Creep and shrinkage using different code

PSZ 19:16 (Pind. 1/07)

UNIVERSITI TEKNOLOGI MALAYSIA
DECLARATION OF THESIS / UNDERGRADUATE PROJECT PAPER AND COPYRIGHT

NG PING CHEW
Author’s full name :

Date of birth : 18 JULY 1982

Title : COMPARISON OF CREEP AND SHRINKAGE

USING DIFFERENT CODE OF PRACTICE

Academic Session: 2007/2008

I declare that this thesis is classified as :

CONFIDENTIAL (Contains confidential information under the Official Secret
Act 1972)*

RESTRICTED (Contains restricted information as specified by the
organization where research was done)*

√ OPEN ACCESS I agree that my thesis to be published as online open access
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Certified by :

SIGNATURE SIGNATURE OF SUPERVISOR

820718-04-5165 ASSOC. PROF. IR. DR. WAHID OMAR
(NEW IC NO. /PASSPORT NO.) NAME OF SUPERVISOR

Date : 18 NOVEMBER 2008 Date : 20 NOVEMBER 2008

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DECLARATION

“I/We* hereby declare that I/We* have read this project report and in my/our*
opinion this project report is sufficient in terms of scope and quality for the
award of the degree of Master of Engineering (Civil – Structure).”

Signature : ………………………….………………..

ASSOC. PROF. IR. DR. WAHID
Name of Supervisor : …………………………….……………..

Date 20 NOVEMBER 2008
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COMPARISON OF CREEP AND SHRINKAGE
USING DIFFERENT CODE OF PRACTICE

NG PING CHEW

A project report submitted in fulfillment of the
requirements for the award of the degree of
Master of Engineering (Civil – Structure)

Faculty of Civil Engineering
Universiti Teknologi Malaysia

NOVEMBER 2008

ii

I declare that this project report entitled “Comparison of Creep and Shrinkage Using
Different Code of Practice” is the result of my own except as cited in the references.
The project report has not been accepted for any degree and is not concurrently
submitted in candidate of any degree.

Signature : …………………………………………

NG PING CHEW
Name : …………………………………………

Date : 18 NOVEMBER 2008
………………………………………….

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DEDICATION

For my beloved family

iv

ACKNOWLEDGEMENT

The author is deeply indebted to his supervisor, Assoc. Prof. Ir. Dr. Wahid
Omar whose help, stimulating suggestions and encouragement helped him in all the
time of the study and preparation of this project.

The author wishes to thank Mr. Edgar T. Almoite, for his helpful guidance.

The author has furthermore to thank all the lecturers and staffs of Faculty of
Civil Engineering UTM, for their advice and assistance throughout the study.

Sincere appreciation also extends to all his friends for their help, support,
interest and valuable hints.

Last but not least, the author would like to express his deepest gratitude to his
parents, Ng Soon and Ken Kim Moy, for unconditional support and encouragement
to pursue his interest.

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ABSTRACT

This project presents a study on the behavior of creep and shrinkage of concrete
specimens. Prediction of creep and shrinkage strain was studied and compared based
on British Standard 8110, Eurocode 2 and Australian Standard. The objective of this
study is to understand the concrete behavior of creep and shrinkage and to produce a
spread sheet of determining creep and shrinkage strain based on the three standards
mentioned. The spread sheet will be used to ease future engineers in estimating
creep and shrinkage strain of concrete in structural design work. Creep and
shrinkage are two important time-dependent properties of concrete as it causes
cracking and adversely affects the functionality, durability and appearance of
structure. There are many parameters that affect the concrete creep and shrinkage
strain such as concrete strength, type of cement, relative humidity, effective
thickness, days of loading, etc. In this study, relative humidity of the environment
was used as the controlled parameter in comparing the creep and shrinkage strain.
Australian Standard was found to be limited in determining creep and shrinkage
strain because it is based on climatic zones in Australia. However, prediction using
Australian Standard is still being considered as each zone has its own range of
relative humidity. Graph and formula method for Eurocode were considered in this
study. Both of the methods gave acceptable results. In creep and shrinkage strain
comparison, Eurocode present acceptable result with more conservative strain. This
code of practice is preferable in determining concrete creep and shrinkage among the
standards researched in this study.

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ABSTRAK

Projek ini membincangkan kajian kelakuan rayapan dan pengecutan atas spesimen
konkrit. Anggapan rayapan dan pengecutan konkrit telah dikaji dan perbandingan
telah dibuat berdasarkan British Standard 8110, Eurocode 2 dan Australian Standard.
Tujuan kajian ini adalah untuk memahami kelakuan rayapan dan pengecutan konkrit
serta menghasilkan spread sheet yang berfungsi untuk menentukan nilai rayapan dan
pengecutan berpandukan ketiga-tiga kod rekabentuk tersebut. Spread sheet ini akan
menyenangkan kerja jurutera dengan menentukan nilai rayapan dan pengecutan
konkrit dalam rekabentuk struktur. Rayapan dan pengecutan merupakan ciri-ciri
penting konkrit. Rayapan dan pengecutan akan menyebabkan keretakan konkrit,
dimana menjejaskan struktur tersebut dari segi fungsi, ketahanan dan persembahan
luarannya. Terdapat banyak parameter yang mempengaruhi rayapan dan pengecutan
konkrit seperti kekuatan konkrit, jenis simen, kelembapan bandingan, kedalaman
berkesan, masa pembebanan dan sebagainya. Dalam kajian ini, kelembapan
bandingan alam persekitaran telah digunakan sebagai parameter pengawal dalam
bandingan rayapan dan pengecutan. Didapati bahawa Australian Standard adalah
terhad dalam menentukan nilai rayapan dan pengecutan disebabkan kod tersebut
adalah berdasarkan zon cuaca Australia. Walau bagaimanapun, anggapan bagi nilai
rayapan dan pengecutan konkrit berdasarkan Australian Standard masih dilaksanakan
dalam kajian ini kerana setiap zon mempunyai kelembapan bandingannya masing-
masing. Bagi Eurocode, kaedah graf dan formula telah digunakan dalam kajian ini.
Kedua-dua kaedah tersebut telah menghasilkan keputusan yang memuaskan. Dalam
perbandingan nilai rayapan dan pengecutan, Eurocode menghasilkan keputusan
dengan konsevatif. Antara kod-kod yang digunakan, kod ini adalah lebih digemari
dalam kajian ini untuk menentukan nilai rayapan dan pengecutan.

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TABLE OF CONTENTS

CHAPTER TITLE PAGE

DECLARATION ii
DEDICATION iii
ACKNOWLEDGEMENT iv
ABSTRACT v
ABSTRAK vi
TABLE OF CONTENTS vii
LIST OF TABLES xii
LIST OF FIGURES xiii
LIST OF SYMBOLS xviii
ABBREVIATION xx

1 INTRODUCTION 1
1.1 Background 1
1.2 Serviceability of Concrete Structures 5
1.3 Problem Statement 7
1.4 Objectives of Project 8
1.5 Scope of Work 9
1.6 Expected Outcome 10

2 LITERATURE REVIEW 11
2.1 Significance of Volume Changes and
Creep 11
2.2 The Gel Structure as Related to Volume
Changes 12

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2.3 Shrinkage of Concrete 13
2.3.1 Type of Shrinkage 14
2.3.1.1 Plastic Shrinkage 14
2.3.1.2 Autogeneous Shrinkage 15
2.3.1.3 Drying Shrinkage 16
2.3.1.4 Carbonation Shrinkage 19
2.3.2 Factors Affecting Shrinkage 22
2.3.2.1 Effect of Composition and
Fineness of Cement 23
2.3.2.2 Effect of Type and
Gradation of Aggregate 23
2.3.2.3 Effect of Cement and Water
Contents 24
2.3.2.4 Effect of Admixtures 25
2.3.2.5 Temperature and Relative
Humidity 27
2.3.2.6 Volume-to-Surface Ratio 27
2.3.2.7 Volume and Type of
Aggregate 28
2.3.2.8 Elastic Modulus of
Aggregate 29
2.3.3 Differential Shrinkage 29
2.3.4 Shrinkage-induced Cracking 32
2.3.5 Effect of Shrinkage 34
2.3.6 Methods of Controlling Shrinkage
Cracking 34
2.3.6.1 Conventional Method 36
2.3.6.2 Innovative Method 39

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2.4 Creep of Concrete 42
2.4.1 Creep Behavior of Concrete 44
2.4.2 Components of Creep Strain 45
2.4.3 Factors Affecting Creep 47
2.4.3.1 Effect of Stress and Age
When First Loaded 47
2.4.3.2 Effect of Water-Cement
Ratio and Mix 47
2.4.3.3 Effect of Composition and
Fineness of Cement 48
2.4.3.4 Effect of Character and
Grading of Aggregate 49
2.4.3.5 Effect of Moisture Condition
of Storage 50
2.4.3.6 Effect of Size of Mass 51
2.4.4 Effect of Creep 52
2.4.5 Test for Creep 53
2.4.5.1 Dead load 54
2.4.5.2 Spring-loaded 54
2.4.5.3 Hydraulic 54
2.4.5.4 Stabilized Hydraulic 55

