A Comparative Study of Shrinkage and Cracking Of

8/6/2019 A Comparative Study of Shrinkage and Cracking Of

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A Comparative Study of Shrinkage and Cracking of

High Performance Concrete Mixtures for Bridge Decks

Jennifer Morris

Thesis submitted to theCollege of Engineering and Mineral Resources

atWest Virginia University

in partial fulfillment of the requirementsfor the degree of

Master of Sciencein

Engineering

Julio F. Davalos, Ph.D., ChairIndrajit Ray, Ph.D., Co-Chair

Karl E. Barth, Ph.D.

Department of Civil and Environmental Engineering

Morgantown, West Virginia

2002

Keywords: high performance concrete, HPC, shrinkage, creep, cracking, latex, SRA


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ABSTRACT

A Comparative Study of Shrinkage and Cracking of High-Performance Concrete

Mixtures for Bridge Decks

Jennifer Morris

To alleviate the deterioration of concrete structures, High-Performance Concrete(HPC) is extensively used, particularly for bridge decks, due to its favorable durabilitycharacteristics. However, there is a problem with the high cracking tendency of HPC dueto its high early shrinkage, low water-cementitious ratio, brittleness and low creep. Thus,the HPC advantages of high compressive strength and low permeability are somewhatoffset by cracking problems, which reduce the service life of bridge decks. To improvethis problem, the goal of this study is to modify conventional HPC mixtures withadmixtures or modifiers to obtain low-cracking or no-cracking formulations using WestVirginia aggregates. Using a normal concrete (NC) as a bench-mark, a total of four

distinct HPC mixtures were studied: conventional HPC termed Class H by the WVDOH,HPC with shrinkage reducing admixture (HPC-SRA), HPC with latex (HPC-L), and HPCwith high latex content or Overlay Latex-Modified Concrete (OLMC) as used by theWVDOH. By maintaining constant aggregate-paste volume ratio, the performance ofthese mixtures was evaluated through a number of tests: compressive strength, splittensile strength, free shrinkage, restrained shrinkage with ring specimens at early age,cracking tendency by measuring crack onset and width, chloride permeability, and creepfor a few cases.

Following ACI 363, the results showed that HPC-L and OLMC are less brittlethan the other mixtures. Free-shrinkage results indicated that shrinkage-reducingadmixture (HPC-SRA) minimized shrinkage significantly, followed by the addition oflatex (HPC-L and OLMC). In relation to a proposed shrinkage model developed in thisstudy, particularly for HPC-SRA and OLMC, the existing ACI 209 predictionoverestimated the values, except for NC and HPC. For cracking response of restrainedshrinkage, OLMC performed the best with no cracks after 90 days, followed by crackingonset for HPC-L at 60 days and HPC-SRA at 33 days. The nearly negligible freeshrinkage of HPC-SRA did not prevent crack formation during restrained shrinkage. Theonset of cracks for both NC and HPC occurred early, at 18 and 27 days respectively, andHPC exhibited the highest number of cracks than all other mixtures; this behavior of HPCis explained by simply analyzing the results for tensile strength, shrinkage and creep.Overall, the performance of OLMC and HPC-L was best in terms of strength, chloridepermeability, and shrinkage and cracking.

This study will provide better focus to continued in-depth research into mixtureoptimization to achieve nearly crack-free HPC mixtures under field restrained shrinkageconditions.


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Table of Contents

Title Page ...................................................................................................................i

Abstract......................................................................................................................ii

Acknowledgement .....................................................................................................iii

Table of Contents.......................................................................................................iv

List of Figures............................................................................................................viii

List of Tables .............................................................................................................x

Chapter 1: Introduction ..........................................................................................1

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

1.2 Research Objectives and Tasks..........................................................2

1.3 Research Plan.....................................................................................2

1.4 Thesis Organization ...........................................................................3

Chapter 2: Background and Literature Review ...................................................5

2.1 Introduction........................................................................................5

2.2 Curing ................................................................................................6

2.3 Shrinkage ...........................................................................................7

2.3.1 Plastic Shrinkage....................................................................8

2.3.2 Autogenous Shrinkage ...........................................................9

2.3.3 Drying Shrinkage ...................................................................9

2.3.4 Carbonation Shrinkage...........................................................11

2.3.5 Factors Influencing Shrinkage ...............................................12

2.3.6 Free Shrinkage Test Method..................................................14

2.4 Creep..................................................................................................15

2.4.1 Factors Influencing Creep......................................................16

2.4.2 Effects of Creep .....................................................................18

2.4.3 Creep Test ..............................................................................19

2.5 Rapid Chloride Permeability Test......................................................19

2.5.1 The RCPT Methods and Information ....................................20

2.5.2 Concerns and Research Regarding the RCPT .......................21

2.6 The Ring Test Method .......................................................................23


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2.6.1 Ring Test Set Up and Theory on Cracking............................24

2.6.2 Test Methods and Results Using Ring Set Up.......................29

2.6.2.1 Study by Li et al. (1999) ............................................29

2.6.2.2 Study by Shah et al. (1992)........................................31

2.6.2.3 Study by Wiegrink et al. (1996).................................33

2.6.2.4 Study by Grzybowski and Shah (1990) .....................35

2.7 NCHRP Report by Krauss and Rogalla .............................................37

2.8 Research Significance........................................................................40

Chapter 3: Experimental Program ........................................................................42

3.1 Introduction........................................................................................42

3.2 Materials ............................................................................................42

3.2.1 Portland Cement.....................................................................42

3.2.2 Coarse Aggregate...................................................................44

3.2.3 Fine Aggregate.......................................................................46

3.2.4 Fly Ash...................................................................................47

3.2.5 Silica Fume ............................................................................48

3.2.6 Chemical Admixtures ............................................................49

3.2.6.1 High Range Water-Reducing Admixture...................50

3.2.6.2 Air Entraining Admixture..........................................51

3.2.6.3 Shrinkage-Reducing Admixture ................................52

3.2.6.4 Latex ..........................................................................53

3.2.6.5 Defoamer....................................................................54

3.2.7 Water......................................................................................54

3.3 Mixture Proportions...........................................................................55

3.4 Mixing Procedures .............................................................................57

3.5 Preparation of Test Specimens...........................................................59

3.5.1 Cylinder Specimens for Compressive Strength Test .............59

3.5.2 Cylinder Specimens for Split Tensile Strength Test..............59

3.5.3 Disk Specimens for Rapid Chloride Permeability Test .........60

3.5.4 Prism Specimens for Length Change Measurements ............61

3.5.5 Ring Specimens for Crack Monitoring..................................62


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3.5.6 Prism Specimens for Creep Test............................................64

3.5.7 Curing of Different Types of Test Specimens .......................66

3.6 Testing Procedures.............................................................................66

3.6.1 Fresh Concrete Tests..............................................................67

3.6.1.1 Slump .........................................................................67

3.6.1.2 Air Content.................................................................67

3.6.1.3 Unit Weight................................................................68

3.6.1.4 Temperature ...............................................................68

3.6.2 Hardened Concrete Tests .......................................................68

3.6.2.1 Compressive Strength Test ........................................69

3.6.2.2 Split Tensile Strength Test.........................................69

3.6.2.3 Length Change Measurement Test ............................70

3.6.2.4 Restrained Shrinkage of Ring Specimens..................72

3.6.2.5 Rapid Chloride Permeability Test..............................73

3.6.2.6 Creep Test ..................................................................75

Chapter 4: Test Results and Discussions ...............................................................77

4.1 Introduction........................................................................................77

4.2 Fresh Concrete Properties ..................................................................77

4.3 Compressive Strength and Split Tensile Strength Results.................79

4.4 Length Change Measurement Results ...............................................89

4.5 Creep Test Results .............................................................................106

4.6 Discussion of Ring Specimen Cracking ...........................................109

4.7 Rapid Chloride Permeability Test Results.........................................119

Chapter 5: Summary and Conclusions..................................................................123

