[m] Technical Report SL-96-21 I July 199811~11US Army Corpsof EngineersWaterways ExperimentStation

Methods of Preparing HorizontalConstruction Joints in Mass Concrete

Report 2Minimizing

by Billy

Laitance

D. Neeley, Toy S. Poole, Charles A. Weiss, Jr.

Approved For Public Release; Distribution Is Unlimited

Prepared for Headquarters, U.S. Army Corps of Engineers

The contents of this report are not to be used for advertising,publication, or promotional purposes. Citation of trade namesdoes not constitute an official endorsement or approval of the useof such commercial products.

The findings of this report are not to be construed as anofficial Department of the Army position, unless so desig-nated by other authorized documents.

(!$aPRINTED ON RECYCLED PAPER

Technical Report SL-96-2July 1998

Methods of Preparing HorizontalConstruction Joints in Mass Concrete

Report 2Minimizing Laitance

by Billy D. Neeley, Toy S. Poole, Chartes A. Weiss, Jr.U.S. Army Corps of EngineersWaterways Experiment Station3909 Halls Ferry RoadVicksburg, MS 39180-6199

Final reportApprovedfor public release; distributionis unlirniied

Prepared for U.S. Army Corps of EngineersWashington, DC 20314-1000

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Contents

Preface . . . . .

IIntroduction

BackgroundObjectives .Scope . . . .

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2ExperirnentalProgram

General . . . . . . . . . .Materials and Mmtures

Materials . . . . . . .Matures . . . . . . .

Test Blocks . . . . . . .

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Tests and Preparation of Test Specimens . . . . . . . . . .Effect of Curing on Concrete Mineralogy . . . . . . . . .

Fabrication of samples for X-ray diffraction analysisX-ray diffraction methodology . . . . . . . . . . . . . .

3Test Results and Statistical Analysis . . . . . . . . . . . .

Direct Tensile Strength . . . . . .Shear Strength . . . . . . . . . . .

Maximum . . . . . . . . . . . .Residual . . . . . . . . . . . . .

Drying Shriige . . . . . . . . .Examination of Surface Material

&Discussion of Results . . . . . . .

Bleeding and Laitance . . . . . . .Joint Strength . . . . . . . . . . . .Surface Material Mineralogy . .

Concrete with silica fhme . .Concrete without silica fime

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5Conclusions and Recommendations . . . . . . . . . . . . .

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . .Recommendations . . . . . . . . . . . . . . . . . . . . . . . .

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vi

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144

5

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2323242425

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Ill

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Appendix A: Photographs of Joint Surfaces PriortoPlacement of Second Lift . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..A1

Appendix B: Failure Envelopes horn Shear Tests . . . . . . . . . . . . . . . . B1

SF 298

List of Figures

Figure 1.

Figure 2.

Figure 3.

Figure 4.

Figure 5.

Figure 6.

Figure 7.

Figure 8.

Figure 9.

Combined grading of coarse aggregate . . . . . . . . . . . . . . . 8

Testarticle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Preparation of cores . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

X-ray diffraction patterns of samples of concretewithout silica fume removed horn the top surface ofconcrete 7daysafler casting.. . . . . . . . . . . . . . . . . . . . . 14

X-ray diffraction patterns of samples of concretecontaining 2.5 mass percent silica fume removed fromthe top surface of concrete 7 days after casting . . . . . . . . . . 15

Comparison of direct tensile strength of joints in lowbleeding concrete with tensile-strength data from jointedspecimens reported in Neeley and Poole (1996) . . . . . . . . . . 17Comparison of maximum shear strength of joints in lowbleeding concrete with tensile-strength data from jointedspecimens reported in Neeley and Poole (1996) . . . . . . . . . . 18Comparison of residual shear strength of joints in lowbleeding concrete with tensile-strength data fkom jointedspecimens reported in Neeley and Poole (1996) . . . . . . . . . . 20Measurements of drying shrinkage from umestrainedprisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