3 PREDICTION METHODS 56
3.1 Introduction 56
3.2 Shrinkage 57
3.2.1 Drying Shrinkage Strain 57
3.2.2 British Standard 61
3.2.3 Australian Standard 64
3.2.3.1 Basic Shrinkage Strain 64
3.2.3.2 Design Shrinkage Strain 65

x

3.2.4 Eurocode 68
3.2.4.1 Eurocode (Annex B) 71
3.3 Creep 72
3.3.1 Creep Strain 72
3.3.3.1 Basic Creep Factor 77
3.3.3.2 Design Creep Factor 78
3.3.4 Eurocode 80
3.3.4.1 Eurocode (Annex B) 82

4 METHODOLOGY 86
4.1 Introduction 86
4.2 Information Gathering 86
4.3 Preparation of Spread Sheet 88

5 ANALYSIS OF RESULTS 89
5.1 Introduction 89
5.2 Shrinkage 89
5.2.1 Shrinkage Strain 90
5.2.4 European Standard 93
5.2.5 Comparison of Shrinkage Using
Different Standards 95
5.3 Creep 97
5.3.1 Creep Strain 98
5.3.4 European Standard 101

xi

5.3.5 Comparison of Creep Using
Different Standard 103

6 CONCLUSION AND RECOMMENDATIONS 105
6.1 Conclusions 105
6.2 Recommendations for Further Studies 107

REFERENCES 105

APPENDIX 109

xii

LIST OF TABLES

TABLE NO. TITLE PAGE

2.1 Shrinkage of neat cement in comparison with the 24
corresponding shrinkages of the same cement diluted
with a single sieve size (No. 4 to 3/8 in.) of gravel
and crushed limestone, respectively

2.2 Methods of controlling drying shrinkage 35

2.3 Aggregate type related to drying shrinkage 37

2.4 Effect of mineral character of aggregate upon creep 50

2.5 Effect of moisture condition of storage upon creep 51

3.1 Nominal unrestrained drying shrinkage values εcd,0 (%) 69
for concrete with cement CEM Class N

3.2 Values for kh 69

3.3 Basic creep factor 77

xiii

LIST OF FIGURES

FIGURE NO. TITLE PAGE

1.1 Relationship between concrete strain and time 5

2.1 Relationship between shrinkage and loss of water from 18
specimens of cement-pulverized silica pastes cured for
7 days at 21ºC and then dried

2.2 Loss of mass of concrete due to drying and carbonation 19

2.3 Drying shrinkage and carbonation shrinkage of mortar 20
at different relative humidity

2.4 Influence of the sequence of drying and carbonation of 21
mortar on shrinkage

2.5 The pattern of shrinkage as a function of cement content, 25
water content and water/cement ratio

2.6 Effect of W/C ratio and aggregate content on shrinkage 28

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2.7 Relation between axial shrinkage and width of square 31
cross-section and length/width ratio of 4

2.8 Relation between ultimate shrinkage and 31
volume/surface ratio

2.9 Schematic pattern of crack development when tensile 33
stress due to restrained shrinkage is relieved by creep

2.10 Typical strain-time plot of concrete under sustained 43
load and after release of load

2.11 Recoverable and irrecoverable creep component 45

2.12 Creep components in drying specimen 46

2.13 Effect of water-cement ratio on creep 48

2.14 Creep in compression and tension for mass-cured 49
concretes

2.15 Effect of size of specimens upon creep 52

3.1 Coefficient KL 59

3.2 Coefficient Kc 59

3.3 Coefficient Ke 60

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3.4 Coefficient Kj 60

3.5 Drying shrinkage of normal-weight concrete 63

3.6 Shrinkage strain coefficient (k1) for various environments 66

3.7 Climatic Zones in Australia 67

3.8 Coefficient KL 73

3.9 Coefficient Km 74

3.10 Coefficient Ke 74

3.11 Effects of relative humidity, age of loading and section 76
thickness upon creep factor

3.12 Creep factor coefficient (k2) for various environments 79

3.13 Maturity coefficient (k3) 79

3.14 Method for determining the creep coefficient for concrete 82
under normal environmental conditions

5.1 Relationship between shrinkage and relative humidity 90
based on code of practice specified by Hong Kong
government

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based on BS 8110

based on AS 3600

using table and formula method based on EC 2

5.5 Relationship between shrinkage and duration based on 95
EC 2

5.6 Comparison of shrinkage using different code of practice 97

5.7 Relationship between creep and relative humidity based 98
on code of practice specified by Hong Kong government

5.8 Relationship between creep and relative humidity 100
based on BS 8110

based on AS 3600

using graph and formula method based on EC 2

5.11 Comparison of creep using different standards 103

xvii

5.12 Relationship between creep and duration based on EC 2 104

xviii

LIST OF SYMBOLS

εcs / εss – shrinkage strain
cs – modification factor to allow for properties of crushed granitic
aggregate
KL – coefficient relating to the environment (shrinkage)
Kc – coefficient relating to the composition of the concrete
(shrinkage)
Ke – coefficient relating to the effective thickness of the section
(shrinkage)
Kj – coefficient defining the development of shrinkage relative to
time
Ks – reinforcement coefficient
αe – modular ratio Es/Ec
ρ – steel ratio As/Ac
As – total area of longitudinal reinforcement
Ac – gross cross-sectional concrete area
Es – modulus of elasticity of the reinforcement
Ec – short-term modulus of concrete
εcs.b – basic shrinkage strain
k1 – shrinkage strain coefficient
εsd(t) – drying shrinkage strain in time
εsd(t) – nominal unrestrained drying shrinkage
εsa – autogeneous shrinkage strain
h0 – effective thickness
u – perimeter of the member in contact with the atmosphere

xix

kh – coefficient depending on h0
fcm – the mean compressive strength (MPa)
αds1 – coefficient which depends on the type of cement
αds2 – coefficient which depends on the type of cement
E28 – static modulus of elasticity at 28 days
Et – modulus of elasticity at an age t
Eu – modulus of elasticity at age of unloading
Øc – creep coefficient depending on KL, Km, Kc, Ke, Kj
Km – coefficient relating to the hardening (maturity) of the concrete
Øcc.b – basic creep factor of concrete
k2 – creep factor coefficient
k3 – maturity coefficient
φ – creep coefficient
kσ – stress-strength ratio σc/fcm
φ0 – notional creep coefficient
φRH – factor to allow for the effect of relative humidity on the notional
creep coefficient
βc(t,t0) – coefficient to describe the development of creep with time after
loading
βH – coefficient depending on the relative humidity and the notional
member size
t – age of concrete in days at the moment considered
t0 – age of concrete at loadings in days
fcu,28 – 28 day cube strength in N/mm2
fcu,t – cube strength at an age t
σc – compressive stress in concrete

xx

ABBREVIATION

BS 8110 - British Standard 8110
AS 3600 - Australian Standard 3600
EC 2 - European Standard Eurocode 2
RH - Relative Humidity

CHAPTER 1

INTRODUCTION

1.1 Background

Concrete is a composite building material made from the combination of
aggregate and cement binder. The most common form of concrete consists of Portland
cement, mineral aggregates (generally gravel and sand) and water. Contrary to common
belief, concrete does not solidify from drying after mixing and placement. Instead, the
cement hydrates, gluing the other components together and eventually creating a stone-
like material. When used in the generic sense, this is the material referred to by the term
concrete.

The quality of concrete can be assessed from several characteristics, namely its
strength, durability, creep and shrinkage. These are the most important and common
criteria used to grade a concrete into its quality level. A concrete of good quality should
be able to work up to the structural ability for which it is designed for, and also to last
for at least the design lifetime for which it is designed for.

2

The behavior of hardened concrete can be characterized in terms of its short-term
(essentially instantaneous) and long-term properties. Short-term properties include
strength in compression, tension, bond, and modulus of elasticity [17]. The long-term
properties include creep, shrinkage, behavior under fatigue, and durability characteristics
such as porosity, permeability, freeze-thaw resistance, and abrasion resistance [17].

Concrete is one of the most durable construction materials. However, cracking
adversely affects its durability, functionality, and appearance. A major cause of
cracking is related to shrinkage-induced strains, creating stresses when concrete is
restrained [4]. The shrinkage of concrete is often attributed to drying of the concrete
over a long period of time, and recent observations have also focused on early age
shrinkage and creep problems. Cracked concrete typically needs to be repaired to
prevent further deterioration due to freezing and thawing, and corrosion of steel
reinforcement resulting from infiltration of water with or without chloride ions from de-
icing salts. The cracking leads to additional costs for repair to prevent premature
deterioration of the concrete and the corrosion of reinforcement steel.

