treated identically up through compression testing at 28 days. 

(a) (b) 
45

Table 12. Cylinder and cube compressive strength results. 

Compressive Strength 

(MPa) 

Specimen No. 

Average St. Dev. 

% Difference 

From Control 

Steam (M1B) 

 76-mm Cylinder † 6 203 4.2 – 

 102-mm Cylinder 5 202 6.3 −0.3 

 Overlength 76-mm Cylinder 3 188 8.9 −7.5 

 51-mm Cylinder 6 203 6.9 0.3 

 100-mm Cube 5 201 12.1 −1.2 

 51-mm Cube 6 214 11.7 5.8 

Steam (L1B) 

 76-mm Cylinder † 6 210 7.0 – 

 102-mm Cylinder 5 215 5.4 2.2 

 Overlength 76-mm Cylinder 3 194 8.2 −7.5 

 51-mm Cylinder 6 200 12.3 −5.0 

 100-mm Cube 5 220 5.0 4.6 

 51-mm Cube 6 220 10.8 4.6 

Untreated (L2B) 

 76-mm Cylinder †

 8 149 3.8 – 

 102-mm Cylinder 5 154 2.1 3.7 

 Overlength 76-mm Cylinder 3 150 1.5 1.1 

 51-mm Cylinder 6 140 14.8 −5.9 

 100-mm Cube 3 161 2.9 8.0 

 51-mm Cube 6 158 8.8 6.1 

1 MPa = 145 psi 

1 mm = 0.039 inch 

Table 13 presents the results from this set of compression tests. These results clearly indicate that 

the age of the UHPC at stripping has a major impact on the compressive strength. The cylinders 

that were stripped as soon as their integrity allowed had a 25 to 30 percent lower compressive 

strength at 28 days compared with the cylinders that were stripped approximately 20 hours later. 

Once the cylinder has gained sufficient integrity to support itself, the primary role of the cylinder 

mold is to retain moisture within the cylinder. The loss of that moisture barrier clearly resulted in 

a decrease in the moisture that was available for hydration and a resulting decrease in 

compressive strength. 
46

Table 13. Demolding age effect on 28-day compressive strength results. 

Specimen Group No. Compressive Strength (MPa) 

Average St. Dev. 

Steam 

 Stripped at 28 hours after casting 6 143 8.6 

 Stripped at 47 hours after casting 6 203 5.9 

 Stripped at 55 hours after casting 6 208 13.0 

Untreated 

 Stripped at 28 hours after casting 6 99 3.4 

 Stripped at 47 hours after casting 6 129 3.5 

 Stripped at 55 hours after casting 6 136 5.2 

1 MPa = 145 psi 

3.3.6 Long-Term Delayed Steam Effect on Compressive Strength 

Although the delayed steam-curing regime was designed to mimic the postponed steam curing of 

UHPC, it is anticipated that longer delays prior to steam treatment could occur. For this reason, 

two batches of UHPC were cast to focus on the compressive strength level that UHPC can attain 

if the steam treatment is significantly delayed. Each of these batches included 36 cylinders, half 

of which were untreated and half of which had a steam treatment applied at a time distant from 

casting. The steam treatment was the same as the standard steam treatment except for the start 

date. The steam treatment always started 6 days before the date of the compression testing. 

Table 14 presents the results of these tests. The first batch, L21, was tested at 8, 12, and 16 

weeks after casting. The second batch, L22, was tested at 8, 20, and 32 weeks after casting. In 

both cases, the untreated cylinders and the cylinders that underwent the long-term, delayed steam 

treatment showed consistent compressive strengths over the months of testing. The diminishing 

impact of the steam treatment, delayed from 1 day to 2 weeks after casting, does not seem to 

continue indefinitely. By 2 months, the impact seems to have leveled off such that steaming at 

this time is as effective as steaming at up to 32 weeks after casting. Additionally, it is important 

to note that the application of the steam treatment increased the compressive strength by 30 

percent, regardless of when it was applied. 

3.3.7 Fiber Effect on Compression Failure 

The steel fiber reinforcement included in UHPC provides a number of advantages, most notably 

in terms of tensile structural behavior. Normally, a compression test on high-strength concrete 

would result in a very brittle, dramatic failure. As noted earlier in the discussion of UHPC 

compression behavior, the UHPC that is reinforced with steel fibers does not exhibit explosive 

failures during compression tests. Recalling figure 6, a compression test on steam-treated UHPC 

would likely result in a rapid load drop, but the cylinder would remain intact. 
47

Table 14. Long-term delayed steam effect on compressive strength. 

Batch Curing Steam Days Test Day No. Compressive Strength (MPa)

Average St. Dev. 

L21 Untreated – 56 days 6 142 3.9 

 Untreated – 84 days 6 138 4.6 

 Untreated – 112 days 6 134 2.3 

 Steam 50 to 52 56 days 6 181 7.2 

 Steam 78 to 80 84 days 6 174 4.1 

 Steam 106 to 108 112 days 6 178 5.1 

L22 Untreated – 56 days 6 130 2.4 

 Untreated – 140 days 6 128 5.7 

 Untreated – 224 days 6 131 6.1 

 Steam 50 to 52 56 days 6 173 6.3 

 Steam 134 to 136 140 days 6 167 8.6 

 Steam 218 to 220 224 days 7 171 4.8 

1 MPa = 145 psi 

A few UHPC cylinders were cast according to the normal mix design and procedures except that 

the steel fiber reinforcement was not added. Thus, the high compressive strength abilities of the 

UHPC matrix are present, but the restraining and confining effects of the fibers are absent. The 

test results from these cylinders indicated a dramatic change in the compression failure behavior. 

Figure 25 shows four still pictures of the failure of one of these cylinders. These pictures were 

captured from a digital video recording. The video was captured at a rate of 30 frames per 

second, thus the smallest increment of time over which to observe changes in behavior is 

1/30 second. The failure of this cylinder occurred when small chips of UHPC began to fly off the 

top and bottom of the cylinder at its intersection with the platens. This chipping continued for a 

few seconds until a larger chip exited the top of the cylinder just 1/6 second before failure. The 

cylinder then rapidly and dramatically failed: Over the course of 1/30 second, the intact cylinder 

shattered into many small pieces of UHPC. 

Although this experiment was not controlled with fiber-reinforced and nonreinforced UHPC 

tested in parallel, the compressive strength results are still of interest. The cylinders tested 

averaged 200 MPa (29 ksi), with the cylinder shown in the video reaching 214 MPa (31 ksi). 

Note that these strengths correspond to loads of approximately 890 kN (200 kips); therefore, the 

Forney 4,450-kN (1,000-kip) capacity testing machine was only at 20 percent of its capacity and 

would not have held an inordinately high level of strain energy before failure. 
48

Figure 25. Photos. Compression failure of a steam-treated UHPC cylinder containing no fiber reinforcem

before failure, (b) 1/30 second before failure, (c) at failure, and (d) 1/10 second after failu

(a) (b) (c) (d) 
49 

3.3.8 Load Rate Effect on Compression Testing Results 

The compression testing discussed so far in section 3.3 was all completed with a load rate of 

1.0 MPa/s (150 psi/s) during the initial elastic portion of the stress-strain response. As previously 

mentioned, this load rate is outside the bounds recommended by the ASTM C39 test method. 

Before initiating the compression testing portion of the UHPC material characterization work, a 

set of UHPC cylinders was cast with the focus being on the effect of various load rates on 

standard compression testing results. 

It is normally accepted that higher load rates will result in higher compression strength and 

modulus of elasticity results. For this reason, using the ASTM C39 load rate of 0.24 ±0.10 MPa/s 

(35 ±15 psi/s) is recommended for any standardized compression testing of concrete. However, 

the high strength results expected from UHPC mean that a single compression test on a concrete 

cylinder could take 15 to 20 minutes or more. This objectionably long time for a single data point 

led to the testing discussed below. 

Twenty-four cylinders with a 76-mm (3-inch) diameter were cast within a single batch of UHPC 

for this testing. The cylinders were all cast and steam treated according to normal procedures. 

The cylinders were then divided into four groups, each of which was tested at a different load 

rate ranging from 0.24 to 1.7 MPa/s (35 to 250 psi/s). Within each group of six cylinders, three 

were tested in compression according to ASTM C39. The remaining three were tested according 

to ASTM C469, including multiple unloads/reloads, before being tested according to ASTM 

C39. This testing differed from the previously discussed modulus testing, because the axial 

deformations were measured via a standard compressometer as described in the test method, and 

data on Poisson’s ratio were also collected. 

Table 15 presents the results of these tests. The results do not clearly indicate a change in 

behavior caused by the increasing load rate. For this particular batch, the compressive strength 

remained around 200 MPa (29 ksi), the modulus of elasticity remained around 56.5 GPa 

(8,200 ksi), and Poisson’s ratio was quite consistent at 0.19. These results indicate that an 

increase in the load rate would not be detrimental. From a practical standpoint, a compression 

test run at 1.0 MPa/s (150 psi/s) can be completed within 5 minutes on a steam-treated cylinder 

and more quickly on lower strengths of UHPC. For these reasons, a load rate of 1.0 MPa/s (150 

psi/s) was chosen for all of the UHPC compression testing. 

Table 15. Load rate effect on compression testing results. 

Compressive 

Strength (MPa) 

Modulus of 

Elasticity (GPa) 

Load Rate Poisson’s Ratio 

(MPa/s) 

No. Ave. St. Dev. No. Ave. St. Dev. No. Ave. St. Dev.

0.24 6 194 11.2 3 55.8 2.1 3 0.195 0.018 

0.34 6 201 6.9 3 57.9 1.5 3 0.193 0.011 

1.0 6 201 6.8 3 56.9 1.8 3 0.199 0.007 

1.7 6 201 3.3 3 55.2 1.0 3 0.184 0.011 

1 MPa/s = 150 psi/s, 1 GPa = 145,000 psi, 1 MPa = 145 psi 
50 

3.4 TENSION TESTING 

Four types of tension tests were implemented in this research program to experimentally 

determine the tensile properties of UHPC. These tests included flexural testing of prismatic 

sections, split tensile testing of cylinders, axial tensile testing of mortar briquettes, and axial 

tensile testing of cylinders. The following sections discuss the test procedures and the test results. 

3.4.1 Flexural Prism 

The ASTM C1018 Standard Test Method for Flexural Toughness and First-Crack Strength of 

Fiber-Reinforced Concrete (Using a Beam With Third-Point Loading) was one test used to 

determine the tensile properties of UHPC.(21) This test involves the four-point flexural loading of 

small-scale concrete prisms. During the test, the load on and the deflection of the prism are 

monitored. These data are then used to determine the cracking strength and postcracking 

toughness response of the concrete. 

This test method requires specialized equipment in order to correctly load the specimen and 

accurately monitor the response. First, the test setup requires that the deflection measuring 

system must measure “net values exclusive of any extraneous effects due to seating or twisting 

of the specimen on its supports or deformation of the support system.” Second, the loading of 

the prism must be completed through a “testing arrangement where specimen net midspan 

deflection is used to control the rate of increase of deflection using a closed-loop, servocontrolled testing system.” 

To meet these requirements, a specialized test setup was devised. In this setup, a yoke similar to 

that shown in figure 2 of ASTM C1018 was used to measure the midspan deflections. LVDTs 

were attached to the yoke on each side of the specimen at midspan, and the yoke was attached to 

the specimen at mid-depth over the support points. The LVDTs bore on a plate that was epoxied 

to the compression face and extended to hang over the sides of the prism. Figure 26 shows two 

examples of the deflection measurement setup for this four-point bending test. 

The loading control of the test was accomplished by completing the test in an MTS® load frame. 

The control signal for the MTS was the midspan net deflection of the specimen as captured by 

the LVDTs. The signals from the two LVDTs were electronically averaged, then submitted to the 

MTS control panel. In this way, the deflection rate of the prism was accurately and consistently 

maintained at the correct level. This loading technique also limited the amount of potential 

energy stored in the load frame that could be transmitted into the concrete prism during the 

temporary stiffness loss associated with cracking. 

The loading apparatus used to test the prisms was reconfigurable to allow for testing of multiple 

sizes of prisms. The rollers and their support blocks are movable to allow for lower support 

spans from 152 to 406 mm (6 to 16 inches) and upper loading spans from 51 to 152 mm 

(2 to 6 inches). As shown in figure 26, the two upper load points and the two lower support 

points are steel rollers that impart no axial restraint on the prism. Torsional effects caused by 

misalignment between the planes of the prism faces and the rollers were overcome by placing 

individual shims between each roller and its bearing block. The bearing blocks under the rollers 
51 

are supported by 152-mm (6-inch) deep solid steel beams that are connected to the heads of the 

MTS machine. These deep steel beams reduce flexibility in the loading apparatus, thus allowing 

for easier and more refined control of the test during cracking of the prism. 

The ASTM C1018 test was performed on prisms from all four curing regimes. The casting and 

curing of the prisms was completed following normal procedures. During casting of each prism, 

special care was taken to ensure that the UHPC flowed from one end of the prism to the other, 

thus ensuring a fiber distribution and alignment system that was similar to that which would 

occur in the large-scale casting of a beam or plate type flexural member. 

Figure 26. Photos. Prism flexural test setup for (a) a 229-mm (9-inch) 

span and (b) a 305-mm (12-inch) span. 

(a) 

(b) 
52 

The test specification recommends a preferred standard size of molded specimen of 356 by 102 

by 102 mm (14 by 4 by 4 inches), resulting in a third-point loading configuration with a 305-mm 

(12-inch) lower span. This specimen size was not used for a number of reasons. First, this depth 

of prism is on the high end of the likely thickness of UHPC plate members subject to flexural 

forces. Second, a shear span-to-prism depth ratio equal to one, as recommended by the 

specification, will create a stress field and deflection response in the prism that is substantially 

both flexural and shear. A larger shear span-to-prism depth ratio would much more accurately 

represent the flexural response of UHPC. Finally, the specification indicates that the prism crosssectional dimensions must only be at least three times the length of a fiber. This requirement can 

easily be met by smaller cross-sectional sizes. 

The test program included two prism cross sections and a total of four loading configurations. 

The cross sections investigated were 51- by 51-mm (2- by 2-inch) prisms and 76-mm (3-inch) 

deep by 102-mm (4-inch) wide prisms. The 51- by 51-mm (2- by 2-inch) prisms were cast in 

lengths of 279 mm (11 inches) and 432 mm (17 inches). The 76- by 102-mm (3- by 4-inch) 

prisms were cast 406 mm (16 inches) long. Four loading configurations were used to test the 

51- by 51-mm (2- by 2-inch) prisms. These included third-point loading on 152-, 229-, and 

305-mm (6-, 9-, and 12-inch) lower spans, as well as loading on a 381-mm (15-inch) lower span 

with 76 mm (3 inches) between the upper load points. The third-point loading with a 305-mm 

(12-inch) lower span was also used for the 76- by 102-mm (3- by 4-inch) cross section prisms. 

These various cross sections and loading configurations allow for the comparison of observed 

tensile behaviors, because the 51- by 51-mm (2- by 2-inch) prism on a 152-mm (6-inch) span is a 

scaled model of the ASTM C1018 recommended prism. Other test setups provide more realistic 

representations of flexural tensile material behaviors. 

The following procedure was used to complete the test: 

1. The prism was centered in the load frame with the screeded face of the prism oriented 

toward the front and the vertical molded faces placed as the compression and tension 

faces. 

2. The load was manually increased to approximately 220 newtons (N) (50 lb). 

3. The setup was checked to ensure that all rollers were bearing on the prism. 

4. The LVDTs were then set in place, and control of the machine was transferred to the 

averaging circuit attached to the LVDTs. 

5. The MTS MicroProfilerTM was then used to apply a constant deflection rate to the prism. 

6. The deflection rate was set so that the expected first crack deflection would occur 

approximately 1 minute into the test. This rate varied depending on the prism cross 

section and loading configuration. 

7. The test was stopped after a deflection of at least 16 times the cracking deflection was 

reached. 

8. The collection of the data for the test was completed through an analog data acquisition 

system. It was set to record data at 10 Hz until well after first cracking of the prism had 

clearly occurred. 

9. The record rate was then manually decreased to 4 Hz for the remainder of the test. The 

time, load, deflection from both LVDTs, and electronic circuit averaged deflection values 

were all recorded. 
53 

The strength and toughness results and analysis presented in the next two sections are highly 

dependent on the correct identification of first cracking in each prism test. Plain concrete and 

fiber-reinforced concrete may begin to behave nonlinearly due primarily to internal 

microcracking before the first overall cracking of the prism; therefore, determining first cracking 

can be somewhat subjective. The behavior of UHPC is such that the first cracking is tensile 

stress cracking on the bottom flange of the prism. Thus, first cracking—recorded by the data 

acquisition system and physically observed on the specimen—is usually quite clear. Figure 27 

shows the early parts of load-deflection response curves for both a steam and an untreated prism. 

This figure shows that the prism response is linear until first cracking when a clearly defined 

decrease in load-carrying capacity occurs. Soon thereafter the load again begins to increase. The 

sawtooth pattern visible in the response is indicative of additional individual cracks forming 

throughout the highly stressed tension face of the prism. 

1 kN = 0.225 kip, 1 mm = 0.039 inch 

Figure 27. Graph. Examples of first crack shown on load-deflection response curves. 

A total of 76 prisms were tested over the five loading/cross section configurations. Seventy-one 

prisms were tested successfully out of this group. The load-deflection results are presented in 

figures 28 through 47. Figures 28 through 31 present the results from the scaled standard ASTM 

C1018 test on 51- by 51-mm (2- by 2-inch) prisms with third-point loading over a 152-mm 

(6-inch) span. Figures 32 through 35 present the results from the 76- by 102-mm (3- by 4-inch) 

prisms tested with third-point loading over a 305-mm (12-inch) span. Figures 36 through 39 and 

40 through 43 present the results from 51- by 51-mm (2- by 2-inch) prisms tested with thirdpoint loading on 229-mm (9-inch) and 305-mm (12-inch) spans, respectively. Finally, figures 44 

through 47 present the results from the 51- by 51-mm (2- by 2-inch) prisms tested over a 

381-mm (15-inch) span with 76 mm (3 inches) between the load points. Sequential results in 

each set have been offset by 0.1 mm (0.004 inch) to allow for clear presentation. The curing 

regime applied to each set of prisms is listed in the lower right corner of each set of results. 

0

2

4

6

8

10

12

14

16

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Deflection (mm)

Load (kN)

First Crack

Steam Treated

Untreated

First Crack
54 

1 kN = 0.225 kip, 1 mm = 0.039 inch 

Figure 28. Graph. ASTM C1018 load-deflection response results for steam-treated 51- by 

51-mm (2- by 2-inch) prisms over a 152-mm (6-inch) span with third-point loading. 

1 kN = 0.225 kip, 1 mm = 0.039 inch 

Figure 29. Graph. ASTM C1018 load-deflection response results for untreated 51- by 

51-mm (2- by 2-inch) prisms over a 152-mm (6-inch) span with third-point loading. 

0

5

10

15

20

25

30

35

0.00 0.30 0.60 0.90 1.20 1.50

Deflection (mm) 

Load (kN)

Steam

0

5

10

15

20

25

30

35

0.00 0.30 0.60 0.90 1.20 1.50

Deflection (mm) 

Load (kN)

Untreated
55 

1 kN = 0.225 kip, 1 mm = 0.039 inch 

Figure 30. Graph. ASTM C1018 load-deflection response results for tempered 

steam-treated 51- by 51-mm (2- by 2-inch) prisms over a 152-mm 

(6-inch) span with third-point loading. 

1 kN = 0.225 kip, 1 mm = 0.039 inch 

Figure 31. Graph. ASTM C1018 load-deflection response results for delayed 

steam-treated 51- by 51-mm (2- by 2-inch) prisms over a 152-mm 

(6-inch) span with third-point loading. 

0

5

10

15

20

25

30

35

0.00 0.30 0.60 0.90 1.20 1.50

Deflection (mm) 

Load (kN)

Tempered Steam

0

5

10

15

20

25

30

35

0.00 0.30 0.60 0.90 1.20 1.50

Deflection (mm) 

Load (kN)

Delayed Steam
56 

1 kN = 0.225 kip, 1 mm = 0.039 inch 

Figure 32. Graph. ASTM C1018 load-deflection response results for steamtreated 76- by 102-mm (3- by 4-inch) prisms over a 305-mm 

(12-inch) span with third-point loading. 

1 kN = 0.225 kip, 1 mm = 0.039 inch 

Figure 33. Graph. ASTM C1018 load-deflection response results for untreated 

76- by 102-mm (3- by 4-inch) prisms over a 305-mm (12-inch) 

span with third-point loading. 

0

10

20

30

40

50

60

70

0.0 0.5 1.0 1.5 2.0 2.5

Deflection (mm)

Load (kN)

Steam

0

10

20

30

40

50

60

70

0.0 0.5 1.0 1.5 2.0 2.5

Deflection (mm) 

Load (kN)

Untreated
57 

1 kN = 0.225 kip, 1 mm = 0.039 inch 

Figure 34. Graph. ASTM C1018 load-deflection response results for tempered steamtreated 76- by 102-mm (3- by 4-inch) prisms over a 305-mm 

(12-inch) span with third-point loading. 

1 kN = 0.225 kip, 1 mm = 0.039 inch 

Figure 35. Graph. ASTM C1018 load-deflection response results for delayed steamtreated 76- by 102-mm (3- by 4-inch) prisms over a 305-mm 

(12-inch) span with third-point loading. 

0

10

20

30

40

50

60

70

0.0 0.5 1.0 1.5 2.0 2.5

Deflection (mm) 

Load (kN)

Tempered Steam

0

10

20

30

40

50

60

70

0.0 0.5 1.0 1.5 2.0 2.5

Deflection (mm) 

Load (kN)

Delayed Steam
58 

1 kN = 0.225 kip, 1 mm = 0.039 inch 

Figure 36. Graph. ASTM C1018 load-deflection response results for steamtreated 51- by 51-mm (2- by 2-inch) prisms over a 229-mm 

(9-inch) span with third-point loading. 

1 kN = 0.225 kip, 1 mm = 0.039 inch 

Figure 37. Graph. ASTM C1018 load-deflection response results for untreated 

51- by 51-mm (2- by 2-inch) prisms over a 229-mm (9-inch) 

span with third-point loading. 

0

5

10

15

20

25

0.0 0.5 1.0 1.5 2.0 2.5

Deflection (mm) 

Load (kN)

Steam

0

5

10

15

20

25

0.0 0.5 1.0 1.5 2.0 2.5

Deflection (mm) 

Load (kN)

Untreated
59 

1 kN = 0.225 kip, 1 mm = 0.039 inch 

Figure 38. Graph. ASTM C1018 load-deflection response results for tempered steamtreated 51- by 51-mm (2- by 2-inch) prisms over a 229-mm 

(9-inch) span with third-point loading. 

1 kN = 0.225 kip, 1 mm = 0.039 inch 

Figure 39. Graph. ASTM C1018 load-deflection response results for delayed steamtreated 51- by 51-mm (2- by 2-inch) prisms over a 229-mm 

(9-inch) span with third-point loading. 

0

5

10

15

20

25

0.0 0.5 1.0 1.5 2.0 2.5

Deflection (mm) 

Load (kN)

Tempered Steam

0

5

10

15

20

25

0.0 0.5 1.0 1.5 2.0 2.5

Deflection (mm)

Load (kN)

Delayed Steam
60 

1 kN = 0.225 kip, 1 mm = 0.039 inch 

Figure 40. Graph. ASTM C1018 load-deflection response results for steamtreated 51- by 51-mm (2- by 2-inch) prisms over a 305-mm 

(12-inch) span with third-point loading. 

1 kN = 0.225 kip, 1 mm = 0.039 inch 

Figure 41. Graph. ASTM C1018 load-deflection response results for untreated 

51- by 51-mm (2- by 2-inch) prisms over a 305-mm (12-inch) 

span with third-point loading. 

0

3

6

9

12

15

18

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Deflection (mm) 

Load (kN)

Steam

0

3

6

9

12

15

18

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Deflection (mm) 

Load (kN)

Untreated
61 

1 kN = 0.225 kip, 1 mm = 0.039 inch 

Figure 42. Graph. ASTM C1018 load-deflection response results for tempered steamtreated 51- by 51-mm (2- by 2-inch) prisms over a 305-mm 

(12-inch) span with third-point loading. 

1 kN = 0.225 kip, 1 mm = 0.039 inch 

Figure 43. Graph. ASTM C1018 load-deflection response results for delayed steamtreated 51- by 51-mm (2- by 2-inch) prisms over a 305-mm 

(12-inch) span with third-point loading. 

0

3

6

9

12

15

18

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Deflection (mm) 

Load (kN)

Tempered Steam

0

3

6

9

12

15

18

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Deflection (mm) 

Load (kN)

Tempered Steam
62 

 kN = 0.225 kip, 1 mm = 0.039 inch 

Figure 44. Graph. ASTM C1018 load-deflection response results for steamtreated 51- by 51-mm (2- by 2-inch) prisms over a 381-mm (15-inch) 

span with 76 mm (3 inches) between loads. 

1 kN = 0.225 kip, 1 mm = 0.039 inch 

Figure 45. Graph. ASTM C1018 load-deflection response results for untreated 

51- by 51-mm (2- by 2-inch) prisms over a 381-mm (15-inch) span with 

76 mm (3 inches) between loads. 

0

2

4

6

8

10

12

0.0 1.0 2.0 3.0 4.0 5.0

Deflection (mm) 

Load (kN)

Steam

0

2

4

6

8

10

12

0.0 1.0 2.0 3.0 4.0 5.0

Deflection (mm) 

Load (kN)

Untreated
63 

1 kN = 0.225 kip, 1 mm = 0.039 inch 

Figure 46. Graph. ASTM C1018 load-deflection response results for tempered steamtreated 51- by 51-mm (2- by 2-inch) prisms over a 381-mm (15-inch) 

span with 76 mm (3 inches) between loads. 

1 kN = 0.225 kip 

1 mm = 0.039 inch 

Figure 47. Graph. ASTM C1018 load-deflection response results for delayed steamtreated 51- by 51-mm (2- by 2-inch) prisms over a 381-mm (15-inch) 

span with 76 mm (3 inches) between loads. 

0

2

4

6

8

10

12

0.0 1.0 2.0 3.0 4.0 5.0

Deflection (mm)

Load (kN)

Tempered Steam

0

2

4

6

8

10

12

0.0 1.0 2.0 3.0 4.0 5.0

Deflection (mm)

Load (kN)

Delayed Steam
64

3.4.1.1 Strength 

The procedure implemented for the ASTM C1018 prism flexure tests allowed for accurate 

recording of the flexural behavior of UHPC from initial elastic behavior, through tensile 

cracking, to tensile fiber pullout. The strength-based results from these tests are presented in 

table 16. The table provides the averaged results from the prisms in each set. The first and most 

important result from these tests is behavior at first cracking of the UHPC matrix. As previously 

discussed, the first crack is an indication of the tensile cracking strength of UHPC. The load, 

strength, and deflection at first cracking are all listed in the table. The first crack strength is 

based on the equation in figure 48, which is referenced in ASTM C1018, provided in ASTM 

C78, and based on mechanics of materials principles assuming pure bending.(22) In the equation, 

fct,flexure is the flexural tensile cracking strength, P is the total load applied to the prism, l is the 

span, b is the width of the prism, and d is the depth. The variable η equals 1.0 for third-point 

loading and 1.2 for the loading configuration with a 381-mm (15-inch) span and 76 mm 

(3 inches) between load points. This factor accounts for the difference in the bending moment at 

midspan on the prism under the differing loading configurations. 