List of Tables

Table 1. Test Matrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Table 2. Aggregate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Table 3. Portland Cement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

iv

Table 4. Fly Ash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Table 5. Silica Fume . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Table 6. Concrete Mixture Proportions.. . . . . . . . . . . . . . . . . . . . 9

Table 7. TestResults, FreshandHardened Concrete . . . . . . . . . . . . 9

Table 8. DirectTensile Strength . . . . . . . . . . . . . . . . . . . . . . . . . 16

Table 9. Maximum Shear Strength . . . . . . . . . . . . . . . . . . . . . . . . 18

Table10. Residual Shear Strength . . . . . . . . . . . . . . . . . . . . . . . . . 19

Preface

The investigation describedin this report was conducted by the Concreteand Materials Division (CMD), Structures Laboratory (SL), U.S. ArmyEngineer Waterways Experiment Station (WES). The work was sponsored byHeadquarters, U.S. Army Corps of Engineers, as a part of Civil WorksInvestigation Studies Work Unit 32767, Horizontal Construction JointTreatment in Mass Concrete.

The study was conducted under the general supervision of Messrs. BryantMather, Director, SL, and John Q. Ehrgott, Assistant Director, SL, andDr. Paul F. Mlakar, Chief, CMD. Direct supervision was provided byMr. Edward F. ONeil, Acting Chief, Engineering Mechanics Branch (EMB),CMD. Mr. Billy D. Neeley, EMB, was the Principal Investigator andcoauthored this report with Drs. Toy S. Poole and Charles A. Weiss, Jr.,Engineering Sciences Branch, CMD. Messrs. Michael Lloyd, Jimmy Hall,Cliff Gill, and Michael Hedrick and Ms. Linda Mayfield, EMB, assisted inpreparing and testing the concrete specimens.

At the time of publication of this report, Director of WES wasDr. Robert W. Whalin. Commander was COL Robin A. Cababa, EN.

Z& contenisof tM vepori are nd & be usedfor odvetiing, publication,or promdionol purposes. CWatbnof tmde names does I@ constituteano@c&d endbtxemeti or apptvvol of the we of such commerchl products.

vi

1 Introduction

Background

Under ideal conditions, any mass concrete structure should be monolithic.However, mass concrete structures usually contain horizontal constructionjoints because it is impractical to place such a large volume of concrete withoutlengthy interruptions. These joints must be capable of transmitting stress com-binations, including tension, compression, and horizontal shear, from one partof the concrete structure to another. As a minimum, a horizontal constructionjoint must have bond, tensile, and shear strengths greater than the stresses towhich it will be subjected. Ideally, the strength of the joint should be equal tothat of the surrounding concrete.

Planes of weakness can result if horizontal construction joints are not pre-pared properly during construction. Structural weakness, leakage, and subse-quent deterioration can result from a poorly prepared horizontal constructionjoint. The quality of a horizontal construction joint in mass concrete dependson both the quality of the concrete, both above and below the joint, and thepreparation of the joint surface.

Experience has shown that the lower surface of a joint plane must becleaned thoroughly prior to placement of fresh concrete to ensure good bondstrength and watertightness of the two layers. Various methods of cleaning thelower surface of a joint plane have been used. Civil Works Guide Specifica-tion CWGS-03305, Mass Concrete (Headquarters, Department of the Army1992), has provisions for cleaning by air-water cutting, high-pressure waterjet, or wet sandblasting. It states that all laitance and inferior concrete shouldbe removed so that clean, well-bonded coarse aggregate particles are exposedover the lift surface. However, the coarse aggregate particles should not beundercut. Use of a surface retarder is permitted to extend the period of timeduring which air-water cutting is effective. CWGS-03305 also states that thesurface of a construction joint should be kept continuously wet for the first12 hr of the 24-hr period prior to placing the fresh concrete, except that thesurface shall be damp with no free water at the time of placement.