The early age of concrete is known to have a significant control on the overall
performance of concrete structures. During this stage, concrete may be subjected to
severe internal actions due to thermal and hydric gradients within concrete itself and at
the same time it may be affected by the external conditions of environment and loading
[18]. All these actions may lead to different deformations within the concrete that is just
building its resistance and stiffness. Creep and shrinkage of concrete are known to have
significant effect at early age of concrete. Thus, discussing the performance of this
young age concrete with special attention to the shrinkage and creep and time dependent
deformations is of interest by many researchers.

3

In predicting the strength and serviceability of reinforced and pre-stressed
concrete structures, appropriate descriptions of the mechanical properties of the
materials are required including the prediction of the long term behavior of the concrete.
The prediction of short-term shrinkage and creep is also important to assess the risk of
concrete cracking and stripping and unshoring times [17]. The mechanical properties of
concrete are significantly affected by the temperature and availability of water during
curing, the environmental humidity and temperature after curing, and the composition of
the concrete, including the mechanical properties of the aggregates.

When concrete is subjected to sustained compressive stress, deformations
continue to increase with time due to creep and shrinkage. Creep strain is produced by
sustained stress. Shrinkage strains are independent of stress and are caused by chemical
reactions in the hydrating cement paste and by the loss of water during the drying
process. The creep and shrinkage deformations in a concrete structure are frequently
larger, and in some cases much larger than the initial deformations produced when the
external loads are first applied [7]. They thus have a significant effect on service-load
behavior.

The resistance to deformation that makes concrete a useful material means also
that volume changes of the concrete itself can have important implications in use. Any
potential growth or shrinkage may lead to complications, externally because of structural
interaction with other components or internally when the concrete is reinforced. There
may even be distress if either the cement paste or the aggregate changes dimension, with
tensile stresses set up in one component and compressive stresses in the other. Cracks
may be produced when the relatively low tensile strength of the concrete or its
constituent materials is exceeded.

4

Cracking not only impairs the ability of a structure to carry its design load but
also affect its durability and damage its appearance. In addition, shrinkage and creep
may increase deflections in one member of a structure, adversely affecting the stability
of the whole. Volume change of concrete is not usually associated with changes that
occur before the hardened state is attained. Quality and durability, on the other hand, are
dependent on what occurs from the time the concrete mix has been placed in the mold.

Control of cracking may also be done by providing appropriate reinforcement.
The reinforcement, however, does not reduce shrinkage but helps to keep cracks from
widening. The use of expansive cements, coal-combustion products containing calcium
sulfite or sulfate, and fibers is one way of counteracting shrinkage. Usually, expansive
cements and clean-coal ash produce expansion by formation of ettringite. When the
expansion is restrained by reinforcement, a compressive pre-stress is induced in
concrete, compensating shrinkage.

Figure 1.1 illustrates the relationship between various measured and derived
strain values. The figure shows that the concrete undergoes autogenous shrinkage
before drying. Once drying commences at time t0, drying shrinkage occurs. Upon
loading, both drying and basic creep occurs in the drying specimen.

5

Figure 1.1: Relationship between concrete strain and time [8]

1.2 Serviceability of Concrete Structures

For a concrete structure to be serviceable, cracking must be controlled and
deflections must not be excessive. The design for serviceability is possibility the most
difficult and least well understood aspect of the design of concrete structures. Service
load behavior depends primarily on the properties of the concrete and these are often not
known reliably at the design stage. Concrete behaves in a non-linear and inelastic
manner at service loads and the non-linear behavior that complicates serviceability
calculations is due to cracking, tension stiffening, creep, and shrinkage.

The control of cracking in a reinforced or pre-stressed concrete structure is
usually achieved by limiting the stress increment in the bonded reinforcement to some

6

appropriately low value and ensuring the bonded reinforcement is suitably distributed.
For deflection control, engineer should select maximum deflection limits that are
appropriate to the structure and its intended use. The calculated deflection must not
exceed these limits.

The quest for serviceable concrete structures must involve the development of
more reliable design procedures. It must also involve designers giving more attention to
the specification of an appropriate concrete mix, particularly with regard to the creep
and shrinkage characteristics of the mix, and sound engineering input is required in the
construction procedures.

When designing for serviceability, engineer must ensure that the structure can
perform its intended function under the day to day service loads. Deflection must not be
excessive, cracks must be adequately controlled and no portion of the structure should
suffer excessive vibration. Shrinkage cause time-dependent cracking, thereby reducing
the stiffness of a concrete structure, and is therefore a detrimental factor in all aspects of
the design for serviceability.

Excessive wide cracks can be unsightly and spoilt the appearance of an exposed
concrete surface. They allow the ingress of moisture accelerating corrosion of the
reinforcement and durability failure. In exceptional cases, they reduce the contribution
of the concrete to the shear strength of a member. Excessively wide cracks in floor
systems and walls may often be avoided by the inclusion of strategically placed
contraction joints, thereby removing some of the restraint to shrinkage and reducing the
internal tension. When cracking occurs, in order to ensure that crack widths remain
acceptably small, adequate quantities of well distributed and well-anchored
reinforcement must be included at every location where significant tension will exist.

7

Deflection problems that may affect the serviceability of concrete structures can
be classified into three main types:

a. Where excessive deflection causes either aesthetic or functional problems.

b. Where excessive deflection results in damage to either structural or non-
structural element attached to the member.

c. Where dynamics effects due to insufficient stiffness cause discomfort to
occupants.

1.3 Problem Statement

Creep and shrinkage are very important time-dependent properties of concrete.
They are in direct relation to the performance of concrete. The prediction of time-
dependent behaviour is the most uncertain part of the design of concrete structures.
Moreover, the prediction of the time-dependent behaviour is important not only for the
structural maintenance after its completion, but also for the stress and deformation
control during the erection stages of the structure.

Most of the engineers today do not consider the concrete behaviour of creep and
shrinkage in their design work because of lacking experience and understanding on the
phenomenon and the effect on concrete specimen. Most of them consider creep and
shrinkage cracks as non-structural cracks which is not important and will not cause any
serious effect on concrete specimen. This assumption and consideration is not true
because cracking deteriorate concrete’s durability and integrity. A number of analytical

8

techniques are available for the prediction of creep and shrinkage on concrete members.
However, each has its own simplifying assumptions, advantages and disadvantages.
Some of those codes are more suited to particular conditions than others such as
parameters used in BS are based on the conditions in Europe which may not be
accurately applicable in Malaysia.

Therefore, the study is mainly concentrates on the understanding of concrete
behaviour due to creep and shrinkage and to study the prediction of creep and shrinkage
strain using different code of practice.

1.4 Objectives of Project

Based on the scope of work, the objectives of the project are defined below:

(i) Study the properties and deformation of concrete due to creep and
shrinkage.

(ii) Evaluate and identify the parameters and method used in determining the
coefficient of creep and shrinkage for British Standard, Eurocode and
Australia Standard.

(iii) Develop spreadsheets that calculating the creep and shrinkage of concrete
for British Standard, Eurocode and Australia Standard.

(iv) Compare the creep and shrinkage strain using BS8110, EC2 and AS3600
under controlled parameters.

9

1.5 Scope of Work

Time-dependent concrete deformation is nowadays one of the concerns in
engineering field as it affects the serviceability and aesthetic of the concrete structures.
The main factors that cause concrete deforms due to environment and applied stress are
shrinkage and creep. Therefore, the research on this topic has been proposed in the Final
Year Project of Master Studies (Civil-Structural) in Universiti Technologi Malaysia.

In Masters Pre Project, the scope of work was mainly focused on the literature
review of related studies. Substantial information on concrete properties such as
modulus of elasticity, creep and shrinkage will be gathered through latest journals and
publications in libraries and also articles from internet. The history of concrete, effect of
admixtures on concrete properties and factors affecting deterioration on concrete and the
effects are studied in this study.

In Masters Project, detail studies on the concrete deformation due to time-
dependent factors (creep and shrinkage) will be made. The formulas, and method used
in predicting concrete deformation due to creep and shrinkage will be identified using
British Standard, EURO Code and Australia Standard. Spread sheet to determine
concrete creep and shrinkage will be produced by inputting the controlling parameters
such as strength of concrete (fcu), relative humidity, type of cement, effective thickness,
provided steel reinforcement, etc.

10

1.6 Expected Outcome

There are some outcomes to be expected through this master research studies
such as:

(i) To understand the concrete properties due to creep and shrinkage.

(ii) To be familiar with the codes in creep and shrinkage of concrete
specification.

(iii) To understand the parameter and method used in calculating concrete
creep and shrinkage for British Standard, Eurocode & Australia Standard.

