The rebound hammer method could be used for:
(i) assessing the likely compressive strength of concrete with the help of suitable corelations
between rebound index and compressive strength,
(ii) assessing the uniformity of concrete,
(iii) assessing the quality of the concrete in relation to standard requirements, and
(iv) assessing the quality of one element of concrete in relation to another.
When the plunger of rebound hammer is pressed against the surface of the concrete, the spring- controlled mass rebounds and the extent of such rebound depends upon the surface hardness of concrete. The surface hardness and therefore the rebound is taken to be related to the compressive strength of the concrete. The rebound is read off along a graduated scale and is designated as the rebound number or rebound index.
S. No. | Application | Approx. Impact Energy required for Rebound hammer (Nm) |
---|---|---|
1 | For Testing Normal Weight Concrete | 2.25 |
2 | For light-weight concrete or small and impact sensitive parts of concrete | 0.75 |
3 | For testing mass concrete for example, in roads, air field pavements and hydraulic structures | 30.0 |
Table 1 : Impact Energy for Rebound hammer for different Applications.
Fig. 2: Schematic of Rebound Hammer |
IS-516 (Part 5/Sec 4):2020 “ Part 5 Non-Destructive Testing of Concrete - Section 4 Rebound Hammer Test”.
It is necessary that the rebound hammer is checked against the testing anvil before commencement of a test to ensure reliable results. The testing anvil should be of steel having Brinell hardness of about 5000 N/mm2. The supplier/manufacturer of the rebound hammer should indicate the range of readings on the anvil suitable for different types of rebound hammers.
The most satisfactory way of establishing a correlation between compressive strength of concrete and
its rebound number is to measure both the properties simultaneously on concrete cubes.
The concrete cube specimens are held in a compression testing machine under a fixed load,
measurements of rebound number taken and then the compressive strength determined as per IS: 516
(part1/Sec 1). The fixed load required is of the order of 7 N/mm2 when the impact energy of the
hammer is about 2.25 Nm. The load should be increased for calibrating rebound hammers of greater
impact energy and decreased for calibrating rebound hammers of lesser impact energy. The test
specimens should be as large a mass as possible in order to minimise the size effect on the
test result of a full scale structure. 150 mm cube specimens are preferred for calibrating rebound hammers of lower impact energy (2.25 Nm), whereas for rebound hammers of
higher impact energy, for example 30 Nm, the test cubes should not be smaller than 300 mm.
If the specimens are wet cured, they should be removed from wet storage and kept in the
laboratory atmosphere for about 24 hours before testing. To obtain a correlation between rebound
numbers and strength of wet cured and wet tested cubes, 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. A direct correlation between rebound numbers on wet cubes and the strength of wet cubes is not
recommended. Only the vertical faces of the cube as cast should be tested. At least nine readings should
be taken on each of the two vertical faces accessible in the compression testing machine when using the
rebound hammers. The points of impact on the specimen must not be nearer an edge than 25 mm and should not be
less than 25 mm from each other. The same points must not be impacted more than once.
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.
Concretes made with high alumina cement can give strengths 100 percent higher than that with ordinary Portland cement. Concretes made with super sulphated cement can give 50 percent lower strength than that with ordinary Portland cement.
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 light weight aggregates require special calibration.
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 over estimate the strength of concrete.
A wet surface will give rise to under estimation 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.
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. It is possible to establish correction factors by removing the carbonated layer and testing the concrete with the rebound hammer on the uncarbonated concrete.
The influence of vertical distance from the bottom of concrete placement on the rebound number is very significant. Generally, higher rebound number is observed near the bottom of concrete placement. As during compaction, concentration of aggregates will be higher at the bottom.
The direct correleation 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.
(a) Date/period of testing.
(b) Identification of the concrete structure/element.
(c) Location of test area(s).
(d) Identification of the rebound hammer.
(e) Details of concrete and its condition.
(f) Date/time of peformance of the test.
(g) Test result and hammer orientation for each test area; and