Proper Procedures

Q:  We’re bidding a paving project and the specification calls for testing of the bond between the setting bed and the stone to verify 50 psi minimum bond strength. We were planning on using a conventional mortar bed with no additives. Will this meet 50 psi or should we plan on purchasing something of higher performance?

A:  I wouldn’t be worried about using the standard mortar. When this test is asked for, the lab will likely follow the ASTM C482 procedure “Standard Test Method for Bond Strength of Ceramic Tile to Portland Cement Paste,” although with some modifications. This isn’t a test that I do very frequently, but when I have done it with a standard Type S mortar I’ve gotten results of between 100 and 150 lbs/in². The higher porosity stones typically do better since there is a better mechanical lock at the stone/mortar interface. As in all cases, the key will be quality control. When test specimens are prepped, whether by the supplier or the lab, extreme care is taken to make sure that the stone surface is clean and that 100% mortar to stone contact is achieved. When setting in the field, such perfection is not the norm. So if the job calls for a field test as opposed to a lab test, you need to have confidence in the workmanship of your crews.

Q:  We’re nearly completed with installing an anchored veneer project. We had the system engineered and we installed the stone per the engineer’s design. Now at the last minute, the owner has retained a curtainwall consultant to do pressure tests on the building to verify that the system will hold up. The project is in a hurricane region, so we’re concerned due to the high loads that will be used in the test.

A:  The normal procedure used for this type of test is ASTM C1201, “Standard Test Method for Structural Performance of Exterior Dimension Stone Cladding Systems by Uniform Static Air Pressure Difference.” While this procedure was written to be performed as a laboratory test, it can, with some modification, be done in the field. I’m not worried about the high loads, since the engineer that designed your anchorage system was using those same high loads when doing the design. Depending on the type of stone, the engineer was using a factor of safety of anywhere from 3.0 to 8.0, while the in situ pressure test will not be taken to that high of a factor of safety. Doing this test in the field creates a challenge in achieving a good seal of the chamber, and leaks in the chamber prevent them from achieving really high pressures. I would be surprised if the field technicians can get the pressures up past about 1.5 times the design loads. If you did the installation according to the engineered design, it should easily pass this test.