CHAPTER 2

LITERATURE REVIEW

2.1 Significance of Volume Changes and Creep

If concrete is free to deform, any volume changes would be of little consequence,
but usually it is restrained by foundations, steel reinforcement, or by adjacent concrete
subject to different conditions. As the potential movement is thus restrained, stresses
will be developed which may rupture the concrete. This is particularly true when
tension is developed; thus, contractions causing tensile stress are more important than
expansions which cause compressive stress. Difference in moisture contents of the
exposed and unexposed faces of thin concrete slabs, such as highways and canal linings,
may cause curling and eventual cracking. Cracking not only may impair the ability of
any structure to carry its designed loads, but it also may affect its durability and damage
its appearance. The durability is affected by the entry of water through cracks, which
corrodes the steel, leaches out soluble components, and deteriorates the concrete when
subjected to freezing and thawing.

12

Creep, in general, tends to relieve the stress in concrete, especially when
reinforced. Thus, when a sustained load is applied to a reinforced concrete column,
creep of the concrete causes a gradual reduction in the load on the concrete and a
corresponding increase in the load on the steel. In various structural elements such as
continuous beams and slabs, creep relieves some of the stress in the most highly stressed
portions and increases the stress in adjacent portions of the concrete, so that finally the
stresses are more uniform throughout the member. This relieving of the higher stresses
serves to reduce the tendency toward cracking. However, creep may cause
objectionable sagging of thin, long-span floor slabs or other structural elements.

2.2 The Gel Structure as Related to Volume Changes

Cement, after hydration, consists of crystalline material plus a calcium silicate
gel resulting from the combination of cement and water. The amount of the gel
increases with the age of hydration and is greater for higher water-cement ratios and for
finer cements. The amount of gel also depends upon the chemical composition of the
cement, as fully hydrated dicalcium silicate is believed to be mostly gel, while hydrated
tricalcium silicate is more than half gel. For the water-cement ratios used in average
concrete, the gel has a larger volume than the crystalline portions.

The crystalline materials in cement are believed to be unaffected by ordinary
drying, but the gel is finely porous and undergoes large volume changes upon wetting
and drying. The quantity and characteristics of the calcium silicate gel, therefore,
largely determine the potential shrinkage upon drying of hydrated cement.

13

Water is held in the pores of the gel by such large attractive forces that when it is
removed from the pores by evaporation, the forces which formerly attracted the water
become effective in compressing and reducing the volume of the gel. All concretes,
then, are subject to moisture volume changes in some degree, and the problem involved
is so to control conditions that the volume changes have small or practically harmless
effects upon the integrity of the structure.

2.3 Shrinkage of Concrete

Concrete deformation due to movement of water from or to the ambient medium
when no external stress is acting is termed shrinkage. It is independent of stress and is
caused by chemical reactions in the hydrating cement paste and by the loss of water
during the drying process. Technically, shrinkage will continue for the life of the
concrete, but most shrinkage will occur within the first 90 days after placement [33].

Shrinkage cracking is a major cause of concern for concrete structures. In
addition to weakening the structure, these shrinkage cracks have the potential to allow
infiltration of moisture and chloride ions that accelerate the corrosion of steel
reinforcement and reduce the durability of concrete.

14

2.3.1 Type of Shrinkage

The four main types of shrinkage associated with concrete are plastic shrinkage,
autogenous shrinkage, drying shrinkage, and carbonation shrinkage.

2.3.1.1 Plastic Shrinkage

Plastic shrinkage is associated with moisture loss from freshly poured concrete
into the surrounding environment. Plastic shrinkage occurs only in fresh concrete. The
most common mechanism is the evaporation of water from the surface of the plastic
concrete. However, the loss of water through the sub-base or formwork can exacerbate
the effects of surface evaporation [33].

In the fresh concrete, the particles are completely surrounded by water. If water
is removed from the system, menisci are formed between particles. These menisci
generate negative capillary pressure, which pulls the cement particles together. By
pulling on the particles, the capillary stresses tend to reduce the volume of the cement
paste. Capillary pressures continue to rise as water is lost at the surface of the concrete.
When the pressures reach a critical value, the water that remains in the concrete
rearranges to form discrete zones with voids between them. Plastic shrinkage cracking
occurs at this point.

15

2.3.1.2 Autogenous Shrinkage

Autogenous shrinkage is the volume change of the cement paste due to self-
desiccation and chemical shrinkage after initial setting has occurred. Autogenous
shrinkage is a microscopic volume change occurring after the initial setting in situations
where the supply of water from outside of concrete is not enough. As the hydration of
cementitious materials progresses, very fine pores are produced within the hardened
cement paste due to the formation of calcium silicate hydrate (CSH) gel. As the
hydration further progresses, capillary pore water and gel water is consumed and
menisci are produced in these pores due to a lack of water supply from outside. As a
result of negative pressure in the pores, hardened paste shows shrinkage [18].

Autogenous shrinkage is the early shrinkage of concrete caused by the loss of
water from capillary pores due to the hydration of cementitious materials, without the
loss of water into the surrounding environment. This phenomenon is known as self-
desiccation of concrete. Self-desiccation occurs in all concrete irrespectively of the
water-cement ratio. However, its effects are very different in normal concrete and high-
performance concrete. In high-performance concrete, significantly more cementitious
materials and less mixing water are used compared with normal concrete. In normal
concrete, there is substantially more water than required for hydration of cementitious
materials particles. This excess amount of water is contained in well-connected
capillaries. Menisci created by the process of self-desiccation occur in large capillaries.
But, stresses generated in large capillaries are very low, resulting in lower autogenous
shrinkage. On the other hand, in case of high-performance concrete, pore network is
essentially composed of fine capillaries due to low water-cement ratio and high amounts
of cementitious hydration products. When self-desiccation starts to take place, very high
tensile stresses are generated in these fine pores, resulting in higher autogenous
shrinkage.

16

Although autogenous shrinkage is three-dimensional, it is usually expressed as a
linear stain so that it can be considered alongside the drying shrinkage. Typical values
of autogenous shrinkage are about 40 x 10-6 at the age of one month and 100 x 10-6 after
five years. Autogenous shrinkage tends to increase at higher temperatures, with higher
cement content, and possibly with finer cements, and with cements which have a high
C3A and C4AF content. At a constant content of blended cement, a higher content of fly
ash leads to lower autogenous shrinkage. As self-desiccation is greater at lower
water/cement ratios, autogenous shrinkage could be expected to increase but this may
not occur because of the more rigid structure of the hydrated cement paste at lower
water/cement ratios. Nevertheless, at very low water/cement ratios, autogenous
shrinkage is very high: a value of 700 x 10-6 was reported for concrete with a
water/cement ratio of 0.17 [1].

2.3.1.3 Drying Shrinkage

Drying shrinkage is different from autogenous shrinkage with regard to the
mechanism of a decrease in humidity. Drying shrinkage is caused by the diffusion of
water from concrete into the outer surrounding environment.

Drying shrinkage refers to the reduction in concrete volume resulting from the
loss of capillary water by evaporation. This shrinkage causes an increase in tensile
stress of restrained concrete, which leads the concrete to cracking, internal warping, and
external deflection, even if the concrete is not subjected to any kind of external loading.

17

According Mehta and Monteiro the change in volume of drying concrete is not
equal to the volume of water removed [2]. The reason is that the loss of water from
large capillaries may be considered as free water, and its removal does not cause volume
change. Loss of water held by capillary tension in small capillaries may cause shrinkage
of concrete. It is also possible that shrinkage is related to the removal of interlayer
water, which is also known as zeolite water. It has been suggested that a monomolecular
water layer between the layers of CSH is strongly held by hydrogen bonding. This
water is associated with CSH structure and the interlayer water is lost only on strong
drying.

The drying shrinkage of hydrated cement paste begins at the surface of the
concrete. Depending on the relative humidity of the ambient air and the size of
capillaries in the cement paste structure, drying shrinkage progresses more or less
rapidly through concrete. The drying in ordinary concrete is, therefore, rapid because
the capillary network is well connected and contains large capillaries. In the case of
high-performance concrete, drying shrinkage is slow because the capillaries are very
fine and soon get disconnected by hydration products.

The influence of the gel particle size on drying is shown by the low shrinkage of
the much more coarse-grained natural building stones and by the high shrinkage of fine
grained shale [3]. Also, high-pressure steam-cured cement paste, which is
microcrystalline and has a low specific surface, shrinks 5 to 10 times [4], and sometimes
even 17 times [5], less than a similar paste cured normally.

It is possible also that shrinkage, or a part of it, is related to the removal of
intracrystalline water. Calcium silicate hydrate has been shown to undergo a change in
lattice spacing from 1.4 to 0.9 nm on drying [6]; hydrated C4A and calcium
sulfoaluminate show similar behavior [7]. It is thus not certain whether the moisture

18

movement associated with shrinkage is inter- or intracrystalline. But, because paste
made with both Portland and high-alumina cements, and also with pure ground calcium
monoaluminate, exhibit essentially similar shrinkage, the fundamental cause of
shrinkage must be sought in the physical structure of the gel rather than in its chemical
and mineralogical character [6].

