Hbrery, U.K. Blag

APR 4 1964

NBS MONOGRAPH 74

Creep and Drying Shrinkage

of Lightweight and

Normal-Weight Concretes

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

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UNITED STATES DEPARTMENT OF COMMERCE • Luther H. Hodges, Secretary

NATIONAL BUREAU OF STANDARDS • A. V. Astin, Director

Creep and Drying Shrinkage

of Lightweight and

Normal-Weight Concretes

T. W. Reichard

National Bureau of Standards Monograph 74

Issued March 4, 1964

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C., 20402 - Price 30 cents
Library of Congress Catalog Card Number: 64-60007
Contents

Page

1. Introduction 1

2. Scope of investigation 1

3. Materials 2

3.1. Aggregates - 2

3.1.1. Lightweight aggregates 2

3.1.2. Natural dense aggregates 3

3.2. Cements 3

3.3. Air-entraining agent 3

4. Concrete mixes 3

4.1. Trial mixes- -i 3

4.2. Test concrete mixes 4

5. Preparation of test specimens 4

5.1. Description of specimens 4

5.2. Casting of specimens 4

5.3. Curing of the series A concretes 4

5.4. Preparation of cylinder bearing surfaces 4

6. Test procedure 6

6.1. Compressive strength test 6

6.2. Dynamic modulus of elasticity and Poisson's ratio 6

6.3. Flexural strength test 6

6.4. Creep and shrinkage text 6

7. Results 7

7.1. Results—Series A tests ^ 7

7.1.1. Compressive strength test results 7

7.1.2. Moduli of elasticity and Poisson's ratio 8

7.1.3. Results of flexural strength tests 9

7.1.4. Creep and shrinkage 10

7.1.5. Moisture expansion 15

7.2. Results—Series B tests 15

7.2.1. Supplemental test I 16

7.2.2. Supplemental test II 16

7.2.3. Supplemental test III 18

7.2.4. Supplemental test IV 19

7.2.5. Supplemental test V 19

7.2.6. Supplemental test VI 20

8. Discussion of results 20

8.1. Significance of slump as a measure of water content of concrete 20

8.2. Statistical analyses of test results 20

8.3. Compressive strength 21

8.4. Modulus of elasticity 21

8.5. Flexural strength 21

8.6. Creep and shrinkage 22

8.7. Conclusions 24

9. Appendix 24

9.1. Lightweight aggregates 24

9.2. Strain measurements 24

9.3. Steam curing of the concrete 1 25

9.4. Preparation of cylinder bearing surfaces 26

9.5. Rate of loading for compressive strength test 26

9.6. Creep test 27

9.6.1. Creep test frame 27

9.6.2. Loading procedure 28

9.6.3. Effect of eccentricity of loading on creep measurements 28

9.6.4. Effect of loading time on creep measurements 28

9.7. Peculiarities of an anomalous lightweight aggregate 29

III

Creep and Drying Shrinkage of Lightweight

and Normal-Weight Concretes

T. W. Reichard

A description is given of a series of tests and test results from an investigation of the mechanical properties of structural-grade lightweight and normal-weight concretes. The

major part of the work was planned to obtain comparative values of compressive creep, drying shrinkage, strengths, and moduli of elasticity for concretes made with 24 lightweight

and 5 natural, normal-weight aggregates and having the same nominal strength at the time

the specimens were placed under load. Data are presented from a total of. 76 different concretes which show that there is a wide

range in values of the mechanical properties of concretes of the same nominal compressive

strength. It is shown that relatively high values of creep and drying shrinkage are not

alwaj's associated with lightweight concretes and that, in fact, some lightweight concretes have practically the same mechanical properties as some normal-weight concretes. In addition to environmental conditions, the two major factors affecting the creep of concrete appear to be the ratio of the applied stress to the strength at the time of loading, and

the aggregate used. Curing conditions, type of cement, mix proportions, and several other

variables appear to affect the creep chiefly insofar as they affect the stress-strength ratio. It was observed that the creep at the age of two years can be estimated from the 90-day

results with reasonable accuracy.

1. Introduction

The trend toward lighter weight in structures coupled with the increasing scarcity, in some parts

of the coimtry, of good-quality natural concrete

aggregates has led to the rapid increase in use of

manufactured lightweight aggregates. Because

structxiral-grade concretes can be readily made

with many of these aggregates, large amounts, are being used in concrete construction. Information

on long-term length changes is needed because the

stresses normally used in concrete construction

are of such magnitude that proper allowances have

to be made for loss of prestress and increase in

deflection. Some research on creep and drying shrinkage of

lightweight aggregate concretes has been reported

in the literatm-e [1, 2, 3, and 4].^ However, it was felt that a large group of the lightweight and

normal-weight aggregates, representative of those

being used in the construction industry, should be

included in a single comprehensive investigation.

The mix design, cure, and loading of the concretes should follow the general practice of the precast

concrete industry. Although the major part of

the data developed through this investigation will be of primary interest to the prestressed concrete

industry, much of the data can be utilized by

others interested in concrete construction.

The investigation was primarily designed to develop information on various mechanical properties of concretes having the same nominal

compressive strength, but made with different

aggregates.

This investigation was undertaken with the

financial support of the Expanded Shale, Clay,

and Slate Institute and with the cooperation of

the producers of the aggregates.

2. Scope of Investigation

The work reported here is part of a continuing

program concerning the long-term voliune changes

in concretes made from lightweight and normalweight aggregates. The creep, drying-shrinkage,

and strength data presented are for concretes made

with 24 expanded shale - lightweight aggregates

and 5 natural, normal-weight aggregates. These

aggregates were from widely separated geographical locations and were to be representative

samples of aggregates being used in the production of structural concretes. For the major part of the work reported, hereafter called series A tests, the concretes were

designed for a nominal 1-day compressive strength

of 4000 psi using a 2-in. slump and 5-percent air

' Figures in brackets indicate the references at the end of this Monograph.

'In this report, "expanded shale aggregates" include shales, clays, and

slates expand.3d in a horizontal rotating kiln.

content. The normal-weight aggregate concretes were designed using the 2-in. slump, although for

comparable workability with the lightweight concretes a 3- or 4-in. slump would have been preferable. Specunens of each concrete were placed

under a long-term compressive creep load of 2000

psi when 1-day old. This creep load, which is a

higher percentage of the strength at the time of

loading than is recommended by a proposed ASTM

creep method, was chosen as being representative

of the practice in prestressing concrete members.

In effect, the series A work was planned to give a comparison of the compressive creep and shrinkage

characteristics of equal-strength concretes made

with the various aggregates. Data are also presented from a series of supplemental tests, hereafter called series B, made to investigate

variables other than the aggregates.

1
The data reported here are from a total of 76 test batches of concrete. Of these, 63 batches were made with the 24 Ughtweight aggregates, 10 were made with the 5 normal-weight aggregates, and

the other 3 batches were made with a combination

of lightweight and normal-weight aggregates.

Creep- and drying-shrinkage data, compressive

and fiexural strengths, modulus of elasticity,

Poisson's ratio, and other data from these concretes are presented.

Details of the procedures and equipment used,

as well as a description of the aggregates, are given in the appendix.

3. Materials

3.1. Aggregates

For the purposes of this investigation, all lightweight aggregates were designated by numbers

and the normal-weight aggregates were designated

by letters. All aggregates were to be representative samples of the production and were used in "as received" condition.

Sieve analysis and unit weights of each aggregate

are given in table 1 . All aggregates were received

in bags holding 1 to 2 ft^ of aggregate. Because

of segregation of the fine aggregate while bagging,

there were some differences in sieve analyses and

unit weights from bag to bag. The values

reported in table 1 are from samples taken from at

least three bags.

The amount of aggregate passing the No. 200

sieve, as listed in table 1, is not considered to be

accurate for some of the lightweight aggregates.

The test method used (ASTM C-136 and C-117) is thought to cause some of the larger particles to break down, especially when the more friable

aggregates are being tested.

3.1.1. Lightweight Aggregates

All 24 lightweight aggregates used in this in- vestigation were representative samples of the

production at the time of shipment. However,

due to changes in processing and/or raw materials

the current production of a particular plant may be

much different from what it was when the aggregate was received for this investigation. It should

be noted that some of the aggregates were slight!}^

heavier than the maximum dry-loose unit weight

specified by ASTM C-330-60f.

The lightweight aggregates were shales, clays, or slates expanded in a horizontal rotary kiln. Although only about one third of these aggregates were considered to be "coated", nearly all contained some "coated" particles. When the aggregate particles are discharged from the rotary kiln,

they are usually of a semirounded shape having a hard surface or shell and are commonly referred

to as "coated particles." In some plants some or

all of this product is crushed after burning. Aggregate particles produced by crushing after burning

are commonly referred to as "crushed particles."

The particle shape and surface texture of lightweight aggregate appear to affect the concrete- making ability in much the same way as they do

for normal-weight aggregate. A fuller description

of each aggregate is given in the appendix.

Table 1. Sieve analysis and unit weight of aggregates

Aggregate

no.

16-

16_

18_ 18-

20., 20-

21.

21.

22. 22.

23-

23.

24.

24-

26-

26.

26- 26.

27. 27-

28.. 28-

30- 30-

Size

GG & WM.

GGA

RG- TR.

Fine

Int. Fine

Int. Fine

Int. Fine

Int.

Fine

Int.

Fine

Int.

Fine

Int. Coarse

Fine

Int. Fine

Coarse

Fine

Int.

Fine

Int.

Fine

Int. Fine

Int. Fine

Int. Fine

Int. Fine

Int.

Fine Coarse Fine

Int. Fine

Int.

Fine

Int.

Fine

Int. Fine

Int. Fine

Int.

Fine

Int. Fine

Coarse

Fines

Fines

Fines

Fines

Unit

weiglit (Dry

loose)

pcf

62.4

44.2

57.3

46.2

69.8

47.2

55.7 45.3

58.7 43.7

63.8

55.2

66.1 55.0

53.7

70.2 60.9

62.0

50.3

46.0 36.4

60.4 44.0

61.5

39.1

69.6 47.4

64.4

38.6

68.6 47.0

44.2

32.0

67.0

38.3

57.5

50.7

73.7

51.4

64.0 42.0

70.5 37.6

61.9

64.0

68.0

43.6

107.6

96.

93.

90.0

Sieve analysis, percent passing H Vi Vs i 8 16 30 50 100 200

100

100

48

78

100

100

100

100

100

100

62 40

1 ASTM 330-60T.

2
3.1.2. Natural Dense Aggregates

All five normal-weight aggregates in this in- vestigation were commercially accepted for use in

structural-grade concretes and were being used in large quantities for that purpose. All these

aggregates were %-m. maximum size. The aggregate designated E was Elgin sand and

gravel as used by Shideler [1] and [2]. The sand

of aggregate E had been screen separated and re- combined before delivery. The coarse aggregate was supplied in three sizes. The aggregate designated ED was also Elgin but was supplied and

used as delivered to the trade in the Chicago area. Aggregate R.G., kno\\Ti commercially as a granite, was a siliceous sand and gravel. This aggregate

consisted of water-worn igneous stones of many

varieties, none of which appeared to be granite.

Aggregate W.AI. was a local sand and gravel,

predominately quartzite. The gravel was separated into three sizes and recombined at batching.

Aggregate T.R. was a crushed trap rock from

the northeastern United States. The fine fraction

supplied with aggregate T.R. was a predominately

quartzite sand.

The aggregate designated G.G. was a crushed

light-gray granite and was supplied with a rela- tively fine quartzite sand. This sand was used in

making one batch of concrete {G.G.A.). A second

batch of concrete (G.G.) was made with this coar§e

aggregate and the local sand used with aggregate

W.M.

3.2. Cements

All the cement of each type was received at one

time and was specified to be from one silo. The

type III cement which was used throughout the

series A tests was stored in a sealed, dehumidified room in the e.xport-style shipping bags. The

type I cement which was used only in the series B tests was stored in sealed steel drums.

Results of physical and chemical tests made by

the NBS cement testing laboratory are shown in

table 2. The results shown are for tests made

about five months after receipt of the cements.

3.3. Air- Entraining Agent

A neutralized vinsol resin was used as an air- entraining agent in all the concretes. This

material, which was marketed as a 30-percent

solution, contained 26 percent of vinsol resin

solids, 4 percent of NaOH solids and 70 percent

of water. This solution was further diluted to a 7}^ percent solution before using.

No other additive was used.

Table 2. Chemical and physical tests of cements '

Chemical analysis, %:

Loss SO3 MeO

SiO AI2O3- _.. Fe203 CaO _.. NazO.,. K2O

Physical tests: Autoclave expansion, %

Initial set, hr:min

Final set, hr:min

Compressive strength, Sin. cubes, psi:

1 day

3 days

7 days

28 days. -

Type I

1.64

2. 16

2. 14 21.39

5.52

2. 44 64.17

.07

.77

.05 4:05

7:05

2100 3130

Type III

0.90

2.59

1.88 21.32 4.78

2.32

66. 05

.11 .36

.02 2:25

4:40

3050

4900

5840

8400

' Federal Test Method Standard 158a.

4. Concrete Mixes

All concretes for series A tests were designed

to have a nominal compressive strength of 4000

psi at 1 day using type III cement. To gain

this high early strength without using excessive

amounts of cement these concretes were cured

for 12 hr at atmospheric pressure in a steam

chamber at 140 °F. This cure is roughly equivalent to the curing procedure used in the precast

and prestressed concrete industry. All concretes were to be placed with a slump of 2-in. and 5- percent air content. The concretes for series B tests were placed

. with the 2-in. slump and 5-percent air, but the

' nominal strength, cure, and type of cement varied

i from test to test. The specifications for each

test within series B will be described later.

4.1. Trial Mixes

The producer of each lightweight aggregate

J had recommended a suitable mix for the series

, A concrete, but because of the variability in cements and other factors, at least three trial batches of concrete were made with each aggregate.

The proportion of fine to coarse aggregate was

as recommended by the producer, but cement

contents were varied so as to get a range in the

compressive strength values. The amount of

air-entraining agent was also adjusted during the

trial batching.

The trial batches were mixed in a %-ft^ pan-type

mixer. In mixing the concrete the lightweight

aggregates were mixed for 1 min with about % of

the estimated mix water and then allowed to stand for 10 min; the air-entraining agent and the

cement were then added and mixed for a minimum

of 2 min, additional water being added as was

judged necessary for the required 2-in. slump.

The same mix cycle was used for the normalweight aggregate concretes except that the 10- min waiting period was not used. Immediately

after the mixer was emptied, samples of the concrete were taken for slump- and air-content

measurements. The air content was determined

by means of an air meter in accordance with ASTM

C-173. Unit weight determinations were made by

using the calibrated bowl of the air meter.

3
4.2. Test Concrete Mixes

The amounts of cement, water, and air-entraining agent for the test concretes were selected

after consideration of the trial batch results. Although these were used as a guide in the makeup

of the test batches, the results of the latter were

not uniformly the same, probably because of the

differences in the two mixers used for trial batches

and test batches and because of the absorption

characteristics of the lightweight aggregates.

