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Advertising supplement provided by Portland Cement Association

by Jennifer Grover Prokopy

Recommended Test Methods

The recommended test methods for field quality control are outlined in ASTM C780: the mortar aggregate ratio test (Annex A4) and the mortar water content test (Annex A5). The mortar aggregate ratio test provides a means to “identify, measure, evaluate, and control differences which may be expected to exist between laboratory and jobsite mortars.” When used together, the two tests provide a powerful QC measure of field mortars, providing an accurate comparison to preconstruction laboratory proportions.

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Avoid the Most Common Testing Mistake

One of the most common mistakes made is to test the compressive strength of field mortars (defined by ASTM C780) as a quality control measure and expect conformance with the property requirements set forth in ASTM C270. This is done even though ASTM C780 clearly dictates against the practice:

This test method [compression testing] establishes testing procedures for determining compressive strength of preconstruction and construction [field] mortars. Strength values for mortars obtained through these testing procedures are not required, nor expected, to meet strength requirements of laboratory specification C270 mortars.

A number of conditions cause compressive strength of field mortar to differ from compressive strength measured in the laboratory under ASTM C270 specifications.

First, mortar mixed in the field will not contain the same amount of water as the laboratory specimen; standard practices (and ASTM specifications) allow masons to adjust water levels on-site to achieve optimum workability. A variety of situations, including environmental conditions that vary from lab to site and from day to day, can result in a difference in water level large enough to skew test results. Second, these environmental (weather) differences can affect the compressive strength of test specimens. Third, the sizes and proportions of test specimens vary greatly from mortar joints in the field, making a fair comparison difficult. Finally, laboratory specimens also include mechanical effects that lower their perceived compressive strength. These factors combine to make it nearly impossible to achieve accurate compressive strength comparisons from field to lab.

Conclusion

Solid testing procedures are an essential part of quality control, and good quality control procedures are a critical part of quality assurance. Use the tools offered in this unit to assist in creating a quality assurance program for mortars.