Rebound Hammer Testing (Schmidt hammer)
The strength of concrete is generally governed by the strength of the cement paste. Measurements of the strength of the paste can therefore provide a reasonable assessment of the strength of the concrete.This strength can be determined by inference from the elasticity of the concrete. A practical assessment of elasticity can be made on site by measuring the rebound of a sprung hammer.
The surface of the concrete is cleaned to remove laitance and shutter board marks.
A series of twelve readings are taken with a 'Schmidt' hammer in a diamond pattern at the test point.
The hammer is pressed against the surface, loading the sprung mass, and releasing this at the end of the stroke.
The percentage rebound is measured by a latched rider on the side of the hammer.
Care is taken to avoid obvious anomalies such as blowholes or exposed aggregate.
The conversion of rebound number to compressive strength can be achieved by preparation of a calibration chart for the concrete concerned. If this is not possible a crude assessment can be made from the manufacturers' data.
Attention must be paid to BS 1881 Pt.. 202 which states 'The use of universal calibrations, such as those produced by the manufacturers of rebound hammers, can lead to serious errors and should be avoided'.
To produce a calibration graph the structure should be tested and locations selected for coring to represent the greatest possible range of rebound numbers.
Cores should be removed from the rebound test location and their compressive strengths determined. Sufficient cores should be taken to enable a correlation to be determined. The greater the number of cores the greater will be the resulting accuracy.
The calibration chart may then be used to convert rebound numbers to cube strength. It is unlikely that 95% confidence limits on the estimation of the in-situ concrete strength by rebound hammer will be better than ±25% under ideal conditions.
In the absence of such calibration, the interpretation of the rebound number is based upon data in the manufacturers handbook.
For a horizontal impact on a vertical surface the probable cube strength of the concrete can be estimated as:
fc = 0.0126 (R + 29.8) - 21.1 N/mm2
where R is the mean rebound number after discarding the highest and lowest values.
In practice the test is very dependent upon the surface condition and moisture content of the concrete as well as the ratio of aggregate to cement paste. Under laboratory conditions, testing carefully cast cubes, it is possible to predict the strength with some accuracy. The manufacturers own estimate of error gives 95% confidence bound as ±70% at low strength and 11% at very high strengths. In the useful range of 20 N/mm2 the error range for 95% confidence is quoted as 30% to 18% respectively.
The manufacturers quoted figures are for concrete in the range 7 to 56 days old. Acknowledgement is made of the hardening of the surface layer with age, primarily due to carbonation. The rebound hammer tests a localised area of concrete to a depth of perhaps 20 or 30mm. In the region the influence of the carbonation can be significant. Removal of the carbonated concrete changes the test from being quick, simple and non-destructive to slow laborious and cosmetically damaging. For these reasons the carbonation layer is rarely removed.
Moisture content can modify the modulus of elasticity by up to 25% (ref. 2 pp. 363). Poor surface condition will tend to lower the rebound number.
Caution must be exercised when assessing a structure using results from rebound hammers. Where the moisture content, surface condition and carbonation are likely to be consistent then it will have a benefit in its ability to detect weaker areas of concrete. In external texture and near surface moisture contents the potential errors can exceed 50% rendering the results less useful.
Operating Instructions - Concrete Test Hammer, Proceq 1989
Properties of Concrete. A. M. Neville 3rd Edition 1981
BS 1881 pt. 202:1986. Recommendations for Surface Hardness Testing by Rebound Hammer