Caution: Rebound Hammer Test results on old buildings could be misleading!
It is mandatory to conduct a "rebound hammer test" as a part of structural audit for old buildings in Mumbai.
Going by the principles of Rebound hammer, a calibrated spring operated instrument, it simply gives the rebound number that corresponds to the hardness of the underlying material. Its calibration is done by a R number reading 80 on an anvil. The anvil has the reference hardness on which the hammer is calibrated after typically 500 readings.
Typical applications - new construction
Rebound hammer is a easy to use, low cost, quick test to assess the compressive strength of concrete and is extensively used as a preliminary non destructive test for new construction.
Carbonation and hardness
In old buildings, however, as the process of carbonation sets in, the cover concrete (the concrete between the rebar and the surface) becomes harder. It could be around 1-2 mm per year in buildings that are not well protected and painted.
False sense of assurance
In such a condition, when rebound hammer test is performed, it gives an unusually higher rebound number, that corresponds to a higher strength (as per the manufacturers calibration chart). This could be misleading and give a false sense of assurance to the client and consultants. Where as in reality, the concrete strength slowly decreases with time due to various environmental factors.
Using rebound hammer on old buildings
One of the methods to get correct results is to remove the carbonated layer of concrete and perform the test. This is usually not easy to do and not feasible when the carbonation has crossed the reinforcement. The other method is to apply a correction factor specified by the manufacturer. This presumes that the underlying concrete strength vs rebound number follows the calibration given by the hammer manufacturer. This may not be always correct has rebound hammers to need to be calibrated by taking rebound number on a cube and then crushing the cubes to get compressive strength. This is not possible in an old building as the cubes taken at the time of construction dont exist anymore!
As a part of the mandatory requirement, it is compulsory to conduct this test. However, it is best that the rebound hammer test results are ignored on carbonated surfaces for old structures
Aaryan follows a practice to inform the clients and consultants while interpreting rebound hammer results by mentioning the above interpretation in its test data sheet. This ensures that Aaryan follows the mandatory requirement, yet cautions its clients and consultants to interpret the results correctly.