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The hydration of cement in concrete is exothermic, which means it gives off heat. In large elements, the heat caused by hydration can dissipate at the surface, but is trapped in the interior, resulting in potentially large thermal gradients. The thermal expansion of concrete is greater at higher temperatures, so if the temperature differential between the surface and the interior becomes too great, the interior will expand more than the exterior. When the thermal stress from this mis-matched expansion exceeds the tensile strength of the material, the concrete will crack. This phenomenon is referred to as thermal cracking. Accurate characterization of the progress of hydration of a concrete mixture is necessary to predict temperature gradients, maximum concrete temperature, thermal stresses, and relevant mechanical properties of concrete that will influence the thermal cracking risk of concrete. Calorimetry is the most direct test method to quantify the heat evolution from a concrete mixture. There is currently no model, based solely on calorimetry, which completely describes the effects of mixture proportions, cement and SCM chemistry, and chemical admixture dosages on the temperature sensitivity and adiabatic temperature rise of concrete. The objective of this study is to develop a comprehensive model to describe these effects. First, the temperature sensitivity of the hydration reaction (described with activation energy, E[subscript a]) is needed to accurately predict the behavior of concrete under a variety of temperature conditions. A multivariate regression model is from isothermal calorimetry testing to describe the effects of water-cementitious materials ratio, cement chemistry, supplementary cementing materials, and chemical admixtures on the E[subscript a] of portland cement pastes. Next, a multivariate regression model is developed from semiadiabatic calorimetry testing that predicts the temperature development of concrete mixtures based on mixture proportions, cement and SCM chemistry, and chemical admixture dosages. The results of the models are validated using data from literature. The final model provides a useful tool to assess the temperature development of concrete mixtures, and thereby reduce the thermal cracking risk of the concrete structure.

A large amount of heat can be liberated during cement hydration, causing very large temperature increases in mass concrete members. The non-uniform temperature field produced by the cement during curing can cause very high internal stresses that may crack the concrete. Concrete thermal cracking in very large structures is a well-known phenomenon and was studied extensively during the height of dam construction in the United States. In recent years concrete bridge member sizes have increased for structural and aesthetic reasons. Recent problems in San Antonio and Houston, Texas with thermal cracking and very high internal temperatures in mass concrete bridge members has renewed interest in studying early-age thermal cracking and its mechanisms. In order to predict the early-age thermal cracking risk of a concrete member, the temperature history, autogenous shrinkage, modulus development, tensile strength development, coefficient of thermal expansion development, creep behavior, and external restraint conditions must be known. A testing procedure has been developed to measure concrete heat of hydration, mechanical property development, and free shrinkage response at different curing temperatures. The concrete free shrinkage includes thermal and autogenous shrinkage components and is measured using a newly developed free shrinkage testing apparatus. The early age concrete creep is calculated from rigid cracking frame tests performed at different varying temperatures. Trends in early age creep behavior for different concrete mixtures common in mass concrete have been found and are used to develop a statistical model relating concrete mixture proportions and constituent material properties for use in mass concrete thermal stress modeling. The results from the test methods described are used in a new concrete early-age cracking risk and durability software package called ConcreteWorks. / text

A thesis submitted to the Faculty of Engineering, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy / The cracking of mass and structural concrete due to thermal stress is a major problem in the concrete construction industry. Concrete will crack when the thermal stress exceeds tbe tensile strength of the concrete, Decisions on the type of concrete mix, cooling facilities and construction techniques to be used in the erection of a concrete structure can only be made if the thermal behaviour and strength of the concrete can be predicted during hydration. This thesis describes the development of a low cost, computer controlled, adiabatic calorimeter to determine tlte heat of hydration and a probe to determine the thermal conductivity or concrere samples. The main thrust of this thesis is the development of the thermal conductivity probe which, for the first time, can measure the thermal conductivity of concrete through all stages of hydration. A thermal model was also developed to verify the results, and the use of the calorimeter for temperature matched curing tests is also discussed. Results, obtained from the test procedures described, will provide far more accurate predictions of the temperatures in concrete structures than was possible in the past. / Andrew Chakane 2018

This diploma thesis is focused on the study of the thermal properties of high-performance fiber reinforced concrete HPFRC. The composites are based on a combination of steel and polymeric fibers. Typical properties of these materials include high mechanical strength, water resistence and salt penetration. HPFRCs are mainly used in the construction industry to build tall buildings. The differential transient method was used to study thermophysical variables. The theoretical part of the thesis described the types of concrete, their production, and their properties. Furthermore, thermophysical quantities and methods of their determination (stationary and transient methods, thermal analysis) are defined in the thesis. The aim of the thesis was to determine thermal properties, namely thermal conductivity and specific heat capacity. The results of the thesis can reveal changes in the composition of studied materials as well as critical temperatures for damaging the materials.

It has been observed that mortar joints, where precast concrete trim and limestone building blocks are used in conjunction, tend to crack in the course of one or two years to such an extent as to admit water. It seemed possible that a difference in the thermal coefficients of the two materials might cause unequal changes in length along the joint, thereby setting up shearing stresses in the mortar, sufficient in time to destroy the bond. This investigation was undertaken in order to determine the relative rates of expansion of the local materials in question and to consider the validity or the theory that such differences might affect the mortar. The investigation and the conclusions drawn therefrom form the first part of this thesis. Part two consists of an investigation into the effects of different mixes on the thermal coefficient of such a concrete as might be suitable for the manufacture of precast trim. / M.S.