The Schmidt rebound hammer is essentially a surface hardness tester with little apparent theoretical relationship between the strength of concrete and the rebound number of the hammer.
However, within certain limits, empirical correlations have been established between strength properties and the rebound number. Furthermore, a scientist has tried to establish a correlation between the rebound number of the hammer and the hardness as measured by the Brinell method.
The rebound hammer method can be used for:
- Assessment of the likely compressive strength of concrete using appropriate correlations between rebound index and compressive strength
- Assessing the uniformity of concrete,
- Assessing the quality of the concrete in relation to the standard requirements, and
- Assessment of the quality of one concrete element in relation to another.
Rebound numbers are affected by a number of factors, such as cement and aggregate types, surface condition and moisture content, age of concrete, and degree of carbonation of concrete.
1. Type of cement
Influence of the type of cement Concrete made with cement with a high alumina content can give a strength that is 100 percent higher than that of ordinary Portland cement. Concrete made with supersulfated cement can provide 50 percent less strength than that made with regular Portland cement.
2. Generator type
Influence of the aggregate type Different aggregate types used in concrete give different correlations between compressive strength and recoil numbers. Normal aggregates such as gravel and crushed stone aggregates give similar correlations, but concrete made with lightweight aggregates requires special calibration.
3. Surface condition and moisture content
Influence of the surface condition and moisture content of concrete The rebound hammer method is only suitable for narrow textured concrete. Open-structure concrete typical of masonry blocks, honeycomb concrete or fine-grained concrete is not suitable for this test. All correlations assume full compaction as the strength of partially compacted concrete has no unique relationship with the rebound numbers. Trowel and floating surfaces are harder than cast surfaces and tend to overestimate the strength of concrete. A wet surface will lead to an 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. Concrete curing and aging
Influence of curing and age of concrete The relationship between hardness and strength varies as a function of time. Variations in initial cure rate, subsequent cure, and exposure conditions also affect the relationship. The effect of age can be ignored for concrete older than 14 days.
5. Carbonation of concrete
Influence of concrete surface carbonation The influence of concrete surface carbonation on the rebound number is very significant. Carbon dioxide concrete gives an overestimation of the strength that in extreme cases can be up to 50 percent. The depth of carbonation should be checked in cases where the age of concrete is more than six months. It is possible to establish correction factors by removing the carbon dioxide layer and testing the concrete with the recoil hammer on the non-carbonated concrete.
6. Test position
Influence of vertical distance from the bottom of the concrete placement. The influence of the vertical distance from the bottom of the concrete placement on the rebound number is very significant. In general, a higher rebound number is observed near the bottom of the concrete placement, because during compaction, the concentration of aggregates will be higher at the bottom.
7. Strength Correlations
Influence of surface conditions used in the development of correlation between compressive strength and rebound number. 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 measurements are taken. The ratio of dry tested cube to wet tested cube in compression is generally about 1.05.