5.1 Introduction........................................................................................123

5.2 Summary of Results...........................................................................123

5.3 Cost Analysis .....................................................................................127

5.4 Conclusions........................................................................................131

5.5 Suggestions for Future Research .......................................................132

References..................................................................................................................134

Appendix A Compressive Strength and Split Tensile Strength Results.................142


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Appendix B Length Change Measurement Test Results........................................147

Appendix C Rapid Chloride Permeability Test Results.........................................158

Appendix D Creep Test Results .............................................................................164

Appendix E Crack Data Results.............................................................................171


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List of Figures

Figure 2.1.......Schematic of the Ring Test ...............................................................28

Figure 3.1 .......Portland Cement.................................................................................44

Figure 3.2 .......River Gravel.......................................................................................45

Figure 3.3 .......River Sand..........................................................................................47

Figure 3.4 .......Fly Ash...............................................................................................48

Figure 3.5 .......Silica Fume ........................................................................................49

Figure 3.6.......High Range Water-Reducing Admixture and Container ...................50

Figure 3.7 .......Air Entraining Admixture and Container ..........................................51

Figure 3.8.......Shrinkage-Reducing Admixture and Container.................................52

Figure 3.9.......Latex Admixture and Container.........................................................53

Figure 3.10 .....Defoamer and Bottle ..........................................................................54

Figure 3.11 .....Rotary Mixer......................................................................................58

Figure 3.12 .....Typical Cylinder Specimen................................................................60

Figure 3.13 .....Typical Disk Specimen ......................................................................61

Figure 3.14 .....Typical Prism Specimen ....................................................................62

Figure 3.15.....Schematic of Ring Set Up..................................................................63

Figure 3.16 .....Typical Ring Specimen......................................................................64

Figure 3.17 .....Typical Beam Specimen ....................................................................65

Figure 3.18.....Length Change Apparatus with Rod..................................................71

Figure 3.19.....Length Change Apparatus with Specimen.........................................71

Figure 3.20.....Microscope with Magnification x40 to Measure Crack Width .........72

Figure 3.21.....Rapid Chloride Permeability Test......................................................75

Figure 3.22 .....Creep Machine ...................................................................................76

Figure 4.1 .......Compressive Strength at Various Ages .............................................85

Figure 4.2 .......Split Tensile Strength at Various Ages..............................................85

Figure 4.3 .......Normalized Compressive Strength for All Mixtures.........................86

Figure 4.4 .......Normalized Split Tensile Strength for All Mixtures..........................86

Figure 4.5 .......Relationship Between Split Tensile Strength and Square Root of

Compressive Strength ........................................................................87


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Figure 4.6.......Free Shrinkage and Moisture Loss for NC........................................92

Figure 4.7 .......Free Shrinkage and Moisture Loss for HPC......................................92

Figure 4.8 .......Free Shrinkage and Moisture Loss for HPC-SRA.............................93

Figure 4.9.......Free Shrinkage and Moisture Loss for HPC-L ..................................93

Figure 4.10.....Free Shrinkage and Moisture Loss for OLMC..................................94

Figure 4.11 .....Free Shrinkage for All Mixture Types...............................................95

Figure 4.12.....Free Shrinkage versus Moisture Loss ................................................95

Figure 4.13 .....Best-Fit Linear Equation for NC Free Shrinkage Strain....................100

Figure 4.14.....Best-Fit Linear Equation for HPC Free Shrinkage Strain .................100

Figure 4.15 .....Best-Fit Linear Equation for HPC-SRA Free Shrinkage Strain ........101

Figure 4.16 .....Best-Fit Linear Equation for HPC-L Free Shrinkage Strain..............101

Figure 4.17 .....Best-Fit Linear Equation for OLMC Free Shrinkage Strain..............102

Figure 4.18.....Predictions for Free Shrinkage Strain in NC .....................................103

Figure 4.19.....Predictions for Free Shrinkage Strain in HPC ...................................103

Figure 4.20 .....Predictions for Free Shrinkage Strain in HPC-SRA..........................104

Figure 4.21.....Predictions for Free Shrinkage Strain in HPC-L ...............................104

Figure 4.22.....Predictions for Free Shrinkage Strain in OLMC ...............................105

Figure 4.23.....Specific Creep for NC and HPC ........................................................109

Figure 4.24 .....Crack Data for NC with Best-Fit Equation........................................110

Figure 4.25 .....Crack Data for HPC with Best-Fit Equation......................................111

Figure 4.26.....Crack Data for HPC-SRA with Best-Fit Equation ............................111

Figure 4.27 .....Crack Data for HPC-L with Best-Fit Equation..................................112

Figure 4.28.....Comparison of Crack Data for All Mixtures .....................................113

Figure 4.29 .....Average Crack Width after 75 Days of Drying .................................113

Figure 4.30 .....Charge Passed in RCPT for All Mixtures..........................................120

Figure 4.31 .....Comparison of Chloride Permeability among Mixtures....................120


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List of Tables

Table 3.1 ........Material Composition of Type I Portland Cement ............................43

Table 3.2 ........Sieve Analysis of Coarse Aggregate..................................................45

Table 3.3 ........Sieve Analysis of Fine Aggregate......................................................46

Table 3.4 ........Mixture Designations.........................................................................56

Table 3.5 ........Mixture Proportions in US Customary Units.....................................56

Table 4.1 ........Fresh Concrete Properties..................................................................78

Table 4.2 ........Compressive Strength at Various Ages for All Mixtures..................82

Table 4.3 ........Split Tensile Strength at Various Ages for All Mixtures...................84

Table 4.4 ........Comparison of Slopes and R2 Values from ACI Relationship ..........88

Table 4.5 ........Compressive Strength Comparison of Batches..................................91

Table 4.6 ........Correction Factors for Each Mixture .................................................97

Table 4.7 ........Constant Used in Free Shrinkage Strain Prediction...........................102

Table 4.8 ........Crack Data for All Mixtures ..............................................................115

Table 4.9 ........Ring Specimen Cracking Predictions Based on Estimated

Stress-Strain Parameters ...................................................................118

Table 4.10 ......Charge Passed (Coulombs) in RCPT for All Mixtures......................119

Table 4.11 ......Total Charge Passed with ASTM Rating for All Mixtures................121

Table 5.1 ........Ranking of the Concrete Mixtures for Various Tests ........................126

Table 5.2 ........Total Points Ranking for All Mixtures ..............................................127

Table 5.3 ........Price per Unit for all Materials and Suppliers ...................................128

Table 5.4 ........Cost Analysis for NC.........................................................................128

Table 5.5 ........Cost Analysis for HPC.......................................................................129

Table 5.6 ........Cost Analysis for HPC-SRA..............................................................129

Table 5.7 ........Cost Analysis for HPC-L...................................................................130

Table 5.8 ........Cost Analysis for OLMC...................................................................130

Table 5.9 ........Price Comparison for All Mixtures....................................................131


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Chapter 1

INTRODUCTION

1.1 Research Background

Concrete is a material used in many civil engineering projects. Most commonly

concrete can be found in buildings, bridges and highways. In order to build more durable

structures a new class of concrete termed high-performance concrete (HPC) has been

implemented. HPC has become exceedingly popular in recent years throughout the

United States due to its high strength, low permeability, long-term durability and high

modulus of elasticity. HPC is being used in highways, bridge decks and overlays at an

increasing rate. Trends have begun to develop in structures containing HPC.

Specifically in bridge overlays, cracks are forming at early ages on the bridge surfaces.

This has become a large concern for the advocates of HPC. Researchers have recognized

that HPC has many benefits and are intent on improving the cracking issues related to

high performance concrete mixtures. Much research has revolved around why this

cracking is occurring and ways to reduce or alleviate it.

Realizing both the benefits and short comings of HPC, the West Virginia

Department of Highways initiated a large-scale and comprehensive study at the

Department of Civil and Environmental Engineering of West Virginia University to

develop HPC mixtures using local aggregates. Previous work by a fellow graduate


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student laid the basis for the HPC mixture proportions (Zhang 2001). The research now

looks to compare five concrete mixtures on the basis of strength, chloride permeability,

free shrinkage, restrained shrinkage, creep and cracking. The goal of this research is to

study the amount and rate of shrinkage and cracking of different mixtures, while still

maintaining acceptable strength and durability.

