Evaluation of rate of deformation for early-age concrete shrinkage analysis and time zero determination

Early age cracking is usually the result of internal tensile stresses induced by self-desiccation shrinkage (SDS) rather than external loading. Hence, the prediction of early age cracking risk is strongly linked to autogenous shrinkage development. Both the ultimate magnitude of shrinkage and the time zero (TZ), at which shrinkage starts to develop an internal stress, could be decisive for high-performance concrete (HPC) durability. The moment TZ can be considered as the borderline between autoplastic shrinkage and effective shrinkage. Deformation rate curves might be used as a framework to identify the three main phases of hardening that occur in cement paste as hydration progresses, and the development rate of autogenous shrinkage (AS). The present study proposes starting shrinkage measurement (autogenous or total) from the moment when the rate of deformation reaches its maximum value (the first peak in the curve) and the shrinkage strain rate curve pattern changes sharply, or at the end of plastic-shrinkage. To determine the TZ with more accuracy and predict the critical moment when internal stresses may start to develop, continuous shrinkage measurement was carried out on different HPC mixtures designed with three water-to-binder ratios (w/b) of 0.26, 0.30, and 0.35, using three binder types. Results have shown that the rate of deformation development at an early age would be a reliable method to distinguish the three main phases of cement hydration reaction: liquid, semiliquid/transition, and hardened. The rate of deformation evolution may also indicate the moment when shrinkage strains evolve quickly and start to build up an internal tensile stress that could lead to cracking. The new method proposed, based on the development of the rate of deformation, has revealed itself to be highly efficient for adequate determination of TZ and could be used for all concrete types independently of specimen size.