Using a Schmidt hammer is an effective method to determine the surface strength of concrete during the curing process. The nondestructive rebound method provides you with the ability to know whether the concrete strength is variable or consistent over your construction project. In this regard, the Schmidt hammer test acts as a precursor to determining if your concrete is ready for further load, such as in the construction of foundations, or if you have problems in concrete quality in specific areas of your build. The testing principles and mechanics of the rebound hammer date back to Ernst Schmidt's hammer design in 1948.
Select an area of concrete to test. Test 10 times in one area with a minimum distance of 1 inch between each test site, as recommended by the U.S. Department of Transportation.
Remove surface lumps of concrete with a carborundum stone to provide a consistent surface across the test area. Press the flat end of the plunger rod on the Schmidt hammer onto a solid surface so that the rod unlocks and extends.
Position the flat end of the plunger rod onto your first test site at a 90-degree angle to the concrete. Hold the barrel of the Schmidt hammer and push the barrel forward slowly using a smooth action. Maintain a strong grasp on the barrel as the plunger rod reaches full resistance on the concrete and triggers the rebound measurement.
Lock the rebound button into position on the side of the Schmidt hammer. Write down the number showing on the indicator in order to note the rebound number or "R-value." Reset the plunger rod as you did previously in Step 2.
Conduct a further nine tests within your concrete test area, making a note of each R-value. Add up the 10 R-values and divide the result by 10 to calculate the average R-value for the test site. Reject any single R-value if it is not within six whole numbers of the average and work out the resulting average using the R-values that remain.
Continue to use the Schmidt hammer to test further concrete areas by repeating Steps 1 to 5. Use the Schmidt hammer manufacturer's correlation graph to compare your average R-value for each test area to gain the expected compression strength of the concrete.