Chepter 1 Introduction

Between 1959 and 1973, four technical reports were published by theU.S. Army Engineer Waterways Experiment Station (WES) describing theresults of an investigation of methods of preparing horizontal constructionjoints (Tynes 1959; Tynes 1963; McDonald and Smith 1966; Tynes andMcC1eese 1973). These investigations generally concluded that (a) wetsandblasting, air-water cutting, and high-pressure water jetting were effectivemethods of cleaning a joint surface, (b) application of mortar to a joint surfacedid not improve the integrity of the joint, and (c) a stronger and moreimpermeable joint will result if the hardened concrete surface is dry when thefresh concrete is placed. Pacelli, Andriolo, and Sarkaria (1993) presented casehistories of investigations regarding the performance of construction joints infive large concrete darns. Test results indicating the bond and shear strengthsof new concrete placed on the cleaned and roughened surface of existing cm-crete were presented. Their investigations generally concluded that (a) high--pressure water jetting and air-water cutting were as effective as wetsandblasting for cleaning a joint surface, (b) properly prepared constructionjoints had shear and tensile strengths equal to at least 85 percent of that of theintact concrete, (c) roughness of the joint surface did not have a significantinfluence on the strength of the joint, (d) application of mortar to a jointimproved the joint strength only if the joint surface was not properly cleaned,and (e) the permeability of a properly prepared joint was essentially the sameas that of the intact concrete. Neeley and Poole (1996) concluded that (a) thesurface was adequately cleaned when the visible laitance had been removed andfine and coarse aggregate particles had been exposed, (b) undercutting thecoarse aggregate particles was unnecessary, and (c) allowing the joint surfaceto dry approximately 24 hr prior to placement of the next lift and placing onthe dry surface would result in a stronger joint.

In light of the conclusions cited above, a new question arose. If surfacecleanliness and moisture content are more important than surface roughness(degree of aggregate interlock created by undercutting the coarse aggregateparticles), could an adequately prepared joint surface be produced withoutusing cleaning procedures such as high-pressure water jetting, air-watercutting, or sandblasting? Or more simply stated, if loose surface laitance couldbe prevented or minimized, would surface cleaning still be required?To answer this question, methods to minimize laitance needed to be examined.

Laitance is defined by the American Concrete Institute (ACI) Commit-tee 116 (ACI 1997) as a layer of weak and nondurable material containingcement and fines from aggregates, brought by bleeding water to the top ofoverwet concrete; the amount is generally increased by overworking or over-manipulating concrete at the surface by improper finishing or by job-traffic. ACI Committee 116 (ACI 1997) defines bleeding as the autogenous flow ofmixing water within, or its emergence from, newly placed concrete or mortar;caused by the settlement of the solid materials within the mass. Generally,most normal strength concrete mixtures are mixed using more water forworkability than is needed in the mixture to fill the space among the solidparticles of aggregate and cement. These particles are initially suspended in

2Chapter 1 Introduction

this water. After concrete has been placed but before the cement has stiffened,some settling of aggregate and cementitious material particles occurs. As thesettlement occurs, some of the excess mixing water is released. The releasedwater, being the lightest component, then migrates (bleeds) to the top surfaceof the concrete. A nominal amount of bleeding is common for most concretesand, if handled properly, is not necessarily bad. However, in some instances ascum of fine particles can be carried to the surface by the bleed water. Theseparticles can create a weak, nondurable surface. In some cases calciumhydroxide can precipitate from solution in the bleed water and can react withC02 in the air at the surface, leading to the formation of CaCOJ. Aftereventual evaporation of the bleed water, the fine particles can be seen as adusting or flaking of loose material on the surface, or in the case of CaCOq, awhite powdery materiaI. This laitance, if not removed, can act as a bondbreaker.

Since laitance is the result of bleeding, it follows that reducing bleedingshould reduce laitance. Mindess and Young (1981) list four ways to reducebleeding: (a) increase fineness of the cement (Portland cement or groundgranulated blast-fimace slag) or add pozzolans or other finely divided mineraladmixtures, (b) increase the rate of hydration of the cement by using cementswith high alkali contents or high GA contents, or use CaC12as an admixture,(c) use air entrainment, and (d) reduce the water content. Use of high alkali,high ~A, or finely ground cement, and CaC12is not desirable in mass cm-crete. In most instances, use of such materials would not be permitted. Use ofCaClz is not permitted in most Corps concrete. When an accelerating admix-ture is allowed, usually only nonchloride materials maybe used. Mostconcrete used by the Corps has entrained air. Water content is minimized bycarefid mixture proportioning and effective use of water-reducing admixtures.Pozzolans, such as fly ash, or ground granulated blast-fhmace slag are used innear]y all Corps mass concrete.

