Testing grout on the job site and in a laboratory should be consistent to ensure accurate testing values. When testing the compressive strength of Non-Shrink Grouts or Post-Tension Grouts, we follow the guidelines from ASTM C1107. This specification states to cover the cube molds with a cover plate that is flat, rigid, nonreactive and no less than ¼ inch thick. Then place a 15 lb weight on the cover plate to ensure the grout is restrained.