1.2 Research Objectives and Tasks

Taking the basic mixture proportions from a previous exploratory study of HPC

(Zhang 2001), modifications were made and more specific aspects will be studied. The

goal of this thesis is to study the shrinkage and cracking characteristics of HPC mixtures

with various admixtures and compare the results, ultimately looking for one or more

outstanding mixes. The present study was organized into the following tasks:

1. Development of varying mixtures

2. Evaluation of performance of mixtures

3. Analysis of test results and selection of optimum mixture

1.3 Research Plan

A total of five different mixtures were prepared, each one a variation of mixture

proportions found in an earlier study (Zhang 2001), which conform to specifications from

the WVDOH. The first mixture was normal concrete (NC) used as a reference material

for the other mixtures. The second mixture was HPC using various admixtures, including


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air entraining admixture (AEA) and high range water-reducing admixture (HRWRA).

The third mixture was a variation on the HPC termed HPC-SRA because of the addition

of a shrinkage-reducing admixture (SRA). The SRA was used to try to reduce shrinkage

and therefore reduce cracking in the test specimens. The fourth mixture contained latex

(HPC-L), and the fifth mixture was a variation on the latex mix. The fifth mixture was an

overlay latex modified concrete (OLMC) designed to represent common overlays on

bridge decks. Fresh concrete properties were determined for each mixture, which

included slump, air content, unit weight and temperature. Tests conducted in the

hardened state of each concrete mixture were compressive strength, split tensile strength,

free shrinkage, restrained shrinkage, crack propagation, chloride permeability and creep.

The data for all mixtures was then analyzed to select optimum mixtures based on the

performance tests and an additional material cost analysis.

1.4 Thesis Organization

Chapter 2 presents the terminology, concepts and procedures associated with the

study of concrete cracking, along with a literature review of similar research programs to

the one performed here at WVU. The importance of curing, types and factors influencing

shrinkage, definition and factors influencing creep, review of the rapid chloride

permeability test, and both the theory and procedure of the ring test method will be

discussed in the beginning part of Chapter 2. The literature review of various published

research studies and the results will follow. Chapter 3 contains the present experimental

program, which discusses the materials, sample preparation and all testing procedures.


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Chapter 4 presents all the data and briefly discusses the results. Chapter 5 summarizes

the results with a conclusion section, compares the results for all the mixtures, performs a

material cost analysis and makes recommendations for further research in this area.


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Chapter 2

BACKGROUND AND LITERATURE REVIEW

2.1 Introduction

Development and usage of HPC has opened new doors in the construction

industry (Bloom and Bentur 1995). HPC has been used more frequently for bridge decks,

roads and construction projects due to its high strength, high elastic modulus, low

permeability, long-term durability and excellent workability. Unfortunately, the concrete

used in the field does not always possess the characteristics listed above (Li et al. 1999).

In the development and recent usage of HPC, it has become clear that high performance

concrete has sensitivity to cracking (Bloom and Bentur 1995). Cracking in bridge decks

has become a growing problem in recent years. Cracks can form in the early stages of

bridge life, sometimes even before the bridge is open to traffic. Looking into the cause of

this cracking, it is well established that concrete shrinks when subjected to a drying

environment. If the shrinkage is restrained then tensile stresses develop and cracking

may result. This cracking is a critical concern with bridge decks because of the presence

of deicing chemicals, which can accelerate deck deterioration by corrosion of

reinforcement and scaling of the deck surface (Weiss et al. 1998).

This chapter presents some of the issues related to the cracking tendencies in

concrete. All the topics are discussed with their influence and effect on HPC in mind,


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since the goal of this study is to look at the cracking mechanisms and tendencies of HPC.

In order to better understand the issue of cracking, this research program studies the

strength, free shrinkage, restrained shrinkage for crack propagation and creep of HPC,

along with the permeability of each mixture using the rapid chloride permeability test.

The importance of curing, various types of shrinkage, creep, the rapid chloride

permeability test and the ring test method utilizing restrained shrinkage will be covered in

this chapter along with a section presenting a literature review of several experimental

programs involving crack propagation, all similar to this present program of study.

2.2 Curing

In order to make a good quality concrete, proper placing and curing is of utmost

importance. Curing is the term for the procedures taken to promote hydration in the

cement by keeping the concrete as saturated as possible. If the loss of water is not

prevented, the concrete fails to develop its full strength, shrinkage occurs, permeability

increases and resistance to abrasion is reduced. For larger concrete structures, only the

outer layer of the concrete is affected by curing, due to the lack of moisture movement

deep within the structure. Curing is important to the outer surface of concrete because

this is where the weathering, carbonation, abrasion and permeability issues occur. In

order for hydration of cement to take place throughout the whole structure, the capillaries

must be filled with water. Evaporation of water from the capillaries must be prevented

and water loss by self-desiccation must be replaced from the outside environment. In


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HPC the problem of self-desiccation is more acute due to its smaller capillary pores and

the chance of early shrinkage is higher than for normal concrete.

There are two main methods of curing and numerous variations of the procedures

for each. When curing, either a wet curing method, which is probably the most common,

or membrane curing method is used. Wet curing involves providing water on the surface

of the structure, which can be absorbed by the concrete. This is accomplished by

continuous spraying of water, called ponding, or covering the concrete surface with wet

burlap, sand, sawdust, straw or cotton mats. This allows the concrete to absorb water as

necessary to continue hydration and avoid drying shrinkage. The second method of

curing relies on the prevention of loss of water from the surface of the concrete without

external sources of water for the concrete to absorb. Common materials used as a water

barrier are polyethylene sheets or a spray of synthetic hydrocarbon resins, which form a

membrane. However, in HPC the former method is recommended. Regardless of the

type of method used, curing is a very important aspect of the concreting process that

helps drastically reduce the amount of shrinkage during drying (Neville 1997).

2.3 Shrinkage

It is common knowledge that when concrete dries, it loses water and a volumetric

change occurs. When water moves out of concrete, contraction takes place, which is

called shrinkage. There are four main types of shrinkage that will be defined: plastic

shrinkage, autogenous shrinkage, drying shrinkage and carbonation shrinkage. Factors


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influencing shrinkage and the method of testing shrinkage are presented after the

definitions are covered.

2.3.1 Plastic Shrinkage

Plastic shrinkage occurs when the concrete is still in the plastic state. This type

of shrinkage is due to the evaporation of water from the surface of the concrete and is

influenced by the ambient temperature, humidity, and presence of wind. If the amount of

water evaporated exceeds the amount of water brought to the surface of the concrete

through the bleeding process, hydrostatic stresses tend to develop thus causing lateral

shrinkage and consequently cracking. Evaporation of water increases when the concrete

temperature is higher than the ambient temperature, and therefore, it is advisable to

protect concrete from the sun and wind, which means placing and finishing quickly and

immediately starting the curing process (Neville 1997). Bloom and Bentur (1995) found

that the use of silica fume accelerates setting time and increases free plastic shrinkage in

an open environment, which again reinforces the fact that the concrete containing silica

fume, which HPC usually has, should be cast quickly and the curing process should be

carefully planned. Plastic shrinkage can easily be controlled with the prevention of

evaporation of water immediately after casting by spraying the surface of the setting

concrete with water, or covering it with wet burlap and plastic until the concrete is out of

its plastic state.


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2.3.2 Autogenous Shrinkage

After concrete sets, volume changes continue to occur. If no moisture movement

to or from the cement paste is permitted, shrinkage takes place due to self-desiccation.

Self-desiccation is the withdrawal of water from the capillary pores to hydrate the

unhydrated cement present in the interior of the concrete mass. Shrinkage due to self-

desiccation is known as autogenous shrinkage. This type of shrinkage is usually included

as part of the drying shrinkage because of its relatively small value in comparison

(Neville 1997).

