Development of a technology to control thermal cracking in concrete by means of a smart material

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Hydration heat of cement in massive concrete causes significant thermal stress leading to cracking at early ages. In order to avoid such a stress, a smart material for controlling the hydration heat is developed in this study. As the first step, the influence of curing temperature history on mechanical properties of concrete is quantitatively discussed as a function of equivalent age. It was confirmed that high temperature curing of concrete at the early ages prevents the increase of strength in the matured condition. On the basis of these results, the melting temperature of paraffin microcapsule was chosen to be less than 60 degree C. Smart materials developed in this study are composed of paraffin microcapsule containing very fine particles of a hydration-retarder agent. Hence the rising rate of temperature of concrete and thermal stress are controlled under the specified value. Performance and applicability for practical purpose of the developed smart materials are verified by a ser … More ies of experimental studies. Conclusively the following results are obtained :1. Timing and technique of mixing the microcapsules with fresh concrete was one of the most difficult subjects in this study. While microcapsule are easily damaged during concrete mixing, adding the microcapsules to fresh concrete in agitator at the construction site just before placing the concrete is most acceptable for the practical purpose.2. Amount of microcapsules necessary for controlling the hydration rate of concrete is strongly dependent on the type of used fine aggregates. Therefore it is essential to examine the necessary amount of the microcapsules in advance.3. Reduction of hydration heating rate does reduce the thermal stress in concrete, though the maximum temperature is the same as that of plain concrete in case of a very thick and massive concrete. For the practical application, the necessary amount of microcapsule needs to be determined by analyzing the thermal diffusion process in the thickness of the member given in the design documents since the maximum temperature is much lower than that of the adiabatic condition as obtained in the experimental study especially when the hydration heating rate is reduced. Less