Our firm is adding a partial second story to an old warehouse to convert it to an office building. The foundation and soil conditions make the project possible, but how can I determine the compressive strength of the existing masonry? How many areas should I test? What methods are most cost effective?
I know four methods for determining the compressive strength of existing walls. The first is to remove a section from the wall and send it for laboratory testing per ASTM E 447. There are some problems with this approach, however. Physically removing samples from the wall affects its appearance. Depending on the building's age, matching the color and texture of the masonry units removed may be nearly impossible. Also, samples may be damaged during removal or shipping, especially if the wall has multiple wythes. A second method is to make two vertical cuts entirely through the wall thickness and remove some of the masonry between these cuts. An in-place compression test can be performed using a hydraulic ram. As with the previous test method, the wall will be damaged by this testing and it may be difficult to perform in multi-wythe systems. Because the test is performed in place, however, samples will not be disturbed in shipping. The third method involves removing masonry units and samples of the mortar and estimating the compressive strength. The masonry units are tested in accordance with ASTM C 67 to determine compressive strength. Mortar is analyzed for chemical composition. The masonry strength can be estimated using Tables and in ACI 530-88. The fourth way to test the compressive strength in the wall is to use a flat jack as described in ASTM C 1196-91. The test involves making a horizontal cut in the mortar joint. A thin steel bladder is installed into this cut. The masonry is loaded by pumping hydraulic fluid to expand the flat jack. Because no bricks are cut, the wall is easily repaired by repointing the joint. I recommend using this fourth method for most instances. To determine how many tests to perform, use ASTM E 122. If you use a factor of 3, an error of 10%, and a coefficient variation of 15%, the procedure indicates that you should perform 20 tests. For a small facility, this may not be practical. However, I recommend performing at least five tests, whatever the facility's size. Performing five tests gives a probability of 1 in 10 that the measured compressive strength will vary from the actual compressive strength by more than 10%. More tests may be needed if the coefficient of variability exceeds 15%. This ignores variability within the test method itself.