Procedure :
The test is sensitive to local variations in the concrete, for e.g. If plunger strikes the aggregate underneath, then it would indicate high rebound number, visa versa if it strikes a void / cement paste , then it will indicate low rebound number. For this reason it is desirable to take at least 12 readings per 300 Sqcm. ( as per BS 1881 : Part 202 (21)) and discard abnormally high / low readings from the average one. The impact / test points shall not be closer than 20 mm to each other.
Influence of Test Conditions
The test is influenced by following factors :
Type of Cement :-
Concrete made with high alumina cement can give strength 100 % higher or that with super sulphated cement can give 50 % lower strength than that with ordinary Portland cement. While Rebound readings on Blended cement concrete are lower by @ 5 to 10 %
Type of Aggregate :-
The influence of aggregate type and proportion can be considerable. Limestone coarse aggregate show rebound numbers approximately 7 points lower than those for concretes made with gravel aggregate
Surface condition & Moisture content :-
Trowelled & floated surfaces are harder than moulded surfaces. Exposed aggregate surfaces are unsuitable for this method.
Readings on Dry concrete are more by @ 10 % and a wet surface will give rise to underestimation of strength of concrete by about 20 % lower than calibrated under dry condition.
Curing & Age of concrete –
The relation between hardness and strength varies as a function of time, subsequent curing and exposure condition will further influence this relationship. But this effect can be ignored for concrete up to 3 months old.
Carbonation of Concrete surface :-
Concrete exposed to atmosphere will normally form a hard carbonated skin. The strength predicted for carbonated concrete may overestimate up to 50 %.
Type of mould :-
Rebound readings on Mild steel, Timber, Plywood and PVC moulds are higher by @ 2 to 10 %
Application :-
The rebound hammer method may be used for
- Assessing the likely compressive strength of concrete with the help of suitable calibration charts.
- Assessing uniformity of concrete
- Assessing the quality of concrete in relation to specified standard requirements
- Assessing the quality of one element of concrete to another
Reliability &Limitations :-
The test determines the hardness of the surface only. The impact of hammer is sensitive to the surface layer of about 25 to 50 mm only. The reliability of this test as alone, for compression strength is less, but it can generally indicate uniformity of concrete
The rebound hammer results are very sensitive to the spring constant, thus after a use of hammer for about 2000 impacts or three months, it is recommended to calibrate the hammer on Anvil.
The influence of aggregate type and proportion can be considerable. Thus it is strongly recommended that every lab, NDT consultant using rebound hammer shall develop co-relation charts for compressive strength verses Rebound number, and shall not use blindly, the graphs given by manufacturers.
The probable accuracy of prediction of concrete strength in laboratory level is ± 15 % and that of structure is ± 25 %. It is recommended to use of Rebound hammer along with ultrasonic concrete testing or core test.