A }^-yd^ turbine-type mixer was used for tht

7-ft^ test batches. Its mixing action was such

that the amount of air-entraining agent necessary was considerably reduced from the estimate ar- rived at from the trial batches. The mix cycle

and the sampling procedure M^ere the same as used for the trial batches, except that the unit

weight of the fresh concrete was determined by means of a J^-ft^ measure. Mix proportions for all concretes of series A

are given in tables 3 and 7.

5. Preparation of Test Specimens

5.1. Description of Specimens

Compressive strength, creep, and shrinkage spec- imens were 6- by 12-in. cylinders cast in machined

cast-iron molds. Specimens for flexural strength

and moisture expansion tests, which were made

only for the series A concretes, were 3- by 4- by

16-in. prisms cast in 4-in. high steel molds. Gage

points for the 10-in. Whittemore strain gage were

cast in the creep, shrinkage, and moisture expansion specimens. The creep and shrinkage speci- mens had three gage lines spaced uniformly around

each cylinder. The moisture expansion specimens

had a gage line cast in each 4- by 16-in. face. A

description of the gage points and the method of

casting them in the specimen is given in the

appendix (sec. 10.2).

5.2. Casting of Specimens

The concrete was consolidated in the molds by

means of a laboratory-type internal vibrator. The

concrete was placed in the mold in two equal

layers. Each layer was vibrated by forcing the

vibrator spud through the concrete once completely around the circumference of the specimen,

or less if the concrete had subsided to a level plane. The spud was inserted through the top

layer of concrete and slightly into the bottom

layer when vibrating the top layer. Approximately 10 sec of vibration were needed for each

layer. No attempt was made to obtain a good

cast surface on the speciinens by external vibration.

The molds were slightly overfilled following the

vibration, and the excess was struck off after the

initial set of the concrete. This vibration method of consolidating the concrete was used instead of rodding as it was

thought to be more representative of the practice

followed in placing concretes in precasting plants.

A preliminary investigation regarding the merits

of this method of vibration indicated that rodded

cyhnders (ASTM C-192) would yield slightly

higher compressive strengths than the vibrated

cylinders.

This method of vibration was designed to give

satisfactory consolidation without overvibrating

the concrete. Over\'ibration of the concrete would result in a loss of entrained air and possibly some segregation of the aggregate from the cement

paste. The generally higher 1-day unit weights as compared with the plastic unit weights (table 3)

is evidence, however, that some of the entrained

air was lost even with the rather short vibration

period used in this investigation.

5.3. Curing of the Series A Concretes

All series A specimens were moved into an

atmospheric-pressure, steam-curing chamber approximately 4 hr after casting. The chamber was steam heated with a 2-stage electric steam

generator. One stage of the generator had

a relatively low capacity so that steam was

being continuously introduced into the chamber throughout the curing cycle. The thermostatically controlled second stage of the generator was used to raise the temperature of the chamber

up to the required 140 °F and to maintain it for the required 12 hr. After the steam-curing period

the specimens were allowed to cool in the chamber

for 2 hr before demolding.

Immediately following removal from the molds,

the specimens were weighed and the unit weight of

the concrete recorded as the 1 day unit weight.

All compressive strength specimens and half the

flexure prisms were then stored in laboratory air controlled at 73.4° ±4° F. The other half of the

flexural-test prisms were stored in the fog room

controlled at 73.4° ±3° F. The creep and shrinkage specimens were stored , in a laborator}^ controlled at 72° ±2° F and

50 ± 3 percent relative humidity. f

5.4. Preparation of Cylinder Bearing Surfaces

All cylinders except the shrinkage specimens ! were prepared for loading by wet grinding the

specunen ends in a surface grinder. A special jig was used to insure that the ends of the specimen

would be ground perpendicular to the sides. This

grinding was done shortly after removal from the

molds. A description of the grinding method is given in the appendix.

This method of preparation of the bearing sur- faces was used to avoid the use of special capping

plates and capping materials for the creep specimens; grinding also eliminated the need of capping

the compressive strength specimens.

4
Table 3. Data Jor series A concretes

Batch

Quantities per cu yd of concrete i

Water 2 Cement

Aggregate

(oven dry)

Fine Coarse

Air

content Slump

Unit weight

Plastic 1 day

28-day

test

1 year

air dried

Oven

dry

Aggregate moisture content when batched

(by weight)

Fines Coarse

Compressive strength of concrete, /J 3

1 day 7 day

28 day90 day1 year

lb 469

462

523

482 440

523 495

635

600

508 434

511

477

484

541 516

445 468

451

596

571

441 439

369

373

465

448

383

369

624

516

567

572

471 473

415

570

357

395

410

431 439

401 365

505 474

261

304

266

310

282

290

244

lb 636

655

654

561 575

605

644

627

630

583

589

610

629

627

590 600

562 597

524

668

657

562

593

544

588

585

591

591 580

644 650

684

672

615

605 M5

570

555

626

613

740

741

604

619

585

578

456

452

462

476

497

455

478

lb 1040

1051 1046

1006

945 885

878

941

902

1015

1032

1032

1162

1177

1074

1127

1076

786

840

1029

1043

1118

1096

1250

1240

929

912

1024

1016

776 784.

1026

1024 1106

1250

1288

1137

1115

832

832

1086

1128

969

974

1240

1234

1226

1184

1133

1225

1304

1189

lb 701 708

708

722

642 686

631 622

707

677

705

712

758

752

768

713

694

716

548

555

637

646

579

565

621 618

623

612

554 550

479 484

771

771 750

714

733

663

650

643

643

521 523

645

2018

2002

1958

1860

1940

2060

1982

1955

%

4.0

5.3

5.0

3.0

5.0

4.0

4.3

3.5

4.5

5.5 4.0

4.8

5.5

5.5

3.3

3.5

4.0 3.5

3.5

4.5

5.4

6.5

3.8

5.0 4.5

4.0

3.8

6.0

5.0

4.5

4.0

3.5

4.5

5.0

4.5

5.5

7.0

6.3

4.0

3.5

5.0 4.3

7.3

5.5

5.0 4.3

4.0

3.8

5.0

6.2

5.5

4.0

4.4

5.0

3.5

3.3

2.5

1.0

1.8

1.5 3.0

2.3 3.0

3.0

1.5 2.8

3.0

2.5

3.3

2.8

2.5

2.5 3.0

2.8

1.8

3.5

2.3

1.8 2.3

3.0

2.5

2.0

2.5

2.3 2.0

2.5

2.5

1.8

2.3

2.5

3.3

2.3

2.0

3.0

3.0 3.0

2.5

2.8

3.0

2.5

1.5 3.0

3.0

2.5

3.0

1.8

2.5

1.5

pc/

104.8

105.2

105.6

100.9 95.4

100.6 99.2

101.8

100.6

100.

104.2

105.8

112.0

111.6

101.4

105.6

101.0

94.6

94.8

96.6 99.0

96.2

96.3

106.6

105.9

91.6 90.0

101.1 100.4

90.6 89.4

104.6

105.4

104.7

105.2

102.3

102.0

97.0

96.2

95.3

96.6

98.6

146.0

146.0

144.6

139.2

141.2

146.2

149.0

141.5

PCf

105.4

106.6

108.6

102.6 99.3

100.6

100.6

102.9

103.5

100.2

100.5

100.8

105.6

107.8

112.8

113.3

103.4 107.0

102.4

96.3 97.0

98.8

100.7

96.6

97.2

108.2

107.3

93.5

91.4

101.7

101.3

92.9 93.0

106.7 106.5

104.

107.7 108.5

104.5

103.2

98.1 98.5

97.5

100.0

99.1

147.6

147.2

146.5

140.2

143.5

149.0

151.0

143.4

VCf

101.4

101.5 102.9

97.1 94.6

96.5

95.7

96.1 96.4

94.1 96.7

95.1

100.4

103.2

106.2

106.8

98.0

100.7

96.1

89.7

90.4

93.3

96.4

93.8

93.8

102.4

102.4

89.4

89.3

96.1 95.9

86.1 86.4

101.5

102.2

98.7

103.5

106.0

101.3

100.8

93.1

95.1

92.2 93.4

94.6

94.2

146.6

145.3

143.4

139.1

141.2

146.9

149.1

143.1

pcf

99.0

99.1 100.4

93.3

90.2

93.2

92.5

92.7 92.7

90.6

"92.'4'

97.8

103.6 103.4

95.1 98.8

94.8

83.7

86.0

90.6

92.9

91.7

92.3

99.2

101.1

87.1

92.4

93.8

81.5 82.7

99.0

96.3

101.0

102.4

98.7

98.4

90.2

92.9

90.4

91.6

91.5

91.3

143.9

142.9

pcf

95.0

95.0

89.0

88.0

86.0

86.0

87.9

89.3

93.9

96.3

92.3 95.2

77.2

79.3

97.4

89. 1

78. 0

78. 0

96. 0

95. 4

93. 0

99. 1 98. 9

95. 2

94. 8

86 4

89. 0

88.6

142.5

%

0.1 0

0

2.3

.5

14.2

14.7

2.9

10. 5.6 10.5

2.7

2.7

3.5

8.0

5.6

2.2

2.2

. 1

. 1

. 1

. 1

00

7.0

6.4

.4

.5

.4

0

0

.3.3

3.2

0

5.8

5.0

0

. 2

0

%

0

0

0

1.8

.5

1.7

1.3 1.5

. 1

. 1

. 1

6.8

6.8

8.0

7.3

00

0

10.0

9.0

1.5

1.5

0

0

.4

.4

0

0

2.0

1.7

.3

00

0

0

.3

.3

0

0

1.0

.5

0

0

0

0

0

0

0

0

psi 3440

3670 4470

3800

3790

4130 3940

4070

4230

3350

4720 3730

4710

4310

4450

3620

4410

3160

4900

5770

3520

4400

3850

4240

4190

4590

4420

3810

4260

4590

4000

3760

5170

4560 4530

4680

5110

4360

4060

4140 4080

3640

3990

3900 3640

3870

3680

3550

4080

3930

3260

4470

4410

psi 4250

4160

5550

4970

4870

5310 4840

5060 4910

4030

6020

4910

5780

5380

5510

4350

5220 4120

5830

6330

4180

5340

4600

4370

6490

5480

4940

4560

5580 5690

4370

4510

6000

6450

6510

5440

6049

5820

5330

4770

5400

4060

4740

4910

4600

5400

5360

4640

4840

4330

5200

5280

psi 4350

4910

6660

5950

6130

5690 5460

6580

5620

6240

6680

5660

5890

6470

6700

5170 5920

4680

6140

6710

5460 6640

5260 4630

6270

6100

6720

5560

6720

6340

4800 4680

7460

6370

6630

6440

6860

7090 6180

6240

6940

4560

5000

5670 5800

6490

6880

5300

5250

5780

4840

5790

6900

psi 4680

6040

7160

6120

6900

6600

5500

5990

5350

6490

5670

5980

6760

7060

6600

6160

5040

5970

6790

5620

6770

4900

5240

6070

6570

5290

6410

6610

7200

4910

5020

7690

7220 5700

6730

7590

6730

6080

6420

6860

4540

5060

6110

5845

6390 6030

psi 4610

4780 6460

6020

5310

5720

6210

5760 6050

5120

'5720'

6490

6490

6660

5660

6160

4960

6740 6480

6530

6230

5040

6120

6370

5250

5360

6340

6730

4380

4570

7180 6770

6300 6540

6560

5900

4650

6460

4650

5020

6630

6750

6500 5720

1 Based on unit weight as demoldcd.

2 Includes moisture in aggregate.

3 Average of 2 specimens.

5
6. Test Procedure

6.1. Compressive Strength Test

The compressive strength was determined for most of the concretes at the ages of 1,7, 28, 90,

and 365 days.^ Two 6- by 12-in. cylinders were

used as a sample at each age. The specimens, which were prepared for test by

the grinding mentioned in sec. 5.4, were loaded

until failure in a 300,000-lb capacity hydraulic

testing machine. The failures in most specimens

of the lightweight concretes were sudden and

sometimes explosive. The type of failure in general was the usual double cone, but in some

specimens vertical splitting occurred.

Stress-strain determinations were made on one

of the specimens ^rom each sample. Strain readings were made, by means of 6-in. bonded wire

strain gages, at convenient load increments without interrupting the continuous application of the

load which was continued until failure of the

specimen. The strain gages were bonded to the

specimens with an epoxy resin cement except for the one-day test. A modified cyanoacrylate was

used to bond the gages on the damp one-day

specmiens.

The secant modulus of elasticity at two stress

levels was computed from the stress-strain data

for each sample. These secant values were determined for stress levels of 0.4 and 0.9 of the compressive strength.

The secant modulus difi'ers theoretically from

the chord modulus specified in ASTM C-469-61T,

but is identical in value for the data developed in this investigation.

The rate of loading for the stress-strain speci- mens was about 22 psi/sec. The other compressive strength specimens were loaded at 44 psi/sec.

6.2. Dynamic Modulus of Elasticity and

Poisson's Ratio

The dynamic modulus of elasticity {Ea) and

Poisson's ratio was determined on all compressive

test specimens using the resonance method described in ASTM C-2 15-60. Ed was calculated

from the fundamental longitudinal frequency.

Poisson's ratio was calculated from the fundamental longitudinal and torsional frequencies.

6.3. Flexural Strength Test

The modulus of rupture of the series A concretes was determined on the 3- by 4- by 16-in. prisnis using ASTM Test Method C-78-57. Determinations were made at the ages of 1, 28, and

3 A limited supply of aggregate reduced the number of specimens cast from

concretes ED, O.O.A., E.G., T.R., and W.M. for series A tests.

365 days. Two specimens were tested at the age

of one day shortly after removal from the molds.

Four specimens were tested at each of the other

ages, with two being laboratory air-dried and the

other pair being continuously moist cured.

The test height of the prism was 3 in. as the

height at casting was 4 in. Two breaks were made on each specimen.

6.4. Creep and Shrinkage Test

All creep specimens representing series A concretes were placed under load when about 28 hr

old, but the series B concretes were loaded at other ages and will be described later. The loading frame was essentially the same as used by

Shideler [1] except that the height was increased

to allow for half-length dummy concrete speci- mens at the top and bottom of the set of three

test specimens in each frame. The dummy cylinders were necessary in order to comply with a

test method being prepared by ASTM Committee

C-9.

Four specimens were used as a sample of each

concrete. One specimen from each sample was

loaded in each of two creep frames. Two cylinders were left unloaded to serve as shrinkage

specimens. Thus, each creep frame contained test specimens representing three different concretes. Considerable care was used in setting up the

creep frame and in placing the specimens in the

frame in order to reduce to a minimum the ec- centricity in the load on the specimens. The load was applied by means of a hydraulic ram through

a load cell. Eccentricity of loading was checked

by observing the variation of the deformation in the three gage lines of the specimens at low loads.

If the variation was observed to be greater than

10 percent, the load was immediately removed and

specimens were rotated relative to each other until

the variation was reduced. The load applied to the specimens was estimated to be within 1 per- cent of the required load. The loading frames,

which were reloaded periodically, had a maximum,

observed drop-off in load, prior to reloading, of

5 percent. Details of the test frame and loading

procedure are given in the appendix.