HPC, however, is extremely prone to autogenous shrinkage due to its low water-

cement ratio (w/c) and small capillary pores. Persson (1998) found that autogenous

shrinkage increases with decreasing w/c and the addition of silica fume. In Persson

(1998) but not personally consulted, Roy and Larrard (1993) and also Sicard (1993)

support the results found in Perssons research program. This type of shrinkage can be

rather large in magnitude unless continuous water curing is started soon after casting of

concrete. The curing practices for HPC are crucial to the minimization of shrinkage and

hence reduced risk of cracking.

2.3.3 Drying Shrinkage

Probably the most important type of shrinkage is drying shrinkage, which occurs

immediately after the concrete ends its plastic state. Drying shrinkage is the withdrawal

of water from concrete stored in an unsaturated air. The amount of drying shrinkage


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possible for a particular specimen is influenced greatly by the aggregates in the concrete,

which provides restraint from shrinkage. The relative humidity of the surrounding

environment also affects the magnitude of drying shrinkage (Neville 1997). The actual

mechanism by which drying shrinkage takes place is complex, but generally accepted

that it involves the loss of adsorbed water from the hydrate cement paste. When concrete

is initially exposed to a condition where there is a difference between the relative

humidity of the environment and the concrete, free water is lost (Nmai et al. 1998). The

total volume of water lost to the surrounding environment does not equal the total change

in volume of the concrete. The initial loss of free water has little or no effect on the

volume of the concrete. Only when the adsorbed water evaporates does a change in

volume occur, which is equal to the loss of a water layer one molecule thick from the

surface of all gel particles. Therefore, the concrete would not be able to absorb back the

full amount of water loss and the total change in volume is not recoverable, leading to the

conclusion that drying shrinkage of concrete is not a reversible process. To decrease the

magnitude of drying shrinkage, a higher relative humidity of the surrounding

environment is needed. Drying shrinkage in HPC is more often than not greater than

both carbonation and autogenous shrinkage combined (Neville 1997). This type of

shrinkage is critical due to the low water-cementitious ratio (w/cm), which is usually

specified and becomes more critical when shrinkage is restrained.


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2.3.4 Carbonation Shrinkage

Carbonation shrinkage occurs in the hardened concrete state caused by the

reactions between the constituents of hydrated cement paste and carbon dioxide, CO2.

These reactions take place even with small amounts of CO2 in the surrounding air. Of the

hydrates in cement paste, the one that reacts most with CO2 is calcium hydroxide,

Ca(OH)2, and the product of the reaction is calcium carbonate, CaCO3. Carbonation

results in an increase in mass of the concrete. When drying shrinkage and carbonation

occur simultaneously an incorrect conclusion that the concrete has reached constant mass

or equilibrium may be assumed, whereas it is merely carbonation and drying shrinkage

canceling each other out. One positive aspect of carbonation is the fact that CaCO3

occupies a larger volume than Ca(OH)2, which it replaces, and thus results in less porous

concrete (Neville 1997).

In HPC, carbonation shrinkage can all but be eliminated when certain precautions

are taken. HPC normally forms less Ca(OH)2 compared to normal concrete, due to

reactions with pozzolanic materials, such as silica fume, or other polymeric materials that

are usually incorporated in an HPC mix design. Results have indicated that the use of

pozzolanic materials at a low w/cm may eliminate carbonation shrinkage altogether in

HPC. Carbonation shrinkage in HPC introduces surface cracking, which affects the

durability of the concrete (Persson 1998).


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2.3.5 Factors Influencing Shrinkage

There have been many studies centered around the factors that influence

shrinkage. Shah et al. (1996) states that the amount of shrinkage depends on many

factors including material properties, temperature and relative humidity of the

environment, the age of the concrete and size of the structure. Nmai et al. (1998) gives a

similar list of factors affecting drying shrinkage, which include characteristics of the

concrete mix ingredients and their proportions, design and construction practices,

environmental influences and time.

The influences of concrete mix ingredients and their proportions on drying

shrinkage will be discussed here because it is the largest shrinkage in magnitude in HPC.

Conflicting data can be found on influences of concrete mix ingredients on drying

shrinkage, but unquestionably, water and coarse aggregate have a profound effect on

minimizing the paste content. Drying shrinkage increases with increasing water and

cement content, and therefore in order to minimize the drying shrinkage of concrete the

total water and cement content must be kept as low as possible. This can be done with

the use of HRWRA. The standard ACI 212 reports that the effects of HRWRA on HPC

are conflicting, but states that a decrease in long term shrinkage may result depending on

the degree to which the water content of the concrete is reduced. Other admixtures such

as AEA have been shown to have little or no effect on drying shrinkage. Also, cement

content and its type, composition and fineness have little effect on concrete shrinkage.

Aggregate, on the other hand, has a two-fold effect on drying shrinkage. First, the use of

high coarse aggregate content minimizes the total water and paste contents of the


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concrete mixture, therefore minimizing the drying shrinkage. At a given w/c, the drying

shrinkage is reduced as the aggregate-cement ratio is increased. Secondly, the drying

shrinkage of the cement paste is reduced by the coarse aggregate due to its restraining

influence. The amount of restraint provided by the coarse aggregate depends on the type

of aggregate, its stiffness, the total amount of aggregate used and the maximum size.

Some types of aggregate that are difficult to compress and provide more restraint to the

shrinkage of the cement include granite, limestone and quartz. Aggregates to avoid if

low drying shrinkage is desired include sandstone and slate. Aggregates with clay

coatings should also be avoided because of the reduction in restraint and the increased

water demand. Finally, the use of pozzolanic materials or other polymeric materials as

admixtures increase the volume of fine pores in the cement hydration product resulting in

an increase in drying shrinkage (Nmai et al. 1998).

Some design and construction practices that affect drying shrinkage are the

amount of reinforcement provided, and the size, shape and surface area-volume ratio of

the concrete member. Steel reinforcement supplies restraint to reduce the drying

shrinkage. In the same ambient environment, a small concrete specimen will shrink more

than a larger concrete member due to its higher surface area-volume ratio. Thus, the

drying shrinkage occurring in a concrete structure has been found to be a fraction of that

obtained in the laboratory setting. Also at the job site, retempering a concrete mixture, by

adding water for workability, will increase the drying shrinkage because of the increase

in water content and should be avoided (Nmai et al. 1998).

The magnitude of drying shrinkage is greatly affected by the relative humidity of

the surrounding environment. The lower the relative humidity is, the greater the drying


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shrinkage in concrete structures. The drying shrinkage is also time dependent. A long-

term study on shrinkage showed that nearly 50% of the drying shrinkage obtained at 20

years occurred within the first 3 months of drying, thus showing that the precautions

taken early on in the life of a concrete structure can greatly increase its service life (Nmai

et al. 1998).

2.3.6 Free Shrinkage Test Method

Now that the various types of shrinkage have been discussed, the testing method

of measuring free shrinkage is presented. To measure free shrinkage strain, ASTM C

157, Standard Test Method for Length Change of Hardened Hydraulic-Cement Mortar

and Concrete and AASHTO T 160, Length Change of Hardened Hydraulic Cement

Mortar, recommend a prismatic specimen 11.25 inches long and with a 1, 3, or 4 inch

square cross section. The free shrinkage strain is calculated by measuring the length

change, defined as an increase or decrease in the linear dimension of a test specimen,

which has been caused to change by any factor other than externally applied forces and

temperature changes (ASTM C 157). It is assumed that the length of the specimen is

much larger than the cross sectional dimensions, and therefore shrinkage takes place only

in the length direction (Grzybowski and Shah 1990). The length change measurement

over time leads to the calculation of one-dimensional shrinkage strain of the material.

This gives an idea of how much concrete will shrink when no restraints are present, but

does not yield any information about the characteristics of restrained concrete. So

ideally, the combination of the free shrinkage test and a type of restrained shrinkage test


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is needed for adequate information on the overall shrinkage characteristics of concrete. A

number of predictions of shrinkage are available, such as ACI 209 and Hou et al. (2001)

who both recommend the known hyperbolic function model. ACI 209 incorporated

correction factors based on member size, drying time and mix proportions, but did not

account for strength. Hou et al. (2001) included a strength factor to accommodate HPC

in their study.

