NISTIR 6399

Deterioration of Iowa Highway Concrete

Pavements: A Petrographic Study

Paul E. Stutzman

Building and Fire Research Laboratory

Gaithersburg, Maryland 20899

United States Department of Commerce

Technology Administration

National Institute of Standards and Technology

NISTIR 6399

Deterioration of Iowa Highway Concrete

Pavements: A Petrographic Study

Paul E. Stutzman

December, 1999

Building and Fire Research Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899

National Institute of Standards and Technology

William M. Daley, Secretary

Technology Administration

Dr. Cheryl L. Shavers, Under Secretary of Commerce for Technology

National Institute of Standards and Technology

Ray Kammer, Director

v

Executive Summary

Major highway concrete pavements in Iowa have exhibited premature deterioration. The

cause(s) of the deterioration has been the subject of much controversy. Various

investigators have used different methods to characterize the processes and have arrived at

different interpretations. The deterioration has been attributed to effects of ettringite

formation (including delayed ettringite formation), alkali-silica expansive reactions, and to

frost attack, or some combination of them. Evidence for all three processes have been

reported. While one or more of these processes is likely to be the cause of the deterioration,

the possibility that there may be other deleterious processes should be considered.

Highway pavements included in this study were constructed in the mid-1980s as nonreinforced, dual-lane, roads ranging in thickness between 200 mm and 300 mm, with

skewed joints reinforced with dowels. Deterioration was initially recognized with a

darkening of joint regions, which occurred for some pavements as soon as four years after

construction. Pavement condition ranges from severe damage to none, and there appeared

to be no unequivocal materials or processing variables correlated with failure.

Based upon visual examinations, petrographic evaluation and application of materials

models the deterioration of concrete highway pavements in Iowa appear related to a freezethaw failure of the coarse aggregate and mortar. For concrete cores extracted from sections

showing no apparent, to minor damage, cracking is initially noted in the coarse aggregate,

and then in the mortar. Crack patterns sub-parallel to the concrete surface transecting the

mortar fraction and the coarse aggregate are indicative of freeze-thaw damage.

The entrained air void system was marginal to substandard, according to guidelines in ACI

212.3R (Chapter 2), and filling of some of the finer-sized voids by ettringite further

degraded the air void system. The filling increases the void spacing factor and decreases

specific surface values beyond those considered necessary for concrete to have adequate

durability when exposed to freeze-thaw conditions. The formation of secondary ettringite

within the entrained air voids probably reflects a relatively high degree of concrete

saturation causing the smaller voids to fill with pore solution when the concrete freezes.

Freeze-thaw cracking of aggregate may be related to both the high degree of saturation and

the loss of the adjacent air void system as no correlation between aggregate texture and

cracking was noted. Modeling simulations of deterioration processes indicate that cracking

may be explained by both aggregate and paste expansions.

The original air void system parameters generally appear to be uniform from top to base of

the concrete. The excessive vibration, considered by some to be contributory [9], appears

to not have adversely affected the entrained air void system. Specimens taken from the

visible "vibrator trails", which mark the insertion points of an immersion probe vibrator,

show a marked increase in the air void spacing factor to a depth of about 15 cm.

Measurement of the void filling shows it to be either uniform or, in some cases, increasing

with depth, suggesting that the degradation is not a “top down” process by infiltration of

gypsum-contaminated road salt solutions. These trends may also reflect an influence of

surface drying and / or a source of water at the base of the slabs.
vi

Microcracking is common in most of the cores occurring as vertical surface cracks

extending 10 mm to 20 mm into the core. These cracks may be found both exclusively in

the cement paste and also in the paste and coarse aggregates, and may be drying shrinkagerelated and / or due to freeze-thaw cycling. Cracks along the vibration trails appear to be

drying shrinkage cracks resulting from increased shrinkage potential of the mortar-rich

vibration trail regions relative to the bulk concrete.

Other failure processes proposed include alkali-silica reaction (ASR) and delayed ettringite

formation (DEF). ASR affects the shale and an occasional quartz sand grain in the fine

aggregate, but is not deemed to be a significant cause of the deterioration. The field survey

found no evidence of permanent expansion such as closure of joints, blowups, nor gel

efflorescence. Significant cracking associated with ASR may occasionally be found when

large (5mm) shale aggregates are located in the upper-most 20 mm of the concrete. Delayed

ettringite formation was not deemed likely as no evidence of a uniform paste expansion

away from aggregates was observed, no massive agglomerations of ettringite in the paste,

near aggregates, or in cracks, and no filling of the capillary pore system was observed. The

lack of observed expansion, as mentioned above, is also evidence against this mode of

deterioration.

The utilization of fly ash does not appear to have affected the deterioration as all pavements

with or without fly ash exhibiting substantial damage also exhibit significant filling of the

entrained air void system, and specimens containing fly ash from sound pavements do not

have significant filling.

US 20 pavements are among some of the best and worst performing pavements in this

study. A set of specimens, reportedly containing the same materials (including fly ash) and

the same mix design, exhibit opposite performance. Cores from the undamaged stretch of

the road appear to have a smaller coarse aggregate maximum size and a more uniform

aggregate gradation. These cores contain an entrained air content almost double, and a

spacing factor half that of the failed pavement. The additional air volume may account for

the improvement in spacing factor and resulted in an air system adequate for freeze-thaw

protection. However, specific surface values still indicate a coarse-sized entrained air void

system. The change in aggregate gradation may have altered the mixture rheology

facilitating development of an improved entrained air void system.

The influence of the mixture design, mixing, and placing must be evaluated with respect to

development of an adequate entrained air void system, concrete homogeneity, long-term

drying shrinkage, and microcracking. A high-sand mix may have contributed to the fieldobserved harsh mixture characteristics and exacerbate concrete heterogeneity, difficulty in

developing an adequate entrained air void system, poor consolidation potential, and

increased drying shrinkage and cracking.

Finally, the availability of moisture, in particular to the base of the pavement slab, must also

be considered, as the secondary precipitation of ettringite in entrained air voids indicates

they were at least partially filled with some pore solution at times. Freeze-thaw failure of

concrete occurs when a critical level of saturation is reached. Water available at the base of

the slabs, in joints, and cracks may have been wicked into the slabs to a point of critical

saturation.
vii

Abstract

Major highway concrete pavements in Iowa have exhibited premature deterioration

attributed to effects of ettringite formation, alkali-silica expansive reactions, and to frost

attack, or some combination of them. These pavements were constructed in the mid1980s as non-reinforced, dual-lane, roads ranging in thickness between 200 mm and 300

mm, with skewed joints reinforced with dowels. Deterioration was initially recognized

with a darkening of joint regions, which occurred for some pavements as soon as four

years after construction. Pavement condition ranges from severe damage to none, and

there appeared to be no unequivocal materials or processing variables correlated with

failure.

Based upon visual examinations, petrographic evaluation, and application of materials

models, the deterioration of concrete highway pavements in Iowa appear related to a

freeze-thaw failure of the coarse aggregate and the mortar. Crack patterns sub-parallel to

the concrete surface transecting the mortar fraction and the coarse aggregate are indicative

of freeze-thaw damage of both the mortar and aggregate. The entrained air void system

was marginal to substandard, and filling of some of the finer-sized voids by ettringite

appears to have further degraded the air void system. The formation of secondary

ettringite within the entrained air voids probably reflects a relatively high degree of

concrete saturation causing the smaller voids to be filled with pore solution when the

concrete freezes. Alkali-silica reaction (ASR) affects some quartz and shale in the fine

aggregate, but is not considered to be a significant cause of the deterioration. Delayed

ettringite formation was not deemed likely as no evidence of a uniform paste expansion

was observed. The lack of field-observed expansion is also evidence against the ASR and

DEF modes of deterioration. The utilization of fly ash does not appear to have affected the

deterioration as all pavements with or without fly ash exhibiting substantial damage also

exhibit significant filling of the entrained air void system, and specimens containing fly

ash from sound pavements do not have significant filling.

The influence of the mixture design, mixing, and placing must be evaluated with respect

to development of an adequate entrained air void system, concrete homogeneity, longterm drying shrinkage, and microcracking. A high-sand mix may have contributed to the

difficult mixture characteristics noted upon placement and exacerbate concrete

heterogeneity problems, difficulty in developing an adequate entrained air void system,

poor consolidation potential, and increased drying shrinkage and cracking. Finally, the

availability of moisture must also be considered, as the secondary precipitation of

ettringite in entrained air voids indicates they were at least partially filled with pore

solution at times. Water availability at the base of the slabs, in joints, and cracks may have

provided a means for absorbing water to a point of critical saturation.
viii

Table of Contents

Executive Summary ............................................................................................................. v

Abstract ..............................................................................................................................vii

1. INTRODUCTION......................................................................................................... 1

1.1 Background................................................................................................................. 1

1.2 Objective of the Study and Project Plans.................................................................... 1

1.3 Previous Studies.......................................................................................................... 2

2. TECHNICAL APPROACH............................................................................................. 4

2.1 Field Observations of Pavements................................................................................ 4

2.2 Analysis of Concrete Mixture Design......................................................................... 8

2.3 Cement Composition Analyses................................................................................ 10

2.4 Specimen Preparation ............................................................................................... 12

2.5 Air Void System Analysis ........................................................................................ 12

3. RESULTS....................................................................................................................... 13

3.1 Air Void Analysis ..................................................................................................... 13

3.2 Petrographic Analysis ............................................................................................... 16

3.2.1 Iowa 175 Test Road............................................................................................ 16

3.2.2 Iowa 175 Cores.................................................................................................. 17

3.2.3.1 Site Description US 20, Mile 134.4................................................................. 22

3.2.3.2 Core Descriptions ............................................................................................ 22

3.2.3.2 US 20 Location 2, Webster County................................................................. 29

3.2.3.2.1 Core Descriptions ......................................................................................... 29

3.2.3.3 US 20, Webster County Location 3................................................................. 37

3.2.3.3.1 Core Descriptions ......................................................................................... 37

3.3 Iowa I-35, Story County ........................................................................................... 50

3.4 Iowa I-80................................................................................................................... 58

4. DISCUSSION AND CONCLUSIONS.......................................................................... 61

4.1 Primary Cause of Pavement Deterioration ............................................................... 61

4.2 The Significance of Secondary Ettringite................................................................. 61

4.3 Freezing and Thawing............................................................................................... 63

4.4 Microcracking........................................................................................................... 65

4.5 Alkali-Aggregate Reaction ....................................................................................... 66

4.6 Delayed Ettringite Formation ................................................................................... 66

4.7 Fly Ash...................................................................................................................... 66

5. ACKNOWLEDGEMENTS ........................................................................................... 67

6. REFERENCES............................................................................................................... 69
ix

List of Figures

Figure 1. Onset of pavement deterioration is visible as a slight darkening of the concrete

in the transverse joint region (top, I-80) and in pavements with advanced

deterioration, a tightly-closed pattern cracking (bottom, US 20). Some of these

pavements also have vibrator trails. .............................................................................. 5

Figure 2. Map cracking oriented in the longitudinal direction of the pavement,

interconnected by slightly finer cracks perpendicular to pavement direction becomes

more prominent with progression of deterioration (top). Advanced degradation shows

cracking paralleling the transverse joint and road edge, and along cracks within

vibrator trails. The degradation would be characterized initially as “low” severity

according to the SHRP Distress Identification Manual [17] and “moderate” as the

deterioration progresses................................................................................................. 6

Figure 3. Pavements exhibiting severe deterioration show significant pattern cracking

(here accentuated by moisture along the crack), cracking along joints, spalling along

joint edges and white efflorescence. Open joints and lack of displacement along

pavement slabs provides evidence against permanent expansive processes such as

alkali-aggregate reaction and sulfate attack................................................................... 7

Figure 4. Concrete materials proportions, by mass, for selected sections of Iowa US 20... 8

Figure 5. Aggregate gradation plots showing coarse, fine, and combined aggregate

gradation and the lack of particles in the intermediate size range................................. 9

Figure 6. Optimum aggregate chart plots of Iowa pavement mix designs fall into the gapgraded region and indicate likely difficulties in workability [20]............................... 10

Figure 7. Cement compositions, expressed as mass percent, from chemical analyses from

the Iowa DOT database and using a modified Bogue calculation described in Taylor

[22]. The y-axis has been expanded to more clearly illustrate total silicates and other

phases........................................................................................................................... 11

Figure 8. Potassium permanganate and barium chloride staining colors sulfate phases

purple facilitating their identification. This allows discrimination of both the original

and effective entrained air void systems...................................................................... 13

Figure 9: Iowa 175 Core 2 shows some aggregate / mortar segregation and cracking

located near the core base (base to the right)............................................................... 18

Figure 10. Iowa 175 Core 2 surface microstructure. Slight discoloration in the upper

mortar indicates approximate depth of carbonation. Micrograph field width: 14 mm 18

Figure 11. Iowa 175 Core 2 base mortar microstructure exhibiting partial filling of the

entrained air void system. Micrograph field width: 4 mm ........................................ 19

Figure 12. Iowa 175 Core 2 base microstructure exhibits cracking in the mortar and coarse

aggregate. Micrograph field width: 14 mm................................................................. 19

Figure 13. Iowa 175 Core 2 air void and materials distribution plots. A decrease in air

near the surface may reflect the loss of entrapped air. The blue triangles represent an

air void parameter estimate for the original concrete; the red box represents that value

as affected by void filling, if present. .......................................................................... 20

Figure 14. Core 7 (road surface is to the left) exhibits cracking of coarse aggregate,

vertical cracks from the surface that are either drying shrinkage or freeze thaw-related.