All deformations in the creep and shrinkage

specimens were measm'ed with the same Whitte- more strain gage. The mild steel standard bar,

used for temperature corrections of the strain gage

readings, was stored on a stone slab in a box of

%-in. plywood. This procedure was used in order

to reduce temperature variations in the standard

bar caused by the forced air currents in the

temperature controlled laboratory. All gage readings were estimated to 0.00002 in. which, for the

10-in. gage length, was a strain of 0.0002 percent.

6
7. Results

7.1. Results—Series A Tests

7.1.1. Compressive Strength Test Results

No special problems were encountered in obtaining the required compressive strength of concrete for the aggregates tested. However,

with the limited experience acquired from eacli aggregate, some errors were made in obtaining the

required slump and air content. No attempt was made to obtain identical concretes when additional

batches were made from a given aggregate. In some instances, such batches were deliberately

designed to be different. The test data listed in tables 3 and 7 and shown

in figure 1 are for the series A concretes. The

data presented in figure 1 for the lightweight

concretes are averages for all the lightweight concretes, while the data for the normal-weight

concretes are averages of the two batches made

from aggregate E.

Probably the most significant aspect of the

compressive-strength data is the slight loss of compressive strength in most of these steam-cured

concretes after 90 days. This retrogression, which

has been reported by others, is usually attributed

to microscopic shrinkage cracks which occur with. the air drj^ing after the steam cure. However,

it may be significant that the continuously moistcured lightweight concrete specimens showed no

increase in flexural strength between 28 days and

one year, whereas the air-dried normal-weight

concrete specimens showed greater flexural strengths than the companion moist-cured spec- imens at all ages.

AGE, MONTHS

"T 1 r

NORMAL WEK3HT

CONCRETE SYMBOL CURE BATCHES

AVERAGED

Light wtl^t —O

Air Dr» 37 —•— Moitt 37

Normal Weight Air Or» 2 Mollt 2

J L

AGE, KWNTHS Figure 1. Average compressive- and flexural-strength test results for series A concretes at various ages.

All concretes were steam-cured for 12 hr at 140 °F and were made with type III

7000

6000 -

5000 -

K 4000 -

(/) uj 3000

o:

a.So

o

2000

1000

"I

\—I—

Botch I7A

1—

1 r I I r

Age Secont Modulus at Compressive

0.4 fc 0.9fc Sfrengtfi 10^ psi psi psi

1 Day Z.Z9 1. 94 4300

70oy 2.30 1.97 5280

28 Day 2.36 2.08 6150

900o« 2.42 227 5840

lyr 2.14 2.05 5740

STRAIN Figure 2. Typical stress-strain curves for a lightweight concrete.

7
7.1.2. Moduli of Elasticity and Poisson's Ratio

A set of typical stress-strain curves determined on compressive strength specimens is shown in

figure 2. From curves such as these the secant

modulus of elasticity can be determined for any

stress level. For the purposes of this investigation the secant modulus (E) was determined for

stress levels of 0.4 and 0.9 of the compressive

strength. These values are listed in table 4. The efi^ect of age on the moduli of elasticity is shown in figure 3. As with the compressive

strength there is retrogression at later ages.

5.4x10^

50

46 r _SECANT

-AVERAGE , 2 BATCHES ELGIN AGGREGAir

AVERAGE ,46 BATCHES LIGHTWEIGHT AGG. DYNAMIC

"^T—

I DAY AVERAGE

LIGHT WaGHT

I YEAR A/ERAGE NORMAL WEIGHT

Points Shown are for 28 days O Lightweighf • Normal weight 3 Mixed

2 3 4 SECANT MODULUS AT 0.4 ti, psi

5x10*

4 5 6 AGE, MONTHS Figure 3. Average secant and dynamic modulus of elasticity for series A concretes. The secant values are at 0.4 of the compressive strength (ft) at the time of test.

FiGUEE 4. Relationship between the secant and the dynamic

modulus of elasticity. The lines shown indicate average values of the l-day and 1-year data omitted from this graph.

Table 4. Moduli of elasticity of series A and series B concretes, 10^ psi

Aggregate &

batch

1 A..

B_. C.

2 A..

B_.

4 A..

B..

5 A_.

B..

6 A_

B.

C_

8 A.

B.

9 A.

B_

10 A_

B.

C_

14 A.

B_

15 A.

B.

16 A.

B_

17 A_

B_

18 A_

B.

Et at0.4/;i

1 day 7 day 28 day 90 day 1 year

1. 86

2. 14

2. 11

2.26

2. 12

1.81

1.68

1.71

1.94 2.15

2.04

2. 10

2. 29

2.03

2. 29

1.85

1.95

2.14

1.62

1.78

1.82 1.96

2.03

2.05

2.29

2. 06

2.04

2.06

2. 21

2. 14 2.24

2.39

2.29

1.95

1.93

1.76

1.78

1.99

2. 25 2.00

2.19

2.38

2. 26 2.37

1.89

1.99

1.77

1.70

1.76

1.94 2.08

2. 25

2. 14

2.30

2.37

2.13

1.98

2. 14 2.26

2.44

2. 35

2. 07

1.96

1.82 1.83

1.98

2. 34

2.21

2.26

2.42

2.34

2.40

2.05

2.00

1.84

1.66

1.76

1.99

2. 22

2.13

2. 36

2. 31

2.16

2.17

1.94

2. 09

2.31

2. 31 2.23

1.99

2. 01

1.87

1.70

1.85

2. 33

2.05

2.21 2.63

2. 26

2. 45

2.00

2. 07 1.85

1.50

1.77

1.93

2. 08

2.08

2. 07

2. 42 2.31

2.15

2. 00

1.92

1.94

2. 23

2. 12

1.98

1.92 1.79

1.67

1.73

1.70

"i'se""

2.12

2. 32

2.20

2. 28

1.85

1.94

1.77

1.40

1.64

1.73

2. 01

2. 03 1.96

2.14

2. 28

1.90

1.90

E; at 0.9 /^i

1.89

1.79

1.60

1.41

1.41

1.41 1.80

1.65 1.65

1.80

1.45

1.60

1.55

1.96

1.80 1.83

1.94 2.06

1.81

1.91

1.93

1.99 1.89

2.04

2. 03

1.70

1.66

1.50

1.52

1.60 1.92

1.69

1.88

2. 00

1.87

1.95

1.71 1.70 1.48

1.48

1.58

1.67 1.88

2. 04 1.99

1.97

2. 08

1.98

1.89

1.90 2.04

2. 10

2. 12

1.88

1.77

1.65 1.62

1.74

2. 10 1.96

1.98

2.16

1.93 2.02

1.94

1.82

1.64

1.58

1.70

1.76

1.98

2.02

2. 08

2. 06

2.00

2.02

90 day 1 year

1.75 1.77

1. 95 1.87

2. 02 2. 05

2.01 1.89

2.05 1.84

1.84 1.85

1.96 1.74

1.68 1.60 1.59 1.67

1.67 1.57 2.18

2. 03 1.75

2. 00 1.97

2. 27 2. 18

1.94 . 1.95

2.10 2. 08

1.86 1.81

1.91 1.86

1.69 1.65

1.64 1.50 1.83 1.70

1.77 1.62

1.92 1.96

2.02 1.97

1.96 1.93

2.27 2. 05

2. 13 2. 21

2. 06 1.87

1.94 1.88

Ed by Resonance method

1 day 7 day 28 day 90 day 1 year

2.31 2. 46 2.33 2. 30 2.17

2.43 2. 51 2. 51 2.46 2.22

2. 43 2. 56 2. 57 2. 65 2.36

2. 47 2. 56 2.56 2.53 2.34

2.33 2. 44 2.63 2. 41 2.17

2.21 2.33 2.33 2.25 2. 12

2. 18 2. 27 2. 25 2.22 2.00

1.86 1.97 1.95 2.02 1.84

1.93 2.00 2.00 1. 95 1.86

2. 10 2.22 2. 24 2.12 1.96

2. 28 2.41 2. 47 2. 45

2. 17 2. 28 2. 32 2. 28 2. 09

2.41 2.50 2.52 2. 49 2.33

2. 57 2. 69 2.76 2. 77 2.53

2. 61 2.70 2. 72 2. 65 2.62

2.70 2.78 2.78 2.80 2.56

2. 19 2.29 2. 26 2. 25 1.98

2. 29 2.39 2.32 2.34 2.18

2.15 2.13 2. 13 1.98

1.81 1.89 1.95 2. 01

2. 09 2.24 2. 21 2.10 1.97

2. 34 2.39 2. 46 2. 33 2.19

2. 28 2.40 2.37 2. 30 2.23

2. 31 2. 41 2. 36 2. 31 2.17

2.56 2. 66 2. 64 2.57 2.39

2. 58 2. 66 2. 66 2. 62 2.50

2.19 2.30 2.27 2.20 2. 10

2. 18 2.25 2. 31 2. 22 2.14 8
Table 4. Moduli of elasticity of series A and series B concretes, ICfi psi—Continued

E-, at 0.4/^1 k; at O.9/0I eI by Eesonance method

1 day 7 day 28 day 90 day 1 year

2. 21 1.87

3.67

2.16

2.20

2.89

2.97

2.35

2.04

1. 98 2.14

2. 16

1.43 1.34

1.53

1.52

2. 67

2. 64

2. 47

2. 68

2. 61 2.54

2.41

2.60 2.582.73

2.33

2. 27 2.50

2.31

1.96

2. 07

1.99

2. 10

1.85 2.082. 01

2. 15

1.59

1. 93 2.02

2.01

3. 26 3.15 3.92

3.53

3.12 3.47

2.44

2. 10

2. 91

2.67

3.11

4.66 4.83

4. 61 4.50

2. 45

1. 94 2.04

2.46

2.00

2.06

1.76

1.75 2.98

2. 29

2.39

4.11 2.30

2. 31

2.93

3.23

2.40

2.05

2. 18

1.58

1.55

2.60

2.68

2. 61

2.55

2.69

2.48

2.36

1.93

2.05

2.03

1.97

2. 13 2.04

4.26

4. 00

3.94

2.41 2.86

3.24

5.00

4.62

2.52 2.00

2.08

2.00

1.96 3.35

2.39

2.46

4.43

2.35 2.33

2.93

3. 24

2.44

2.14

1.65

1.49

2.11 2.13

1.46 1.53

2.70

2. 53 2.46

2.55 2.70

2. 48

2.32

1.84

1.94

2.06

2. 13

2. 07

1.97

4.49

4.12

2. 47

1.98 2.15

2.05

2. 07

3.64

2.36

2.39

2. 81 3.22

2.50

2. 14 1.72

1.65

1 day 7 day 28 day 90 day 1 year

1.93

2.08

1.27

1.31

2. 45 2.35 2.32

2. 36

•2.72

2.25

2. 26

1.65

1.72

1.95

2.00

1.83

1.88

4.41 4.19

1.97

1.93 3.94

1.73

1.74

1.25

1.16

2.37

2. 36 2.19

2. 06

2. 23

2.00

1.92

1.72 1.87

1.65

1.89

1.35

1. 69

2. 61

2. 77

2. 27

2.18

2.35

3. 46

3.72

2.05

1.54

3.17

1.89

2.02

2.34

2.33

1.86

1.85

1.87

1.38

1.38

2. 36

2. 35

2. 26

2. 23

2.35

2.15

2.02

1.81

1.41 1.18 2.00

2.12

2.18

3.34 2.06

2. 14

2.42

2. 67

1.89

1.87 2.00

1.53 1.47

2. 35 2.50

2. 42

2. 18

2. 35

2. 23

2.02

1.89 1.84

1.89 2. 01 1.98 1.86

1.70 1.91 1.76 1.80

3.21 3. 59 2.92 3.28

2.98 3.94

2. 49 2. 49

2. 56

2.64 2.74

3.73 4.03

3. 81 4. 01

1.75 1.50 2.18

2.23

2. 25

3.68

2.15

2.16

2. 53 2.74

1.97

2. 01 1.66

1.58

1.85

1.93

1.41

1.4fi

2. 52 2.30

2. 31

2. 26

2. 42

2. 23 2.03

1.65 1.73

1.90 2.03

1.90

1.76

3. 66

3. 25

2.32

1.87 2.00

1.86

1.67

2. 29

2. 23

2. 27

2.33

2.74 2.07

2.03

1.78

1.70

1 day 7 day 28 day 90 day 1 year

1.82 2.01

1.26

1.28

2. 38

2. 28

2.20

2. 21 2.58

2. 13

2.09

1.64 1.68

1.88

1.98

1.73

1.78

3.85

3. 45

1.85

1.65

2.80

2. 26 2.23

1.66

1.63

2.78

2.78

2. 63

2.76

2. 92

2.70

2. 55

2. 10 2.23

2.19

2. 28

2. 22

2.17

4.64

4. 28

3.55

3.50

3.92

5. 72

5. 25

2. 57

2. 06 2.20

2 46

2 16

5 03

2 41

2 51

3. 14 3 45

2 70

2. 31

1 84

1 79

2.40

2.41

1.71 1.75

2.84

2.72

2.88

3.04

2.80

2.71

2.20

2.34

2. 27

2. 39

2. 37

2. 32

5. 22

4.89

4. 77

3.74

4. 27

5.92

5.69

2. 63

2.11 2.19

1.98

1.97

4. 17

2. 58 2.64

5. 47

2. 51

2. 57

3. 27

3.78

2. 81

2. 37

2. 41

1.69

1.68

2. 87

2.91 2.78

2.86

3.05

2.84

2.70

2. 17

2. 34

2. 22

2. 29

2.34

2. 35

5.49

5. 13

4.93

3. 87 4.06

4. 14

fi. 01

5. 81

2. 67

2. 13

2. 22

2. 09

2. 12 4.44

2. 61 2.61

5.70

2. 57

2. 61

3. 30 3.80

2. 79

2.33

2. 25 2.40

1.61 1.C7

2. 86 2.79

2. 67

2.82

3.04

2.71 2.62

2.08

2. 22

2. 19

2. 31

2.32

2. 25

5. 36

5. 01

2. 57

2.11 2.29

2.11 2.16 4.47

2. 56

2. 59

3.20 3.82

2.80

2. 32

i/i=Compressive strength at age tested.

- E, = Secaiit modulus of elasticity.

3 Kd=T>ynam\c modulus from the longitudinal resonance frequency.

I The similarity between the dynamic and secant

I modulus curves is rather close for both types of

i concrete. A direct comparison of the dynamic with the

secant values of modulus, shown in figure 4, in- dicates that the ratio of the moduli is higher for the normal weight concretes than for the lightweight. The average ratio of the dynamic to the secant modulus at the age of 28 days was 1.11 for

i the lightweight and 1.32 for the normal-weight

concretes. This ratio decreased Avith age for both

types of concrete. For the one-day test these

ratios were 1.1.3 and 1.37 for the lightweight and

normal weight respectively. For the 1-year test the ratios were 1.10 and 1.16. Poisson's ratio, which was determined only by

the resonance method, varied little with age,

strength or aggregate except for a slight drop in value after 90 days. For the lightweight concretes the indiAadual values of the ratio varied

from 0.16 to 0.25 with the average being 0.21. The average Poisson's ratio for the normal-weight

concretes was 0.20 with individual values ranging

from 0.16 to 0.23.

