The final part of a three-part series, this article discusses the role top-down stiffening of concrete plays in delaminations. Concrete gradually stiffens, or loses plasticity, as water is lost and hydration products build up. Although a small amount of water immediately combines with cement during the first few minutes of mixing, most of it remains uncombined as hydration reactions enter a dormant period. During this dormant period, the stiffening rate is less affected by hydration-product buildup than by water loss. Thus, for most slabs, timing of finishing operations may depend more on jobsite factors affecting water loss.
The authors believe that directional stiffening can have a big impact on the probability of delaminations occurring. Concrete that stiffens from the bottom up is less likely to trap bleedwater than concrete that stiffens from the top down. Stiffening rate at the surface is influenced by wind speed, relative humidity, air temperature and radiant heat from sunlight. Field observations and tests performed by the authors indicate that higher wind speeds accelerate stiffening, allowing finishers to start floating and complete troweling earlier. However, premature finishing shouldn't automatically be cited as the cause of delaminations. The probability of slab delaminations increases markedly when air-entrained concrete is used or conditions that increase bleeding capacity are added to those that produce top-down stiffening.