Segregation of mortar and coarse aggregate is visible in the whole core cross section

(upper image) and the upper-core microstructure (lower)........................................... 23

Figure 15. Core 8 exhibits less cracking but does have surface cracking to depths of 20

mm. Some of these cracks as seen in the lower image (13 mm field width) are

associated with cracking within the coarse aggregate................................................. 24
x

Figure 16. Cracking (red) and ASR (yellow) in Core 7 as observed using SEM. Cracking

of aggregate is common. Surface cracks appear carbonated and terminate in both the

mortar and the coarse aggregate. Micrograph field width is approximately 10 cm. .. 25

Figure 17. Filling of the smaller entrained air voids (purple) has significantly increased the

air void spacing factor while only slightly decreasing the total entrained air void

volume for both Cores 7 and 8. Field width: 4 mm..................................................... 25

Figure 18: Air void distribution, spacing factors, and concrete component distribution

versus depth for Core 7. The red boxes represent the current value, the blue triangle,

the original value, and the red line in the spacing factor plot denotes the ASTM C 457

recommended limit for freeze-thaw protection. .......................................................... 26

Figure 19: Air void distribution and spacing factors versus depth, and concrete component

distribution for Core 8, mid-panel. .............................................................................. 27

Figure 20. Core 13 cross sections show segregation within the concrete and only minor

cracking of the mortar and coarse aggregate............................................................... 30

Figure 21. Core 14 exhibits some cracking of both the paste and coarse aggregate and

some mortar / aggregate segregation in middle........................................................... 31

Figure 22. Clustering of air voids may indicate difficulties in mixing and development of a

properly sized, disseminated entrained air void system. Field: 7 mm......................... 32

Figure 23. A SEM image of Core 13 paste shows ettringite-filled entrained air voids, the

irregularly-shaped capillary voids (black), and a reactive shale grain in the lower-left.

Note absence of cracking outside of this shale grain and lack of paste / aggregate gaps

that would be typical of an overall paste expansion. Field: 200 μm. ......................... 32

Figure 24. X-ray images corresponding to image in Figure 23. While the shale has

undergone alkali-silica reaction, no cracking is apparent within the paste. Common

locations of aluminum, sulfur and calcium in the X-ray images delineate ettringite.. 33

Figure 25. Air void distribution and spacing factors versus depth, and concrete component

distribution for Core 13. The air volume appears uniform with depth while the spacing

factor appears substandard in the upper 100 mm. Filling has resulted in a slight

increase in the spacing factor....................................................................................... 34

Figure 26. Air void distribution and spacing factors versus depth, and concrete component

distribution for Core 14. The total air volume appears similar to that of Core 13 from

the joint and appears uniform with depth (deviations from the median probably reflect

entrapped air voids). The spacing factor appears marginal to sub-standard at depths

below 6 cm, and filling of the smaller entrained air voids has resulted in an increase in

void spacing factor....................................................................................................... 35

Figure 27. Core 18 contains about 12 % entrained air volume and a spacing factor of 0.08

mm. Aggregate gradation appears more uniform, and possibly smaller maximum size

than the degraded pavement concretes. Low specific surface and common entrapped

air voids are features common to other cores in this study. ........................................ 38

Figure 28. Air void and component distribution plots for core 18. Little loss of air void

volume is evident with only a slight, uniform loss of air void spacing factor.

Component distribution appears uniform from top to bottom..................................... 39

Figure 29. Core 18 mortar microstructure includes numerous shale fragments that appear

to have undergone alkali-silica reaction, however little cracking of the mortar was

evident. SEM examination indicated that some of the reaction product permeated the

paste filling the capillary voids adjacent to the shale. Field width: 10 mm. ............... 40
xi

Figure 30. Friedel's salt in hardened paste is identified here using X-ray microanalysis. It

appears to be a primary hydration product in the paste throughout the cores,

suggesting that it may be a result of the presence of chlorides in the original mix as

opposed to the infiltration of road salts. ...................................................................... 40

Figure 31. SEM of Core 18 shows an air void partially filled with ettringite (lower-right)

and partially reacted fly ash particles (circular). X-ray element distribution (lower

image) shows regions of high sulfur and aluminum that delineate locations of

ettringite. Regions of chlorine and aluminum delineate regions of chloroaluminate.

Field width: 75 μm. ..................................................................................................... 41

Figure 32. Core 19 exhibits extensive cracking of both the paste and coarse aggregate.

Cracks trending parallel to the pavement surface (to the left) intersect perpendicular

cracks; features typical of freeze-thaw cracking. The oblique view of the upper

portion of core 19 shows cracking in both the surface (polished for clarity) and crosssection orientations...................................................................................................... 43

Figure 33. SEM image of a surface crack (road surface is at the top, 5 mm field width)

extending through the paste portion of the mortar and terminating in a coarse

aggregate...................................................................................................................... 44

Figure 34. Back scattered electron image of Core 19 paste microstructure showing ASRaffected shale (left) and fly ash (circular). X-ray image regions of high sulfur (yellow)

and intermediate aluminum (purple) mark locations of ettringite while regions of high

chlorine (lower left image) denote locations of chloroaluminate................................ 45

Figure 35. Core 19 (left) appears to have a finer-sized sand and possibly a larger

maximum coarse aggregate size when compared to Core 18 (right). ......................... 46

Figure 36. Core 19 data show a decrease in spacing factor with depth and possibly a trend

to an increased loss of spacing factor with depth. The component distribution plot

shows a possible increase in aggregate and decrease in paste in the upper half of the

core. ............................................................................................................................. 47

Figure 37. Top portion of core 19 with cracks (red) and ASR-affected shale grain (yellow)

shows cracking through both the cement paste and coarse aggregate. A color-coded

stress image resulting from a simulation of aggregate expansion showing regions of

high stresses in red correlates well with some, but not all of the observed cracking. . 49

Figure 38. Cores 11, joint (top) and 12, mid-panel (bottom) cross section with horizontal

crack in lower-most third. Some segregation is visible in both cores. ........................ 52

Figure 39. Surface cracking in Core 11 to about 20 mm passes through both the mortar

and coarse aggregate. Lower image shows minor surface carbonation and cracking

within the mortar. Field width 8 mm........................................................................... 53

Figure 40. Surface crack in core 12 (approx. center-left) appears to be entirely within the

mortar, and the lower image shows the crack plane through mortar highlighted using

ink. 8 mm field width. ................................................................................................ 54

Figure 41: Core 11 air void parameters and component distribution show a loss of air void

spacing yet relatively uniform air void volume and materials distribution................. 55

Figure 42. Core 12 air void volume and spacing parameters, and component distribution

shows an increased degree of entrained air void filling with depth. ........................... 56

Figure 43. Core 29 cracking near surface passes through both the mortar and coarse

aggregate. Cracking also follows close along paste-aggregate interface and is typical

of a shrinkage crack. Cracking in aggregate and in the mortar at depth may reflect

freeze-thaw damage, as the air void system in the upper half of this core is much

poorer than that in the lower half................................................................................. 59
xii

Figure 44. Core 29, centered on a well-defined vibration trail exhibits a significant

increase in spacing factor in the upper half. This probably represents the zone of

influence of the immersion vibrator probe. Note that the spacing factor is generally

not satisfactory at any depth. ....................................................................................... 60

List of Tables

Table 1. Cores Selected for Linear Traverse Analysis: C 457, Modified Point Count...... 14

Table 2. ASTM C-457 Modified Point Count Results of Original and Effective Entrained

Air Void Parameters. ................................................................................................... 15

Table 3. Iowa 175, West of Ellsworth, Iowa: Materials and Properties [28].................... 16

Table 4. US 20 Location 1, Webster County: Materials and Properties [28]................. 21

Table 5. US 20 Location 2, Webster County: Materials and Properties [28]................. 28

Table 6. US 20 Location 3, Webster County: Materials and Properties [28]................. 36

Table 7. I-35 Story County Location 5: Materials and Properties [28].......................... 51

Table 8. I-80, Milepost 210.40: Materials and Properties [28]. ..................................... 57
1

1. INTRODUCTION

1.1 Background

Major highway concrete pavements in Iowa have exhibited premature deterioration. The

Iowa Department of Transportation (DOT) has responsibility for determining the cause of

the deterioration and implementing measures for avoiding similar future problems. The

cause(s) of the deterioration has been the subject of much controversy. Various

investigators have used different methods to characterize the processes and have arrived at

different interpretations. The deterioration has been attributed to effects of ettringite

formation (including delayed ettringite formation), alkali-silica expansive reactions, and to

frost attack, or some combination of them. Evidence for all three processes have been

reported. While one or more of these processes is likely to be the cause of the

deterioration, the possibility that there may be other deleterious processes should be

considered.

1.2 Objective of the Study and Project Plans

The objective of this study is to identify probable processes responsible for premature

deterioration of the concrete pavement of selected major state highways in Iowa and, if

possible, to determine the major causes of the premature deterioration of concrete

pavements.

Our project consists of three major tasks outlined in a research plan which has been

developed in collaboration with the Iowa DOT. The first task consisted of field

inspections of deteriorated concrete and collection of as much information as available on

the pavement projects such as the concrete mix designs and constituent materials, the

mixing, consolidation and curing of concretes, test records, and inspection reports.

Specimens of both deteriorated and un-distressed pavements, constructed from the same

concretes, have been collected.

In the second task, the concrete specimens were subjected to petrographic examination

using visible light microscopy and scanning electron microscopy coupled with image

analysis, and X-ray powder diffraction analysis. The intent of these investigations was to

identify the major degradation process(es) causing premature deterioration of the

pavement and if more than one major process was identified, assess their relative

contributions.

Task three involved the attempt to identify the causes of deterioration by application of

microstructure simulation, hydration simulation, and micro-mechanical models developed

at NIST. Through a combination of backscattered electron and X-ray imaging, the

microstructure of the distressed concretes was established. Segmenting images into

constituent phases and “healing” the cracks via image processing provides a starting

microstructure for operation of the models. Modeling of deterioration processes involves

selected expansion of phases in accordance with proposed deterioration mechanisms.

Analysis of the stress fields after a series of expansion cycles formed a basis for predicting

the crack patterns associated with the different modes of deterioration. Comparison of the

predicted stress fields and resulting crack patterns provided additional information

regarding the potential mode(s) of deterioration.
2

1.3 Previous Studies

Highway pavements in this study were constructed in the mid-1980s as non-reinforced,

dual-lane, roads ranging in thickness between 200 mm and 300 mm, with skewed joints

reinforced with dowels. Deterioration was initially recognized by Iowa DOT staff as a

darkening of joint regions, which occurred for some pavements as soon as four years after

construction. Pavement condition ranges from severe damage to none, and there appeared

to be no unequivocal materials or processing variables correlated with failure.

Numerous studies of cores from these pavements have produced a variety, and some

conflicting, opinions as to the causes of the deterioration, and the significance of

microstructural features. Jones [1] examined cores drilled from US 20 in Webster County

and found low air contents in regions where cracking was occurring. He speculated that

the paver vibration and supplemental vibration around joint regions might be responsible

for the low air void content leaving the pavements vulnerable to freeze-thaw damage.

Ouyang [2] examined Wisconsin and Iowa pavements and concluded that internal and

external sulfate attack and freeze-thaw attack were the most probable causes for premature

deterioration of the US 20 pavements. He also felt that improper construction methods

resulted in a low air content in the surface layer of the pavements. He noted that deposits

of ettringite in the entrained air voids may have reduced resistance to sulfate attack and to

freeze-thaw damage, and that freeze-thaw cracking could have facilitated the transport of

external sulfate ions into the pavements.

Pitt et al. [3] examined the role of sulfate impurities originating from deicing salts on the

durability of Portland cement mortar and pavements. They speculated that deicing salt

impurities may have contributed to early pavement failures previously attributed to Dcracking. They note that concrete incorporating aggregate that passed freeze-thaw tests

can exhibit failure in the field. These failures were observed to be especially severe in

joint areas, and may actually have been the result of sulfate attack, even though the crack

patterns appeared similar to that of D-cracking. Specimens exposed to gypsum-containing

brines appeared to deteriorate more rapidly compared to those exposed to pure water.

They observed an increase in ettringite and Friedel’s salt after exposure to deicing salts. It

was thought that these phases filled voids making the mortars more vulnerable to freezethaw cycling damage.

