The Schmidt rebound hammer is principally a surface hardness tester with little apparent theoretical relationship between the strength of concrete and the rebound number of the hammer.
However, within limits, empirical correlations have been established between strength properties and the rebound number. Further, scientist has attempted to establish a correlation between the hammer rebound number and the hardness as measured by the Brinell method.
The rebound hammer method could be used for:
Assessing the likely compressive strength of concrete with the help of suitable correlations between rebound index and compressive strength
Assessing the uniformity of concrete,
Assessing the quality of the concrete in relation to standard requirements, and
Assessing the quality of one element of concrete in relation to another.
Factors Influencing Rebound Hammer Test
The rebound numbers are influenced by a number of factors like types of cement and aggregate, surface condition and moisture content, age of concrete and extent of carbonation of concrete.
1. Type of Cement
Concretes made with high alumina cement can give strengths 100 percent higher than that with ordinary Portland cement. Concretes made with supersulphated cement can give 50 percent lower strength than that with ordinary Portland cement.
2. Type of Aggregate
Different types of aggregate used in concrete give different correlations between compressive strength and rebound numbers. Normal aggregates such as gravels and crushed rock aggregates give similar correlations, but concrete made with lightweight aggregates require special calibration.
3. Surface Condition & Moisture Content
The rebound hammer method is suitable only for close texture concrete. Open texture concrete typical of masonry blocks, honeycombed concrete or no-fines concrete are unsuitable for this test. All correlations assume full compaction, as the strength of partially compacted concrete bears no unique relationship to the rebound numbers. Trowelled and floated surfaces are harder than moulded surfaces, and tend to overestimate the strength of concrete. A wet surface will give rise to underestimation of the strength of concrete calibrated under dry conditions. In structural concrete, this can be about 20 percent lower than in an equivalent dry concrete.
4. Curing and Age of Concrete
The relationship between hardness and strength varies as a function of time. Variations in initial rate of hardening, subsequent curing and conditions of exposure also influence the relationship. The effect of age can be ignored for concrete above 14 days old.
5. Carbonation of Concrete
The influence of carbonation of concrete surface on the rebound number is very significant. Carbonated concrete gives an overestimate of strength which in extreme cases can be up to 50 percent. The carbonation depth shall be checked in cases where the age of concrete is more than six months. It is possible to establish correction factors by removing the carbonated layer and testing the concrete with the rebound hammer on the uncarbonated concrete.
6. Test Position
The influence of vertical distance from the bottom of concrete placement on the rebound number is very significant. Generally, a higher rebound number is observed near the bottom of concrete placement as during compaction concentration of aggregates will be higher at the bottom.
7. Strength Correlations
The direct correlation between rebound numbers and strength of wet cured and wet tested cubes is not recommended. It is necessary to establish a correlation between the strength of wet tested cubes and the strength of dry tested cubes on which rebound readings are taken. The ratio of dry tested cube to wet tested cube in compression is generally about 1.05.