Figure 48. Equation. Flexural cracking strength of a concrete prism. 

It has been widely observed that the actual tensile cracking strength of concrete, fct, is 

overestimated by the tensile cracking strength results of a small-scale flexural test, fct,flexure. 

Carpinteri and Chiaia summarized extensive amounts of previous research on this topic.(23) They 

indicated that the overestimation is usually caused by depth and strain gradient effects on the 

flexural cross section. Chanvillard and Rigaud indicate that the concrete ahead of the crack front 

tends to microcrack, thus reducing stress concentrations.(24) They performed research on the 

UHPC that is the subject of this report. Their results indicated that the overestimation of the 

tensile strength is caused by the fiber reinforcement. 

Various correction factors have been suggested to account for this overestimation. The 

Association Française de Génie Civil (AFGC) Interim Recommendations for Ultra High 

Performance Fibre-Reinforced Concretes recommends the equation in figure 49, which varies 

depending on the depth of the cross section in millimeters, d, as compared to a reference depth, 

d0, of 100 mm (4 inches).(1) Note that this empirical equation is based on experimental data, but 

it has been verified for UHPC by Chanvillard and Rigaud.(24) The first crack tensile strength 

results as modified by this correction factor are provided in table 16. 

Table 16 also includes results related to the average peak load carried by each set of prisms. The 

peak load values tended to be between 170 and 200 percent of the cracking load values. The 

equivalent flexural strength corresponding with the peak load is listed. This strength is based on 

the equation in figure 48 and substitutes the peak load for the cracking load. This calculation is 

presented purely for comparative purposes, as this equivalent flexural strength has no physical 

meaning. At peak loading, the UHPC prism is exhibiting extensive cracking, and its midspan 

neutral axis no longer resides at mid-depth. Thus, the assumptions of pure bending on a uniform, 

elastic cross section, which are inherent in this equation, are not met. 

, 2 bd

Pl f ct flexure =η
65

Figure 49. Equation. AFGC correction factor for concrete prism flexural strength. 

Finally, this table includes the average effective modulus of elasticity of each set of prisms. The 

equation in figure 50 provides the relationship between the centerline deflection of a simply 

supported beam with two point loads and the cross-sectional and material properties of the beam. 

This equation accounts for both the flexural and shear responses in the beam. In this equation, Δcl

is the centerline deflection, P is the total load applied to the prism, L is the span, a is the shear 

span, E is the modulus of elasticity, and I is the moment of inertia. In the shear term, A’ is the 

effective shear area (i.e., 80 percent of the cross-sectional area for a prism) and G is the shear 

modulus. To calculate the shear modulus, it is assumed that the Poisson’s ratio of UHPC is 0.18. 

Manipulation of this equation shows that the modulus of elasticity can be calculated based on the 

elastic slope of the load-deflection curve and a constant term representing the load configuration 

and beam cross section. 

Figure 50. Equation. Centerline deflection of a simply supported prismatic beam. 

The load-deflection responses presented previously were analyzed to determine the average 

elastic response between 20 and 50 percent of the load at first cracking. These stiffness values, 

which are dependent on the test setup, were then used in conjunction with the equation in figure 

50 to determine the effective modulus of elasticity. The effective modulus results tend to be 

higher than those based on the compression tests presented earlier in this chapter. The exception 

to this tendency is in the prisms where lower modulus values were observed with a span-to-depth 

ratio of 3. 

The lower modulus value in the smaller span-to-depth ratio prisms is likely due to the effects of 

local disturbed regions near the load application points. With this short span and these close load 

points, nearly the entire prism could be considered to be locally disturbed, including 

deformations that could add to the flexure and shear deformations. Any additional deformation 

would result in a lower calculated effective modulus of elasticity. 

Prism flexure testing of five different loading configurations for each curing regime was 

intended to identify the benefits and detriments of varying the prism span and cross section. 

From a qualitative standpoint, the easiest completion was of the 229-mm (9-inch) and 305-mm 

(12-inch) span tests on the 51- by 51-mm (2- by 2-inch) cross section. The behavior of the prisms 

in these loading configurations was observed to be more consistent than in other configurations. 

Also in these configurations, the moderate elastic load-displacement response decreased the 

difficulty encountered with the stiffer and more flexible configurations in the test setups. 

⎟

⎟

⎟

⎟

⎟

⎠

⎞

⎜

⎜

⎜

⎜

⎜

⎝

⎛

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

+

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

= 0.7

0

0.7

0

,

1 2.0

2.0

d

d

d

d

f f ct ct flexure

2 ' (3 4 ) 48

2 2

GA

Pa L a

EI

Pa Δcl = − +
66

Quantitatively, the results from the 229-, 305-, and 381-mm (9-, 12-, and 15-inch) spans for the 

51- by 51-mm (2- by 2-inch) cross section prisms were most consistent. 

Also, it must be mentioned that the basic intent of the ASTM C1018 test seems to be to create a 

state of pure bending in a concrete prism to allow for quantification of the tensile properties. As 

the span-to-depth ratio of the beam decreases, the proportion of the behavior that is shear based 

increases. Figure 51 shows the ratio of the shear to flexural deflection of a 51-mm (2-inch)-deep 

prism. This prism was assumed to be loaded at its third points, and it has a shear modulus equal 

to 61 percent of its elastic modulus based on a Poisson’s ratio of 0.18. With a span-to-depth ratio 

of 3, the shear deflections are 17.1 percent of the flexural deflections, thus the assumption of 

flexural behavior is questionable. As the span increases toward 305 mm (12 inches), the 

assumption of flexural behavior becomes much more reasonable. In the longer spans, the state of 

stress on the cross section in the constant moment region is primarily composed of stresses 

normal to the face of the cross section. However, the same does not hold true for the shorter 

spans and the more influential shear forces. 

1 mm = 0.039 inch 

Figure 51. Graph. Ratio of shear to flexural deflection for a third-point loaded prism.

0.00

0.04

0.08

0.12

0.16

0.20

0.24

0.28

100 150 200 250 300 350 400

Prism Span (mm)

Ratio of Shear to Flexural 

Deflection

51 mm Deep Prism

152 mm span Third-Point Loading

17.1%

229 mm span

7.6% 305 mm span

4.3%
67

Table 16. ASTM C1018 strength results. 

1 MPa = 145 psi, 1 GPa = 145,000 psi, 1 mm = 0.039 inch 

Prism 

Cross Section 

and Setup*

No. First Crack 

Deflection 

(mm) 

First Crack 

Strength† 

(MPa) 

Corrected 

First Crack 

Strength‡

(MPa) 

Modulus of 

Elasticity§

(GPa) 

Peak Load 

Deflection 

(mm) 

51x51, 152, 51 3 0.044 18.6 10.3 51.1 0.503 

76x102, 305, 102 3 0.083 17.5 10.9 60.5 1.055 

51x51, 229, 76 3 0.069 17.7 9.9 59.8 0.761 

Steam 51x51, 305, 102 4 0.117 16.7 9.2 58.9 1.858 

51x51, 381, 76 4 0.156 15.9 8.8 59.0 3.174 

51x51, 152, 51 4 0.039 16.2 9.0 47.7 0.532 

76x102, 305, 102 3 0.092 15.6 9.7 49.8 1.083 

51x51, 229, 76 3 0.074 15.9 8.8 52.3 1.088 

51x51, 305, 102 4 0.134 15.9 8.8 49.3 1.528 

Untreated 

51x51, 381, 76 4 0.193 16.8 9.3 50.7 2.216 

51x51, 152, 51 4 0.041 19.6 10.8 54.0 0.486 

76x102, 305, 102 3 0.075 14.6 9.1 56.4 0.855 

51x51, 229, 76 4 0.076 17.7 9.9 55.5 0.990 

51x51, 305, 102 4 0.131 18.2 10.1 56.3 1.265 

Tempered 

Steam 

51x51, 381, 76 2 0.185 18.2 10.1 56.1 1.933 

51x51, 152, 51 4 0.040 17.2 9.6 56.1 0.486 

76x102, 305, 102 3 0.107 17.7 11.0 58.4 0.918 

51x51, 229, 76 4 0.069 16.1 9.0 55.6 0.779 

Delayed 

Steam 51x51, 305, 102 4 0.138 16.9 9.4 55.8 1.721 

51x51, 381, 76 4 0.167 16.1 8.9 56.1 2.240 

* Prism depth x width, span length, distance between upper load points 

† Calculated using equation in figure 48, which assumes pure bending on a uniform, elastic, uncracked cros‡Calculated using equation in figure 49 

§ Calculated using equation in figure 50, which allows for bending and shear on a uniform, elastic, uncrack
68

3.4.1.2 Toughness 

Within concrete materials characterization testing, toughness is a term that provides some 

indication of the concrete’s energy absorption capability. Usually, toughness is quantified in 

terms of the area under a load-deflection response curve. Toughness values are specific to the 

testing procedure implemented. 

The ASTM C1018 test method presents one means of determining the toughness of fiberreinforced concrete. The test results are analyzed in terms of the area under the load-deflection 

curve up to specific deflection levels. The toughness results are then normalized by dividing the 

total area under the curve up to the specified deflection by the area under the curve up to the 

deflection at first cracking. 

ASTM C1018 defines a set of toughness indices in terms of the behavior that might be expected 

from a material that exhibits an elastic-plastic, flexural, load-deflection response. (Although not 

clearly stated in the test method, note that this reference response does not correspond to the 

response that would be observed for a material that exhibits elastic-plastic uniaxial stress-strain 

behavior.) Table 17 is reproduced from the appendix of the test method. The table provides basic 

information relating to the calculation of toughness indices and the expected results from various 

materials. Additional indices can be created in a similar fashion. Results for I30 and I40 are also 

presented in the following discussion. 

Table 17. Definition of toughness indices (from ASTM C1018 FIG. X1.1). 

 Values of Toughness Indices 

Index 

Designation 

Deflection 

Criterion*

Plain 

Concrete 

Elastic-Plastic 

Material†

Observed Range for 

Fibrous Concrete 

I5 3.0δ 1.0 5.0 1 to 6 

 I10 5.5δ 1.0 10.0 1 to 12 

 I20 10.5δ 1.0 20.0 1 to 25 

* δ is the deflection at first cracking. 

† This refers to a material that exhibits an elastic-plastic flexural response. 

Table 18 presents the toughness results for the sets of prismatic flexural tests. The information 

layout in the table is similar to table 16. Note that some results are missing for I40. In addition, 

some other results for I40 are from fewer than the total number of prisms listed. This is due to 

some prisms exhibiting fiber pullout and subsequent failure prior to reaching 20.5 times the 

cracking deflection. Figures 52 through 55 graphically present the toughness results from the 

four curing regimes. All of these results show increasing toughness through I40. 
69

Table 18. ASTM C1018 toughness results. 

Prism Toughness Index Residual Str

Cross Section and 

Setup*

No. First 

Crack 

Deflection 

(mm) 

First 

Crack 

Strength†

(MPa) 

I5 I10 I20 I30 I40 

R5,10 

R10,20 

51x51, 152, 51 3 0.044 10.3 5.6 12.5 28.3 44.2 62.8 138 158 

76

x102, 305, 102 3 0.083 10.9 5.3 11.8 25.9 40.6 54.4 129 141 51

x51, 229, 76 3 0.069 9.9 5.6 12.7 28.7 45.3 60.6 143 160 

Steam 51

x51, 305, 102 4 0.117 9.2 6.2 14.4 32.8 53.0 77.4 165 183 

51

x51, 381, 76 4 0.156 8.8 5.8 13.9 32.0 51.6 72.6 162 180 

51x51, 152, 51 4 0.039 9.0 5.7 12.9 29.0 46.3 63.0 144 161 

76

x102, 305, 102 3 0.092 9.7 5.7 12.8 28.5 45.0 59.9 142 158 51

x51, 229, 76 3 0.074 8.8 5.7 13.1 29.5 46.8 64.2 147 165 

51

x51, 305, 102 4 0.134 8.8 5.6 12.8 28.8 48.3 65.5 143 160 

Untreated 

51

x51, 381, 76 4 0.193 9.3 5.8 13.1 28.7 50.1 67.6 147 156 

51x51, 152, 51 4 0.041 10.8 5.8 12.9 28.2 44.2 65.5 141 153 

76

x102, 305, 102 3 0.075 9.1 6.1 14.1 31.9 53.7 73.0 161 178 51

x51, 229, 76 4 0.076 9.9 6.0 13.8 31.1 50.8 69.6 155 173 

51

x51, 305, 102 4 0.131 10.1 5.8 13.0 28.0 43.1 – 144 150 

Tempered 

Steam 

51

x51, 381, 76 2 0.185 10.1 6.1 13.9 31.1 47.6 – 156 172 

51x51, 152, 51 4 0.040 9.6 5.5 12.6 28.5 43.2 63.7 143 159 

76

x102, 305, 102 3 0.107 11.0 5.5 12.4 27.6 46.7 61.0 138 151 51

x51, 229, 76 4 0.069 9.0 5.8 13.5 30.9 48.9 65.6 153 175 

Delayed 

Steam 51

x51, 305, 102 4 0.138 9.4 5.8 13.4 30.3 48.3 61.7 152 169 

51

x51, 381, 76 4 0.167 8.9 6.0 14.1 32.2 50.8 74.6 162 181 

* Prism depth 

x width, span length, distance between upper load points 

† Corrected tensile cracking strength based on the equation in figure 49 

1 mm = 0.039 inch, 1 MPa = 145 psi 
70

Figure 52. Graph. ASTM C1018 toughness results for steam-treated UHPC prisms. 

Figure 53. Graph. ASTM C1018 toughness results for untreated UHPC prisms. 

0

20

40

60

80

I5 I10 I20 I30 I40

Toughness Value

51x51,152,51

76x102,305,102

51x51,229,76

51x51,305,102

51x51,381,76

Steam

I5 I10 I20 I30 I40

0

20

40

60

80

I5 I10 I20 I30 I40

Toughness Value

51x51,152,51

76x102,305,102

51x51,229,76

51x51,305,102

51x51,381,76

Untreated

I5 I10 I20 I30 I40
71

Figure 54. Graph. ASTM C1018 toughness results for tempered 

steam-treated UHPC prisms. 

Figure 55. Graph. ASTM C1018 toughness results for delayed 

steam-treated UHPC prisms. 

0

20

40

60

80

I5 I10 I20 I30 I40

Toughness Value

51x51,152,51

76x102,305,102

51x51,229,76

51x51,305,102

51x51,381,76

Tempered Steam

I5 I10 I20 I30 I40

0

20

40

60

80

I5 I10 I20 I30 I40

Toughness Value

51x51,152,51

76x102,305,102

51x51,229,76

51x51,305,102

51x51,381,76

Delayed Steam

I5 I10 I20 I30 I40
72

Figure 56. Graph. ASTM C1018 residual strength results for steam-treated prisms. 

Figure 57. Graph. ASTM C1018 residual strength results for untreated prisms. 

0

50

100

150

200

250

R5,10 R10,20 R20,30 R30,40

Residual Strength Value

51x51,152,51 76x102,305,102

51x51,229,76 51x51,305,102

51x51,381,76

Steam

R5,10 R10,20 R20,30 R30,40

0

50

100

150

200

250

R5,10 R10,20 R20,30 R30,40

Residual Strength Value

51x51,152,51 76x102,305,102

51x51,229,76 51x51,305,102

51x51,381,76

Untreated

R5,10 R10,20 R20,30 R30,40
73

Figure 58. Graph. ASTM C1018 residual strength results 

for tempered steam-treated prisms. 

Figure 59. Graph. ASTM C1018 residual strength results 

for delayed steam-treated prisms. 

0

50

100

150

200

250

R5,10 R10,20 R20,30 R30,40

Residual Strength Value

51x51,152,51 76x102,305,102

51x51,229,76 51x51,305,102

51x51,381,76

Tempered Steam

R5,10 R10,20 R20,30 R30,40

0

50

100

150

200

250

R5,10 R10,20 R20,30 R30,40

Residual Strength Value

51x51,152,51 76x102,305,102

51x51,229,76 51x51,305,102

51x51,381,76

Delayed Steam

R5,10 R10,20 R20,30 R30,40
74

Residual strength factors also are presented in table 18. These values are calculated by 

subtracting one toughness index from a subsequent index then multiplying the result by a 

normalizing factor related to the idealized elastic-plastic, flexural response. Regardless of the 

toughness indices chosen, the residual strength factor for the idealized elastic-plastic, flexural 

material will equal 100. For example, the residual strength factor, R10,20, equals 10(I20–I10). 

The toughness exhibited by the UHPC is quite impressive, regardless of the curing regime or test 

configuration. The results all tend to be at the upper end or above the ASTM C1018 predicted 

range for toughness of fiber-reinforced concrete. The UHPC also exhibits residual strength 

values that tend to increase at least through I30, and these values through this deflection level are 

all above the 100 reference level for elastic-plastic flexural behavior. Figures 56 through 59 

provide a graphical representation of the residual strength factors. 

3.4.2 Split Cylinder 

The tensile strength of UHPC was also measured through the ASTM C496 Standard Test 

Method for Splitting Tensile Strength of Cylindrical Concrete Specimens.(25) This test, often 

referred to as the split-cylinder test, indirectly measures the tensile strength of concrete by 

compressing a cylinder through a line load applied along its length. This test can be completed in 

a standard concrete compression testing machine, with only one special requirement: the hinged 

bearing that loads the specimen. 

This load configuration creates a lateral tensile stress in the cylinder across the vertical plane of 

loading. A relatively uniform tensile stress field is created over the middle 75 percent of the 

cylinder’s diameter along the plane of loading, and the maximum tensile stress occurs at the 

center of the cylinder. ASTM C496 indicates that the maximum tensile stress can be calculated 

based on the equation in figure 60. In this equation, P is the load applied to the cylinder, l and d

are the length and diameter, and fθ is the tensile stress. 

Figure 60. Equation. Tensile stress in an ASTM C496 split-cylinder test. 

The split-cylinder test does not determine the uniaxial tensile cracking strength of concrete. The 

loading configuration used in this test actually creates a biaxial stress state inside the cylinder 

that has been described many times.(26,27) For the purposes of this report, it is sufficient to 

indicate that the vertical compressive stress in the center of the cylinder is approximately three 

times the lateral tensile stress. 

This test is normally completed on standard concrete that does not contain fiber reinforcement. 

As such, the tensile strength results are normally clear. The cylinder will fail when its tensile 

strength is reached; therefore, the peak load carried by the cylinder can be used to determine the 

splitting tensile strength through the equation in figure 60. Fiber-reinforced concrete, and in 

particular UHPC, tends to behave differently. In these concretes, the initiation of cracking 

signifies the beginning of a new phase in the material’s behavior, but does not signify failure of 

the material. With UHPC in particular, the load will continue to increase after cracking, and the 

ld

P f

π

θ

2 =
75

cracks that form during cracking will be so small that identification without microscopic 

investigation may not be possible. 

A supplemental feature was added to the ASTM 496 test because the cracking strength is the 

material property of interest. As the UHPC cylinder cracks, the measured length of the lateral 

diameter of the cylinder will show a marked increase. A lateral expansion measuring apparatus 

was devised and is shown in figure 61. This spring-loaded device clamps across the cylinder and 

measures the lateral expansion of the cylinder from load initiation through failure. The small 

lateral compressive force exerted by the apparatus on the specimen (less than 0.02 MPa (2 psi)) 

is considered negligible. Two LVDTs located near the front and back of the cylinder 

electronically capture the displacements, which are sent to a data acquisition system along with 

the load. This lateral expansion measuring apparatus is similar to a device used by Nanni in work 

that he completed on fiber-reinforced concrete.(28)

The typical load versus lateral displacement response for a UHPC cylinder is presented in figure 

62. The curve shown is the average of the results from the two LVDTs. The lateral deflection 

behavior is basically linear until the UHPC cracks. At cracking, a discontinuity occurs in the 

displacement response while the load level remains relatively constant. The total jump in 

displacement is usually less than 0.025 mm (0.001 inch) and could be nonexistent in a specimen 

that has not undergone a steam-based treatment. After cracking, a clear change occurs in the 

slope of the response curve. The load then continues to increase at a decreasing rate until the 

peak load is reached. 

Normal UHPC mix design and casting procedures were used for this series of tests. Twelve 

4-inch diameter cylinders were cast for each curing regime. The screeded end of the cylinder was 

ground prior to testing, and the length of each cylinder was approximately 200 mm (7.9 inches). 

The cylinders were tested in groups of four at three different ages for each curing regime. The 

steam, untreated, and tempered steam regimes were tested at 5, 14, and 28 days after casting. The 

delayed steam regime was tested at 14, 21, and 28 days after casting. At 14 days, the delayed 

steam specimen results are more comparable to the untreated specimens than to the other delayed 

steam treated specimens, because the steam treatment did not occur until days 15 through 17. 

The initial load rate for these tests was set at 3.4 MPa/min (500 psi/min) of tensile stress 

according to the equation in figure 60. The rate was set above the ASTM specified rate of 

0.7 to 1.4 MPa/min (100 to 200 psi/min) because of three reasons: (1) the higher tensile cracking 

strength of the UHPC, (2) the significant displacement that must be traversed before the peak 

load is reached, and (3) the reluctance to change the load rate after test initiation. Preliminary 

testing on UHPC prior to the initiation of these tests indicated that this increased load rate should 

not cause significant changes in material behaviors. 

Figure 63 presents the average tensile cracking results from the split-cylinder tests. As previously 

mentioned, cracking is defined to occur when an abrupt or semiabrupt change in specimen lateral 

stiffness occurs. The number of days after casting is indicated in parentheses after the curing 

regime’s name. The figure shows both the average stress and the ±1 standard deviation from the 

average. 
76

Figure 61. Photos. Split-cylinder tensile test including (a) standard test setup, (b) lateral 

expansion measuring apparatus, and (c) UHPC cylinder during test. 

All of the groups that underwent a steam-based curing regime exhibited a split-cylinder tensile 

cracking strength of between 11.0 and 12.4 MPa (1.6 and 1.8 ksi). The untreated group exhibited 

decidedly lower strength values, along with a clear increase in strength over time. At 5 days, the 

untreated group had a tensile cracking strength of 6.9 MPa (1.0 ksi), and by 28 days this strength 

had increased to over 9.0 MPa (1.3 ksi). 

(a) (b) 

(c) 
77

1 kN = 0.225 kip 

1 mm = 0.039 inch 

Figure 62. Graph. Typical response for a UHPC cylinder during the ASTM C496 test. 

1 MPa = 145 psi 

Figure 63. Chart. Average tensile cracking results from the ASTM C496 test. 

0

200

400

600

800

1000

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Lateral Displacement (mm)

Load (kN)

Cracking

Peak

12.4

11.6

10.9

6.6

8.8

9.2

11.0

11.5

11.8

8.6

11.4

11.9

0 5 10 15

Steam, 5d

Steam, 14d

Steam, 28d

Untreated, 5d

Untreated, 14d

Untreated, 28d

Tempered Steam, 5d

Tempered Steam, 14d

Tempered Steam, 28d

Delayed Steam, 14d

Delayed Steam, 21d

Delayed Steam, 28d

Tensile Cracking Strength (MPa)
78

The split-cylinder loading configuration causes vertical compressive stress and lateral tensile 

stress in the cylinder. This biaxial stress state has a definite effect on the postcracking behavior. 

The vertical compressive stresses that are parallel to any cracks cause the fiber reinforcement 

bridging the cracks to carry higher loads prior to pulling out of the UHPC matrix. For this 

reason, results derived from the peak load carried by a cylinder and passed through the equation 

in figure 60 are not accurate general representations of the tensile strength of UHPC. 

However, these peak load results may be useful for comparing fiber pullout behavior following 

different curing treatments. These variations in peak load results may be useful in determining 

whether curing was properly applied to a UHPC specimen. The average peak stress carried by 

these groups of specimens is presented in figure 64. This stress is calculated based on both the 

peak load reached and the equation in figure 60. The results show that fibers in untreated 

cylinders pull out earlier under this stress state, and the cylinder as a whole can only carry an 

equivalent stress of around 19.3 MPa (2.8 ksi). The steam treatment in the tempered steam 

regime enhances the behavior such that 21.4 MPa (3.1 ksi) of equivalent stress can be carried. 

The steam and delayed steam regimes carried the largest peak loads with average equivalent 

stresses of around 24.1 MPa (3.5 ksi). 

1 MPa = 145 psi 

Figure 64. Chart. Average split cylinder peak strength from the ASTM C496 test. 

26.5

24.1

24.2

15.1

19.4

19.0

21.1

21.6

22.0

18.3

25.2

22.9

0 5 10 15 20 25 30

Steam, 5d

Steam, 14d

Steam, 28d

Untreated, 5d

Untreated, 14d

Untreated, 28d

Tempered Steam, 5d

Tempered Steam, 14d

Tempered Steam, 28d

Delayed Steam, 14d

Delayed Steam, 21d

Delayed Steam, 28d

Split Cylinder Peak Strength (MPa)
79

Frequently, the tensile strength of concrete is discussed as a percentage of the compressive 

strength. The results from the split cylinder tests have been normalized by the 28-day 

compressive strength of their control cylinders and are presented in table 19. Only the postcuring treatment results are presented, because only the 28-day compressive strengths are known 

for these particular batches of UHPC. The cracking stress tends to be between 5 and 7 percent of 

the compressive strength. The equivalent peak stress carried by the cylinders is higher, with 

values ranging from 12 to 16 percent. 

The cracking behavior must be monitored for the ASTM C496 test to provide useful results in 

terms of tensile cracking strength. The lateral expansion measuring apparatus (described above) 

allowed for quantification of this cracking behavior. Monitoring of the cracking behavior may 

also be possible in certain instances via audible observations. Table 20 presents the first crack 

parameters for each group of specimens. Aural monitoring throughout the test allowed the load 

at first cracking to be estimated in nearly all of the cylinders that had undergone a steam-based 

curing treatment. As discussed previously, the data collection during the test allowed for a 

specific determination of the load at first crack. The size of the first crack was also estimated 

based on the instantaneous lateral expansion of the cylinder at first cracking. The values 

presented in the table are the crack size (measured by the front or back LVDT, whichever 

displayed a larger instantaneous increase). These results provide a clear sense of the width of 

UHPC tensile stress cracks when they first occur as well as the type of instrumentation that is 

required to monitor or capture this behavior. 

Table 19. Split tensile strength normalized by 28-day compressive strength. 