Stark [4] concluded that the expansion caused by alkali-silica reaction (ASR) was the

cause of the pavement deterioration. He felt that the ASR reaction was associated with

chert and shales in the coarse sand sizes and that the problem pavements incorporated

high alkali cements and Class C fly ash. Similarly, Thalow [5] found alkali-silica

reactivity involving chert in the sand fraction and concluded that it was the primary cause

of deterioration. He considered the filling of the entrained air voids to reflect water

saturation. Evidence of an overall paste expansion, possibly indicating a delayed

ettringite formation, was not found. Kofoed [6] examined a set of cores and found alkalisilica reaction involving shale in the sand and a poor entrained air void system and

concluded that both ASR-related expansion and freeze thaw cycling led to cracking. In

contrast to these findings, Marks [7] tested properties of selected Iowa sands used in

construction and noted that while many failed the ASTM P-214 test (now ASTM C 1260

[8]), that in the past they had a good performance record in pavements.
3

Marks and Dubberke [9] observed the filling of entrained air voids with ettringite and felt

that the voids would serve as a center-of-pressure as ettringite precipitation progressed,

resulting in cracking. While no measurements were performed, they considered the

volume of ettringite to be excessive and, that sodium chloride, likely from road salts, may

have accelerated the deterioration by causing the ettringite to expand and then dissolve.

This is in contrast to observations of Stark [10] finding ettringite to be stable in sodium

chloride solutions, and that monosulfate appears to be converted to ettringite under such

conditions. Schlorholtz and Amenson [11] determined that two or more deterioration

processes were acting simultaneously. They concluded that freeze-thaw damage was most

probable for the cracks in the cores extracted from I-35 and US 20. Alkali-silica reaction

products were observed in all cores but were considered secondary in importance relative

to the freeze-thaw processes. In regions showing less severe deterioration, damage was

attributed to mixing or plastic concrete problems. D-cracking and marginal air void

systems were also thought to be potential causes of deterioration.

Gress [12] noted a relationship between tine groove depth and distress. Regions with

shallow tine depths and apparently poor finishability, generally denoted regions where

distress was more likely. He considered these an indication of possible setting problems

and that the loss of entrained air in these regions may be due to over vibration. Cracking

was observed within the matrix with some cracks in the coarse aggregates and small

entrained air voids were filled with what is thought to be ettringite. Empty cracks

intersecting these filled voids led Gress to conclude that growth of ettringite in the voids

created the pressure that cracked the concrete. Additionally, he felt these concretes

contained “excessive ettringite”, though no measurements were made.

Ouyang and Lane [13] examined freeze-thaw-cycled laboratory specimens prepared with

and without Class C fly ash. They found a relationship between filling of entrained air

voids and loss of freeze-thaw durability. They also felt that the degree of filling coincided

with the length of moist curing prior to cycling, as well as the usage of Class C fly ash.

This led to a subsequent study by Ouyang and Lane [14] where they devised an

experimental program to examine the role of ettringite filling of entrained air voids and

freeze-thaw durability in which a correlation between filling and freeze thaw performance

was found. They felt that the filling was influenced by the period of moist curing and wetdry test cycling, and by the materials used in the concretes. They proposed limits for

materials in an effort to minimize the formation of ettringite.

A study by Northwestern University [15] of pavements from the Midwest, including those

from Iowa, concluded that damage resulted from of multiple factors. They recommended

investigation of the role of fly ash in pavements, avoidance of high alkali and high sulfate

cements, the use of a permeable base, and also stressed the need to account for synergistic

effects of combinations of variables in future testing.
4

2. TECHNICAL APPROACH

2.1 Field Observations of Pavements

A field examination of the pavements was organized by Vernon Marks of Iowa DOT.

Site visits to pavements of Iowa US 20, Iowa 175, I-35, US 218, and I-80 allowed

examination and designating coring locations. Sites were selected from pavements

exhibiting no apparent deterioration, those slightly affected, and pavements with relatively

severe damage. In the following paragraphs, the observations are summarized. More

detailed observations are given in the results of petrographic examinations of specific

cores.

In some cases, onset of pavement deterioration was visible as a slight darkening of the

concrete in the transverse joint region. The darkening may reflect accumulation of dirt and

other road debris and moisture migration within fine-sized cracks along the joints.

Additional degradation resulted in development of cracking along the transverse joint and

road edge, and along cracks parallel to vibrator trails (Figure 1). The degradation would

be characterized initially as “low” severity according to the Strategic Highway Research

Program (SHRP) Distress Identification Manual [16] and “moderate” as the deterioration

progresses. The most severely deteriorated regions exhibited pattern cracking over the

entire slab, more substantial darkening at joints, cracking paralleling joints and the road

edge, along with minor spalling of concrete adjacent to the joint. Map cracking oriented

in the longitudinal direction of the pavement, interconnected by slightly finer cracks

perpendicular to pavement direction became more prominent with the progression of

deterioration (Figure 2). Damage observed during the field examination does not exceed

the “moderate” rating, although some of the sections of US 20 considered to be the most

severely affected, were paved with an asphalt overlay at the time of the site visit.

Longitudinal cracks with widths ranging from less than 1 mm to 2 mm wide are present

along some vibrator trails. Vibrator trails exhibit a slightly different texture, probably

related to lack of coarse aggregate as observed in regions where the pavement surface had

been ground. Gress [12] noted that the tining penetrated deeper across the vibrator trails.

The cracking paralleling the longitudinal cracks may also be classified as “low” to

“moderate” (Figure 3).

There was no evidence of any permanent volume change, deflection, closing / opening of

joints, or tilting and shearing of slabs. All roads had a tined surface reportedly cut after the

concrete had set [21] and the condition of the tine grooves varied from cleanly cut to

having a torn appearance. The exposed pavement surfaces appeared satisfactory, though

some surfaces reportedly experienced difficulty in finishing during the paving operation.

Iowa DOT noted many of the mixes were "harsh" and were difficult to place [21].

Secondary deposits in the form of a white efflorescence (probably carbonated calcium

hydroxide) were noted along US 20 on surfaces and in cracks within the spalling adjacent

to the joints, but only in regions with the most severe deterioration.
5

Figure 1. Onset of pavement deterioration is visible as a slight darkening of the concrete

in the transverse joint region (top, I-80) and in pavements with advanced deterioration, a

tightly-closed pattern cracking (bottom, US 20). Some of these pavements also have

vibrator trails.
6

Figure 2. Map cracking oriented in the longitudinal direction of the pavement,

interconnected by slightly finer cracks perpendicular to pavement direction becomes more

prominent with progression of deterioration (top). Advanced degradation shows cracking

paralleling the transverse joint and road edge, and along cracks within vibrator trails. The

degradation would be characterized initially as “low” severity according to the SHRP

Distress Identification Manual [17] and “moderate” as the deterioration progresses.
7

Figure 3. Pavements exhibiting severe deterioration show significant pattern cracking

(here accentuated by moisture along the crack), cracking along joints, spalling along joint

edges and white efflorescence. Open joints and lack of displacement along pavement slabs

provides evidence against permanent expansive processes such as alkali-aggregate

reaction and sulfate attack.
8

2.2 Analysis of Concrete Mixture Design

Figure 4 gives the concrete materials proportions from US 20 for mixes with and without

fly ash while Figure 5 indicates the gap grading of the aggregates. Evaluation of the mix

design by two separate internationally recognized engineers [18,19] found agreement in

that the high sand content and gap-grading probably led to difficulties in workability and

consolidation, and may have resulted in drying shrinkage problems. The aggregate

distributions plotted on a Shilstone Coarseness Factor Chart [20] fall in a region reflecting

gap-graded concrete and where workability difficulties may be present (Figure 6). The

parallel blue lines on this chart define a region of optimum grading for low water demand

and optimum response to vibration. While the potential difficulty in workability were not

unexpected to the Iowa Department of Transportation staff [21], it is significant in that it

may also affect the development of the entrained air void system during mixing. Failure

to develop an adequate entrained air void system appears to be contributory to the

deterioration of these pavements.

US 20 Site 2

Cement

13%

Water

7%

C. Agg.

42%

F. Agg.

36%

Fly Ash 

(C)

2%

US 20 Site 4

Cement

15%

Water

6%

C. Agg.

43%

F. Agg.

36%

Fly Ash

0%

Figure 4. Concrete materials proportions, by mass, for selected sections of Iowa US 20.
9

Figure 5. Aggregate gradation plots showing coarse, fine, and combined aggregate

gradation and the lack of particles in the intermediate size range.

US 20, IA #2 - Ft. Dodge Coarse and Yates Fine Aggregates

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

38.0 25.0 19.0 12.7 9.5 4.8 2.4 1.2 0.6 0.3 0.15 0.07

Size (mm)

Mass Percent Passing

Total Aggregate % Coarse % Fine

US 20, No. 4 - Ft. Dodge Coarse and Croft Fine Aggregates

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

38.0 25.0 19.0 12.7 9.5 4.8 2.4 1.2 0.6 0.3 0.15 0.07

Size (mm)

Mass Percent Passing

Total Aggregate % Coarse % Fine
10

Figure 6. Optimum aggregate chart plots of Iowa pavement mix designs fall into the gapgraded region and indicate likely difficulties in workability [20].

2.3 Cement Composition Analyses

Estimates of cement compositions were made using a modified Bogue calculation

described by Taylor [22]. Chemical data for these analyses were taken from the Iowa

State University testing results provided in the Iowa Department of Transportation

materials database. As data were not available, assumptions for free lime (0.3 %) were

made for all estimates. These data show that, if one considers total silicates as a group,

cement compositions appear generally uniform across the job interval. Variations in the

silicate phase fractions may reflect deviations of the free lime content from that assumed

in the calculations (Figure 7), errors in sampling, and analysis. These data were later used

in conjunction with the mix design to estimate the potential volume of ettringite formation

to test the hypothesis of excess ettringite formation. Data were not available to allow

matching of cement analyses with pavement locations.

Optimum Aggregate Relationship

20

25

30

35

40

45

100 90 80 70 60 50

Coarseness Factor (CF)

% Retained on 2.38 mm sieve & also retained on .95 mm sieve

Workability Factor (WF),

% passing 2.38 mm

 sieve adjusted for cementitious material

s

Optimum

Well Graded 13 mm to 38 mm

Rocky

Sticky

"Gap-Graded"

tends to segregate

Data CF (x) WF (y)

I-80 74.9 37.6

I-80 75.3 35.4

I-80 82.6 38.5

US 20 81.0 32.7

US 20 78.2 30.3

I-218 83.7 33.5

I-175 77.8 35.2 
11

US 20 Cements ID #4

20

30

40

50

60

70

80

90

100

AC84-244AC85-001

AC85-583

AC86-007

AC86-163

AC87-192AV88-190AC89-566AC89-764AV89-053AV89-242AC90-008AC90-433AC91-024

Na2SO4

K2SO4

Gypsum

Periclase

F.Lime

Ferrite

Aluminate

Belite

Alite

C3S C2S C3A C4AF Free Lime Periclase Gypsum K2SO4 Na2SO4 Ckr. CaSO4

Avg. 51.2 28.0 4.8 7.4 0.3 1.5 5.7 1.0 0.0 0.0

Stdev. 18.7 20.4 1.4 0.5 0.0 0.3 0.5 0.0 0.1 0.0

Io w a U S 2 1 8 C e m e n ts ID # 2

7 5

8 0

8 5

9 0

9 5

100

AC82-283

AC83-001

AC83-277

AC83-459

AC84-003

AC84-239

AC85-005

AC85-007

AC85-025

AC85-025a

AC85-027

AC85-275

AC86-001

AC86-097

AC86-297

AC87-1

Na2SO4

K2SO4

G ypsum

P e ric la s e

F . L im e

F e rrite

A lu m in a te

B e lite

A lite

C3S C2S C3A C4AF Free Lime Periclase Gypsum K2SO4 Na2SO4 Ckr. CaSO4

Avg. 45.7 35.2 3.4 8.3 0.3 1.8 4.4 0.9 0.2 0.0

Stdev. 9.4 10.3 1.3 0.8 0.0 0.3 0.6 0.1 0.1 0.1

Figure 7. Cement compositions, expressed as mass percent, from chemical analyses from

the Iowa DOT database and using a modified Bogue calculation described in Taylor [22].

The y-axis has been expanded to more clearly illustrate total silicates and other phases.
12

2.4 Specimen Preparation

A set of cores was prepared to allow examination of full-depth concrete cross sections of

specimens from both the joint and mid-panel, outside of a wheel path, and both between

and on vibrator trails. Examination of cores followed recommendations provided in

ASTM C 856 [23], Standard Practice for Examination of Hardened Concrete, while

procedures for preparation and examination may also be found in Walker [24].

Core numbers were logged upon receipt and core surfaces examined and described.

Examination of the entire core allows description of the overall concrete condition, the

coarse and fine aggregate characteristics, cement paste condition, qualitative assessment

of entrapped and entrained air void system, cracking, and identification of any foreign

objects. Photographing the prepared surfaces provided a record of the specimen condition.

