Heat resistance during operating service is the most important physical property for grout. A grout that can’t stand the heat means premature grout failure, lost downtime and lost money. The critical time period for a foundation grout is after two (2) hours at temperatures in excess of 150 °F. After 2 hours, the heat softens most grouts, causing alignment failure. Cooling off the grout will reharden it, but the damage is already done. If the grout does not have high heat deflection, the continuous softening and hardening will allow the machinery to wallow out or lose alignment. The knife test at operating temperatures will indicate if the grout is softening. Some grouts will test in excess of 12,000 psi at ambient temperature, but will test at less than 1,000 psi after 2 hours at 160 °F. Any solvents added to the grout will further decrease the grout’s heat resistance at operating temperatures. Grouts with low flashpoints usually have added solvents.