Why the higher water/cement ratios?
Shrinkage-compensating cement needs about 10% to 15% more mixing water than standard cement to accommodate the formation of the ettringite. Water cement ratios are typically 0.45 to 0.50 for interior concrete and 0.50 to 0.55 for exterior. We like to see the concrete batched at a seven-inch slump at the concrete plant because of the high demand for water, which usually occurs initially when mixing water with shrinkage-compensating cement. This high demand is caused by the (1) hydration of the cement, (2) the ettringite growth starts immediately, and (3) we are wetting materials. We can look for a one-to-three inch slump loss within the first 30 minutes. Shrinkage-compensating concrete should be placed at a five plus/minus one inch slump.
