To determine the compressive strength of hardened concrete by using the Schmidt hammer

To determine the compressive strength of hardened concrete by using the Schmidt hammer.

1.      Eqpt / Materials

Rebound hammer, carbomndum stone, Mass of concrete.

2.      Procedure

a.      Selection of test surface: Concrete member to be tested shall be at least 4 in thick and fixed with a structure. Smaller specimens must be rigidly supported. Area exhibiting honey combing, scaling rough texture high prosity should be avoided. Concrete should be approximately the same age and moisture. Dry concrete with high rebound numbers than the wet concrete.

b.      Preparation of test surface: A test area should be at least 6” in diameter. Heavily textured, soft or surface with loose mortar shall be ground smooth with the carborudum stone.

c.       Testing Method:

(1)   Firmly hold the instrument in apposition that allows the plunger to strike perpendicular to the surface tested.

(2)   Gradually increase the pressure on the plunger until the hammer impacts.

(3)   After impact, record the rebound number from the graduated scale given on the hammer.

(4)   Take ten readings from each test area, and take the mean.

(5)   Strength of concrete can be known by reading the rebound numbers from abscissa and drawing the line which cut the curve at some ordinate, which will give the comp strength.

3.      Precautions:

a.       To avoid load variation take 12 to 14 readings at different places.

b.      Discard maximum and minimum no of readings.

c.       Test should be done against smooth surface

4.      Details of readings/obsns/data to be recorded/collected

a.       Structure identification

b.      Location of testing area in structure,e.g. 3” from bottom of column 2 bent 3 or one meter from the north end etc.

c.       Description of testing area,e.g. wood, floated surface, dry ground with abrasion stone etc.

d.      Proportion of concrete if known.

e.       Average rebound number for each test area.

S/No	Direction of Hammer Used	Rebound Number	Average	Comp Str (N/mm2)	Comp Str (psi)
1.
2.
3.
4.
5.
6.
7.
8.
9.

5.      Interpretation/ collection of obsns/reading, graph etc leading to the results

To be done with the standard specification of ASTM C 805-97

6.      Comments:

Hammer must kept vertical with talking reading at different places.