PSZ 19:16 (Pind. 1/07)                                 UNIVERSITI TEKNOLOGI MALAYSIA    DECLARATION OF THESIS / UNDERGRADU...
DECLARATION   “I/We* hereby declare that I/We* have read this project report and in my/our*     opinion this project repor...
COMPARISON OF CREEP AND SHRINKAGE USING DIFFERENT CODE OF PRACTICE              NG PING CHEWA project report submitted in ...
iiI declare that this project report entitled “Comparison of Creep and Shrinkage UsingDifferent Code of Practice” is the r...
iii   DEDICATIONFor my beloved family
iv                               ACKNOWLEDGEMENT        The author is deeply indebted to his supervisor, Assoc. Prof. Ir. ...
v                                      ABSTRACTThis project presents a study on the behavior of creep and shrinkage of con...
vi                                   ABSTRAKProjek ini membincangkan kajian kelakuan rayapan dan pengecutan atas spesimenk...
vii                TABLE OF CONTENTSCHAPTER                 TITLE                            PAGE          DECLARATION    ...
viii2.3   Shrinkage of Concrete                       13      2.3.1 Type of Shrinkage                     14             2...
ix    2.4   Creep of Concrete                             42          2.4.1 Creep Behavior of Concrete              44    ...
x          3.2.4 Eurocode                        68                  3.2.4.1 Eurocode (Annex B)    71    3.3   Creep      ...
xi          5.3.5 Comparison of Creep Using                Different Standard              1036   CONCLUSION AND RECOMMEND...
xii                            LIST OF TABLESTABLE NO.                        TITLE                               PAGE   2...
xiii                            LIST OF FIGURESFIGURE NO.                         TITLE                                 PA...
xiv2.7    Relation between axial shrinkage and width of square      31       cross-section and length/width ratio of 42.8 ...
xv3.4    Coefficient Kj                                               603.5    Drying shrinkage of normal-weight concrete ...
xvi5.2    Relationship between shrinkage and relative humidity       92       based on BS 81105.3    Relationship between ...
xvii5.12   Relationship between creep and duration based on EC 2   104
xviii                            LIST OF SYMBOLSεcs / εss   –   shrinkage straincs          –   modification factor to all...
xixkh         –   coefficient depending on h0fcm        –   the mean compressive strength (MPa)αds1       –   coefficient ...
xx                        ABBREVIATIONBS 8110   -   British Standard 8110AS 3600   -   Australian Standard 3600EC 2      -...
CHAPTER 1                                    INTRODUCTION1.1     Background        Concrete is a composite building materi...
2       The behavior of hardened concrete can be characterized in terms of its short-term(essentially instantaneous) and l...
3       In predicting the strength and serviceability of reinforced and pre-stressedconcrete structures, appropriate descr...
4       Cracking not only impairs the ability of a structure to carry its design load butalso affect its durability and da...
5              Figure 1.1: Relationship between concrete strain and time [8]1.2    Serviceability of Concrete Structures  ...
6appropriately low value and ensuring the bonded reinforcement is suitably distributed.For deflection control, engineer sh...
7       Deflection problems that may affect the serviceability of concrete structures canbe classified into three main typ...
8techniques are available for the prediction of creep and shrinkage on concrete members.However, each has its own simplify...
91.5     Scope of Work        Time-dependent concrete deformation is nowadays one of the concerns inengineering field as i...
101.6    Expected Outcome       There are some outcomes to be expected through this master research studiessuch as:       ...
CHAPTER 2                               LITERATURE REVIEW2.1    Significance of Volume Changes and Creep       If concrete...
12       Creep, in general, tends to relieve the stress in concrete, especially whenreinforced. Thus, when a sustained loa...
13       Water is held in the pores of the gel by such large attractive forces that when it isremoved from the pores by ev...
142.3.1   Type of Shrinkage        The four main types of shrinkage associated with concrete are plastic shrinkage,autogen...
152.3.1.2 Autogenous Shrinkage       Autogenous shrinkage is the volume change of the cement paste due to self-desiccation...
16       Although autogenous shrinkage is three-dimensional, it is usually expressed as alinear stain so that it can be co...
17          According Mehta and Monteiro the change in volume of drying concrete is notequal to the volume of water remove...
18movement associated with shrinkage is inter- or intracrystalline. But, because pastemade with both Portland and high-alu...
192.3.1.4 Carbonation Shrinkage       In addition to shrinkage upon drying, concrete undergoes shrinkage due tocarbonation...
20       Carbonation shrinkage is probably caused by the dissolving of crystals ofCa(OH)2 while under a compressive stress...
21       The sequence of drying and carbonation greatly affects the total magnitude ofshrinkage. Simultaneous drying and c...
222.3.2   Factors Affecting Shrinkage        Generally, plastic shrinkage results from surface evaporation due toenvironme...
232.3.2.1 Effect of Composition and Fineness of Cement       Cement properties and cement content in concrete influence co...
24shrinkages of the same cement diluted with a single sieve size (No. 4 to 3/8 in.) of graveland crushed limestone, respec...
25increase in the cement content because this results in a larger volume of hydrated cementpaste which is liable to shrink...
26use of a set retarder. Conversely, set accelerators increase the heat of hydration andearly-age shrinkage.    This combi...
27       Entrainment of air has been found to have no effect on shrinkage [15]. Addedcalcium chloride increases shrinkage,...
282.3.2.7 Volume and Type of Aggregate       Drying shrinkage of concrete is a fraction of that of neat cement because the...
292.3.2.8 Elastic Modulus of Aggregate        Modulus of elasticity is the most important property of aggregate that direc...
30Because relaxation occurs only slowly, it may prevent cracking when shrinkagedevelops slowly; however, the same magnitud...
31    Figure 2.7: Relation between axial shrinkage and width of concrete prisms of                square cross-section and...
322.3.4   Shrinkage-induced Cracking        As mentioned in connection with differential shrinkage, the importance ofshrin...
33moisture movement, and therefore is advantageous from the standpoint of crackingtendency. On the other hand, the presenc...
342.3.5     Effect of Shrinkage          Virtually all concrete is subject to some form of restraint, such as steelreinfor...
35shrinkage cracking cannot be entirely prevented. For example, concrete in hot, dry, andwindy conditions can have much hi...
362.3.6.1 Conventional Method       Shrinkage cracking in concrete is currently being controlled throughconventional metho...
37              Table 2.3: Aggregate type related to drying shrinkage [18]                 Aggregate                      ...
382.3.6.1.3   Admixtures       Fly ash, silica fume, set retarders, and accelerators are all admixtures that havebeen inve...
392.3.6.2 Innovative Method       Because of the extreme variance of the conventional methods used to controldrying shrink...
40       Low volume of polypropylene fibers can significantly reduce the plasticshrinkage of concrete. For low-volume fibe...
412.3.6.2.2   Shrinkage-Compensating Concrete       Shrinkage-compensating concrete is an innovative material that causes ...
422.4    Creep of Concrete       Creep of concrete, resulting from the action of a sustained stress, is a yielding ofthe c...
43        Figure 2.10: Typical strain-time plot of concrete under a sustained load                                  and af...
44        iii.       Drying Creep               -   Is the additional creep that occurs when the specimen under load is al...
452.4.2   Components of Creep Strain        There are two components of creep strain which occur in a concrete member,reco...
46Meanwhile drying creep is the additional creep which occurs in a drying specimen.However, drying creep, like drying shri...
472.4.3   Factors Affecting Creep        The magnitude of the creep depends upon several factors relating to the qualityof...