Lapping provides a smooth surface necessary for examination of the concrete

microstructure using a stereo microscope [23,24]. Cracks were marked using India ink and

a draftsman’s pen by touching the pen tip to the crack and allowing capillary suction to

draw the ink along the crack plane [24]. This technique was also used to identify open

aggregate cracks in the limestone coarse aggregate and shale in the sand. Selected cores

were then marked for sampling for scanning electron microscopy (SEM). The cores were

cut in half lengthwise and a slab approximately 15 mm was taken from the core center.

The core slab was sectioned into four pieces and dried in an oven at 50 oC. After 48 h,

these sections were imbedded using a low-viscosity epoxy in a low vacuum to assist in

infiltration of the pore structure, and the epoxy was cured at 60 oC for 24 hours. The

embedded specimens were then ground to expose the concrete face, and polished using a

series of (6, 3, 1, and 0.25) μm diamond polish; the surfaces were cleaned using a lint-free

cotton cloth. A thin carbon coating was deposited on the surface for conduction of excess

SEM beam charge. The slabs were stored in a vacuum dessicator to protect them from

laboratory humidity. Remnant core halves were lapped and polished similar to the

procedure for petrographic analysis.

2.5 Air Void System Analysis

Characterization of the entrained air void system of selected cores was performed using

the modified point count procedure of ASTM C 457 [25]. Data were obtained from a slab

sawn from the center of each core, carefully smoothed and polished to present a surface

suitable for microscopical examination. The prepared surface was then stained using a

potassium permanganate, barium chloride stain per Poole and Thomas [26] to highlight air

voids filled with ettringite (Figure 8). The point count was performed using a semiautomated linear traverse system [27] with an additional point count tabulator. Each data

set represents a total traverse length of at least 2540 mm, in accordance with ASTM C 457

requirements. Uncertainties in the measurements are estimates of two standard deviations,

corresponding to a 95 % level of confidence. The linear traverse analysis counted all

voids, that is, all effective and those rendered ineffective due to filling.
13

Figure 8. Potassium permanganate and barium chloride staining colors sulfate phases

purple facilitating their identification. This allows discrimination of both the original and

effective entrained air void systems.

The second tabulator was used to track the filled air voids, permitting recalculation of the

data to reflect the effective entrained air void system. This facilitated counting of two air

void systems, that of the concrete as placed, and that of the concrete at the time of

sampling. These values differ because of the subsequent filling of some of the entrained

air voids with ettringite. Criterion for distinction between an ineffective and effective

void was that the void must appear completely filled. Voids rimmed or partially filled

with secondary mineralization, and those with open cores were considered effective as

entrained air voids.

3. RESULTS

3.1 Air Void Analysis

The cores selected for air void analysis are given in Table 1 along with their conditions

based upon visual observations. Table 2 presents results of the air void analysis. Trends in

total air and air void spacing as both original and effective values are plotted versus core

depth and will be presented in the petrography section (section 3.2). Examination of these

plots, as well as a plot of concrete component distribution proved helpful in understanding

the deterioration of the pavements.
14

Table 1. Cores Selected for Linear Traverse Analysis: C 457, Modified Point Count

Core # Road Location Condition, comments

2 Iowa 175 J Test section, 15 % Class C fly ash, appears sound

11 I-35, Story J No vibrator trail evident, North-bound lane,

12 I-35, Story M Pattern cracking at mid-panel joints

7 US 20 JB Mix 3, severe pattern and longitudinal cracking

8 US 20 MB slight transverse cracking, joint spalling, popouts

13 US 20 JB Mix 2, appears sound

14 US 20 MB

18 US 20 JB Mix 1, sound in this location

19 US 20 J Mix 1, severe map cracking, cracking along joints,

Paved over before site visit

29 I-80 MO Longitudinal and slight pattern cracking

J = joint

M = midpanel

B = between vibrator trail

O = on vibrator trail

Specimens without a vibration trail code reflect the lack of a clearly discernable vibration

trail at that location.
15

 Modified Point Count Results of Original and Effective Entrained Air Void Parameters.

Volume Percent Paste/Air Ratio Specific Surface (mm-1) Spacing Factor (mm)

Effective Paste Aggregate Org. Eff. Org. Eff. Org. Eff.

11.7 21.2 66.9 1.8 1.8 17.5 17.1 0.10 0.11

± 1.6 ± 2.1 ± 2.7 ± 2.6 ± 2.6 ± .01 ±. 02

6.0 27.0 65.4 3.4 4.5 21.3 15.2 0.16 0.30

± 1.4 ± 2.4 ± 2.6 ± 3.8 ± 3.3 ± .03 ± .06

7.9 21.4 67.3 2.5 2.7 17.4 10.1 0.18 0.34

± 1.3 ± 2.1 ± 2.0 ± 3.1 ± 2.0 ± .03 ± .06

4.3 28.8 65.2 4.8 4.5 29.1 9.0 0.16 0.74

± 0.9 ± 2.0 ± 2.0 ± 4.9 ± 1.9 ± .03 ± .16

4.6 25.3 68.9 4.3 5.5 35.4 21.1 0.12 0.26

± 1.1 ± 2.3 ± 2.1 ± 7.4 ± 5.3 ± .03 ± .06

6.2 25.6 67.9 3.9 4.1 22.1 18.4 0.18 0.22

± 1.2 ± 2.3 ± 2.7 ± 4.6 ± 3.9 ± .04 ± .05

6.3 25.0 68.2 3.8 3.9 18.1 17.3 0.21 0.23

± 1.3 ± 2.3 ± 2.7 ± 3.8 ± 3.8 ± .04 ± .05

12.0 21.1 67.6 1.7 1.8 20.8 18.5 0.08 0.09

± 1.8 ± 2.1 ± 2.7 ± 3.1 ± 2.8 ± .01 ± .01

7.9 17.8 72.0 1.9 2.3 14.6 7.7 0.13 0.29

± 1.4 ± 2.0 ± 2.2 ± 2.6 ± 1.5 ± .02 ± .06

6.2 32.2 61.2 5.2 5.2 15.4 14.3 0.34 0.36

± 1.2 ± 2.3 ± 2.4 ± 3.4 ± 3.1 ± .07 ± .08
16

3.2 Petrographic Analysis

3.2.1 Iowa 175 Test Road

Iowa 175, west of the town of Ellsworth, is a test road with sections containing portland

cement concrete, and concrete panels containing partial cement replacement by Class C

or Class F fly ash. These pavements, constructed about 1981, have performed very well

and exhibit no evidence of vibrator trails. This location was of interest as these concretes

appear to be in excellent condition, yet are slightly older than those exhibiting problems,

and used similar materials, including fly ashes. The materials and properties of the

specimens taken from Iowa 175 were provided by the Iowa Department of Transportation

and are given in table three.

Table 3. Iowa 175, West of Ellsworth, Iowa: Materials and Properties [28]

Cement: Type I,

2.16 % SO3,

0.35 % equivalent alkali

Coarse Aggregate: Alden (Weaver) Limestone, open quarry,

1.5 % freeze-thaw loss, 2.7 % absorption,

LA Abr. 40 % loss, durability factor 79

Fine Aggregate: Ames sand

Admixtures: AEA

Fly Ash: Class C, 15 % replacement of cement

1.65 % SO3,

1.46 % equivalent alkali,

0.20 % loss on ignition

Concrete Batch Mass / m3

 (IDOT A-3 Mix)

Cement 278 kg 0.49 w/s Slump: 90 mm to30 mm, avg. 50 mm

Water 161 Air: 7.8 % to 5.4 %, avg. 6.6 %

Fly Ash 49

C. Agg. 973 Slipform paver - spreader used

F. Agg. 844 Temperature: 32 to 7 (°C), 

no precipitation, clear

WR 0 Mix Characteristics: no reported problems

AEA no value provided 27.1 MPa compressive strength

Cores:

No. Mile Post Location Vibrator Trail Designation Condition/Comments

2 224+95 Joint 2j Excellent Condition
17

3.2.2 Iowa 175 Cores

Iowa 175 cores appear mostly sound with few cracks, strong paste and aggregate, clean

saw cuts in preparation, good paste / aggregate bonding. Their surfaces exhibit a

rounded-edged tining with possible minor freeze-thaw-related spalling of the concrete

surface. Coarse aggregate is a crushed limestone and the sand is a blend of quartz,

feldspar, and shale. Occasional cracking related to alkali-silica reactivity of the shale

fragments was observed. This cracking appears minor and is seldom greater than a few

tenths of a millimeter in length. Lateral segregation of coarse aggregate and mortar are

seen in Core 2 as mortar-rich regions (Figure 9). The cement paste from Core 2 is slightly

darker than that from cores extracted from other sections of the I-175 test road and

probably reflects color influence of the Class C fly ash. The paste is even-colored,

appears strong, with no evidence of bleeding, and carbonation depths of a few

millimeters (Figure 9, Figure 10). Entrapped air voids are common, especially in the

center of the core where void diameters approach 20 mm. Entrapped air voids were less

common in Cores 4 and 6. The entrained air void systems appear adequate, though some

filling of the voids is apparent near the core base (Figure 11). Cracking is confined to the

base region of the core, and traverses the core through both coarse aggregate and paste

(Figure 12). Examination of the air void data presented in (Table 2) and a graphical

depiction presented in Figure 13 shows that the air void spacing factor is within the range

recommended for durability in moderate and severe environments. The decrease in total

air near the surface probably reflects the influence of consolidation vibration on the

removal of entrapped air. Loss of some of the air void system was noted with the filling

of some of the air voids with ettringite, primarily near the base of the core. The large,

open fracture in the base of Core 2 transecting both coarse aggregate and paste is within

the region where this filling occurs and, while the spacing factor is still sufficient for

freezing and thawing durability according to ACI 201, the cracking appears to be freezethaw-related. Therefore, this may indicate a high degree of concrete saturation at the

base of this pavement.
18

Figure 9: Iowa 175 Core 2 shows some aggregate / mortar segregation and cracking

located near the core base (base to the right).

Figure 10. Iowa 175 Core 2 surface microstructure. Slight discoloration in the upper

mortar indicates approximate depth of carbonation. Micrograph field width: 14 mm
19

Figure 11. Iowa 175 Core 2 base mortar microstructure exhibiting partial filling of the

entrained air void system. Micrograph field width: 4 mm

Figure 12. Iowa 175 Core 2 base microstructure exhibits cracking in the mortar and

coarse aggregate. Micrograph field width: 14 mm.
20

Air Void Distribution: Core 2

0.0

5.0

10.0

15.0

20.0

25.0

0 5 10 15 20 25 30

Depth (cm)

Volume Percent Air

Total

Effective

Air Void Spacing Factor

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

0.22

0 5 10 15 20 25 30

Depth (cm)

Spacing Factor (mm)

Total

Effective

Component Distribution

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Depth (cm)

Volume Percent

Aggregate

Paste

Filled

Effective

Figure 13. Iowa 175 Core 2 air void and materials distribution plots. A decrease in air

near the surface may reflect the loss of entrapped air. The blue triangles represent an air

void parameter estimate for the original concrete; the red box represents that value as

affected by void filling, if present.
21

3.2.3 US 20, Webster County

US 20 provided a set of specimens that represented the best- and poorest-performing

pavements, concretes with and without fly ash replacement of the cement, and pavements

using a single mix design exhibiting opposite performance characteristics. Four sampling

sites were identified with the assistance of Iowa DOT to facilitate comparison of poorlyperforming with sound pavements. Materials and properties information (Table 4) was

taken from IDOT reports supplied to the Pooled Pavement Study by Northwestern

University [28].

Table 4. US 20 Location 1, Webster County: Materials and Properties [28].

Cement: Type I

3.5 % SO3,

0.77 % equivalent alkali,

Coarse Aggregate: Ft. Dodge Limestone, Gilmore City bed, ledge 36-42

1 % Freeze-thaw loss, 0.9 % absorption.,

LA abrasion 28 % loss, durability factor 94

Fine Aggregate: Yates sand

Admixtures: AEA (air entraining admixture)

Fly Ash: Class C,

3.23% SO3,

2.95% equivalent alkali, 0.28% loss on ignition

Concrete Batch Mass / m3

Cement 274 kg 0.48 w/s Slump: 50 mm to 44 mm, avg. 46 mm

Water 160 Air: 7.8 % to 6.0 %, avg. 6.7 %

Fly Ash 46

C. Agg. 980 slipform paver with spreader

F. Agg. 821 Temp: 31 to 10 (°C), no precipitation, clear/sunny

WR na Mix Characteristics: Harsh

AEA no value 31 MPa compressive strength

(IDOT Mix 3 - actual material mass estimated from mass proportions [28]

Completed 5/86, deterioration noted 1990

Cores

No. Mile Post Location Vibrator Trail Designation Condition

 7 134.4 Joint Between 7jb severe pattern, parallel to

 8 134.4 Midpanel Between 8mb joints, and longitudinal

cracking
22

3.2.3.1 Site Description US 20, Mile 134.4

Sampling location IDOT site 2 was located at milepost 134.4 in the west-bound lane of

US 20 in Webster county. Pavements exhibit cracks paralleling the transverse joints,

severe mid-panel pattern cracking, joint spalling, popouts, and longitudinal cracking

along vibration trials.

