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The calculation of shrinkage-induced stresses and evaluation of cracking risk in concrete structures require information on shrinkage strain that always relates to the moisture decrease caused by drying or cement hydration. In the present paper, the development of internal relative humidity (RH) and global deformation in early-age normal and high-strength concrete were experimentally measured simultaneously by monitoring humidity and deformation sensors, respectively, starting from the time of concrete casting. The experimental results show that the development of the internal RH of concrete first follows a vapour-saturated stage (RH = 100%, stage I), followed by a gradually reducing stage (RH ≤ssthan 100%, stage II). Meanwhile, the global deformation behaves first as a plastic swelling for the initial several hours after casting and then shrinking occurs with a gradually reduced rate. The end of swelling normally corresponds to the point of concrete setting, at which point the transformation of fresh concrete from a liquid-like plastic state to a solid state is finished. Shrinkage has been measured even in the vapour-saturated stage with RH = 100%. In the humidity reduction stage, a linear relationship between free shrinkage strain and humidity reduction exists. In practice, the relationship of free shrinkage strain and local RH may be used to calculate the free shrinkage strain field and further to calculate the shrinkage-induced stresses in structures as well.

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