Book Description

Early-age thermal cracking occurs when the tensile strain, arising from either restrained thermal contraction or a temperature differential, exceeds the tensile strain capacity of the concrete. In high strength concretes autogenous shrinkage may also contribute to early contraction. Numerous factors influence the risk of early-age cracking including the temperature rise; the coefficient of thermal expansion of the concrete; the restraint to movement offered either by adjacent elements or by differential strain within an element; and the ability of the concrete to resist tensile strain. This guide provides a method for estimating the magnitude of crack-inducing strain and the risk of cracking; and where cracking will occur guidance is provided on the design of reinforcement to control crack widths. Significant changes since the last revision to this report in 1992 include compatibility with EN1992-3:2006 and EN1992-1-1 with regard to the properties of concrete and the design of reinforcement.

113 pages

Contents: Notation Abbreviations 1 Introduction 1.1 Scope 1.2 Background to updating the former report R91 1.3 The cause of early-age thermal cracking 1.4 The design process 1.5 Responsibilities 1.6 Other effects of temperature 1.7 Advances 15, 1.8 Format of the book 2 Significance of cracking 2.1 The acceptability of cracking 2.2 The relationship between crack width and functionality 2.3 Cracks which lead to durability problems 2.4 Cracks which lead to a malfunction of the structure 2.5 Cracks which are aesthetically unacceptable 2.6 The role of autogenous healing 2.7 Measures to avoid cracking 3 The design process 3.1 Introduction 3.2 Estimating the risk of cracking and the crack-inducing strain 3.2.1 External restraint 3.2.2 Internal restraint 3.2.3 Input data 3.3 Minimum reinforcement area 3.3.1 General equation 3.3.2 Tensile strength of concrete 3.3.3 Tensile strength of steel 3.4 Crack spacing .5 Crack width 3.5.1 The requirements of EN1992-3 3.5.2 A member restrained along one edge 3.5.3 A member restrained at it ends only 3.5.4 A member subject to internal restraint 3.6 The Simplified Method with default values 3.6.1 Estimating the risk of cracking and the crack-inducing strain 3.6.2 Crack control 3.7 Selecting options to reduce the risk of cracking 4 Factors affecting early-age cracking and data required for design 4.1 Information and assumptions 4.2 Temperature rise 4.2.1 Influencing factors 4.2.2 Estimating T1 4.2.3 Adjustments to T1 4.3 Annual temperature change, T2 4.4 Temperature differentials, DT 4.5 Coefficient of thermal expansion, ac 4.6 Shrinkage strains 4.6.1 Autogenous shrinkage 4.6.2 Drying shrinkage, ecd 4.6.3 Appropriate use of shrinkage strains 4.7 Restraint, R 4.7.1 General guidance 4.7.2 External edge restraint 4.7.3 End and local restraint 4.7.4 Internal restraint 4.7.5 Combined internal and external restraint 4.8 Tensile strain capacity, ectu 4.9 Coefficients and inherent safety factors 4.9.1 Coefficient for creep, K1 4.9.2 Coefficient for Sustained loading, K2 4.9.3 Inherent safety factors 4.10 Tensile strength of concrete, fct 4.11 Estimating the minimum area of reinforcement 4.11.1 Changes in approach 4.11.2 External restraint 4.2.3 Internal restraint is dominant 4.12 The ratio of tensile strength to bond strength fct/fb 5 Application of the design process using worked examples 5.1 Example 1: Retaining wall on a rigid foundation (continuous edge restraint) 5.2 Example 2: Suspended slab cast between core wall and column end restraint) 5.3 Example 3: Massive raft foundation (internal restraint) 6 Measures to mitigate cracking and minimise crack widths 6.1 Planning pour sizes and construction sequence 6.2 Movement joints 6.3 Additional measures to cool the concrete 6.3.1 Pre-cooling the constituent materials 6.3.2 Cooling of the fresh concrete before placing 6.3.3 In situ cooling 6.3.4 Application of cooling techniques 7 Specification, testing and monitoring 7.1 Specification 7.2 Test methods for obtaining relevant concrete properties 7.2.1 Heat generation and temperature rise 7.2.2 Coefficient of thermal expansion 7.2.3 Tensile strain capacity 7.3 Measurement and assessment of in situ temperature and strain 7.3.1 Measuring temperature differentials 7.3.2 Deriving restraint factors 7.4 Measurement of crack width 5 Actions in the event of non compliance 8 Conclusions and recommendation 9 References Appendices A1 Heat generation, temperature rise and temperature differentials A2 A model for predicting the temperature rise and temperature differentials using adiabatic temperature data A3 Estimating drying shrinkage using the method of EN1992-1-1 A4 Estimating autogenous shrinkage A5 Estimating restraint A6 Estimating tensile strain capacity A7 Evaluating risk/extent of cracking using a strain based approach A8 Reinforcement design to control crack widths A9 The effect of peak temperature on the in situ strength A10 Assessment of the in situ tensile strength of concrete Spreadsheet calculators Model for prediction of temperature rise and thermal gradients EN1992-1-1 drying shrinkage calculator EN1992-1-1 autogenous shrinkage calculator Restraint calculator based on the ACI method for wall Crack width calculator for edge restraint and end restraint