10.048 Validity of Mortar Testing

Q During a preconstruction meeting, concern was voiced that the mortar testing should be suspended since it would not represent the actual field strength conditions. What circumstances can be stated to explain the concern?

— Project Manager in Southern California

A 12/08 – Response prepared by Kurtis K. Siggard, Executive Director of the Concrete Masonry Association of California & Nevada (CMACN).

Care must be taken in how we sample, test, and report mortar tests. Mortar, by its nature, retains water for an extended period of time. The water in the mix is necessary to maintain the workability of the mortar and to create the bond between the masonry unit and mortar. There have been significant problems lately with field-testing pre-blended mortars that are proportioned to meet the property requirements of ASTM C270. Many of these pre-blended products have constituents that retain a greater amount of water requiring less retempering of the mortar. Due to the high water/cement ratio of these mortars when sampled soon after mixing, and the molding of samples in watertight containers, sampled strength may be much lower than the strength of the mortar used in construction. ASTM C1586 states: Measuring mortar compressive strength of field-sampled mortar has no relevance unless preconstruction testing is performed in the laboratory using similar mixing equipment, mortar materials, and the same specimen geometry. Even when this is done, the field compressive strength data can only be compared to the preconstruction mortar strength data in general, due to other factors, such as weather, temperature of mortar, and the absorption properties of the specific masonry units being used.

Reference Documents

ASTM C270 Standard Specification for Mortar for Unit Masonry

ASTM C1586 Standard Guide for Quality Assurance of Mortars