Normalized by 28-day 

Compressive Strength 

Group 28-day 

Compression 

Strength 

(ksi) 

ASTM C496 

Cracking Stress

ASTM C496 

Peak Stress 

Steam (5) 181 0.051 0.148 

Steam (14) 181 0.064 0.132 

Steam (28) 181 0.060 0.134 

Untreated (28) 121 0.076 0.161 

Tempered Steam (5) 168 0.066 0.125 

Tempered Steam (14) 168 0.068 0.127 

Tempered Steam (28) 168 0.070 0.133 

Delayed Steam (21) 170 0.067 0.147 

Delayed Steam (28) 170 0.070 0.133 

1 ksi = 6.895 MPa C
80

Table 20. First-crack parameters determined by instantaneous lateral expansion 

of cylinder and aural observations. 

Group Sound First Crack Size (mm) 

 (Audible/Inaudible) Average Minimum Maximum 

Steam (5) Audible 0.0211 0.0170 0.0249 

Steam (14) Audible 0.0221 0.0127 0.0290 

Steam (28) Audible 0.0236 0.0157 0.0391 

Untreated (5) Inaudible 0.0079 0.0038 0.0122 

Untreated (14) Inaudible 0.0091 0.0000 0.0216 

Untreated (28) Inaudible 0.0180 0.0071 0.0277 

Tempered Steam (5) Audible 0.0206 0.0163 0.0257 

Tempered Steam (14) Audible 0.0178 0.0084 0.0371 

Tempered Steam (28) Audible 0.0193 0.0150 0.0279 

Delayed Steam (14) Inaudible 0.0147 0.0066 0.0216 

Delayed Steam (21) Audible 0.0180 0.0086 0.0236 

Delayed Steam (28) Audible 0.0084 0.0008 0.0185 

1 mm = 0.039 inch C

3.4.3 Mortar Briquette 

Another means of concrete tensile strength determination is the briquette tension test. This test 

method, described in AASHTO T132, normally involves the direct tension testing of a small 

briquette cast from cement mortar.(29) The dogbone-shaped briquette is 76 mm (3 inches) long, 

25 mm (1 inch) thick, and has a 645-mm2

 (1-inch2) cross section at midlength. Special selfaligning grips allow for passive gripping of the specimen in the test machine and ensure uniform 

loading. 

The standard UHPC mix was used, including the steel fibers, in this test program. The casting of 

the briquettes is assumed to have caused some preferential alignment of the fibers because the 

minimum cross section of the briquette is only 645 mm2

 (1 inch2). This preferential alignment 

would be expected to be parallel to the walls of the mold; therefore, the fiber percentage aligned 

across the anticipated failure plane would be higher than normal. 

The normal UHPC mix design and casting procedures were used. Eighteen briquettes were cast 

for each of the four curing regimes: three sets of six briquettes that were tested at 28, 56, and 

84 days after casting. Figure 65 shows both the grips that were used to test the briquettes as well 

as a briquette in the grips. 

The testing was completed in a 98-kN (22-kip) capacity MTS testing machine. The tests were 

controlled based on the displacement of the crosshead of the testing machine to enable the 

observation of the post-peak load-displacement response. AASHTO T132 recommends loading 

the briquettes at 2.7 kN/min (600 lb/min). This portion of the test method was modified, and the 

tests were conducted at a displacement rate of 0.025 mm/s (0.001 inch/s). Given the stiffness of 

the load carrying apparatus between the crossheads, this displacement rate equates to 
81

approximately 4.0 kN/min (900 lb/min) throughout the initial elastic loading portion of each 

briquette’s response. 

Figure 65. Photos. AASHTO T132 setup including (a) test grips and (b) specimen. 

Figures 66 through 77 present the results from the 18 tests for each curing regime. Six replicates 

are included in each plot, with the results shifted by 1 mm (0.039 inch) to allow for clarity. The 

curing regime and age (in days) at testing are included in the upper right corner of each plot. 

These results show that the UHPC behaved linear-elastically up to first cracking. After cracking, 

a slight decrease in load usually occurred followed by a load increase to a level near or above the 

cracking load. All of the results show a significant amount of postcracking load-carrying 

capacity. A more detailed discussion of these test results that focuses on strength and toughness 

is included in the following two sections. 

Similar to all batches of UHPC cast for this research program, compression tests on 7.62-cm 

(3-inch) diameter control cylinders were completed. The results from the control cylinders for 

the four batches associated with the briquette testing indicate that three of these batches exhibited 

strengths below the average values. The steam-treated and untreated batches showed 

compressive strengths 15 percent below the overall test program average, and the delayed steamtreated batch was 8 percent below the average. These results should be kept in mind as the results 

in the remainder of this section are discussed. 

(a) (b) 
82

1 kN = 0.225 kip, 1 mm = 0.039 inch 

Figure 66. Graph. Load-displacement response for steam-treated briquettes (28 days). 

1 kN = 0.225 kip, 1 mm = 0.039 inch 

Figure 67. Graph. Load-displacement response for steam-treated briquettes (56 days). 

1 kN = 0.225 kip, 1 mm = 0.039 inch 

Figure 68. Graph. Load-displacement response for steam-treated briquettes (84 days). 

0

2

4

6

8

0 2 4 6 8 10 12 14 16

Cross-Head Displacement (mm)

Load (kN)

Steam (28)

0

2

4

6

8

0 2 4 6 8 10 12 14 16

Cross-Head Displacement (mm)

Load (kN)

Steam (56)

0

2

4

6

8

0 2 4 6 8 10 12 14 16

Cross-Head Displacement (mm)

Load (kN)

Steam (84)
83

1 kN = 0.225 kip, 1 mm = 0.039 inch 

Figure 69. Graph. Load-displacement response for untreated briquettes (28 days). 

1 kN = 0.225 kip, 1 mm = 0.039 inch 

Figure 70. Graph. Load-displacement response for untreated briquettes (56 days). 

1 kN = 0.225 kip, 1 mm = 0.039 inch 

Figure 71. Graph. Load-displacement response for untreated briquettes (84 days). 

0

2

4

6

8

0 2 4 6 8 10 12 14 16

Cross-Head Displacement (mm)

Load (kN)

Untreated (28)

0

2

4

6

8

0 2 4 6 8 10 12 14 16

Cross-Head Displacement (mm)

Load (kN)

Untreated (56)

0

2

4

6

8

0 2 4 6 8 10 12 14 16

Cross-Head Displacement (mm)

Load (kN)

Untreated (84)
84

1 kN = 0.225 kip, 1 mm = 0.039 inch 

Figure 72. Graph. Load-displacement for tempered steam-treated briquettes (28 days). 

1 kN = 0.225 kip, 1 mm = 0.039 inch 

Figure 73. Graph. Load-displacement for tempered steam-treated briquettes (56 days). 

1 kN = 0.225 kip, 1 mm = 0.039 inch 

Figure 74. Graph. Load-displacement for tempered steam-treated briquettes (84 days). 

0

2

4

6

8

0 2 4 6 8 10 12 14 16

Cross-Head Displacement (mm)

Load (kN)

Tempered Steam (28)

0

2

4

6

8

0 2 4 6 8 10 12 14 16

Cross-Head Displacement (mm)

Load (kN)

Tempered Steam (56)

0

2

4

6

8

0 2 4 6 8 10 12 14 16

Cross-Head Displacement (mm)

Load (kN)

Tempered Steam (84)
85

1 kN = 0.225 kip, 1 mm = 0.039 inch 

Figure 75. Graph. Load-displacement for delayed steam-treated briquettes (28 days). 

1 kN = 0.225 kip, 1 mm = 0.039 inch 

Figure 76. Graph. Load-displacement for delayed steam-treated briquettes (56 days). 

1 kN = 0.225 kip, 1 mm = 0.039 inch 

Figure 77. Graph. Load-displacement for delayed steam-treated briquettes (84 days). 

0

2

4

6

8

0 2 4 6 8 10 12 14 16

Cross-Head Displacement (mm)

Load (kN)

Delayed Steam (28)

0

2

4

6

8

0 2 4 6 8 10 12 14 16

Cross-Head Displacement (mm)

Load (kN)

Delayed Steam (56)

0

2

4

6

8

0 2 4 6 8 10 12 14 16

Cross-Head Displacement (mm)

Load (kN)

Delayed Steam (84)

0

2

4

6

8

0 2 4 6 8 10 12 14 16

Cross-Head Displacement (mm)

Load (kN)

Delayed Steam (28)
86

3.4.3.1 Strength 

The tensile cracking strength results are presented in figure 78. First cracking was defined as a 

discontinuity in the load-displacement curve caused by an instantaneous decrease in load. The 

results from the six replicates in each group have been averaged, and the average value is shown 

on the bar in the chart. The standard deviation within the results for each group is provided by 

the error bar, which indicates a ±1 standard deviation. 

1 MPa = 145 psi 

Figure 78. Chart. Tensile cracking strength of UHPC briquettes. 

The UHPC that underwent the steam-based curing treatment showed consistent strength levels 

from 1 to 3 months after casting. The steam-treated UHPC tensile cracking strength is 

approximately 8.3 MPa (1.2 ksi), while the tempered steam-treated UHPC is 9.7 MPa (1.4 ksi). 

The delayed steam-treated UHPC shows a lower value of around 1.0 ksi. The untreated UHPC 

shows a steady increase in strength between the first and third months, with the cracking tensile 

strength increasing 20 percent to 7.6 MPa (1.1 ksi) by 3 months. Again, recall that the 

compressive strength results for three of these curing regimes were lower than anticipated, thus 

the tensile strength results were likely affected as well. 

The postcracking strength results are presented in figure 79, which has the same format as 

figure 78. The postcracking peak strength was defined as the highest load level reached after the 

tensile cracking load decreased. These results show significantly more scatter than the tensile 

cracking results. Of most importance, these results show that the postcracking load-carrying 

8.4

8.1

8.9

6.3

6.6

7.7

10.1

10.0

9.9

6.7

6.8

7.4

0 3 6 9 12

Steam (28)

Steam (56)

Steam (84)

Untreated (28)

Untreated (56)

Untreated (84)

Tempered Steam (28)

Tempered Steam (56)

Tempered Steam (84)

Delayed Steam (28)

Delayed Steam (56)

Delayed Steam (84)

Tensile Cracking Strength (MPa)
87

capacity is very similar to the precracking capacity. The fiber-reinforced nature of the UHPC 

matrix allows the fibers bridging a crack to carry a similar load level after cracking as the 

cement-based matrix did before cracking. 

3.4.3.2 Toughness 

The toughness exhibited by UHPC after cracking is of utmost importance if the UHPC will carry 

tensile forces after cracking. The UHPC needs to maintain load-carrying capacity as multiple 

cracking occurs and as individual cracks widen. As discussed in section 3.4.3.1, one means of 

measuring toughness is to calculate the area under the load-displacement curve. Frequently, the 

area under the curve after cracking is compared with the precracking area. Using this means of 

comparison, normal concrete would have a toughness of nearly zero with no postcracking loadcarrying capacity. On the other hand, an elastic-plastic material such as mild steel would exhibit 

a high post-yield toughness result. 

1 MPa = 145 psi 

Figure 79. Chart. Postcracking peak strength of UHPC briquettes. 

To compare toughness results from one test program to another, the means of measuring the 

displacement must accurately measure the actual displacement of the concrete in question. Given 

the nature of the AASHTO T132 test, measuring only the displacement of the UHPC briquette is 

not possible. Thus, the results presented here include the overall displacement of the loading 

system. A quantitative comparison of the areas contained under the load-displacement curves 

9.3

8.1

9.4

6.9

5.9

7.9

9.3

9.4

9.5

5.6

6.7

5.7

0 3 6 9 12

Steam (28)

Steam (56)

Steam (84)

Untreated (28)

Untreated (56)

Untreated (84)

Tempered Steam (28)

Tempered Steam (56)

Tempered Steam (84)

Delayed Steam (28)

Delayed Steam (56)

Delayed Steam (84)

Postcracking Peak Strength (MPa)
88

from this test program to other test programs is not possible. However, a quantitative comparison 

within this test program and qualitative external comparisons are definitely warranted. 

Figure 80 presents the average calculated areas under the load-displacement curves after 

cracking for the 12 groups of briquettes tested. The area calculation includes the entire area 

under the curve from cracking until the load level dropped to 890 N (200 lb), which is equivalent 

to a stress of 1.38 MPa (200 psi) on the original minimum cross section. These results show a 

much larger scatter, but the average results are relatively consistent within each curing regime. 

The larger area results in the steam-treated regime as compared with the tempered steam-treated 

regime are quite instructive. Recall that the tempered steam briquettes in figures 72 through 74 

show higher tensile strengths but that the shape of the postcracking portion of the curve exhibits 

a more rapid decrease in load capacity. The steam-treated regime clearly exhibits the best 

postcracking behavior, with almost twice the area as compared with the untreated briquettes. The 

tempered steam and delayed steam-treated regimes both show approximately 75 percent of the 

postcrack area as compared with the steam-treated regime. 

1 MPa = 145 psi 

1 N-m = 0.74 kip-ft 

Figure 80. Chart. Area under the load-displacement response curve after cracking. 

For completeness, figure 81 presents the results for the ratio of the postcracking area to the 

precracking area under the load-deflection curve. As previously mentioned, this ratio is the 

standard means of measuring the toughness of a semibrittle material like fiber-reinforced 

concrete. The area under the curve before cracking is based on the linear elastic portion of the 

load-displacement response, thus eliminating the seating behaviors that occur early in each test. 

11.9

12.7

11.5

6.1

6.0

6.3

9.8

8.4

8.8

8.5

11.0

8.2

0 5 10 15

Steam (28)

Steam (56)

Steam (84)

Untreated (28)

Untreated (56)

Untreated (84)

Tempered Steam (28)

Tempered Steam (56)

Tempered Steam (84)

Delayed Steam (28)

Delayed Steam (56)

Delayed Steam (84)

Postcrack Area (N-m)
89

These results seem to indicate that the tempered steam briquettes displayed the poorest toughness 

results while the delayed steam- and steam-treated briquettes had the best results. However, these 

results are an artifact of the higher cracking strength exhibited by the tempered steam briquettes 

and not by their low postcracking toughness. 

1 MPa = 145 psi 

Figure 81. Chart. Ratio of postcracking to precracking areas 

under the load-displacement curve. 

The postcracking behavior of UHPC is dependent primarily on the steel fibers and their 

attachment to the UHPC matrix. Clearly, a reduction in the number of fibers in the UHPC mix 

eventually must lead to a decrease in the sustained tensile load capacity after cracking. The 

curing treatment applied to the UHPC could likely have affected the bond between the fibers and 

the matrix. 

Considering these two points, the failed specimens from the briquette tension tests were 

evaluated. A significant portion of the briquettes in 9 of the 12 briquette test groups were studied 

to determine the number of fibers that intersected the failure plane. The fibers extending from 

each of the failure surfaces in each briquette were counted. The totals are shown in table 21. 

These results are then compared with the postcrack area under the load-displacement curve to 

determine what portion of the area could be attributed to each fiber. Obviously, this calculation 

glosses over many qualifying factors that would cause different fibers to carry different loads 

(i.e., fiber orientation relative to crack, fibers extension on either side of crack). However, this 

calculation is instructive in providing some results related to the effect of curing and fiber 

percentages. 

3.77

4.36

3.40

3.54

3.13

2.41

2.08

1.97

2.18

4.33

5.53

3.60

0.0 2.0 4.0 6.0 8.0

Steam (28)

Steam (56)

Steam (84)

Untreated (28)

Untreated (56)

Untreated (84)

Tempered Steam (28)

Tempered Steam (56)

Tempered Steam (84)

Delayed Steam (28)

Delayed Steam (56)

Delayed Steam (84)

Ratio of Areas Under Postcracking and Precracking

Load-Displacement Curves
90

Table 21. Fiber influence on postcracking behavior. 

Fibers Crossing 

Failure Plane 

Postcrack Area per 

Fiber (N-m/fiber) 

Group No. 

Samples

Average Standard 

Deviation 

Average Standard 

Deviation

Steam (56) 6 337 47 0.038 0.015 

Steam (84) 5 276 46 0.040 0.011 

Untreated (28) 5 370 45 0.017 0.003 

Untreated (56) 6 356 42 0.017 0.005 

Untreated (84) 6 329 77 0.019 0.003 

Tempered Steam (56) 3 384 30 0.024 0.003 

Tempered Steam (84) 6 321 35 0.028 0.009 

Delayed Steam (56) 6 179 63 0.064 0.018 

Delayed Steam (84) 6 149 19 0.054 0.014 

1 N-m = 0.74 kip-ft 

Aside from the delayed steam regime, these results indicate that the curing treatment seems to 

affect the ability of fibers to bond to the UHPC matrix. All of these groups contained similar 

numbers of fibers crossing the failure plane, but show three distinct levels of postcrack area per 

fiber. The steam-treated regime exhibits more than twice the toughness as the untreated regime 

on a per-fiber basis. Additionally, the delayed steam treatment results show that these briquettes 

had distinctly fewer fibers crossing the failure plane. Although this result probably could not 

explain the lower precracking tensile strength of these specimens, it definitely could explain the 

lower-than-expected postcracking peak strength. 

3.4.4 Direct Tension 

The fourth tension test used to quantify the material behaviors of UHPC was a direct-tension test. 

Unlike the flexural or split-cylinder tests, a direct-tension test applies a uniaxial tensile stress 

onto the concrete and monitors tensile behaviors as tensile strains are increased. In this test 

program, 102-mm (4-inch) diameter concrete cylinders were subjected to a uniaxial tensile load 

applied through the flat-end surfaces of each cylinder. 

Direct-tension tests are rarely performed on concrete due to the difficulties inherent in this test 

method. Among other things, direct-tension tests generally require complicated test setups 

because of the tensile gripping of the concrete and the custom equipment required for the loading 

and data collection. However, many researchers have investigated this topic and have attempted 

to develop reliable and repeatable means of testing the tensile stress-strain properties of concrete. 

(See references 30 through 45.) The direct-tension test setup and procedures used in this research 

program are an agglomeration of these testing techniques and rely most heavily on USBR 4914 

Procedure for Direct Tensile Strength, Static Modulus of Elasticity, and Poisson’s Ratio of 

Cylindrical Concrete Specimens in Tension(42) and RILEM TC 162-TDF Uniaxial Tension Test 

for Steel Fibre Reinforced Concrete.(39) 

Concrete cylinders that had a 102-mm (4-inch) diameter and a 203-mm (8-inch) length were cast 

for the direct-tension tests. These cylinders were cast following the normal casting and curing 
91

procedures detailed in section 3.2. Six cylinders were prepared for each curing regime. Note that 

no special attention was given toward achieving the proper distribution and orientation of the 

fiber reinforcement within the cylinder, thus it is expected that the fibers were not necessarily 

randomly distributed and oriented within the cylinder. This topic will be discussed in further 

detail later in this section. The direct tension tests were completed between 2 and 4 months after 

casting. Finally, these cylinders were cast from the same four batches of UHPC as the mortar 

briquettes discussed in section 3.4.3. To reiterate the discussion presented in this section, three of 

these four batches of cylinders exhibited somewhat low compressive strengths, which indicates 

that the tensile behaviors could be diminished as well. 

Two loading configurations were used in this test program, each for half of the cylinders from 

each curing regime. In the first configuration, the cylinders were tested as cast. In the second 

configuration, the cylinders were circumferentially notched to create a plane of reduced crosssectional area and thus higher stresses per applied load. The notch was created by milling a 

parabolic-shaped groove into the surface of the cylinder. This groove was 6.4 mm (0.25 inch) 

deep, thus resulting in a reduced cylinder diameter of 89 mm (3.5 inches). The total height of the 

groove was 12.7 mm (0.5 inch). Figure 82(a) shows a notched cylinder, while figure 82(b) shows 

an unnotched cylinder ready for testing. 

These tests were completed in a 445-kN (100-kip) capacity MTS testing machine. The axial 

displacement rate of a portion of the cylinder was used as the control signal. This displacement 

rate was captured through three LVDTs mounted on a testing apparatus similar to the one that 

was discussed in section 3.3.2 and shown in figure 82(b). The signal from these LVDTs was 

electronically averaged and then was sent to the MTS controller. The unnotched cylinders used a 

102-mm (4-inch) gage length centered on the specimen, and the notched cylinders used a 38-mm 

(1.5-inch) gage length centered over the notch. All cylinders were tested with a displacement rate 

of approximately 0.005 mm/min (0.0002 inch/min), which is the rate recommended in the 

RILEM test procedure. 

Before testing, the cylinders had their ends ground and checked for planeness. The testing 

machine heads and the ends of the specimen were then lightly sandblasted. A high-strength, 

high-modulus, rapid-setting epoxy was then applied to the testing machine heads as the cylinder 

was placed in the machine. A compressive load of approximately 22 kN (5 kip) was then applied 

to the specimen until the epoxy had cured. Note that the epoxy required at least 8 hours of cure 

time; thus, testing was completed on a daily cycle. 

The test setup and procedures discussed above were implemented in order to capture the full 

tensile stress-strain behavior of UHPC. Unfortunately, no behaviors after initial tensile cracking 

could be reliably observed and monitored. The MTS controller temporarily lost control of the 

cylinder during the brittle initiation of the first tensile crack. When control was regained, the 

cylinder had undergone some tensile fiber pullout across the crack, and the critical portion of the 

UHPC tensile behavior just after tensile cracking had been bypassed. This uncontrolled behavior 

was probably caused by a lack of sensitivity of the MTS control system due to its larger-than 

necessary load capacity. However, accurate postcracking results could probably not have been 

obtained anyway, because failure surfaces have indicated that the fiber distribution and 

orientation within the cylinders was not sufficiently random. 
92

Figure 82. Photos. (a) Notched cylinder and (b) testing of an unnotched cylinder. 

For these reasons, the test results presented in this section focus only on the tensile cracking 

strength and on the modulus of elasticity of the UHPC. Table 22 provides the results from the six 

cylinders in each curing regime. The table indicates which specimens were notched, the location 

of the first tensile crack, the tensile cracking strength, and the modulus of elasticity. The modulus 

of elasticity was determined based on the displacement readings across the 102-mm (4-inch) 

gage length between the applied stresses of 2.5 MPa (360 psi) compressive and 2.5 MPa 

(360 psi) tensile. Recall that the cylinders were initially compressed as the epoxy cured; thus, 

continuous data were recorded during the test over this transition from compressive to tensile 

behavior. 

The table indicates that 11 of the 12 notched specimens failed within the notch. Aside from 

possible local stress concentrations caused by the notch, it is anticipated that these results 

accurately represent the tensile cracking strength of the UHPC. For the purposes of this research, 

local stress concentration effects are ignored, and these tensile cracking strengths are viewed to 

be accurate representations of pure tensile behavior. Only half of the unnotched cylinders failed 

remotely from the bearings; therefore, the unnotched specimen results are more limited. It is 

expected that the specimens that failed within 25.4 mm (1 inch) of the bearing had their strengths 

reduced by local bearing effects. 

(a) (b) 
93

Table 22. Direct tension test results. 

 Specimen Notched Cracking Location Cracking 

Strength 

(MPa) 

Modulus of 

Elasticity 

(GPa) 

Steam 

 M1D00 Yes Notch 10.8 N/A 

 M1D01 Yes Notch 10.3 N/A 

 M1D02 Yes Notch 12.1 N/A 

 M1D10 No Within 25 mm of bearing 10.4 †

 51.9 

 M1D11 No Center of specimen 9.9 50.5 

 M1D12 No Within 25 mm of bearing 9.4 †

 53.3 

Untreated 

 M2D00 Yes Notch 5.7 N/A 

 M2D01 Yes Notch 5.9 ‡

 N/A 

 M2D02 Yes Notch 6.5 ‡

 N/A 

 M2D10 No Within 25 mm of bearing 6.3 †

 46.4 

 M2D11 No Within 25 mm of bearing 6.6 †

 47.9 

 M2D12 No Within 25 mm of bearing 7.0 †

 47.9 

Tempered Steam 

 M3D00 Yes Notch 7.2 N/A 

 M3D01 Yes Notch 8.9 N/A 

 M3D02 Yes Notch 7.5 N/A 

 M3D10 No Center of specimen 7.8 52.1 

 M3D11 No Center of specimen 9.4 52.9 

 M3D12 No Center of specimen 9.6 51.2 

Delayed Steam 

 M4D00 Yes Unnotched section 8.2 *

 N/A 

 M4D01 Yes Notch 11.5 N/A 

 M4D02 Yes Notch 10.9 N/A 

 M4D10 No Within 25 mm of bearing 3.5 †

 50.9 

 M4D11 No Center of specimen 9.1 †

 53.9 

 M4D12 No Center of specimen 10.4 51.4 

†

 Cracking strength results may have been influenced by crack location. 

‡

 Minimum cracking strength value due to difficulty with test apparatus leading to 

 poor data resolution. 

*

 Cracking did not occur in the notch that had a peak tensile stress of 10.7 MPa.

1 MPa = 145 psi, 1 GPa = 145,038 psi, 1 mm = 0.039 inch 

The limited total data set available from the direct tension tests precludes the presentation of 

definitive conclusions. However, the results do provide a range in which tensile cracking of 

UHPC can be anticipated. For steam-treated UHPC, tensile cracking likely occurs between 

9.7 and 11.0 MPa (1.4 and 1.6 ksi). For untreated UHPC, the range is from 5.5 to 6.9 MPa 
94

(0.8 to 1.0 ksi). For tempered steam specimens, the range was from 7.6 to 9.0 MPa (1.1 to 

1.3 ksi). For delayed steam specimens, the range was from 9.0 to 11.0 MPa (1.3 to 1.6 ksi). 

The modulus of elasticity results are not affected by cracking or other localized behaviors. For 

this reason, three results are available for each curing regime. The steam-treated UHPC modulus 

of elasticity averaged 51.7 GPa (7,500 ksi). The untreated UHPC averaged 47.6 GPa (6,900 ksi). 

The tempered steam and the delayed steam-treated UHPC specimens both averaged 52.1 GPa 

(7,550 ksi). Aside from the steam-treated results, these results are somewhat higher than those 

presented in section 3.3 that were obtained from purely compressive testing of cylinders. 

Finally, the linearity of the tensile stress-strain response from minimal compressive load through 

tensile cracking was studied. The same stress-strain responses recorded from the testing of the 

unnotched cylinders that were used for the modulus of elasticity determination were also used to 

determine the nonlinearity of the response as cracking was approached. Overall, the results 

indicate that UHPC, regardless of the curing regime, exhibits very linear behavior up through 

tensile cracking. The theoretical linear-elastic stress-strain response based on the calculated 

modulus of elasticity was used to determine the actual stress deviation (reduction) at any level of 

tensile strain. The decrease from the linear-elastic predicted level was less than 0.14 MPa 

(20 psi) for 10 of the cylinders and was less than 0.21 MPa (30 psi) for the remaining 2 

specimens. 

3.5 FRACTURE TESTING 

Understanding the cracking processes that occur when UHPC is stressed beyond its tensile 

strength is of great importance. Experience with UHPC structures has indicated that individual 

cracks tend to grow rapidly to a significant length, but that the width of any such crack is very 

small. However, this experience is based on structural testing of UHPC members with loading 

configurations that were not intended to create and to propagate stable cracks. 

