Question: Our job has just been shut down because the mortar doesn't meet the specified strength. The architect specified Type M mortar (2500 psi) under ASTM C 270, Specification for Mortar for Unit Masonry. The testing laboratory molded mortar cubes on the job from samples taken from the mixer, then took the cubes to the laboratory for curing and testing. The engineer at the testing laboratory told us the tests were done according to ASTM C 109, Test Method for Compressive Strength of Hydraulic Cement Mortars, as directed by C 270.The architect has called a conference to resolve the problem.

We think he'll require us to add more cement to the mortar. Because of the hot weather, we're already having trouble with short boardlife and are afraid more cement will make the problem worse. What other alternatives can we suggest?

Answer: You're caught in the middle of a misunderstanding about ASTM C 270. Mortar that meets the requirements of C 270 must be made in the laboratory using the same materials and proportions that will be used on the job. The entire C 109 testing procedure--proportioning, mixing, molding, curing, and testing must be done in the laboratory. Mortar cubes made in the field and tested in the laboratory do not have to meet C 270 requirements.You can test job-mixed mortar using ASTM C 780, Preconstruction and Construction Evaluation of Mortars for Plain and Reinforced Unit Masonry. But results of C 780 tests also don't have to meet strengths specified in C 270.At the moment, the project architect has no qualified test data to help him determine whether or not the mortar meets specifications. Here are two suggestions you could make at the meeting:

  • Require that laboratory tests be made on laboratory-prepared mortar using materials and proportions to be used on the job. If the results meet the requirements of ASTM C 270, the mortar is acceptable.
  • Require that C 780 tests be made on job-mixed mortar and their results correlated with results from laboratory tests on laboratory-prepared mortar. Correlated C 780 results would then be a guide for determining if mortar is acceptable. Be aware that the correlation may change with changing job conditions.