7.1.3. Results of Flexural Strength Tests The average flexural-strength age data, which

are shown graphically in figure 1, illustrate one

difference between the lightweight and normalweight concretes. Examination of figure 1 and

table 5 will show that air-dried specimens of lightweight concrete have significantly lower

flexural strengths than moist specimens, but for the normal-weight concrete the opposite is true.
Table 5. Results of flexural strength test

Ratio of moduli Modulus of rupture, psl

of rupture- Aggregate Batch wet/dry

1 day 28 day 1 year 1 'Par

Dry Wet Dry vVn

1. A 500 690 710 620 730 1.20 1.18

1 B 530 570 740 620 710 1.30 1.15

2 A 500 460 680 570 720 1.48 1.26 2 B 440 480 660 540 690 1.38 1.28

4-- --- A 520 500 650 710 730 1.30 1.02

4 B 470 550 660 510 640 1.20 1.25

5 A 420 380 630 490 650 1.66 1.33

5 B 370 390 620 510 560 1.59 1.10

6.- - -. A 490 550 640 DUU DoU 1.16 1.14

6 C 510 570 750 580 720 1.32 1. 24

8.- --- --- h- 540 600 660 560 630 1. 10 1. 14 8 B 520 620 700 640 770 L14 1.20

9. A 550 620 760 770 870 1. 23 1. 13 9 B 570 790 840 740 820 l!06 1. 11

10 A 490 490 790 600 720 1. 61 1. 20 10 B 450 580 730 560 730 L26 1.30 10 C 440 570 690 510 690 1.21 1.35

14 A 400 150 590 260 3.90

14 B 420 140 590 350 500 4. 20 1.43

15. ... - A 500 590 660 590 670 1.12 1.13

15 B 530 550 770 650 670 1.40 1.02

16.. . — . A 540 530 730 590 740 1.38 1.25

16 B 560 570 740 610 680 1.30 1. 12

17 .. A 570 550 700 710 770 1.27 1.08

17 B 540 520 760 730 740 1.46 1.01

18. . — . A 450 520 660 610 660 1.27 1.08

18 B 440 490 620 530 550 1.27 1.04

20- .- A 600 570 780 610 700 1.37 1. 15 20 B 510 500 750 690 720 1.50 1.05

21. . A 390 390 590 400 490 1.51 1.23

21 B 400 370 580 410 550 1.57 1.34

23 A 550 670 830 750 850 1.24 1.13 23 B 510 600 800 700 770 1.33 1.09

24 ... A 540 570 740 580 710 1.30 1.22 24 B 550 580 710 660 750 1. 22 1.14

25. . A 570 670 860 700 900 1.28 1.30

25 B 510 600 820 640 770 1..37 1.20

26 A 490 590 660 550 670 1.13 1.20

26 B 440 570 660 530 660 1.16 1.25

27 - A 450 440 610 560 650 1.39 1. 16 27 B 470 450 630 590 620 1. 40 1.05

30.. . _ . .. . A 490 500 650 580 720 1.30 1.24 30 B 450 550 730 600 660 1.33 1. 10

ill . . .. . A 650 890 880 1150 920 .99 .80 B 610 880 800 880 870 .91 .99

Averages: Lightweight 490 520 890

700 840590 690 1.45 1.18 Elgin 630 1020 900 .95 .90

1 Each value is average of 4 breaks on 2 specimens.

This observation is similar to the results reported

by Hanson [5] for the indirect tension test on

concretes made from aggregate E. Other investigators [6, 7], on the contrary, have reported

results which indicated lower flexural strengths for dry than for continuously moist normal-weight

concretes. This apparent conflict among the

findings of different investigators may be a

reflection of the intrinsic differences in tlie aggregates used.

Differences between concretes made from dif- ferent lightweight aggregates can be illustrated by

comparing the flexural-strength data for aggregate

14 with those for the other lightweight aggregates

in table 5. The drying of aggregate 14 specimens

reduced the flexural strength to a much greater extent than for the other lightweights, especially

for the 28-day test. Shrinkage cracks became

visible on all air-dried test specimens including

the loaded creep specimens of concretes made from

this aggregate* when they were about 3 weeks old,

but none were visible on specimens of the other

concretes. These shrinkage cracks were no longer

visible on the loaded creep specimens after 1-year

drying, but were still visible on the unloaded

specimens.

The average flexural strength of lightweight

concrete kept damp until tested at 28 days was 45

percent greater than that of the air-dried concrete. This advantage decreased to 18 percent at 1 year

due to the fact that there was no increase in the

flexural strength of the moist-cured specimens

while the dry flexural strength increased.

There does not appear to be any direct correlation between the fle.xural and the compressive

strengths of the lightweight concretes for the range

in values found in this investigation.

7.1.4. Creep and Shrinkage

The creep and shrinkage characteristics of a concrete are very important in the design of any

concrete structural member which will be under a

sustained load. These characteristics are important since the deformation of the member with

time can be three or more times as great as the

elastic deformation occurring upon application of

the load.

For design purposes the creep of a concrete is usually considered separately from the drying

shrinkage, since the amount of the creep is dependent on the magnitude of the load. The term

"creep plus shrinkage," as used in this report, is: the deformation, exclusive of the elastic deformation, of the test specimen under the sustained

compressive load. Although it is thought [8] that

the drying shrinkage of the concrete specimen is affected by the load, the term "shrinkage" is taken here as the shortening of the companion,

unloaded specimen, while "creep" is taken as the

difference between the deformations of the loaded

and unloaded specimens.

The shrinkage values reported here are assumed

to be a combination of drying and some carbonation shrinkage. It has been shown by Verbeck [9] and others that carbonation effects are at a maximum when the relative humidity is about 50 per- cent—the storage conditions for the shrinkage and

creep specimens in this investigation. The effect of carbonation on the creep of concrete is not' known.

Creep and shrinkage data for the series A

concretes are given in table 6 and figure 5. The-

"adjusted creep" values given in this table were

* The fines of this aggregate, which were used in the series A tests, are seldom used. A discussion of the results obtained from concretes made fromi

this aggregate, which is considered to be quite different than the other light-i weight aggregates, is given in the appendix. 10
0 0 0 0 0 0 0 0 0 0 0 0 0 0

I I I I I I I I I I I I I I I I I I I I I I

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

T—n—I—I—:—I—I—I—I—I—I—I—r-i—i—i—i—i—r—

i—i—;—i—r—i—i—r—1—i—i—i—i—i i i—i—rn—I J I

0 0 0 0 0 0

TIME. DAYS Figure 5. Creep and shrinkage data for series A concretes.

Different types of lines indicate different batches. The plotted values are the average of 2 specimens.

adjusted to a stress-strength ratio of 0.5 by assuming that the creep was proportional to the ratio of the sustained stress to the compressive strength

of the concrete at the time of loading. This

adjustment of the measured creep values was made

necessary because, although the creep stress on all the series A concretes was 2000 psi, the compressive strengths of the concretes varied above and

below the nominal 4000 psi. The adjusted values

are thought to be better than the measured values

as a basis for comparison of the various concretes. Neville [10] has presented data which justified this use of the stress-strength ratio.

The creep-rate factor, F{k), given in table 6 and

used in estimating the 20-year creep values for these concretes is the factor in the empirical

formula e =F{k) log« (t+l) where e is the creep

at time, t, in days. This formiila is specified in the

proposed ASTM creep test method and is used by

the Bureau of Reclamation [11] and others. Although this formula does not acciu"ately describe

the data at early ages (less than 28 days), it was

used because the fit was satisfactory at later ages. Although this formula has been shown to be fairly

reliable under certain circumstances, the reliability of the 20-year creep values estimated from this

11
Table 6. Creep and shrinkage data for series A concretes

1-year values 2-year values

Measured Adjusted 2 Measured Adjusted 2

Creep plus Shrinkage Creep Creep Creep plus Creep Shrinkage Creep Creep plus shrinkage shrinkage shrinkage % % % % % % %

0. 255 0.070 0. 186 0.159 0. 229 0.208 0. 073 0. 179 0. 252 0.030

.230 .074 .156 .144 .218 .175 .072 . 162 .234 . 029

.251 .084 .167 .185 .269 . 033

.211 .066 . 145 . 137 .203

.225 .070 .155 .146 .216 .029

.203 .072 .131 .136 .208 . 152 .076 .158 .234 . 029

.239 .079 . 160 . 157 .236 . 181 .080 . 177 .257 . 034

.271 .098 .173 .177 .275 029

.276 . 104 . 172 . 183 .287 . 031

.247 .070 .177 .148 .218 .203 .075 .169 .244 .030

. 188 .067 . 121 . 144 .211 029

.233 .074 .159 .147 .221 . 177 .079 . 164 .243 !029

.219 .076 .143 . 146 .222 .163 .080 .166 .246 028

.187 .068 . 119 . 142 . 210 . 135 .076 .160 .236 !032

.248 .080 .168 .183 .263 .191 .083 .207 .290 037

.234 .082 .152 .169 .251

'

. 170 .083 . 188 .271 034

.207 .066 .141 .128 .194 .157 .073 .143 .216 .025 .216 .077 .139 .154 .231 . 159 .078 . 178 .256 031 .253 . 067 . 186 . 148 . 215 . 208 .073 . 165 .238 !027

. 214 .081 . 133 . 162 .243 . 161 . 089 . 196 .285 035

. 182 . 065 . 117 . 167 .23? . 135 . 082 . 193 . 275 !033

.242 .070 .172 . 151 . 221 . 185 . 075 . 162 . 237 028

.203 . 074 . 129 . 143 . 217 . 148 . 072 . 164 . 236 !029

. 157 .056 .101 .097 . 153 . 117 .058 . 113 . 171 026

. 168 . 061 . 107 . 114 . 175 . 122 .062 . 130 . 192 !025

.200 .066 . 134 .140 .206 . 152 .071 .158 .229 027

.181 .070 .111 . 126 .196 .125 .073 . 142 .215 ;026

. 164 .058 . 106 . 118 .176 . 123 . 061 . 137 . 198 024

. 170 .060 . 110 . 106 . 166 . 127 .058 . 122 . 180 ;022

.226 .084 .142 .151 .235 .165 .089 .176 .265 . 031 .213 .084 .129 .147 .231 . 151 .083 . 172 .255 . 030

.278 .099 .179 . 179 .278 034

.292 . 102 . 190 . 179 .281 032

.150 .057 .093 .119 .176 .105 .061 .135 .196 .025

. 166 .059 .107 . 122 . 181 . 120 .061 . 136 . 197 025

. 174 . 060 . 114

.099

. 130 . 190 !024

.155 .056 . 113 .169 .111 .062 . 126 .188 021

. 148 . 050 . 098 . 126 . 176 . 107 . 057 . 137 . 194 !024

.187 .058 . 129 . 140 . 198 . 142 . 064 . 154 . 218 028

. 196 . 065 . 131 . 134 . 199 . 147 . 066 . 150 .216 !027

.283 .086 . 197 .205 .291 .222 .086 .231 .317 043

. 281 . 087 . 194 . 198 . 285 . 219 . 086 .223 . 309 ]041

. 192 . 064 . 128 . 116 . 180 . 149 . 068 . 135 . 203 025

'. 174 !065 !l09 !l09 '. 174 . 120 .069 . 120 .189 !026

. 222 . 065 . 157 . 154 . 219 . 180 . 069 . 176 . 245 . 030

'.216 ]068 .148 135 .203 . 165 .072 . 150 .222 ]027

.133 .049 .084 .081 . 130 .090 .053 .087 . 140 .015

. 168 . 053 . 115 . 106 . 159 . 125 . 055 . 116 . 171 .018

.150 .049 .101 .090 . 139 .110 .054 .098 .152 .019

. 173 .049 . 124 . 127 . 176 .134 .053 . 137 .190 .025

.165 .053 .112 .110 .163 . 121 .061 .119 .180 .023

.225 .076 . 149 . 122 . 198 .161 .086 . 132 .218 .027

.148 .037 .111 .123 .160 . 121 .040 .134 . 174 .028

.118 .035 .083 .092 . 127 .090 .038 .100 . 138 .021

Batch

Stress 1 strength

ratio

0. 58

.54

.45

.53

.53

.51

.49

.47

.60

.42 .54

.46 .45

.55

.45

.63

.41 .35

.57

.45

.52

.47

.45

.52

.47

.44

.50

.53

.39

.44 .44

.44 .39

.46 .49

.48

.49

.55

.50

.51 .55

.52

.54

.51

.61

.45

.45

' Stress=2000 psi creep load; strength =compressive strength, psi, at the time of creep loading.

2 Creep adjusted to the stress-streneth ratio of 0.50.

3 Creep rate factor for formi'la, f = F(k) log (^+1). Where e=creep m percent for stress-strength ratio of 0.50 at time (J.) in days since loading. F(k) is the

slope of the line drawn through a semilog plot of the creep data.

12
90 DAY CREER percent

Figure 6. Relationship between creep at 90 days and at two

years for series A concretes. . stress-strength ratio is the ratio of the sustained creep stress (2000 psi) to the strength o( the concrete at the time of loading (approximately 4000 psi).

formula should, be considered questionable at this point.

Because of the nature of the phenomena, in- vestigations of creep and shrinkage constitute

long-term studies, and several relationships were

investigated with the view of finding one which

might be useful for predicting creep. Figures

6 and 7 were plotted with the thought that there

might be a fairly reliable relationship between

early creep and shrinkage data and results at

later ages. The average line through the data in figure 6 indicates that the average value of creep

for concretes at 2 years is about 1.6 times the creep

at 90 days. For comparison, the empirical formula

given above estimates that the creep at 2 years

woidd be about 1.5 times the creep at 90 days.

Extrapolating beyond the data shown as figure

6 by assuming the creep-time curve is logarithmic,

it is estimated that the creep at 20 years will be

about 1.4 times the 2-year value or 2.2 times the

90-day value.

The correlation between the 90 day and 2 year

shrinkage was not as well defined as that for the

creep data. The shrinkage data, plotted as

figure 7, shows a considerable scatter, with the data

for the concretes made from aggregate 14 ^ showing

a particularly large divergence from the other

concretes. The creep and shrinkage data for the series A

concretes are plotted as histograms in figure 8. These histograms provide a convenient means of comparing the creep and shrinkage values of light-

.03 .04 .05 .06 90 DAY SHRINKAGE, percent

Figure 7. Relationship between shrinkage at 90 days and

at two years for series A concretes.