3.2.3.2 Core Descriptions

Cores 7 and 8 exhibit a white exhudation from some large shale fragments indicating

some alkali-aggregate reaction, though no significant cracking was observed. Large (to

30 mm) entrapped air voids were common. The top surface appears rough with uneven

tining grooves. The paste appears strong and cuts cleanly, and the paste / aggregate bond

appears strong. The coarse aggregate is a crushed limestone that exhibits some internal

cracking, and the sand is silicious and angular. The larger-sized sand particles appear

more rounded and contain some shale grains. Segregation is common with a lack of

coarse aggregate in the upper third of the core (Figures 14, 15). The partial loss of the

core in the upper image of Figure 14 resulted from the core transecting a joint dowel

(circular void). Surface cracking terminating in both limestone aggregate and paste is

common (Figure 16). These cracks were empty and closed, and were identified through a

subtle change in paste coloration due to carbonation along the crack plane. Entrained air

void system analysis indicates a lower volume of air near the concrete surface and a

significant loss of entrained air void capacity due to filling with ettringite (Figure 17,

Table 2). The total air volume decreased from 5.9 % to 4.6 % while the spacing factor

has increased from 0.12 mm to 0.26 mm. The loss of the smaller entrained air voids is

reflected by the change in specific surface from 35 mm-1 to 21 mm-1. The increased

spacing factor is in excess of that recommended in ACI 201 for freeze-thaw protection.

Graphical representation of the total air, spacing factors, and component distribution

(Figure 18, Figure 19) illustrates the alteration of the air void system resulting from the

ettringite filling. Air distribution, as measured by total air volume, appears uniform from

the core top to base. Core 7 may show a slight decrease in air in the upper 50 mm,

possibly reflecting the elimination of entrapped air in the upper-most portion of the

concrete. This also indicates that, outside the immediate regions if the immersion

vibrators, the air void spacing factors do not appear to be adversely affected. The

decrease in ettringite precipitation near the surface (upper 5 mm) may reflect the

influence of surface drying. At greater depths, filling of voids appears uniform from both

a total air void volume and loss of spacing factor. Component distribution appears

uniform indicating no apparent vertical segregation of materials due to vibration.

SEM examination found no evidence of an overall paste expansion typified by gaps

around aggregates. Some cracking was seen within aggregates but similar to the other

specimens, there does not appear to be any relation to their rock texture. Rims on outer

surfaces of sand-sized chert grains may indicate ASR reactivity but little, if any, cracking

appears associated with these grains.
23

Figure 14. Core 7 (road surface is to the left) exhibits cracking of coarse aggregate,

vertical cracks from the surface that are either drying shrinkage or freeze thaw-related.

Segregation of mortar and coarse aggregate is visible in the whole core cross section

(upper image) and the upper-core microstructure (lower).
24

Figure 15. Core 8 exhibits less cracking but does have surface cracking to depths of 20

mm. Some of these cracks as seen in the lower image (13 mm field width) are associated

with cracking within the coarse aggregate.
25

Figure 16. Cracking (red) and ASR (yellow) in Core 7 as observed using SEM. Cracking

of aggregate is common. Surface cracks appear carbonated and terminate in both the

mortar and the coarse aggregate. Micrograph field width is approximately 10 cm.

Figure 17. Filling of the smaller entrained air voids (purple) has significantly increased

the air void spacing factor while only slightly decreasing the total entrained air void

volume for both Cores 7 and 8. Field width: 4 mm.
26

Figure 18: Air void distribution, spacing factors, and concrete component distribution

versus depth for Core 7. The red boxes represent the current value, the blue triangle, the

original value, and the red line in the spacing factor plot denotes the ASTM C 457

recommended limit for freeze-thaw protection.

Component Distribution

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

Depth (cm)

Volume Percent

Aggregate

Paste

Filled

Effective

Air Void Distribution: Core 7

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

0 5 10 15 20 25 30

Depth (cm)

Volume Percent Air

Total

Effective

Air Void Spacing Factor

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

0 5 10 15 20 25 30

Depth (cm)

Spacing Factor (mm)

Total

Effective
27

Air Void Distribution: Core 8

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

0 5 10 15 20 25 30

Core Depth (cm)

Volume Percent Air

Total

Effective

Air Void Spacing Factor

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0 5 10 15 20 25 30

Core Depth (cm)

Spacing Factor (mm)

Total

Effective

Component Distribution

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

Depth (cm)

Volume Fraction

Aggregate

Paste

Filled

Effective

Figure 19: Air void distribution and spacing factors versus depth, and concrete

component distribution for Core 8, mid-panel.
28

Table 5. US 20 Location 2, Webster County: Materials and Properties [28].

Cement: Type I,

3.5 % SO3,

0.77 % equivalent alkali,

Coarse Aggregate: Fort Dodge Limestone, Gilmore City

< 1% FT Loss, 0.9 % absorption,

LA abrasion 28 % loss, durability factor 94

Fine Aggeregate: Croft sand

Admixtures: WR, AEA

No Fly Ash

Concrete Batch Mass / m3 (IDOT Mix #2)

Cement 340 kg 0.40 w/s Slump: 50 mm to 40 mm, avg. 46 mm

Water 136 Air: 7.5 % to 5.6 %, avg. 6.5 %

Fly Ash 0

C.Agg. 1009 Slipform paver with spreader

F.Agg. 825 Temp: 23 to 24 (°C), no precipitation, clear/mild

WR 0 Mix Chara: no data

AEA no value 31 MPa compressive strength

Cores:

No. Mile Location Vibrator Trail Designation Condition

13 127 Joint Between 13jb appears sound

14 127 Midpanel Between 14mb same
29

3.2.3.2 US 20 Location 2, Webster County

Cores 13 and 14 are from a section of US 20 placed in late 1986 that still appears sound.

This pavement used a concrete mix design without fly ash replacement of cement. This

sampling site was selected as the use of fly ash was considered to be a potential source of

the durability difficulties and therefore provides a reference from which to compare to

pavements that incorporated fly ash.

3.2.3.2.1 Core Descriptions

Cross sections of the cores are presented in Figures 20 and 21. The cores exhibit a roughtextured tined surface with an occasional white efflorescence along the core exterior that

appears related to shale fragments. The concrete appears sound and strong, it cuts cleanly

and has good cement / aggregate bonding. The cement paste is a uniform gray and

exhibits only a thin carbonation layer near the core surface. The coarse aggregate is a

limestone up to 40 mm in cross section, and the fine aggregate is a silicious sand. Some

cracking occured in the coarse aggregate. Shale particles in the sand appear to have

undergone some alkali-silica reaction but cracking associated with this reaction appears

limited. Sand-sized shale grains at the core surface occasionally exhibit minor popouts.

Some cracking was noted at the top surface about 4 mm in length traversing both paste

and an occasional sand grain, with some cracking terminating in a limestone coarse

aggregate. Segregation of mortar and coarse aggregate is apparent within the cores.

Segregation near the surface was not observed. Entrapped air voids as large as 12 mm

occur in the lower half of the core. Clustering of entrained air voids in paste, and also

along aggregate boundaries is common (Figure 22). The entrained air void system is

marginal to substandard with respect to spacing factor, and has been altered slightly due

to filling with ettringite. The spacing factor values, especially when examined across the

core, are often in excess of those recommended for freeze-thaw resistance (Table 2). SEM

imaging of a mid-core paste region (Figure 23) shows the filling of some air voids. Some

of the smaller, irregularly shaped capillary voids (black regions) were probably locations

of monosulfate that decomposed, serving as the source material for the ettringite. X-ray

imaging of the same field (Figure 24) highlights the ettringite locations using its

characteristic X-ray signature of high sulfur, intermediate aluminum, and intermediate

calcium. Additionally, the reacted shale in the lower-left does not have any associated

cracking. Air void volume and spacing factor plots indicate a relatively uniform air

volume from top to the base, while the spacing factor appears marginal to about 100 mm

below the surface (Figure 25, Figure 26). Void filling, as shown by the spacing factor

plot, appears uniform from top to bottom. The component distribution plot also indicates

a uniform material distribution, with possibly a slight increase in aggregate with depth.

The entrained air void system is substandard when considering the specific surface (Table

2), void distribution, and possibly spacing factor. The poor air void system and the mortar

/ coarse aggregate segregation throughout the core may reflect the harshness of the mix,

incomplete mixing, and so, difficulties in achieving dispersion of the air entraining agent

and the ability to generate a good entrained air void system.
30

Figure 20. Core 13 cross sections show segregation within the concrete and only minor

cracking of the mortar and coarse aggregate.
31

Figure 21. Core 14 exhibits some cracking of both the paste and coarse aggregate and

some mortar / aggregate segregation in middle.
32

Figure 22. Clustering of air voids may indicate difficulties in mixing and development of

a properly sized, disseminated entrained air void system. Field: 7 mm.

Figure 23. A SEM image of Core 13 paste shows ettringite-filled entrained air voids, the

irregularly-shaped capillary voids (black), and a reactive shale grain in the lower-left.

Note absence of cracking outside of this shale grain and lack of paste / aggregate gaps

that would be typical of an overall paste expansion. Field: 200 μm.
33

Figure 24. X-ray images corresponding to image in Figure 23. While the shale has

undergone alkali-silica reaction, no cracking is apparent within the paste. Common

locations of aluminum, sulfur and calcium in the X-ray images delineate ettringite.

The difference between these cores and those from pavements incorporating fly ash is the

increased clustering of entrained air voids. The filling of entrained air voids, as seen in

these specimens, is not a feature unique to the fly ash-containing concrete pavements.

The entrained air void system may also be considered to be marginal to sub-standard.

Subsequent filling of the air voids with ettringite resulted in a decrease in total air volume

and an increase in spacing factors. While these pavements appear sound, the air void

data would indicate that they would exhibit deterioration as time progresses.
34

Air Void Distribution: Core 13

0.0

5.0

10.0

15.0

20.0

25.0

30.0

0 5 10 15 20 25

Depth (cm)

Volume Percent Air

Total

Effective

Air Void Spacing Factor

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0 5 10 15 20 25

Depth (cm)

Spacing Factor (mm)

Total

Effective

Component Distribution

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Depth (cm)

Volume Percent

Aggregate

Pa ste

Filled

Effe ctive

Figure 25. Air void distribution and spacing factors versus depth, and concrete

component distribution for Core 13. The air volume appears uniform with depth while the

spacing factor appears substandard in the upper 100 mm. Filling has resulted in a slight

increase in the spacing factor.
35

Air Void Distribution: Core 14

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

0 5 10 15 20 25 30

Depth (cm)

Volume Percent

Total

Effective

Air Void Spacing Factor

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0 5 10 15 20 25 30

Depth (cm)

Spacing Factor

Total

Effective

Component Distribution

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

Depth (cm)

Volume Percent

Aggregate

Paste

Filled

Effective

Figure 26. Air void distribution and spacing factors versus depth, and concrete

component distribution for Core 14. The total air volume appears similar to that of Core

13 from the joint and appears uniform with depth (deviations from the median probably

reflect entrapped air voids). The spacing factor appears marginal to sub-standard at

depths below 6 cm, and filling of the smaller entrained air voids has resulted in an

increase in void spacing factor.
36

Table 6. US 20 Location 3, Webster County: Materials and Properties [28].

Cement: Type I,

3.5 % SO3,

0.77 % equivalent alkali,

Coarse Aggregate: Fort Dodge Limestone, Gilmore City

< 1 % FT Loss, 0.9 % absorption,

LA abrasion 28 % loss, durability factor 94

Fine Aggregate: Croft Sand

Admixtures: WR, AEA

Fly Ash: Class C:

3.62 % SO3,

3.73 % equivalent alkali,

0.45 % loss on ignition

Concrete Batch Mass / m3

Cement 274 Slump: 50 mm to 38 mm, avg. 43 mm

Water 160 Air: 6 % to 7.2 %, avg. 6.6 %

Fly Ash 46

C.Agg. 980 Slipform paver, spreader

F.Agg. 821 Placing conditions: 29 °C, clear/sunny

WR 7 Mix Characteristics: very harsh

AEA no value 31 MPa compressive strength

Cores:

No. Mile Post Location Vibrator Trail Designation Condition/Comments

18 124 Joint Between 18jb appears sound

19 125.55 Joint Between 19jb severe cracking

bituminous overlay
37

3.2.3.3 US 20, Webster County Location 3

Cores from this sampling site were extracted from US 20, west bound. This pavement

was placed using IDOT concrete Mix 1 containing a class C fly ash. Cores from mile 124

(Core 18) were extracted from a short segment that was closed to traffic and not salted for

at least the first year. This pavement exhibits no evidence of distress and led to

speculation that deicing salts may be significant in the degradation [21].. In contrast,

specimens from Mile 125.55 (Core 19) are severely damaged, showing longitudinal

cracking along vibrator paths, cracks paralleling transverse joints, and map cracking. The

distress was first noted in 1990, about four years after placement, and the crack density

appears to decrease with distance from edges and joints.