A series of tests were completed on UHPC prisms to determine some of the basic behaviors of 

individual cracks. These tests focused on loading a prenotched prism in 3-point bending, with the 

load being incremented based on the opening at the mouth of the crack. The tests were loosely 

based on a portion of ASTM E1820 Standard Test Method for Measurement of Fracture 

Toughness.(46)

The prisms and test setup conformed to the ASTM test method for the single-edge beam test 

discussed in Annex A1 of the specification. The prism had a height of 102 mm (4 inches), a 

width of 51 mm (2 inches), and a span of 406 mm (16 inches). The only deviation from the 

ASTM test method was in the size and shape of the notch cut into the prism to act as the crack 

initiation point. In these tests, the crack was cut into the tension flange to a distance of 

approximately 25 mm (1 inch). The crack was cut with a 4.75-mm (3/16-inch) thick diamondtipped saw blade. The crack’s tip had a rounded profile and was not sharpened through fatigue 

cycling or by any other means. 

Four prisms were cast for both the steam-treated and untreated curing regimes. These prisms 

were part of the M1P and M2P batches discussed in section 3.1. The casting of these prisms 

followed normal procedures except that they were cast upside-down from the orientation in 
95

which they would be tested. These prisms were cast in special steel molds that allowed for partial 

enclosure of the top of the prism at the ends. Therefore, the only nonmolded surface on the 

finished prism was the center of the bottom or tension flange in the finished specimen. 

These tests were completed in a 98-kN (22-kip) capacity, MTS-controlled load frame. The tests 

were controlled based on the crack mouth opening displacement (CMOD). The CMOD was 

measured via a 12.7-mm (0.5-inch) full-range-capacity clip gage that was attached to knife edges 

epoxied to the bottom flange on either side of the starter notch. The entire test-loading protocol 

was preprogrammed into the MTS Microprofiler. The testing included an initial CMOD rate of 

approximately 0.013 mm/min (0.0005 inch/min), as well as a postcracking rate 2.5 and 5.0 times 

larger. The protocol included periodic unloadings set to occur at predefined CMOD values. 

Unfortunately, for unknown reasons these unloadings frequently did not occur as planned. Tests 

were stopped after 2.5 mm (0.1 inch) of CMOD. Figure 83 shows one of the prisms undergoing 

this fracture test. 

Figure 83. Photo. Test setup for 102- by 51-mm (4- by 2-inch) notched prisms 

loaded on a 406-mm (16-inch) span. 

Two methods were used to monitor the crack extension during the test. The primary method 

involved using an optical microscope to view the fracture process zone just ahead of the tip of 

the crack. This microscope had a field of view of 2.5 mm (0.1 inch), which allowed the crack tip 

to be located with relative ease. As the test progressed, alcohol was used as a volatile penetrant 

to help indicate the extent of cracking. The crack extension measurements were all taken from 

the bottom of the prism. 

The second method used to monitor crack extension involved a crack propagation resistance 

gage. Figure 84 shows one of these gages during a test as a crack is traversing the grid. The 

resistance across the gage changes as more of the lines are broken due to crack extension. This 

method of crack monitoring was used on two prisms from each of the steam-treated and 

untreated curing regimes. 
96

Figure 84. Photo. Resistance foil gage to monitor crack propagation. 

Figures 85 and 86 show photographs of crack propagation on two prisms. Alcohol was used to 

help indicate the locations of the cracks in both figures. Some important findings observed 

throughout these tests are illustrated in the figures. First, a single crack formed initially in each 

prism because the starter notch had sufficient depth. The crack then extended up through the 

prism as shown in figure 85(a). However, the fibers binding the initial crack together soon 

allowed for a redistribution of stresses resulting in the formation of other cracks that ran parallel 

to the first crack. Second, the cracks tended to extend very close to the compression flange. 

Figure 85(b) shows a crack extending to within 3.8 mm (0.15 inch) of the compression flange. 

The load and crack extension results in terms of the CMOD for each specimen are presented in 

figures 87 through 94. In each figure, (a) shows the overall response of the prism with the load 

versus CMOD behavior plotted with regard to the left y-axis and the crack extension versus 

CMOD behavior plotted with regard to the right y-axis. In each figure, (b) shows the initial 

behavior and first cracking of the UHPC prisms. Additionally, this plot shows the geometric 

construction used to determine when first cracking occurred. 

The ASTM E1820 specification defines cracking for materials that exhibit both stable and 

unstable first cracking. The data collected from these eight prisms indicate stable cracking 

behavior occurred with no clear evidence of a pop-in in any specimen. Thus, first cracking is 

defined by creating a linear best-fit line to the initial elastic portion of the load-CMOD response, 

then decreasing the slope of this line by 5 percent. The intersection of the new line with the 

original data is defined as the load and CMOD at first cracking. 

Part (b) of the referenced figures shows the required construction and the values at first cracking. 

Note that the initial load-CMOD response between 1.3 and 2.6 kN (300 and 600 lb) was used to 

define the elastic portion of the behavior, because it was beyond any initial seating-based 

nonlinearities and was before the initiation of cracking-based nonlinearities. 

The original intent of these tests had been to determine the fracture toughness, KIc, of steamtreated and untreated UHPC according to ASTM E1820. Unfortunately, the calculations 
97

associated with this specification indicate that the test as performed did not meet the standards of 

a qualified and size-independent KIc test. 

Figure 85. Photos. Prism M1P00 after (a) 86 mm (3.6 inches) and 

(b) 98 mm (3.8 inches) of crack extension. 

Figure 86. Photo. Prism M2P03 after 93 mm (3.63 inches) of crack extension.

(a) (b) 
98

1 kN = 0.225 kip 

1 mm = 0.039 inch 

(a) Overall response including periodic unloadings. 

1 kN = 0.225 kip 

1 mm = 0.039 inch 

(b) Initial response including elastic stiffness and 95 percent of elastic stiffness curves. 

Figure 87. Graph. Load-CMOD response for steam-treated prism M1P00. 

0

2

4

6

8

10

12

14

0.0 0.5 1.0 1.5 2.0

Crack Mouth Opening Displacement (mm) 

Load (kN)

0

20

40

60

80

100

Crack Extension (mm)

Load - CMOD

Optical Crack Length

0

1

2

3

4

5

6

0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040

Crack Mouth Opening Displacement (mm) 

Load (kN)
99

1 kN = 0.225 kip 

1 mm = 0.039 inch 

(a) Overall response including periodic unloadings. 

1 kN = 0.225 kip 

1 mm = 0.039 inch 

(b) Initial response including elastic stiffness and 95 percent of elastic stiffness curves. 

Figure 88. Graph. Load-CMOD response for steam-treated prism M1P01. 

0

2

4

6

8

10

12

14

0.0 0.5 1.0 1.5 2.0

Crack Mouth Opening Displacement (mm) 

Load (kN)

0

20

40

60

80

100

Crack Extension (mm)

Load - CMOD

Optical Crack Length

Multiple Cracks

0

1

2

3

4

5

6

0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040

Crack Mouth Opening Displacement (mm) 

Load (kN)
100

1 kN = 0.225 kip 

1 mm = 0.039 inch 

(a) Overall response including periodic unloadings and crack length from tension flange. 

1 kN = 0.225 kip 

1 mm = 0.039 inch 

(b) Initial response including elastic stiffness and 95 percent of elastic stiffness curves. 

Figure 89. Graph. Load-CMOD response for steam-treated prism M1P02. 

0

2

4

6

8

10

12

14

0.0 0.5 1.0 1.5 2.0

Crack Mouth Opening Displacement (mm) 

Load (kN)

0

20

40

60

80

100

Crack Extension (mm)

Load - CMOD

Foil Gage Crack Length

Optical Crack Length

Multiple Cracks

0

1

2

3

4

5

6

0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040

Crack Mouth Opening Displacement (mm) 

Load (kN)
101

1 kN = 0.225 kip 

1 mm = 0.039 inch 

(a) Overall response including periodic unloadings and crack length from tension flange. 

1 kN = 0.225 kip 

1 mm = 0.039 inch 

(b) Initial response including elastic stiffness and 95 percent of elastic stiffness curves. 

Figure 90. Graph. Load-CMOD response for steam-treated prism M1P03. 

0

2

4

6

8

10

12

14

0.0 0.5 1.0 1.5 2.0

Crack Mouth Opening Displacement (mm) 

Load (kN)

0

20

40

60

80

100

Crack Extension (mm)

Load - CMOD

Optical Crack Length

Foil Gage Crack Length

0

1

2

3

4

5

6

0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040

Crack Mouth Opening Displacement (mm) 

Load (kN)
102

1 kN = 0.225 kip 

1 mm = 0.039 inch 

(a) Overall response including periodic unloadings. 

1 kN = 0.225 kip 

1 mm = 0.039 inch 

(b) Initial response including elastic stiffness and 95 percent of elastic stiffness curves. 

Figure 91. Graph. Load-CMOD response for untreated prism M2P00. 

0

2

4

6

8

10

12

14

0.0 0.5 1.0 1.5 2.0

Crack Mouth Opening Displacement (mm) 

Load (kN)

0

20

40

60

80

100

Crack Extension (mm)

Load - CMOD

Optical Crack Length

Multiple Cracks

0

1

2

3

4

5

6

0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040

Crack Mouth Opening Displacement (mm) 

Load (kN)
103

1 kN = 0.225 kip 

1 mm = 0.039 inch 

(a) Overall response including periodic unloadings. 

1 kN = 0.225 kip 

1 mm = 0.039 inch 

(b) Initial response including elastic stiffness and 95 percent of elastic stiffness curves. 

Figure 92. Graph. Load-CMOD response for untreated prism M2P01. 

0

2

4

6

8

10

12

14

0.0 0.5 1.0 1.5 2.0

Crack Mouth Opening Displacement (mm) 

Load (kN)

0

20

40

60

80

100

Crack Extension (mm)

Load - CMOD

Optical Crack Length

Multiple Cracks

0

1

2

3

4

5

6

0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040

Crack Mouth Opening Displacement (mm) 

Load (kN)
104

1 kN = 0.225 kip 

1 mm = 0.039 inch 

(a) Overall response including periodic unloadings and crack length from tension flange. 

1 kN = 0.225 kip 

1 mm = 0.039 inch 

(b) Initial response including elastic stiffness and 95 percent of elastic stiffness curves. 

Figure 93. Graph. Load-CMOD response for untreated prism M2P02. 

0

2

4

6

8

10

12

14

0.0 0.5 1.0 1.5 2.0

Crack Mouth Opening Displacement (mm)

Load (kN)

0

20

40

60

80

100

Crack Extension (mm)

Load - CMOD

Optical Crack Length

Foil Gage Crack Length

Multiple Cracks

0

1

2

3

4

5

6

0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040

Crack Mouth Opening Displacement (mm) 

Load (kN)
105

1 kN = 0.225 kip 

1 mm = 0.039 inch 

(a) Overall response including periodic unloadings and crack length from tension flange. 

1 kN = 0.225 kip 

1 mm = 0.039 inch 

(b) Initial response including elastic stiffness and 95 percent of elastic stiffness curves. 

Figure 94. Graph. Load-CMOD response for untreated prism M2P03. 

0

2

4

6

8

10

12

14

0.0 0.5 1.0 1.5 2.0

Crack Mouth Opening Displacement (mm) 

Load (kN)

0

20

40

60

80

100

Crack Extension (mm)

Load - CMOD

Foil Gage Crack Length

Optical Crack Length

Multiple Cracks

0

1

2

3

4

5

6

0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040

Crack Mouth Opening Displacement (mm) 

Load (kN)
106

3.6 PENETRATION RESISTANCE TESTING 

Penetration resistance testing was completed on early age UHPC to determine the times of initial 

and final setting. The testing was completed nominally in accordance with the AASHTO T197 

standard test method.(47) The primary difference between the T197 test procedure and the one 

undertaken was that the UHPC was used as cast and was not sieved to obtain a mortar sample. 

The lack of coarse aggregate and the ease with which a penetration probe could be inserted into 

fresh UHPC allowed for this change in test procedure. 

The T197 penetration resistance test is performed by forcing a flat-headed probe into a sample of 

fresh concrete. The surface area of the probe and the force required to insert it are used to 

determine the penetration resistance in psi. Initial setting is defined as when the concrete reaches 

a penetration resistance of 3.4 MPa (500 psi). Final setting is defined as when the concrete 

reaches a penetration resistance of 27.6 MPa (4,000 psi). The probe sizes range from 645 mm2

 to 

16 mm2

 (1 inch2 to 0.025 inch2). Preliminary testing indicated that the smallest probe could be 

used to provide consistent results in both the initial and final setting ranges. Additionally, the use 

of a single probe size eliminates one source of error within the results. For these reasons, the 

smallest probe was used exclusively in this study. 

The penetration testing was performed in conjunction with the compression stress-strain 

response testing described in section 3.3.2. The requirements of these tests along with the 

delayed set times inherent in UHPC made the collection of penetration resistance data 

troublesome. In general, UHPC tends to exhibit virtually no setting for at least 12 hours. 

Sometime thereafter, the concrete will begin to set and will then quickly reach both initial and 

final set. This timetable led to the collection of partial penetration resistance results from four 

different batches of concrete. 

Table 23 provides the penetration resistance results. The table indicates the batch from which the 

penetration resistance specimens came and the subsequent curing action which was applied to all 

other specimens in that batch. However, it must be reiterated that no curing actions were 

performed on any of the penetration resistance specimens. These results show that the initial set 

for this particular UHPC mix design, cast and maintained under laboratory conditions, occurred 

around 15 hours. The final set occurred a few hours later at approximately 17 hours after casting. 

Table 23. Penetration resistance results. 

Subsequent 

Curing Regime 

Batch 

Name 

Initial Set 

(hours) 

Final Set 

(hours) 

Steam N1A 15.25 18 to 20 

Untreated N2A less than 14.5 16 

Tempered Steam N3A less than 15 15.75 

Delayed Steam N4A between 9.5 and 17.5 between 9.5 and 17.5 
107

3.7 SHRINKAGE TESTING 

3.7.1 Long-Term Shrinkage Testing 

Long-term shrinkage testing of the unrestrained, hardened UHPC was completed according to 

ASTM C157.(48) Three prisms, 76 mm by 76 mm by 280 mm (3 inches by 3 inches by 

11 inches), were cast for each curing regime. Gage studs were cast into the end of each prism so 

that the length change could be measured according to ASTM C490.(49) After casting, the prisms 

were kept in a laboratory environment until demolding, which occurred at approximately 

22.5 hours. Table 24 shows the demolding time for each curing regime. 

Table 24. Long-term shrinkage. 

Curing Regime 

Premix Age at 

Casting 

(days) 

Demolding 

Time 

(hours) 

Ultimate 

Shrinkage 

(microstrain) 

Steam 105 22.5 766 

Untreated 55 22.0 555 

Tempered Steam 50 22.0 620 

Delayed Steam 47 23.0 657 

Each prism had its initial length reading recorded within 30 minutes after demolding. The time of 

this initial reading was set by the demolding time as shown in table 24 and was not precisely 

24 hours as recommend by the specification. The reading also did not capture some of the early 

age shrinkage of the UHPC, which will be discussed in the following section. All prisms were 

stored and measured in a temperature and humidity controlled room per the specification except 

for the duration of any steam-based treatment. 

Measurements of the changes in length were recorded on a daily, then weekly, then monthly 

basis for 1 year. A special emphasis was placed on recording a measurement both before and 

after any curing treatment was applied to any prism. Figure 95 shows the results of these tests up 

to 250 days after demolding. After that time, very little change was observed in the recorded 

values. 

Figure 95 also provides best-fit approximations of the shrinkage behavior. The approximating 

equation, shown in figure 96, is a modification of the equation recommended in ACI 209R-92, 

where it is recommended that A equal 35.(50) In the research, st is the shrinkage at a given time, t

is the time in days, sult is the ultimate shrinkage that the concrete will undergo, and A is a variable 

defining the shape of the curve. 

Figure 95 shows that the prisms that underwent steam or delayed steam treatment exhibited no 

discernible post-treatment shrinkage. This result corroborates Acker’s shrinkage behavior theory 

presented in chapter 2.(12) In comparison, the untreated and tempered steam-treated prisms show 

continued shrinkage past 4 months of age. Even so, all the prisms reached at least 95 percent of 

their ultimate shrinkage by 2 months after demolding. For comparison, the ACI 209R-92 
108

recommended equation indicated that a normal concrete might have reached 60 percent of its 

ultimate shrinkage at this age. 

Figure 95. Graph. Long-term shrinkage results. 

Figure 96. Equation. Shrinkage as a function of time after casting. 

3.7.2 Early Age Shrinkage Testing 

The results presented in section 3.7.1 show that this concrete can exhibit large, unrestrained 

shrinkage strains regardless of the curing applied. More importantly, the shrinkage exhibited 

tends to occur much earlier in the overall behavior than would normally be expected. For these 

reasons, the ASTM C157 specification and its requirement that the concrete needs to have set 

prior to the recording of the initial measurement seems ill-suited for determining the full, 

unrestrained shrinkage behavior of this concrete. 

To quantify this early age shrinkage behavior, a test was devised to measure the unrestrained 

shrinkage of a prism starting at casting. An embedded strain measuring device (Geokon 4202 

Vibrating Wire Strain Gage) was cast into the center of a prism. This type of gage, shown in 

figure 97, measures strain by monitoring the resonant frequency of a wire tensioned between two 

end blocks that are embedded in the concrete. Aside from the gage embedment, the mixing, 

casting, and demolding procedures were the same as the ones followed for the long-term 

shrinkage prisms discussed above. Vibrating wire gages were embedded into one prism that was 

receiving the steam treatment and into another prism that was subjected to the untreated curing 

regime. The gages monitored the shrinkage for the first 17 days after casting. After demolding, 

standard ASTM C490 length change measurements were also recorded for verification purposes. 

t ult s

A t

t

s

+ =

0

200

400

600

800

0 50 100 150 200 250

Elapsed Days After Demolding

Shrinkage (microstrain)

Delayed Steam

Tempered Steam

Untreated 

Steam
109

Figure 97. Photo. Embeddable vibrating wire gage. 

Figure 98 shows the unrestrained shrinkage results from these embedded strain gages. The 

premix for this particular batch was over 150 days old, thus the initial setting was slower and the 

prisms were demolded at 28 hours. The steam-treated prism was steamed from hours 29 to 77 as 

shown in the figure, while the untreated prism was maintained in a laboratory environment. 

Figure 98. Graph. Early age shrinkage. 

The figure shows that this concrete exhibits rapidly occurring, large-value, early age shrinkage. 

The steam-treated prism reached a total of 850 microstrain, and the untreated prism was 

continuing to exhibit some shrinkage beyond the 790 microstrain recorded at 40 days after 

casting. Of greater interest is the rate at which the shrinkage occurs. Table 25 provides shrinkage 

rates in microstrain per hour that were obtained at discrete times during the early age of these 

two prisms. In particular, note that the shrinkage rate is over 60 microstrain per hour at 

1.18 days, which is just after the prisms are demolded. This high rate of shrinkage soon drops 

off, but in total over 400 microstrain of shrinkage occurred in the untreated prism in the 24 hours 

following the 20-hour mark when shrinkage started to occur. For reference, concrete usually has 

a tensile cracking strain of between 150 and 250 microstrain. 

0

200

400

600

800

1000

0 2 4 6 8 10 12 14 16

Elapsed Days After Casting

Shrinkage (microstrain)

Steam

Untreated
110

3.8 CREEP TESTING 

3.8.1 Long-Term Creep Testing 

The long-term compressive creep testing of the UHPC was conducted according to ASTM 

C512.(51) Four cylinders, 102 mm (4 inches) in diameter by 204 mm (8 inches) long, were cast 

for each curing regime for these tests. Two additional half-height cylinders were cast for each 

curing regime to act as loading blocks. All cylinders had their ends ground to within 0.5 degree 

of parallel. The testing was completed using hydraulically actuated load frames in a room that 

was controlled for temperature and humidity. No attempt was made to retard moisture gain or 

loss from the cylinders either prior to or during the test. 

Table 25. Early age shrinkage rate. 

Elapsed Time 

Since Casting 

(days) 

Steam Treated 

Shrinkage Rate 

(microstrain per hour) 

Untreated 

Shrinkage Rate 

(microstrain per 

hour) 

0.0 0.0 0.0 

0.8 0.0 0.0 

1.0 20 20 

1.1 36 36 

1.2 64 64 

1.3 –*

 18 

1.5 34*

 3.5 

1.7 11*

 3.5 

2.0 6.5*

 2.9 

2.5 2.8*

 2.4 

3.0 1.2*

 0.8 

3.5 0.0 0.7 

4.5 0.2 1.9 

6.0 0.1 1.3 

8.0 0.0 0.8 

10.0 0.0 0.5 

* Prism was undergoing Steam treatment 

The four cylinders for each curing regime were instrumented with Whittemore points before 

being stacked into a load frame. The Whittemore points were attached to each cylinder at three 

locations around the circumference with a 152-mm (6-inch) nominal gage length. Measurement 

of the points was completed before loading, immediately after loading, and periodically for 1 

year. Figure 99 shows creep cylinders in the load frame as well as the method used to measure 

the Whittemore points. 

The creep testing was initiated for each curing regime after the curing treatment was applied. For 

the steam and tempered steam cylinders, the creep loading was initiated 4 days after casting. 

Loading was initiated 21 days after casting for the delayed steam cylinders and 28 days after 
111

casting for the untreated cylinders. The load level applied to each set of cylinders was 77 MPa 

(11.2 ksi), which is 40 percent of 193 MPa (28 ksi), the anticipated compressive strength of a 

steam-treated cylinder. 

Figure 99. Photos. (a) Creep cylinders in load frame and (b) measurement of creep. 

Table 26 provides the creep testing results for the four curing regimes. The 28-day average 

compressive strength is also shown. Note that in all cases the load applied was in excess of the 

ASTM recommended load. The initial elastic strain was mechanically measured on the cylinders. 

The final creep strain is the estimated long-term value that the strain in the cylinders 

asymptotically approached. Intermediate creep strain values were calculated by subtracting the 

long-term shrinkage and the initial elastic strain from the overall measured strain. A best-fit 

approximation, discussed below, allowed for the determination of the final asymptotic value. 

Note that the subtraction of the long-term shrinkage from the recorded creep strain assumes that 

creep and shrinkage are uncoupled behaviors. However, research has indicated that these 

behaviors are dependent on one another in UHPC.(12) In this research, the shrinkage strain was 

subtracted from the creep strain for two reasons. First, some of the measured creep strain was 

necessarily due to shrinkage. Second, the overall shrinkage strains were small or nonexistent, 

thus the total error introduced was minimal. If the shrinkage strain were not subtracted from the 

creep strain for the untreated and tempered steam-treated specimens, the creep coefficients 

would have been less than 10 percent higher. 

(a) (b) 
112

Table 26. Long-term creep results. 

Curing Regime 

Control 

Strength 

(MPa) 

Stress / 

Strength

Initial 

Elastic 

Strain 

(με) 

Final 

Creep 

Strain 

(με) Ccu

δcu 

(με/MPa)

Steam 188 0.41 1500 440 0.29 5.7 

Untreated 114 0.67 2057 1600 0.78 21.2 

Tempered Steam 177 0.43 1670 1100 0.66 14.2 

Delayed Steam 168 0.46 1580 485 0.31 6.4 

1 MPa = 145 psi 

The creep coefficient, Ccu, is defined as the final asymptotic amount of additional creep strain 

that occurs over time divided by the initial elastic strain that occurs when load is first applied. 

The specific creep, δcu, is defined as the creep coefficient divided by the elastic modulus of the 

concrete. For reference, creep coefficients for concrete are normally in the range of 1.5 to 3.0, 

and specific creep values normally range from 35 to 145 microstrain per MPa. 

Recall the discussion in section 2.4.3 regarding the creep and shrinkage of UHPC. The results 

presented here correlate well with that discussion in that steam and delayed steam-treated UHPC 

exhibit extremely low creep coefficients, while the tempered steam-treated and untreated UHPC 

exhibits higher creep coefficients. Specifically, note that the delayed and tempered steam 

specimens had similar compressive strengths and load levels, but that the tempered steam 

exhibited more than twice as much creep strain. The more severe steam treatments should cause 

more rapid and more complete self-desiccation of the UHPC, thus leading to less creep of the 

concrete. 

A plot of the creep data acquired during the year of testing is shown in figure 100. The creep 

strain shown is the additional strain observed after the initial loading of the cylinders. The data 

presented are the average results from the four cylinders in each curing regime. The figure also 

includes best-fit approximations for each dataset. The best-fit curves are based on the equation 

shown in figure 101, wherein t is the time in days since load initiation, εct is the creep strain at 

that time, and A and B are variables that define the shape of the curve. This equation is loosely 

based on a creep equation suggested by Branson.(52)

3.8.2 Early Age High-Stress Creep Testing 

A test program was initiated to investigate the dimensional stability of UHPC that was subjected 

to compression loading early in its strength gain. This test program was designed to answer 

questions related to the delay required prior to stressing a prestressed UHPC girder. Although the 

rapid-strength gain of UHPC is beneficial with regard to the stressing of a girder, it is important 

to know what portion of that strength gain is actually useable. 

Short-term creep tests were conducted on UHPC cylinders that were of moderate compressive 

strength. Two strength levels were investigated: 59 and 86 MPa (8.5 and 12.5 ksi). Within one 

batch of UHPC, 102-mm (4-inch) diameter cylinders were cast for creep tests and 76-mm 
113

(3-inch) diameter cylinders were cast to determine the concurrent compressive strength of the 

UHPC. The specimen preparation procedures for the creep cylinders mimicked the procedures 

that were followed for the ASTM C39 compression tests that were previously discussed, 

including sulfur capping of the cylinder ends due to the relatively low compressive strengths. 

Figure 100. Graph. Long-term creep results. 

 

Figure 101. Equation. Creep as a function of time after loading. 

The creep testing was completed in a 445-kN (100-kip) capacity MTS testing machine. Each 

cylinder rested one end on a flat steel bearing and was loaded through the other end by a 

spherical bearing. Each cylinder was loaded at a computer-controlled constant load rate of 

1.0 MPa/s (150 psi/s) until the desired compressive load level was reached. This load level was 

then maintained for 30 minutes, after which the load was removed at 1.0 MPa/s (150 psi/s). The 

deformation of the cylinder was monitored throughout the test via three LVDTs that were 

mounted on two parallel rings attached to the cylinders. This deformation measurement system 

was the same as the one used in the elastic modulus testing (previously discussed in section 

3.3.2). A photograph of the test setup is provided in figure 102. 

The early age creep results are presented in table 27. The table provides the cylinder identifier, 

the compressive test at the time of creep testing based on concurrently tested 76-mm (3-inch) 

diameter cylinders, and the stress level maintained during the creep test. The stress levels tested 

ranged from 60 percent of the compressive strength to just over 90 percent. 

B

A t

t ct 0.6

0.6

+

ε =

0

200

400

600

800

1000

1200

1400

1600

0 100 200 300

Elapsed Days Under Load

Creep (microstrain)

Delayed Steam

Tempered Steam

Untreated

Steam
114

Figure 102. Photo. Short-term creep test setup. 

Table 27. Early age creep results. 