The relation between the mass of water lost and shrinkage is shown in Figure
2.1. For neat cement pastes, the two quantities are proportional to one another as no
capillary water is present and only adsorbed water is removed. However, mixes to
which pulverized silica has been added and which, for workability reasons, require a
higher water/cement ratio, contain capillary pores even when completely hydrated.
Emptying of the capillaries causes a loss of water without shrinkage but, once the
capillary water has been lost, the removal of adsorbed water takes place and causes
shrinkage in the same manner as in a neat cement paste.

Figure 2.1: Relationship between shrinkage and loss of water from specimens of
cement-pulverized silica pastes cured for 7 days at 21ºC and then dried [3]

19

2.3.1.4 Carbonation Shrinkage

In addition to shrinkage upon drying, concrete undergoes shrinkage due to
carbonation, and some of the experimental data on drying shrinkage include the effects
of carbonation. Drying shrinkage and carbonation shrinkage are, however quite distinct
in nature.

Carbonation shrinkage is caused by the chemical reactions of various cement
hydration products with carbon dioxide present in the air. This type of shrinkage is
usually limited to the surface of the concrete. Because of carbon dioxide is fixed by the
hydrated cement paste, the mass of the latter increases. Consequently, the mass of
concrete also increases. When the concrete dries and carbonates simultaneously, the
increase in mass on carbonation may at some stage give the misleading impression that
the drying process has reached stage of constant mass, i.e. equilibrium (see Figure 2.2).

Figure 2.2: Loss of mass of concrete due to drying and carbonation [8]

20

Carbonation shrinkage is probably caused by the dissolving of crystals of
Ca(OH)2 while under a compressive stress (imposed by the drying shrinkage) and
depositing of CaCO3 in spaces free from stress; the compressibility of the hydrated
cement paste is thus temporarily increased. If carbonation proceeds to the stage of
dehydration of C-S-H, this also produces carbonation shrinkage.

Figure 2.3 shows the drying shrinkage of mortar specimens dried in CO2 – free
air at different relative humidity, and also the shrinkage after subsequent carbonation.
Carbonation increases the shrinkage at intermediate humidity, but not at 100 per cent or
25 per cent. In the latter case, there is insufficient water in the pores within the cement
paste for CO2 to form carbonic acid. On the other hand, when the pores are full of
water, the diffusion of CO2 into the paste is very slow; it is also possible that the
diffusion of calcium ions from the paste leads to precipitation of CaCO3 with a
consequent clogging of surface pores.

Figure 2.3: Drying shrinkage and carbonation shrinkage of mortar at different relative
humidity [9]

21

The sequence of drying and carbonation greatly affects the total magnitude of
shrinkage. Simultaneous drying and carbonation produces lower total shrinkage than
when drying is followed by carbonation (Figure 2.4) because, in the former case, a large
part of the carbonation occurs at relative humidity above 50 per cent; under such
conditions carbonation shrinkage is reduced. Carbonation shrinkage of high-pressure
steam-cured concrete is very small.

Figure 2.4: Influence of the sequence of drying and carbonation of mortar on
shrinkage [9]

When concrete is subjected to alternating wetting and drying in air containing
CO2, shrinkage due to carbonation becomes progressively more apparent. The total
shrinkage at any stage is greater than if drying took place in CO2 – free air. However,
carbonation of concrete prior to exposure to alternating wetting and drying reduces the
moisture movement.

22

2.3.2 Factors Affecting Shrinkage

Generally, plastic shrinkage results from surface evaporation due to
environmental conditions, such as humidity, wind speed or ambient temperature. ACI
305R, Hot Weather Concreting [10], provides guidance for placement of concrete to
minimize plastic shrinkage cracking.

Several factors which may be expected to influence the magnitude of volume
changes in mortars and concretes caused by variations in moisture conditions, which
take place with time and the simultaneous hardening of the cement paste are [34]:

(i) Composition and fineness of the cement
(ii) Cement and water contents
(iii) Type and gradation of aggregate
(iv) Admixtures
(v) Age at first observation
(vi) Duration of tests
(vii) Moisture and temperature conditions
(viii) Size and shape of specimen
(ix) Absorptiveness of forms
(x) Amount and distribution of reinforcement

23

2.3.2.1 Effect of Composition and Fineness of Cement

Cement properties and cement content in concrete influence concrete shrinkage.
As the fineness of cement increases, so does the hydration rate of cement, leading to an
increase to an increase in autogenous shrinkage of concrete. Small autogenous
expansion as opposed to shrinkage may be produced through the use of coarser cements.
Therefore, early age cracking could be possibly being avoided. Although coarser
particles of cement are relatively beneficial in minimizing early age cracking, they may
be detrimental to long-term strength. Mehta and Monteiro [2] state that the variation in
fineness and composition of Portland cement affect the rate of hydration, but not the
volume and characteristics of hydration products. Therefore, normal changes in fineness
and composition of cement have negligible effect on drying shrinkage of concrete.
Higher cement content with lower W/C in concrete results in higher autogenous
shrinkage due to self-desiccation and chemical shrinkage, but may reduce drying
shrinkage due to dense microstructure and poor pore connectivity.

2.3.2.2 Effect of Type and Gradation of Aggregate

The drying shrinkage of concrete is not related to a fraction of neat cement as the
aggregate particles not only dilute the paste but they reinforce it against contraction.
Tests have shown that if the aggregate were readily compressible, as when using porous
but nonabsorbent rubber particles, the concrete would shrink as much as neat cement.
The ability of normal aggregates to restrain the shrinkage of a cement paste depends
upon (1) extensibility of the paste, (2) degree of cracking of the paste, (3)
compressibility of the aggregate, and (4) volume change of aggregate due to drying. In
table A is shown the shrinkage of neat cement in comparison with the corresponding

24

shrinkages of the same cement diluted with a single sieve size (No. 4 to 3/8 in.) of gravel
and crushed limestone, respectively. The reduction in shrinkage due to the aggregate is
greater than would be expected considering its relative volume. It is possible that
internal cracking of the paste due to the restraint of the aggregate is a factor.

Table 2.1: Shrinkage of neat cement in comparison with the corresponding shrinkages
of the same cement diluted with a single sieve size (No. 4 to 3/8 in.)
of gravel and crushed limestone, respectively [35]

2.3.2.3 Effect of Cement and Water Contents

The water content is probably the largest single factor influencing the shrinkage
of cement paste and concrete. Tests have shown that for cements having normal
shrinkage characteristics, the shrinkage of the cement paste varies directly with the
water-cement ratio [33].

Figure 2.5 shows the pattern of shrinkage as a function of cement content, water
content, and water/cement ratio where the concrete is moist-cured for 28 days, thereafter
dried for 450 days [11]. At a constant water/cement ratio, shrinkage increases with an

25

increase in the cement content because this results in a larger volume of hydrated cement
paste which is liable to shrinkage. However, at a given workability, which
approximately means a constant water content, shrinkage is unaffected by an increase in
the cement content, or may even decrease, because the water/cement ratios is reduced
and the concrete is therefore, better able to resist shrinkage.

Figure 2.5: The pattern of shrinkage as a function of cement content, water content and
water/cement ratio [11]

2.3.2.4 Effect of Admixtures

Admixtures can adversely affect the shrinkage potential of concrete. For
instance, water reducers can be used to reduce the paste volume and thereby enhance the
creep capacity without the loss of workability. Set retarders can be used to delay set and
to decrease the amount of heat of hydration. A lower heat of hydration will decrease the
thermal shock on the hydrating concrete [33]. However, overly long retardations will
increase the potential for plastic shrinkage cracking. Proper curing is necessary with the

26

use of a set retarder. Conversely, set accelerators increase the heat of hydration and
early-age shrinkage. This combination will increase transverse shrinkage and the
resulting cracking.

Shrinkage-reducing admixtures (SRAs) are also available. These admixtures
reduce the drying shrinkage by reducing the surface tension of the water in the capillary
pores. If the surface tension of the water is reduced, there is less tension transferred to
the capillary walls, and consequently less shrinkage. Laboratory evaluations have
shown a slight decrease in compressive strength when an SRA is used. Taking
advantage of the water-reducing properties of SRAs can offset the decrease in strength.

Shrinkage of concrete made with high-alumina cement is of the same magnitude
as when Portland cement is used, but it takes place much more rapidly [12]. Including
either fly ash or ground granulated blastfurnace slag in the mix increases shrinkage.
Specifically, at a constant water/cement ratio, a higher proportion of fly ash or slag in
the blended cement leads to higher shrinkage by some 20 percent with the former
material and by up to 60 percent at very high contents of slag [13]. Silica fume
increases the long-term shrinkage [14].

Water-reducing admixtures per se probably cause a small increase in shrinkage.
Their main effect is indirect in that the use of an admixture may result in a change in the
water content or in the cement content of the mix, or in both, and it is the combined
action of those changes that influences shrinkage. Superplasticizers have been found to
increase shrinkage by some 10 to 20 percent. However, the changes in the observed
shrinkage are too small to be accepted as reliable and generally valid.

