H-Meter Operating Instructions

Chapter 1

GENERAL DESCRIPTION

The concrete rebound hammer is an instrument which is easy to use, for quick and approximate measurement of the resistance to pressure of manufactured concrete products.

The principles on which it works are based on the rebound impact of a hammer on a piston which rests against the surface of the concrete products: the greater resistance of the concrete, the greater the rebound impact.

By reading this rebound impact on a scale and relating it to curves on graphs supplied with the instrument, the resistance to compression in MPa or PSI can be found. The test hammer may be used for nondestructive control on cement during the normal construction of factories and bridges.

The test makes it possible to learn the strength of impact, which depends on the resistance of the agglomerate in the absence of large inert lumps or clusters of sand or gravel.

From the force of the impact the resistance of the agglomerate surface can be deduced and subsequently the resistance of the concrete.

With the aid of the test hammer the quality of the concrete in every part of a construction can quickly be examined, and in this time the hardening of the various castings can be followed.

Chapter 2

INSTRUCTIONS FOR USE

1. Having taken the test hammer from its case, gently push the piston inwards pressing it against a surface. The piston will release itself from its catch and emerge from the body of the device which will then be ready for testing.
   
   
2. Press the piston against the surface of the concrete to be tested, holding the device perpendicularly to the surface. Gradually apply increasing pressure until the mass impact is released. Keep the device firmly pressed against the surface being tested and look at the rebound reading on the scale. Do not touch the side button whilst pressing on the piston. On removing the device from the surface being tested, the piston once more emerges completely from the device which is immediately ready for a second test.
   
   
3. If it is difficult to see the reading under the conditions specified in paragraph 2), PRESS THE SIDE BUTTON ONLY AFTER IMPACT and remove the device in order to read the scale.
   
   
4. If the button is used, as in the above case, the piston remains inside the device when it is removed from the surface being tested. In such cases, in order to prepare the test hammer for further tests, activate in by the method indicated in Paragraph 1). The position of the device when the piston has withdrawn inside is also the position used for packing away the device in its case.
   
   
5. The «H» values of rebound have been defined in such a way that they can be converted, by means of diagrams which are provided with the device , into terms of resistance to compression for tests carried out on the cylinder or on the cube. On each diagram, 5 different curves have been reproduced, which take into account the angle of the instrument, which thus is also suitable for tests on floors; ceiling and any surfaces which are at slope to the horizontal.
   
   However, when carrying out tests on non vertical surface and hence not holding the instrument horizontally, it is essential to take into account the factor of gravity acting upon the impact shock. The four supplementary curves of each graphs take into account precisely this.
   
   For example, for a test carried out in a floor, hence with the instrument in a vertical, downward position (a = -90°), which gives a reading of «H» 35, we can obtain, by means of the diagram a resistance to compression on a cube of 36.96 MPa (5359 PSI). Similarly, the same reading of «H» 35 would indicate resistance of 31.49 MPa (4566 PSI) if the instrument were horizontal and the test were carried out against a vertical wall (a = 0°); and a resistance of 24.61 MPa (3568 PSI) would be obtained with the instrument perpendicular and facing upwards, in a test being carried out against a ceiling (a = +90°).
   
   
   
   The curves are valid for compact cements made with Portland cement, with sand and gravel resistant materials. The cement must be 14 to 56 days old, with a smooth, dry surface.
   
   
6. The tests must be carried out on smooth and uniform surface obtained from castings. Avoid uneven and porous surfaces, lumps of gravel and joints in the concrete. Tests on thin sections or zones less 10 cm, wide require particular care in the interpretation of the results, because of the distortion due to the elasticity of the section. If possible, the back of the section should be held stiff by some means.
   
   
7. Take care to remove any materials or paint covering the concrete. If the surface is rough, smooth it down with the rubbing stone provided with the device.
   
   
   
   
8. It is advisable to take 15 readings in order to obtain a reliable «H» value. For each reading change, the spot being tested by 2 or 3 cm. The «H» value can be considered reliable if 10 to 15 readings are not further from the average than the amount specified below:
   
   

   «H» value	15	30	45
   Discard reading +/-	2,5	3,0	3,5

   
   The value of «H» to be introduced into the graphs will be obtainable from the average of the 10 best readings.
   
   
9. The conversion diagrams have been derived from experiments on sample of cements left in the air to age, from between 7 to 90 days, under normal conditions. Experience has shown that such diagrams are not applicable in the following cases:
   
   
   1. When there are elements of stone or cement products which differ from normal. In such as cases it is possible to obtain new curves by experimentation.
      
      
   2. Concrete with soft aggregates such as pumice or shale give too low a value of resistance to compression. Also in this case, it is possible to obtain new curves by experimentation.
      
      
   3. In hard-mix cements or cements which are not properly compacted internal leaks often occur. In such case, the results may not correspond correctly to the calibration curves.
      
      
   4. Very old and hard cements have a surface which is harder than the interior and they give rebound impact readings which are higher than they should be.
      
      
   5. New cements with damp surface give lower rebound impact readings than they should.
      
      
   6. Frozen cements do not give reliable results.
      
      
10. The device does not require special maintenance. Avoid letting dust collect in the piston and penetrating inside the device. Beware of surface of contact between the piston and the hammer being dirty from oil or dust as this could create errors in the rebound impact.