48size of the pores in the paste structure, so that water may the more readily escape, andthen under a sustained load the ...
49    Figure 2.14: Creep in compression and tension for mass-cured concretes [34]       Figure 2.14 indicates that in both...
50       The effect of mineral character of aggregate is shown in Table 2.4 for sixconcretes. Same mineral aggregate was u...
51thus reduces the seepage. Another factor affecting compressive creep is that dryingshrinkage at or near the surface resu...
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
Creep and shrinkage using different code
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Creep and shrinkage using different code

  1. 1. 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 (full text) I acknowledged that Universiti Teknologi Malaysia reserves the right as follows: 1. The thesis is the property of Universiti Teknologi Malaysia. 2. The Library of Universiti Teknologi Malaysia has the right to make copies for the purpose of research only. 3. The Library has the right to make copies of the thesis for academic exchange. 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 2008NOTES : * If the thesis is CONFIDENTAL or RESTRICTED, please attach with the letter from the organization with period and reasons for confidentiality or restriction.
  2. 2. 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 : …………………………………..………* Delete as necessary
  3. 3. COMPARISON OF CREEP AND SHRINKAGE USING DIFFERENT CODE OF PRACTICE NG PING CHEWA 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
  4. 4. iiI declare that this project report entitled “Comparison of Creep and Shrinkage UsingDifferent 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 concurrentlysubmitted in candidate of any degree. Signature : ………………………………………… NG PING CHEW Name : ………………………………………… Date : 18 NOVEMBER 2008 ………………………………………….
  5. 5. iii DEDICATIONFor my beloved family
  6. 6. iv ACKNOWLEDGEMENT The author is deeply indebted to his supervisor, Assoc. Prof. Ir. Dr. WahidOmar whose help, stimulating suggestions and encouragement helped him in all thetime 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 ofCivil 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 hisparents, Ng Soon and Ken Kim Moy, for unconditional support and encouragementto pursue his interest.
  7. 7. v ABSTRACTThis project presents a study on the behavior of creep and shrinkage of concretespecimens. Prediction of creep and shrinkage strain was studied and compared basedon British Standard 8110, Eurocode 2 and Australian Standard. The objective of thisstudy is to understand the concrete behavior of creep and shrinkage and to produce aspread sheet of determining creep and shrinkage strain based on the three standardsmentioned. The spread sheet will be used to ease future engineers in estimatingcreep and shrinkage strain of concrete in structural design work. Creep andshrinkage are two important time-dependent properties of concrete as it causescracking and adversely affects the functionality, durability and appearance ofstructure. There are many parameters that affect the concrete creep and shrinkagestrain such as concrete strength, type of cement, relative humidity, effectivethickness, days of loading, etc. In this study, relative humidity of the environmentwas used as the controlled parameter in comparing the creep and shrinkage strain.Australian Standard was found to be limited in determining creep and shrinkagestrain because it is based on climatic zones in Australia. However, prediction usingAustralian Standard is still being considered as each zone has its own range ofrelative humidity. Graph and formula method for Eurocode were considered in thisstudy. Both of the methods gave acceptable results. In creep and shrinkage straincomparison, Eurocode present acceptable result with more conservative strain. Thiscode of practice is preferable in determining concrete creep and shrinkage among thestandards researched in this study.
  8. 8. vi ABSTRAKProjek ini membincangkan kajian kelakuan rayapan dan pengecutan atas spesimenkonkrit. Anggapan rayapan dan pengecutan konkrit telah dikaji dan perbandingantelah dibuat berdasarkan British Standard 8110, Eurocode 2 dan Australian Standard.Tujuan kajian ini adalah untuk memahami kelakuan rayapan dan pengecutan konkritserta menghasilkan spread sheet yang berfungsi untuk menentukan nilai rayapan danpengecutan berpandukan ketiga-tiga kod rekabentuk tersebut. Spread sheet ini akanmenyenangkan kerja jurutera dengan menentukan nilai rayapan dan pengecutankonkrit dalam rekabentuk struktur. Rayapan dan pengecutan merupakan ciri-ciripenting konkrit. Rayapan dan pengecutan akan menyebabkan keretakan konkrit,dimana menjejaskan struktur tersebut dari segi fungsi, ketahanan dan persembahanluarannya. Terdapat banyak parameter yang mempengaruhi rayapan dan pengecutankonkrit seperti kekuatan konkrit, jenis simen, kelembapan bandingan, kedalamanberkesan, masa pembebanan dan sebagainya. Dalam kajian ini, kelembapanbandingan alam persekitaran telah digunakan sebagai parameter pengawal dalambandingan rayapan dan pengecutan. Didapati bahawa Australian Standard adalahterhad dalam menentukan nilai rayapan dan pengecutan disebabkan kod tersebutadalah berdasarkan zon cuaca Australia. Walau bagaimanapun, anggapan bagi nilairayapan dan pengecutan konkrit berdasarkan Australian Standard masih dilaksanakandalam 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. Dalamperbandingan nilai rayapan dan pengecutan, Eurocode menghasilkan keputusandengan konsevatif. Antara kod-kod yang digunakan, kod ini adalah lebih digemaridalam kajian ini untuk menentukan nilai rayapan dan pengecutan.
  9. 9. vii TABLE OF CONTENTSCHAPTER 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
  10. 10. viii2.3 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
  11. 11. ix 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 553 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
  12. 12. x 3.2.4 Eurocode 68 3.2.4.1 Eurocode (Annex B) 71 3.3 Creep 72 3.3.1 Creep Strain 72 3.3.2 British Standard 75 3.3.3 Australian Standard 77 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) 824 METHODOLOGY 86 4.1 Introduction 86 4.2 Information Gathering 86 4.3 Preparation of Spread Sheet 885 ANALYSIS OF RESULTS 89 5.1 Introduction 89 5.2 Shrinkage 89 5.2.1 Shrinkage Strain 90 5.2.2 British Standard 91 5.2.3 Australian Standard 92 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 5.3.2 British Standard 99 5.3.3 Australian Standard 100 5.3.4 European Standard 101
  13. 13. xi 5.3.5 Comparison of Creep Using Different Standard 1036 CONCLUSION AND RECOMMENDATIONS 105 6.1 Conclusions 105 6.2 Recommendations for Further Studies 107 REFERENCES 105 APPENDIX 109
  14. 14. xii LIST OF TABLESTABLE 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
  15. 15. xiii LIST OF FIGURESFIGURE 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
  16. 16. xiv2.7 Relation between axial shrinkage and width of square 31 cross-section and length/width ratio of 42.8 Relation between ultimate shrinkage and 31 volume/surface ratio2.9 Schematic pattern of crack development when tensile 33 stress due to restrained shrinkage is relieved by creep2.10 Typical strain-time plot of concrete under sustained 43 load and after release of load2.11 Recoverable and irrecoverable creep component 452.12 Creep components in drying specimen 462.13 Effect of water-cement ratio on creep 482.14 Creep in compression and tension for mass-cured 49 concretes2.15 Effect of size of specimens upon creep 523.1 Coefficient KL 593.2 Coefficient Kc 593.3 Coefficient Ke 60
  17. 17. xv3.4 Coefficient Kj 603.5 Drying shrinkage of normal-weight concrete 633.6 Shrinkage strain coefficient (k1) for various environments 663.7 Climatic Zones in Australia 673.8 Coefficient KL 733.9 Coefficient Km 743.10 Coefficient Ke 743.11 Effects of relative humidity, age of loading and section 76 thickness upon creep factor3.12 Creep factor coefficient (k2) for various environments 793.13 Maturity coefficient (k3) 793.14 Method for determining the creep coefficient for concrete 82 under normal environmental conditions5.1 Relationship between shrinkage and relative humidity 90 based on code of practice specified by Hong Kong government
