Rebound Hammer Testing

The rebound hammer (Schmidt hammer) test is a non-destructive method for assessing the compressive strength of concrete without causing damage to structures. This test adheres to the ASTM C805 Standard of the American Society for Testing and Materials. The rebound hammer functions through a spring-loaded mass triggered when the plunger is pressed against the concrete. The resulting impact energy is transferred to the concrete surface, and the rebound or reflection of this energy is indicated by a “rebound number” on the hammer unit’s scale. A higher rebound number suggests a greater compressive strength of the concrete, with values displayed in the range of 10 to 100. Calibration of the rebound hammers is always performed before the test, using a standard steel anvil to ensure a reading close to 80.

The rebound hammer is widely employed for assessing concrete compressive strength due to its non-destructive nature, time efficiency for conducting multiple tests, and cost-effectiveness compared to other testing methods. The greater the data acquired through this technique, the more beneficial it becomes in illustrating data dispersion. However, the rebound hammer test has limitations, as the index of higher impact energy reflection obtained can be influenced by the smoothness of the test surface, the size of aggregates in the concrete, the distance between the outer surface and aggregates, and the moisture content of the concrete.

Mechanism and Operational Procedures of Schmidt Hammers

  1. The operator firmly holds the cylinder, pressing the plunger against the concrete surface. Simultaneously, the steel mass, depicted by the dark blue area, is secured at the plunger’s top end, causing the spring to stretch (illustrated by the yellow line).
  2. As the top end of the plunger nears the cylinder’s upper end, the tension force in the spring releases the steel mass. Consequently, the steel mass travels into the concrete surface.
  3. The steel mass impacts the collar on the plunger, initiating the propagation of stress waves through the plunger onto the concrete surface. The reflection of the stress wave, denoted by the yellow arrow, travels through the plunger rod, ultimately reaching the steel mass.
  4. The reflection of the impact force causes the steel mass to retract from the concrete surface, simultaneously pushing the needle that indicates the rebound reading.
  5. The needle remains at the maximum travel distance of the steel mass. It is at this stage that the rebound number reading is conducted.