_

'

1

'

1 ' 1

' — O- Lightweight o

•-Normal Walglit o

o

o

o

• 0

• • 0 o

o • o o o

o o o o o

• • o 0 o 0 o o o o - • o o o 0 o o o o o

1 • ° 1 °

1 1 1 1 ° 0 ° °

.10 14 .16 .IS ADJUSTED ONE YEAR CREEP, percent

'

1

'

1 1 0-Ligl<tw«ight 0

•-Normal Weight oo o

o o

o o

O 0 o o

o o o o

0 o o 0 • o

• • 0 o o o 0 o

• • • o o o o o o o o o •l O ° ° o ° ° ° °l

o

1 .03 .04 .05 .06 .07 .08 j09 .10 .11 ONE YEAR SHRINKAGE, percent Figure 8. Frequency histograms of creep and shrinkage

data for series A concretes. The creeps were adjusted to a stress-strength ratio of 0.5, by assuming that the

creep was proportional to the ratio of the sustained stress to the compressive

strength of the concrete at the time of loading.

' It will be recalled that concrete with aggregate 14 developed visible shrinkage cracks in air-dried specimens at an age of 3 weeks.

13

714-538 O—64 3
*f .36

FiGUKE 9. Bar chart showing the adjusted creep plus shrinkage for each aggregate

at one year. The creeps were adjusted tc a stress-strength ratio of 0.5. Values shown are an average of 2 specimens from each of at least 2 batches.

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

D- —

- /X

CREEP PLUS SHRINKAGE

O — LIGHT WEIGHT CONCRETES O—NORMAL WEIGHT CONCRETES

SHRINKAGE n

- ^.^^^ffi^^

p 1 1 1 i 1 1 1 1 1 1 1 1 1 1 160 200 240 TIME, DAYS

Figure 10. Creep- and shrinkage-time data for the average

series A lightweight and normal-weight concretes. The values shown are averages for 4fi batches of lightweight and for 8 batches

of normal-weight concretes. weight and normal-weight concretes. As can be

seen, there is considerable overlapping of the data,

but in general the normal-weight concretes ex- hibited less creep and shrinkage than the hghtweight concretes. However, some lightweights

exhibited less creep and shrinkage than some nor- mal-weight concretes. A bar chart comparison of

the creep plus shrinkage of the concretes made

from each aggregate is given as figure 9.

_ O Lightweight • Normal weight

Z .12 O

O

UJ

I-

(n^ .08

'•o '

.02 04 .06 .08 .10 .12 ONE YEAR SHRINKAGE, percent Figure 11. Relationship between the creep and the shrinkage

values at one year. The creep values were adjusted to a stress-strength ratio of 0.5. The average adjusted creep of the lightweight

concretes was 0.145 percent at 1 year, while the

average for the normal-weight concretes was 0.106

percent. The average 1-year shrinkages of the

lightweight and normal-weight concretes were

0.072 percent and 0.050 percent, respectively. The

grand average creep and shrinkage curves for the

lightweight and normal-weight concretes are shown

in figure 10. Figure 1 1 indicates a rough correlation between

the adjusted creep and the shrinkage of a concrete observed in specimens 1 year old. Figure 12 shows the relationship between the

14
Table 7. Test data for additional series A concretes under test for less than 1 year

Table 7A. Mix proportions, unit weight, and strength

Aggregate Batch

Quantities per cu yd

. Air content Slump

Unit weight

Aggregate

moisture when

batched Compressive strength

Water Cement

Aggregate

Plastic 1 day 28 day Oven

dry

Fine C oarse 1 day 7 day 28 day 90 day

Fine Coarse

lb lb lb lb . % in. pcf pcf pcf pcf % % psi psi psi psi

_ A 452 660 1030 836 3.5 3.0 108.4 110.3 105.6 99.9 3.2 1.6 4700 5830 6800 7040 B 484 623 1030 836 3.5 3.0 108.0 110.0 105.6 100.

3.2 1.6 4690 5810 6750 6730

22 A 508 651 907 620 3.5 2.0 98.2 98.1 91.5 85.6 .4 . 1 3880 4590 5300 5330 B 495 668 900 615 3 2.5 99.0 99.

93.0 87.3 .4 . 1 4530 4840 5300 5190

28 A 462 600 801 884 4.0 99.4 101.6 97.0 90.3 .2 .4 4490 5710 7250 7040 B 431 588 782 867 3.0 96.0 98.5 94.0 87.8 .2 .4 3870 5140 6000 6630

Table 7B. Moduli of elasticity,^ 10^ psi

E, at 0.4 f'c E, at 0.9 f'c Ed by resonance method

Aggregate Batch

1 day 7 day 28 day 90 day 1 day 7 day 28 day 90 day 1 day 7 day 28 day 90 day

11 A 2. 04 2.16 2. 31 2.24 1.61 1.85 2. 10 2. 03 2.60 2. 70 2. 73 2.67 B 2.06 2.23 2.09 2. 19 1.65 1.83 1.88 1.98 2.60 2. 72 2. 75 2.69

22 A 1.43 1.67 1.85 1.69 1.30 1.61 1.86 1.63 1.85 1.99 2. 00 1.97 B 1.63 1.77 1.80 1.77 1.46 1.61 1.68 1.64 2. 01 2. 04 2. 06 2. 06

28 A 2. 16 2. 18 2. 25 2.20 1.86 1.92 2. on 2.02 2. 36 2.43 2. 51 2. 43 B 1.93 2.06 2. 11 2. 07 1.71 1.77 1.86 1.86 2.20 2. 33 2.33 2.30

Table 7C. Flexural strength, creep, and shrinkage

Aggregate Batch

Flexural strength test 90-day measured values

Stress- strength

ratio

Adjusted

90-day

creep

Estimated creep <

Modulus of rupture, psi Ratio, wet- dry Creep

plus

shrinlcage

Shrinkage Creep 2 year 20 year

1 day

28

Dry

day

Wet

28 day

psi psi psi % % % % % % %

11 A 560 630 820 1.30 0. 175 • 0. 055 0. 120 43 0. 140 0. 22 0. 31 B 510 580 760 1.31 .169 .054 .115 43 ,134 .21 .29 22 A 480 480 660 1.38 . 182 .061 .121 52 .116 .18 .26 B 440 410 650 1.59 .162 .063 .099 44 .113 .18 .25 28 A 530 600 690 1.15 .161 .052 .109 45 .121 .19 .27 B 490 570 710 1.25 .180 .054 .126 52 . 121 .19 .27

' See footnotes for table 3.

2 See footnotes for table 4.

' See footnotes for tables 5 and 6.

* 2-year creep=1.6 times 90-day adjusted creep. 20-year creep=2.2 times 90-day adjusted creep.

cement contents of some series A concretes and their

"creep plus shrinkage "values. Data for all series A

concretes which had 1-day strengths between 3,800

and 4,000 psi were used in this plot so that no

adjustment of creep for stress-strength ratio was

needed. Although the data show considerable

scatter, this graph indicates that in general the

greater the amount of cement needed to develop

a given strength, the greater is the "creep plus

shrinkage". Lyse presented some date [12] which

also indicates that creep and shrinkage may be

proportional to the percentage of cement paste in a concrete.

7.1.5 Moisture Expansion

The possibility of an expansive reaction between

an aggregate and the cement or even long-term

moisture expansion of the aggregate itself was

investigated by determining the length change

with time of a specimen of each concrete stored in

the fog room.

After about 1 year of storage in the fog room,

the maximum expansion recorded was 0.014 percent. The average expansion of all the concretes was 0.007 percent. An average increase in weight

of 1.7 percent was recorded. The maximum in- crease in weight was 2.3 percent. There was a very slight increase in expansion, and weight for a 2d year of exposure.

7.2. Results—Series B Tests

A series of supplemental tests was designed to investigate certain variables within the series A

tests described in the preceding section. These

supplemental tests, six of which are described in

15
• NORMAL WEIGHT O LIGHT WEIGHT <a> -

Oo

400 450 500 5S0 600 650 700 750 CEMENT CONTENT, lb/yd'

Figure 12. Relationship between creep plus shrinkage and

the Cement content. The values shown are for series A concretes with

3800 and 4200 psl. 24 hr strength between

this report, were called the series B tests. The

data from these six tests are given in tables 8

through 14 with the test number being designated

by Roman numerals. Because a correlation with

series A data was necessary, the mix design, test procedure, etc., of series A were followed in the

series B tests where applicable. Significant mix

data for the series B concretes are given in table S.

7.2.1. Supplemental Test I Test I was designed to develop data on the

effect of the type of cement on the length change

characteristics of steam-cured concretes. The

concretes for this test were designed to have the same cement content and strength at the time of

loading as those of series A, except that they were

to be made with a type I cement, steam-cured 36

hr, and placed under load when 2 days old. The

concretes were loaded in creep frames at three

stress levels, 1,000, 2,000, and 2,500 psi.

Table 9 gives 2,000 psi creep load data obtained

for test I and series A concretes made with three

aggregates. These data indicate that the concretes made with the type I cement exhibit less creep

and drying shrinkage then the concretes made

with the type III cement. The reductions in adjusted creep for aggregates 18 and 27 due to the use of type I cement were 17 and 10 percent,

respectively. The reduction of 36 percent

observed for aggregate 1 was probably due to some

extent to the large difference in the slumps of the

concretes.

7.2.2. Supplemental Test II The concretes for test II were designed to be

representative of mixes used in ordinary in situ concrete work with a 28-day compressive strength

of 3,000 psi. These concretes were made with

the type I cement, damp cured for 6 days, and

placed under creep test at loads of 600 and 1,200

psi when 7 days old. Creep test data for these concretes stressed to 1,200 psi and concretes of

series A and test I made from the same aggregates

are given in table 10. Although the cement contents of the test II concretes were less, the 1 year creep data for comparable concretes of series A are similar when

compared on the basis of equal stress-strength

ratio of 0.5, except for aggregate 30 concretes. The concretes of aggregate 30 differed considerably

in the slump, stress-strength ratio, as well as the

cement content. A comparison of the data for the aggregate 27

concretes of test II and test I indicates that the

amount of creep for the moist-cured and steamcured concretes are about the same. The data for the aggregate 30 concretes appear to indicate a

considerable eft'ect of slump on creep and shrinkage

as mentioned in the previous paragraph. This

effect is similar to that noted in test I for aggregate

1 concretes. However, the data for the concretes

Table 8. Data for series B concretes

Test no. Aggregate Type of no. cement

I 1 I 18 I-.-- 27.__ I n 27 I 30 I_.__ E I m 27 III IV 20 III

27 III- E Ill V 25S III 25G III 14S III VI 4 III 14D ni_ 14W in

Mix quantities per yard of concrete

Cement

lb677

572 630

483

402

304

603

658

490

542

552

475

604 632

Water i

lb463 355

362

407

502

277 390

511 423

280 333

367

402

405

544

693

Aggregate moisture when

batched

% by wt. 0.1

0

.1 2.7 0

01.0

1.5 2.6

00

0

.1 1.1 1.2 28.5

Unit weight of concrete

1 day

pfc 108.8 90.4

97.6

92.0

97.2

146.0 99.6

102.9 102.0

151.0

116.9 124.0

112.6

99.1

91.1

96.1

28-day

air dried

pcf

104.4 87.2

93.2

89.2

92.0

142.9 97.3

98.1 97.2

98.9

148.3

113.7

120.9 106.7

94.

85.4

86.5

Oven

dry

pcf

93.6

81.1

88.1 82.5 84.4

138.6 91.5

89.3

92.7

144.3

108.8

116.5

101.4 88.2

76.1

75.8

Cures

Steam.

Steam.

Steam. MoistMoistMoist.

Steam.

Moist.

Steam.

Steam. Steam

Steam.

Steam.

Steam. Steam.

Steam.

Steam.

Age at creep loading

days

2

2

2

Compressive

strength

Creep 28 days

loading

psi 2)Si 3980 4970 3280 4200

3590 4240 2390 3300

1890 2910 2070 3040

5220 5650 5030 5620

5370 7620 4680 5560 4940 6540

3680 5240 4200 5750 3430 4960

4600 6070

5430 6720

4640 6120

1 Includes moisture in aggregate when batched.

2 Steam-cured concretes made from type I cement were steam-cured 36 hours, those made from type III were steam-cured 12 hours.

16
Table 9. Effect of type of cement on creep and shrinkage of steam-cured concretes

Aggregate Type of cement Slump

Cement

content

Stress- strength

ratio

28-day

strength

Measured values at 1 year

Shrintage

Creep

plus

shrink- age

Creep

I—

in.

I--. m.

m.

111. 1.5 3.0

2.0

2.0

2.3

2.5

lb/yd' 677

609

572

586

630

611

%

50

52

6149

56

psi 4970

5310

4200

5640

4240

4780

%

0. 052

.076

.049

.059

.054

.065

%

0. 148

.245

.161 .167 .169

.184

%

0. 096 .169

.112

.108

.115

.119

1 Creep adjusted to a stress-strength ratio of 0.5.

T.\BLE 10. Comparison of creep test data from moist-cured and steam-cured concretes made

with same aggregates

Test Aggregate Type of Cement Cure

Stress- strength

ratio

Compressive strength when

loaded

Slump of concrete

Measured values at 1 year

Shrini- age

Creep

plus

shrink- Creep

Adjusted

creep i at 1 year

U

ISeries A

II

Series A

II.

Series Series A

27.. 27..

27-

30..

30.. E.. E.. ED

I.. I._ ni

I., ni

I., mm

Moist.. Steam..

Steam..

Moist.. Steam..

Moist.

Steam.. Steam..

50

56

52

63

53* 5853

56

psi 2390

3590

3820

1890 3770

2070 3780

3550

in. 2.0

2.3

2.5

1.0 2.8

1.5 2.3 3.0

%

0. 058

.054

.065

.055

.066

.051

.051 .049

%

0. 161 .169

.184

.178

.218

.176

.151 .150

0. 103 .115

.119

.123 .152

.125

.100

.101

%

0. 103

.103 .114

.097

.143 .108

.095

.090

' Creep adjusted to a stress-strength ratio of 0.5.

T.\BLE 11. Effect of type of cure on creep, shrinkage and strength of a lightweight concrete made

with aggregate 27

Curing method ^ Compressive strength 1 year measured values Adjusted

creep ' at 1 Steam year Moist

storage 1 day 3 day 7 day 28 day 1 year Shrinkage Creepplus shrinkage Creep

12 Hrs

12 Hrs None

None

7 Days

7 Days

psi 4,690

3, 180

psi 5,030

4,700

4,240

psi 5,220 5,220

5.030

psi 5,670

5. 630

5,620

psi 5,270

5, 700 5.810

%

0.063

.068

.072

%

0. 162

.165

.176

%

0.099

.097 .104

%

0. 130

.128 .130

'Creep adjusted to a stress-strength ratio of 0.5.

T.\BLE 12. Effect of ininus 200-mesh aggregate on creep and shrinkage

Test Aggregate Cement

content

Amount

of -200

material

in fine aggregate by weight

Stress- Strength

ratio

1 year measured values 1 year adjusted

values 1

Shrinkage Creep

Creep

plus shrinkage Creep

Creep

plus shrinkage

Scries \ _._ ._. 20

to/yd 3 647 684

611 658

*fA

%

112

3

119

%

45

37

52

43

53

41

%

0.084

.071 .065

.066

.051 .043

%

0. 135

.114

.119

.100

.100

.063

%

0. 219

. 186

.184 .166

.151 .106

%

0. 149

.154 .114

.117

.095

.077

%

0.233

.225

.179 .183

.146

.120

IV 20.. __ Series A . _ 27 _ IV 27

Series A __. . E

IV E 490 1 8

Creep adjusted to a stress-strength ratio of 0.5.
made with normal-weight aggregate E indicate

that the slump did not appreciably affect the creep

and shrinkage.