  18. 18. xvi5.2 Relationship between shrinkage and relative humidity 92 based on BS 81105.3 Relationship between shrinkage and relative humidity 93 based on AS 36005.4 Relationship between shrinkage and relative humidity 94 using table and formula method based on EC 25.5 Relationship between shrinkage and duration based on 95 EC 25.6 Comparison of shrinkage using different code of practice 975.7 Relationship between creep and relative humidity based 98 on code of practice specified by Hong Kong government5.8 Relationship between creep and relative humidity 100 based on BS 81105.9 Relationship between creep and relative humidity 101 based on AS 36005.10 Relationship between creep and relative humidity 102 using graph and formula method based on EC 25.11 Comparison of creep using different standards 103
  19. 19. xvii5.12 Relationship between creep and duration based on EC 2 104
  20. 20. xviii LIST OF SYMBOLSεcs / εss – shrinkage straincs – modification factor to allow for properties of crushed granitic aggregateKL – 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 timeKs – reinforcement coefficientαe – modular ratio Es/Ecρ – steel ratio As/AcAs – total area of longitudinal reinforcementAc – gross cross-sectional concrete areaEs – modulus of elasticity of the reinforcementEc – short-term modulus of concreteεcs.b – basic shrinkage straink1 – shrinkage strain coefficientεsd(t) – drying shrinkage strain in timeεsd(t) – nominal unrestrained drying shrinkageεsa – autogeneous shrinkage strainh0 – effective thicknessu – perimeter of the member in contact with the atmosphere
  21. 21. xixkh – coefficient depending on h0fcm – the mean compressive strength (MPa)αds1 – coefficient which depends on the type of cementαds2 – coefficient which depends on the type of cementE28 – static modulus of elasticity at 28 daysEt – modulus of elasticity at an age tEu – modulus of elasticity at age of unloadingØc – creep coefficient depending on KL, Km, Kc, Ke, KjKm – coefficient relating to the hardening (maturity) of the concreteØcc.b – basic creep factor of concretek2 – creep factor coefficientk3 – maturity coefficientφ – creep coefficientkσ – stress-strength ratio σc/fcmφ0 – notional creep coefficientφRH – factor to allow for the effect of relative humidity on the notional creep coefficientβc(t,t0) – coefficient to describe the development of creep with time after loadingβH – coefficient depending on the relative humidity and the notional member sizet – age of concrete in days at the moment consideredt0 – age of concrete at loadings in daysfcu,28 – 28 day cube strength in N/mm2fcu,t – cube strength at an age tσc – compressive stress in concrete
  22. 22. xx ABBREVIATIONBS 8110 - British Standard 8110AS 3600 - Australian Standard 3600EC 2 - European Standard Eurocode 2RH - Relative Humidity
  23. 23. CHAPTER 1 INTRODUCTION1.1 Background Concrete is a composite building material made from the combination ofaggregate and cement binder. The most common form of concrete consists of Portlandcement, mineral aggregates (generally gravel and sand) and water. Contrary to commonbelief, concrete does not solidify from drying after mixing and placement. Instead, thecement 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 termconcrete. The quality of concrete can be assessed from several characteristics, namely itsstrength, durability, creep and shrinkage. These are the most important and commoncriteria used to grade a concrete into its quality level. A concrete of good quality shouldbe able to work up to the structural ability for which it is designed for, and also to lastfor at least the design lifetime for which it is designed for.
  24. 24. 2 The behavior of hardened concrete can be characterized in terms of its short-term(essentially instantaneous) and long-term properties. Short-term properties includestrength in compression, tension, bond, and modulus of elasticity [17]. The long-termproperties include creep, shrinkage, behavior under fatigue, and durability characteristicssuch as porosity, permeability, freeze-thaw resistance, and abrasion resistance [17]. Concrete is one of the most durable construction materials. However, crackingadversely affects its durability, functionality, and appearance. A major cause ofcracking is related to shrinkage-induced strains, creating stresses when concrete isrestrained [4]. The shrinkage of concrete is often attributed to drying of the concreteover a long period of time, and recent observations have also focused on early ageshrinkage and creep problems. Cracked concrete typically needs to be repaired toprevent further deterioration due to freezing and thawing, and corrosion of steelreinforcement resulting from infiltration of water with or without chloride ions from de-icing salts. The cracking leads to additional costs for repair to prevent prematuredeterioration of the concrete and the corrosion of reinforcement steel. The early age of concrete is known to have a significant control on the overallperformance of concrete structures. During this stage, concrete may be subjected tosevere internal actions due to thermal and hydric gradients within concrete itself and atthe 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 justbuilding its resistance and stiffness. Creep and shrinkage of concrete are known to havesignificant effect at early age of concrete. Thus, discussing the performance of thisyoung age concrete with special attention to the shrinkage and creep and time dependentdeformations is of interest by many researchers.
  25. 25. 3 In predicting the strength and serviceability of reinforced and pre-stressedconcrete structures, appropriate descriptions of the mechanical properties of thematerials 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 ofconcrete cracking and stripping and unshoring times [17]. The mechanical properties ofconcrete are significantly affected by the temperature and availability of water duringcuring, the environmental humidity and temperature after curing, and the composition ofthe concrete, including the mechanical properties of the aggregates. When concrete is subjected to sustained compressive stress, deformationscontinue to increase with time due to creep and shrinkage. Creep strain is produced bysustained stress. Shrinkage strains are independent of stress and are caused by chemicalreactions in the hydrating cement paste and by the loss of water during the dryingprocess. The creep and shrinkage deformations in a concrete structure are frequentlylarger, and in some cases much larger than the initial deformations produced when theexternal loads are first applied [7]. They thus have a significant effect on service-loadbehavior. The resistance to deformation that makes concrete a useful material means alsothat volume changes of the concrete itself can have important implications in use. Anypotential growth or shrinkage may lead to complications, externally because of structuralinteraction with other components or internally when the concrete is reinforced. Theremay even be distress if either the cement paste or the aggregate changes dimension, withtensile stresses set up in one component and compressive stresses in the other. Cracksmay be produced when the relatively low tensile strength of the concrete or itsconstituent materials is exceeded.
  26. 26. 4 Cracking not only impairs the ability of a structure to carry its design load butalso affect its durability and damage its appearance. In addition, shrinkage and creepmay increase deflections in one member of a structure, adversely affecting the stabilityof the whole. Volume change of concrete is not usually associated with changes thatoccur before the hardened state is attained. Quality and durability, on the other hand, aredependent 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 fromwidening. The use of expansive cements, coal-combustion products containing calciumsulfite or sulfate, and fibers is one way of counteracting shrinkage. Usually, expansivecements and clean-coal ash produce expansion by formation of ettringite. When theexpansion is restrained by reinforcement, a compressive pre-stress is induced inconcrete, compensating shrinkage. Figure 1.1 illustrates the relationship between various measured and derivedstrain values. The figure shows that the concrete undergoes autogenous shrinkagebefore drying. Once drying commences at time t0, drying shrinkage occurs. Uponloading, both drying and basic creep occurs in the drying specimen.