Cylinder 

Identifier 

Compressive 

Strength 

(MPa) 

Applied 

Stress 

(MPa) 

Stress / 

Strength Cc-30

55 to 65 MPa Compressive Strength 

 A1 65 39 0.60 0.42 

 A2 59 44 0.75 0.66 

 A3 64 54 0.84 0.79 

 A4 55 49 0.88 0.80 

83 to 90 MPa Compressive Strength 

 B1 86 52 0.60 0.32 

 B2 84 61 0.72 0.39 

 B3 84 65 0.77 0.44 

 B4 84 70 0.83 0.52 

 B5 88 75 0.85 0.85 

Failed Under Load 

 A5 66 59 0.91 N/A 

 B6 84 77 0.92 N/A 

1 MPa = 145 psi 

Table 27 lists the 30-minute creep coefficient, Cc-30, for each of the cylinders that was able to 

carry the load through the end of the test. This creep coefficient is similar to the standard creep 

coefficient, except that here it equals the amount of additional creep strain that occurred during 

the 30 minutes divided by the initial elastic strain when the constant load level was reached. The 

two cylinders loaded to stress levels above 90 percent of the compressive strength both failed, 

and thus no value of Cc-30 is listed. 
115

Figures 103 and 104 show plots of the results of these tests. The observed strain values have 

been normalized on the strain when the constant stress level is first reached. In this way, the 

effect of stress level on early age creep is clearly visible. At both strength levels, stressing to 

above 85 percent causes twice the creep strain to occur, and stressing above 90 percent caused 

failure. Even stressing to only 60 percent caused a significant amount of creep strain to 

accumulate in a short time. 

Figure 103. Graph. Early age creep behavior of 55 to 65 MPa (8.0 to 9.5 ksi) UHPC. 

Figure 104. Graph. Early age creep behavior of 86 MPa (12.5 ksi) UHPC. 

0.0

0.5

1.0

1.5

2.0

2.5

-5 0 5 10 15 20 25 30 35

Elapsed Time (minutes)

Total Strain / Initial Loaded Strain

Stress/Strength

0.85, 0.83, 0.77, 0.72, 0.60

Stress/Strength = 0.92

0.0

0.5

1.0

1.5

2.0

2.5

-5 0 5 10 15 20 25 30 35

Elapsed Time (minutes)

Total Strain / Initial Loaded Strain

Stress/Strength

0.88, 0.84, 0.75, 0.60
116

3.9 COEFFICIENT OF THERMAL EXPANSION 

The coefficient of thermal expansion of UHPC was measured through the use of the provisional 

AASHTO test specification, TP60-00.(53) The test method determines the coefficient of thermal 

expansion (CTE) by measuring the length change of a concrete cylinder in a variable temperature 

water bath. Three 102-mm (4-inch) diameter UHPC cylinders from each of the four curing 

regimes were tested. 

Normally, the degree of water saturation of a concrete will influence its CTE. To counteract this 

fact, the test method requires that the concrete needs to be in a saturated condition before testing. 

In the current testing of UHPC, the saturation of the cylinders before testing was considered to 

be detrimental to the final results for two reasons. First, one of the primary variables in the 

overall test program is the curing condition of the UHPC at the time of testing. Saturating the 

UHPC would change the condition of the UHPC by introducing more water into the matrix, 

which could then react with unhydrated cement. Second, because saturation of the UHPC is 

problematic due to its low permeability, determining when a specific degree of saturation was 

reached would be difficult. Thus, the exterior surfaces of the cylinders, aside from the bearing 

points of the supports and LVDT, were sealed with epoxy before testing. 

The testing of the 12 cylinders was completed after they had undergone their complete curing 

treatment. All the cylinders were at least 2 months old. The untreated cylinders were the oldest at 

4.5 months at the time of testing. 

The results from the CTE testing are shown in table 28. In general, the CTE of UHPC is around 

15 x 10-6 mm/mm/°Celsius (C) (8.3 x 10-6 inch/inch/°Fahrenheit (F)). This value is somewhat 

higher than the normally expected value for concrete of around 10 x 10-6 mm/mm/°C (5.6 x 10-6

inch/inch/°F). However, recall that UHPC contains both high cement content and no coarse 

aggregate. Hydrated Portland cement paste has been reported to have a CTE of between 11 x 10-6

and 16 x 10-6 mm/mm/°C (6.1 x 10-6 and 8.9 x 10-6 inch/inch/°F), and aggregates tend to exhibit 

lower CTE values.(54)

Table 28. Coefficient of thermal expansion results. 

Coefficient of Thermal Expansion 

Curing Regime 

Results 

(x10-6 mm/mm/°C) No.

Average 

(x10-6 mm/mm/°C) 

Standard 

Deviation 

(x10-6 mm/mm/°C) 

Steam 15.3, 15.7, 15.8 3 15.6 0.3 

Untreated 14.4, 14.6, 15.1 3 14.7 0.4 

Tempered Steam 15.0, 15.5, 15.8 3 15.4 0.4 

Delayed Steam 15.0, 15.2, 15.5 3 15.2 0.3 

1 mm/mm/°C = 1.8 inches/inches/°F 
117

3.10 HEAT OF HYDRATION

The amount of heat that UHPC generates during initial curing is of interest for two reasons: (1) it 

can be used as an indicator of other behaviors and (2) it could cause detrimental behaviors if not 

accounted for in the design. Time-temperature data were gathered for 15.24-cm (6-inch) 

diameter cylinders during the first few days after casting. This testing was completed on two 

UHPC mixes. The first mix was the standard UHPC mix that has been discussed throughout this 

report. The second mix included a slight modification of the mix design in that no accelerator 

was added. Specifically, 30.0 kg/m3

 (1.87 lb/ft3) of Rheocrete CNI was eliminated from the mix, 

and an extra 32.0 kg/m3

 (2.00 lb/ft3) of water was added. 

The time-temperature results were collected for cylinders in three environments. Figure 105 

shows the heat that was generated by cylinders in a normal laboratory environment. These results 

indicate that the accelerator can have a significant effect on set time, but that the overall level of 

heat generation is not significantly different. Figure 106 shows the heat that was generated by 

cylinders that were kept in a normal laboratory environment until setting began, after which the 

steam treatment was applied. Again, the accelerator’s primary impact seems to be in the rate of 

setting and not in the peak heat generated. Note that the steam treatment of the accelerated mix 

was started earlier in the setting behavior, thus impacting the comparative level of the peak 

temperatures recorded. 

°F = 1.8 (°C) + 32 

Figure 105. Graph. Heat generated in 152-mm (6-inch) diameter 

cylinders during initial curing. 

The difficulty with the previously discussed methods of measuring the heat generated by the 

UHPC is that local conditions within a laboratory environment can vary. Air currents that aid in 

the dissipation of heat can have a significant impact on final results. To address this issue, timetemperature data were also collected using a well-insulated calorimeter. For these tests, each 

152-mm (6-inch) diameter cylinder was placed in a Quadrel iQdrumTM heat signature calorimeter 

and monitored for 6 days. The results are shown in Figure 107. Through the use of the 

18

20

22

24

26

28

30

32

0 20 40 60

Elapsed Hours After Mixing

Temperature ( oC)

With Accelerator Without 

Accelerator

Surrounding Air Temperature
118

calorimeter, the effect of the accelerator becomes very clear. The accelerator causes a slow 

temperature rise to occur before setting and caused setting to begin over 1 day earlier. Heat was 

generated more quickly during setting, but the peak temperature reached by the cylinder was 

only 3 oC (5.4 oF) higher than in the unaccelerated mix. Note that these relative changes between 

mix designs are instructive; however, the actual time-temperature and setting behaviors of any 

UHPC mix will depend on many other factors as well (i.e., age of premix, precise environmental 

conditions, quantity of accelerator). 

°F = 1.8 (°C) + 32 

Figure 106. Graph. Heat generated in 152-mm (6-inch) diameter 

cylinders from casting through steaming. 

°F = 1.8 (°C) + 32 

Figure 107. Graph. Heat signature for 152-mm (6-inch) diameter 

cylinders in a well-insulated calorimeter. 

10

30

50

70

90

110

0 20 40 60 80 100 120 140

Elapsed Hours After Mixing

Temperature ( oC)

Without 

Accelerator

With 

Accelerator

10

20

30

40

50

60

70

80

0 24 48 72 96 120

Elapsed Hours After Casting

Temperature ( oC)

Without accelerator

With accelerator
119

3.11 AIR VOID ANALYSIS

An air void analysis was completed on the UHPC. The modified point-count method of ASTM 

C457 was used to determine the air void structure.(55) The variables investigated in this testing 

included the effect of the analysis location within individual cast cylinders, the effect of the time 

a cylinder was held on a vibrating table after casting, the effect of mix stiffness, and the effect of 

using an accelerator in the mix design. 

The casting procedure was as follows. The UHPC was mixed as usual, following the normal mix 

design and procedure, except that an unaccelerated mix (as described in section 3.2) was 

included. The UHPC was then cast into 152-mm (4-inch) diameter plastic cylinder molds. The 

filling of the molds differed from the usual procedure in that some of the molds were filled 

without using the vibrating table while others were filled on the vibrating table and were then 

held in place on the table for a specific length of time. After casting, the cylinders were screeded, 

covered in plastic, and left in a laboratory environment to set. 

The preparation and testing of the cylinders for the air void structure analysis was as follows. 

After setting, the cylinders were cut in half lengthwise. One of the two halves was then polished 

using up to 800-grit sandpaper. Each cylinder’s half was then divided into a top and bottom 

section on its flat face. Modified point-count measurements were then completed on each of 

these sections. Because this UHPC did not contain any coarse aggregate, the fibers were counted 

as coarse aggregate within the data collection procedure. In total, each air void data collection set 

covered an area of 6,770 mm2

 (10.5 inches2) and traversed a length of 1,270 mm (50 inches) 

while counting 1,429 points. 

The results from these tests are presented in table 29. The results are grouped into sets for each 

of the three mixes tested. For reference, the stiff mix has a flow table reading of 165 mm 

(6.5 inches) after 20 impacts, and the normal flow mixes both have readings of 190 mm 

(7.5 inches) after 20 impacts. Each pair of rows shown with a common vibrating table time is the 

top and bottom results from the same cylinder half. 

Although some slight differences were observed in various groupings of specimens, the limited 

sample set makes drawing specific conclusions difficult. In general, the air content across the 

range of mixes and specimen locations was approximately 6.5 percent. The bottom of the 

cylinders tended to have slightly more air, likely due to air in the top of the cylinders being 

removed by vibration. The cylinders that were held on the vibrating table for 300 seconds after 

filling also tended to have a decrease in air content. The stiffer mix tended to have slightly less 

air than the more flowable mix, and the results of the accelerated and unaccelerated mixes had no 

clear differences. 

3.12 STEEL FIBER DISPERSION TESTING 

Dispersion of fibers throughout UHPC is a necessary part of achieving many of its material 

properties. In this section, a simple set of tests is described in which the dispersion of fiber 

throughout UHPC cylinders was measured. This batch of UHPC, L03, exhibited a relatively fluid 

rheology with a flow table result after 20 impacts of 235 mm (9.25 inches). 
120

Table 29. Air void analysis results. 

1 mm = 0.039 inch, 1 mm2/mm3 = 25.4 inches2/inches3 

Vibration 

Table Time 

(seconds) Location 

Air 

(%) 

Paste 

(%) 

Fine 

Agg. 

(%) 

Fiber 

(%) 

Voids 

Counted 

Mean 

Chord 

Length 

(mm) 

Voids 

per 

mm 

Speci

Surfa

(mm2/m

Stiff Mix w/ Accelerator (M1J) 

 0 Top 5.7 47.2 43.3 3.8 249 0.29 0.20 13. 0 Bottom 6.2 51.2 38.8 3.8 267 0.29 0.21 13.7  Fill + 20 Top 5.5 36.4 55.1 3.0 274 0.26 0.22 15.6  Fill + 20 Bottom 6.9 30.7 59.5 3.0 257 0.34 0.20 11. Fill + 60 Top 6.7 43.2 47.0 3.0 289 0.29 0.23 13.5  Fill + 60 Bottom 7.4 42.4 47.2 3.0 287 0.33 0.22 12.2  Fill + 300 Top 6.5 31.5 59.8 2.2 233 0.36 0.19 11.3  Fill + 300 Bottom 4.7 25.1 66.3 3.9 221 0.27 0.17 14.

Normal Flow Mix w/ Accelerator (M1JxxA) 

 0 Top 6.2 43.7 48.0 2.1 282 0.28 0.22 14.3  0 Bottom 7.1 44.7 46.9 1.3 302 0.30 0.24 13.3  Fill + 20 Top 6.4 45.8 46.5 1.4 291 0.28 0.23 14.4  Fill + 20 Bottom 7.6 44.0 46.9 1.5 385 0.25 0.30 15.9  Fill + 60 Top 7.8 29.9 60.8 2.0 349 0.28 0.28 14. Fill + 60 Bottom 9.2 27.6 61.1 2.1 375 0.31 0.30 12.9  Fill + 300 Top 6.9 42.2 49.3 1.5 234 0.38 0.19 10.6  Fill + 300 Bottom 6.1 41.5 50.9 1.5 218 0.36 0.17 11.3 

Normal Flow Mix w/o Accelerator (M1K) 

 0 Top 6.3 49.0 41.1 3.6 281 0.28 0.22 14. 0 Bottom 7.3 48.1 41.9 2.7 251 0.37 0.20 10.9  Fill + 20 Top 6.7 43.0 47.9 2.4 244 0.35 0.19 11.5  Fill + 20 Bottom 4.8 39.2 51.8 4.2 207 0.30 0.16 13.5  Fill + 60 Top 7.5 49.5 39.7 3.3 233 0.41 0.19 9.8  Fill + 60 Bottom 7.2 49.6 39.7 3.4 285 0.32 0.22 12.4  Fill + 300 Top 3.2 44.0 50.5 2.2 107 0.38 0.08 10.5  Fill + 300 Bottom 4.6 42.8 50.0 2.6 151 0.39 0.12 10.3 
121

The 76-mm (3-inch) diameter cylinders for these tests were cast without the use of a vibrating 

table, and some of the cylinders were then impacted on an ASTM C230 flow table.(56) After the 

cylinders had gained sufficient strength, they were cut in half along their length. Four 645-mm2

(1-inch2

) areas were then marked on the flat face of the cylinders. The delineated areas were 

located in a line along the length of the cylinder at a 12.7-mm (0.5-inch) spacing. The fibers 

contained within each area were then counted. 

Five cylinders were tested according to this procedure. Each of these cylinders underwent a 

different number of impacts on the flow table, ranging from 0 to 100. The fiber counting results 

are presented in figure 108. The results seem to indicate that more fibers tend to be located 

toward the bottom of each cylinder. Whether this result is caused by the impacts administered on 

the flow table or is an artifact of the method used to pour the UHPC into the cylinder molds is 

not clear. 

1 mm2 = 0.002 inch2

Figure 108. Graph. Fiber dispersion analysis results for cylinders 

impacted on an ASTM C230 flow table. 

Of greater importance is the wide range of fiber concentrations evident throughout the five 

cylinders. Figure 109 shows the four areas measured on the cylinder that underwent 25 impacts. 

One green dot has been placed on the head of each individual fiber. Clearly, casting technique is 

very important if equal dispersion of fibers throughout the mix is desired. 

3.13 DURABILITY TESTING 

A series of durability tests were conducted to determine the resistance of UHPC to various 

environmental aggressors. These tests included measurement of the chloride ion penetrability, 

scaling and freeze-thaw resistance, abrasion resistance, and alkali-silica reaction susceptibility. 

The results are detailed throughout this section. 

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Bottom Lower Middle Upper Middle Top

Fibers per Square Millimeter

0 impacts 25 impacts 50 impacts

75 impacts 100 impacts
122

Figure 109. Photos. Fiber dispersion analysis photographs for a 645-mm2 (1-inch2) area in 

the (a) bottom, (b) lower middle, (c) upper middle, and (d) top of a cast cylinder. 

3.13.1 Rapid Chloride Ion Penetrability Testing 

The ability of concrete to resist ingress of chloride ions can result in a significantly more durable 

concrete. The ASTM C1202 test for the Electrical Indication of Concrete’s Ability to Resist 

Chloride Ion Penetration.(57) frequently referred to as the rapid chloride ion penetrability test, was 

conducted on specimens from the four UHPC curing regimes. This test approximates the 

resistance that a concrete may exhibit to chloride ion penetration by measuring the amount of 

electrical current that passes through a 51-mm (2-inch) thick slice of concrete over 6 hours. A 

60-volt direct current (DC) potential is applied across the slice of a 102-mm (4-inch) diameter 

cylinder, while a sodium chloride solution is applied to one side of the slice, and a sodium 

hydroxide solution is applied to the other side. 

Figure 110 shows a cylinder slice before being tested and while a test is in progress. The 

cylinders were cast and cured following normal procedures. For these tests, each cylinder was 

cast in a 102-mm (4-inch) diameter mold that was filled to 76 mm (3 inches). After both 

demolding and the application of any curing treatment, the cylinders were cut to length, and both 

ends of the cylinder were then ground to create a uniform finish. The outside circumference of 

each cylinder was sealed with epoxy. The standard amount of steel fiber reinforcement was 

included in all cylinders. The short, discontinuous nature of the steel fibers in the UHPC matrix 

allowed for this test to be completed without shortening the circuit or generating significant heat 

in the ponded fluids. 

Results from these tests are presented in table 30. Three tests were performed for each curing 

regime at 28 days after casting. An additional three tests were performed on the untreated and 

tempered steam-treated regimes at 56 days, because these regimes had exhibited slightly higher 

results during the first tests. 

Aside from the untreated tests from 28 days, all of the results are in the negligible range as 

defined by ASTM C1202. Also, the untreated and tempered steam-treated regimes exhibited 

significant reductions in charge passed between 28 and 56 days. Figure 111 shows averaged 

results for current versus time from three sets of cylinders. For reference, if 0.003 amps of 

current were passed through the concrete for the duration of the test, then the total charge passed 

would be 65 coulombs. 

(a) (b) (c) (d) 
123

Figure 110. Photos. (a) Cylinder and (b) setup for rapid chloride ion penetrability test. 

Table 30. Rapid chloride ion penetrability results. 

Curing Regime Age Coulombs Passed

(days)

No.

Average Standard 

Deviation

Chloride Ion 

Penetrability 

Steam 28 3 18 1 Negligible 

Untreated 28 2 360 2 Very Low 

Untreated 56 3 76 18 Negligible 

Tempered Steam 28 3 39 1 Negligible 

Tempered Steam 56 3 26 4 Negligible 

Delayed Steam 28 3 18 5 Negligible 

3.13.2 Chloride Penetration 

In conjunction with the chloride ion tests described in section 3.13.1, standard chloride ion 

penetration testing was conducted according to the AASHTO T259 specification.(58) This test, 

often referred to as the chloride ponding test, entails ponding a 3-percent sodium chloride 

solution on the surface of the concrete for 90 days. After 90 days, the level of migration of 

chloride ions into the concrete is determined. 

This test was conducted on three specimens from each of the curing regimes. The specimens 

were cast in 102-mm (4-inch) diameter steel cylinder molds that were filled at least 76 mm 

(3 inches) full during casting. After both demolding and the application of any curing treatment, 

the circumferential surface of each cylinder was coated in epoxy. A section of rubber hose was 

then slid over the cast face end of the cylinder and affixed with a hose clamp. Finally, 28 days 

after casting, the sodium chloride solution ponded on the cylinder face. Note that the solution 

was ponded on the steel mold cast face of the cylinder to replicate the UHPC surface that would 

have been achieved in a precast concrete production facility. 

(a) (b) 
124

Figure 111. Graph. Average current passed versus time results for three sets of cylinders. 

The AASHTO T260 standard test method was used to sample and test the concrete following the 

ponding.(59) Samples were taken from each cylinder at depths of 13, 25, 38, 51, 64, and 76 mm 

(0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 inches) from the ponded surface. The samples were obtained by 

using a drill press with a 6.3-mm (1⁄4-inch) masonry bit to drill into the side of the cylinder at the 

correct depth. Approximately 15 grams of the pulverized concrete were captured at each 

specified depth. The steel fiber shavings were then magnetically removed from each sample of 

the concrete powder. Thus, at least 10 grams of powder were available for the chloride ion 

testing. 

The chloride ion testing was conducted according to Procedure A, Section 5.2: Procedure for 

Total Chloride Ion Content. The chlorides were unlikely to penetrate any significant distance into 

the concrete because of the penetrability of this concrete. This allowed a large collection of 

background chloride content samples to be obtained at 64 and 76 mm (2.5 and 3.0 inches). The 

average background chloride ion content in kg/m3

 of concrete was determined to be 0.051 with a 

standard deviation of 0.004. 

Figure 112 presents the corrected results for these tests. The removal of the background chloride 

content from each sample caused some of the samples to seem to exhibit negative chloride ion 

contents. The AASHTO specification indicates that these results should be rounded up to zero. 

Overall, the chloride ion content in the UHPC for all of the curing regimes is extremely low. 

There is some trend toward higher chloride ion contents near the ponded surface; however, the 

amount of chlorides that migrated into the concrete over 90 days is still extremely small. 

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0 60 120 180 240 300 360

Minutes Elapsed

Current (Amps)

Untreated (28 days)

Untreated (56 days)

Steam (28 days)
125

1 kg/m3 = 1.686 lb/yd3

, 1 mm = 0.039 inch

Figure 112. Graph. Chloride ion content results after 90 days of ponding. 

An additional benefit of this set of tests is that the behavior of the surface of this steel fiberreinforced UHPC can be qualitatively observed. Figure 113 shows a tempered steam-treated 

cylinder just before the 90 days of ponding began and 1 day after it concluded. This specimen is 

representative of all 12 tests that were completed because minor corrosion of steel fibers on and 

very close to the surface occurred. This corrosion would probably more aptly be described as 

surface staining caused by exposed fibers. There was no indication that the corrosion of the 

fibers was progressing into the interior of the UHPC. 

Figure 113. Photos. (a) Cylinder before and (b) after 90 days of chloride ponding. 

3.13.3 Scaling Resistance 

The UHPC scaling resistance to deicing chemicals was evaluated through the use of the ASTM 

C672 standard test method.(60) In this test method, a solution of calcium chloride is ponded on the 

(a) (b) 

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0 20 40 60 80

Depth from Ponded Surface (mm)

Added Chloride Ion Content

(kg/m3 of Concrete)

 1 1 1 1 

 2 2 2 2 

 3 3 3 3 

Steam Untreated

Tempered Delayed

Steam Steam
126

surface of the concrete. The concrete specimen is then placed in a −18 oC (−0.4 oF) freezing 

environment for 18 hours followed by a 23 oC (73.4 oF) thawing environment for 6 hours. This 

cycle is repeated daily, while ensuring periodically that the surface of the specimen remains 

covered in the ponding solution. 

The ASTM C672 testing included two slabs from each curing regime. Each slab was cast in a 

356-mm by 356-mm (14-inch by 14-inch) square steel mold that was 76 mm (3 inches) deep. 

The cast surface on the bottom of the slab became the surface to be ponded after demolding. The 

curing treatments were applied to the slabs, and then the dams were epoxied to the ponding 

surface. The dams were composed of steam-treated UHPC. A rapid-setting, two-part epoxy was 

used as the bonding agent slab and the dam. Figure 114 shows one of the slabs before testing was 

initiated. The total exposed ponding area on the surface of each slab was approximately 0.065 m2

(100 inches2

). 

The testing of these slabs commenced in two phases. In the first phase, a walk-in freezer was 

used to generate the freezing condition, while the standard laboratory environment generated the 

thawing environment. The capabilities of this freezer were such that a consistent temperature 

could not be maintained. Over the course of 4 months, 70 cycles were completed. Up through 

cycle 50, the temperature in the freezer was usually below −9 oC (15.8 oF). For the next 20 

cycles, the temperature was usually in between −8 and −3 oC (17.6 and 26.6 oF). Due to the 

inability of this freezer to maintain a sufficiently cold temperature, this phase of the testing was 

halted. At the cessation of these tests, no scaling was observed on the surface of any of the slabs. 

Figure 114. Photo. Scaling slab before initiating ASTM C672 testing. 

The second phase of the ASTM C672 testing used an environmental chamber to carefully control 

the freezing and thawing portions of each cycle to within ±1.67 oC (±3 oF) of the prescribed 

values. A total of 145 additional cycles were completed on all eight slabs. Every few cycles, the 

slabs were checked for leaking of the dams and for the proper solution level on the ponded 

surface. The slabs were also drained, flushed, inspected, and refilled after cycles 20 and 50. No 
127

scaling was observed. After cycle 145, the slabs were drained through a #30 sieve to capture any 

small scaling particles that might have been present. Each slab produced less than 0.5 grams 

(0.02 ounces) of material, which included small bits of concrete, pieces of epoxy, and other small 

detritus that had made its way onto the slabs over the course of 95 cycles. 

A final inspection of the ponded surface indicated that no scaling had occurred, thus an ASTM 

C672 surface condition rating of 0 was warranted. No differences were observed between the 

slabs from any of the four curing regimes. The surfaces did tend to show a slightly rougher 

texture after testing, with many more of the small air bubbles visible just under the ponded 

surface. Figure 115 shows the same slab that was pictured in figure 114 after having undergone 

the 70-plus-145 cycles of scaling as described above. 

Figure 115. Photo. Scaling slab after ASTM C672 testing. 

Figure 116 shows a byproduct of the ASTM C672 scaling tests. Some of the dams on top of the 

slabs did not seal properly with the slab surface. At the joint, the calcium chloride solution 

slowly leaked out and ran down the side of the slab. This leak allowed for an extremely 

aggressive environment on the vertical surfaces of these slabs. Even after the 70-plus-145 cycles 

of freezing and thawing in this environment, these vertical surfaces only showed minor surface 

deterioration of the type shown in the figure. The corrosion of the fibers did not seem to 

penetrate into the slabs, and no scaling, spalling, or chipping of the concrete surface was 

apparent. 
128

Figure 116. Photo. Surface deterioration of a vertical surface after 70-plus-145 cycles of 

wetting/drying with a chloride solution in a freezing/thawing environment. 

3.13.4 Abrasion Resistance 

The abrasion resistance of UHPC was measured through the standard test method described in 

ASTM C944.(15) This test determines abrasion resistance by measuring the amount of concrete 

abraded off a surface by a rotating cutter in a given time period. The cutter consists of a series of 

dressing wheels mounted on a rod that is attached to a drill press. Figure 117 shows a picture of 

the cutter head bearing on a specimen. The drill press is used to apply a constant force through 

the cutter into the specimen and to rotate the cutter at 200 revolutions per minute. 

The test procedure specifies that the force exerted by the cutter on the specimen surface should 

be 98 N (22 lb) unless the test is conducted on concrete that is particularly abrasion-resistant. 

Preliminary testing on UHPC indicated that doubling the load to 196 N (44 lb) as recommended 

by the standard would provide better results, thus this load level was used for the testing. 

Cylinders with a 152-mm (6-inch) diameter were cast for these tests. The specimens were cast 

following normal procedures except that steel molds were used, and the molds were only filled 

approximately 76 mm (3 inches) full. Three cylinders were cast for each curing regime. 

