Drying shrinkage of cementitious materials at early age
Throughout its service life, concrete undergoes volume changes occurring both in the fresh and in the hardened state. Drying shrinkage is considered one of the most significant deformation phenomena in hardened concrete, because of its magnitude and because it occurs throughout its whole service life. If the shrinkage is restrained by other structural elements (external restraint) or due to differential shrinkage in the cross-section (self-restraint), internal stresses develop that may result in visible cracking and/or micro-cracking. Cracking negatively affects the long-term performances of concrete elements, jeopardizing aesthetics and durability, eventually shortening their service life and increasing the maintenance costs.
At early-age, the premature exposure to drying (and hence occurrence of shrinkage) may further exacerbate the cracking susceptibility, as the material properties of cement-based materials are not yet completely developed. A fundamental understanding of the early-age properties and their development becomes of paramount importance to minimize drying shrinkage and in general the likelihood of cracking. At early-age, however, the continuous evolution of the microstructure due to hydration results in inconsistent and incomparable ime-dependent and moisture-dependent measurements when approached at different early ages (or hydration stages). Hence, decoupling the hydration from the effect of drying by stopping the hydration reaction to achieve an unchanging system becomes essential.
In this work, the equivalent system approach "to arrest cement hydration" and enable the investigation of cement-based materials at early age on unchanging systems and at fixed degree of hydration is presented. The use of the equivalent approach allows investigations of moisture-dependent and time-dependent properties at early age that are otherwise difficult or impossible to perform because of the ongoing hydration and/or concurrence of drying, as for example in the case of drying shrinkage, creep/relaxation, and transport properties. These phenomena involve water transport and drying, which notoriously need long time to come to equilibrium. Specifically to this work, this approach is used to investigate the early-age drying shrinkage behavior and the (early-age) elastic properties as a function of the moisture content in cement-based materials.
The main idea behind the equivalent system approach for investigating cement based materials at fixed degree of hydration lies on the replacement of a specific amount of binder, which would be still unhydrated at a certain hydration age, by fine quartz filler. This can be accomplished at the mixing stage, knowing the degree of hydration at a given age measured on thereal cementitious system and substituting it with quartz (replacement by volume).