It can be concluded that efforts to reduce bleeding are already incorporatedinto the proportioning of Corps mass concrete. The purpose of this researcheffort was to determine if bleeding could be minimized further through use ofother pozzolans or chemical admixtures. Pozzolans such as silica fume have amuch higher water demand than does fly ash. One characteristic of silica fumeconcretes is that bleeding is either minimal or nonexistent. Silica fume is nor-mally used to produce high-strength or highly impermeable concrete. Whileneither of these two attributes is needed for typical mass concrete, it is possiblethat a small addition of silica fume to the total cementitious material couldsignificantly reduce bleeding without significantly affecting other fresh andhardened properties. Another possible solution is with the addition of achemical admixture, antiwashout admixture (AWA). AWAs are normally usedto increase the cohesiveness of concrete to be placed underwater, thus minimiz-ing washout of the cementitious paste from the aggregate particles. Anotherresult is that bleeding is reduced. Therefore, it is possible that a small additionof an AWA to the mass concrete mixture could significantly reduce bleedingwithout significantly affecting other fresh and hardened properties.

Chapter 1 Introduction 3

Objectives

The objectives of this research program were as follows:

a. Determine if loose, flaky laitance could be minimized, or prevented, onthe surface of a mass concrete placement.

b. Determine the strength of a horizontal construction joint where bleed-ing had been minimized and surface cleaning had not been used.

Scope

Mass concrete monolithic models similar to those used in the earlierinvestigations were constructed. An addition of 2.5 percent of silica fume, byvolume of cementitious material, was chosen as the method to minimize bleed-ing. Three surface moisture conditions were evaluated. Properties tested weredirect tensile strength and shear strength of the joint. Drying shrinkage of theconcrete was also measured to evaluate the effect of the silica fume on theshrirhge properties of the concrete. A test matrix is given in Table 1. Afterproperty measurements on concrete from the monolithic models had beencompleted, a second small experiment was executed in an attempt to determinethe cause of some unexpected results. Many of the measurements were madeand recorded in non-SI units and converted to S1 units using conversion valuesin American Society for Testing and Materials (ASTM) E 380 (ASTM 19960).

Table 1Test Matrix

BlodcIdontifii Type of Joint Preparation Moistura Condition of Joint

M None Wet continuously.

N Nona Wet continuously, than dry 16 hr priorto placement, then moisten surfaceimmadiataly before placement.

o None Dry 24 hr before placemant.

Chapter 1 introduction

2 Experimental Program

General

The materials and concrete mixtures used in this investigation were typicalof those currently used in mass concrete construction. A brief description ofthe materials, mixtures, and test specimens is given below.

Materials and Mixtures

Materials

The coarse aggregate was 75-mm nominal-maximum-size (NMS) crushedlimestone. The fine aggregate was a natural sand. The grading (ASTM C 136(ASTM 1996e)) of each aggregate and values of absorption and specific gravity(ASTM C 127 (coarse aggregate) and C 128 (fine aggregate) (ASTM 1996c,d))are given in Table 2. A graph showing grading of the coarse aggregate isshown as Figure 1.

The cement was portland cement, conforming to Type II requirements ofASTM C 150 (ASTM 1996h). The fly ash conformed to Class F requirementsof ASTM C 618 (ASTM 1996m). The silica fume conformed to therequirements of ASTM C 1240 (ASTM 1996n). Physical and chemical prop-erties of the cement, fly ash, and silica fume are given in Tables 3, 4, and 5,respectively.