The abrasion resistance of a concrete surface will vary depending on the finish of its surface. For 

this reason, the ASTM C944 test was completed on the finishes of three different concrete 

surfaces for each of the four curing regimes. The surface finishes included a steel cast surface, a 

sandblasted surface, and a ground surface. In the test program, the 12 cylinders were all tested in 

the steel cast surface state. The cylinders were then sandblasted, after which they were tested 

again. Finally, the cylinders were ground and tested a third time. 
129

Figure 117. Photo. ASTM C944 abrasion test setup. 

The ASTM test procedure indicates that the testing should include abrading the surface with the 

rotating cutter for 2 minutes. At the conclusion of the 2 minutes, the amount of concrete that has 

been abraded away is reported. In this test program, a modification was made to the test 

procedure in which the surface abrasion was repeated so that a total of between three and five 

abrasion sequences, with a duration of 2 minutes each, were applied to each specimen. The 

number of abrasion sequences varied depending on the abrasion depth capacity of the cutter and 

the resistance of the specimen. 

Figure 118 shows two of the steel cast surface specimens after abrasion testing. The untreated 

cylinder on the left underwent four abrasion sequences that lasted 2 minutes each. The steamtreated cylinder on the right underwent five abrasion sequences that also lasted 2 minutes each. 

The photo clearly shows that the steel cast surface of steam-treated UHPC is much more 

abrasion-resistant than the untreated UHPC. The rotating cutter had difficulty breaking into the 

surface on the steam-treated cylinder and tended to skid, with only slight abrasion occurring near 

the center of rotation. 

The results for the ASTM C944 abrasion testing are presented in figures 119 through 121. 

Throughout these figures, the results for each curing regime and surface condition set are the 

averages of the results for the three cylinders tested. Figure 119 presents the ASTM C944 

abrasion weight loss as defined in the test specification. As detailed above, this result only 

includes the first 2 minutes of abrading for each specimen. The other two figures present the 

results from the entire sequence of tests completed on each specimen. Figure 120 presents the 

average amount of concrete per abrading over the sequence of abradings. Figure 121 presents the 

least-squares linear approximation of the weight loss per abrading. 
130

Figure 118. Photo. Steel cast surface untreated and steam-treated abrasion 

specimens after 8 and 10 minutes of abrading, respectively. 

1 gram (g) = 0.035 ounces (oz)

Figure 119. Chart. ASTM C944 weight loss (grams) per abrading. 

Untreated Steam 

0.13

0.30

0.17

1.00

2.20

0.73

0.07

0.30

0.20

0.10

0.13

0.13

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Steam, Cast

Steam, Blasted

Steam, Ground

Untreated, Cast

Untreated, Blasted

Untreated, Ground

Tempered Steam, Cast

Tempered Steam, Blasted

Tempered Steam, Ground

Delayed Steam, Cast

Delayed Steam, Blasted

Delayed Steam, Ground

 ASTM C944 Weight Loss (g) per Abrading
131

1 gram (g) = 0.035 ounces (oz)

Figure 120. Chart. Average weight loss (grams) per abrading. 

1 gram (g) = 0.035 ounces (oz)

Figure 121. Chart. Linear best-fit weight loss (grams) per abrading. 

0.09

0.21

0.19

2.56

2.24

1.60

0.14

0.30

0.41

0.07

0.15

0.19

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Steam, Cast

Steam, Blasted

Steam, Ground

Untreated, Cast

Untreated, Blasted

Untreated, Ground

Tempered Steam, Cast

Tempered Steam, Blasted

Tempered Steam, Ground

Delayed Steam, Cast

Delayed Steam, Blasted

Delayed Steam, Ground

 Average Weight Loss (g) per Abrading

0.08

0.22

0.18

1.89

2.10

1.18

0.14

0.31

0.34

0.07

0.14

0.18

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Steam, Cast

Steam, Blasted

Steam, Ground

Untreated, Cast

Untreated, Blasted

Untreated, Ground

Tempered Steam, Cast

Tempered Steam, Blasted

Tempered Steam, Ground

Delayed Steam, Cast

Delayed Steam, Blasted

Delayed Steam, Ground

 Best-fit Weight Loss (g) per Abrading
132

These results show that a steam-based curing treatment of any kind has a significant impact on 

the abrasion resistance of UHPC. In general, the untreated cylinders lost approximately an order 

of magnitude more concrete per abrading than did any of the other cylinders. Quantitatively, the 

steam-based treated cylinders usually lost between 0.1 and 0.3 grams (0.0035 and 0.01 ounces) 

per abrading, while the untreated cylinders usually lost between 1 and 3 grams (0.035 and 0.1 

ounce) per abrading. Also, as would be expected given its smoother texture, the steel cast surface 

performed the best compared with the sandblasted and ground surfaces. 

These results can be compared to the results from Horszczaruk(14) presented in section 2.4.4. The 

testing of 83- to 100-MPa (12- to 14.5-ksi) steel fiber-reinforced concretes produced results 

ranging from 0.14 grams to 0.25 grams (0.005 to 0.009 ounces) of mass loss per abrading for the 

more abrasion-resistant concretes. (These results are presented in a form comparable to the 

UHPC results presented in figure 121.) However, these results are based on ASTM C944 with 

the standard 98-N (22-lb) load, as opposed to the double load used in the present research 

program. The UHPC mass loss would have been significantly lower with the decreased abrasive 

normal force on the cutter head. Also, note that Horszczaruk’s research indicated that higherstrength concretes exhibited constant abrasion resistance, from test initiation until conclusion. 

This seems not to be the case with UHPC, which displayed increased abrasion resistance prior to 

the breaching of the smooth cast surface. 

3.13.5 Freeze-Thaw Resistance 

The resistance of UHPC to freezing and thawing degradation was quantified through the use of 

the ASTM C666 (Procedure A) standard test method.(61) In this test, concrete prisms are 

subjected to freezing and thawing while submerged in a water bath. The aggressive environment 

created in this accelerated durability test helps to determine if the concrete has a microstructure 

that can resist the thermal expansion and contraction effects of water. The concrete could achieve 

this resistance by either resisting the initial water penetration or by allowing the thermal 

expansion of any penetrated water to occur within a voided microstructure. 

The specified lower and upper temperature value targets for the freezing and thawing 

environments are −18 oC and 4.4 oC (0 and 40 oF), respectively. The automated equipment used 

in this test program allowed for five cycles of freezing and thawing to be completed per day. In 

this test method, each prism is housed in a water-filled container that is only slightly larger than 

the prism itself. The containers are placed in an environmental chamber that freezes the prisms 

and their surrounding water layer using cold air, and then thaws the prisms using water. 

The 76- by 102- by 406-mm (3- by 4- by 16-inch) UHPC prisms produced for this test were cast 

following normal procedures. Three prisms were cast for each curing regime. After casting, any 

curing treatments were applied. The freeze/thaw testing began between 5 and 6 weeks after 

casting, following 2 days in which the prisms were submerged in 4.4 oC (39.92 oF) water to 

prepare them for the initial test measurement. In total, 690 cycles of freezing and thawing were 

conducted over the course of 9 months. During stoppages in testing, the prisms were stored in a 

frozen state in a walk-in freezer. In a few instances, however, machine malfunctions did allow 

the prisms to soak in room temperature water for up to a few days until the malfunction was 

corrected. 
133

Periodically, the freeze/thaw cycling was halted, and the prisms were measured. The data 

collection included mass determination and capture of the fundamental transverse frequency of 

each prism. Figure 122 is a photograph showing the setup used to determine the fundamental or 

resonant frequency. In this test, a transducer records the vibrations induced in a prism supported 

on two wires by an instrumented hammer. The capture and determination of this frequency 

response was completed according to ASTM C215. 

Figure 122. Photo. Resonant frequency testing of a freeze/thaw prism. 

Figure 123 presents pictures of an untreated prism both before and near the end of testing. The 

second photograph shows that the prism experienced some deterioration of fibers that were 

exposed on the surface. Also, the surface of the prism tended to become slightly pitted as the 

cycling progressed. Overall, this pitting was very minor and was much more prevalent on the 

Untreated prisms than on any of the steam-treated regimes. 

Figure 123. Photos. Freeze-thaw prism (a) before testing and (b) after 564 cycles. 

(a) (b) 
134

The resonant frequency results from this test program are presented in figure 124. These results 

are the average values from the three prisms that were tested for each curing regime. Note the 

clear difference between the initial resonant frequency results from each curing regime. All 

prisms in each regime resonated within 16 Hz of one another, indicating a tight band of results. 

Figure 124. Graph. Resonant frequency of freeze/thaw prisms. 

The relative dynamic modulus (RDM) is a value defined in the ASTM C666 test method as the 

percent difference in the squares of the frequency at any cycle compared with the initial 

frequency before freeze/thaw cycling. This calculation normalizes the results for ease of 

comparison to other tests. Figure 125 shows these results. In general, this test method assumes 

that freeze-thaw cycling will cause the RDM to decrease as the concrete deteriorates. This RDM 

decrease would occur due to the resonant frequency decreasing as the prism develops internal 

microcracking. The figure shows that the RDM changed very little in the tempered steam and 

delayed steam regimes, decreased slightly in the steam-treated regime, and increased 

significantly in the untreated regime. Overall, these results confirm that UHPC is very resistant 

to deterioration caused by freezing and thawing. However, it is premature to conclude that the 

untreated UHPC exhibits the best freeze-thaw resistance, because these RDM values are being 

influenced by other factors beyond those intended in the test. 

One indicator that an additional unintended behavior was occurring during the freeze-thaw 

cycling was the mass change exhibited by the prisms. Figure 126 presents the mass change 

results. All the regimes exhibited mass increase throughout the testing, with the untreated prisms 

averaging a 0.2 percent increase by 125 cycles. In a normal freeze-thaw test, a prism will lose 

mass as it deteriorates. As mentioned previously, these prisms showed very little deterioration 

throughout the test. However, it seems that instead of deteriorating during the cycling, these 

prisms were taking on water and possibly even hydrating. 

2400

2450

2500

2550

2600

2650

0 100 200 300 400 500 600 700

Cycles

Resonant Frequency (Hz)

Steam Untreated

Tempered Steam Delayed Steam
135

Figure 125. Graph. Relative dynamic modulus of elasticity of freeze/thaw prisms. 

Figure 126. Graph. Mass change of prisms during freeze-thaw testing. 

Further experiments were conducted to investigate the increase in RDM in the untreated prisms. 

To determine if the prisms were taking on water and possibly hydrating, an additional batch of 

eight prisms were cast. This batch included two prisms that were treated according to each curing 

regime. After 28 days, initial freeze-thaw measurements were completed on the prisms, and then 

one prism from each curing regime was placed in a water bath. The other prism from each curing 

regime was maintained in a laboratory environment. The prisms were kept under these 

conditions for 250 days, during which mass and dynamic modulus of elasticity testing was 

periodically completed. 

The resonant frequency results from these eight prisms are presented in figure 127. The RDM 

results are presented in figure 128. The legend for each series indicates the curing regime and 

whether the prism was placed in air or water after day 28. These figures show that for the three 

steam-based treatment regimes, a steady increase in frequency occurred throughout the testing in 

the prisms maintained in the laboratory environment. The submerged prisms from these same 

95

100

105

110

115

120

125

0 100 200 300 400 500 600 700

Cycles

Relative Dynamic Modulus (%)

Steam Untreated

Tempered Steam Delayed Steam

-0.10

0.00

0.10

0.20

0.30

0.40

0 100 200 300 400 500 600 700

Cycles

Mass Change (%)

Steam Untreated

Tempered Steam Delayed Steam
136

regimes show a more rapid increase initially followed by the same type of steady increase. The 

untreated prisms show different behavior, with the submerged prisms exhibiting a rapid increase 

in frequency response during the first 10 days and both untreated prisms showing large overall 

increases in frequency response. The mass change of these prisms is also instructive, with the 

results provided in figure 129. The masses on all of the prisms increased throughout the testing, 

with the untreated prisms tending to show a larger increase than their counterparts in the three 

steam-based treatment regimes. 

One final experiment was conducted to determine if the prisms were hydrating as they were 

gaining water. A set of 76-mm (3-inch) diameter cylinders were cast along with the eight prisms 

previously discussed. These cylinders were divided into four groups, paralleling the conditions 

that the prisms in the untreated and steam treatment regimes underwent through day 56 after 

casting. An additional two sets of control cylinders were cast for testing in compression at 

28 days. 

Table 31 presents the results of these tests. These compression results show that placing UHPC 

in a water bath can increase the compressive strength. As would be expected, the strength 

increase is greater in the untreated UHPC than in the steam-treated UHPC. This greater increase 

is likely partially due to the greater permeability of the untreated UHPC. 

Figure 127. Graph. Resonant frequency of prisms maintained at room temperature 

in a laboratory environment or in a water bath. 

2300

2350

2400

2450

2500

2550

2600

2650

0 50 100 150 200 250

Time (Days)

Resonant Frequency (Hz)

Steam, Air Steam, Water

Untreated, Air Untreated, Water

Tempered Steam, Air Tempered Steam, Water

Delayed Steam, Air Delayed Steam, Water
137

Figure 128. Graph. Relative dynamic modulus of elasticity of prisms maintained 

at room temperature in a laboratory environment or in a water bath. 

Figure 129. Graph. Mass change of prisms maintained at room temperature 

in a laboratory environment or in a water bath. 

95

100

105

110

115

120

125

0 50 100 150 200 250

Time (Days)

Relative Dynamic Modulus (%)

Steam, Air Steam, Water

Untreated, Air Untreated, Water

Tempered Steam, Air Tempered Steam, Water

Delayed Steam, Air Delayed Steam, Water

-0.10

0.00

0.10

0.20

0.30

0.40

0 50 100 150 200 250

Time (Days)

Mass Change (%)

Steam, Air Steam, Water

Untreated, Air Untreated, Water

Tempered Steam, Air Tempered Steam, Water

Delayed Steam, Air Delayed Steam, Water
138

Table 31. Effect of a water bath on the compressive strength of steam-treated 

and untreated UHPC. 

Curing Regime Subsequent Compression Strength (MPa)

Environment

Age 

(days)

No.

Average Standard 

Deviation 

Steam N/A 28 4 143 5.0 

Steam Air 56 5 150 4.1 

Steam Water Bath 56 5 153 8.0 

Untreated N/A 28 4 105 1.4 

Untreated Air 56 5 108 5.1 

Untreated Water Bath 56 5 120 8.1 

1 MPa = 145 psi 

These additional tests, which were completed following the initial freeze-thaw testing, have 

shown that the procedure followed for the freeze-thaw testing could lead to unintended water 

permeation and to additional hydration of the UHPC. This effect has been shown to be limited in 

the steam-based treatment curing regimes; however, it can be significant in the untreated regime. 

The added hydration creates the possibility of a 15 percent increase in the RDM if a prism 

spends 250 days in a water bath and a 10-percent increase in compressive strength if a cylinder is 

placed in a water bath from day 28 to 56. 

3.13.6 Alkali-Silica Reaction 

Alkali-silica reaction (ASR) testing was performed in accordance with ASTM C1260.(62) This 

accelerated test method allows for the possible detection of the deleterious effects of ASR far 

more rapidly that could be achieved using the standard test method, ASTM C1293.(63) The 

ASTM C1260 test involves submerging mortar bars in an 80 oC (176 oF) sodium hydroxide 

solution for 2 weeks, while periodically measuring the length of the bar. These tests were 

extended to 4 weeks. 

Standard mortar bar molds were used to cast the specimens for this series of tests. Because this 

UHPC contains no coarse aggregate, no special preparation of the batch ingredients was 

necessary before casting the bars. Six mortar bars were cast for each curing condition. The 

mortar bars had a 25.4- by 25.4-mm (1- by 1-inch) cross section and were 280 mm (11 inches) 

long. The bars had gage studs cast into each end so that ASTM C490 length change 

measurements could be recorded throughout the testing.(49) Figure 130 shows one of the mortar 

bars in the length comparator for measurement. 

The standard test method includes specific instructions regarding timing throughout the test. 

These instructions include demolding the mortar bars at 24 hours, placing the bars in an 80 oC 

(176 oF) water bath for the following 24 hours, then placing the bars in the 80 oC (176 oF) sodium 

hydroxide solution bath until test completion. Due to the curing regimes being investigated in 

this research program, the timetable recommended in the standard test method could not be 

followed. Table 32 provides the timetable for each of the five sets of specimens that were tested. 
139

Submerging the UHPC in the 80 oC (176 oF) bath is somewhat similar to the steam-based curing 

regimes wherein the UHPC is placed in a high-heat, high-humidity environment. For this reason, 

it is anticipated that the test method described in ASTM C1260 may inadvertently provide 

additional curing to the UHPC, especially UHPC that did not undergo a steam-based curing 

treatment. Thus, two sets of specimens were tested in the untreated curing regime. In what is 

deemed the untreated (standard) set in table 32, the ASR testing was not initiated until 28 days 

after casting. In the untreated (modified) group, the ASR testing was initiated immediately after 

demolding. 

Figure 130. Photo. Length comparator for ASR measurements. 

Table 32. Timetable for ASTM C1260 specimens. 

Curing Regime Demold Curing 

Treatment 

24-hr Water 

Bath 

NaOH Bath 

Start 

Steam 25 hrs 26th–74th hr 98th–122nd hr hr 122 

Untreated (Standard)†

 25 hrs 25th hr–28th day 28th–29th day day 29 

Untreated (Modified)‡

 26 hrs None 26th–51st hr hr 51 

Tempered Steam 26 hrs 26th–74th hr 98th–122nd hr hr 124 

Delayed Steam 26 hrs 15th–17th day 18th–19th day day 19 

†

 ASR testing initiated 28 days after casting ‡

 ASR testing initiated 26 hours after casting

Table 33 presents the results from this series of tests. The standard test method indicates that 

expansion values less than 0.10 percent after 14 days of testing are indicative of innocuous 

behavior, whereas expansion values over 0.20 percent are potentially deleterious. The UHPC 

expansion results are an order of magnitude below this lower threshold after 14 days and even 
140

after 28 days of testing. Figure 131 presents the entire set of test results, including approximately 

10 length readings for each curing regime throughout the 28 days of testing. 

These results indicate that there should be no concern of ASR problems with the concrete after a 

steam-based curing regime is applied. Given the procedures used in this test method to check for 

ASR susceptibility, Untreated UHPC may not be as resistant to ASR as determined in these tests. 

However, for ASR to occur in any concrete, free water must be present. Given the low 

permeability of UHPC, it seems unlikely that ASR would be an issue under any curing regime. 

Table 33. ASTM C1260 alkali-silica reactivity expansion results. 

Curing Regime Bars 14-day Expansion (%) 28-day Expansion (%) 

Tested Average Standard 

Deviation 

Average Standard 

Deviation 

Steam 6 +0.013 0.008 +0.009 0.005 

Untreated† 6 +0.011 0.002 +0.012 0.002 

Untreated‡

 6 −0.004 0.002 +0.012 0.001 

Tempered Steam 6 +0.005 0.002 +0.004 0.003 

Delayed Steam 6 +0.001 0.002 +0.002 0.002 

†

 ASR test initiated 28 days after casting ‡

 ASR test initiated 1 day after casting 

Figure 131. Graph. ASTM C1260 alkali-silica reactivity expansion results. 

3.14 SPLIT-CYLINDER TENSION TESTING ON CRACKED CYLINDERS 

Any material that is affected by external environmental aggressors will necessarily be more 

affected by those aggressors if pathways into the interior of the material are present. The 

durability of uncracked UHPC has been well covered by the topics discussed in section 3.13. 

However, in some applications cracked UHPC could be subjected to aggressive environments. 

The cracks could allow for ingress of contaminants and for a more rapid deterioration of the 

-0.02%

0.00%

0.02%

0.04%

0.06%

0.08%

0.10%

0 5 10 15 20 25 30

Elapsed Time Since ASR Test Initiation (days)

Morar Bar Expansion

Steam

Air (standard)

Air (modified)

Tempered Steam

Delayed Steam
141

UHPC than would be expected based on uncracked durability results. Recall the research 

presented in section 2.4.2 wherein Rapoport et al. indicated that the permeability of fiberreinforced normal concrete could be increased by cracking if the cracks were larger than 

0.10 mm (0.004 inch).(10) 

To begin to access the durability of cracked UHPC, a series of tests were conducted to determine 

if the ponding of a sodium chloride solution on cracked UHPC would negatively impact the 

postcracking tensile strength. Although these tests are not standardized, they are a combination 

of the ASTM C496 split-cylinder tensile tests discussed in section 3.4.2 and the AASHTO T259 

ponding tests discussed in section 3.13.2.(25,58) The split-cylinder tensile test portion of this 

program was conducted identically to the tests discussed earlier in the chapter, including the 

measurement of lateral deformation using a pair of LVDTs. The only difference was in the 

staging of the test wherein specimens were not necessarily loaded directly to failure (as discussed 

below). The ponding portion of these tests was identical to the AASHTO T259 tests described 

previously. 

This series of tests included 30 cylinders with a 102-mm (4-inch) diameter; half of the cylinders 

came from batch L1R and half came from L2R. Only the steam-treated and untreated cases were 

investigated. The casting and curing of the specimens followed standard procedures, and the 

testing discussed herein was initiated on day 28 after casting. 

The 15 cylinders from each batch were divided into 5 groups of 3. The first group of cylinders, 

the control group, was loaded to failure. The second group was loaded until cracking had 

occurred, then unloaded, then reloaded to failure. The third group was loaded identically to the 

second group, except that after unloading the cylinders were stored in a laboratory environment 

for 90 days. The fourth group was identical to the third group except that, during the 90-day 

storage, water was ponded on a cracked end of the cylinder. Finally, the fifth group was identical 

to the fourth group, except that a 3-percent sodium chloride solution was ponded on a cracked 

surface. 

The cracking of each cylinder was determined by monitoring the lateral deformation behavior. 

The two means of determining if a crack had occurred included a clear jump in the lateral 

deformation or a definite reduction in stiffness of the load versus lateral deformation response. 

Most cylinders exhibited a clear jump in their response, but a few untreated cylinders only 

exhibited the stiffness change. Cylinders from the second through the fifth groups were unloaded 

immediately after cracking was observed. 

The cracks on these specimens were then identified and measured using an optical microscope. 

First, alcohol was used as a volatile penetrant to highlight each crack. A crack that was made 

visible using this technique can be seen in figure 132. The cracks were then located using the 

microscope set at 350x magnification. In general, each end of each cylinder only had one crack. 

Once located, each crack was traced to find its largest width. The cracks were then photographed 

for later analysis of the crack widths. A photograph of a crack from one of the steam-treated 

cylinders is shown in figure 133. At this optical magnification and given the resolution of the 

digital photograph acquired, the crack shown in this figure is only 2 pixels wide, corresponding 

to a width of 0.0036 mm (0.00014 inch). 
142

Figure 132. Photo. Crack in a split cylinder tensile specimen. 

Figure 133. Photo. Crack in a split-cylinder tensile specimen under 350x magnification. 

The crack widths on the ponded faces of the 12 ponded cylinders ranged from 0.0018 mm 

(0.00007 inch) to 0.012 mm (0.00047 inch). Ten of the 12 cylinders had crack widths between 

0.0036 and 0.0069 mm (0.00014 and 0.00027 inch). Table 34 provides the crack width and splitcylinder peak strength results for the cylinders that were ponded. 

Crack

Crack
143

Table 34. Crack width and split-cylinder peak strength results for ponded cylinders. 

Crack Width (mm) Split-Cylinder 

Peak Strength 

(MPa) 

Curing 

Regime 

Ponding 

Solution 

Cylinder 

Number 

Ponded 

End 

Nonponded 

End 

Steam Water 1 0.0051 0.0036 23.4 

Steam Water 2 0.0069 0.0036 25.8 

Steam Water 3 0.0069 0.0036 21.4 

Steam NaCl 1 0.0036 0.0051 22.1 

Steam NaCl 2 0.0036 0.0018 24.7 

Steam NaCl 3 0.0036 0.0018 26.8 

Untreated Water 1 0.0119 0.0051 19.6 

Untreated Water 2 0.0069 No Crack 22.0 

Untreated Water 3 0.0051 0.0036 20.3 

Untreated NaCl 1 0.0018 No Crack 22.1 

Untreated NaCl 2 0.0051 0.0051 21.5 

Untreated NaCl 3 0.0051 0.0036 21.9 

1 MPa = 145 psi, 1 mm = 0.039 inch 

The results of these tests indicate that the ponding of neither the water nor the sodium chloride 

solution had little effect on the peak tensile load carried by the cylinders. Figure 134 provides the 

postcracking strength results for all five groups of cylinders for both curing regimes. The peak 

load carried shows no clear change, and it is clear that the ponding of the water or the sodium 

chloride did not have a serious detrimental effect on the fibers crossing the crack in each 

cylinder. However, it must also be noted that the sodium chloride solution did cause corrosion of 

fibers exposed on the ground end of the cylinder, similar to what was observed in the AASHTO 

T259 and ASTM C672 tests. Further research into the permeability of cracked UHPC is 

necessary to determine what crack size results in a change in the permeability characteristics of 

the material. 
144

1 MPa = 145 psi 

Figure 134. Chart. Split-cylinder peak strength results.

26.8

24.8

25.1

23.5

24.6

19.5

20.3

21.9

20.6

21.8

0 5 10 15 20 25 30

Load to Failure

Crack, Unload, Fail

Crack, Unload, Wait 90d, Fail

Crack, Unload, Pond Water 90d, Fail

Crack, Unload, Pond NaCl 90d, Fail

Load to Failure

Crack, Unload, Fail

Crack, Unload, Wait 90d, Fail

Crack, Unload, Pond Water 90d, Fail

Crack, Unload, Pond NaCl 90d, Fail

Split Cylinder Peak Strength (MPa)

Untreated

Steam
145

CHAPTER 4. DISCUSSION OF RESULTS 

4.1 TENSILE BEHAVIOR OF UHPC 

Four small-scale experimental test methods were used to study the tensile behavior of UHPC. 

These test methods included the ASTM C1018 prism flexure test, the ASTM C496 split-cylinder 

test, the AASHTO T132 mortar briquette test, and the cylinder direct tension test. Individual test 

results were presented in section 3.4. In this chapter, the results will be combined and compared 

and the test methods and their applicability and practicality for UHPC will be discussed. 

4.1.1 Summary of Experimental Results 

The four small-scale concrete tension testing methods used in this research provided varied 

bodies of results. Table 35 presents the primary quantifiable results from the four test methods. 

Each test provided an indication of the tensile cracking strength of UHPC. The direct tension and 

prism flexure tests also provided an indication of the modulus of elasticity of UHPC. Finally, the 

prism flexure tests provided an indication of the postcracking toughness of UHPC. For reasons 

discussed in section 3.4.1, the third-point loaded UHPC prism with the 305-mm (12-inch) span is 

taken as the standard prism flexure loading configuration, and its results are presented in this 

section. 

The tensile cracking strength of UHPC is critical to the design of UHPC structures; however, it is 

difficult to determine experimentally. All of the four test methods provide realistic tensile 

cracking strengths that could be assumed to be accurate representations of actual tensile 

behavior. However, table 35 also shows that, for each curing regime, the tensile cracking 

strength can vary by approximately 3.4 MPa (0.5 ksi) depending on the test method used. For 

this reason, engineering judgment must be used to predict the true tensile cracking behavior. 

