Curing and Environmental Control for a Large Grout Placement
A 6 m × 4 m grout bed is placed outdoors in late summer. The ambient temperature is 32 °C, the relative humidity is 30%, and there is a steady 15 km/h wind. The grout is a cementitious non-shrink grout with a specified 28-day strength of 45 MPa and a maximum shrinkage limit. The crew places the grout and immediately covers it with a single layer of polyethylene sheeting, but the sheeting is not weighted down and flaps in the wind. The next morning, the surface shows plastic shrinkage cracks across approximately 20% of the area, and the edges are dry and crumbly. The specification requires curing for 7 days and a surface moisture retention method.
1. Explain the cause of the plastic shrinkage cracks and the dry, crumbly edges. What environmental controls were missing?
Read the worked answer
Plastic shrinkage cracks occur when the rate of evaporation from the fresh grout surface exceeds the rate of bleed water rising to the surface. At 32 °C, 30% RH, and 15 km/h wind, the evaporation rate is high - likely well above the threshold (roughly 1 kg/m²/h) at which plastic shrinkage cracking becomes likely. The single, unweighted polyethylene sheet flapping in the wind provided intermittent coverage, allowing evaporation and wind to reach the surface. The dry, crumbly edges indicate rapid moisture loss and incomplete hydration at the perimeter, where the sheet was least effective. Missing controls: windbreaks, fogging or misting, placing during cooler periods, and a properly secured curing cover. The crew should have used a weighted, continuous cover, or a curing compound compatible with the grout, and monitored evaporation rate.
2. What curing method should be used, and how should the crew verify that curing is effective over the 7-day period?
Read the worked answer
The crew should use a moisture-retention method: a continuous, weighted polyethylene sheet or a curing blanket, kept in contact with the surface, or an approved curing compound applied uniformly. Given the wind, the sheet must be weighted or taped at edges and seams. Fogging or misting can be used to maintain surface moisture during placement. Verification: check the surface daily for dampness under the cover; measure surface temperature and ambient conditions; and confirm the cover remains in place. If a curing compound is used, verify coverage rate and compatibility. After 7 days, inspect for cracks and surface hardness. Any cracks should be evaluated for depth and whether they compromise the grout's function; shallow plastic shrinkage cracks may be acceptable, but deep cracks require repair.