2.4 Creep

Creep is defined as the increase in strain over time of concrete subjected to

constant stress (Neville 1964). Creep is a complex phenomenon whose mechanisms are

uncertain. There are varying types of creep just as there are different types of shrinkage,

and a specimen undergoing creep has various categories of deformations. If the specimen

is allowed to dry while loaded, drying creep and shrinkage occur from the moisture

movement between the concrete specimen and the surrounding environment. If no

moisture movement to or from the ambient surroundings is permitted, there is some

nominal elastic strain and basic creep, which are due to an applied load. For the most

part, in experiments, basic creep and drying creep are not distinguished, and an additive

approach is used to account for both shrinkage and creep of a given specimen. A

companion concrete specimen is used to gather shrinkage strain data, which are

subtracted from the total strain collected from the creep specimen to produce the creep

strain. Even though it has been found that this is not entirely accurate, it is the most

convenient and practical method of creep data collection and calculation used (Neville


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1997). Now that the introduction to creep and its terminology have been presented, a

discussion of the factors influencing creep will follow.

2.4.1 Factors Influencing Creep

There are many factors that influence the amount of creep in concrete. The

hydrated cement paste is actually the part of concrete that undergoes creep and the

aggregate acts as a restraint, therefore the strength and amount of aggregate is important.

According to Neville (1997), the porosity of the aggregate has been found to influence

the creep of concrete, but since a more porous aggregate generally has a lower modulus

of elasticity, the conclusion is reached that porosity is not an independent factor. Another

explanation of porous aggregates affecting creep may be that the pores act to transfer

moisture within the concrete, thus leading to the development of drying creep. General

statements regarding different aggregate types and their influence on creep are typically

not given because of the great variation of aggregates within any mineralogical and

petrological type.

A second and rather obvious influence on creep is stress and there is a direct

proportionality between creep and applied stress. The applied stress limit expressed as a

fraction of the ultimate strength usually falls between 0.4 and 0.6 for normal concrete and

occasionally as high as 0.75 for high strength concrete. Creep is inversely proportional to

the strength of concrete at the time of load application. This leads to the stress-strength

ratio approach, which involves specifying the strength of concrete by laboratory testing

and calculation of the ultimate strength and then taking a fraction of the strength for the


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sustained load. This is a more practical method than taking into consideration factors

such as type of cement, w/c ratio and age, which can lead to tedious computations.

The type of cement used affects creep by influencing the strength of the concrete

and should be recognized if comparing mixtures with different types of cement. Other

cementitious materials also cause changes in creep, such as the presence of silica fume,

which has no effect on basic creep but reduces the drying creep. Creep is generally also

reduced in concretes containing fly ash and ground granulated blast furnace slag.

Concrete made with expansive cement has a larger amount of creep than concrete made

with Portland cement. There have been no definite patterns on the influences of water-

reducing and set-retarding admixtures on creep.

In dealing with external factors, relative humidity is one of the most important

influences on creep, and generally speaking, the lower the relative humidity the higher

the creep. In reality, concrete exposed to any sort of relative humidity allows for drying

and therefore the occurrence of drying creep. Another external factor involves storing

the concrete specimen in alternating air and water. This alternating wetting and drying

increases the magnitude of creep, which shows that laboratory tests may underestimate

creep of concrete under normal weather conditions. The size of the specimen influences

the amount of creep; an increase in specimen size decreases creep. This is probably due

to the effect of shrinkage, the volume-surface ratio, and the increase in strength of the

core of a mass of concrete. Consequently a lower drying creep value in the inner portion

of the concrete mass results in an overall decrease in creep. Finally, it has been found

that creep under cyclic loading is higher than that under static loading equal to the mean


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cyclic stress. Therefore, static creep results underestimate the amount of creep under

cyclic loading conditions (Neville 1997).

2.4.2 Effects of Creep

Strains, deformations and stress distributions are all affected by creep. In

statically indeterminate structures, creep actually relieves stress concentrations induced

by shrinkage, temperature changes or support movements. In all structures, creep

reduces internal stresses due to non-uniform shrinkage and therefore reduces cracking.

Some undesirable effects from creep include excessive deflections of structural members

and the presence of other serviceability problems, often in high rise buildings and long

span bridges. In general the effects of creep, like shrinkage, can be harmful, but creep is

beneficial in relieving stress concentrations. Research at the University of Illinois under

Lange et al. (1997) showed that creep is significant in relaxing shrinkage stress at an

early age. However, creep is very dependent on mix composition and stress levels. More

often than not, creep has been incorporated into the designs of structures and is used as an

advantage wherever possible. There is little advantage on creep response in structures

using HPC, since HPC has low creep due to its very low w/c ratio, high cement contents,

silica fume admixtures and other ingredients that produce very high early strengths and

moduli of elasticity. The low creep values have little reduction influence on the tensile

stresses developed from restrained drying shrinkage and thermal effects, and therefore the

HPC structures have a greater tendency of cracking. This is a problem in early age HPC

bridge decks.


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2.4.3 Creep Test

The test method to measure creep is described in ASTM C 512, Standard Test

Method for Creep of Concrete in Compression. The test method covers the

determination of creep of molded concrete specimens subjected to a sustained

longitudinal compressive load. Creep from different concrete mixtures can be compared

using this testing method. The test specimens are loaded to no more than 40% of the

compressive strength at the age of loading. Strain readings are taken immediately before

and after loading and then on the given time frame as the experiment indicates. The load

should vary by no more than 2% throughout the test or it must be adjusted. When placing

specimens in the creep frame, care must be taken to avoid eccentricities in initial set up

and while loading.

2.5 Rapid Chloride Permeability Test

The strength of a mixture has long been associated with all other concrete

properties and used as an indicator of a good quality concrete design. In recent years,

evidence has shown that the strength of a concrete mixture may not be a good measure of

the durability of a mix (Myers et al. 1997). A method of measuring the durability of

concrete directly was needed rather than indirectly via strength tests. Chloride

permeability was deemed the most relevant property affecting the durability of concrete.

Two main testing methods surfaced for the determination of concretes permeability to

chloride ions, AASHTO T 277, Rapid Determination of the Chloride Permeability of


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Concrete and AASHTO T 259, Resistance of Concrete to Chloride Ion Penetration.

AASHTO T 277 and the equivalent ASTM C 1202, Standard Test Method for Electrical

Indication of Concretes Ability to Resist Chloride Ion Penetration, have become

increasingly popular among engineers, due to the continuing demand for faster and less

expensive test methods. The next section will present the testing methods for the rapid

chloride permeability test (RCPT).

2.5.1 The RCPT Methods and Information

The RCPT consists of two parts, the conditioning of the concrete specimen and

the actual test, which involves monitoring the amount of electrical current passing

through the specimen. The test specimen is made from the top 2 inches of a 4-inch

diameter and 8-inch long concrete cylinder cut by a diamond saw. The disk is surface

dried and the cylindrical surface is covered with silicone and allowed to dry. Next, the

specimen is vacuumed for 3 hours and then covered with de-aerated water. The vacuum

is on for the duration of the test and continues to run for another hour after the water has

been added. The disk specimen is then soaked for 18 hours before it is removed, and its

two faces are mounted to voltage cells with silicone and again allowed to dry. The

testing procedure involves filling one voltage cell with a sodium chloride solution and the

other with a sodium hydroxide solution. After connecting the voltage cells to a 60-volt

direct current power supply, the apparatus is turned on and the current passing through

the concrete disk is monitored for 6 hours. The amount of electrical current passed is

given in Coulombs, which are Amperes-second. The more permeable the concrete the


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higher the Coulomb value, and likewise, the lower the Coulomb value the less permeable

the concrete. The RCPT is usually performed on 28-day old concrete specimens

(Suprenant 1991). Even with its increasing popularity, many concerns have been voiced

regarding the RCPT and many research programs were designed to look into some of

these questions.