Mixtures

The concrete mixtures were proportioned in accordance with ACI211. 1,Standard Practice for Selecting Proportions for Normal, Heavyweight, andMass Concrete (ACI 1996). The mortar content for the mixtures was withinthe range recommended by ACI 211.1 for concrete mixtures containing 75-mmNMS aggregate. The combined grading of the coarse aggregate is given inFigure 1. The total cementitious material in the mixture without silica fume

Chapter 2 ExperimentalProgram 5

Table 2Aggregate II

IICumulative Percent Paaaing

75-mm NMS 37.5-mm NMS 19.O-mm NMSSiive sue Coarse Agg. Coarse Agg. Coarse Agg. Fine Agg.

75 rrmll 100

6orrnn 50 100

37.5 rrvn 8 96

25.0 mm 1 29 100

19.0 mm 7 97

12.5 mm 3 65

9.5 mm 3 39

4.75 mm 2 6 100

2.36 mm 1 80

1.18mm 68

6oo Jml 57

300 Jnll 23

160 Jan 2

SpesifIo Gravity 2.65 2.74 2.71 2.60

Absorption, % 1.6 0.3 0.2 1.2

CMD ID I 950122 I CL-2 MG-2 920048 I 950057 1

Table 3Portland Cement

A120, I 3.4

Fe,O. I 2.4

Cao I 63.7MgO I 3.8

so. I 2.8

Loas on ignition I 1.1

-

Alkeliea - total as NazO 0.67

-no, 0.15

P20, I 0.10

CMD ID 920044

Property Reauit

C3A, % 6

C3S, % I 61

C,s, % I 16

C4AF, % 7

Heat of hydration, kJ/kg 305

final set (Gillmore), min 265

Air content, % 9

Compressive strength, 3 days, MPa 21.8

Compressive strength, 7 days, MPa 28.1

False set 88

6Chapter 2 ExperimentalProgram

Table 4Fly Ash

CMD ID 930028

Property. % Rsautt Property Result

SiOz 53.9 Loss on ignition, % 1.6

AlzO, 27.1 Available eJkalies,% 0.9

FezO, 11.0 Fineness, residue 45-p_rrsieve, % 15

Sum SiOzr AlzO~, Fez03 91.9 Weter requirement, % 97

MgO 0.9 Density, Mg/m3 2.36

S03 0.3 Autocleve expansion, % -0.06

Mokture content 0.2 Pozzolanic activity with lime, MPe 7.9

Strength ectivity with cement, % 86

Table 5Silica Fume

1CMD ID 950570

I

Propany, % Result Property Resutt

SiOz 85.9 Density, Mg/m2 2.24

II Moisture content I 0.3 I Surfece area, mzhcg I 3,740

was 60 percent portland cement and 40 percent fly ash by volume. The totalcementitious material in the mixture with silica fume was 58.5-percent portlandcement, 39-percent fly ash, and 2.5-percent silica fume by volume. The con-crete mixture proportions are given in Table 6. Tests were conducted on thefresh concrete to determine slump (ASTM C 143) (ASTM 1996g), unit weight(ASTM C 138) (ASTM 1996~, and air content (ASTM C 231) (ASTM1996k). Cylindrical specimens (152-mm diam by 305-rnm high) were preparedaccording to ASTM C 192 (ASTM 1996j) and cured in a moist curing roommeeting the requirements of ASTM C 511 (ASTM 19961)until time of testing.Sp@rnens were tested in unconfined compression at 7-, 14-, 28-, and 90dayage according to ASTM C 39 (ASTM 1996a). Results of tests on the unhard-ened concrete and the unconfined compression tests are given in Table 7.Unrestrained prisms (76- by 76-by 254-mm) were prepared and curedaccording to ASTM C 157 (ASTM 1996i) for determination of dryingshrinkage.