  27. 27. 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 anddeflections must not be excessive. The design for serviceability is possibility the mostdifficult and least well understood aspect of the design of concrete structures. Serviceload behavior depends primarily on the properties of the concrete and these are often notknown reliably at the design stage. Concrete behaves in a non-linear and inelasticmanner at service loads and the non-linear behavior that complicates serviceabilitycalculations is due to cracking, tension stiffening, creep, and shrinkage. The control of cracking in a reinforced or pre-stressed concrete structure isusually achieved by limiting the stress increment in the bonded reinforcement to some
  28. 28. 6appropriately low value and ensuring the bonded reinforcement is suitably distributed.For deflection control, engineer should select maximum deflection limits that areappropriate to the structure and its intended use. The calculated deflection must notexceed these limits. The quest for serviceable concrete structures must involve the development ofmore reliable design procedures. It must also involve designers giving more attention tothe specification of an appropriate concrete mix, particularly with regard to the creepand shrinkage characteristics of the mix, and sound engineering input is required in theconstruction procedures. When designing for serviceability, engineer must ensure that the structure canperform its intended function under the day to day service loads. Deflection must not beexcessive, cracks must be adequately controlled and no portion of the structure shouldsuffer excessive vibration. Shrinkage cause time-dependent cracking, thereby reducingthe stiffness of a concrete structure, and is therefore a detrimental factor in all aspects ofthe design for serviceability. Excessive wide cracks can be unsightly and spoilt the appearance of an exposedconcrete surface. They allow the ingress of moisture accelerating corrosion of thereinforcement and durability failure. In exceptional cases, they reduce the contributionof the concrete to the shear strength of a member. Excessively wide cracks in floorsystems and walls may often be avoided by the inclusion of strategically placedcontraction joints, thereby removing some of the restraint to shrinkage and reducing theinternal tension. When cracking occurs, in order to ensure that crack widths remainacceptably small, adequate quantities of well distributed and well-anchoredreinforcement must be included at every location where significant tension will exist.
  29. 29. 7 Deflection problems that may affect the serviceability of concrete structures canbe 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 thestructural maintenance after its completion, but also for the stress and deformationcontrol during the erection stages of the structure. Most of the engineers today do not consider the concrete behaviour of creep andshrinkage in their design work because of lacking experience and understanding on thephenomenon and the effect on concrete specimen. Most of them consider creep andshrinkage cracks as non-structural cracks which is not important and will not cause anyserious effect on concrete specimen. This assumption and consideration is not truebecause cracking deteriorate concrete’s durability and integrity. A number of analytical
  30. 30. 8techniques 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 asparameters used in BS are based on the conditions in Europe which may not beaccurately applicable in Malaysia. Therefore, the study is mainly concentrates on the understanding of concretebehaviour due to creep and shrinkage and to study the prediction of creep and shrinkagestrain 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.
  31. 31. 91.5 Scope of Work Time-dependent concrete deformation is nowadays one of the concerns inengineering 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 areshrinkage and creep. Therefore, the research on this topic has been proposed in the FinalYear Project of Master Studies (Civil-Structural) in Universiti Technologi Malaysia. In Masters Pre Project, the scope of work was mainly focused on the literaturereview of related studies. Substantial information on concrete properties such asmodulus of elasticity, creep and shrinkage will be gathered through latest journals andpublications in libraries and also articles from internet. The history of concrete, effect ofadmixtures on concrete properties and factors affecting deterioration on concrete and theeffects 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 usedin predicting concrete deformation due to creep and shrinkage will be identified usingBritish Standard, EURO Code and Australia Standard. Spread sheet to determineconcrete creep and shrinkage will be produced by inputting the controlling parameterssuch as strength of concrete (fcu), relative humidity, type of cement, effective thickness,provided steel reinforcement, etc.
  32. 32. 101.6 Expected Outcome There are some outcomes to be expected through this master research studiessuch 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.
  33. 33. CHAPTER 2 LITERATURE REVIEW2.1 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 concretesubject to different conditions. As the potential movement is thus restrained, stresseswill be developed which may rupture the concrete. This is particularly true whentension is developed; thus, contractions causing tensile stress are more important thanexpansions which cause compressive stress. Difference in moisture contents of theexposed 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 ofany structure to carry its designed loads, but it also may affect its durability and damageits appearance. The durability is affected by the entry of water through cracks, whichcorrodes the steel, leaches out soluble components, and deteriorates the concrete whensubjected to freezing and thawing.
  34. 34. 12 Creep, in general, tends to relieve the stress in concrete, especially whenreinforced. 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 acorresponding increase in the load on the steel. In various structural elements such ascontinuous beams and slabs, creep relieves some of the stress in the most highly stressedportions and increases the stress in adjacent portions of the concrete, so that finally thestresses are more uniform throughout the member. This relieving of the higher stressesserves to reduce the tendency toward cracking. However, creep may causeobjectionable 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 silicategel resulting from the combination of cement and water. The amount of the gelincreases with the age of hydration and is greater for higher water-cement ratios and forfiner cements. The amount of gel also depends upon the chemical composition of thecement, as fully hydrated dicalcium silicate is believed to be mostly gel, while hydratedtricalcium silicate is more than half gel. For the water-cement ratios used in averageconcrete, the gel has a larger volume than the crystalline portions. The crystalline materials in cement are believed to be unaffected by ordinarydrying, but the gel is finely porous and undergoes large volume changes upon wettingand drying. The quantity and characteristics of the calcium silicate gel, therefore,largely determine the potential shrinkage upon drying of hydrated cement.
  35. 35. 13 Water is held in the pores of the gel by such large attractive forces that when it isremoved from the pores by evaporation, the forces which formerly attracted the waterbecome 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 involvedis so to control conditions that the volume changes have small or practically harmlesseffects upon the integrity of the structure.2.3 Shrinkage of Concrete Concrete deformation due to movement of water from or to the ambient mediumwhen no external stress is acting is termed shrinkage. It is independent of stress and iscaused by chemical reactions in the hydrating cement paste and by the loss of waterduring the drying process. Technically, shrinkage will continue for the life of theconcrete, but most shrinkage will occur within the first 90 days after placement [33]. Shrinkage cracking is a major cause of concern for concrete structures. Inaddition to weakening the structure, these shrinkage cracks have the potential to allowinfiltration of moisture and chloride ions that accelerate the corrosion of steelreinforcement and reduce the durability of concrete.
  36. 36. 142.3.1 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 concreteinto the surrounding environment. Plastic shrinkage occurs only in fresh concrete. Themost common mechanism is the evaporation of water from the surface of the plasticconcrete. However, the loss of water through the sub-base or formwork can exacerbatethe effects of surface evaporation [33]. In the fresh concrete, the particles are completely surrounded by water. If wateris removed from the system, menisci are formed between particles. These meniscigenerate negative capillary pressure, which pulls the cement particles together. Bypulling on the particles, the capillary stresses tend to reduce the volume of the cementpaste. 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 concreterearranges to form discrete zones with voids between them. Plastic shrinkage crackingoccurs at this point.
  37. 37. 152.3.1.2 Autogenous Shrinkage Autogenous shrinkage is the volume change of the cement paste due to self-desiccation and chemical shrinkage after initial setting has occurred. Autogenousshrinkage is a microscopic volume change occurring after the initial setting in situationswhere the supply of water from outside of concrete is not enough. As the hydration ofcementitious materials progresses, very fine pores are produced within the hardenedcement paste due to the formation of calcium silicate hydrate (CSH) gel. As thehydration further progresses, capillary pore water and gel water is consumed andmenisci are produced in these pores due to a lack of water supply from outside. As aresult of negative pressure in the pores, hardened paste shows shrinkage [18]. Autogenous shrinkage is the early shrinkage of concrete caused by the loss ofwater from capillary pores due to the hydration of cementitious materials, without theloss of water into the surrounding environment. This phenomenon is known as self-desiccation of concrete. Self-desiccation occurs in all concrete irrespectively of thewater-cement ratio. However, its effects are very different in normal concrete and high-performance concrete. In high-performance concrete, significantly more cementitiousmaterials and less mixing water are used compared with normal concrete. In normalconcrete, there is substantially more water than required for hydration of cementitiousmaterials particles. This excess amount of water is contained in well-connectedcapillaries. Menisci created by the process of self-desiccation occur in large capillaries.But, stresses generated in large capillaries are very low, resulting in lower autogenousshrinkage. On the other hand, in case of high-performance concrete, pore network isessentially composed of fine capillaries due to low water-cement ratio and high amountsof cementitious hydration products. When self-desiccation starts to take place, very hightensile stresses are generated in these fine pores, resulting in higher autogenousshrinkage.
