Question: When using a manual straightedge how high a slump can be allowed while still maintaining control of shrinkage and minimizing curling? What else can be done to prevent or minimize curling?

Answer: Slump should be kept as low as practicable for proper finishing. Usually this means 2 to 3 inches, but if the placing and straightedging are to be done entirely by hand it can be 4 inches. The water content can be minimized, however, by using water reducing admixtures and maintaining the same slump.Other measures that can be undertaken to minimize shrinkage, curling and slump are as follows:

  • Avoid using aggregates that have a high shrinkage potential.
  • Organize the job for prompt placement of concrete so that there are no delays with consequent need to retemper the concrete with additional water.
  • Reduce the spacing interval between joints to minimize curling in each panel.
  • After curling is complete apply a coating, sealer or wax to slow down the rate of moisture loss and at the same time minimize carbonation which could contribute to surface shrinkage.