How to Calculate Swiss Hammer Readings

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The Swiss hammer test is the most common non-destructive test used for measuring in-situ compressive strength of concrete. An in-situ test is a test performed in place -- not in a laboratory setting -- such as a bridge deck, bedrock or a concrete floor slab. The Swiss hammer test is also known as the rebound hammer test or Schmidt hammer test after the inventor of the device, Swiss engineer Ernst Schmidt. Since the Swiss hammer test is basically a surface hardness test, readings from the device must be interpreted carefully to correlate results to a compressive strength of concrete.

Things You'll Need

  • Swiss hammer
  • Calculator
  • Pencil
  • Straight edge
  • Push the hammer scale reading slider to zero (if present). Some Swiss hammers have an internal reading scale and do not require resetting.

  • Push the plunger end of the Swiss hammer against a hard surface to unlock its position. Keep applying pressure until the plunger rod is pressed into the hammer body and the internal latch catches the steel hammer. Once the internal latch catches the hammer, stop applying pressure.

  • Hold the Swiss hammer perpendicular to the concrete test material. Slowly press the hammer perpendicular towards the concrete until the latch releases, allowing the hammer to drop.

  • Press the button on the side of the hammer to lock the slide indicator in place. Read the value nearest to the pointer on the scale. The reading from the hammer is also known as the R-value. R-values vary from 0 to 100. The hammer slide scale values correlate to the hardness of the concrete. Higher R-value readings equal a greater concrete hardness. Record the R-value for the test.

  • Perform 10 test readings no closer than 1 inch apart on the concrete sample. If the test rod crushes the surface of the concrete because of voids near the surface, throw out the test result.

  • Add up the 10 R-values and divide the sum by 10 to calculate the average. If any of the 10 R-values vary from the average by more than six, exclude the value and recalculate the average by dividing the sum of the remaining nine results by nine. If more than two R-values vary from the average by more than six, throw out all the results, choose 10 new test locations and go back to the beginning of this step.

  • Reference the average R-value to the calibration graph provided with the Swiss hammer or with a calibration graph created using Federal Highway Administration publication FHWA-SA-97-105. The graph is sometimes printed on the side of the hammer body. Using the R-value, go to the x-axis of the graph and match the calculated value. Draw a vertical line up to the calibration line. Draw a horizontal line to the left to intersect the y-axis. The value intersected on the y-axis is the estimated compressive strength of the concrete based on the Swiss hammer test.

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