2.5.2 Concerns and Research Regarding the RCPT

This new method of testing concrete chloride permeability using RCPT has not

gone without controversy. Many researchers have voiced concern in the repeatability of

the test and how various factors influence the test results.

Certain factors within the physical test itself have been questioned and some of

them will be detailed. Mobasher and Mitchell (1998) found that for high permeability

concretes the copper screens accumulate corrosion products, which deteriorate the

screens. The operator of the RCPT should be aware of the screens potential corrosion

and be cautious of its consequences. Mobasher and Mitchell (1998) suggested using a

stainless steel screen as a more durable alternative. Dubois and Peabody (1998) thought

that the 6 hour duration for a RCPT was rather long, and if the time could be cut down,

both time and money would be saved. After 131 tests, Dubois and Peabody (1998)

concluded that the 360 minutes could actually be reduced to 30 minutes and a multiplier

of 14 could be used to represent the value obtained in the full 360 minute test, but

recommend that additional concretes be tested before altering the standard procedures.

Myers et al. (1997) and Mobasher and Mitchell (1998) both did extensive research on the


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effects of various materials in the concrete mixture on the chloride permeability. Myers

et al. (1997) found that the addition of AEA had no effect on the chloride permeability of

the test specimens. Mobasher and Mitchell (1998), on the other hand, found that as the

amount of AEA increases so does the permeability. Both studies indicated that as the w/c

ratios increase so did the permeability. Myers et al. (1997) studied the effect of fly ash in

concrete mixes and discovered that the use of fly ash decreases the permeability.

Concerns regarding the repeatability of the RCPT have been stated by many

researchers. Hooton (1998) shows large variability in the chloride permeability tests of

cylinders taken from the same batch. The coefficient of variation for three sets of data

with six replicates was 41.1%. Hooton (1998) concludes that concrete is much more

variable with respect to chloride permeability than with strength. Mobasher and Mitchell

(1998) also studied the precision from various laboratories. For two RCPTs on the same

material by the same operator, the results should not differ by more than 35% with a

coefficient of variation of 12.3%. For multi-laboratory work, the coefficient of variation

was found to be 18% for a single test, and two tests run from different laboratories on the

same material should not differ by more than 51%. Mobasher and Mitchell (1998)

attribute the high precision values on variability of the samples and deviations from the

test schedule and procedure by the participating laboratories. In addition, Dubois and

Peabody (1998) found, after 300 tests, that there is little consistency from one sample to

the next and the average of at least two samples should always be used.

Even with all the concerns, the chloride permeability test seems to be the most

cost effective and time efficient method of measuring chloride permeability in concrete.

The RCPT has shown difficulty in repeatability but with proper equipment usage and a


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cautious and careful technician there should be no problem with the accuracy of the

physical test.

2.6 The Ring Test Method

In shrinkage research, the free shrinkage test alone does not yield sufficient

information on cracking tendencies because almost all concrete structures undergo some

type of restraint. Virtually all steel and precast concrete girder bridges incorporate cast-

in-place concrete elements in their design, including curbs, sidewalks, barriers and most

importantly, bridge decks. The precast bridge elements are restrained by girders and this

produces tension when the concrete shrinks and therefore leads to cracking. The cracking

of bridge decks poses a threat to the long-term life of the bridge. Therefore, it is

necessary to have the ability to quantify cracking for comparative purposes of different

mix designs to develop more durable structures. The development of a test method,

which enables the evaluation of stresses under restrained conditions, is needed. There is

no standard test to assess the cracking of concrete due to restrained shrinkage, but there

are some testing methods used to measure shrinkage cracking behavior (Li et al. 1999).

The ring test is the most effective evaluation of cracking, due to restrained shrinkage,

among the available tests. The ring test method is discussed in this section along with

mention of other types of test methods for restrained shrinkage, and later in this section,

summaries of various studies by the ring test method are discussed.


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2.6.1 Ring Test Set Up and Theory on Cracking

While the free shrinkage potential of concrete is significant, it is not the only

factor in determining the vulnerability of concrete to cracking. Concrete that is

unrestrained can develop freely a certain amount of shrinkage, but when all or part of the

shrinkage is restrained, tensile stresses develop. When the induced tensile stresses

exceed the tensile strength of the concrete the onset of cracking can be expected. The

cracks provide easy access for oxygen, moisture, chlorides and other aggressive

chemicals into the concrete matrix and can decrease the durability and life span of the

structure. Therefore, both the crack width and orientation of the crack are important

factors (Nmai et al. 1998), and in general, the cracking characteristics of concrete are

important in concrete research.

Three different specimen geometries have been used in studies to measure the

effect of restraint in concrete: rings, which are extensively discussed in this paper, bars

and plates. The bar specimens provide uni-axial stress development, but the end

conditions often complicate testing (Weiss et al. 1998). Flared ends are often used in the

development of closed loop testing machines as in studies by Parilee et al. (1988), Kovler

(1994) and Bloom and Bentur (1995). These testing frames are used to determine the

stress maintained in a specimen under a prescribed displacement. Significant insights

into the relationship between drying shrinkage and creep have been obtained (Weiss and

Shah 1997). In addition to bar specimens, plate specimens have been tested to evaluate

both biaxial restraint and plastic shrinkage cracking. When restraint to shrinkage is

provided in two directions, a biaxial state of stress is produced characterized by specimen


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geometry in addition to material properties (Grzybowski and Shah 1990). This makes it

more difficult to test the effects of varying materials in concrete. On the other hand, both

the bar and plate test specimens simulate planar elements that easily correlate to

structural elements (Weiss and Shah 1990). Similar to the plate test, Weiss et al. (1998)

used a slab type specimen to simulate the restraint experienced in highway pavements

and bridge decks. The specimen was thin and long to avoid moisture gradients and

ensure the ability to develop sufficient tensile stress to induce cracking. The bar and

plate specimens have advantages but the big drawback is the complication in the end

restraints.

Due to laboratory difficulties in providing sufficient end restraint and avoiding

eccentricities, ring type specimens are used. In the ring test, concrete is cast around a

solid ring, usually steel. The dimensions most commonly used in this set up are shown in

Figure 2.1. The diameter of the steel ring affects the concrete shrinkage restraint.

Therefore, a diameter of 305 mm is used to approximate the worst case shrinkage

restraint on a bridge deck. Stress analysis of the ring test set up with varying dimensions

shows that the stress distribution is non-linear. But using the dimensions in Figure 2.1

and assuming the concrete ring is subjected to an internal pressure from the restraint of

the steel ring, the difference between the tensile hoop stresses on the outer and inner

surfaces is only 10 percent, and the maximum radial stress is 20 percent of the maximum

hoop stress. Given the steel ring diameter of 305 mm, it can be assumed that the concrete

is subjected to a state of uniaxial stress (Grzybowski and Shah 1990; Shah et al. 1992;

Weiss and Shah 1997, and Wiegrink et al. 1996).


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A steel ring of 305 mm in diameter and 140 mm in height is used as the inside

formwork of the concrete ring and either PVC pipe or sonnet tubing is used for the

outside formwork having a diameter of 375 mm. The thickness of the concrete ring

around the steel is therefore 35 mm, which is one-fourth its height of 140 mm and leads

to the assumption that uniform drying takes place along the height (Shah et al. 1992).

The steel ring and PVC pipe are centered on a wooden base to finish the formwork.

During casting, the concrete is rodded or vibrated to ensure proper compaction. The

concrete ring is allowed to set for 24 hours under wet burlap. The outer form work,

either PVC pipe or sonnet tube, is peeled off and the top of the ring immediately sealed

with a silicon rubber sealant to prevent moisture loss from the top of the ring and ensure

drying from the outer circumferential surface. The specimen is then stored in a

controlled environment where the temperature and humidity often varies from experiment

to experiment.