Data from the concretes from test I of series B,

which were obtained in creep tests at three stress

levels, are combined and analyzed with similar

data from test II. In making the adjustments to measured creep values throughout this paper it was assumed that over the usually narrow ranges over which the adjustments were made the

creep of a concrete was proportional to the ratio

of the applied stress to the concrete strength at the

time of loading. Test I and II were set up so that this assumption could be tested. Creep data for the various stress-strength ratios

used in tests I and II are plotted in figure 13. In drawing the curves shown on figures 13 and

14 (which was derived from fig. 13) it was assumed

that the origin was a point on each curve. The

curve shown in figure 14 represents the average

of the five curves of figure 13, but with the creep

values expressed as percentages of the average

creep at a stress-strength ratio of 50 percent.

It can be seen that the assumption made pre- viously that the creep is proportional to the stress- strength ratio is not strictly true. Since the error from making the adjustments does not appear to have been great, all adjustments made to creep

values hereafter in this report are also based on

the assumed straight line proportionality.

Upon studying figure 13 it will be noticed that

although different cures, aggregates and strengths,

as well as cements, are involved, the spread in the creep values at any particular stress-strength

ratio is not great. To show more clearly tlie effect of the stress-strength ratio on the creep

K .10

AGG. TEST I (Steam cured) TEST n

(Moist cured

18 A

27

30

E

10 20 30 40 50 60 70 80 90 STRESS -STRENGTH RATIO, percent

Figure 13. Relationship between the creep and the stress- strength ratio for some series B concretes.

I '

I

'

I

'

I Averages From 6 Concretes Loaded For I Yeor.

10 20 30 40 50 60 70 STRESS- STRENGTH RATIO, percent Figure 14. Relationship between relative creep and the

stress-strength ratio for some series B concretes. The curve represents the average from the curves presented as figure 13.

CL 10 -

I

1 1 1 r O TYPE HI CEMENT STEAM CURED

TYPE I CEMENT STEAM CURED H TYPE I CEMENT MOIST CURED

Aggregote No. 27

I I L

20 30 40 50 60 70 80 STRESS- STRENGTH RATIO, percent Figure 15. Relationship between the creep and the stress- strength ratio for concretes made from aggregate 27.

of concrete, all available creep data for concretes made with aggregate 27 are plotted in figure 15. The correlation between creep and stress-strength

ratio appears ratlier remarkable when it is considered that three cures, two types of cenient, and

two nominal strength- and cement-content ranges

are represented in this figure.

7.2.3. Supplemental Test III Test III was designed to investigate whether

creep and shrinkage of steam-cured concretes are

less than for comparable moist-cured concretes, as reported by Shideler [2]. A single batch of concrete was made with aggregate 27 and type III cement. Two-thirds of the

36 test cylinders cast were steam cured. The

18
balance of the cylinders and half of the steamcured cylinders were moist cured untU 7 days old. At this age, when all the specimens had nearly

the same compressive strength, one cylinder rep- resenting each of the three cm'es was placed under

2000 psi load in a single creep test frame. Pertinent information on the test III specunens and

data from test III are given in tables 8 and 11. It can be seen that after 1 year the steam-cured

specimens have exhibited slightly less creep and

shrinkage than the specmiens with only the moist

cure. Shideler [2] reported that steam-cured concretes exhibited as much as 50 percent less creep

than moist-cm-ed concretes. However, by adjusting his data for the stress-strength ratio this apparent difierence in creep is decreased to less than 25 percent.

As the reduction in creep and shrinkage of the

steam-cured compared with, the moist-cured concrete does not appear to be as significant in these

tests as reported elsewhere, it is thought that

differences in the reactions of different cements

to the steam cure may account for the difference

in the beha\-ior of the concretes in the different

investigations.

7.2.4. Supplemental Test IV Test IV was designed to explore more fully the

indications of the early results from the series A

test, which appeared to indicate that concretes made with aggregates containing larger amounts

of material passing the 200 sieve exhibited greater

creep and shrinkage. It was reasoned that if the

amount of this fine material were changed significantly, a significant change should occur in the

creep and shrinkage.

The concretes for test IV were designed, cured,

and tested the same as series A concretes, except

for the amount of material passing 200 mesh sieve

which was introduced as a variable. The pertinent data obtained in this test are given in table

12. These data would seem to indicate that there

is no significant or consistent effect on the creep

and shrinkage due to the changes in the quantities

of the minus 200 material used in this investigation. One explanation of this apparent lack of agree- ment between the earlier results and the subsequent tests is the possibility that the softer aggregates, which might be expected to exhibit

greater creep and shrinkage, might also contain a greater quantity of very fine material because of breakdown of the aggregate with handling.

At the present time there is no standard test method for rating the hardness of these aggregates,

so that any comparison may be questionable.

7.2.5. Supplemental Test V

Test V was designed to determine the effect of replacement of some lightweight aggregate by a normal-weight aggregate on creep and shrinkage.

It is the practice in some parts of the country to use natural sand to replace lightweight fines when

making lightweight concretes. The sand is used

in some cases to increase the modulus of elasticity of the concrete and in other cases is used for reasons of economy.

Table 13. Effect of partial replacement of lightweight with normal-weight aggregate

Test

Aggregate used

Stress- Strength

ratio

28-day

strength 28-day

secant modulus

28-day

unit weigh t

Oven dry

unit weight

Measured values at 1 year 1 year adjusted

values 1

Fine Coarse Shrinkage Creep and

shrinkage Creep Creep

Creep and

shrinkage

VI Sand 25 %

5448

58

48

38

psi 5240

5750 4060

6640

6430

10' psi 2.94

3. 26 2.44

2.42

1.71

pcf

114.

121.

107.

101.

90.

pcf

109.

116. 102.

95.

78.

%

0.056

.051 .063

.061

.073

%

0.212

. 173 .240

. 191 .198

%

0. 156

.122

.177

.130

. 125

%

0. 145

. 127

. 153

.137 .167

%

0. 201 .178 .216

. 198 .240

VI 25 Gravel..- VI Sand 14 Series A- 25 25

Series A — 14 14.

Creep adjusted to a stress-strength ratio of 0.5. Table 14. Effect of varying the moisture content of aggregates at the time of mixing on creep and shrinkage

Test Aggregate

Aggregate moisture content when batched

(by weight)

Unit weight

Stress- strength

ratio 28-daystrength

Measured values at 1 year Adjusted values at

1 year 2

Fine Coarse 1 day 28-day

air dry Oven

dry

Shrinkage

Creep

plus

shrinkage

Creep Creep

Creep

plus

shrink- age

VI.. 4... %

2.0

14.5 2.4

29.9

6.8

%

0.2

1.5 027.0

9.5

pcf

99

101

9196

97

pcf

949685

86

90

pcf

88

89

76

7678

%

43

6037

43

38

psi 6070 5570 6720

6120

6430

%

0. 073

.076

.077

.085 .073

%

0.200

.221

. 199 .238

.198

%

0. 127 .145

. 122 .153 .125

%

0. 147 .146

.165

.177 .167

%

0. 220

.222

.242 .262

.240

Series A 4 VI 14D

VI 14W

Series A.. . . 14 1....

1 As received.

- Creep adjusted to a stress-strength ratio of 0.5. 19
Three batches of concrete were made with the

type III cement and two lightweight aggregates.

One batch (designated 25S) was made by combining the local natural sand with the coarse fraction

of aggregate 25. The second batch (designated

25G) was made using the lightweight fines of aggregate 25 with the %-m. maximum size WM

gravel. The third batch was made with the local

sand and the coarse fraction of aggregate 14. The mixes were designed to have approximately

the same strength as the series A concretes. The

cure and test procedure were also the same in series A.

From an analysis of the data presented in table

13 it appears that the practice of replacing lightweight aggregate fines with natural sand may

reduce the drying shrinkage. The replacement of

lightweight fines by natural sand reduced the drying

shrinkage for aggregates 14 and 25 by 14 and 8

percent, respectively; the substitution of WAd

gravel for the coarse fraction of aggregate 25

resulted in a reduction of shrinlcage of 16 percent.

However, the adjusted values of creep at 1 year were not consistently reduced. The substitution

of the sand for the lightweight fines resulted in a

slight increase in the creep for concrete made with

aggregate 14, but a decrease for aggregate 25

concrete. The substitution of gravel for the

aggregate 25 coarse fraction resulted in a slight

reduction in creep.

7.2.6. Supplemental Test VI The effect of varying the amounts of moisture in

the aggregates at the time of mixing on the creep

and shrinkage of lightweight concretes was investigated in test VI. In the series A tests most of the aggregates were

batched when relatively dry (see table 3) except

for the fines of aggregate 4 which had been shipped

and stored in plastic-lined bags. The first of three concretes for test VII was made with the aggregate

4 after it had been dried. The second and third concretes were made with air-dry and very moist

aggregate 14 respectively. Table 14 gives pertinent data for test VI and comparable data from

series A concretes.

It appears from the data in table 14 that the

practice of soaking lightweight aggregates before

batching probably causes only a slight increase in creep and shrinkage of the concrete over that

exhibited by concretes made with dry-batched

aggregates. It should be remembered that al- though the moisture condition of the aggregates

varied considerably when batched, the 10-min

waiting or soaking period used in mixing the concretes eliminated much of the apparent difference

between the dry- and wet-batched aggregates.

The absorption characteristics of the lightweight

aggregates vary considerably, but approximately

50 percent of the 24-hr absorption occurs in the

first 10 min that free water is available. The

absorption after the first 10 min, although at a

relatively slow rate contributes to a loss of slump

after mixing. This absorption of water by the

aggregate after mixing is usually called "watergrab".

8. Discussion of Results

It is the usual practice to compare lightweight

with "regular" normal-weight concrete as if the

"regular" concrete had properties which are standard regardless of the aggregates used. However, the data obtained in this study and by other

investigators indicate that there is a considerable

spread in the properties of concretes of the same

nominal strength made either with natural, nor- mal-weight aggregates or with manufactured,

lightweight aggregates. In fact, there is such an

overlapping of the data from the two types of concrete that it would seem to indicate that

except for flexural strength the mechanical properties of some lightweight concretes are equivalent

to the properties of some normal-weight concretes.

8.1. Significance of Slump as a Measure of

Water Content of Concrete

With normal-weight concretes nuide with lowabsorption normal-weight aggregates the measured

slump can be regarded as a fair index of the

relative water-cement ratio, but with the lightweight concretes some judgment must be used in comparing the concretes on the basis of the

reported values of slump. The lightweight mixes

were proportioned on a cement content-slump

basis because for most lightweight aggregates the

absorption characteristics are such that the net

water-cement ratio cannot be determined accurately. Moreover, these same absorption characteristics can also introduce errors in determining

the slump except when the aggregates are satu- rated or nearly so. Consequently when we

consider the possible effect of slump on creep as we did in sec. 7.2.1, we must also consider whether

mixing conditions were identical, since the slump

of lightweight concrete refiects both the net watercement ratio as well as the "water-grab" of the

aggregates.

As the rate of absorption is relatively high

during the early part of the period when free moisture is available to the aggregate, a difference

of a few niinutes can appreciably affect the mixing

water necessary for a given slump. The "watergrab" was very noticeable when mixing many of

the air-dry lightweight aggregates, even with the

10 min soaking period used in this investigation.

All values of slump reported here are for concretes as discharged from the mixer and do not

necessarily indicate the consistency as placed since

the "water-grab" results in a loss of slump.

8.2. Statistical Analysis of Test Results

It is impractical to tabulate all the data from

the individual tests performed in this investigation.

20
Table 15. Statistical analyses of test results

Test Test ages Number ofsets of specimens

Number of

tests per

set

Average of test results

Within-Test

Standard deviation CoefiScient

of variation

Unit weight of concrete. Compressive test

Flexural test Creep test Shrinkage test.

7 days, 28 days and 1 year.

1, 7, 28, 90 days and 1 year.

1 day, 28 days and 1 year.

1 year resiilts do

174

295

242

5457

104.2 pcf

5220 psi

610 psi

0.200% strain 0.068% strain

0.48 pcf

188 psi..

41 psi_.

0.0044% strain 0.0018% strain

0. 46

2.6

2.2

2.6

However, as an aid in determining the significance

of apparent differences in test values between

different concretes, witliin-test values of the standard de-vaations and coefficients of variation were

determined by the method recommended in [13]. These values, which are given in table 15, are a convenient method of expressing the efficiency of the test procedures. However, for these test results the within-test factors, with the exception

of those for unit weight also include the variabilitj^ of the concretes. The variation in the unit weights

of duplicate specimens of the same batch of concretes allows some insight into the variability of the concretes and is included in table 15 for this reason. A spread of one standard deviation in the unit weights between 2 lightweight concretes made from the same aggregate, corresponds to a spread in the 28 day compressive strength of about 200 psi.

8.3. Compressive Strength

As was mentioned in sec. 7.1.1 the most significant finding from the compressive-strength

data for the series A concrete was the relatively

small gain in strength from 28 to 90 days and the

slight drop in strength after 90 days. This

finding for steam-cured concretes may appear to be opposite to the usual finding for moist-cured

I concretes. However, it has been reported by

Price [14] and others, that concretes moist-cured

for a limited time and then air dried will exhibit

I a similar retrogression in compressive strength.

This would seem to indicate that the retrogression

j in the strength of the steam-cured concretes was

not so much the result of the steam cure, per se, as

it was a result of the interruption in the moisture

supply.

Hanson [15] and Higginson [16] have shown that

( even 28-day compressive strengths are lower for

I steam-cured than for continously moist-cured

j concretes. When it is remembered that steamcured specimens are usually relatively dry when

tested and that the continuously moist-cured

I specimens are tested damp this difference acquires

added significance.

It seems that the compressive strength-time

curves for steam-cured and limited moist-cured

concretes are similar in shape, but the steam-cure

strength level is below the moist-cure at 28 days

and older.

The actual gain in strength from 1 day to 28

days varied considerably for the series A concretes.

It appears that the gain in strength is probably

dependent on some characteristic of the aggregate

as well as on the type of cure and cement. It is possible that the small rate of gain in strength of concretes for some of the aggregates reflects the

fact that the concretes are appraoching the max- imum strengths which can be obtained with those

aggregates.

8.4. Modulus of Elasticity

Many attempts have been made to devise an

empirical formula with which values for the

modulus of elasticity of concrete can be computed

from strength and unit-weight data with reasonable accuracy. The most recent formula proposed [17] is

where

static modulus of elasticity, psi

T^=unit weight of the concrete at the time

of test, pcf

/'c= compressive strength of concrete at the

time of test, psi.

The data from this investigation were plotted

in figure 16 to show the relationship between the secant modulus and W^'V/'c- can be seen from figure 16 there is considerable scatter from

the proposed line. However, if the data from the

granite concretes, indicated in this plot by the

letter 6, are excluded the correlation is better.

