Concrete Non-Destructive Testing – Schmidt’s Hammer

The Schmidt Hammer, also known as a Swiss Hammer or Sclerometer, is a portable measuring instrument for testing the elastic properties or force of concrete or stone, especially surface hardness and resistance to penetration.

It was invented by the Swiss engineer Ernst O. Schmidt in the early 1950s and introduced to the market by Proceq. It is considered one of the main non-destructive test methods of concrete.

The compressive strength of the concrete is one of the most important parameters considered by engineers to evaluate the condition of the structure. The removal of testimonies from concrete pieces is often restricted or even prohibited, and therefore the Non Destructive Method is often the only viable alternative. The time to present the results is much lower than the rupture tests, since the test is performed in-loco. As it is unnecessary to remove testimonials, no aggressive equipment is required.

 

The hammer measures the energy recovery from the impact of a spring mass against the sample surface. The test hammer hits against concrete with a defined energy. Its recovery is dependent on the hardness of the concrete and is measured by the test equipment. By means of a conversion table, the value of this recovery can be used to determine the compressive strength. The test should be performed with the hammer at right angles to the surface which in turn should be flat and smooth. The recovery reading will be affected by the orientation of the hammer when used upright (at the bottom of a suspended slab, for example) gravity will increase the return distance of the mass and reduce to a test performed on a floor slab. The Schmidt hammer has an arbitrary scale ranging from 10 to 100.

 

The test is also sensitive to other factors:

Local variation of the sample. To minimize this, it is recommended to take a variety of readings and take an average value.

The water content of the sample, a saturated material will give different results of a dry.

 

The state-of-the-art equipment has been switched from analogue to digital, and integrated with a software platform that analyzes various measurements, statistically improving curves according to in-situ conditions. Some models already have almost no influence of the angle of inclination and with the aid of the software can make the correction of form and carbonation.

 

Prior to testing, the Schmidt hammer must be calibrated using an anvil calibration test provided by the manufacturer for the purpose. 12 readings should be taken, scorning the largest and the smallest, then take the average of the remaining ten. Using this test method is classified as indirect as it does not give a direct measurement of the strength of the material. It simply gives an indication based on the surface properties, it is only appropriate to make comparisons between samples.

 

At the end of the video, a demonstration of an ultrasonic device test, which the manufacturer recommends for comparison of results, can be observed.

The Schhmidt Hammer can also be used on rocks and even rolls of paper.

This test method for testing concrete is governed by ASTM C805. It is also in accordance with EM 12504-2 and EM 13791.

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