Now that the various test methods have been discussed, ways to decrease the

shrinkage cracking of concrete will be presented. The simplest of all solutions is to

reduce shrinkage, which can be accomplished many different ways. The reduction in the

amount of water reduces the presence of water available for evaporation and therefore

reduces shrinkage. Increasing the curing time allows for the concrete to develop stiffness

and toughness, therefore cutting down on the initial shrinkage. Reinforcing fibers are

used to decrease crack widths, though the fibers do not prevent cracking (Weiss and Shah

1997). Grzybowski and Shah (1990) found that the addition of fibers in concrete

significantly reduces the crack widths resulting from restrained drying shrinkage.

Shrinkage compensating concretes are used when concrete is restrained. Shrinkage


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compensating concretes are produced with expansive cements, which expand the

concrete during curing and place any reinforcing steel in precompression, thus

counteracting the development of tensile stresses and while the total shrinkage maybe

about the same, the cracking due to restraint can be delayed. Precautions must be taken

and careful attention must be paid to avoid warping and bending of concrete slabs due to

uneven restraints. Non-expansive shrinkage reducing admixtures are also used to reduce

the shrinkage and consequently cracking of concrete (Weiss and Shah 1997). Studies by

Nmai et al. (1998), Shah et al. (1996) and Weiss et al. (1998) have shown that SRAs

effectively reduce shrinkage and delay cracking in concrete. These are just some of the

methods used to reduce shrinkage and therefore cracking in concrete specimens.

The tendency for concrete to crack is due to restrained shrinkage, which is related

to free shrinkage, creep and tensile strength. In the case of cracking, creep acts as a

mechanism of stress relief and reduces the tensile stress. HPC often has a higher tensile

strength, which is an advantage when combined with creep. But, tensile creep does not

significantly reduce tensile stress development, and the potential for cracking is still

present. The study of early age tensile creep is critical for full understanding of shrinkage

cracking (Weiss et al. 1998). Shah et al. (1996) emphasized this by stating that creep

effects in the stress analysis at early ages should not be ignored. In HPC and high

strength concretes, the combined effect of higher free shrinkage, lower specific creep and

higher modulus of elasticity leads to early cracking (Wiegrink et al. 1996).


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Concrete Specimen

Steel Ring

Wooden Base

305 mm (12.25 in)

140 mm (6 in)

375 mm (16 in)

254 mm (11.75 in)

Silicone Rubber Sealant

Figure 2.1 Schematic of Ring Set Up

Steel Ring

Concrete Specimen


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2.6.2 Test Methods and Results Using Ring Set Up

Using the ring test enables the study of different concrete mixtures and the effects

of various admixtures and additions of reinforcing fibers in a mix. Study programs

targeting the influences of various admixtures and the use of reinforcing fibers will be

presented in this section. The experimental set up for each study is presented due to the

importance of the experimental conditions to make the results more meaningful. This

section describes various experimental programs similar to the one devised for the

present study.

2.6.2.1 Study by Li et al. (1999)

Li et al. (1999) used the ring test to study the effects of silica fume, fly ash,

ground granulated blast furnace slag and calcium nitrite inhibitor on restrained shrinkage

cracking. The ring test set-up was constructed using the same dimensions as in Figure

2.1, and Li et al. (1999) used the ring specimen for both restrained and free shrinkage.

The restrained shrinkage test used a steel ring as the inner mold that remained throughout

the experiment and for the free shrinkage, both inner and outer molds were stripped

away.

Eight mixes were cast using 10 mm limestone aggregate and natural river sand.

Six mixes included fly ash, to decrease bleeding and segregation, and therefore improve

workability and gain higher compactness. Three mixes used silica fume to obtain high

strength, low permeability and gains in long-term durability of the concrete. One mix


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replaced 50% of the cement with ground granulated blast furnace slag to increase

workability, decrease bleeding and segregation, and increase the durability of the

concrete. Retarder and water-reducer were used for all mixes to delay set time, decrease

slump loss and the amount of water needed. Four mixes contained calcium nitrite

inhibitor for improved anticorrosive properties of the concrete.

The concrete was allowed to set for 24 hours before the outer mold was stripped

off of the restrained shrinkage ring, after which the top surface of the ring was sealed

with epoxy resin. The resin was used to avoid moisture loss from the top of the ring and

allow drying only from the outer circumferential surface. Both the inner and outer molds

were removed for the free shrinkage and the top and inner surface of the concrete was

sealed with epoxy resin to prevent drying. For the first four days, the specimens were

moist cured at 100% relative humidity and 20oC. After four days, the specimens were

exposed to a drying environment of 40% relative humidity and 20oC. The specimens

were monitored for strain in the free shrinkage test and crack development for the

restrained shrinkage test.

Free shrinkage measurements were taken using a dial-gage extensometer with a

gage length of 200 mm. Five pairs of brass studs fixed on the top of the circumferential

surface were used to take the free shrinkage readings and averaged to find the free

shrinkage strain. Measurements were gathered once every 24 hours for 10 weeks. For

the restrained shrinkage test, the appearance of any cracks was noted and the crack width

measured with a microscope with a magnification x30. Crack width at three positions,

one-quarter, one-half and three-quarter points of the vertical distance from the top surface


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of the ring were measured every 24 hours for 10 weeks. The average of the three values

was used as the crack width.

The study by Li et al. (1999) found that the crack width increases with increasing

silica fume, fly ash and calcium nitrite inhibitor. The use of ground granulated blast

furnace slag did not drastically change the shrinkage strain, but did change the restrained

shrinkage cracking behavior; the appearance of the initial crack was delayed and its rate

of growth was decreased.

2.6.2.2 Study by Shah et al. (1992)

Shah et al. (1992) studied the effects of three different types of shrinkage reducing

admixtures on the free shrinkage and restrained shrinkage of concrete mixes, while

maintaining the same amount of water. Shah et al. (1992) also used the ring test set up

shown in Figure 2.1. In order to monitor the free shrinkage of the concrete mixes,

shrinkage prisms, 285 mm long and 100 mm square, were used in compliance with

ASTM C 157. The materials used included 9 mm maximum aggregate size pea gravel,

dried natural river sand with a maximum grain size of 3 mm and Type I Portland cement.

Three types of shrinkage-reducing admixtures were used: the first, SRA1 is a commercial

material containing an alkoxylated alcohol; the second, SRA2 is a similar alkoxylated

alcohol-based oligomer, and the third, SRA3 is an experimental alcohol-based material.

The mix proportions by weight for the mix design were 1:2:2:0.5 (cement: sand: coarse

aggregate: water). Shrinkage reducing admixture of 1, 2, and 4 percent was incorporated

into the mixtures along with a control mix having no shrinkage-reducing admixture.


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When casting the specimens, the rings were filled half way and vibrated for a minute and

then filled to the top and vibrated for another minute to assure proper compaction.

The free shrinkage prisms and restrained shrinkage rings were cured for 4 hours at

20oC and 100% relative humidity. The short curing period was chosen in order to start

shrinkage measurements as early as possible. After demolding, the specimens were

stored in an environment of 20oC and 40% relative humidity and free shrinkage readings

were taken every 24 hours for 42 days with a dial-gage extensometer. The weight of

each specimen was also recorded when the free shrinkage readings were taken.

Restrained shrinkage readings were taken by a mounted movable microscope. The crack

widths reported were an average of three readings: one at the center of the ring and the

other two at the center of the top and bottom halves of the ring. Surface examinations for

new cracks and measurements of existing cracks were performed every 24 hours for the

first 7 days and every 48 hours up to 42 days. For each mix, 75 x 150 mm cylinders were

cast and tested for compressive strength at 1, 7, and 28 days. The cylinders were

subjected to the same drying environment as the ring specimens and free shrinkage

prisms.

Test results of the Shah et al. (1992) study showed that the addition of SRAs

reduces the compressive strength of the concrete. The amount of reduction in strength

depended on the amount and type of SRA used. It was also found that free shrinkage is

significantly reduced with the use of SRA; the larger the percentage of SRA used, the

greater the reduction in free shrinkage. Depending on the type and amount of shrinkage

reducing admixture used, a considerable reduction in crack width occurred when

compared with the control concrete mix.


