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.

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