As an architect, I would like to be able to perform quality control checks on the mortar mixing. In the past I have required the contractor to prepare mortar cubes for every 5,000 square feet of masonry and, on most projects, this has worked out well. On a few, however, the seven-day test results came back very low. We waited three more weeks to test the remaining cubes from that batch at 28 days. These results also were low, as were the results of subsequent cube tests from other batches. By this time, much of the masonry had been completed. The contractor felt the problem was in the method of fabricating and curing the cubes, not in the masonry work itself. So we cut prisms from the wall and tested mortar bond by using the bond wrench test (ASTM C 1072) and compressive strength by using ASTM E 447. The results of these tests were within normal ranges. This experience has made me rethink the whole quality control process. What can we do to evaluate the mortar mixing that will give timely results and be less dependent on the method of fabrication and curing?
The best way to monitor the mortar is to watch it being mixed. Obviously, this is not always convenient. An alternate approach would be to determine the mortar aggregate ratio in accordance with A4 of ASTM C 780. This test determines the ratio of cementitious components to sand in the mortar mix. It should be performed in conjunction with A5 in ASTM C 780, which is the test for mortar water content. These tests can be performed on any fresh mortar, either out of the mixer or off the board. They do not, however, determine the ratio of portland cement to lime when portland cement/lime mortars are used. This could be monitored by recording the number of bags of each that are used throughout the project. Another test that may be useful is the mortar air content test described in A6 of ASTM C 780. With it, you can monitor the mortar mixing time and efficiency. Large variations in the air content may indicate a problem with the mixing procedures. For a masonry quality control program to be meaningful, it should be conducted in conjunction with a preconstruction testing program. Preconstruction tests should be done on the same materials that will be used in the construction. Changes in the mortar air content can be studied by varying the mixing time during preconstruction tests. The mortar aggregate ratio can be evaluated using sand with different moisture contents. Both these tests can be performed on the mortar being used to build prisms for preconstruction compression or flexural testing.