27

Entrainment of air has been found to have no effect on shrinkage [15]. Added
calcium chloride increases shrinkage, generally between 10 and 50 percent [16],
probably because a finer gel is produced and possibly because of greater carbonation of
the more matures specimens with calcium chloride.

2.3.2.5 Temperature and Relative Humidity

A high temperature and low relative humidity of the ambient environment
accelerate the diffusion of the adsorbed water and capillary water into the atmosphere,
and consequently, increases the drying shrinkage of concrete. An increase in the
atmospheric humidity slows down the rate of moisture flow from the interior to the outer
surface of concrete. Mehta and Monteiro [2] states that at 0% relative humidity, it is
assumed that the drying shrinkage of concrete is zero.

2.3.2.6 Volume-to-Surface Ratio

The size and shape of a concrete element have a considerable effect on the rate
and total amount of shrinkage. The size and shape are often considered together as the
volume-to-surface area ratio. A high volume-to-surface ratio usually results in lower
shrinkage magnitudes.

28

2.3.2.7 Volume and Type of Aggregate

Drying shrinkage of concrete is a fraction of that of neat cement because the
aggregate particles not only dilute the paste but reinforce it against contraction. The size
and grading of aggregate do not, by themselves, influence the magnitude of shrinkage,
but an aggregate incorporating larger sizes permits the use of a mix with less cement and
hence a lower shrinkage.

The shrinkage of aggregates themselves may be of considerable importance in
determining the shrinkage of concrete. Some fine-grained sandstones, slate, basalt, trap
rock and aggregates containing clay show large shrinkage while concretes low in
shrinkage often contain quartz, limestone, granite or feldspar. The pore structure of
aggregate particles may have a strong effect on autogenous shrinkage. Aggregate
particles may contain water in coarse pores, which provides the “internal curing” for
hydrating cement paste hence reducing autogenous shrinkage. Figure 2.6 indicates the
relationship between W/C ratio, aggregate content and shrinkage.

Figure 2.6: Effect of W/C ratio and aggregate content on shrinkage [36]

29

2.3.2.8 Elastic Modulus of Aggregate

Modulus of elasticity is the most important property of aggregate that directly
influences drying shrinkage of concrete. When readily compressible aggregate is used,
concrete will shrink as mush as neat cement, and that expanded shale leads to shrinkage
more than that of ordinary aggregate. Steel aggregate on the other hand, leads to
shrinkage less than that of ordinary concrete. The drying shrinkage of concrete
increased 2.5 times [17] when an aggregate with high elastic modulus was substituted by
an aggregate with low elastic modulus.

2.3.3 Differential Shrinkage

It was mentioned earlier that the potential shrinkage of neat cement paste is
restrained by the aggregate. In addition, some restraint arises also from non-uniform
shrinkage within the concrete member itself. Moisture loss takes place only at the
surface so that a moisture gradient is established in the concrete specimen, which is thus
subjected to differential shrinkage. The potential shrinkage is compensated by the
strains due to internal stresses, tensile near the surface and compressive in the core.
When drying takes place in an unsymmetrical manner, warping (curling) can result.

It may be useful to point out that the values of shrinkage generally quoted are
those of free shrinkage, or potential shrinkage, that is, contraction unrestrained either
internally or by external constraints on a structural member. In considering the effect of
the constraining forces on the actual shrinkage, it is important to realize that the induced
stresses are modified by relaxation, which may prevent the development of cracking.

30

Because relaxation occurs only slowly, it may prevent cracking when shrinkage
develops slowly; however, the same magnitude of shrinkage occurring rapidly may well
induce cracking. It is shrinkage cracking that is of paramount interest.

Because drying takes place at the surface of concrete, the magnitude of shrinkage
varies considerably with the size and shape of the specimen, being a function of the
surface/volume ratio. A part of the size effect may also be due to the pronounced
carbonation shrinkage of small specimens. Thus, for practical purposes, shrinkage
cannot be considered as purely an inherent property of concrete without reference to the
size of the concrete member.

Many investigations have, in fact, indicated an influence of the size of the
specimen on shrinkage. The observed shrinkage decreases with an increase in the size
of the specimen but, above some value, the size effect is small initially as shown in
Figure 2.7. The shape of the specimen also appears to enter the picture but, as a first
approximation, shrinkage can be expressed as a function of the volume/surface ratio of
the specimen. There appears to be a linear relation between this ratio and the logarithm
of shrinkage as shown in Figure 2.8.

The effect of shape is secondary. I-shaped specimens exhibit less shrinkage than
cylindrical ones of the same volume/surface ratio, the difference being 14 percent on the
average [18]. The difference, which can be explained in terms of variation in the mean
distance that the water has to travel to the surface, is thus not significant for design
purposes.

31

Figure 2.7: Relation between axial shrinkage and width of concrete prisms of
square cross-section and length/width ratio of 4 [37]

Figure 2.8: Relation between ultimate shrinkage and volume/surface ratio [18]

32

2.3.4 Shrinkage-induced Cracking

As mentioned in connection with differential shrinkage, the importance of
shrinkage in structures is largely related to cracking. Strictly speaking, we are
concerned with the cracking tendency because the advent or absence of cracking
depends not only on the potential contraction but also on the extensibility of concrete, its
strength, and its degree of restraint to the deformation that may lead to cracking.
Restraint in the form of reinforcing bars or a gradient of stress increases extensibility of
concrete in that it allows it to develop strain well beyond that corresponding to
maximum stress. A high extensibility of concrete is generally desirable because it
permits concrete to withstand greater volume changes.

The schematic pattern of crack development when stress is relieved by creep is
shown in Figure 2.9. Cracking can be avoided only if the stress induced by the free
shrinkage strain, reduced by creep, is at all times smaller than the tensile strength of the
concrete. Thus, time has two-fold effect: the strength increases, thereby reducing the
danger of cracking but, on the other hand, the modulus of elasticity also increases so that
the stress induced by a given shrinkage becomes larger. Furthermore, the creep relieves
decreases with age so that the cracking tendency becomes greater. A minor practical
point is that, if the cracks due to restrained shrinkage form at an early stage, and
moisture subsequently has access to the crack, many of the cracks will become closed by
autogenous healing.

One of the most important factors in cracking is the water/cement ratio of the
mix because its increase tends to increase shrinkage and, at the same time, to reduce the
strength of the concrete. An increase in the cement content also increases shrinkage and,
therefore, the cracking tendency, but the effect on strength is positive. This applies to
drying shrinkage. Carbonation, although it produces shrinkage, reduces subsequent

33

moisture movement, and therefore is advantageous from the standpoint of cracking
tendency. On the other hand, the presence of clay in aggregate leads both to higher
shrinkage and to greater cracking.

The use of admixtures may influence the cracking tendency through interplay of
effects on hardening, shrinkage, and creep. Specifically, retarders may allow more
shrinkage to be accommodated in the form of plastic shrinkage and also probably
increase the extensibility of concrete, and therefore reduce cracking. On the other hand,
if concrete has attained rigidity too rapidly, it cannot accommodate the would-be plastic
shrinkage and, having a low strength, cracks.

The temperature at the time of placing determines the dimensions of concrete at
the moment when it ceases to deform plastically. A subsequent drop in temperature will
produce potential contraction. Thus, placing concrete in hot weather means a high
cracking tendency. Steep temperature or moisture gradients produce severe internal
restraints and thus represent a high cracking tendency. Likewise, restraint by the base of
a member, or by other members, may lead to cracking.

Figure 2.9: Schematic pattern of crack development when tensile stress due to
restrained shrinkage is relieved by creep [33]

34

2.3.5 Effect of Shrinkage

Virtually all concrete is subject to some form of restraint, such as steel
reinforcement, forms, subgrade, or adjacent members. Each of these forms of restraint
involve the imposition of a gradually increasing tensile force on the concrete which may
lead to time-dependent cracking, increases in deflection and a widening of existing
cracks.
The advent of shrinkage cracking depends on the degree of restraint to shrinkage,
the extensibility and strength of the concrete in tension, tensile creep and the load
induced tension existing in the member. Cracking can be avoided if the gradually
increasing tensile stress induced by shrinkage, and reduced by creep, is at all times less
than the tensile strength of the concrete. The existence of load induced tension in
uncracked regions accelerates the formation of time-dependent cracking. The control of
such cracking requires two important steps. First, the shrinkage-induced tension and the
regions where shrinkage cracks are likely to develop must be recognized by the
structural engineer. Second, an adequate quantity and distribution of anchored
reinforcement must be included in these regions to ensure that the cracks remain fine
and the structure remains serviceable.

