For Standard Test Method for Rebound Number of Hardened Conc

For Standard Test Method for Rebound Number of Hardened Concrete,

1) describe 3 possible sources of error, 2) describe the accuracy of the results 3) account for the accuracy of the results using 3 or more sources of error

Solution

Rebound hammer test is the most appropriate and useful testing methods for comparing the relatively weaker and stronger areas of concrete. The rational use of the various available methods, as well as a possible combination of more than one method, can be particularly interesting from the viewpoint of the validity of results. Rebound hammer devices give a measure of the only surface hardness of the concrete, and then the relationship to any other property of the concrete is empirical

Sources of error for Rebound Number of Hardened Concrete:

1. The Smoothness of the Surface: It is not applicable for the open textured surface, i.e. a trowelled surface.

2. Size, Shape and Rigidity of the Specimen: 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 index or number.

3. Type of Cement

4. Carbonation of the Concrete Surface: In older concrete, the carbonation depth can be several millimetres thick. In such cases, the rebound numbers can be up to 50% higher than those obtained on an un-carbonated concrete surface.

Accuracy of results:

The rebound depends on the surface hardness of the concrete and is measured by test equipment. By referring to the some conversion tables, the rebound result of the test can be used to determine the compressive strength of the concrete. Although past investigations showed that there is a general relationship between compressive strength of concrete and the rebound number, there is a wide range of disagreement among various research workers regarding the accuracy of estimation of strength from Schmidt hammer. In fact, there is about a variation of 15-20% in concrete strength measured by the method