A new approach for the determination of the starting point of autogenous shrinkage strains (ASS)

The determination of the starting point of autogenous shrinkage strains is still a point of discussion within the scientific community. Several approaches, each one more or less easy to implement, have been proposed to determine this important point when dealing with low water to binder ratio (W/B) concrete. It is at this moment that external water curing must be applied to control the detrimental effects of the development of early autogenous shrinkage strains because the development of early cracking can be very detrimental for concrete durability. Concrete is continuously subjected to volumetric changes, particularly at an early age, when hydration heat and autogenous shrinkage evolve very fast. It is difficult to separate thermal and plastic shrinkage from shrinkage due to chemical contraction. In large concrete elements, it is appropriate to calculate isothermal shrinkage to reach the starting point of autogenous shrinkage. Neither the temperature criterion alone, nor the penetration resistance test can predict the starting point of autogenous shrinkage, and therefore, the risk of early-age cracking. In this research, the concept of threshold of solidification was coupled with that of temperature rise to more accurately determine the starting point of autogenous shrinkage. An experimental study on the development of isothermal shrinkage of large concrete elements made with different binders was carried out. In order to clarify the effect of the W/B on the importance of the determination of the starting point of autogenous shrinkage, concrete mixtures were made at three W/B: 0.45, 0.35 and 0.26. Isothermal shrinkage was measured using vibrating wire gauges imbedded in a large concrete element. Experimental results show the importance of the correct determination of the starting point of autogenous shrinkage when the W/B is low.