Inspection: Evaluating Existing Masonry
Remodeling or restoring existing masonry construction generally requires some knowledge of the material properties of the construction. If you’re lucky, there will be existing drawings that define the masonry compressive strength (f 'm), from which some of the other masonry property values can be derived. What if the original construction documents are nowhere to be found or the drawings have no information on the material properties of the masonry? Typically, drawings for most buildings prior to about 1930 wouldn’t have information on the specified masonry compressive strength. Fortunately, there are in place or “in situ” test methods that can be used to evaluate the masonry state of stress, the deformability and compressive strength, the typical shear strength of the mortar and the general mortar type.
An in situ stress test setup. When the original distances between the upper and lower gage points (circled) are restored, the pressure in the flatjack (center) has restored the original compressive stress in the wall.
Existing Stress in Masonry
The compressive stress in clay masonry can be evaluated with a stress relief test using a thin metal bladder called a flatjack. The test is described in detail in ASTM C1196, Standard Test Method for In Situ Compressive Stress Within Solid Unit Masonry Estimated Using Flatjack Measurements. Briefly, the test involves installing gage points, typically four courses apart and taking and recording very accurate measurements of the distance between gage point pairs. Next a mortar bed joint, midway between the gage points is removed by either drilling or saw cutting. If the masonry is loaded in compression, the gage points will move slightly closer together after the removal of the bed joint. In order to restore the original dimensions of the gage points, a flatjack is inserted into the empty bed joint and slowly pressurized. Once the original gage point dimensions have been restored, the pressure in the flatjack is essentially the compressive stress in the wall that existed prior to the test. This information is valuable in understanding how loads are being transmitted through the structure and to verify the stresses are consistent with calculations.
The test setup shown above is basically a prism test conducted in the wall. The flatjacks, top and bottom, apply load to the masonry between them. The LVDT’s (center) measure the displacement as the pressure is increased. The wall above and below the test provide the necessary reaction forces.
Masonry Compressive Behavior and Strength
Another common test in clay masonry and occasionally in stone masonry is ASTM C1197, Standard Test Method for In Situ Measurement of Masonry Deformability Properties Using the Flatjack Method. This test measures the deformation of the masonry as it is subjected to loading from two flatjacks. Flatjacks are inserted into two evacuated mortar joints, typically, separated by five courses of brick or by a similar dimension in stone. As the flatjacks are simultaneously pressurized, accurate measurement devices, known as LVDTs, measure the deformation of the masonry. The developed pressure vs. deformation curve gives engineers information on the stiffness of the masonry and its likely ultimate compressive strength.
Mortar joint shear test in clay masonry. The hydraulic ram (left) applies load to the brick to the right until slip occurs. The dial indicator is used to monitor movement of the test unit.
Mortar Joint Shear Strength
In any major renovation of an existing masonry building, there will likely be an analysis of the resistance of the structure to horizontal forces such as wind and seismic loads. The in-plane shear behavior of masonry walls is dependent on the strength of the bond between the mortar and the unit. This bond strength can be tested in place using a test method outlined in ASTM C1531, Standard Test Methods for In Situ Measurement of Masonry Mortar Joint Shear Strength Index. In this test, a clay, concrete or stone masonry unit is isolated for the test by removing the mortar head joint on one side and either the head joint or entire unit on the other side of the test unit. The test can be conducted either with a hydraulic ram placed in the space occupied by the removed unit or with a small flatjack in one of the empty head joints. A force is applied to the the test unit and the force required to slide the unit is measured to provide a value for the mortar joint shear strength. Engineers use this number to evaluate the adequacy of the existing walls to resist horizontal loads or devise strengthening methods or additional shear walls as necessary.
Pendulum hardness test. The pendulum has swung from the inverted position and rebounded to a value of approximately 60 (no units).
Mortar Hardness
Mortar hardness can be related to the approximate compressive strength of the mortar using a device called a rebound hammer. The only rebound hammer specifically designed for testing mortar in place is manufactured by Proceq and is known as the Schmidt, Type PM hammer. The device is a simple pendulum that swings from an inverted position through approximately 180 degrees to impact the mortar in a bed joint. The pendulum then rebounds to a maximum angle where one-way clutches stop the pendulum. The rebound number can be read from the side of the device and the higher the rebound number, the harder the mortar. The rebound hardness can be related to the mortar type (e.g. Type N), and can indicate mortar uniformity on a project. The pendulum hammer can also provide a rough idea of the compressive strength of the mortar.
In summary, it is possible to get masonry material properties for an existing or historic masonry building and a number of tests can be done in place, eliminating the need for prism removal and associated repairs. Further, in place testing eliminates potential damage to the sample that can occur when sawing, removing and transporting prisms or other material samples.