Chapter 2 ExperimentalProgram

100

60

80

70

30

20

10

0

I I

75 50 37.5 25.0 190 12.5 9.5 4.75SIEVE SIZE, mm

I

Figure 1. Combined grading of coarse aggregate

Test Blocks

The three moisture conditions described in the experimental program wereeach represented by a single test block of concrete, designated M, N, and O, asshown in Table 1. Each block was 0.92 m long, 0.53 m wide, and 0.75 mhigh and was cast in two lifts. The first lift was 0.45 m deep and incorporatedsilica fime as 2.5 percent of the total cementitious material. The surface of theIlesh concrete was not finished. Block M (continuously wet) was cured withwet burlap and sheet plastic for 14 days, after which the covering wasremoved. The surface was vacuumed to remove loose particles, and the secondlift (0.3 m thick) was placed. Block N (dry then rewet) was cured with burlapand sheet plastic for 13 days, after which the surface was aIlowed to dry for16 hr. The surface was then demoistened and vacuumed, and the second lift(0.3 m thick) was placed. Block O (dry) was cured with the burlap and plasticfor 13 days, after which the surface was allowed to dry for 24 hr. The surfacewas vacuumed, and the second lift (0.3 m thick) was placed. Photographs ofthe concrete surfaces as prepared are shown in Figures Al through A6 inAppendix A.

Chapter 2 Experimental Program

Table 6Concrete Mixture Proportions

Material 1 cubic metro

Mii without Silica FrJma Maaa,kg Volume. m

Portland 109 0.035

Fly ash 55 0.023

fine aggragata 635 0.244

19.O-mm NMS coarse aggregate 362 0.134

37.5-mm NMS coarse aggregate 381 0.139

75-mm NMS coarse aggregate 752 0.284

Water 100 0.100

Akantraining admixture 0.19 L

Miira with silica Fumai

Podanrf cament 109 0.03!5

Fiy ash 55 0.023

Silica fume 3 0.001

Fine aggregate 634 0.244

19.O-mm NMS coarse aggregate 361 0.133

37.5-rnrn NMS coarse aggregate 380 0.139

75-mm NMS coarse aggregate 750 0.283

Water 101 0.101

Air-entrainingadmixture 0.19 L

Table 7Test Results, Fresh and Hardened Concretel

Testmocks

M,N,O

Layerz

8ottomTop

I I

Slumpmm

4020

&5.3 23.05.1 20.9

Avarage of two batches for eech layer.z 8ottom - concrate with silicafume; top - concrete without silicafume.3 In that portion of the concrete containing aggregate smallerthan the 37.5-mm sieve.

Chapter 2 Experimental Program 9

Tests and Preparation of Test Specimens

A minimum of twelve 152-mmdiarn cores were taken from each test blockusing a diamond-bit core barrel, according to ASTM C 42 (ASTM 1996b).Cores were cut perpendicular to the horizontal joint, completely through thetest block, as shown in Figure 2. Cores were randomly selected for direct ten-sile testing or shear testing. Test specimens were then cut from the cores asillustrated in Figure 3. The experimental design required six intact specimensand six specimens with joints for direct-tensile strength testing according toCRD-C 164 (WES 1949), six intact specimens and six specimens with jointsfor shear-strength testing according to RTH-203 (WES 1989), and six intactspecimens for compressive-strength testing according to ASTM C 42 (ASTM1996b). Shear strength was measured at three levels of normal loading.Nominal values of normal stress were 192, 383, and 766 kPa, although actualvalues were recorded. Two specimens were tested at each level of normal loadto determine the maximum shear strength at failure of the joint plane. A linearregression line was calculated with normal stress as the independent variable(X) and measured shear strength as the dependent variable (Y). The Yintercept (shear at zero normal loading) was taken as the cohesion. The stan-dard error of the intercept was used to compare results among treatment condi-tions. The coefficient of internal friction, $, is the arctan of the slope of theregression line. The standard error of the slope was used for statist~ca.1evalua-tions of $. After maximum shear determinations had been completed, sheartesting was repeated on the broken specimens to determine the residual valuesof cohesion and +. Residual values of cohesion and @were calculated by thesame linear regression procedure as for maximum shear strength.

