The rebound (Schmidt) hammer is one of the oldest and best known methods of comparing the concrete in various parts of a structure and indirectly assessing concrete strength. The hammer weighs about 1.8 kg and is suitable for use both in a laboratory and in the field. The rebound of an elastic mass depends on the hardness of the surface against which its mass strikes.

Applications

• Finding the elastic properties like compressive strength
• Assessing –  the uniformity of concrete.
• Determining variations of strength within a structure

Codes

• IS 13311, IS 516: 1959
• BS EN 12504-2
• ASTM C805

Methodology

• Before commencement of a test, the rebound hammer should be tested against the test anvil, to get reliable results,
• Apply light pressure on the plunger. it will release it from the locked position and allow it to extend to the ready position for the test.
• Press the plunger against the surface of the concrete, keeping the instrument perpendicular to the test surface. Apply a gradual increase in pressure until the hammer impacts
• Take the average of about 15 readings.

Limitations

• Hammer has to be used against a smooth surface, preferably a formed one.
• If the concrete does not form part of a large mass any movement caused by the impact of the hammer will result in a reduction in the rebound number
• Not a very good test for old concrete and low strength concrete.
• Moisture content affects the readings significantly.
• Type of cement also affects the readings.
• Surface Carbonation increases the hammer reading significantly than it actually is.