The ASTM C 270 Standard Specification for Mortars for Unit Masonry recognizes two methods of specifying and mixing mortar: by proportion and by property. Which method is better? We've asked two knowledgeable engineers, Wisconsin consultant Al Isberner and Diane Throop of the Masonry Institute of Northwestern Ohio, to spell out their reasons for advocating one method over the other. The Proportion Method: Simpler is Better
by Al IsbernerIdeally, the "Mortars" portion of all project specifications should simply state: "All masonry mortar shall be Type N (or S or M, as applicable) in accordance with ASTM Specification C 270." Nothing more needs to be stated to have the mortar requirements understood by all parties familiar with this national standard.Masonry contractors do appreciate being able to select constituent materials and adjust the mortar's aggregate content to suit the units being placed and the climatic conditions during construction. They also appreciate the elimination of onsite sampling and testing of the prepared mortar. This latitude in proportioning the mixtures and ease of conforming to both the project specification and ASTM C 270 allow construction to proceed with a minimum of peripheral interference. The Property Method: We Need the Numbers
by Diane ThroopThe original authors of ASTM C 270 began their work by agreeing to develop a property specification. They listed the desirable properties of a masonry mortar and began creating test methods to measure those properties so a specification could be written. At some point, however, it became clear that to get an ASTM specification approved by consensus, a proportion table would also have to be added. We have lived with this confusing compromise ever since. For several reasons, the industry needs to move toward the property method. First, using the property specification gives you test results for mortar so you know its laboratory properties. Second, specifying desired properties avoids the potential for getting mortar with properties significantly different from those listed in C 270 Table 2. Third, the industry is moving toward "engineered masonry" and the property standard for mortar is consistent with that direction. Fourth, shifting to the property standard is a step toward expansion of the property table.