The procedure for testing a concrete structure is given below:

• All members and points of a concrete structure selected for testing should be marked for identification; they should also be in dry condition.

• Testing should be conducted on surfaces that are smooth and uniform, preferably surfaces created by casting against a form. Avoid rough spots, hony-comb or porous areas. Free or trowelled surface may also be satisfactory if appropriate corrections are applied or a special calibration is prepared. If loosely adhering scale, plaster work or coating is present, this should be rubbed off with a grinding wheel or stone.

• For concrete section less than 100 mm thick, the rebound of the hammer will be affected by the elastic deformation of the section, and it should be backed up by a heavy mass placed on the back side.

• At each of selected points, made smooth and clean, take six rebound readings. For each reading shift the hammer 25 mm and take care not to rebound the same spot twice. The point of impact should be at least 20 mm away from any edge or sharp discontinuity. Small air pockets near the surface under the point of impact cause low rebound, on the other hand, immediately over a hard aggregate the impact will result in a high rebound.

Note: Manufacturer of Concrete hammer recommends at selected points 5 or better 10 impact reading. It is recommended at least 9 valid reading and not more than 25. It is normally better to confine the readings of a test (9 to 25 readings) to an area not exceeding about 300 mm x 300 mm rather than to carry out random testing extending over the whole structure or unit.

• The usual directions of test are either horizontal or vertically down, but any direction of test can be used as long as it is consistent. Calibration or corrections for a given direction of test are supplied with the hammer or can be derived.

• The rebound values usually are considered reliable when at least six readings deviate not more than +2.5 to 3.5 on the impact scale. The compressive strength is then determined by taking average of rebound reading.

• Compressive strength of the concrete can be determined from the relationship between the rebound number and the strength given by the curve. For reliable results the calibration curve will be derived from the given set of materials and conditions.