3.2.3.3.1 Core Descriptions

Core 18 (Figure 27), from a sound pavement at mile 124.4, is approximately 300 mm in

length and appears different from Core 19 in that it seems to have a more uniform

aggregate gradation, and a substantially better entrained air void system. The concrete

appeared sound, though some microcracking was evident in the outer few millimeters

under SEM examination. The concrete cut cleanly and exhibits good aggregate - paste

bonding. Occasionally, a millimeter-sized void filled with alkali-silica reaction gel was

seen, but little cracking was observed adjacent to the gel.

The cement paste is a uniform gray and appears strong and carbonation is limited to a

thin layer at the wearing surface. Entrapped air voids as large as 10 mm are common

throughout the core. The entrained air void system appears adequate and fairly uniform

from top to base, although some filling of the voids by ettringite is present throughout the

core. Linear traverse analysis (Table 2) indicates the original and effective air void

characteristics as changing little, though some ettringite filling has occurred and the

specific surface is lower than that recommended in ASTM C 457. The spacing factor is

within that recommended for freeze-thaw durability. This may have been achieved as a

result of the relatively high total air volume. Graphical representation of the air void

characteristics and concrete composition shows a uniform distribution of entrained air,

spacing factor, and component distribution (Figure 28). While much less pronounced

than at other sampling locations, the loss of entrained air voids due to filling appears to

be uniform with depth, and the decrease in specific surface values indicate a coarsening

of the void system. Shale fragments show evidence of alkali-silica reactivity, but little

significant cracking is present (Figure 29). No evidence of any overall paste expansion

was observed. X-ray imaging revealed a significant amount of chlorine in discrete

regions. These regions appear to be the phase chloroaluminate, or Friedel's salt

(C3A·CaCl2·10H2O) (Figures 30, 31). The presence of Friedel's salt is not unique to these

cores, nor to any specific locations within the core. This mode of occurrence suggests

that the chloride may have been present in the plastic concrete, possibly as an accelerator.
38

Figure 27. Core 18 contains about 12 % entrained air volume and a spacing factor of 0.08

mm. Aggregate gradation appears more uniform, and possibly smaller maximum size

than the degraded pavement concretes. Low specific surface and common entrapped air

voids are features common to other cores in this study.
39

Air Void Distribution: Core 18

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

0 5 10 15 20 25 30

Depth (cm)

Volume Percent Air

Total

Effe ctive

Air Void Spacing Factor

0.00

0.05

0.10

0.15

0.20

0.25

0 5 10 15 20 25 30

Depth (cm)

Spacing Factor (mm)

Total

Effective

Component Distribution

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Depth (cm)

Volume Percent

Aggregate

Paste

Filled

Effective

Figure 28. Air void and component distribution plots for core 18. Little loss of air void

volume is evident with only a slight, uniform loss of air void spacing factor. Component

distribution appears uniform from top to bottom.
40

Figure 29. Core 18 mortar microstructure includes numerous shale fragments that appear

to have undergone alkali-silica reaction, however little cracking of the mortar was

evident. SEM examination indicated that some of the reaction product permeated the

paste filling the capillary voids adjacent to the shale. Field width: 10 mm.

Figure 30. Friedel's salt in hardened paste is identified here using X-ray microanalysis. It

appears to be a primary hydration product in the paste throughout the cores, suggesting

that it may be a result of the presence of chlorides in the original mix as opposed to the

infiltration of road salts.
41

Figure 31. SEM of Core 18 shows an air void partially filled with ettringite (lower-right)

and partially reacted fly ash particles (circular). X-ray element distribution (lower image)

shows regions of high sulfur and aluminum that delineate locations of ettringite. Regions

of chlorine and aluminum delineate regions of chloroaluminate. Field width: 75 μm.
42

Core 19, from Mile 125.5 on US 20 is from a pavement reportedly containing the same

materials and mixture design as for Core 18. Pavements in this location have performed

poorly and show extensive deterioration. These sites present an opportunity to compare

sound vs. poorly performing concrete with similar, if not the same, materials,

proportioning, and placement history.

Core 19 was extracted from the joint region of a pavement. This section of US 20

exhibited the greatest amount of cracking of sampling sites in this study. The core

exhibits medium and fine open cracks throughout trending sub-parallel to the road

surface (Figure 32). Vertical cracks appear to pass around aggregates and many surface

cracks also pass through the coarse aggregate (Figure 33). Cracking within the aggregate

is common and may often be traced into the mortar. The coarse aggregate often exhibited

a de-lamination of the outer portions of the aggregate, much like the peeling of an onion

skin. This feature is similar to that seen in aggregate from a laboratory-prepared concrete

beam with known freeze thaw-susceptible aggregates. Damage in the mid-panel cores

from this location occurs as cracking sub-parallel to the surface and cracking in

aggregates. The mid-panel specimens show slightly less damage further from the

surface. All cores cut cleanly and, aside from the cracking, appear sound and strong.

Reactive shale in the sand fraction is present and a gel filling in voids adjacent to the

shale is occasionally found. Cracking associated with these fragments appears to be very

fine and limited in extent. The cement - aggregate bond appears strong. The cement

paste is a uniform gray with no evidence of bleeding and only a few millimeters of

carbonation along the road surface. Entrapped air voids to 10 mm are common

throughout each of the cores.

Figure 34 shows SEM images of the cement paste from Core 19. The upper,

backscattered electron, image shows a region that includes a shale grain (left side), and

hydration products, fly ash (circular), and pores (black). Alkali-silica reaction gel within

the aggregate may be located using the potassium X-ray image (upper-left of the nine

image series) yet no cracking is seen within the paste. Ettringite in the cement paste may

be identified by its platy parting and by the combined X-ray images of intermediate

aluminum, high sulfur, and intermediate calcium intensities. The occurrence of ettringite

within the cement paste is common and may be either primary or a replacement of

monosulfate. The gaps surrounding the ettringite grains are, in part, shrinkage phenomena

due to the microscope vacuum but may also be a result of dissolution. The relatively

large, irregular-shaped pores (black) may indicate dissolution of monosulfate and calcium

hydroxide. Field examinations of this pavement did not find any evidence of overall

expansion, making the occurrence of ettringite apparently innocuous. Regions of the

chlorine (lower-left) and intermediate-intensity aluminum images delineate

chloroaluminate, or Friedel's salt.
43

Figure 32. Core 19 exhibits extensive cracking of both the paste and coarse aggregate.

Cracks trending parallel to the pavement surface (to the left) intersect perpendicular

cracks; features typical of freeze-thaw cracking. The oblique view of the upper portion

of core 19 shows cracking in both the surface (polished for clarity) and cross-section

orientations.
44

Figure 33. SEM image of a surface crack (road surface is at the top, 5 mm field width)

extending through the paste portion of the mortar and terminating in a coarse aggregate.
45

Figure 34. Back scattered electron image of Core 19 paste microstructure showing ASRaffected shale (left) and fly ash (circular). X-ray image regions of high sulfur (yellow)

and intermediate aluminum (purple) mark locations of ettringite while regions of high

chlorine (lower left image) denote locations of chloroaluminate.
46

Air void analysis (Table 2) show that total air decreased 1.6 % and the spacing factor

more than doubled to 0.29 mm, while the specific surface dropped indicating a

substantial coarsening of the effective entrained air void size distribution. The specific

surface value of 14.67 is much lower than the recommended interval of between 23.6 and

43.3 indicating an air void size distribution much coarser than that recommended by

ASTM C 457 [44].

Visual examination of Cores 18 and 19 found a likely difference in aggregate grading

(Figure 35) with Core 18 appearing to have a more uniform, and perhaps smaller,

aggregate gradation. The apparent change in aggregate gradation may have affected the

development of the higher-volume entrained air void system in Core 18. However,

specific surface values indicate a coarse air void size distribution.

Graphical representation of the air void and component distributions indicates a decrease

in spacing factor and an increase in cement paste content with depth (Figure 36). The

spacing factors for both the total (original) and effective systems fall beyond that

necessary for frost protection at depths greater than 150 mm.

Figure 35. Core 19 (left) appears to have a finer-sized sand and possibly a larger

maximum coarse aggregate size when compared to Core 18 (right).
47

Air Void Distribution: Core 19

0.0

5.0

10.0

15.0

20.0

25.0

0 5 10 15 20 25 30

Depth (cm)

Volume Percent

Total

Effective

Air Void Spacing Factor

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0 5 10 15 20 25 30

De pth (cm )

Spacing Factor

Tota l

Effective

Component Distribution

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

De pth (cm )

Volume Percent

Aggrega te

Paste

Filled

Effe ctive

Figure 36. Core 19 data show a decrease in spacing factor with depth and possibly a trend

to an increased loss of spacing factor with depth. The component distribution plot shows

a possible increase in aggregate and decrease in paste in the upper half of the core.
48

Figure 37 shows the results of modeling aggregate expansion to simulate freeze-thaw

failure of the coarse aggregate. Using a finite element program on the digital images,

selected components of that image, such as the aggregate or the paste, are allowed to

expand. Other components in the concrete serve to restrain this expansion, giving rise to

stresses in the concrete. To simulate the field conditions, the section was surrounded on

three sides by an effective material having the same material properties as the bulk

concrete, while the upper surface was free to expand. This technique is still in

development and is limited in that it uses a linear elastic finite element algorithm. Only

stress analysis is considered, and the code uses finite-sized, 2-dimensional images.

However, it should provide some insight as to how differing deterioration modes may

affect the crack patterns [29,30].

The upper image is of the specimen examined in the electron microscope. Cracking (red)

and ASR-affected shales (yellow) have been highlighted to provide a clearer picture of

their occurrence within the concrete. The model was configured such that the core section

was surrounded by similar material and only the top surface was free to move. In this

example, expansion of the aggregate produced the color-coded stress image where

intensity increases with a color change from black to yellow to red. High stress regions

occur in the paste where two aggregates lie within close proximity, and within the

aggregates. Many of these sites are also locations of cracking. However, not all of the

cracks may be explained using this simulation, so aggregate expansion can explain only

some of these cracks. Simulations where the paste was allowed to expand, such as in

freeze-thaw cycling of paste, also showed high stresses in regions of cracking.

This data indicates that both coarse aggregate expansion and paste expansion may explain

many of the cracks in this specimen. As no alkali-aggregate reaction products are

associated with cracking in the coarse aggregate, these cracks must be attributed to

freezing of water within the aggregate pore system. The paste expansion must be a result

of freezing as the lack of evidence of a permanent expansion and lack of paste-aggregate

interfacial zone gaps eliminates the overall, uniform paste expansion of delayed ettringite

formation. Additionally, if the relatively coarser-sized pore systems of the aggregate are

critically saturated, then the paste pore systems must also be saturated.
49

Figure 37. Top portion of core 19 with cracks (red) and ASR-affected shale grain

(yellow) shows cracking through both the cement paste and coarse aggregate. A colorcoded stress image resulting from a simulation of aggregate expansion showing regions

of high stresses in red correlates well with some, but not all of the observed cracking.
50

3.3 Iowa I-35, Story County

Iowa I-35, Story County at mile post 122.1 (Table 7) exhibits longitudinal cracking and

joint staining, along with pattern cracking at mid-panel joints. These pavements exhibits

spotty damage that is less well-developed, and, according to Iowa DOT, were slower to

show deterioration.

Concrete appears sound, well-proportioned, uniform, cuts cleanly with a strong appearing

cement paste-aggregate bond (Figure 38). A single horizontal crack was found in the

lower-third of the Core 11. The coarse aggregate consists of an angular crushed limestone

predominantly of a grainstone texture. These specimens appear to have a finer coarse

aggregate size and more uniform aggregate size distribution compared to those from US

20. The fine aggregate is composed of angular, silicious sand with a small proportion of

oblate shale grains of up to 5 mm in size. The cement paste appears uniform and gray

with carbonation limited to the outer few millimeters of mortar. Entrapped air voids were

common in the regions about 4 cm above and below the surfaces. Vertical cracks from

the surface were common and appear to terminate both within the mortar and a coarse

aggregate (Figures 39, 40).

The entrained air void system (Table 2) was measured at 8 % to 9 % air by volume, with

a spacing factor of 0.16 mm to 0.18 mm, and specific surface of 17 mm-1 to 21 mm-1.

These values are within recommended limits with the exception of the specific surface.

Filling of some of the voids by ettringite results in a doubling of the spacing factors to

0.30 mm and 0.32 mm, a decrease in specific surface, and a loss of 2 % air volume for

the joint core. This was the greatest volume loss for the set examined in this study. The

original air void systems for cores 11 and 12 appear uniform with depth. Filling, in

contrast, appears uniform for core 11 (Figure 41), but increases with depth for Core 12

(Figure 42), the mid-panel specimen. This may reflect the availability of water at the

joint as compared to the mid-panel. The increased filling with depth may reflect a source

of water beneath the slab or the effects of surface drying.
51

Table 7. I-35 Story County Location 5: Materials and Properties [28].

