The operation of rebound hammer is shown in the fig.1. When the plunger of rebound hammer is pressed against the surface of concrete, a spring controlled mass with a constant energy is made to hit concrete surface to rebound back. The extent of rebound, which is a measure of surface hardness, is measured on a graduated scale. This measured value is designated as Rebound Number (rebound index). A concrete with low strength and low stiffness will absorb more energy to yield in a lower rebound value.
Fig: Operation of the rebound hammer
Fig: Rebound Hammer
IS 13311 Pt-2 – 1992 and BS 6089-81 and BS: 1881: Pt-202 explains the standard procedure for test and correlation between concrete cube crushing and strength rebound number. The results are significantly affected by the following factors:
a) Mix characteristics
- Cement type
- Cement content
- Coarse aggregate type
b) Angle of inclination of direction of hammer with reference to horizontal (fig2).
c) Member characteristics
- Surface type
- Age, rate of hardening and curing type
- Surface carbonation
- Moisture condition
- Stress state and temperature
Fig.2 – cube compressive strength is N/mm2 plotted against rebound number
Since each of these may affect the readings obtained, any attempts to compare or estimate concrete strength will be valid only if they are all standardised for the concrete under test and for the calibration of specimens.
A) Strength Assessment
This test is conducted to assess the relative strength of concrete based on the hardness at or near its exposed surface. Carrying of periodic calibration of rebound hammer using standard anvil is desirable. However for new concrete construction, rebound hammer is calibrated on concrete test cubes for a given source of constituent materials (i.e. cement, sand, stone aggregates), this calibration data can be used with reasonable accuracy In arriving at equivalent in-situ cube strength of relatively new concrete (not more than three months old concrete. This calibration exercise may be carried out in a concrete lab by casting cubes of designed mix and testing these under controlled condition with rebound hammer as well as test to destruction in compression. Calibration graphs then can be drawn. Large number of readings are desirable to reduce the affects of variability in readings due to various localized as well as instrument factors. This method may give highly erroneous results for concrete whose surface is exposed to atmosphere for longer periods say more than three months. This is due to hardening of concrete surface due to carbonation, which may cause overestimation as much as 50% for old structure. Hence strength assessment by rebound hammer test should generally be restricted to relatively new structures only,
B) Survey of weak and delaminating concrete
as the test requires a flat surface and large number of readings to reduce variability, this test is not generally suitable for use on spalled concrete surfaces of distressed structures. However, comparison of rebound numbers which indicate the near surface hardness of the concrete will help to identify relative surface weakness in cover concrete and also can be used to determine the relative compressive strength of concrete. Locations possessing very low rebound numbers will be identified as weak surface concrete and such locations will be identified for further investigations like corrosion distress, fire damage and or any other reason including original construction defects of concrete. This survey is to be carried our on each identified member in a systematic way by dividing the member into well-defined grid points. The grid matrix should have a spacing of approximately 300mm x 300mm. Table 1 gives guidelines for qualitative interception of rebound hammer test results with reference to quality.
Table: 1 Quality of concrete from rebound values
Quality of Concrete
30 – 40
20 – 30
Poor and / or delaminated
Very poor and/or delaminated
i) Before commencement of a test, the rebound hammer should be tested against the test anvil, to get reliable results, for which the manufacturer of the rebound hammer indicates the range of readings on the anvil suitable for different types of rebound hammer.
ii) 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.
iii) 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. (Do not touch the button while depressing the plunger. Press the button after impact, in case it is not convenient to note the rebound reading in that position.)
iv) Take the average of about 15 readings.