Table 35 shows that the results for the cylinder direct tension and mortar briquette test cracking 

tensile strength were generally similar for each curing regime. This similarity is not surprising, 

because both test methods are based on the uniaxial application of tensile stresses. The prism 

flexure results, after the application of the correction factor discussed in section 3.4.1.1, are 

generally slightly higher than the direct tension results. Note that without the correction factor, 

the prism flexure results would be approximately two times the direct tension results. Finally, the 

split-tensile results are approximately one-third larger for all curing regimes, a result that is not 

unexpected when comparing split-cylinder test results with direct tension results.(27) Direct 

tension results are normally more highly regarded than split-cylinder results. However, direct 

tension tests may possibly be disproportionately impacted by local heterogeneities as compared 

to the larger structures that they are supposed to mimic. Therefore, lower tensile cracking 

strengths may be reported than what would actually be observed in practice. 
146

Table 35. UHPC material characterization results for average tensile properties of UHP

Material Characteristic Steam Untreated Tempered 

Steam 

Delayed 

Steam 

Supplemental D

First Cracking Strength (MPa) Split Cylinder 11.7 9.0 11.7 11.7 ASTM C496

 Prism Flexure 9.0 9.0 10.3 9.7 ASTM C1018; 305 mm s

 Mortar Briquette 8.3 6.2 9.7 6.9 AASHTO T132

 Direct Tension 9.7–11.0 5.5–6.9 7.6–9.0 9.0–11.0 Axial tensile load

 Combined Result, fct 9.0 6.2 9.0 9.0 Best estimate of tensile c

Modulus of Elasticity (GPa)

 Direct Tension 51.7 47.6 52.1 52.1 Axial tensile stress and s

 Prism Flexure 58.9 49.3 56.3 55.8 Flexure and shear based; 

Postcracking Strength (MPa)

 Mortar Briquette 9.0 6.2 9.0 6.2 AASHTO T132

Flexural Toughness  Prism Flexure: I10 14.4 12.8 13.0 13.8 ASTM C1018; 305 -mm 

 Prism Flexure: I30 53.0 48.3 43.1 48.3 ASTM C1018; 305-mm 

Compression Testing Results 

 Compressive Strength, fc’ (MPa) 193 126 171 171 ASTM C39; 28-day stren

 Modulus of Elasticity (GPa) 52.4 42.7 51.0 50.3 ASTM C469; 28-day mo

Tension, Compression 

fct = x (fc’)0.5 in MPa 0.65 0.55 0.69 0.69 fct = x (fc’)0.5 in psi 7.8 6.7 8.3 8.3 fct = x fc′ 0.046 0.049 0.052 0.052 

1 MPa = 145 psi 1 psi = 6,895 Pa 1 GPa = 145,000 psi 1 mm = 0.039 inch 
147

The results from these four test methods were combined together to determine the tensile 

cracking strength. The combining of these results takes into account the overall body of results, 

the number of tests, and the deviation of results from the average value. When these factors are 

combined, the tensile cracking strength of steam-treated UHPC is 9.0 MPa (1.3 ksi). The tensile 

cracking strength of untreated UHPC is 6.2 MPa (0.9 ksi). The tensile cracking strength of 

tempered steam and delayed steam-treated UHPC are both 9.0 MPa (1.3 ksi). 

These tensile cracking strength results can be compared with the compressive strength results 

discussed previously in this report. The bottom of Table 35 shows that the tensile cracking 

strength, fct, is approximately 5 percent of the compressive strength, fc′, in all four curing 

regimes. Alternatively, the equation in figure 135 is frequently used to approximate a tensile 

strength from a given compressive strength. In this equation, the square root of the compressive 

strength is related to the tensile strength by a linear multiplier, x. In English units (psi), the 

UHPC that underwent the steam treatment has a multiplier of 7.8, the untreated UHPC has a 

multiplier of 6.7, and the tempered and delayed steam-treated regimes have a multiplier of 8.3. 

 

Figure 135. Equation. Concrete tensile strength approximation. 

The modulus of elasticity results obtained from the tension tests are also presented in the table. 

Given the limited number of direct tension tests completed, these results can be considered to be 

similar to the compression test modulus of elasticity results discussed in chapter 3. Although the 

untreated result is 11 percent higher, the remainder of the results are within 3.5 percent. 

However, the prism flexure modulus of elasticity results are generally between 10 and 15 percent 

higher than the comparable compression test results. This result indicates that the UHPC prism 

flexure tests tend to overestimate the modulus of elasticity as compared to the standard 

compression testing methods, because nearly all of the prism groups exhibit this behavior, 

regardless of cross section and loading configuration. 

The postcracking tensile behavior is much more difficult to quantify, but this research program 

has resulted in a number of both general and specific findings. The mortar briquette tests provide 

the most direct indication of the postcracking behavior. Figures 66 through 77 show the full loaddisplacement curves for all of the mortar briquette tests. First and foremost, these figures indicate 

that UHPC tends to display postcracking strength levels similar to its precracking strength levels. 

The table lists the average postcracking strength values, which are all within 0.8 MPa (0.1 ksi) of 

the mortar briquette first tensile cracking strength levels. Note, however, that postcracking 

behavior is very dependent on fiber dispersion and orientation. The small-scale mortar briquettes 

most likely did not accurately represent the large-scale tensile members for these two factors. 

Limited postcracking strength results are also available from the split-cylinder tests. These tests 

indicate that, under the biaxial state of stress present in this test configuration, UHPC exhibits 

significant postcracking load-carrying capacity. For each curing regime, the peak load carried 

after cracking is approximately twice the cracking load. However, the compressive forces 

paralleling the tensile crack in this loading configuration increase the fiber/matrix bond and thus 

contribute to the postcracking load-carrying capacity. 

'

ct c f = x f
148

Finally, comparative results were also obtained from the prism flexure tests. The results from this 

standardized test can easily be compared both within this study and with normal and fiberreinforced concretes tested by other researchers. Comparisons with the standard toughness levels 

set in ASTM C1018 and with the results from other researchers indicate that UHPC exhibits very 

high postcracking toughness values. Specifically, all the UHPC curing regimes were at or above 

the stated fiber-reinforced concrete toughness upper limit for the values of I5, I10, and I20. Also, 

these tests indicate that a relatively uniform level of toughness will be achieved throughout the 

curing regimes. However, these results do not mean that all UHPC display identical postcracking 

load-carrying behavior, because the ASTM C1018 toughness measure relates postcracking to 

precracking behaviors and thus is biased by the precracking strength. 

4.1.2 Summary of Experimental Test Methods 

Sections 4.1.1 and 3.4 describe the small-scale tension tests employed in this research program 

and the relevant results from each test. This section builds on those presentations to present and 

to compare the test methods. Test capabilities and the difficulties associated with completing 

each test will be discussed. 

The split-cylinder test is the simplest UHPC tension test to complete successfully. This test 

program has shown that consistent UHPC tensile cracking strength results can be obtained by 

following the ASTM C496 test procedure. The only addition to the test is the inclusion of a 

cylinder lateral expansion measurement and recording device. Thus, a measure of the tensile 

cracking strength of UHPC can be obtained using a standard hydraulic compression testing 

machine, a simple data acquisition system, and a pair of displacement transducers. Alternatively, 

an evaluation technique capable of detecting a crack in UHPC, such as ultrasonic inspection, 

could be used to note cracking in steam-based treated UHPC, thus eliminating the need for data 

acquisition and full-range transducers. Test results presented in section 4.1.1 indicated that 

conversion of the split-tensile cracking strength result to the overall tensile cracking strength 

result could be obtained by multiplying the split-cylinder result by a factor of 0.75. 

Unfortunately, the unknown postcracking stress distribution in the cylinder does not allow for the 

quantitative determination of any postcracking tensile behaviors. 

A slightly more advanced loading setup is required for completing the AASHTO T132 mortar 

briquette test. The test performed in this research program required a test machine capable of 

loading under crosshead displacement control and a data acquisition system capable of recording 

load and displacement readings throughout the test. The primary benefit of this test over the 

split-cylinder test is that it directly measures uniaxial tensile properties of UHPC. This benefit 

allows for a more accurate rendering of the UHPC tensile cracking strength. Also, this test 

method can provide some indication of the postcracking tensile strength of UHPC; however, the 

difficulties inherent in the casting of these small briquette specimens decrease the confidence in 

any postcracking strength results. As such, the benefits of this test over the split-cylinder test are 

marginal unless a qualitative verification of postcracking load-carrying capacity is desired. 

The ASTM C1018 prism flexure test is significantly more intensive with regard to testing 

equipment than either of the previous two test methods. To successfully complete this test, a test 

machine capable of loading under an external displacement transducer control is necessary, 

along with a data acquisition system to record load and displacement. Additionally, an electronic 
149

averaging circuit to combine the deflection results from the specimen can provide more 

consistent, uniform loading. The benefit of the prism flexure test is that it provides an indication 

of the tensile cracking strength, the modulus of elasticity, and an arbitrarily defined but widely 

used measure of the postcracking tensile toughness. Because a cross comparison between 

different fiber-reinforced concretes or concretes cast in different laboratories is important, this 

test can provide needed results that are unavailable through simpler test setups. However, as 

discussed in section 3.4.1.1, tensile cracking strength results from this test method must be 

corrected using an empirical relationship. The acquisition of pure tensile cracking behaviors 

from this test method is not possible. 

The final test method, the direct tension test, is the most difficult test to implement and also 

provides the most complete body of results. This method requires a test machine capable of 

loading under external displacement control, an electronic averaging circuit for at least three 

displacement transducers, and a data acquisition system to record the load and displacements. 

Additionally, a means of gripping the tensile specimen, such as the high-strength, high-modulus 

epoxy used in the research, is necessary. Finally, great care must be taken in the fabrication of 

the small-scale specimens to ensure that they replicate the regions in large-scale structures that 

are subjected to tensile stresses. If all these requirements are met, this test has the possibility of 

capturing the full tensile stress-strain or stress-crack opening behavior. These results would 

include the tensile cracking strength, the modulus of elasticity, the postcracking tensile strength, 

and the load-carrying capacity from tensile cracking through fiber pullout. The test program 

discussed in this report did not achieve the goal of successfully completing direct tension tests, 

and thus was not able to determine the full tensile stress-strain response of UHPC from smallscale tension tests. 

The selection of a particular tension test for UHPC must be predicated on the results desired. The 

tests discussed in this section could be used to meet certain needs, but other tests or 

modifications of these tests could also be used. For example, it is likely that UHPC used in 

structures will require a minimum level of tensile strength. As a quality control issue, a 

hydraulically operated test machine under load control could be used to ensure that the minimum 

level of tensile strength is achieved. This could be accomplished by an experienced quality 

control professional performing a mortar briquette or split cylinder test wherein the UHPC was 

required to hold a sustained stress level prior to the audible indication of tensile cracking. 

4.2 LOCAL AND GLOBAL MECHANICAL FAILURE MODES OF UHPC 

UHPC exhibits a number of macrostructural mechanical failure modes. These failure modes can 

be categorized into three specific types: compressive, tensile cracking, and tensile fiber pullout. 

A fourth failure mode, the high-cycle fatigue failure of crack-bridging steel fibers, has been 

observed under special loading conditions but will not be discussed in this report. 

The compressive failure of UHPC can be considered to be similar to the compressive failure of 

any fiber-reinforced concrete. In general terms, UHPC fails under axial compressive load 

through lateral tensile expansion. This lateral expansion is partially restrained by the internal 

steel fiber reinforcement, thus allowing for a more ductile failure than may be expected. As with 

any concrete, higher-strength UHPC tends to fail in a more brittle manner than lower-strength 

UHPC. 
150

Compression failures in hundreds of UHPC cylinders were observed while performing the 

ASTM C39 compression test. The failure of any cylinder that underwent steam-based treatment 

was brittle, with a rapid load decrease occurring immediately after the peak load was achieved. 

Even as such, these cylinders remained largely intact with relatively few small fragments leaving 

the cylinder. The failure of the cylinders in the untreated regime occurred in a much more ductile 

manner. Some of these cylinders, particularly the ones with strengths above the average of 

126 MPa (18.3 ksi), did exhibit brittle behavior. However, most of these cylinders failed through 

a continuous, nonabrupt decrease in load after reaching the peak. The failures of these lowerstrength (and less cured) cylinders were significantly more ductile and exhibited quantifiable 

post-peak behavior that was not achievable on a hydraulically-actuated load testing machine with 

the higher-strength cylinders. Finally, the compression testing of a 76-mm (3-inch) diameter 

steam-treated UHPC cylinder that did not contain any fiber reinforcement resulted in an 

extremely brittle failure with significant fragmentation of the UHPC. 

The tensile behavior of UHPC allows for continuing tensile load-carrying capacity across a 

cracked plane. The design of the structure will determine whether the tensile cracking of UHPC 

is a failure mode. Regardless, for the purposes of this discussion the UHPC behavior at this 

critical junction will be described. 

Similar to the compression failures described above, the tensile cracking of UHPC can be either 

brittle or ductile. Although these differences in behaviors were observed in all small-scale tensile 

tests, the differences were most clearly observed in the split-cylinder tensile tests. In these tests, 

the cylinders that had been subjected to steam-based treatments tended to exhibit a clear aural 

indication of first cracking and displayed discontinuous load-displacement behavior as the crack 

abruptly formed, then was arrested by the bridging fibers. Conversely, the untreated cylinders did 

not exhibit an aural indication of first cracking and sometimes displayed continuous loaddisplacement behavior at cracking. Also, the crack width at crack arrest tended to be larger in the 

cylinders that underwent the steam-based treatment, which was likely due to the higher tensile 

strengths at which cracking occurred. 

Final tensile failure of UHPC generally occurs when the steel fiber reinforcement begins to 

debond from and to pull out of the UHPC matrix. Because fibers are randomly distributed and 

oriented in the UHPC, individual fiber loads vary at any particular global load level. 

Mechanically, pullout occurs when the load carried by an individual fiber overcomes the ability 

of the UHPC to grip the fiber. Pullout by any fibers increases the load that other nearby fibers 

have to carry. Multiple pullouts in a specific location that require gross load redistribution 

through alternate load paths can be defined as fiber pullout failure. 

Fiber pullout across a plane perpendicular to the principal tensile stresses was the failure 

mechanism that concluded all small-scale tension tests. The fibers protruding from each failure 

surface tended to align perpendicular to the surface, likely due to the inelastic bending of each 

fiber at its exit point from the UHPC matrix as the crack widened and pulled the fiber straight 

across the crack. 
151

4.3 EFFECT OF CURING PROCEDURE ON UHPC PROPERTIES 

One of the primary focuses of this research program was to determine the effect that the 

application of a curing treatment had on the behaviors exhibited by UHPC. To quantify this 

effect, the majority of the research completed within the material characterization portion of this 

study included four different curing regimes. The curing regimes included the manufacturerrecommended steam treatment, a delayed version of the same treatment, a reduced temperature 

version of the same treatment, and an untreated case in which the UHPC remained in a 

laboratory air environment until testing. 

Table 36 presents a compilation of results from the material characterization study. The results 

provided in the table are average values that were summarized for ease of discussion. A full 

discussion of each portion of this test program is provided in chapter 3. 

The application of a curing treatment clearly impacts UHPC behavior. A comparison of the 

steam-treated and untreated regime results indicates that the application of the recommended 

steam treatment has a significant effect on some properties. In terms of mechanical properties, 

steam treatment will increase the compressive strength, tensile cracking strength, and elastic 

modulus. In terms of long-term stability, steam treatment will decrease creep and speed the 

realization of the asymptotic shrinkage, virtually eliminating any shrinkage after the treatment is 

complete. In terms of durability, steam treatment seems to decrease the permeability of UHPC, 

thus increasing its resistance to chloride penetration. A more durable mechanical matrix that is 

better able to resist abrasive forces is also created. 

The application of intermediate curing treatments, such as in the tempered steam and delayed 

steam regimes, also has a significant—although generally slightly reduced—impact on the 

UHPC behavior. While the compressive strength and elastic modulus are increased by these 

treatments, they both fall short of the level achieved with the full steam treatment. Interestingly, 

tensile cracking behavior enhancements seem to be similar regardless of the type of steam 

treatment applied. Note that the creep behavior is significantly enhanced by the delayed steam 

treatment, while it is only slightly enhanced by the tempered steam treatment. In terms of 

durability, the chloride ion penetrability is extremely low regardless of the steam treatment 

applied. 
152

Table 36. Average UHPC material properties presented according to curing treatment

Material Characteristic Steam Untreated Tempered 

Steam 

Delayed 

Steam 

Supplemen

Compressive Strength (MPa) 193 126 171 171 ASTM C39; 28-daModulus of Elasticity (GPa) 52.4 42.7 51.0 50.3 ASTM C469; 28-dSplit Cylinder Cracking Strength (MPa) 11.7 9.0 11.7 11.7 ASTM C496 Prism Flexure Cracking Strength (MPa) 9.0 9.0 10.3 9.7 ASTM C1018; 30Mortar Briquette Cracking Strength (MPa) 8.3 6.2 9.7 6.9 AASHTO T132 Direct Tension Cracking Strength (MPa) 9.7–11.0 5.5–6.9 7.6–9.0 9.0–11.0 Axial tensile load Prism Flexural Tensile Toughness (I30) 53.0 48.3 43.1 48.3 ASTM C1018; 30

Long-Term Creep (Ccu) 0.29 0.78 0.66 0.31 ASTM C512; 77-M

Long-Term Shrinkage (microstrain) 766 555 620 657 ASTM C157; initiTotal Shrinkage (microstrain) 850 790 – – Embedded vibratiCoeff. of Thermal Exp. (x10-6 mm/mm/oC) 15.6 14.7 15.4 15.2 AASHTO TP60-0

Chloride Ion Penetrability (coulombs) 18 360 39 18.00 ASTM C1202; 28Chloride Ion Permeability (kg/m3) < 0.06 < 0.06 < 0.06 < 0.06 AASHTO T259; 1

Scaling Resistance No Scaling No Scaling No Scaling No Scaling ASTM C672 Abrasion Resistance (grams lost) 0.17 0.73 0.20 0.13 ASTM C944 2x w

Freeze-Thaw Resistance (RDM) 96% 112% 100% 99% ASTM C666A; 60Alkali-Silica Reaction Innocuous Innocuous Innocuous Innocuous ASTM C1260; tes

1 MPa = 145 psi 1 GPa = 145,000 psi 1 psi = 6,895 Pa 1 mm = 0.039 inch 1 mm/mm/°C = 1.8 inches/inches/°F 1 kg/m3 = 1.69 lb/yd3

1 g = 0.035 oz 
153

Another curing treatment procedure, illuminated during this test program, is slightly outside of 

the clearly defined boundaries of the curing regimes. In section 3.3.5, the effect of demolding 

cylinders in terms of their early age setting and strength gain was discussed in terms of 

compressive strength. These tests showed that demolding UHPC, and thus exposing it to a low 

humidity environment prior to sufficient setting and strength gain, could have a significant 

impact on the 28-day compressive strength. It is extremely important to wait until a sufficient 

strength level has been achieved before demolding UHPC and to keep UHPC in a moist 

environment during the continued strength gain. This procedure was not always followed within 

this material characterization study and, as discussed in chapter 3, some of the observed 

behaviors were reduced as a result. 

The overriding result of the entire UHPC curing treatment comparison is that, regardless of the 

curing treatment applied, UHPC exhibits significantly enhanced properties compared with 

standard normal strength and HPCs. The application of the steam treatment is clearly beneficial; 

however, this procedure is also not always necessary as long as the user is willing to accept 

decreases in strength, long-term stability, and durability. 

4.4 EARLY AGE STRENGTH GAIN OF UHPC 

The compressive strength gain behavior of UHPC is an important characteristic of the concrete. 

Results detailed in chapter 3 indicated that UHPC does not have any compressive strength for 

nearly 1 day after casting. Then, once initial set occurs, UHPC rapidly gains strength over the 

course of the next few days until over 69 MPa (10 ksi) of strength is achieved. At that point, the 

rate of strength gain decreases, but the strength gain continues until over 124 MPa (18 ksi) of 

compressive strength is achieved by 28 days. 

Figure 136 presents a compilation of the compressive strength data for the untreated cylinders 

tested between 1 and 56 days (the results are discussed in section 3.3.2). These results include 

both cylinders in the untreated group and cylinders in the other treatment groups that had not yet 

undergone their own steam treatment. Also, recall from section 3.6 that initial set tended to occur 

at approximately 15 hours, and final set occurred 2 hours later. 

A regression analysis was completed to fit a transition function to the data presented in figure 

136. The delayed then rapid early age strength gain behavior of UHPC results in a somewhat 

complex approximating function. The Weibull Cumulative function, provided as the equation in 

figure 137 and plotted in figure 136, accurately describes the strength gain behavior for any time 

after 0.9 days following casting. This equation includes the time in days after casting, t, and the 

28-day compressive strength in MPa, fc′, with the result being the compressive strength at time t

in MPa. The initial and final set times of UHPC can vary depending on the age of the premix and 

the environmental conditions, thus this equation may not be applicable to UHPC exhibiting 

different setting behaviors. 
154

1 MPa = 145 psi 

Figure 136. Graph. Compressive strength gain as a function of time after casting. 

 

Figure 137. Equation. Compressive strength at any age after casting. 

4.5 COMPARISON OF CYLINDER AND CUBE COMPRESSION STRENGTH 

RESULTS 

The cylinder is the standard concrete compression test specimen geometry used in the United 

States. This is understandable, because cylinder molds are inexpensive and the casting of 

concrete cylinders is relatively easy. However, as opposed to traditional concrete, UHPC 

cylinders cannot easily be prepared for testing. In any concrete compression test, the even and 

parallel loading of the ends of the cylinder is critical to achieving accurate test results. The high 

compressive strength of UHPC eliminates the possible use of end capping materials and 

predicates grinding or milling to ensure plane, parallel loading surfaces. Unfortunately, the 

equipment necessary to perform this end preparation on UHPC cylinders is not yet locally 

available to all quality control and testing departments. 

One method of eliminating the need for cylinder end preparation is to use cubes as the standard 

means to measure the compressive strength of UHPC. Cubes are cast with two pairs of plane, 

parallel faces, thus greatly simplifying the specimen preparation process. The primary drawbacks 

to the use of cubes are the expense of the molds, the unfamiliarity with the test methods, and the 

indirect nature of the results obtained from the test. In general, cube compression test results are 

higher than cylinder compression test results on the same concrete due to the aspect ratio of the 

specimens and the confining effect of the machine platens. To account for these differences, a 

0

20

40

60

80

100

120

140

0 10 20 30 40 50 60

Days after Casting

Compressive Strength (MPa)

Figure 137 equation with f′c = 126 MPa 

⎥

⎥

⎦

⎤

⎢

⎢

⎣

⎡

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ ⎟

⎠

⎞ ⎜

⎝

⎛ − = − −

0.6

' '

, 3

0.9 1 expt f f c t c
155

strength reduction factor normally needs to be applied to a cube compression test result. This 

factor can vary significantly depending on many concrete properties, but has been reported to be 

in the vicinity of 0.82 for ordinary concrete and increases toward 1.0 as the concrete strength 

increases.(64)

Section 3.3.4 presented the results from a series of cube and cylinder compression tests. Recall 

that the 51-, 76-, and 102-mm (2-, 3-, and 4-inch) diameter cylinders, as well as the 51-mm 

(2-inch) and 100-mm (4-inch) cubes were tested and that the series of tests were repeated three 

times on different batches of UHPC. If the 76-mm (3-inch) diameter cylinder is considered to be 

the control specimen, the results showed that the compressive strength exhibited by the 100-mm 

(4-inch) cube specimens ranged from 1.2 percent below to 8.0 percent above the control 

specimen strength. The strength of the 51-mm (2-inch) cube ranged from 4.6 to 6.1 percent 

above. These increases of less than 10 percent are sufficiently small; therefore, cube tests should 

be able to be used as a direct substitute for cylinder compression tests in some applications. 

Although most owners will probably require cylinder compression tests as the standard strength 

test result reported for the foreseeable future, the UHPC component fabricator internal quality 

control testing using the cube specimens would not be precluded. 

One additional finding from the results presented in section 3.3.4 relates to the size of the 

compression specimen (for either the cylinder or the cube). The results indicate that smaller 

specimens, particularly the 51-mm (2-inch) diameter cylinders and the 51-mm (2-inch) cubes, 

tended to exhibit larger standard deviations. The casting-based heterogeneities (i.e., entrapped air 

voids) in the UHPC that are proportionally larger in smaller specimens are likely behind these 

results. For this reason, the use of compression specimens that have a minimum dimension 

smaller than 76 mm (3 inches) is not recommended. 

4.6 SHRINKAGE BEHAVIOR OF UHPC 

A significant portion of this research program has focused on the tensile behaviors of UHPC and 

the applicability of those tensile behaviors to structural engineering. Assurance of those tensile 

properties in a completed structural member is a factor of many things, including both 

casting/placing techniques and curing treatments. Another important factor is the formwork and 

its ability to restrain shrinkage forces. 

The test results presented in section 3.7 indicate that UHPC exhibits shrinkage behaviors that are 

somewhat different than those exhibited by normal concrete. Overall long-term shrinkage of 

UHPC is somewhat large, with an asymptotic value nearing 850 microstrain. More importantly, 

the results indicate that UHPC shrinks very rapidly in the 24 hours after the initiation of setting 

behaviors. This rapid shrinkage can include up to 400 microstrain of shrinkage during this time 

frame with a shrinkage rate of up to 60 microstrain per hour. The shrinkage continues at an 

increased rate as compared to normal concrete with 95 percent of its ultimate shrinkage 

occurring by 2 months after casting. 

These early age shrinkage behaviors are important to the successful casting of UHPC structural 

members, because UHPC will crack at tensile strains significantly below these shrinkage strains. 

Thus, the casting of UHPC must mitigate or eliminate shrinkage restraints on the cast member. 

For highway bridges, standard I-shaped girders are not particularly susceptible to restrained 
156

shrinkage cracking. However, in the casting of a member with restrained areas, such as a doubletee section or a box section, special care must be taken to allow for monitoring and the release of 

shrinkage strains. 

4.7 LONG-TERM STABILITY OF UHPC 

The basic premise behind the application of prestressing forces into concrete girders is that the 

application of an eccentric compressive axial force onto a beam will delay inelastic behaviors 

and increase the ultimate capacity. To achieve these goals, the prestressing forces must be 

maintained indefinitely. Time-dependent concrete behaviors such as creep and shrinkage can 

both negatively impact the maintenance of the prestressing force, thus reducing the enhancement 

provided by the prestress. This section addresses the general long-term stability of UHPC, 

including the creep and shrinkage behaviors that can be expected and the possible methods to 

alleviate those behaviors. 

In large-scale, prestressed concrete girder fabrication facilities, the time required to fabricate a 

girder from initial form setup to cutting of strands and removing the girder from the form is 

critical. This amount of time determines the turnover for the casting bed and directly influences 

the cost of each girder. In general, precasters want to create a rapid turnover, thus they are 

interested in stressing the girder as soon as possible after casting. The results of this method, for 

any concrete, are that the expected creep and shrinkage values will be larger and the prestress 

losses will be greater. 

