Next: Conclusions Up: Main Previous: Autogenous Strategies
If concrete is allowed to freely imbibe water from the surroundings during curing, self-desiccation shrinkage will be delayed and cracking may be avoided. The water may be supplied to the concrete, for example, through membrane or fleece-lined forms [58], which may be considerd as a reverse type of a controlled permeability formwork [59,60]. Also, fog misting or water ponding of the concrete surface may be used. Wet curing should be started as soon as hydration begins [61]. In high-performance concretes, however, it must be recognized that during the hydration, a point will be reached where the capillary porosity becomes depercolated [62], basically eliminating the ingress of curinnnecessary to achieve this depercolation decreases as the w/c decreases [62,63]. Over fifty years ago, Powers [64] suggested maintaining saturated curing conditions only until this depercolation occurs. It is for this reason that the use of the internal curing strategies discussed above is especially important for high-performance concretes. Still, the ingress of water from the external environment during just the first few days of hydration/curing can result in substantial improvements in the mechanical and transport properties of the hardened concrete.
Of course, the effectiveness of water exchange between the environment and a concrete element will depend to some extent on its thickness. For young concrete, water rearrangement due to capillary forces can evidently move water large distances (tens of millimeters) [65], so that at early ages, water may be drawn from the exposed surface deep into the concrete. As the cement paste hydrates, however, and the permeability of the concrete is reduced by many orders of magnitude, after a few days, water movement will become effectively limited to within the top few millimeters of the exposed concrete surface. Still, since the surface layer of the concrete is particularly critical for durability performance, it is normally worthwhile to moist cure even this top few millimeters for as long as feasible.
Ingress of certain substances from the surroundings may lead to expansion of the concrete. Examples of these include sulfates and alkalis. This may potentially counteract autogenous shrinkage of the concrete, but in reality, it may be difficult, if not impossible, to utilize this mitigation strategy in practice.
Swayze has shown that autogenous shrinkage can be used to offset thermal expansion of concrete [66]. Conversely, thermal expansion can be used to mitigate autogenous shrinkage. Of course, the concrete will eventually cool down, but this approach gives the mechanical properties of the concrete a chance to fully develop before the concrete is subjected to significant thermal stresses. For field concrete, the viability of this approach may be compromised due to the simple fact that the weather can not be predicted precisely. Thus, unless the field concrete is cured in a controlled environment, its thermal history and thermal expansion can not be predicted a priori.
The autogenous shrinkage cracking that develops in a structure depends not only on the autogenous shrinkage of the concrete, but also on the external restraint on the concrete. Such restraint may be due to adjacent members, formwork or reinforcement. Whereas internal restraint, e.g. from aggregate particles, may result in diffuse micro-cracking, external restraint may lead to macro-cracking across the section. Uncontrolled cracking due to restraint from adjacent members may be avoided by the use of contraction joints or control joints in the shrinking concrete or alternatively in the adjacent member. In either case, this can be considered a type of controlled cracking. Flexibile formwork can be useful [67,68], especially if the deformation of the concrete is restrained only due to its geometry. The restraining effect from the reinforcement may also lead to cracking. For certain restraining conditions, however, reinforcement may facilitate the formation of more evenly spaced and narrower cracks as opposed to a few wide cracks. This means that reinforcement may be required to provide crack control and to ensure the integrity of the structure [69,70].