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A Schmidt hammer, also known as a rebound hammer, tests the compressive strength of concrete by striking it with a spring-loaded metal rod. The concrete must be fully set, the surface smooth and clean, and the tool calibrated for accurate results. The operator can read the results from a digital or analog scale.
A Schmidt hammer is a tool that can test the compressive strength of concrete. These tools are also referred to as rebound hammers due to the way the mechanism works. A typical design consists of a portable cylinder that holds a smaller spring-loaded rod. The strength of a concrete structure can be determined by striking it with the metal rod and then examining the rebound. A Schmidt hammer can have a digital readout or an analog scale that the operator must read and interpret manually.
To get accurate results from a Schmidt hammer, a number of conditions must be met. The concrete must be fully set, which means it should typically be two weeks to over a month old, depending on conditions. If the concrete is still green, it will have too much sag and may not return useful readings. It may be necessary to prepare concrete that is more than two months old by removing some of the surface material. The surface must also be smooth and clean or the results may be inaccurate.
There are a few other variables that can change the outcome of a rebound hammer test. The results of a test may vary slightly depending on whether the tool is held horizontally, used to test a concrete floor, or used to strike the underside of a slab. This can manifest itself in a reading that is slightly too high or too low. The thickness of the material to be tested can also be important.
Once you have checked the age of the concrete and prepared the surface, you can use the Schmidt’s hammer. This process typically involves releasing the hammer rod and then pressing it into the concrete. The operator can slowly increase the applied pressure until a mechanism inside the hammer activates and strikes the shaft against the concrete. This is usually automated by the tool and the operator will only need to hold it steady while reading a scale on the body of the hammer. A digital readout may show the exact compressive strength, or the operator may need to look at a mechanical scale.
The spring or other internal components inside a Schmidt hammer can wear out over time, causing inaccurate readings. This is usually fixed by calibrating the device from time to time. Some type of metal anvil that has a known compressive strength will usually be used for this type of calibration work. If the hammer is used on one of these anvils and returns a different result than expected, the device can be calibrated to give the correct readings.
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