  38. 38. 16 Although autogenous shrinkage is three-dimensional, it is usually expressed as alinear stain so that it can be considered alongside the drying shrinkage. Typical valuesof autogenous shrinkage are about 40 x 10-6 at the age of one month and 100 x 10-6 afterfive years. Autogenous shrinkage tends to increase at higher temperatures, with highercement content, and possibly with finer cements, and with cements which have a highC3A and C4AF content. At a constant content of blended cement, a higher content of flyash leads to lower autogenous shrinkage. As self-desiccation is greater at lowerwater/cement ratios, autogenous shrinkage could be expected to increase but this maynot occur because of the more rigid structure of the hydrated cement paste at lowerwater/cement ratios. Nevertheless, at very low water/cement ratios, autogenousshrinkage is very high: a value of 700 x 10-6 was reported for concrete with awater/cement ratio of 0.17 [1].2.3.1.3 Drying Shrinkage Drying shrinkage is different from autogenous shrinkage with regard to themechanism of a decrease in humidity. Drying shrinkage is caused by the diffusion ofwater from concrete into the outer surrounding environment. Drying shrinkage refers to the reduction in concrete volume resulting from theloss of capillary water by evaporation. This shrinkage causes an increase in tensilestress of restrained concrete, which leads the concrete to cracking, internal warping, andexternal deflection, even if the concrete is not subjected to any kind of external loading.
  39. 39. 17 According Mehta and Monteiro the change in volume of drying concrete is notequal to the volume of water removed [2]. The reason is that the loss of water fromlarge capillaries may be considered as free water, and its removal does not cause volumechange. Loss of water held by capillary tension in small capillaries may cause shrinkageof concrete. It is also possible that shrinkage is related to the removal of interlayerwater, which is also known as zeolite water. It has been suggested that a monomolecularwater layer between the layers of CSH is strongly held by hydrogen bonding. Thiswater is associated with CSH structure and the interlayer water is lost only on strongdrying. The drying shrinkage of hydrated cement paste begins at the surface of theconcrete. Depending on the relative humidity of the ambient air and the size ofcapillaries in the cement paste structure, drying shrinkage progresses more or lessrapidly through concrete. The drying in ordinary concrete is, therefore, rapid becausethe capillary network is well connected and contains large capillaries. In the case ofhigh-performance concrete, drying shrinkage is slow because the capillaries are veryfine and soon get disconnected by hydration products. The influence of the gel particle size on drying is shown by the low shrinkage ofthe much more coarse-grained natural building stones and by the high shrinkage of finegrained shale [3]. Also, high-pressure steam-cured cement paste, which ismicrocrystalline and has a low specific surface, shrinks 5 to 10 times [4], and sometimeseven 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 ofintracrystalline water. Calcium silicate hydrate has been shown to undergo a change inlattice spacing from 1.4 to 0.9 nm on drying [6]; hydrated C4A and calciumsulfoaluminate show similar behavior [7]. It is thus not certain whether the moisture
  40. 40. 18movement associated with shrinkage is inter- or intracrystalline. But, because pastemade with both Portland and high-alumina cements, and also with pure ground calciummonoaluminate, exhibit essentially similar shrinkage, the fundamental cause ofshrinkage must be sought in the physical structure of the gel rather than in its chemicaland mineralogical character [6]. The relation between the mass of water lost and shrinkage is shown in Figure2.1. For neat cement pastes, the two quantities are proportional to one another as nocapillary water is present and only adsorbed water is removed. However, mixes towhich pulverized silica has been added and which, for workability reasons, require ahigher water/cement ratio, contain capillary pores even when completely hydrated.Emptying of the capillaries causes a loss of water without shrinkage but, once thecapillary water has been lost, the removal of adsorbed water takes place and causesshrinkage 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]
  41. 41. 192.3.1.4 Carbonation Shrinkage In addition to shrinkage upon drying, concrete undergoes shrinkage due tocarbonation, and some of the experimental data on drying shrinkage include the effectsof carbonation. Drying shrinkage and carbonation shrinkage are, however quite distinctin nature. Carbonation shrinkage is caused by the chemical reactions of various cementhydration products with carbon dioxide present in the air. This type of shrinkage isusually limited to the surface of the concrete. Because of carbon dioxide is fixed by thehydrated cement paste, the mass of the latter increases. Consequently, the mass ofconcrete also increases. When the concrete dries and carbonates simultaneously, theincrease in mass on carbonation may at some stage give the misleading impression thatthe 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]
  42. 42. 20 Carbonation shrinkage is probably caused by the dissolving of crystals ofCa(OH)2 while under a compressive stress (imposed by the drying shrinkage) anddepositing of CaCO3 in spaces free from stress; the compressibility of the hydratedcement paste is thus temporarily increased. If carbonation proceeds to the stage ofdehydration of C-S-H, this also produces carbonation shrinkage. Figure 2.3 shows the drying shrinkage of mortar specimens dried in CO2 – freeair at different relative humidity, and also the shrinkage after subsequent carbonation.Carbonation increases the shrinkage at intermediate humidity, but not at 100 per cent or25 per cent. In the latter case, there is insufficient water in the pores within the cementpaste for CO2 to form carbonic acid. On the other hand, when the pores are full ofwater, the diffusion of CO2 into the paste is very slow; it is also possible that thediffusion of calcium ions from the paste leads to precipitation of CaCO3 with aconsequent clogging of surface pores.Figure 2.3: Drying shrinkage and carbonation shrinkage of mortar at different relative humidity [9]
  43. 43. 21 The sequence of drying and carbonation greatly affects the total magnitude ofshrinkage. Simultaneous drying and carbonation produces lower total shrinkage thanwhen drying is followed by carbonation (Figure 2.4) because, in the former case, a largepart of the carbonation occurs at relative humidity above 50 per cent; under suchconditions carbonation shrinkage is reduced. Carbonation shrinkage of high-pressuresteam-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 containingCO2, shrinkage due to carbonation becomes progressively more apparent. The totalshrinkage 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 themoisture movement.
