FIND THE COMPRESSIVE STRENGTH USING REBOUND HAMMER

AIM:

To find out the compressive strength of concrete using rebound hammer.

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EQUIPMENT'S:

Rebound hammer.

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PRINCIPLE:

The Rebound hammer is principally a surface hardness tester. It works on the principle that the rebound of an elastic mass depends on the hardness of the surface against which the mass impinges. There is 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, Kolek has attempted to establish a correlation between the hammer rebound number and the hardness as measured by the Brinell method.

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PROCEDURE:

1) 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.

2) Choose suitable test location and remove plaster, if any, at test location then make surface smooth and clean & mark gridlines as 20mm to 50mm apart in and area should be 300mm X 300mm

3) 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.

4) 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.)

5) Take the average of about 15 readings and calculate mean of each set of reading after discarding extreme readings i.e. too low and too high.

6) Apply correction factors and correlate strength & rebound number correlation curve.

RANGE AND LIMITATIONS:

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1. Smoothness of the test surface

Hammer has to be used against a smooth surface, preferably a formed one. Open textured concrete cannot therefore be tested. If the surface is rough, e.g. a trowelled surface, it should be rubbed smooth with a carborundum stone.

2. Size, shape and rigidity of the specimen

If the concrete does not form part of a large mass any movement caused by the impact of the hammer will result in a reduction in the rebound number. In such cases the member has to be rigidly held or backed up by a heavy mass.

3. Age of the specimen

For equal strengths, higher rebound numbers are obtained with a 7 day old concrete than with a 28 day old. Therefore, when old concrete is to be tested in a structure a direct correlation is necessary between the rebound numbers and compressive strengths of cores taken from the structure. Rebound testing should not be carried out on low strength concrete at early ages or when the concrete strength is less than 7 MPa since the concrete surface could be damaged by the hammer.

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4. Surface and internal moisture conditions of concrete

The rebound numbers are lower for well-cured air dried specimens than for the same specimens tested after being soaked in water and tested in the saturated surface dried conditions. Therefore, whenever the actual moisture condition of the field concrete or specimen is unknown, the surface should be pre-saturated for several hours before testing. A correlation curve for tests performed on saturated surface dried specimens should then be used to estimate the compressive strength.

5. Type of cement

High alumina cement can have a compressive strength 100% higher than the strength estimated using a correlation curve based on ordinary Portland cement. Also, super sulphated cement concrete can have strength 50% lower than ordinary Portland cement.

6. Carbonation of the concrete surface

In older concrete the carbonation depth can be several millimeters thick and, in extreme cases, up to 20 mm thick. In such cases the rebound numbers can be up to 50% higher than those obtained on an un-carbonated concrete surface.

PRECAUTIONS:

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This test is not generally recommended where accurate strength estimate are needed. Because the rebound number is affected by the near surface conditions, erratic results may occur if the plunger is located directly over a course aggregate particle or a subsurface air void. To account for these possibilities, ASTM C 805 requires that 10 rebound numbers be taken for the test. If a reading differs from the average by more than seven units, that reading should be discarded and a new average should be computed based on the remaining readings. If more than two readings differ from the average by seven units, the entire set of reading is discarded.