Rebound Hammer Test

Nondestructive and in-place test methods are valuable in overall quality assurance of concrete. They cannot be used to evaluate strength until laboratory studies develop correlations with traditional strength test results on concretes made with the same materials and mix proportions. Without such correlations, the nondestructive tests can serve to evaluate relative strengths of hardened concretes.

A widely used in-place test is done with a rebound hammer, frequently referred to as a Schmidt hammer, shown in Figure 10. It is a spring-controlled hammer that slides on a plunger.

When you press the plunger of the hammer against a concrete surface, the hammer retracts against the force of the spring. When the hammer retracts completely, the spring automatically releases. The hammer impacts the shoulder area of the plunger, and the spring-controlled mass rebounds. The rebound distance, known as the “rebound number,” is measured on a scale attached to the instrument.

The results of a Schmidt rebound hammer test performed according to ASTM C 805 are affected by surface smoothness; size, shape, and rigidity of the specimen; the type of coarse aggregate, and the carbonation of the concrete surface. When you recognize these limitations and calibrate the hammer for the particular materials used in the concrete, this instrument is useful to determine the relative compressive strength and uniformity of concrete in the structure.

Although rebound numbers are not a precise indication of the concrete strength, higher numbers mean greater strength compared to concretes made of the same materials and placed at about the same time.

Using the impact rebound hammer seems simple, but it is easy to get misleading readings. There are three main steps to this test procedure:

1. Press the plunger against the concrete surface until the hammer impacts.
2. Press the button to lock the plunger.
3. Read the scale.

The operator must take the following precautions:

• The concrete surface must be dry and smooth. Use an abrasive stone or a grinder to smooth a rough surface.
• Keep the plunger at right angles to the surface.
• Take and record ten readings. No two test points should be closer together than 1 inch.
• Calculate the average reading as follows:

1. Average the ten readings.
2. Discard any readings that vary from the average by more than 7.
3. Average the remaining readings.

If more than two readings vary by more than 7 from the average, discard all 10 readings and repeat the procedure.