  44. 44. 222.3.2 Factors Affecting Shrinkage Generally, plastic shrinkage results from surface evaporation due toenvironmental conditions, such as humidity, wind speed or ambient temperature. ACI305R, Hot Weather Concreting [10], provides guidance for placement of concrete tominimize plastic shrinkage cracking. Several factors which may be expected to influence the magnitude of volumechanges in mortars and concretes caused by variations in moisture conditions, whichtake 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
  45. 45. 232.3.2.1 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 anincrease to an increase in autogenous shrinkage of concrete. Small autogenousexpansion as opposed to shrinkage may be produced through the use of coarser cements.Therefore, early age cracking could be possibly being avoided. Although coarserparticles of cement are relatively beneficial in minimizing early age cracking, they maybe detrimental to long-term strength. Mehta and Monteiro [2] state that the variation infineness and composition of Portland cement affect the rate of hydration, but not thevolume and characteristics of hydration products. Therefore, normal changes in finenessand composition of cement have negligible effect on drying shrinkage of concrete.Higher cement content with lower W/C in concrete results in higher autogenousshrinkage due to self-desiccation and chemical shrinkage, but may reduce dryingshrinkage 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 theaggregate 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 porousbut 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 dependsupon (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. Intable A is shown the shrinkage of neat cement in comparison with the corresponding
  46. 46. 24shrinkages of the same cement diluted with a single sieve size (No. 4 to 3/8 in.) of graveland crushed limestone, respectively. The reduction in shrinkage due to the aggregate isgreater than would be expected considering its relative volume. It is possible thatinternal 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 shrinkageof cement paste and concrete. Tests have shown that for cements having normalshrinkage characteristics, the shrinkage of the cement paste varies directly with thewater-cement ratio [33]. Figure 2.5 shows the pattern of shrinkage as a function of cement content, watercontent, and water/cement ratio where the concrete is moist-cured for 28 days, thereafterdried for 450 days [11]. At a constant water/cement ratio, shrinkage increases with an
  47. 47. 25increase in the cement content because this results in a larger volume of hydrated cementpaste which is liable to shrinkage. However, at a given workability, whichapproximately means a constant water content, shrinkage is unaffected by an increase inthe cement content, or may even decrease, because the water/cement ratios is reducedand 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. Forinstance, water reducers can be used to reduce the paste volume and thereby enhance thecreep capacity without the loss of workability. Set retarders can be used to delay set andto decrease the amount of heat of hydration. A lower heat of hydration will decrease thethermal shock on the hydrating concrete [33]. However, overly long retardations willincrease the potential for plastic shrinkage cracking. Proper curing is necessary with the
  48. 48. 26use of a set retarder. Conversely, set accelerators increase the heat of hydration andearly-age shrinkage. This combination will increase transverse shrinkage and theresulting cracking. Shrinkage-reducing admixtures (SRAs) are also available. These admixturesreduce the drying shrinkage by reducing the surface tension of the water in the capillarypores. If the surface tension of the water is reduced, there is less tension transferred tothe capillary walls, and consequently less shrinkage. Laboratory evaluations haveshown a slight decrease in compressive strength when an SRA is used. Takingadvantage 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 magnitudeas when Portland cement is used, but it takes place much more rapidly [12]. Includingeither 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 inthe blended cement leads to higher shrinkage by some 20 percent with the formermaterial and by up to 60 percent at very high contents of slag [13]. Silica fumeincreases 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 thewater content or in the cement content of the mix, or in both, and it is the combinedaction of those changes that influences shrinkage. Superplasticizers have been found toincrease shrinkage by some 10 to 20 percent. However, the changes in the observedshrinkage are too small to be accepted as reliable and generally valid.
  49. 49. 27 Entrainment of air has been found to have no effect on shrinkage [15]. Addedcalcium chloride increases shrinkage, generally between 10 and 50 percent [16],probably because a finer gel is produced and possibly because of greater carbonation ofthe more matures specimens with calcium chloride.2.3.2.5 Temperature and Relative Humidity A high temperature and low relative humidity of the ambient environmentaccelerate the diffusion of the adsorbed water and capillary water into the atmosphere,and consequently, increases the drying shrinkage of concrete. An increase in theatmospheric humidity slows down the rate of moisture flow from the interior to the outersurface of concrete. Mehta and Monteiro [2] states that at 0% relative humidity, it isassumed 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 rateand total amount of shrinkage. The size and shape are often considered together as thevolume-to-surface area ratio. A high volume-to-surface ratio usually results in lowershrinkage magnitudes.
  50. 50. 282.3.2.7 Volume and Type of Aggregate Drying shrinkage of concrete is a fraction of that of neat cement because theaggregate particles not only dilute the paste but reinforce it against contraction. The sizeand 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 andhence a lower shrinkage. The shrinkage of aggregates themselves may be of considerable importance indetermining the shrinkage of concrete. Some fine-grained sandstones, slate, basalt, traprock and aggregates containing clay show large shrinkage while concretes low inshrinkage often contain quartz, limestone, granite or feldspar. The pore structure ofaggregate particles may have a strong effect on autogenous shrinkage. Aggregateparticles may contain water in coarse pores, which provides the “internal curing” forhydrating cement paste hence reducing autogenous shrinkage. Figure 2.6 indicates therelationship between W/C ratio, aggregate content and shrinkage. Figure 2.6: Effect of W/C ratio and aggregate content on shrinkage [36]
  51. 51. 292.3.2.8 Elastic Modulus of Aggregate Modulus of elasticity is the most important property of aggregate that directlyinfluences drying shrinkage of concrete. When readily compressible aggregate is used,concrete will shrink as mush as neat cement, and that expanded shale leads to shrinkagemore than that of ordinary aggregate. Steel aggregate on the other hand, leads toshrinkage less than that of ordinary concrete. The drying shrinkage of concreteincreased 2.5 times [17] when an aggregate with high elastic modulus was substituted byan aggregate with low elastic modulus.2.3.3 Differential Shrinkage It was mentioned earlier that the potential shrinkage of neat cement paste isrestrained by the aggregate. In addition, some restraint arises also from non-uniformshrinkage within the concrete member itself. Moisture loss takes place only at thesurface so that a moisture gradient is established in the concrete specimen, which is thussubjected to differential shrinkage. The potential shrinkage is compensated by thestrains 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 arethose of free shrinkage, or potential shrinkage, that is, contraction unrestrained eitherinternally or by external constraints on a structural member. In considering the effect ofthe constraining forces on the actual shrinkage, it is important to realize that the inducedstresses are modified by relaxation, which may prevent the development of cracking.
  52. 52. 30Because relaxation occurs only slowly, it may prevent cracking when shrinkagedevelops slowly; however, the same magnitude of shrinkage occurring rapidly may wellinduce cracking. It is shrinkage cracking that is of paramount interest. Because drying takes place at the surface of concrete, the magnitude of shrinkagevaries considerably with the size and shape of the specimen, being a function of thesurface/volume ratio. A part of the size effect may also be due to the pronouncedcarbonation shrinkage of small specimens. Thus, for practical purposes, shrinkagecannot be considered as purely an inherent property of concrete without reference to thesize of the concrete member. Many investigations have, in fact, indicated an influence of the size of thespecimen on shrinkage. The observed shrinkage decreases with an increase in the sizeof the specimen but, above some value, the size effect is small initially as shown inFigure 2.7. The shape of the specimen also appears to enter the picture but, as a firstapproximation, shrinkage can be expressed as a function of the volume/surface ratio ofthe specimen. There appears to be a linear relation between this ratio and the logarithmof shrinkage as shown in Figure 2.8. The effect of shape is secondary. I-shaped specimens exhibit less shrinkage thancylindrical ones of the same volume/surface ratio, the difference being 14 percent on theaverage [18]. The difference, which can be explained in terms of variation in the meandistance that the water has to travel to the surface, is thus not significant for designpurposes.
