When should mortar cube testing be performed?

ANSWER: 

Mortar is typically specified to meet the “proportion specification”, which is the default in ASTM C270.  The only circumstance in which mortar cube testing is required is when the mortar has been specified by the “property specification” and the proposed mix design must be verified to meet the property requirements of ASTM C270.  In that circumstance, the proposed mortar mix is batched in the laboratory using the laboratory proportions and conditions defined within ASTM C270. Laboratory-prepared mortars differ from project site-mixed mortars in the type of sand, the water content (which is less), the curing temperature, and humidity of the curing environment.  Once the laboratory establishes a mix design that satisfies the property requirements of ASTM C 270, it defines the mortar’s component materials and their proportions for the contractor to batch in the field.  An appropriate quality assurance measure for field-batched mortars is inspection during batching and mixing.  If test reports are desired for a “paper trail” of compliance, mortar aggregate ratio testing in compliance with ASTM C780 should be considered. ASTM C270 is not a specification to determine mortar strength through field testing.

Many designers incorrectly assume that mortar compressive strength testing during the project is a good approach.  However, mortar cube testing is of little value for several reasons:

  • Mortar compressive strength determined by testing a two-inch cube does not represent mortar compressive strength in the wall, where the mortar is typically 3/8-inch thick. Furthermore, mortar cubes are made in non-absorptive molds while mortar used to erect masonry is subject to water absorption from the mortar into the masonry units. Mortar in the wall, with its lower water content (due to absorption), will have greater compressive strength than that in the molded cube formed from the same batch.
  • Mortar compressive strength testing takes 28 days. If there truly is a problem with the mortar, a significant amount of masonry will have been erected in those 28 days.  Mortar aggregate ratio testing in accordance with ASTM C780 can be completed in a half-day.
  • Compressive strength testing of field-mixed mortar yields inconsistent results because the water content in the mortar will vary with weather conditions. During hot dry weather, the masonry contractor will mix the mortar with more water to compensate for evaporation.
  • The contribution of mortar compressive strength to compressive strength of the masonry assembly is small – the masonry units are the greatest factor in assembly compressive strength. Refer to the FAQ “Is the compressive strength of mortar important?”

Consequently, compressive strength testing of field-mixed mortar does not provide meaningful results.

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