4. TESTS ON HARDENED CONCRETE
4.1 NON-DESTRUCTIVE TESTS
4.1.1 REBOUND HAMMER
AIM:-
- To assess the likely compressive strength of concrete by using rebound hammer as per IS: 13311 (Part 2) - 1992, (Reaffirmed- May 2013), Non Destructive Testing of Concrete-Methods of Test (Rebound hammer).
PRINCIPLE:
- The rebound of an elastic mass depends on the hardness of the surface against which its mass strikes. When the plunger of the rebound hammer is pressed against the surface of the concrete, the spring-controlled mass rebounds and the extent of such a rebound depends upon the surface hardness of the concrete. The surface hardness and therefore the rebound is taken to be related to the compressive strength of the concrete. The rebound value is read from a graduated scale and is designated as the rebound number or rebound index. The compressive strength can be read directly from the graph provided on the body of the hammer.
APPARATUS:-
- Rebound hammer
PROCEDURE:-
- 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.
- Before commencement of a test, the rebound hammer should be tested against the test anvil, to get reliable results, for which the manufacturer of the rebound hammer indicates the range of readings on the anvil suitable for different types of rebound hammer.
- Apply light pressure on the plunger - it will release it from the locked position and allow it to extend to the ready position for the test.
- Press the plunger against the surface of the concrete, keeping the instrument perpendicular to the test surface. Apply a gradual increase in pressure until the hammer impacts. (Do not touch the button while depressing the plunger. Press the button after impact, in case it is not convenient to note the rebound reading in that position.)
- Take the average of about 15 readings.
INTERPRETATION OF RESULTS:-
- The rebound reading on the indicator scale has been calibrated by the manufacturer of the rebound hammer for horizontal impact, that is, on a vertical surface, to indicate the compressive strength. When used in any other position, appropriate correction as given by the manufacturer is to be taken into account.
Points to Remember in Rebound Hammer Test
- The concrete surface should be smooth, clean and dry.
- Ant loose particles should be rubbed off from the concrete surface with a grinding wheel or stone, before hammer testing.
- Rebound hammer test should not be conducted on rough surfaces as a result of incomplete compaction, loss of grout, spalled or tooled concrete surface.
- The point of impact of rebound hammer on concrete surface should be at least 20mm away from edge or shape discontinuity.
- Six readings of rebound number is taken at each point of testing and an average of value of the readings is taken as rebound index for the corresponding point of observation on concrete surface.
Interpretation of Rebound Hammer Test Results:-
- After obtaining the correlation between compressive strength and rebound number, the strength of structure can be assessed. In general, the rebound number increases as the strength increases and is also affected by a number of parameters i.e. type of cement, type of aggregate, surface condition and moisture content of the concrete, curing and age of concrete, carbonation of concrete surface etc.