As would be expected, an even better correlation

is shown in figure 17 where the plotted data is for concretes made with but one aggregate—No. 27. For purpose of comparison figure 18 presents some data for insulating concretes from a previous

unpublished NBS investigation. The proposed

formula is shown to be reliable for the low-density

and low-strength concretes also.

8.5. Flexural Strength

Walker and Bloem [7] found that partially dried concretes have lower flexural strength than continuously moist-cured concretes. They also found

indications that, other factors being comparable,

thoroughly dry concrete produced higher flexural strength than did moist concrete.

21
Figure 16. Relationship between the secant modulus of elasticity and the compressive strength and unit weight of concretes. The secant moduli were those obtained for 0.4 I'c.

1

1

'

1

' I'll I'll 1

' 1

' CEMENT STEAM CURE MOIST CURE

Type HI o A

Type I • A /

No. 27 Aggregate All Ages. A

~

range 2400 to 5900 psi

A

O O/

• O /

o /o

/ O

% fTT-

"

"V 'c —

1 1 1 l/ 1,1, 1 1 1 I III, "40 4.4 4.8 5.2 5.6 6.0 6.4 6.8 7.2 76X10'

Figure 17. Relationship between the secant value of

the modulus of elasticity (E) and the compressive

strength (f'c) and unit weight (W) of concretes made

with aggregate 27.

Plotted values of E were at 0.4 the compressive strength. I'c varied from

2400 to 5900 psi. W varied from 84 to about 102 pcf.

The results from this iiiA^estigation indicate that

for concretes made with certain dense aggregates

the flexural strength of the partially dried concrete may be higher than for the moist concrete. However, the partially dried lightweight concretes always had a lower flexural strength than the moist

concretes. No correlation could be found between

the wet-dry flexural strength ratio and the drying

shrinkage of the concrete as measured on the

6- by 12-in. cylinders. However, the effect of the

partial drying on the flexural strength appears to be related to some as yet imidentified property of the concrete aggregate. In a supplementary test, not presented in this report, it was found tliat 1-hour air drying re- duced the flexural strength of specimens made

from lightweight concretes about the same as 27-dav air drying.

8.6. Creep and Shrinkage

It has been noticed by many observers that the

shrinkage of concrete members in the field is sometimes much less than for similar laboratory

specimens even when average humidity and temperature conditions are the same and the members

are protected from rain. This phenomenon

is generally explained by the fact that concrete

22
S. . 16

- o

. 14

0

1

1

1 1 1

1 r \

-

-

- /

A /

A

n A /

- /m

a /

/ o INSULATING CONCRETES - Type Tested Moist Tested Dry

Perlite A A

Vormiculits - /

Cellular • o

1 1 1

1

1 1 1 1 1 } .2 4 .6 .8 .10X10

Figure 18. Relationship between the secant modulus of

elasticity (E) and the compressive strength (f'c) and unit

weight (W) of very lightweight concretes.

ft varied from 75 to 600 psi. W varied from about 23 to 48 ptf.

will take up moisture much faster than it will release it. Under field conditions of alternate

high and low humidities the concrete behaves as

if it were exposed to much higher than the average

ambient humidity.

In applying shrinkage values from a laboratory

investigation to actual structures, exposure conditions should be considered. It is thought that

the 1-3-ear values reported here are probably

close to the maximum which would be encountered

over most of the United .States for similar concretes. However, in certain areas, such as the

Southwest, a combination of high ambient temperatures and low humidities would probablj^

result in shrinkage values greater than those

reported here. The creep and shrinkage data presented here

indicate that the creeps are roughly proportional

to the drying shrinkages.

At the present time, long-term testing appears

to be the only reliable method which can be used

to judge the effect an aggregate \vill have on the

creep and shrinkage of concrete. The data shown

in figures 6 and 7 show that it might be possible

to obtain a reliable estimate of long-term creep

from a relatively short-term test, but the estimate

is considerably less reliable for shrinkage. In interpreting the creep data it should be

remembered that the specimens tested for this investigation were kept loaded at a constant

stress level. In a sense this test method is directly

applicable only to structural members subjected

to a constant sustained stress uniform over the

cross section of the member; it is somewhat

um'ealistic in its application to prestressed concrete since the stress in a prestressed member is not uniform and is reduced in proportion to the

length change of the concrete and wdth relaxation

of the steel tendons. It has been shown [18] that a considerable loss in stress can occur due to relaxation of the steel. Three methods which

can be used in determining prestress losses, when

creep, shrinkage, and relaxation data are available, have been discussed in [18, 19, 20, and 21].

From the compressive-creep data presented

here it is obvious that creep, as it was defined for

this report, is not proportional to the applied

load. Among other factors, this lack of proportionality can be explained if we assume that the

drying shrinkage of a concrete is increased by an

applied load. XeviUe [9] made some creep

determinations on mortar specimens which were

air dried before being placed under creep load and

concluded that creep is proportional to the applied

load when no drying shrinkage can occur. The data presented in figures 13 and 15 show

a relationship which is very significant in a structural design. These data indicate that one of the

most significant factors affecting the creep of concrete is the stress-strength ratio. The effect of the stress-strength ratio on the creep overshadows

the effect of most other factors. Variables such

as type of cement, curing conditions, and aggregate

gradation are factoi-s affecting creep mainly insofar as the}" affect the stress-strength ratio. The aggregate used in making the concrete appears to be another significant factor in creep. From the data presented in table 13 for aggregate

25 there is an indication that the creep and shrinkage are affected to a greater extent by the properties of the coai-se particles than by those of the

fines of the aggregate.

O LIGHT WEIGHT CONCRETE • RG CONCRETE a OTHER NORMAL WEIGHT

O2I > I ' i ' 1 ' ^ '

1 ,

L 2. 3. 4. 5. 6.x 10' SECANT MODULUS AT 04tj,psi

Figure 19. Relationship between the drying shrinkage and

the secant modulus of elasticity of concrete. The modulus Indicated is for the 28 day test. 23
Carlson [22] concluded that the relative compressibility of the aggregate particles appeared to be the most important factor causing different

aggregates to produce concretes of different shrinkage. The validity of this assumption was investigated in this study by examining the relationship

between the moduli of elasticity and the drying

shrinkage of the concretes. Data from this investigation which are plotted in figure 19, indicate

that such a correlation probably does exist between

the shrinkage and modulus of elasticity. However, the creep data indicate poor or no correlation

between the creep and the modulus of elasticity

of the concrete.

8.7, Conclusions

1. Some lightweight concretes have practically

the same mechanical properties as some normalweight concretes, except for fiexural strength.

2. It appears that the use of a steam-curing

method which increases the early strength of a concrete is detrimental to the long-term strength

gain.

3. If data from granite-aggregate concretes are

excluded, the proposed empirical formula for the

modulus of elasticity, ^=33 W^NT7, will yield

values for both lightweight and normal-weight

concretes reasonably close to observed values.

4. Partially dried lightweight concretes have

lower fiexural strength than continuously moist

concretes. There are indications that other factors

being comparable, the fully dried concretes may

be equal in fiexural strength to the moist concretes.

5. Partially dried concretes made from aggregate E, the only normal-weight concrete tested in

flexure, had higher fiexural strength than the

moist concretes.

6. A concrete which exhibits relatively low

values of creep wOl exhibit low values of drying

shrinkage.

7. Two-year creep values may be estimated with

good accuracy from 90-day creep values. The

same is not true for the shrinkage values.

8. Creep as defined for this report is not strictly

proportional to the stress-strength ratio, even at

relatively low loads.

9. The stress-strength ratio appears to be one

of the two significant factors affecting the creep

of concrete. The eft'ect of this ratio on creep

overshadows the eft'ect of many other variables,

when exposure conditions are constant.

10. The type of aggregate used in making the concrete is the other significant factor in the creep

of concrete. The property of the aggregate which

determines the creep is as yet unidentified. It appears that the coarse aggregate is more eftective

than the fines in aft'ecting the creep.

11. There appears to be a correlation between

the shrinkage and the modulus of elasticity of the

concrete. This correlation would be expected

since the modulus of the concrete is an index of the

compressibility of the aggregate used.

12. There was no evidence of any disruptive

expansion reaction between any aggregate tested

and the cement, or even any significant long-term

moisture expansion of the aggregate itself.

9. Appendix

9.1. Lightweight Aggregates

The lightweight aggregates used in this investigation were either expanded shale, clay, or slate. However, neither an examination of the aggregates, nor an analysis of the performance of the

aggregates in concrete appeared to furnish a means

of identifying the raw material in the aggregates.

This type of lightweight aggregate is produced

in rotary kilns, generally ranging in diameter from

8 to 12 feet and in length from 80 to 200 feet. The typical production process consists of introducing raw materials, grading from approximately one inch to the No. 50 screen, into the kiln where

the temperature is raised until it reaches a pyroplastic state at which time gases are generated and

bloat the aggregate particles. The material is quickly discharged from the kiln and cooled. The

bloating temperatures range from 1800 to 2100

°F, depending upon the raw material. After

cooling, the material is screened and oversize

particles are crushed to fill out the gradation of both fine and coarse aggregate.

There are slight variations in the process, for example, in some cases raw material is separated

into different size fractions which are expanded

separately. Sometimes the kiln feed is presized

to such an extent that no crushing is required

after burnins;.

Examples of the aggregates are shown in figure

20. The sample designated A has a "coated"

appearance. One of these particles has been sliced

to show the cellular interior. Sample B illustrates aggregate particles that have been crushed after processing so that the cellular interior structure is exposed. Table 16 gives a brief physical description of each lightweight aggregate used in this investigation.

9.2. Strain Measurements

The gage used for determining the changes in length of the creep, shrinkage, and moisture

expansion specimens was a 10-in. Whittemore

strain gage. This mechanical gage required two

metal inserts spaced 10-in. apart in the concrete. The two pointed legs of the gage are seated into

small holes in the inserts when making measurements.

Accuracy of the Whittemore is determined by

the accuracy of the dial indicator. The sensitivity of the gage is about ± 2X 10"** in./in., but

practical considerations such as the precision of the

gage holes in the inserts, consistency of measuring

technique, and accuracy of temperature compensation increases the uncertainty of the measurements

to an estimated ±10X10~® in. /in. 24
The gage point inserts used in this investigation were No. 12-24 stainless steel screws with a No.

59 drill-hole drilled in the end. The method of

holding the insert in place on the mold during

casting is shown in figure 21. The brass holding screw was removed when the concrete was removed

from the mold.

Upon removal from the mold each gage point was cleaned and the gage holes were lightly

reamed with a 60° reamer until the pointed legs

of the measuring gage could be well seated.

Since the No. 12-24 screw is no longer considered

a stock size it was thought that another size screw

should be used for future investigations. Also

the standard nut used at the end of the insert did

not pro%dde sufficient thread length for foolproof

use. At the present time a brass knurled nut

with twice the thread length is being used.

Figure 20. Two types of exvanded-shale aggregate. A indicates typical coated particles with one particle cut open to show the

cellular interior. B indicates typical crushed particles.

Table 16. Physical appearance of aggregate particles

Aggregate Color

with Medium gray.. Medium gray some brown. Reddish brown.

Light gray Dark gray Mixed gray and brown.

Grayish brown

Reddish brown.. Medium gray Medium gray some

tans. Tan to medium gray. .

Reddish brown

Brown.. Dark brown. _. Medium gray, some tan. Dark gray and tan Dark gray to brown... Tan to gray Reddish brown

Brownish gray... Medivim gray Reddish brown Medium gray to brown

Gray to tan.

Shape '

Irregular Cubical to rounded. Rounded

Cubical

Irregular to cubical. Cubical to rounded.

Irregular Rounded

Cubical do

do.... Rounded

Irregular to cubical

Rounded

Cubical do

do....

Irregular to cubical. Cubical

.do.

.do. Rounded.

Cubical..

do....

Surface texture

Smooth. Mostly smooth. Smooth. Smooth to rough. Do.

Do.

Smooth. Do. Rough.

Do.

Do. Mostly smooth. Do. Do.

Smooth. Mostly rough. Rough.

Smooth. Mostly smooth. Rough.

Do. Mostly smooth. Smooth. Rough.

' "Rounded" particles are approximately spherical or egg-shaped. "Cubical" particles are approximately equal in all three dimensions but not

rounded. "Irregular" particles are other than rounded or cubical.

' "Rough" texture means that the cellular interior is at least partly exposed.

9.3. Steam Curing of the Concrete

The steam-curing chamber was a fairly well- insulated room approximately 6- by 7- by 8-ft. The walls and ceiling of the room were lined with

heavy aluminum foil. A 1-in. concrete overlay on the floor had been placed over a vapor barrier.

The steam source was a 2-stage generator as

sold for use in steam rooms of health clubs. The

generator was heated with two separate 4 kw

electric units which were individually thermostatically controlled. One unit, called the mainte- nance heater, was on all the time so that there was a continuous flow of steam into the curing

chamber. The second 4 kw unit was used to increase the steam flow until the preset temperature was reached.

The steam was fed into the chamber through

}^-in. copper tubing in which Ys-in. holes were

drilled for distribution of steam.

The two stages of the generator were controlled

by timers so that the rate of temperature rise as well as the curing period could be automatically

controlled. Figure 22 shows a typical curing

MOLD

STAINLESS STEEL 12-24

HEX HEAD SCREW 8 NUT

WITH No, 59 HOLE 4" DEEP

Figure 21. Stainless-steel Whittemore gage point used in

the creep and shrinkage tests. The brass screw and nut were used to maintain the position of the gage

point until the concrete hardened. 25
O 80

70

1 1 1 1 1 1 1 1

HEAT OFF S. / CHAMBER OPENED

-

p

'

/ 1 p

' TEMPERATURE IS AVERAGE f 1

/ 1

1

OF 6 THERMOCOUPLES

sikole'

1 1 1 1 1 1 0 2 4 6 8 10 12 14 16 TIME, HOURS Figure 22. Typical steam-curing cycle for concretes made

with type III cement. The first heating stage of the steam generator was on for about 1!^ hr before the second stage was turned on. The concrete was allowed to set about

4 hr before steam curing.

cycle in the steam chamber. Concretes made

with type III cement were cured for 12 hr at 140 °F, while those made with type I cement were

cured 36 hr at 140 °F.

9.4. Preparation of Cylinder Bearing

Surfaces

In lieu of the usual capping procedure, all creep

and strength specimens were prepared for loading

by wet-grinding their ends flat and parallel in a

vertical-spindle surface grinder. The concrete

specimens were held to the 36-in. rotary magnetic

chuck by a special steel jig which also assured

that the specimens ends were ground perpendicular

to their axes. Figure 23 shows the grinder with

the holding jig and specimens in grinding position.

The grinding was done as soon as possible

following removal of the specimens from the mold.

It took approximately 15 min to grind both ends

of a set of four specimens.

A short preliminary test was made to determine if there would be any obvious difference in the compressive strength due to the difference in the bearing surfaces of ground and capped specimens.