  53. 53. 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]
  54. 54. 322.3.4 Shrinkage-induced Cracking As mentioned in connection with differential shrinkage, the importance ofshrinkage in structures is largely related to cracking. Strictly speaking, we areconcerned with the cracking tendency because the advent or absence of crackingdepends not only on the potential contraction but also on the extensibility of concrete, itsstrength, 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 ofconcrete in that it allows it to develop strain well beyond that corresponding tomaximum stress. A high extensibility of concrete is generally desirable because itpermits concrete to withstand greater volume changes. The schematic pattern of crack development when stress is relieved by creep isshown in Figure 2.9. Cracking can be avoided only if the stress induced by the freeshrinkage strain, reduced by creep, is at all times smaller than the tensile strength of theconcrete. Thus, time has two-fold effect: the strength increases, thereby reducing thedanger of cracking but, on the other hand, the modulus of elasticity also increases so thatthe stress induced by a given shrinkage becomes larger. Furthermore, the creep relievesdecreases with age so that the cracking tendency becomes greater. A minor practicalpoint is that, if the cracks due to restrained shrinkage form at an early stage, andmoisture subsequently has access to the crack, many of the cracks will become closed byautogenous healing. One of the most important factors in cracking is the water/cement ratio of themix because its increase tends to increase shrinkage and, at the same time, to reduce thestrength 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 todrying shrinkage. Carbonation, although it produces shrinkage, reduces subsequent
  55. 55. 33moisture movement, and therefore is advantageous from the standpoint of crackingtendency. On the other hand, the presence of clay in aggregate leads both to highershrinkage and to greater cracking. The use of admixtures may influence the cracking tendency through interplay ofeffects on hardening, shrinkage, and creep. Specifically, retarders may allow moreshrinkage to be accommodated in the form of plastic shrinkage and also probablyincrease 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 plasticshrinkage and, having a low strength, cracks. The temperature at the time of placing determines the dimensions of concrete atthe moment when it ceases to deform plastically. A subsequent drop in temperature willproduce potential contraction. Thus, placing concrete in hot weather means a highcracking tendency. Steep temperature or moisture gradients produce severe internalrestraints and thus represent a high cracking tendency. Likewise, restraint by the base ofa 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]
  56. 56. 342.3.5 Effect of Shrinkage Virtually all concrete is subject to some form of restraint, such as steelreinforcement, forms, subgrade, or adjacent members. Each of these forms of restraintinvolve the imposition of a gradually increasing tensile force on the concrete which maylead to time-dependent cracking, increases in deflection and a widening of existingcracks. 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 loadinduced tension existing in the member. Cracking can be avoided if the graduallyincreasing tensile stress induced by shrinkage, and reduced by creep, is at all times lessthan the tensile strength of the concrete. The existence of load induced tension inuncracked regions accelerates the formation of time-dependent cracking. The control ofsuch cracking requires two important steps. First, the shrinkage-induced tension and theregions where shrinkage cracks are likely to develop must be recognized by thestructural engineer. Second, an adequate quantity and distribution of anchoredreinforcement must be included in these regions to ensure that the cracks remain fineand the structure remains serviceable.2.3.6 Methods of Controlling Shrinkage Cracking Specific methods to properly control shrinkage cracking have been developedand researched. Conventional methods, which include proper material selection,mixture proportioning, and good construction techniques, can be used to a certain extentto control and limit the shrinkage cracking of concrete. Unfortunately, because thesemethods are hard to control, and environmental conditions can vary so much, the
  57. 57. 35shrinkage cracking cannot be entirely prevented. For example, concrete in hot, dry, andwindy conditions can have much higher rates of water evaporation, thus making themmore susceptible to shrinkage cracking. Innovative methods of controlling shrinkagecracking have been found in literature and developed by numerous researches to helpcontrol and eliminate shrinkage cracking. These include using fiber-reinforced concrete,shrinkage-reducing admixtures, shrinkage-compensating concrete, and extensibleconcrete. 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
  58. 58. 362.3.6.1 Conventional Method Shrinkage cracking in concrete is currently being controlled throughconventional methods, which consist of the proper selection of materials and concretemixtures, along with good construction techniques.2.3.6.1.1 Aggregates The type of aggregate used in concrete mixtures, as well as the aggregatecontent, can influence the amount of shrinkage in concrete. The aggregate type was themost significant factor affecting when concrete cracked [19]. Specifically, limestone-aggregate concretes proved to be the most resistant to cracking, while Eau Claire rivergravel had the shortest time-to-cracking of the aggregates tested. Burrows (1998) alsostudied 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 dryingshrinkage while, in the study, sandstone exhibited the highest amount of dryingshrinkage. The amount of aggregate used in a concrete mixture can also help to reduceshrinkage. Research has shown that a higher aggregate content can help to reduceshrinkage. Table 2.3 shows the aggregate type related to drying shrinkage according toBurrows report in 1998.
  59. 59. 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 0.0402.3.6.1.2 Cement Content and Type The amount of cement proportioned in concrete mixtures has an impact on theamount of shrinkage that concrete will undergo. Specifically, concrete cracking hasbeen 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 ofthe concrete mixes was increased [19]. Water-cement ratio also influences shrinkagebehavior in concrete. Krauss and Rogalla found that the concrete with more watershrinks and creeps more than concrete with less water, but it may not crack soonerbecause it has higher creep [19]. Burrows contends that although concrete mixes withlower water-cement ratios produce stronger concrete, that same concrete can be muchmore vulnerable to cracking. The type of cement used also plays an important role inreducing shrinkage cracking. Krauss and Rogalla noted that cements that are groundfiner and have higher sulfate contents increase the early strength of concrete while alsoincreasing the early modulus of elasticity and heat of hydration [19]. For example, TypeIII cement could increase the risk of cracking because of the rapid early strength gains.
  60. 60. 382.3.6.1.3 Admixtures Fly ash, silica fume, set retarders, and accelerators are all admixtures that havebeen investigated for shrinkage by a number of researchers. Fly ash has been found to reduce early concrete temperatures and the rate ofstrength gain, thus reducing concrete cracking. The process of using fly ash to replacecement is referred to as the creation of extensible concrete and is described in detailfollowing this section. Silica fume, a by-product of silicon metal or ferrosilicon alloys in electric arcfurnaces, has been found to increase the cracking of concrete. The silica fume producthas 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 lossresulting from evaporation cannot be replaced. Silica fume is found to be a problemwith cracking tendency specifically when the concrete is not cured properly. Retarders have not been proven either to be the cause of concrete cracking or tohelp reduce the risk of thermal cracking. Plastic cracking could be caused by theaddition of retarders, while retarders have also been found to reduce the risk of thermalcracking by reducing early heat of hydration in concrete.
  61. 61. 392.3.6.2 Innovative Method Because of the extreme variance of the conventional methods used to controldrying shrinkage, innovative methods should be used to help reduce cracking tendenciesof 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 reducesshrinkage cracking. Various parameters that were investigated include the addition offibers at low volumes as compared to high volumes, as well as the different types offibers to be used. Steel fibers can affect the properties of concrete, but the reinforced propertiesdepend on the percentage of fiber addition, the aspect ratio of the fibers, and the strengthof 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. Steelfiber reinforced concrete has been shown to increase the tensile strength, flexuralstrength, and compressive strength of concrete through research. Tests have shown thatsteel fibers do not affect the shrinkage strain of concrete, but the fibers can reduce theamount of cracking associated with the shrinkage strain.
  62. 62. 40 Low volume of polypropylene fibers can significantly reduce the plasticshrinkage of concrete. For low-volume fiber reinforcement typically 0.1%-0.3% haslittle 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 highvolumes, polypropylene fibers can be used to prevent shrinkage cracking. Theshrinkage stress produced in the concrete is transferred to the fibers, which can betterwithstand 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 SRAsused to control shrinkage cracking of concrete. These chemical admixtures, which areadded to concrete work by lowering the surface tension of the pore water insidehardened concrete. The pore water evaporates from capillary pores in the hardenedconcrete 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 thesurface tension of the pore water solution. This surface tension is lowered by SRAs,thus reducing the overall drying shrinkage. Therefore, there are fewer tendencies forshrinkage and resultant stress to occur in the concrete when the pore water initiallyevaporates. SRAs affect the nature of the pore water, rather than limiting or reducingthe amount of water from concrete during drying.
