Top Ten errors while doing rebound hammer test on structures
These the common errors committed while doing the rebound hammer test. Make sure that your agency or team is not committing these, by proper processes and methodology.
Being accredited by NABL, mitigate the risks associated with these errors to a large extent.
1. Taking readings on cube without using a CTM. Rebound hammer reading taken on a cube, without putting in sample in a compression testing machine. As a result, when the rebound hammer reading is taken, the cube moves! It is vital that the reading be taken at the specified load as per the code.
2. Taking readings on plaster. Rebound hammer reading is taken on plaster, without chipping the plaster. There are cases where in old buildings the plaster is very hard at places and removing it also removes the underlying cover concrete together. in such cases, alternate locations to be chosen
3. Taking readings on old carbonated concrete. Taking rebound readings on old concrete with carbonation and interpreting strength as higher strength due to higher hardness. Pl see this link to know more.
4. Not noting direction of hammer. Taking rebound readings without noting the noting the direction of the hammer. The calibration curves of the hammer are slightly different for vertical and horizontal orientation. Noting the orientation is critical for correct interpretation.
5. Reporting in decimal figures. As the average rebound number is reported, there are instances where the result is reported with a decimal number. it is not be noted that the rebound number is an integer and should be reported accordingly. .
6. Using standard calibration curve. Cube results are to be calibrated with compressive strength to corelated hardness and strength to a reasonable extent. Just referring to the manufacturers calibration curve could be misleading.
7. Not calibrating the hammer with anvil regularly. Rebound hammer is a spring operated instrument. Regular calibration is critical ad minor adjustments are needed to the instrument to give correct readings. As per prevailing codes, or methods standardized by the lab, calibration using an anvil is a must.
8. Using concrete hammer on steel and wood. The concrete hammer is calibrated for a certain range of hardness. it does not give correct results on wood and steel as teh hardness of these materials is different. Different rebound hammers are required to be used for these materials.
9. Taking readings with incorrect spacing. Too close or too far apart spacing does not give a correct distribution of results. Spacing to be maintained as per codes specifications.
10. Not removing outlier readings. Too diverse from the average, leading to a high standard deviation, indicate non uniform quality of concrete. Outlier readings are to be removed while taking averages
Avoiding these top 10 errors in conducting rebound hammer tests will go a long way in ensures consistency of testing procedure and results.
Aaryan, with its NABL accredited laboratory, strives to provide quality testing services to its clients.