The shrinkage behavior of UHPC has been discussed in sections 3.7 and 4.6. UHPC can be 

expected to undergo nearly half (400 microstrain) of its long-term shrinkage (850 microstrain) 

during the first 24 hours after setting. Also, steam treatment will hasten the achievement of the 

ultimate shrinkage value and will effectively stabilize the UHPC against further shrinkage 

indefinitely. For these reasons, delaying the stressing of the girder is clearly beneficial. 

The creep behavior is also influenced by the concrete strength at girder stressing. The long-term 

creep results from section 3.8 indicate that creep coefficients between 0.3 and 0.8 can be 

expected for UHPC that is treated according to one of the curing regimes. However, the prestress 

would likely be applied before any of these treatments are completed in a production 

environment. For this reason, the creep behavior at earlier ages, and thus at lower compressive 

strengths, is important. The early age creep testing presented in section 3.8.2 focuses on this 

behavior for UHPC that is still of relatively low strength. Creep at two compressive strength 

levels was investigated, namely 62 and 86 MPa (9 and 12.5 ksi). These results indicate that 

significantly less creep will occur after moderate increases in strength; however, the creep strains 

observed only 30 minutes after loading a 84 MPa (12.25 ksi) cylinder to 70 MPa (10.15 ksi) 

were still over 50 percent of the elastic strains from the initial loading. This 30-minute effective 

creep coefficient of 0.52 is quite high considering that the long-term final creep coefficient for a 

114-MPa (16.5-ksi) UHPC loaded after 28 days of curing is only 0.78. 

Similar to the shrinkage results discussed above, the creep results indicate that delaying the 

stressing of the girder until higher strengths are achieved can be beneficial in terms of long-term 

prestressed girder behavior. These higher strengths and the better resistance to creep can come 
157

either from purely delaying the stressing as in the untreated regime or from steam treating the 

UHPC prior to application of the prestress. 

4.8 MODULUS OF ELASTICITY OF UHPC 

Because compression testing of cylinders is a frequently used quality control method for 

structural concrete, engineers often attempt to relate other characteristics of concrete’s behavior 

to the compression test results. Many researchers have performed work that focuses on the 

relationship between the compressive strength of concrete and its elastic modulus. This section 

compares the strength and modulus results of this test program with the results from predictor 

equations developed elsewhere. Note that this discussion is presented primarily in English units, 

as these correlation relationships are widely known in the United States in this format, and a 

conversion to metric units would lessen the relevance of the results. 

Recall that the modulus of elasticity testing was completed on compression cylinders according 

to the ASTM C469 test method. The results from these tests were presented in section 3.3. Table 

37 summarizes the overall 28-day results from these tests for each curing regime along with the 

compressive strength values. 

Table 37. Compressive strength and modulus of elasticity results. 

Curing 

28-Day Compressive Strength 

(MPa (ksi)) 

Modulus of Elasticity 

(GPa (ksi)) 

Steam 193 (28.0) 52.7 (7,650) 

Untreated 126 (18.3) 42.7 (6,200) 

Tempered Steam 171 (24.8) 51.0 (7,400) 

Delayed Steam 171 (24.8) 50.3 (7,300) 

The American Concrete Institute’s Building Code and Commentary (ACI 318) provides two 

relationships for the modulus of elasticity.(65) The equation in figure 138 shows the first 

relationship wherein the square root of the compressive strength of concrete is related to the 

modulus of elasticity through a scalar factor. In this equation, both the compressive strength, fc′, 

and the modulus of elasticity, E, are in psi. This equation was derived from, and is most relevant 

to, normal strength and normal weight concrete. 

Figure 138. Equation. ACI 318 approximation of modulus of elasticity. 

ACI 318 provides a second relationship for the modulus of elasticity wherein the unit weight of 

the concrete is included. This modification allows for estimation of the modulus of elasticity for 

concrete with a unit weight between 1,440 and 2,480 kg/m3

 (90 and 155 lb/ft3). This 

modification of the estimation equation is important, because both the weight of the concrete and 

the modulus of elasticity are normally heavily dependent on the type of aggregate used. The 

equation in figure 139 presents this relationship in English units, with ρ as the unit weight of 

concrete in lb/ft3

. Recall that the unit weight of UHPC is approximately 2,480 kg/m3 (155 lb/ft3). 

' 57000 c E = f
158

Using this unit weight, this equation would transform into an equation similar to the one in figure 

138 with a scalar of 63,700. Note that the equation shown in figure 139 is also the relationship 

provided in the AASHTO Load and Resistance Factor Design (LRFD) Bridge Design 

Specification.(66)

 

Figure 139. Equation. ACI 318 approximation of modulus of elasticity including density. 

The Comité Européen du Beton has presented a different relationship between the compressive 

strength and the modulus of elasticity.(20) This relationship is shown in figure 140 for metric units 

and in figure 141 after being converted into English units (psi). 

Figure 140. Equation. Comité Européen du Beton approximation for modulus 

of elasticity (metric units). 

Figure 141. Equation. Comité Européen du Beton approximation for modulus 

of elasticity (English units). 

Another relationship between the compressive strength and the modulus of elasticity was 

developed by Kakizaki et al.(67) This research focused on high-strength concretes (83 to 138 MPa 

(12 to 20 ksi)). After an algebraic manipulation into English units (psi), the equation can be 

expressed as shown in figure 142. 

Figure 142. Equation. Kakizaki approximation for modulus of elasticity. 

ACI 363 presents a relationship between the compressive strength and the modulus of elasticity 

that was developed specifically for high strength concretes.(68) In particular, this relationship was 

developed for concretes up to 83 MPa (12 ksi). It is shown in figure 143. 

Figure 143. Equation. ACI 363 approximation for modulus of elasticity. 

Finally, Ma et al. developed an equation based on experimental results from another UHPC 

containing no coarse aggregates.(69) The relationship is shown in figure 144. 

Figure 144. Equation. Ma approximation for modulus of elasticity. 

1.5 ' 33 c E = ρ f

' 0.33 = 9.5( + 8) c E f

' 0.33 = 266600( + 1160) c E f

' 43960 c E = f

3

'

10

525000 c f E =

40000 1000000 ' E = f c +
159

A comparison of these published relationships to the data obtained in this study indicates that 

some relationships are more applicable than others. Figure 145 plots the equations presented in 

figures 138, 141, 142, 143, 144, and 146, as well as the data presented in table 37. Additional 

data from untreated cylinders less than 28 days old is not shown in this figure, but was included 

in the analysis and is shown in the more detailed figure 147. It is clear that the ACI 318 and the 

Comité Européen du Beton equations overestimate the modulus of elasticity in this strength 

range; however, it must be stated that this strength range is well outside the typical applicability 

of these equations. The Kakizaki equation underestimates the results with a reasonable level of 

accuracy. The ACI 363 and Ma equations provide the best approximations of any published 

equations, as will be discussed below. 

Figure 145. Graph. Modulus of elasticity as a function of 28-day compressive strength. 

0

2000

4000

6000

8000

0 5 10 15 20 25 30

28-day Compressive Strength (ksi)

Modulus of Elasticity (ksi) m

Steam

Untreated

Tempered Steam

Delayed Steam

57000(fc')^0.5

9.5(fc'+8)^0.33

3.65(fc')^0.5

40000(fc)^0.5+1M

243500(fc)^(1/3)

46200(fc')^0.5

57000 fc'

0.5

46200 fc'

0.5

525000 (fc'/10)0.33

43960 fc'

0.5

266600 (fc'+1160)

0.33

40000 fc'

0.5+1000000

1 ksi = 6.895 MPa
160

Figure 146. Equation. Approximation for UHPC modulus of elasticity (in psi). 

All of these equations have a similar form, some differing only by the scalar factor. The UHPC 

modulus of elasticity results were reevaluated to determine what type of relationship and what 

scalar factor would provide the best fit to the observed results. It was determined that the form of 

the ACI 318 equation accurately represented the shape of the UHPC relationship and only a 

modification of the scalar factor was required. The result of this analysis is the equation shown in 

figure 146 which has an R-squared value of 0.967 for the UHPC experimental results with 

compressive strengths above 25 MPa (3.6 ksi). This relationship is also shown in figures 145 and 

147. 

Figure 147 presents the most relevant equations along with the data that was used in this 

analysis. The figure also included the data from compressive strengths below 25 MPa (3.6 ksi) 

which was excluded from this analysis. The figure shows that the equation in figure 146 is 

applicable to UHPC over a wide range of strengths, not just to the final strength values between 

124 and 193 MPa (18 and 28 ksi). Therefore, this equation is considered applicable over the 

compressive strength range of 28 to 193 MPa (4 to 28 ksi). Note that the ACI 363 and Ma 

equations displayed lower R-squared values (0.957 and 0.881, respectively), and they both 

overestimate the observed modulus of elasticity results at a large margin at lower compressive 

strengths. 

Figure 147. Graph. Modulus of elasticity as a function of compressive strength. 

0

1000

2000

3000

4000

5000

6000

7000

8000

0 5 10 15 20 25 30

Compressive Strength (ksi)

Modulus of Elasticity (ksi) m

Steam

Untreated

Tempered Steam

Delayed Steam

Untreated (< 28 days)

Figure 148

Figure 144

Figure 143

Figure 146

1 ksi = 6.895 MPa

' 46200 c E = f
161

The equation in figure 146 does not accurately estimate the modulus of elasticity for UHPC with 

a compressive strength in the 7 to 21 MPa (1 to 3 ksi) range. The equation can overestimate the 

modulus of elasticity by as much as 50 percent. To rectify this lapse in the applicability of the 

equation, a more sophisticated equation was used to approximate the untreated curing regime 

data shown in figure 147. A best-fit regression analysis was applied to these data to determine a 

function that accurately represented the behavior. The simplest function to meet the requirements 

was the Log Normal function as presented in figure 148. In this equation, fc′ is the compressive 

strength of the concrete at a particular age and E is the corresponding modulus of elasticity, both 

in psi. This equation can be used to approximate the modulus of elasticity for compressive 

strengths as large as 131 MPa (19 ksi).

Figure 148. Equation. UHPC modulus of elasticity approximation (in psi) 

for compressive strengths up to 131 MPa (19 ksi). 

4.9 COMPRESSIVE STRESS-STRAIN BEHAVIOR OF UHPC 

The compressive stress-strain behavior of UHPC has been discussed extensively in this report, 

including the material property characterization results presented in section 3.3 and a further 

analysis of those results presented in section 4.8. The current section presents an analysis 

wherein an equation is determined that represents the ascending branch of the compressive 

stress-strain response for each of the four curing regimes. This analysis is based on the 

experimental results presented in section 3.3.3. 

The compressive stress-strain responses of different concretes vary because concrete is a 

heterogeneous material without standardized mix designs. Many researchers have presented 

analytical approximations for the ascending branch of the compressive stress-strain behavior, all 

of which have been based on specific sets of experimental data. However, this body of research 

has resulted in minimal consensus on the equation’s formulation or results.(20) 

From an experimental standpoint, gathering consistent, accurate data from the full range of 

compressive behavior response is very difficult. This fact is primarily due to the increasingly 

nonlinear behaviors that concrete tends to exhibit as the strain at the compressive strength is 

reached and surpassed. Even if the descending branch of the behavior is ignored as the 

compressive strength is approached, the observed straining behavior of the concrete becomes 

very dependent on the experimental loading and strain measurement techniques that have been 

used. 

The preceding discussion leads to the conclusion that any strength values and strain values from 

earlier in the concrete response are likely more accurate than strain values from later in the 

concrete response. The concrete compressive strength, fc′, and the concrete modulus of elasticity, 

⎥

⎥

⎥

⎥

⎥

⎦

⎤

⎢

⎢

⎢

⎢

⎢

⎣

⎡

⎟

⎟

⎟

⎟

⎟

⎠

⎞

⎜

⎜

⎜

⎜

⎜

⎝

⎛

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

= −

2 '

1.7

44000

ln

2

1 7100000exp

c f

E
162

E, can both be considered to be relatively accurate based on experimental results. However, the 

concrete strain at the compressive strength and the associated secant modulus are both based on 

strain measurements that are more difficult to capture accurately and are thus less accurately 

known. This fact points to a weakness of many other models of concrete compressive stressstrain behavior, because they are based on an accurate knowledge of the compressive strain at the 

peak strength. 

Recall from section 3.3 that a number of specific parameters were quantified within the 

compressive stress-strain responses of the UHPC. Intermediate stress and strain level 

benchmarks were identified in addition to the standard strength and stiffness measures. These 

benchmarks were defined by the decrease in observed stress compared with a theoretical linear 

elastic response at a particular strain. Average results for the 1, 3, and 5 percent decrease levels 

were presented in table 11. For this analysis, additional benchmarks were defined at the 

10 percent decrease level and at the cessation of the continuous, steady increase in stress-strain 

response. Figure 149 illustrates the stress decrease factor compared with the linear elastic 

behavior, α, on a sample untreated UHPC compressive stress-strain curve. Sample actual stress, 

fc, and strain, εc, values are shown along with the stress and strain differences from the linear 

elastic response. 
163

1 MPa = 0.145 ksi 

Figure 149. Graph. Compressive stress-strain behavior compared 

with linear elastic response. 

These benchmarks were used to define the behavior of each compressive specimen. The results 

were then normalized based on the compressive strength and modulus of each specimen. Note 

that although the compressive stress normalization is based on the experimentally obtained 

compressive strength, the strain normalization is based on the theoretical linear elastic strain at 

the compressive strength. This normalization technique minimizes the inaccuracies discussed 

above by avoiding the use of the actual strain at the compressive strength. Finally, the results 

from each curing regime were compiled into individual data sets. 

Figure 150 presents the normalized stress-strain response benchmark data points for the steamtreated UHPC cylinders discussed in section 3.3. This figure shows the general shape that the 

approximation must match to accurately represent the UHPC behavior. However, it does not 

allow for easy differentiation between potential curves that fit the data. A more accurate 

representation of the overall behavior can be obtained by focusing on the deviation of the actual 

behavior of the concrete compared with the theoretical linear elastic response. Figure 151 

presents the same benchmarks in terms of the decrease from the linear elastic response compared 

with the normalized strain. This presentation highlights the behaviors that must be captured in 

the model. 

0

20

40

60

80

100

120

140

0.000 0.001 0.002 0.003 0.004

Axial Strain

Compressive Stress (MPa)

E

1

(ε c,fc)

αε c

αε cE
164

Figure 150. Graph. Normalized compressive stress-strain results for steam-treated UHPC. 

Figures similar to figure 151 were created for the other three curing regimes as well. In all cases, 

the figures indicate that the UHPC remains close to the theoretical linear elastic response through 

much of its behavior. Approximation curves were fit to each of these data sets. The simplest 

equation that fits the data moderately well was an exponential function with two constants as 

provided in figure 152. These constants, a and b, help fit the equation to the data, and their 

values for each curing regime are provided in table 38. The approximation curve for the steamtreated UHPC is shown in figure 151. 

The analysis discussed above leads to a simple means of defining the ascending branch of the 

compressive stress-strain response of UHPC. Figure 153 presents the equation for the standard 

linear elastic relationship between stress and strain with the inclusion of a modifying factor. This 

factor, specifically (1-α), determines the extent to which the actual curve deviates from the linear 

elastic response. Recall that α is defined by the equation in figure 152. Dividing the equation in 

figure 153 by the compressive strength normalizes the equation and allows for the direct 

inclusion of the equation in figure 152 and therefore a single equation defining the normalized 

compressive stress in terms of the normalized compressive strain. Recall that this equation was 

derived to be applicable only for the ascending branch of the curve. 

These relationships were used to create general stress-strain responses for the four curing 

regimes. These responses were based on the compressive strength and modulus of elasticity 

results presented in chapter 3. The curves are shown in figure 154. 

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Compressive Strain (Normalized by fc'/E)

Compressive Stress

(Normalized by f c')

1

1
165

Figure 151. Graph. Deviation from linear elastic compressive behavior 

for steam-treated UHPC. 

Figure 152. Equation. Deviation of compressive stress-strain response 

from linear elastic behavior. 

Table 38. Constants for equation in figure 152. 

Curing Regime A b 

Steam 0.0010 0.243 

Untreated 0.0114 0.440 

Tempered Steam 0.0041 0.341 

Delayed Steam 0.0044 0.358 

Figure 153. Equation. Compressive stress-strain behavior defined as a function of the 

deviation from linear elastic behavior. 

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

Compressive Strain (Normalized by fc'/E)

α

ae a c

c

bf

E

= −

'

ε

α

f = ε E(1−α) c c
166

1 MPa = 145 psi 

Figure 154. Graph. Compressive stress-strain response approximations. 

0

50

100

150

200

0.000 0.001 0.002 0.003 0.004 0.005

Compressive Strain

Compressive Stress (MPa)

Untreated

Steam

Tempered Steam

Delayed Steam
167

CHAPTER 5. CONCLUSIONS AND FUTURE RESEARCH 

5.1 INTRODUCTION 

UHPC is a new type of concrete that exhibits properties of enhanced strength, durability, and 

long-term stability. The objective of this research was to evaluate the material characteristics of 

UHPC for potential use in highway bridge applications. 

The experimental phase of this research focused on determining the mechanical and durability 

behaviors of UHPC. More than 1,000 individual specimens were tested to determine the material 

characteristics of UHPC. The tests determined the compressive and tensile behaviors, the longterm stability, and the durability of UHPC. The analytical phase of this research combined, 

analyzed, and elaborated on the results from the experimental phase. This phase included 

developing predictor equations for some basic properties of UHPC. 

The conclusions of this study are presented in section 5.2. A brief discussion of ongoing and 

potential future research topics follows in section 5.3. 

5.2 CONCLUSIONS 

The following conclusions are based on the research presented in this report. 

1. UHPC displays significantly enhanced material properties compared with normal and 

HPC. 

2. Steam-based treatment of UHPC tends to significantly enhance its material 

properties. Three steam-based treatments—steam, delayed steam, and tempered 

steam—were investigated and compared with UHPC that was not subjected to a 

curing treatment after casting. In general terms, steam treatment increases UHPC’s 

compressive strength by 53 percent to 193 MPa (28 ksi), increases its modulus of 

elasticity by 23 percent to 52.4 GPa (7,600 ksi), decreases its creep coefficient from 

0.78 to 0.29, and virtually eliminates long-term shrinkage. Steam treatment also 

decreases chloride ion penetrability to a negligible level and significantly enhances 

abrasion resistance. The enhancements of material properties affected by the delayed 

steam and tempered steam treatments are similar to those of the steam treatment but 

of a slightly lesser magnitude. 

3. UHPC exhibits very high compressive strengths, regardless of the curing treatment 

applied. The average 28-day compressive strengths of steam, delayed steam, 

tempered steam, and untreated UHPC were found to be 193, 171, 171, and 126 MPa 

(28.0, 24.8, 24.8, and 18.3 ksi). The compressive strength of steam-treated UHPC was 

found to have stabilized by the completion of the curing treatment. Thus, steamtreated UHPC can reach its full compressive strength within 4 days after casting. 

4. The mixing time and rheological properties of fresh UHPC are influenced by the 

concrete mixer design, the ambient environmental conditions in the mixer, and the 

elapsed time since blending of the premix. A 1934 vintage pan mixer was used 
168

successfully to mix the UHPC; however, the inability of this mixer to impart 

significant energy into the mix resulted in extended mixing times. Low humidity 

within the mixer and in the mix room can result in stiffer UHPC. Older UHPC premix 

requires more mixing to achieve the correct rheological properties, likely due to the 

agglomeration of fine particles in the premix during storage. 

5. The set time of UHPC is significantly delayed compared with normal concrete; final 

set does not occur until 12 to 24 hours after casting. This time to set could also be 

longer depending on the admixtures and on other constituents in the mix. 

6. Once setting has initiated, UHPC gains compressive strength very rapidly. If 

maintained at normal laboratory temperatures, UHPC compressive strength will 

increase to over 70 MPa (10 ksi) by 2 days after setting. Subsequently, the rate of 

strength gain will decrease; 97 MPa (14 ksi) will be reached by 10 days after setting. 

7. The compressive strength of UHPC is not affected by the specimen geometry used to 

determine the result. Cylinders with 51-, 76-, and 102-mm (2-, 3-, and 4-inch) 

diameters were tested according to ASTM C39, and 51-mm (2-inch) and 100-mm 

(4-inch) cubes were tested according to ASTM C109. The testing was conducted for 

two batches of steam-treated UHPC and for one batch of untreated UHPC. In all 

cases, the compressive strength results did not vary by more than 8 percent from the 

76-mm (3-inch) cylinder control result. However, the 51-mm (2-inch) cubes and 

cylinders did tend to exhibit a larger standard deviation. The minimal preparation 

requirements for a cube specimen and the similarity of results mentioned above make 

the 100-mm (4-inch) cube a viable specimen geometry for UHPC compressive 

strength determination. 

8. The curing conditions present during and just after the setting of UHPC can 

significantly affect the final properties of the concrete. In the untreated case, concrete 

cylinders that were stripped as final set was being reached exhibited 25 percent lower 

28-day compressive strengths compared with those stripped 1 day after setting was 

complete. A 30-percent difference was observed in the steam-treated case. These 

strength differences are likely due to the relatively more permeable nature of UHPC 

at earlier ages combined with the very low moisture content in the UHPC. This low 

moisture content results in a loss of water to the surrounding atmosphere and thus 

reduced hydration of the concrete. 

9. The tensile strength of UHPC, both before and after tensile cracking, is significantly 

higher than the strength that occurs in normal concrete. Four test methods were 

implemented to capture the tensile strength of UHPC. The combined results of these 

tests indicate that the tensile cracking strength of UHPC is approximately 9.0 MPa 

(1.3 ksi) after the steam-based curing treatment and approximately 6.2 MPa (0.9 ksi) 

without any treatment. Qualitatively, UHPC was observed to exhibit similar levels of 

tensile strength after cracking; however, in general, these tests were unable to indicate 

specific postcracking strengths. 
169

10. The ASTM C496 split-cylinder tension test, modified to capture first cracking, 

provides the clearest indication of the tensile cracking strength of UHPC. Through the 

use of a hydraulically controlled test machine and minimal instrumentation, the 

tensile cracking strength of UHPC can be obtained. Other tensile tests require more 

extensive instrumentation, specialization of specimen geometry, and more 

sophisticated loading equipment. Unfortunately, the split-cylinder test is not useful 

for determining the postcracking tensile stress capacity, because the biaxial state of 

stress applied to the cylinder does not accurately mimic the conditions that are 

normally associated with tensile regions in a structural member. 

11. The modulus of rupture defined by the ASTM C1018 prism flexure test overestimates 

the tensile cracking strength of UHPC by approximately 60 percent. This result was 

confirmed through the completion of this test on 51-mm (2-inch) square cross 

sections with 152-, 229-, 305-, and 381-mm (6-, 9-, 12-, and 15-inch) spans, and on 

76- by 102-mm (3- by 4-inch) cross sections with a 305-mm (12-inch) span. 

12. The ASTM C1018 prism flexure test provides a clear means of comparing the 

postcracking tensile behavior of various fiber-reinforced concretes. UHPC, regardless 

of curing treatment, performed exceptionally well according to the toughness indices 

defined by this test. For example, the I20 toughness index normally ranges from 1 to 

25 for fiber-reinforced concretes. In UHPC, the I20 results ranged from 28 to 32. 

Although these results cannot be directly reinterpreted to apply to full-scale structural 

members, they do indicate that UHPC can continue to carry significant tensile loads 

after cracking. 

13. UHPC displays durability properties that are significantly beyond those normally 

associated with concrete. Regardless of the curing treatment, the ASTM C666 relative 

dynamic modulus was at least 95 percent of the original value after more than 600 

freeze-thaw cycles. UHPC exhibited no scaling under the ASTM C672 test, even after 

undergoing approximately 200 cycles. The chloride ion penetrability as measured by 

ASTM C1202 was below 50 coulombs for UHPC that had undergone the steam-based 

treatment, and was 360 coulombs for untreated UHPC 28 days after casting. The 

untreated UHPC results dropped to 76 coulombs by 56 days after casting. UHPC was 

found to be innocuous to alkali-silica reaction. 

14. Exposing cracked UHPC split cylinders to an aggressive environment did not result in 

any noticeable decrease in the peak tensile load-carrying capacity. Tight cracks, as 

might be observed in a highly stressed tensile flexural region of an I-girder, were 

created by loading cylinders in a split-cylinder configuration. These cracks were on 

the order of 0.005 mm (0.0002 inch) wide. A cracked face of the cylinder was then 

ponded with a sodium chloride solution as specified in AASHTO T259. After 90 days 

of ponding, the cylinders were tested for peak split-cylinder tensile strength. The peak 

load-carrying capacity of either steam-treated or untreated UHPC did not have a 

discernable decrease after cracking, thus indicating that the sodium chloride solution 

did not enter the cracks and did not cause the fiber reinforcement to deteriorate. 
170

15. UHPC exhibits shrinkage behaviors that are somewhat different from those of normal 

concrete. In total, UHPC tends to exhibit approximately 800 microstrain of shrinkage 

as measured from casting through 1 year. However, shrinkage initiation is affected by 

the delayed set times associated with UHPC, and the majority of the shrinkage occurs 

in a short time frame just after the concrete has set. Unrestrained shrinkage rates of 

over 60 microstrain per hour were observed during the period of rapid strength gain 

just after setting. Without any curing treatment, UHPC will continue to shrink at an 

ever-decreasing rate. Steam treatment accelerates the shrinkage to such an extent that 

the entirety of the shrinkage occurs during the 2-day treatment, and the UHPC is then 

stabilized against further shrinkage. Also, the total shrinkage in steam-treated UHPC 

tends to be slightly higher than the asymptotic shrinkage approached by untreated 

UHPC. 

16. Large compressive stresses on relatively low-strength UHPC can cause significant 

short-term creep. This situation is akin to the stressing of prestressed girders. Eight to 

13 ksi compressive strength UHPC was loaded to compressive stresses between 60 

percent and 90 percent of the strength. During the 30 minutes following the load 

application, the UHPC exhibited 30-minute creep coefficients between 0.32 and 0.85. 

UHPC loaded to over 90 percent of its compressive strength failed under the 

sustained load. The creep that occurred over this short load duration indicates that the 

total long-term creep of UHPC loaded at this compressive strength would be much 

higher than that observed in the long-term creep testing. 

5.3 ONGOING AND FUTURE RESEARCH 

The findings from this report suggest a number of potential topics for future research: 

1. Develop optimized bridge girders that take advantage of the material properties of 

UHPC. These bridge girders should use the tensile and compressive capacities of 

UHPC, while also enhancing the design life of the bridge as a whole by eliminating 

many of the less durable components of a normal bridge. 

2. Fabricate full-scale, optimized UHPC bridge girders to resolve problems associated 

with casting slender concrete members with fiber-reinforced concrete. 

3. Develop a practical test to quantitatively determine the postcracking uniaxial tensile 

behavior of UHPC. 

The research program discussed herein has already been extended to encompass a portion of the 

topics listed above. 
171

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