  63. 63. 412.3.6.2.2 Shrinkage-Compensating Concrete Shrinkage-compensating concrete is an innovative material that causes expansionof concrete during curing, which in turn reduces the effects of drying shrinkage. If theexpansion is properly restrained, the concrete will be subjected to compression the firstfew days after concrete placement. Although the shrinkage-compensating concrete willshrink as much as normal concrete once exposed to drying conditions, the net shrinkagewill be negligible because the concrete started out with an initial expansion. Themechanism of expansion in the shrinkage-compensating concrete is a result of the earlyformation and stability of ettringite. The ettringite crystals need water to expand, andtherefore, 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 areuseful for reducing the cracking in concrete. Basically, some of the conventionalmaterials and methods mentioned previously can be used in an innovative manner toachieve this type of behavior. A typical extensible concrete would have a high volumeof fly ash, low cement content, and a high water-cement ratio. These factors wouldproduce a low heat of hydration, thereby reducing thermal stresses in the concrete whilealso producing a low elastic modulus and high creep, minimizing shrinkage cracking.
  64. 64. 422.4 Creep of Concrete Creep of concrete, resulting from the action of a sustained stress, is a yielding ofthe concrete. It may be due partly to viscous flow of the cement-water paste, closure ofinternal voids and crystalline flow in aggregates, but it is believed that the major portionis caused by seepage of colloidal water from the gel that is formed by hydration of thecement. Although water may exist in the mass as chemically combined water, and asfree water in the pores between the gel particles, neither of these is believed to beinvolved in creep. The rate of expulsion of the colloidal water is a function of theapplied compressive stress and of the friction in the capillary channels. The greater thestress, the steeper the pressure gradient with resulting increase in rate of moistureexpulsion and deformation. The phenomenon is closely associated with that of dryingshrinkage. Creep is defined as the increase in strain under a sustained stress. There is aninstantaneous strain on concrete which is called the “elastic” deformation when thesample is unloaded. It is observed that there is a gradual increase in strain for days aftera stress has been applied to concrete. This is called the “creep” strain. Since thisincrease can be several times as large as the strain on loading, creep is of considerableimportance in structural mechanics. The deformation of concrete with time isschematically shown in Figure 2.10. The rate of creep is relatively rapid at early agesafter loading and then decreases gradually, until after a few years it becomesinsignificant. Roughly, about one-fourth of the ultimate creep occurs within the firstmonth or so, and one-half occurs within the first year.
  65. 65. 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 shrinkageand viscoelastic behavior such as creep usually take place simultaneously. Consideringthe various combinations of loading, restraining, and humidity conditions, the followingterms 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
  66. 66. 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 / εE2.4.1 Creep Behavior of Concrete Creep in concrete can have both positive as well as negative effects on theperformance of concrete structures. On the positive side, creep can relieve stressconcentrations induced by shrinkage, temperature changes, or the movement of supports.For indeterminate beam with two fixed ends, creep deformation will be very useful inreducing 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 otherserviceability problems, especially in high-rise building, eccentrically loaded columnsand long bridges [8]. In mass concrete, creep may be a cause of cracking when arestrained concrete mass undergoes a cycle of temperature change due to thedevelopment of heat of hydration and subsequent cooling [8].
  67. 67. 452.4.2 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 recoverablecomponent also know as delayed elastic strain εd(t), which is caused by the elasticaggregates 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 issubdivided into rapid initial flow εfi(t), basic flow εfb(t) and drying flow εfd(t). Rapidinitial flow happens in the first 24 hours after loading and is the remaining flow whichdevelops gradually with time. While basic flow or basic creep is not dependent upon theloss of moisture from the concrete and will occur with concrete protected from drying.
  68. 68. 46Meanwhile drying creep is the additional creep which occurs in a drying specimen.However, drying creep, like drying shrinkage, is dependent upon the loss of moisturefrom 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]
  69. 69. 472.4.3 Factors Affecting Creep The magnitude of the creep depends upon several factors relating to the qualityof the concrete such as the aggregate-cement ratio, water-cement ratio, kind of aggregateand its grading, composition and fineness of cement, and the age at time of loading. Italso 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 theexpulsion of moisture from the gel becomes more difficult as the porosity is decreasedthrough hydration. Since the extent of the hydration is indicated by the strength of agiven 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 ofconcrete reduces with the increasing of water/cement ratio. Concrete experience highercreep due to higher water/cement ratio because the concrete has insufficient restraint dueto the tension force induced in the concrete. A higher water-cement ratio increases the
  70. 70. 48size of the pores in the paste structure, so that water may the more readily escape, andthen under a sustained load the water of adsorption may be expelled more readily tocause 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 pasteis the source of the phenomenon. The influence of cement is twofold: that arising fromthe physical and chemical properties of the cement. The composition of cement affectsthe creep primarily through its influence upon the degree of hydration. Slow-hardeningcements such as low-heat Portland and Portland-pozzolan cements creep more thancements which hydrate more rapidly. Creep seems to be inversely proportional to therapidly of hardening of the cement used. The more hardened the paste the more rigid itis and the lower its creep potential at a given applied stress.
  71. 71. 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 concretemade with low-heat cement is about one-third greater than for concrete made withnormal cement. This serves to explain why low-heat Portland and Portland-pozzolancements have served so effectively in relieving stresses in large dams as the mass coolsand 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 aggregatereduces creep deformation by reducing the cement paste content and restraining thecement paste against contraction. Generally, concretes made with an aggregate, which ishard and dense and have low absorption and high modulus of elasticity, are desirablewhen low creep strain is needed.
  72. 72. 50 The effect of mineral character of aggregate is shown in Table 2.4 for sixconcretes. Same mineral aggregate was used from fine to coarse; the grading was thesame for all mixes. After carrying a sustained stress of 800 psi for about 5 years themaximum creep (1300 millionths) was exhibited by the sandstone concrete and theminimum (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 theirabsorption factors were generally low, the large variations in creep were not due to themoisture conditions and the aggregates. Neither were they due to seepage from theidentical paste used in each mix. It is possible that variations in crystalline slip, particleshape, surface texture, and pore structure of the aggregates may have had someinfluence.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 theseepage of moisture from the concrete. Naturally, an increase in the humidity of theatmosphere reduces the rate of loss of moisture or water vapor to the surroundingatmosphere, slows down the flow of moisture or water vapor to the outer surface, and
  73. 73. 51thus reduces the seepage. Another factor affecting compressive creep is that dryingshrinkage at or near the surface results in a reduction of the cross-sectional arearemaining in compression and therefore causes higher stresses on the central core. Ahigh temperature and low relative humidity of the ambient environment accelerate thediffusion of the adsorbed water and capillary water into the atmosphere, andconsequently, increases the creep of concrete. Therefore, the creep of concrete can beconcluded to be inversely proportional to the relative humidity. The magnitude of creep for various moisture conditions of storage is shown inTable 2.5. Although these values indicate that for a concrete loaded to 800 psi at the ageof 28 days the creep in air at 70 percent relative humidity was about double that forwater storage, for similar concrete loaded at 3 months to 1,200 psi the creep for the airstorage 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 isprobably due to the reduced seepage, as the path traveled by the expelled water is greaterwith 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

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