I have heard that ready mixed mortar has higher air content than conventional mortar and thus lower bond strength. Is this true, and if so, how much less bond?
According to Sylvester Schmidt of Addiment, Inc., which makes extended-life retarders for ready mixed mortar, ready mixed masonry cement mortars have about the same or slightly less air content than job-mixed masonry cement mortars (16% to 17%). Ready mixed portland cement-lime mortars, on the other hand, have much greater air content (16% to 18%) than conventional, job-mixed cement-lime mortars (4% to 6%). Despite the higher air content, some tests have shown that the bond strengths of ready mixed cement-lime mortars equal those of job-mixed cement-lime mortars. More research is needed to prove this. Many other factors in addition to air content influence bond strength of masonry. I recommend performing bond strength tests on prisms made from the proposed brick and mortar. Construct the prisms before making the final mortar selection. Build and cure them in conditions like those expected on the job. Norbert V. Krogstad is a senior engineer at Wiss, Janney, Elstner Associates, Inc., Northbrook, Illinois.