The sclerometer (also known as Schmidt's hammer) is a measuring instrument created, initially, for the determination of the compressive strength of concrete[1]​ whether in pillars, walls, pavements, etc. In some countries it is no longer used to determine the strength of hardened concrete, but is only used to evaluate the uniformity of in-situ concrete, delineate areas of deteriorated or poor quality concrete or estimate in-situ strength development.

Its operation consists of a weight tensioned with a spring. Said tensioned weight is thrown against the concrete and its rebound is measured.

Although it is not an overly reliable method, its use is widespread. It provides approximate values ​​and is mainly used as a verification method, being less used than the compression test.

In 1950, the first sclerometer was designed for non-destructive measurement of concrete. Patented under the name SCHMIDT, its rebound value "R" allows the hardness of this material to be measured. It has become the most used procedure, worldwide, for non-destructive control in concrete.

Digital sclerometer: compared to conventional sclerometers, the advantage of incorporating a screen in which minimum and maximum values ​​of a series are collected whose number of measurements is established by the user. In addition to automatically establishing concrete age tables, discarding erratic values, displaying the standard deviation and directly correcting the direction of impact, it translates the average R value into conventional strength units, assigns a number to the measurement and can transfer all data to digital documents via a PC.

It is a very subjective method. Depending on who does it and how they do it, it will yield more or less reliable values. Some of the causes that can alter the value are:

• If the sclerometer is not perfectly perpendicular to the surface, it will give us a smaller rebound.

• If the surface is not very clean and polished it will give us lower values ​​(layers of paint or dust create a soft layer that cushions the blow).

• Concrete is a mixture of water, cement, gravel, sand and additives. If we hit a thick aggregate it will give us greater hardness.

• If the concrete is reinforced, we run the risk of hitting close to a corrugated steel bar, with a much higher hardness.