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2.6.2.3 Study by Wiegrink et al. (1996)

Wiegrink et al. (1996) studied the restrained shrinkage cracking of several

different strengths of concrete mixes. The mix proportions of 1:2:2 (cement: sand: coarse

aggregate) were used for all specimens. The materials used in all batches included Type I

Portland cement, pea gravel with a maximum aggregate size of 9 mm and dried natural

river sand with a maximum grain size of 3 mm. It was important to eliminate the

influence of aggregate, so the aggregate paste ratio was kept constant at 2:1. Five mixes,

with various strengths, were obtained by the use of water-reducing admixtures and partial

replacement of cement with silica fume. During mixing, the fresh concrete properties

measured were slump, unit weight and air content as prescribed in ASTM Standards.

After batching took place, the specimens were covered with a plastic sheet until

demolded.

Again, Wiegrink et al. (1996) used the same dimensions of the ring set up as

shown in Figure 2.1. The inner mold was made of a steel ring and the outer mold of PVC

pipe. The top of each ring was sealed with silicon rubber to avoid drying from the top

surface and promote drying on the outer circumferential surface. Crack width growth in

the ring test was monitored by averaging of the crack width measurements with a

microscope at the quarter points along the height of the specimen. For the first 7 days,

the specimens were checked for new cracks and measurements of existing cracks were

taken every 24 hours, then checked every 48 hours after the first week up until 90 days.

Along with the ring specimens, free shrinkage prisms 400 mm length and 100 mm square

were also cast to monitor one-dimensional shrinkage of the concrete mixes. A dial-gage


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extensometer, gage length 250 mm, was used to measure the change in length of brass

studs embedded in the prism specimens. Readings were taken every 24 hours for the first

4 weeks and then 3 times a week up to 90 days. Creep tests were performed on

specimens with identical dimensions as the free shrinkage prisms in accordance with

ASTM C 512. Measurements of the length change due to creep were taken on the same

schedule as the free shrinkage readings. Cylinder specimens 75 x 150 mm were used for

compressive strength tests, and split cylinder tensile strength tests were done with 100 x

100 mm specimens. Both compression and tension tests were performed on concrete at

3, 7, and 28 days of age.

The ring specimens and two free shrinkage prisms were allowed to cure for 6

hours, before demolding, then subjected to an environment of 20oC and 50 % relative

humidity. Cylinder specimens for compressive and split tensile tests were demolded after

24 hours and cured for an additional 7 days in water before being stored in a drying

environment at 20oC and 50% relative humidity. The creep specimens were stored

unloaded in a room at 20oC and 50% relative humidity before being loaded at 40% of the

maximum compressive strength as obtained from the cylinder compression tests.

From this research program, Wiegrink et al. (1996) found that concrete made with

higher water content and demolded after 24 hours lost more water, but the free shrinkage

remained virtually the same. This phenomenon demonstrates that free shrinkage is

dependent on more than just weight loss. A reduction in specific creep was found with

increasing silica fume and decreasing water content. The rate of creep deformation for

concrete containing silica fume stabilized before normal concrete. Since high strength

concrete tends to crack earlier than normal concrete, the value of specific creep at early


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ages may be a factor influencing the early cracking of high strength concrete. Even

though the high strength concrete had a higher tensile strength than normal strength

concrete, the shrinkage cracking performance of the high strength concrete was rather

poor. Wiegrink et al. (1996) believes this is due to higher free shrinkage when demolded

after 6 hours, lower specific creep, higher modulus of elasticity and lower aggregate

interlock along crack faces for high strength concrete.

2.6.2.4 Study by Grzybowski and Shah (1990)

Grzybowski and Shah (1990) studied the effects of two types of reinforcing fiber,

polypropylene and steel, on the shrinkage and cracking properties of concrete. Again,

Grzybowski and Shah (1990) used the same ring test set up as in Figure 2.1. Both

restrained and free shrinkage rings were made similar to those from the study by Li et al.

(1999). The inner ring was made from mechanical tubing and the outer ring was a

cardboard tube. Both inner and outer tubing were removed for the free shrinkage ring,

and the top and inner surfaces were also sealed to prevent moisture loss. Two test series

were included in this study; the cracking of early age concrete and the cracking between

one and six weeks were both investigated. The early age cracking involved curing the

ring specimens for only 2.5 hours at 20oC and 100% relative humidity. The rings were

demolded, sealed and exposed to a drying environment at 20oC and 40% relative

humidity. Three types of specimens were used, plain with no fibers, 0.25% steel fibers

and 0.1% polypropylene fibers. In the one to six weeks cracking study, the specimens

were cured for 4 days at 20oC and 100% relative humidity and then exposed to 20oC and


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40% relative humidity, and the various fiber contents used for the specimens were 0.25,

0.50, 1.00 and 1.50 percent for the steel fibers and 0.1, 0.25, 0.50, and 1.0 percent for the

polypropylene fibers. Plain specimens were also tested as controls. Free shrinkage

prisms measuring 225 x 75 x 25 mm accompanied each of the ring specimens, along with

free shrinkage ring specimens, without the inner steel tube to test the influence of the

geometry of the specimen.

The mix proportions by weight were 1:2:2:0.5 (cement: sand: coarse aggregate:

water). The maximum size aggregate was 9 mm. The steel fibers were 25 mm long and

had a diameter of 0.4 mm, while the polypropylene fibers measured 19 mm long.

Identical mix proportions were used for all mixtures except for the amount and type of

fiber reinforcing. For mixing, half the water and aggregates were mixed for a minute,

and then the cement was added and mixed for an additional minute. Finally, the

remaining water, super plasticizer and fibers were added and mixed for 5 minutes.

The ring specimens were monitored for cracks, and the crack widths measured at

the quarter points by a movable mounted microscope with a magnification x100. The

average of the quarter point measurements was used as the crack width. Free shrinkage

of the prisms was measured using a dial gage extensometer, while the free shrinkage of

the ring used an SR-4 resistance strain gage. Grzybowski and Shah (1990) found that

both specimens yielded similar results indicating that the shrinkage is independent of the

specimen geometry. The addition of fibers did not substantially alter the free shrinkage,

but 0.25 percent reinforcing fibers considerably reduced the crack widths resulting from

restrained drying shrinkage. The steel fibers were more effective than the polypropylene

fibers in this study.


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2.7 NCHRP Report by Krauss and Rogalla (1996)

Many concrete bridge decks develop cracks soon after construction. This is a

prevalent problem in the United States, which leads to a potentially shortened service life

with increased maintenance costs, acceleration in corrosion of reinforcing steel, concrete

deterioration and a damaged appearance of bridge decks. Sponsored by the National

Cooperative Highway Research Program (NCHRP), this study by Krauss and Rogalla

(1996) determined the major factors that contribute to early deck cracking and identified

construction methods, concrete materials and structural design procedures that reduce or

eliminate cracking in bridge decks.

Concrete bridge decks develop cracks when tensile stresses in the deck exceed the

tensile strength of the concrete. Temperature changes in the concrete, concrete shrinkage

and bending from self-weight and traffic loads cause these tensile stresses. Shrinkage

and temperature stresses develop in all bridges because of the restraint in the girders and

the natural thermal and shrinkage movement in the deck. The thermal and shrinkage

stresses are caused by geographic location, material properties and bridge geometry.

Some of the material property factors that affect deck cracking include cement

content, cement composition, early age elastic modulus, creep, aggregate type, concrete

temperature during placement, heat generated during hydration and drying shrinkage.

The amount of restraint provided to the deck by the girders restricts the natural shrinkage

and thermal movement in a bridge deck, causing stresses and the risk of cracking. The


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construction practice, curing and weather conditions can also affect the cracking

tendencies of bridge decks.

The research in this study included the ring test, which enables the study of

effects of mix variations on the time required for cracking to develop in the ring

specimen. Variations in the concrete mixture that would affect cracking include

aggregate type and gradation, cement type and amount, water

A Comparative Study of Shrinkage and Cracking Of

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