Cement: Type I,

3.15 % SO3,

0.63 % equivalent alkali

Coarse Aggregate: Alden, open quarry, limestone,

1.50 % freeze-thaw loss, 2.70 % absorption

LA Abr. 40 % loss, durability factor 79

Fine Aggregate: Hallet (Ames pit)

Admixtures: WR, AEA

Fly Ash: Class C,

3.36 % SO3,

2.87 % equivalent alkali,

0.33 % loss on ignition

Concrete Batch Mass / m3

Cement 303 Slump: 76 mm to 19 mm, avg. 48 mm

Water 150 Air: 8 % to 5.3 %, avg. 6.1 %

Fly Ash 52

C.Agg. 946 slipform paver, with spreader

F.Agg. 828 Temp: 32 °C, no precipitation, cloudy / cool

WR No Value Mix Characteristics: no problems

AEA no value 33 MPa Compressive strength

actual material mass estimated from mass proportions [28]

Cores:

No. Mile Location Vibrator Trail Designation Condition

11 122.1 Joint not visible 11j minor pattern cracking, joint

12 122.1 Midpanel 12m staining, and longitudinal

cracking
52

Figure 38. Cores 11, joint (top) and 12, mid-panel (bottom) cross section with horizontal

crack in lower-most third. Some segregation is visible in both cores.
53

Figure 39. Surface cracking in Core 11 to about 20 mm passes through both the mortar

and coarse aggregate. Lower image shows minor surface carbonation and cracking

within the mortar. Field width 8 mm.
54

Figure 40. Surface crack in core 12 (approx. center-left) appears to be entirely within the

mortar, and the lower image shows the crack plane through mortar highlighted using ink.

8 mm field width.
55

Air Void Distribution: Core 11

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

0 5 10 15 20 25

Depth (cm)

Volume Percent Air

Total

Effective

Air Void Spacing Factor

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0 5 10 15 20 25

Depth (cm)

Spacing Factor (mm)

Total

Effective

Component Distribution

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Depth (cm)

Volume Percent

Aggregate

Paste

Filled

Effective

Figure 41: Core 11 air void parameters and component distribution show a loss of air

void spacing yet relatively uniform air void volume and materials distribution.
56

Figure 42. Core 12 air void volume and spacing parameters, and component distribution

shows an increased degree of entrained air void filling with depth.

Air Void Distribution: Core 12

0.0

5.0

10.0

15.0

20.0

25.0

0 5 10 15 20 25 30

Depth (cm)

Volumer Percent

Total

Effe ctive

Air Void Spacing Factor

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0 5 10 15 20 25 30

Depth (cm)

Spacing Factor (mm)

Total

Effe ctive

Component Distribution

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

Depth (cm)

Volume Percent

Aggregate

Pa ste

Filled

Effective
57

Table 8. I-80, Milepost 210.40: Materials and Properties [28].

Cement, Type I,

SO3 2.41 %; 

Equivalent Alkali 0.64 %; 

Coarse Aggregate: Conklin Quarry, Limestone

Freeze-thaw: 5 % loss; absorption 0.72 %; 

LA Abrasion: 39 % loss; durability factor 79

Fine Aggregate: Kimmich Pit (Conklin Quarry)

Admixtures: WR; AEA

Fly Ash: Class C,

1.97 % SO3,

1.88 % equivalent alkali

0.25 % loss on ignition

Concrete Batch Weights:

Cement 299 kg 0.42 w/s Slump: 76 mm to 19 mm, average- 48 mm

Air 10.5 % to 5.4 %,

Water 148 Temps: 37 °C to 12 °C, no rain, clear/warm

Fly Ash 54 CMI SF-550 Slipform paver, hyd. stinger

C.Agg. 900 Mix Characteristics No data located

F.Agg. 896 Strength average 30 MPa

AEA

Cores:

No. Mile Location Vibrator Trail Designation Condition

29 210.40 Midpanel on 29mb pattern and longitudinal

cracking, distinct vibrator

trails.
58

3.4 Iowa I-80

Core 29 was selected from I-80 to examine microstructural differences of concretes

containing distinct vibration trails. Tining appeared uniform on the surface to a depth of

about 3 mm. Minor pattern cracking is present along with pronounced vibration trails,

and longitudinal cracking along these trails. The pattern cracking density increased close

to the longitudinal cracks, running parallel to both the longitudinal cracks and the

transverse joints. Core 29 was centered upon one of these cracks and was selected for air

void and materials distribution analysis.

Core 29 contains a large surface crack about 0.5 mm wide running parallel to the long

axis of the pavement (Figure 43). The crack appears to extend about 40 mm into the

core. Other less prominent surface cracks trending vertically extend a few centimeters

into the core through the mortar. The plots of air void parameters clearly illustrate the

lower half of the core having greater entrained and entrapped air compared to the upper

half (Figure 44). These plots also indicate that the spacing factors are substandard the

entire depth of the core for both the original and effective air void system. Materials

distribution appears fairly uniform with mortar-rich regions common throughout the core.

The plot of materials distribution shows a slight increase in paste in the upper half of the

core. Cracking observed using optical microscopy appears confined to the upper portions

of the core and pass through both the mortar and coarse aggregate. Filling of the

entrained air voids occurs in the upper portion of the core and was not seen in the top 4

cm. This may reflect the availability of water along the surface crack. The decrease in

filling in the upper-most 4 cm may reflect the zone of drying. The lack of filling in the

lower portion of the core may indicate a lack of a microcrack network at depth and / or

lack of water at the slab base.
59

Figure 43. Core 29 cracking near surface passes through both the mortar and coarse

aggregate. Cracking also follows close along paste-aggregate interface and is typical of a

shrinkage crack. Cracking in aggregate and in the mortar at depth may reflect freeze-thaw

damage, as the air void system in the upper half of this core is much poorer than that in

the lower half.
60

Figure 44. Core 29, centered on a well-defined vibration trail exhibits a significant

increase in spacing factor in the upper half. This probably represents the zone of

influence of the immersion vibrator probe. Note that the spacing factor is generally not

satisfactory at any depth.

IDOT Core 29 (vt)

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

0 5 10 15 20 25 30

Depth (cm)

Volume Percent Air

Total

Effective

Air Void Spacing Factors

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

0 5 10 15 20 25 30

Depth (cm)

Spacimg Factor (cm)

Total

Effective

Component Distribution

0.00

20.00

40.00

60.00

80.00

100.00

13579

1113151719212325

Depth (cm)

Volume Fraction

Aggregate

Paste

Filled Air

Effective Air
61

4. DISCUSSION AND CONCLUSIONS

4.1 Primary Cause of Pavement Deterioration

Based on visual examinations, petrographic evaluation, and an application of materials

models, the deterioration of concrete highway pavements in Iowa appears to be related to

a freeze-thaw failure of the coarse aggregate and the mortar. Surface cracks traceable into

the coarse aggregate, cracking of coarse aggregate and crack patterns sub-parallel to the

concrete surface transecting the mortar fraction and the coarse aggregate are indicative of

freeze-thaw damage of these constituents.

The entrained air void system was marginal to substandard, according to

recommendations provided in ACI 201. Filling of the finer-sized (<125 μm) entrained

air voids by secondary ettringite further degraded the air void system. The filling

approaches up to 2 % by volume resulting in an increase in void spacing factor and

decrease in specific surface values generally in excess of those considered necessary for

freeze-thaw exposure.

4.2 The Significance of Secondary Ettringite

The formation of secondary ettringite within the entrained air voids reflects a relatively

high degree of concrete saturation causing the smaller voids to be filled with pore

solution when the concrete freezes. Freeze-concentration of dissolved solids in the pore

solution and temperature changes may result in a condition contributory to precipitation

of secondary ettringite. Over time, the cyclic freezing and thawing accompanied by

secondary ettringite precipitation would result in filling of some of the entrained air

voids.

The source of material for ettringite formation is probably from dissolution of calcium

monosulfoaluminate within the cement paste. The dissolution of these phases (less than

10 % by volume) and subsequent precipitation as ettringite in the air voids may increase

permeability of the paste facilitating the deterioration. Calculations on potential volume

of ettringite based upon mix design and cement composition estimates indicate that more

than sufficient ettringite (8 % to 10%) may be produced as a result of cement hydration to

fill the entrained air void system. Therefore, losses of 2 % and less, by volume of the

entrained air void system due to secondary ettringite formation, should not be considered

to indicate excessive ettringite formation as proposed in some studies [9].

The occurrence of ettringite in concretes is not new, nor unique, and in many cases its

presence in voids is attributed to the specimen being from "old concrete". Terzaghi [31]

observed secondary ettringite crystallized in air voids in cores extracted from a gate

structure of a shipway in Virginia. She considered this to reflect the presence of

seawater in the pores of the concrete that provided the conditions necessary for formation

of ettringite. Idorn [32] noted that ettringite is a common occurrence in sound, older

concretes and will often be found to accumulate in voids and cracks. Trägårdh and

Lagerblad [33] examined concretes damaged by a combined alkali-silica reactivity and
62

freeze-thaw attack. They commonly observed calcium hydroxide and ettringite in air

voids and attributed their occurrence to dissolution and leaching caused by water

movements during recurrent cycles of freezing and thawing. The dissolved constituents

precipitate in cracks and air voids as secondary products.

Taylor [34] examined microstructural and chemical aspects of sulfate reactions in

concrete and concluded that the ettringite occurring in cracks and voids in both sound and

deteriorated concretes is not necessarily the cause of the cracking but a product of

recrystallization in a pre-existing crack or void of some other origin. He states that in

early stages of hydration, ettringite is replaced by monosulfate as a result of continued

hydration of aluminate. Ettringite is frequently observed in mature concretes, whereas

monosulfate is not. Studies using backscattered electron imaging and X-ray diffraction

(XRD) at NIST do find monosulfate in mature cement pastes but the XRD data do not

exhibit very strong diffraction peaks. This may indicate a poorly-crystalline monosulfate,

or changes due to dehydration or relative humidity during powder preparation. Taylor

[34] feels that the most likely cause for the presence of ettringite in concretes is from

carbonation, resulting in the monosulfate being replaced by a mixture of ettringite and

hemicarbonate. He considered the carbonate could come from small amounts of calcite in

the gypsum and notes that only 2.4 % (by mass of monosulfate) CO2 is necessary.

Atkins et al. [35] performed solubility measurements of ettringite (C6A 3H32),

hydrogarnet (C3AH6), monosulfate (C4A H12), and tetracalcium aluminate hydrate

(C4AH13) and found that dissolution of pure monosulfate lead to the precipitation of

ettringite with sucessive dispersions. These findings were in agreement with those of

Brown [36] that the tetra-calcium aluminate hydrates are thermodynamically metastable

relative to C3AH6 and ettringite. C4AH13 was also found to be metastable relative to

C3AH6.

Ludwig, Stark, and Eckart [37,38,39,40] exposed a suspension of monosulfate in water to

freezing and thawing cycles and found monosulfate will transform to ettringite. They

considered a change in thermodynamic stability at low temperatures to account for the

transformations where formation of ettringite is favored and felt the sulfate required for

this reaction was derived through partial decomposition of the monosulfate. They also

found that freeze-thaw cycling in 3 % NaCl solution resulted in transformation of

monosulfate to Friedel’s salt and ettringite. In these experiments, they observed no

indications of carbonation, and thought the SO3 required for ettringite formation may

come from the conversion of monosulfate to monochloride. The apparent uniform

distribution of Friedel's salt in the Iowa cores would indicate, for these pavements, that

either the chloride was present upon mixing or, less likely, that they have been permeated

by a road salt solution.

Skalny et al. [41] discuss ettringite formation in concrete as a consequence of an aging

via Ostwald ripening. They consider a high moisture content to facilitate this process of

dissolution, transport, and recrystallization into the available empty spaces and, that this

mode of ettringite formation is non-expansive. Neville [49] also recognizes the role of

excess moisture in this process as hydration products may only be formed in water. The
63

formation of ettringite in the air voids therefore must indicate they are at least partially

filled with water.

Detwiler and Powers-Couche [42] examined the effects of sulfates in concrete on its

resistance to freezing and thawing through a series of laboratory freeze-thaw tests using

both 3 % NaCl and 3 % NaCl and gypsum solutions. They concluded that the freezethaw damage precedes the filling of voids with ettringite with deposition occurring in

cracks and voids after damage occurred. These data do not explain the occurrence of

filled voids in concretes from this study where filled voids may be observed in concretes

exhibiting little or no damage, but they do show how easily the formation of secondary

ettringite in voids can occur in concretes exposed to cyclic freezing and thawing.

4.3 Freezing and Thawing

Freeze-thaw cracking of aggregate may be related to both the high degree of saturation

and the loss of the adjacent air void system as no correlation between aggregate lithology

and cracking was noted. Modeling simulations of deterioration processes indicate that

cracking may be explained by both aggregate and paste expansions.