2.3.6 Methods of Controlling Shrinkage Cracking

Specific methods to properly control shrinkage cracking have been developed
and researched. Conventional methods, which include proper material selection,
mixture proportioning, and good construction techniques, can be used to a certain extent
to control and limit the shrinkage cracking of concrete. Unfortunately, because these
methods are hard to control, and environmental conditions can vary so much, the

35

shrinkage cracking cannot be entirely prevented. For example, concrete in hot, dry, and
windy conditions can have much higher rates of water evaporation, thus making them
more susceptible to shrinkage cracking. Innovative methods of controlling shrinkage
cracking have been found in literature and developed by numerous researches to help
control and eliminate shrinkage cracking. These include using fiber-reinforced concrete,
shrinkage-reducing admixtures, shrinkage-compensating concrete, and extensible
concrete. The categories of methods are summarized in Table 2.2.

Table 2.2: Methods of controlling drying shrinkage [31]
Methods Description
Conventional • Proper Material Selection
o Aggregates
o Cement type
o Admixtures
• Mixture Proportioning
o Cement Content
Innovative • Fiber Reinforcement
o Polypropylene
o Steel
• Shrinkage-Compensating Concrete
• Shrinkage-Reducing Admixtures
• Extensible Concrete

36

2.3.6.1 Conventional Method

Shrinkage cracking in concrete is currently being controlled through
conventional methods, which consist of the proper selection of materials and concrete
mixtures, along with good construction techniques.

2.3.6.1.1 Aggregates

The type of aggregate used in concrete mixtures, as well as the aggregate
content, can influence the amount of shrinkage in concrete. The aggregate type was the
most significant factor affecting when concrete cracked [19]. Specifically, limestone-
aggregate concretes proved to be the most resistant to cracking, while Eau Claire river
gravel had the shortest time-to-cracking of the aggregates tested. Burrows (1998) also
studied the effect of the type of aggregate used on the drying shrinkage of concrete.
Again, limestone was found to be one of the aggregates exhibiting the least drying
shrinkage while, in the study, sandstone exhibited the highest amount of drying
shrinkage. The amount of aggregate used in a concrete mixture can also help to reduce
shrinkage. Research has shown that a higher aggregate content can help to reduce
shrinkage. Table 2.3 shows the aggregate type related to drying shrinkage according to
Burrows report in 1998.

37

Table 2.3: Aggregate type related to drying shrinkage [18]
Aggregate One-year shrinkage (percent)
Sandstone 0.097
Basalt 0.068
Granite 0.063
Limestone 0.050
Quartz 0.040

2.3.6.1.2 Cement Content and Type

The amount of cement proportioned in concrete mixtures has an impact on the
amount of shrinkage that concrete will undergo. Specifically, concrete cracking has
been more prevalent when higher cement contents have been used. Krauss and Rogalla,
using a ring shrinkage test, found that cracking occurred sooner as the cement content of
the concrete mixes was increased [19]. Water-cement ratio also influences shrinkage
behavior in concrete. Krauss and Rogalla found that the concrete with more water
shrinks and creeps more than concrete with less water, but it may not crack sooner
because it has higher creep [19]. Burrows contends that although concrete mixes with
lower water-cement ratios produce stronger concrete, that same concrete can be much
more vulnerable to cracking. The type of cement used also plays an important role in
reducing shrinkage cracking. Krauss and Rogalla noted that cements that are ground
finer and have higher sulfate contents increase the early strength of concrete while also
increasing the early modulus of elasticity and heat of hydration [19]. For example, Type
III cement could increase the risk of cracking because of the rapid early strength gains.

38

2.3.6.1.3 Admixtures

Fly ash, silica fume, set retarders, and accelerators are all admixtures that have
been investigated for shrinkage by a number of researchers.

Fly ash has been found to reduce early concrete temperatures and the rate of
strength gain, thus reducing concrete cracking. The process of using fly ash to replace
cement is referred to as the creation of extensible concrete and is described in detail
following this section.

Silica fume, a by-product of silicon metal or ferrosilicon alloys in electric arc
furnaces, has been found to increase the cracking of concrete. The silica fume product
has an average fineness of about two orders of magnitude finer than Portland cement,
which causes the bleeding rate of concrete to decrease, and the subsequent water loss
resulting from evaporation cannot be replaced. Silica fume is found to be a problem
with cracking tendency specifically when the concrete is not cured properly.

Retarders have not been proven either to be the cause of concrete cracking or to
help reduce the risk of thermal cracking. Plastic cracking could be caused by the
addition of retarders, while retarders have also been found to reduce the risk of thermal
cracking by reducing early heat of hydration in concrete.

39

2.3.6.2 Innovative Method

Because of the extreme variance of the conventional methods used to control
drying shrinkage, innovative methods should be used to help reduce cracking tendencies
of concrete. These include fiber-reinforced concrete, shrinkage-reducing admixtures,
shrinkage-compensating concrete, and extensible concrete.

2.3.6.2.1 Fiber-Reinforced Concrete

Many studies have shown that adding fibers to concrete significantly reduces
shrinkage cracking. Various parameters that were investigated include the addition of
fibers at low volumes as compared to high volumes, as well as the different types of
fibers to be used.

Steel fibers can affect the properties of concrete, but the reinforced properties
depend on the percentage of fiber addition, the aspect ratio of the fibers, and the strength
of the concrete paste. Longer fibers provide more strength but decrease workability.
For this reason, fibers with an aspect ratio of less than 100 are commonly used. Steel
fiber reinforced concrete has been shown to increase the tensile strength, flexural
strength, and compressive strength of concrete through research. Tests have shown that
steel fibers do not affect the shrinkage strain of concrete, but the fibers can reduce the
amount of cracking associated with the shrinkage strain.

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Low volume of polypropylene fibers can significantly reduce the plastic
shrinkage of concrete. For low-volume fiber reinforcement typically 0.1%-0.3% has
little effect on the properties of the hardened concrete. However, high volumes of fiber,
generally greater than 2%, can increase the ductility and toughness of concrete. At high
volumes, polypropylene fibers can be used to prevent shrinkage cracking. The
shrinkage stress produced in the concrete is transferred to the fibers, which can better
withstand the tensile stresses than the concrete.

2.3.6.2.2 Shrinkage-Reducing Admixtures

A great deal of research has been performed regarding the development of SRAs
used to control shrinkage cracking of concrete. These chemical admixtures, which are
added to concrete work by lowering the surface tension of the pore water inside
hardened concrete. The pore water evaporates from capillary pores in the hardened
concrete during drying, and the tension in the liquid is transferred to the capillary walls,
resulting in shrinkage. Any stresses generated during drying are proportional to the
surface tension of the pore water solution. This surface tension is lowered by SRAs,
thus reducing the overall drying shrinkage. Therefore, there are fewer tendencies for
shrinkage and resultant stress to occur in the concrete when the pore water initially
evaporates. SRAs affect the nature of the pore water, rather than limiting or reducing
the amount of water from concrete during drying.

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2.3.6.2.2 Shrinkage-Compensating Concrete

Shrinkage-compensating concrete is an innovative material that causes expansion
of concrete during curing, which in turn reduces the effects of drying shrinkage. If the
expansion is properly restrained, the concrete will be subjected to compression the first
few days after concrete placement. Although the shrinkage-compensating concrete will
shrink as much as normal concrete once exposed to drying conditions, the net shrinkage
will be negligible because the concrete started out with an initial expansion. The
mechanism of expansion in the shrinkage-compensating concrete is a result of the early
formation and stability of ettringite. The ettringite crystals need water to expand, and
therefore, moist curing must provide this water, or else minimal expansion will result.

2.3.6.2.3 Extensible Concrete

Extensible concrete is a term that refers to a combination of factors that are
useful for reducing the cracking in concrete. Basically, some of the conventional
materials and methods mentioned previously can be used in an innovative manner to
achieve this type of behavior. A typical extensible concrete would have a high volume
of fly ash, low cement content, and a high water-cement ratio. These factors would
produce a low heat of hydration, thereby reducing thermal stresses in the concrete while
also producing a low elastic modulus and high creep, minimizing shrinkage cracking.

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2.4 Creep of Concrete

Creep of concrete, resulting from the action of a sustained stress, is a yielding of
the concrete. It may be due partly to viscous flow of the cement-water paste, closure of
internal voids and crystalline flow in aggregates, but it is believed that the major portion
is caused by seepage of colloidal water from the gel that is formed by hydration of the
cement. Although water may exist in the mass as chemically combined water, and as
free water in the pores between the gel particles, neither of these is believed to be
involved in creep. The rate of expulsion of the colloidal water is a function of the
applied compressive stress and of the friction in the capillary channels. The greater the
stress, the steeper the pressure gradient with resulting increase in rate of moisture
expulsion and deformation. The phenomenon is closely associated with that of drying
shrinkage.

Creep is defined as the increase in strain under a sustained stress. There is an
instantaneous strain on concrete which is called the “elastic” deformation when the
sample is unloaded. It is observed that there is a gradual increase in strain for days after
a stress has been applied to concrete. This is called the “creep” strain. Since this
increase can be several times as large as the strain on loading, creep is of considerable
importance in structural mechanics. The deformation of concrete with time is
schematically shown in Figure 2.10. The rate of creep is relatively rapid at early ages
after loading and then decreases gradually, until after a few years it becomes
insignificant. Roughly, about one-fourth of the ultimate creep occurs within the first
month or so, and one-half occurs within the first year.