----- ----- ----- _____ __

IWEE!E 1.I ! 1.%%

10

Figure 2. Test article

Chapter 2 Experimental Program

FOR

FOR

FOR

DIRECT SHEAR

DIRECT SHEAR

COMPRESSIVE STRENGTH

DIRECT SHEAR

~ FOR DIRECT TENSION

~ FOR DIRECT TENSION

DIRECT TENSION

Chapter 2

Figure 3. Preparation of cores

ExperimentalProgram 11

Numbers of specimens actually tested for each property differed from thenumber in the experimental design because some were broken during drillingor sawing. Actual numbers tested are indicated in the tabulation of results.

Effect of Curing on Concrete Mineralogy

Fabrication of samples for X-ray diffraction analysis

To ascertain if the hydration of the cement phases aided in the ability toobtain a good bond on the lift surface between the subsequent lifts of concrete,a series of samples were fabricated. In this experiment, only the first lift wasplaced. The concrete mixtures (with and without 2.5-percent silica fume byvolume of total cementitious material) were the same as were used in fabrica-tion of the jointed monolithic models. Three test specimens (152-by 152-by533-mm prisms) were cast from each mixture. After the concrete was placedin the molds, the specimens were covered with a plastic sheet for approxi-mately 24 hr. Bleed water was collected periodically from the concrete surfacefor the first few hours after casting until bleeding ceased. After approximately24 hr, all specimens were covered with wet burlap. One specimen fkom eachmixture was maintained in a moist state for the entire curing period of 6 days.One specimen fkom each mixture was maintained in a moist state for 5 days,and then the surface was allowed to airdry for 24 hr. The remaining twospecimens (one from each mixture) were maintained in a moist state for 5 days,the surface allowed to airdry for 24 hr, and then demoistened. This simulatedthe curing conditions used in fabrication of the monolithic models except thatthe total curing time was shorter. Another difference was that the depth of theconcrete in the test samples was less in this experiment than was the case in themonolithic models. This resulted in less bleed water on the surface of thesesmaller specimens than was evident on the lift surface of the larger models.After 6 days, samples were taken from the surface of the concrete specimensfor examination by X-ray diffraction (XRD).

XRD patterns of the surface material from the plain concrete and the con-crete with silica fume are given in Figures 4 and 5, respectively.

X-ray diffraction methodology

In preparation for XRD examination, a sample was scraped off the surfaceof each prism and ground in a mortar and pestle to pass a 45-pm sieve. XRDpatterns were collected from these powdered samples using standard techniquesfor phase identification. The equipment used in this analysis was a PhilipsPW1800 Automated Powder Diffractometer system. The examination condi-tions included use of CuKa radiation and step-scanning from2 to 6520,8 seconds per step, and 0.05 per step with collection of the diffraction

12Chapter 2 Experimental Program

patterns accomplished using PC-based windows 95 versions of DataScan(Materials Data, Inc.) and analysis using Jade.

Chapter 2 Experimental Program 13

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xisting guidance or, if necessary, update it. This is the second and final report.

The joint moisture conditions at the time of concrete placement were (a) continuously moist, (b) dry, and (c) drymd then demoistened. There was no cleaning of the joint surface prior to placement of a second lift. The concretemixture used in the first lift was proportioned to minimize bleeding and the resulting laitsnce. Jointed specimens were=ted for direct tensile strength and shear strength.

The results indicated that an addition of a small quantity of silica fume does decrease bleeding and raiuce loose,flaky lsitance. However, a smooth, somewhat glassy surface appears to interfere with the bonding of the next lift ofmncrete. Bond strengths are not as good as those when the horizontal construction joints are cleaned by high-pressurewater cutting or by air-water cutting, and then allowed to dry approximately 24 hr immediately prior to placement ofhe next lift of concrete.

This technique is not rrwmnmended. Current guidance concerning joint cleaning procedures should be followed.

14. SUBIECT TERMS 15. NUMBER OF PAGESOoncrete Horizontal construction joint 482oncrete construction Mass concrete 16. PRICE COOE

17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION 20. LIMITATION OF ASSTRACTOF REPORT OF THIS PAGE OF ABSTRACT

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