Thirty-one 6- by 12-in. cylinders were cast from a

single batch of sand and gravel concrete. The

specimens were all steam-cured 12 hr at 140 °F

and then air-dried until tested. The bearing

surfaces of 20 cylinders were prepared by grinding

and the other 11 were capped with a standard

sulfur-sUica capping compound. The grinding was done when the concrete was 24 to 26 hr old

and the capping the day before testing.

Eleven specimens prepared by each method

were loaded in compression at 80,000 Ib/min

until failure in a hydraulic testing machine when 3 days old. The results are listed in table 17. From these data it would seem that grinding the

ends of the specimen may have a slight advantage

over the capping. The coefficient of variation

(V) was computed as 4.5 percent for the capped

cylinders and 2.6 percent for the ground cylinders.

Figure 23. Method of holding and grinding the bearing

surfaces of four 6- by 12-in. cylinders. Both ends of each cylinder were ground. The height of the cylinders after grinding averaged about 11.86 in. A C-80 grit stone was used in the

grinder.

Table 17. Effect of capping method on the compressive

strength test results

Cylinder Breaking loads

Ground ends Capped ends

1 _ _._

lb

111, 400

107, 100

104, 500

106, 300

108, 100

111, 100

110, 400

105, 000

106, 600

110, 200

104, 200

lb

102, 000

110,900

106, 600

92, 500

108, 800

104, 800

107, 500

106, 700

104, 500

108, 700

111, 400

9

3

4 _ 56 - 7 . — _ - 8

910

11 _._ Avg. load, lb 107, 700 3,810

105, 700 Avg. strength, psi- - - — 3, 750

9.5. Rate of Loading for Compressive

Strength Test

In the compressive strength tests of the cylinders

for this investigation two different rates of loading were used. The- lowest rate, about 22 psi/sec, was adopted as the maximum rate at which stress- strain data could be reliably determined "on-the- run". As the sample size for each concrete at each

age was two cylinders, the second cylinder could

be tested at some other rate for comparison.

26
ASTM method C39-61 specifies that the rate of loading for the compressive test should be within

the range of 20 to 50 psi/sec. It was decided

that the second cylinder of each set would be

loaded at double the first, or 44 psi/sec. Both

rates are within the ASTM specification.

An analysis of 295 sets of cylinders indicate that

the faster rate of loading increased the measured

compressive strength only 0.2 percent over the

slow rate. This increase which is insignificant

when compared with the within-test coefficient of

variation of 3.6 percent would seem to show that

within the allowable ASTM rates the rate of loading will have no effect on the measured compressive strength values.

9.6. Creep Test

9.6.1. Creep Test Frame

All creep test frames used in this investigation were essentially as used by Shideler [1] and as shown in figure 24. The %-in. rods for most of the

frames were made from an AISI C-1040 Steel with

a yield point of 87,000 psi. The double nuts over

FREE MACHINING HIGH STRENGTH STEEL ROD

7/8"-l4NF (THREAD 30 - IN.) RUNNING FIT

l-l/2"x II-I/2" DIA STEEL PLATE

3-1" OIA HOLES @ 120°

II- 1/2" DIA STEEL PLATE

3-1" DIA HOLES @ 120°

2" X 6" DIA STEEL BLOCK

2 DIA BALL BEARING

(1/4" INDENTATION FOR BALL BEARING

SEAT, TOP a BOTTOM)

14 DIA STEEL PLATE

COILED COMPRESSION SPRING

X 14" DIA STEEL PLATE

DRILL a TAP 7/8-14 NF 3 HOLES EQUALLY

SPACED (FOR WRENCH TIGHT FIT OF RODS )

SECTION A

Figure 24. Construction details of creep frame used for the 2000 psi load on

three 6- by 12-in. cylinders.

27
the top plates were thought to be necessary for the

higher-load frames as the threads on the rods had

been made for a loose running fit of the nuts. All

plates were torch cut to size. The springs were

designed so that the deflection at the expected load

would be approximately 2-in.

9.6.2. Loading Procedure

When the investigation was being planned, it had been decided to limit the variation in strain

due to eccentricity of the load to 10 percent. It had been thought that the eccentricity might be

easily determined by measuring the strain in each

rod as the load was applied. It was found that

this procedure could not be used as the strains in the three rods were about equal even when the

eccentricity was rather high. The eccentricity was defined as the ratio of strain in the gage line

with the highest strain to that with the lowest.

There were some indications that factors other

than load eccentricity contributed to the differ- ences in strain. It was found quite early that much time and

effort would be saved if considerable care were

taken in setting up each frame prior to loading.

The frame was leveled on the floor so that the rods were as nearly vertical as could be determined.

The top two plates were adjusted so that they were

parallel to each other and perpendicular to the

rods.

Two special jigs were used in setting up the test frames. One jig was used to make the springs

symmetrical and concentric with the ball bearing

seat in the plate. The other jig was used in checking that the test specimens were centered

within the three rods at the top of the stack.

After the stack of three test cylinders and the

two dummy cylinders were placed and centered in

the frame, a small load (about 200 psi) was

applied. Variation in the strain around the top

test specimen was then determined. If the loading was judged to be more than 10 percent eccentric the load was I'emoved, and the top test specimen was rotated slightly relative to the other

specimens. When the loading was within the

10 percent eccentricity, the load was increased to about K the creep load (usually 1000 psi) iind the

eccentricity was determined as before. If the

eccentricity was within acceptable limits at this load, the final load increment was applied.

The determination of the actual load transferred

to the frame from the hydraulic ram presents

experimental difficulties with spring-loaded test frames. The usual procedure in transferring the

load from the hydraulic ram to the three reaction

rods is to tighten the nuts while the ram carries

the entire creep load. However, the degree of tightening, which is difficult to evaluate, determines the loss of the applied load upon the release

of the ram. Accordingly, for this investigation

bonded wire strain gages were attached to the

rods for their use as dynamometers in transferring

the load. In effect, these dynamometers were

calibrated every time the frame was loaded by

measuring the strain in the rod when the required

creep load was applied to the specimens by the ram.

9.6.3. Effect of Eccentricity of Loading on Creep

Measurements

The eccentricity of loading could be determined

and controlled at the initial loading, but this control was not possible when the needed adjustments

of the applied load were made. For this reason

it was assumed that, although the eccentricity

probably changed slightly each time a frame was

reloaded, the average long-term eccentricity could

be determined by comparing the total deformations measured in each gage line. It was thought

that through a comparison of the results from the

two companion specimens the effect of eccentricity on the values of creep could be determined. An

analysis was made, and it was concluded that if load eccentricity had any effect on the measured

creep values, it was masked by the effect of other

factors. The eccentricity of loading caused an average

variation of strain in the three gage lines of ±7

percent; the individual strains differed from the I mean by 0 to 16 percent.

9.6.4. Effect of Loading Time on Creep

Measurements

The measured change in length of the creep [ specimen obtained immediately after the initial i ( application of load is usually considered to be [ elastic movement. However, this strain can

include some creep strain if the period of time \ from initial loading until measurements are made ( is considerable. Thus, the elastic strain for the „ last specimen in a given frame to be measured

would include more creep strain than that for the j first specimen measured. Another factor affect- „ ing this measurement would be the time required

to make adjustments due to eccentricity of loading. |

ii,. As a check on the extent of the creep strain ^ whicli might be included in the elastic strain, a / ^ comparison was made between the stress-strain j jj data for the compressive test specimen and the ) |j, companion creep specimens. This comparison [ j was made by comparing the strains at the same ^ stress level for both methods of loading. It was j|, assumed that on the aver-ige, results from the 64 , ,,| batches of concrete considered, the effect of '

|j individual differences between companion speci- [ mens would be cancelled out, and only differences

due to the different methods of loading would , iffect the results. , The results indicated that, on the average, the , measured elastic strain in the creep specimens was [ ,| 4 percent greater than the strain in the companion .^^ compressive test specimen. This result means ; r that, on the average, 4 percent of the initial strain ' \

which was thought to be elastic was probably , fi-eep. I, I

28
0 0 0 0

STRAIN FiGUEE 25. Stress-strain data for a concrete made with aggregate 14-

9.7. Peculiarities of an Anomalous

Lightweight Aggregate

When the series A concretes made from aggregate no. 14 were about three weeks old, it was

noticed that the concretes were considerably

different than the others. For instance, the measured drying shrinkage was less than expected.

This observation led to the discovery of surface

shrinkage racks on all drying specimens of these

concretes. These cracks were visible with the

naked eye even on the loaded specimens.

The 28 day flexural test revealed a remarkable

decrease in modulus of rupture for the drying

specimens. (See fig. 1 and table 5.) WTien the concrete was 90 days old, the cracks

had progressed to such an extent that the resonantfrequency determinations for the dynamic test were meaningless, and the strain readings at low

stress levels for the stress-strain test were such

that the stress-strain curve was concave upwards.

The effect of these cracks on the stress-strain curve can be seen by comparing the typical stress- strain curves of figure 2 and the curves for aggregate 14 concrete shown in figure 25. It can be

seen that this effect is the greatest for the 90-day

test. Since the shrinkage stresses were being relieved

by cracking, the measurements continued to indicate low shrinkage values for this concrete

until the 1-year readings. A comparison of the

shrinkage-time values for the aggregate 14 concrete with the average lightweight is shown in figure 26. The shrinkage cracks were still visible on the

unloaded 6- by 12-in. cylinders when 1 year old,

but were not visible on the loaded specimens.

The shrinkage cracking affected the shrinkage,

flexural strength, and stress-strain determinations.

1 1 1 1 1 1 1 1 — <

A

O

'' SYMBOL TYPE OF CONCRETE

BATCHES AVERAGED

No OF AGGREGATES M.' A o LIGHT WEIGHT 49 20 NORMAL WHGHT 9 5 A AGG. 14 4 A SAND a 14 1 {III 1 1 1 1 1 1 1

DRYING TIME, months FiGUBE 26. Cornparison of shrinkage-time data for steam- cured concretes made with aggregate 14 and with other

aggregates.

but did not appear to effect the compressive

strength.

When this cracking was noticed, specimens of the other concretes were examined for evidence of cracking. However, no sign of cracking could be

observed on any other concrete. Shrinkage cracking in the surface of concrete specimens of various sizes have been noted by

Grieb and Werner [2.3]; and Kennedy [23] in his

discussion of this worlv has reported measuring

the buildup of a compressive stress exceeding 120

psi in the center of a drying 30- by 60-in. concrete

cj'-linder. The extent of the surface cracking

which might be expected in a concrete is probably

affected greatly by the size of the specimen and

the drying conditions.

It should be emphasized that this particular

aggregate (No. 14) is usually produced only in

the coarse fraction which is intended to be used

29
with natural sand in place of the lightweight

fines used in making the series A concrete. vShrinkage-time values for a concrete made from

the coarse fraction of this aggregate and a natural

sand for test V of series B are included in figure

26 for comparison. This semilightweight concrete did not exhibit the unusual behavior noted

above for the all-lightweight concretes.

References

[1] Shideler, J. J., Lightweight aggregate concrete for structural use, ACI Journal, Proc. 54, 299-328

(Oct. 1957).

[2] Shideler, J. J., Manufacture and use of lightweight

aggregates for structural concrete, PCA Research

and Development Laboratory Bull. D 40.

[3] Best, C. H., and Polivka, M.] Creep of lightweight

concrete, Mag. Concrete Res., 11, No. 33, 129-134

(Nov. 1959).

[4] Jones, Truman R., Hirsch, T. J., and Stephanson,

Henson K., The physical properties of structural quality lightweight aggregate concrete. Final Report from Research Project RP-7 for the Texas Highway Dept.. Texas Transportation Institute, Texas A. and M. College System (Aug. 1959).

[5] Hanson, J. A., Tensile strength and diagonal tension

resistance of structural lightweight concrete, ACI

Journal, Proc. 58, 1-37 (July 1961).

[6] Abrams, Duff A., Flexural strength of plain concrete, ACI Journal Proc. 18, 20-45 (1922).

[7] Walker, Stanton and Bloem, Delar L., Studies of

flexural strength of concrete—Part 3: Effects of variations in testing procedures, Proc. ASTM 57, 1122-1142 (1957).

[8] Glucklich, Joseph, and Ishai, Ori, Creep mechanism

in cement mortar, ACI Journal, Proc. 59, 923-948

(July 1962).

[9] Verbeck, G. J., Carbonation of hvdrated portland

cement, ASTM Special Publication No. 205, 17-

36 (Feb. 1958).

[10] Neville, A. M., Role of cement in the creep of mortar,

ACI Journal, Proc. 55, 963-983 (March 1959).

[11] Hanson, J. A., A 10-year study of creep properties of concrete. Concrete Laboratory Report No. SP-38,

U.S. Bur. of Reclamation (1953).

[12] Lyse, Inge, The shrinkage and creep of concrete, Mag. Concrete Res., 11, No. 33, 143-150 (Nov.

1959).

[13] Recommended practice for evaluation of compressive

test results of field concrete, ACI Journal, Proc.

54, 1-9 (July 1957).

[14] Price, Walter H., Factors influencing concrete

strength, ACI Journal, Proc. 47, 417-432 (Feb.

1951).

[15] Hanson, J. A., Optimum steam curing procedure in , precasting plants, ACI Journal, Proc. 60, 75-99 * (Jan. 1963).

[16] Higginson, Elmo C, Effect of steam curing on the important properties of concrete, ACI Journal,

Proc. 58, 281-296 (Sept. 1961).

117] Pauw, Adrian, Static modulus of elasticity of concrete

as affected by density, ACI Journal, Proc. 57, [I 679-687 (Dec' 1960). ' ' [18] Kingham, R. I., Fisher, J. W., Viest, I. M., Creep

and shrinkage of concrete in outdoor exposure and

relaxation of prestressing steel. Highway Res. Board Special Report 66 (Publication 933), 103-

131 (1961).

[19] Corley, W. G., Sozen, M. A., Siess, C. P., Time

dependent deflections of prestressed concrete beams,

HRB Bull. 307 (Publication 939), 1-25 (1961).

[20] Freudenthal, A. M., and Roll, F., Creep and creep recovery of concrete under high compressive stress, : ACI Proc. 54, 111-1142 (June 1958).

[21] McHenry, D., A new aspect of creep in concrete and ' its application to design, ASTM 43, 1069-1084

(1943).

[22] Carlson, R. W., Drying shrinkage of concrete as

affected by many factors, Proc. ASTM 38, 419-

437, Part II (1938).

[23] Grieb, W. E., and Werner, George, Comparison of

splitting tensile strength of concrete with flexural , and compressive strengths, Proc. ASTM 62, 972- \ 975 (1962). Author's Closure and Discussion by

T. B. Kennedy.

30

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Radio Standards Physics. Frequency and Time Disseminations. Radio and Microwave Materials. Atomic Frequency

and Time-Interval Standards. Radio Plasma. Microwave Physics.

Radio Standards Engineering. High Frequency Electrical Standards. High Frequency Calibration Services. High Fre- quency Impedance Standards. Microwave Calibration Services. Microwave Circuit Standards. Low Frequency

Calibration Services. NBS LABORATORY ASTROPHYSICS GROUP

(Joint Institute for Laboratory Astrophysics at University of Colorado)