Wang et al. [43] found that ice did not completely fill the pores in cement paste and that

supercooled water was always present due to alkalinity of the cement paste. Ice

propagation in aggregates is faster than in cement paste and at -10 °C, ice was found in

large pores but not small ones. Excess water in aggregates is expelled when concrete is

frozen. Depending on size, permeability, and aggregate saturation, water flow from the

aggregate may be too rapid to diffuse into the surrounding paste creating a pressure that

may disrupt the paste at the paste / aggregate interface. Ice formation is from pure water

so pore solution becomes increasingly concentrated. This creates an osmotic pressure

that may be sufficient to result in failure.

The original air void system parameters generally appear to be uniform from top to base

of the concrete. The excessive vibration, considered to be contributory [9], appears to not

have adversely affected the entrained air void system for specimens taken away from the

direct points of immersion probe vibrator insertion. The single vibrator trail specimen

showed a substandard entrained air void system in the upper third of the core, and at the

depth of the probe. Joint specimens exhibiting greater filling of the entrained air voids,

which may reflect greater water availability to concrete adjacent to joints. The filling of

the entrained air void system generally appears uniform with depth, although some midpanel specimens appear to have increased filling with depth. These observations suggest

that the degradation is not a “top down” process by infiltration of gypsum-contaminated

road salt solutions. The increased filling at depth of some of the mid-panel specimens

indicates a significant influence of surface drying and / or a source of water at the base of

the slab. The low-permeability base may actually have facilitated deterioration by

wicking water to the base of the pavement or by hindering drainage.

Entrained air affects plastic concrete by providing a potential for improved workability,

improved resistance to segregation, settlement, and bleeding. The improvement of these
64

properties is dependent upon the total air volume, the size distribution and dispersion of

the voids, and the material properties of the concrete. However, the primary purpose of

the entrained air void system is to facilitate frost resistance in hardened concrete. Air

voids, being much larger than the capillary pores, remain open with water filling them

only at sub-freezing temperatures, or when under pressure [44].

Early theories of frost damage in concrete were based upon the expansion of ice upon

freezing and the subsequent stress increase [45]. Later, consideration that concretes

contain enough air space to accommodate this expansion resulted in thoughts that stresses

are produced by the flow of the water during freezing. The development of an entrained

air void system for protection of concrete allowed room for excess water to migrate upon

freezing. Finally, an osmotic pressure theory was developed based upon ice formation in

voids leaving an alkali-concentrated supercooled water. Subsequent movement of this

unfrozen water toward the air voids may create an osmotic pressure sufficient for

concrete disintegration. Other factors in frost damage that must be considered are the

availability of water, with the volume of freezable water (the degree of saturation)

influenced by initial porosity, concrete age, curing conditions, and environmental

exposure [44].

ASTM C 457 [46] provides a means to measure air-void size and size-distribution.

Entrained air void bubble sizes range from a few micrometers to one millimeter. Largersized voids, while contributing to frost resistance, are considered to be entrapped air. The

total paste volume protected by these large voids is much less than that for the smaller

voids. The air volume is a measure of the total volume of the air bubbles in the hardened

concrete. The spacing factor, considered to be the most significant measure of frost

resistance, is defined as the theoretical maximum distance from any point in the paste to

the edge of the nearest air void. ASTM C 457 recommends a spacing factor between 0.10

mm to 0.20 mm and notes that these values are for moderate exposures. Concretes in

more severe environments, such as pavements that may be exposed to water upon

freezing, may require smaller values than those stated here. Specific surface is

comparable to “fineness” in cements, and is calculated by dividing the cumulative surface

area of the voids by their cumulative volume. This variable is expressed as a surface area

per unit volume with higher values indicating a finer size distribution. ASTM C 457

provides acceptable values for typical concrete being 24 mm2

/mm3 to 43 mm2/mm3.

Freezing of water within aggregate pore systems may also pose problems in concrete

durablity [ 45, 47]. Similar to freezing of water in cement paste, saturated aggregate pore

systems will expel water during freezing. This may result in a hydraulic pressure induced

stress within the aggregate and in the aggregate / cement paste interfacial zone. The

'onion skin'-appearing failure of many of the aggregates may result from this pressure.

This effect is increased with aggregate size and is influenced by freezing rate and degree

of saturation. T.C. Powers reports that the effects of entrained air in accommodating

pressures from freezing of aggregates appear to be minimal [45].

The influence of immersion-type vibrators on the entrained air void system has been

shown to have a limited zone of influence. Simon et al. [48] demonstrated that concrete
65

close to the immersion probe may experience a reduction of 50 % of the total air and a

100% increase in specific surface. They found a decrease in the large entrained air voids

at the point of insertion, along with an increase in small air voids. They concluded that

some of the larger bubbles are being broken up into smaller bubbles. However, beyond a

radius of 125 mm, vibration has little effect on air void systems. For the Iowa concretes,

the air void system quality, if judged primarily on spacing factor did not appear to be

adversely affected by vibration outside of the vibrator trails, and in many cases was

improved near the surface. The core taken directly from a vibrator did exhibit a substandard air void system for about half of its depth, roughly the probe immersion depth.

Mehta and Monteiro [47] indicate that aggregate gradation affects the volume of

entrained air, whose volume may be decreased by excess sand and the presence of finesized mineral admixtures. Mixing, transportation, and placement procedures need to be

optimized as insufficient mixing, over mixing, excessive transport and handling time, and

over vibration all can reduce the air void system.

Neville [49] also acknowledges the influence of mixing on air content: “the cement

should be well dispersed and the mix uniform before the air entraining agent is

introduced. If the mixing time is too short, the air entraining agent does not become

sufficiently dispersed, but overmixing may expel air - there is an optimum mixing

procedure.”

Kosmatka and Panarese [50] identify mixing action as one of the most important factors

in production of entrained air in concrete. The lack of a suitable entrained air void system

in most of these cores indicates a need to examine the effects of production procedures on

the development of the entrained air void system.

4.4 Microcracking

Microcracking in the form of drying shrinkage, or freeze-thaw cracks is common in most

of the cores as vertical surface cracks extending 10 mm to 20 mm into the core. These

cracks may be found both exclusively in the cement paste, and also through the paste and

coarse aggregate. Cracks along the vibration trails appear to be drying shrinkage cracks

resulting from the greater shrinkage potential of the mortar-rich vibration trail regions

and the bulk concrete. The pattern cracking observed on the surface may result from

similar phenomena. Cores through vibration trails exhibit a lower entrapped air volume,

poor air void distribution in upper half, and appear to be enriched in the mortar fraction

due to segregation.

Bakharev and Struble [51] examined microstructural features associated with freeze-thaw

deterioration. An early manifestation was microcracking extending vertically several mm

into the specimen. Additional freeze-thaw cycles resulted in a horizontal cracking that,

when intersecting the vertical cracks, resulted in spalling. They considered the crack

development to result from localized expansive shear stresses from hydraulic pressure

and localized stresses from differential volume changes. They postulate as the top layer

of the concrete is frozen, water is expelled to the surface and to the interior causing
66

hydraulic pressure. As the permeability of the frozen surface layer is low, water moves

inward. Horizontal crack development may be due to development of shear forces at

some distance from the surface due to differential volume change of saturated and

unsaturated layers, that is, the surface layers are expanding while unsaturated interior

layers may be contracting.

4.5 Alkali-Aggregate Reaction

Alkali-silica reaction affects the shale and an occasional quartz sand grain in the fine

aggregate, but is not deemed to be significant cause of the deterioration. The field survey

found no evidence of permanent expansion such as closure of joints, blowups, nor gel

efflorescence. The cement paste pore structure adjacent to the shale grains appears filled

with reaction product, with only minor cracking (about the length of the sand grain). The

accommodation of reaction products by the cement paste pore structure may have limited

expansive stresses created by the reaction of the shale with alkali hydroxides in the pore

solution. Significant cracking associated with ASR may occasionally be found when a

large (5 mm) shale aggregate is located in the upper-most 20 mm of the concrete.

4.6 Delayed Ettringite Formation

Delayed ettringite formation was not deemed likely as no evidence of a uniform paste

expansion away from aggregates was observed, no massive agglomerations of ettringite

in the paste, near aggregates, or in cracks, and no filling of the capillary pore system was

observed. The lack of a permanent expansion, as mentioned above, is also evidence

against this mode of deterioration.

4.7 Fly Ash

The utilization of fly ash does not appear to have significantly affected the deterioration

as 1) earlier test pavements (Iowa 175) incorporating fly ash are sound, 2) a US 20

pavement considered sound, around mile post 124, utilized concrete containing fly ash.

Test pavements on I-175, utilizing Class F and C fly ashes and a control section,

exhibiting good performance are characterized by a good entrained air void system and

some material heterogeneity. Cracking in one core bottom occurs in regions exhibiting

filling of the entrained air voids with secondary ettringite. Spacing factors and specific

surface estimates in the core base appear adequate, that is, indicates a freeze-thaw

resistant concrete. Therefore, this damage indicates the lower portion of the concrete was

critically saturated, probably with the water source at its base. US 20 pavements are

among some of the best and worst performing pavements in this study. A set of

specimens, reportedly containing the same materials (including fly ash) and the same mix

design, exhibit opposite performance characteristics. Cores extracted from the

undamaged stretch of the road appear to have a smaller coarse aggregate maximum size

and a more uniform aggregate gradation. These cores contain an entrained air content

almost double, and a spacing factor half that of the failed pavement. The additional air

volume may account for the improvement in spacing factor and resulted in an air system
67

adequate for freeze-thaw protection. Specific surface values for these pavements still

indicate a coarse-sized entrained air void system. The change in aggregate gradation may

have altered the mix rheology and facilitated development of a more protective entrained

air void system.

The influence of the mix design, mixing, and placing must be evaluated with respect to

the development of an adequate entrained air void system, concrete homogeneity, longterm drying shrinkage, and microcracking. A high-sand mix may contribute to the fieldobserved harsh mix characteristics and exacerbate concrete heterogeneity, difficulty in

developing an adequate entrained air void system, poor consolidation potential, and

increased drying shrinkage and cracking. The availability of moisture, in particular to the

base of the pavement slab, must also be considered, as excess water is necessary for these

failures.

5. ACKNOWLEDGEMENTS

The support of the State of Iowa, the Portland Cement Association, and the National

Institute of Standards and Technology is acknowledged. The assistance of Dr. Steven

Forester and Marcia Simon of the Turner-Fairbanks Research Laboratory of the Federal

Highway Administration provided access to their air void analysis system. The

assistance of Vernon Marks and the Iowa Department of Transportation during the field

study and collection of the cores was appreciated. The assistance of the reviewers, James

Clifton, Clarissa Ferraris, and Steve Lane is appreciated.

The contents of this report reflect the views of the author, who is responsible for the facts

and accuracy of the data presented. The contents do not necessarily reflect the views of

Iowa DOT or the Portland Cement Association.
68

 Appendix A

Estimate of Potential Ettringite Formation in Concrete

I. Physical Properties of Cementitious Materials Relevant to Ettringite Formation.

Phase Molar Volume Molecular Weight Specific Gravity

(cm3

/mole) (g/mole) (g/cm3)

Ettringite 735.0 1249.5 1.70

C3A 89.1 269.9 3.03

C4AF 128.0 477.4 3.73

Gypsum 74.2 172.1 2.32

To determine the potential volume fraction of ettringite in concrete, we find:

Assuming each mole of C3A or C4AF forms one mole of ettringite we can write:

Examples using median values of Iowa concrete cements

US 20: C3A= 0.0864, C4AF 0.0541, Air = 7 %

Cement content calculated at 0.2906 Mg cement/m3

 concrete

Substituting into equation 4, the potential ettringite volume fraction of 0.0926 m3

ettringite / m3 concrete is greater than the 0.07 (7%) air void content

US 218: C3A = 0.066, C4AF = 0.0541, Air Content = 6.5 %

Cement Content = 0.3362 Mg cement / m3

 concrete

Potential ettringite volume fraction of 0.0957 m3 ettringite / m3 concrete is greater than

the .0065 (6.5%) air void content.

The potential ettringite volumes exceed that of the air void system. This suggests that the

entire entrained air void system could potentially be filled by ettringite (if sufficient

sulfate is available) and so, the 2 % maximum volume filling observed in the cores does

not necessarily indicate an external sulfate source.

m concrete

Mg ettringite

m concrete

m ettringite

3 3

3

1.7

(1)

× =

m concrete

MgC A MgC AF

3

3 4

1.7

477.44

1249.5

269.97

1249.5

(2)

×

× + ×

=

( )

m concrete

C Acontent C AFcontent Mgcement

3

3 4

1.7

477.44

1249.5

269.97

1249.5

(3) ×

× + × ×

=

m concrete

C Acontent C AFcontent Mgcement

3

3 4 1.5395

269.97

1249.5

(4)

×











× + ×

=
69

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