Estimation of Early Age Thermal Behaviour of Mass Concrete

The application of mass concrete increases with the increasing construction of massive structures like bridges/ flyovers, dams etc. However, the challenges of high heat generation vis-� -vis heat dissipation problems with thermal behavior become a concern. The excessive heat of mass concrete may lead to cracking and other temperature related damage to the concrete structures. Thermal stress developed from concrete heat development and environmental conditions could potentially lead to structural cracking. Thermal cracking mainly develops due to volume change when relatively large temperature difference exists between the interior and the surface of a concrete placed. When cracking occurs, repair can be very expensive and may delay project completion. In order to improve the durability of mass concrete, mix design and construction methods should be planned to properly control the temperature, especially the maximum temperature and temperature difference between the interior and the surface of the concrete. Concrete releases the majority of its heat and gain its strength due to the formation of calcium silicate hydrate and calcium hydroxide. It is also important to apply the curing procedures before concrete starts hardening to prevent water from evaporating, thus reducing the risk of plastic shrinkage cracking. The cooling stage or the deceleration stage occurs right after final set. Final set is a critical point when the concrete achieves a defined stiffness. The buildup C-S-H and CH begins to limit access of water to the cement, so silicate reactions become slow. Generally, final set occurs at 10 to 13 hours after mixing for Portland cement with or without addition of supplementary cementitious materials (SCMs). During this stage, the heat peak is reached and the temperature begins to drop. Once these reactions are greatly slowed, the concrete begins to cool and experience strain. Mass concrete thermal cracking occurs during the cooling stage when the concrete has strength and begins to cool. When the exterior of the concrete cools much faster than the interior, stress from the contraction of the exterior relative to the interior will cause cracking. This study was conducted to investigate the thermal conductivity and specific heat of the two concrete mixes (M15A80 & M20A40) which are proposed to be used in the construction of Kalisindh Dam, Rajasthan. The thermal properties viz. thermal conductivity and specific heat values of concrete are useful for predicting its thermal behaviour and taking necessary decisions about the construction methodology such as fixing the lift height, methods of cooling system etc., in order to control heat dissipation.