43

Figure 2.10: Typical strain-time plot of concrete under a sustained load
and after release of load [16]

Creep in concrete is a post-elastic phenomenon. In practice, drying shrinkage
and viscoelastic behavior such as creep usually take place simultaneously. Considering
the various combinations of loading, restraining, and humidity conditions, the following
terms is defined:

i. True or Basic Creep

Defined as the creep that occurs under conditions that there is no drying
shrinkage or moisture movement between concrete and ambient
environment.

ii. Specific Creep

- Defined as creep strain per unit of applied stress.
- Specific Creep = εcr / σ

44

iii. Drying Creep

- Is the additional creep that occurs when the specimen under load is also
drying.

iv. Creep Coefficient

- Is defined as the ratio of creep strain to elastic coefficient.
- Creep Coefficient = εcr / εE

2.4.1 Creep Behavior of Concrete

Creep in concrete can have both positive as well as negative effects on the
performance of concrete structures. On the positive side, creep can relieve stress
concentrations induced by shrinkage, temperature changes, or the movement of supports.
For indeterminate beam with two fixed ends, creep deformation will be very useful in
reducing tensile stress caused by shrinkage and temperature variation.

In some concrete structures, creep can do hard to the safety of the structures.
Creep can lead to an excessive deflection of structural members, buckling or other
serviceability problems, especially in high-rise building, eccentrically loaded columns
and long bridges [8]. In mass concrete, creep may be a cause of cracking when a
restrained concrete mass undergoes a cycle of temperature change due to the
development of heat of hydration and subsequent cooling [8].

45

2.4.2 Components of Creep Strain

There are two components of creep strain which occur in a concrete member,
recoverable creep and irrecoverable creep [30], as show in Figure 2.11. the recoverable
component also know as delayed elastic strain εd(t), which is caused by the elastic
aggregates acting on the viscous cement paste after the applied stress is removed.

Figure 2.11: Recoverable and irrecoverable creep component [30]

While the irrecoverable component, also can be referred as flow, εf(t). It is
subdivided into rapid initial flow εfi(t), basic flow εfb(t) and drying flow εfd(t). Rapid
initial flow happens in the first 24 hours after loading and is the remaining flow which
develops gradually with time. While basic flow or basic creep is not dependent upon the
loss of moisture from the concrete and will occur with concrete protected from drying.

46

Meanwhile drying creep is the additional creep which occurs in a drying specimen.
However, drying creep, like drying shrinkage, is dependent upon the loss of moisture
from the concrete to its environment [31]. In normal structural engineering application,
one does not distinguish between basic and drying creep [33].

Therefore, the creep strain can be expressed as [30]

εf(t) = εd(t) + εf(t)
or
εf(t) = εd(t) + εfi(t) + εfb(t) + εfd(t)

as illustrated in Figure 2.12.

Figure 2.12: Creep components in a drying specimen [30]

47

2.4.3 Factors Affecting Creep

The magnitude of the creep depends upon several factors relating to the quality
of the concrete such as the aggregate-cement ratio, water-cement ratio, kind of aggregate
and its grading, composition and fineness of cement, and the age at time of loading. It
also depends upon the intensity and duration of stress, moisture content of the concrete,
the humidity of the ambient air, and the size of the mass.

2.4.3.1 Effect of Stress and Age When First Loaded

The greater the degree of hydration of the cement at the time of load application,
the lower the rate and total amount of creep. One explanation of this is that the
expulsion of moisture from the gel becomes more difficult as the porosity is decreased
through hydration. Since the extent of the hydration is indicated by the strength of a
given concrete, it can be said that creep varies inversely as the strength.

2.4.3.2 Effect of Water-Cement Ratio and Mix

Strength of concrete is determined by the water/cement ratio. The strength of
concrete reduces with the increasing of water/cement ratio. Concrete experience higher
creep due to higher water/cement ratio because the concrete has insufficient restraint due
to the tension force induced in the concrete. A higher water-cement ratio increases the

48

size of the pores in the paste structure, so that water may the more readily escape, and
then under a sustained load the water of adsorption may be expelled more readily to
cause a high rate of creep as shown in Figure 2.13.

Figure 2.13: Effect of water-cement ratio on creep [34]

2.4.3.3 Effect of Composition and Fineness of Cement

Cement is the most important factor in creep because the hydrated cement paste
is the source of the phenomenon. The influence of cement is twofold: that arising from
the physical and chemical properties of the cement. The composition of cement affects
the creep primarily through its influence upon the degree of hydration. Slow-hardening
cements such as low-heat Portland and Portland-pozzolan cements creep more than
cements which hydrate more rapidly. Creep seems to be inversely proportional to the
rapidly of hardening of the cement used. The more hardened the paste the more rigid it
is and the lower its creep potential at a given applied stress.

49

Figure 2.14: Creep in compression and tension for mass-cured concretes [34]

Figure 2.14 indicates that in both tension and compression the creep of concrete
made with low-heat cement is about one-third greater than for concrete made with
normal cement. This serves to explain why low-heat Portland and Portland-pozzolan
cements have served so effectively in relieving stresses in large dams as the mass cools
and have shown superior resistance to cracking.

2.4.3.4 Effect of Character and Grading of Aggregate

Aggregates play an important role in creep of concrete. Coarse aggregate
reduces creep deformation by reducing the cement paste content and restraining the
cement paste against contraction. Generally, concretes made with an aggregate, which is
hard and dense and have low absorption and high modulus of elasticity, are desirable
when low creep strain is needed.

50

The effect of mineral character of aggregate is shown in Table 2.4 for six
concretes. Same mineral aggregate was used from fine to coarse; the grading was the
same for all mixes. After carrying a sustained stress of 800 psi for about 5 years the
maximum creep (1300 millionths) was exhibited by the sandstone concrete and the
minimum (550 millionths) by the limestone.

Table 2.4: Effect of Mineral Character of Aggregate upon Creep [28]

As all aggregates were batched in a saturated, surface-dry condition, and their
absorption factors were generally low, the large variations in creep were not due to the
moisture conditions and the aggregates. Neither were they due to seepage from the
identical paste used in each mix. It is possible that variations in crystalline slip, particle
shape, surface texture, and pore structure of the aggregates may have had some
influence.

2.4.3.5 Effect of Moisture Conditions of Storage

Creep appears to be influenced by the humidity of the air in so far as it affects the
seepage of moisture from the concrete. Naturally, an increase in the humidity of the
atmosphere reduces the rate of loss of moisture or water vapor to the surrounding
atmosphere, slows down the flow of moisture or water vapor to the outer surface, and

51

thus reduces the seepage. Another factor affecting compressive creep is that drying
shrinkage at or near the surface results in a reduction of the cross-sectional area
remaining in compression and therefore causes higher stresses on the central core. A
high temperature and low relative humidity of the ambient environment accelerate the
diffusion of the adsorbed water and capillary water into the atmosphere, and
consequently, increases the creep of concrete. Therefore, the creep of concrete can be
concluded to be inversely proportional to the relative humidity.

The magnitude of creep for various moisture conditions of storage is shown in
Table 2.5. Although these values indicate that for a concrete loaded to 800 psi at the age
of 28 days the creep in air at 70 percent relative humidity was about double that for
water storage, for similar concrete loaded at 3 months to 1,200 psi the creep for the air
storage condition at 70 percent relative was about 2½ times that for water storage.

Table 2.5: Effect of Moisture Condition of Storage upon Creep [5]

2.4.3.6 Effect of Size of Mass

The larger the mass subjected to sustained loading, the less the creep. This is
probably due to the reduced seepage, as the path traveled by the expelled water is greater
with a resulting increase in the frictional resistance to the flow of water from the interior.
The general effect of size of specimen upon creep is shown in Figure 2.15, which

52

includes 6-, 8-, and 10-in. diameter cylinders stored in fog to eliminate the effect of
surface drying. These results show that creep in the 10-in. cylinder is only about one-
half that for the 6-in. cylinder. For storage in dry air, seepage may occur much more
readily from small specimens, and the effect of size on creep may become more
pronounced. Models were prepared using aggregate-cement ratio of 6.95 by weight;
water-cement ratio 0.61 by weight; age at loading 28 days; sustained stress 800 psi;
storage in fog before and after loading.

Figure 2.15: Effect of size of specimens upon creep [5]

2.4.4 Effect of Creep

Creep of plain concrete does not by itself affect strength, although under very
high stresses creep hastens the approach of the limiting strain at which failure takes
place. The influence of creep on the ultimate strength of a simply supported, reinforced
concrete beam subjected to a sustained load is insignificant, but deflection increases
considerably and may in many cases be a critical consideration in design. Another

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