When concrete is struck with a steel hammer the degree of rebound is a measure of the surface hardness and the quality of concrete in the surface region.
The Schmidt hammer is an instrument in which the steel hammer is driven with a constant and reproducible amount of energy onto the concrete surface.
The Schmidt hammer automatically measures the degree of rebound which is expressed as the rebound number.
There are two base types of Schmidt hammer; type N and type P. The type N is suitable for normal strength concrete (20-60 MPa). For lower strength material the type P instrument is to be preferred. This method statement deals specifically with type N, although many of the comments are universal.
The type of N hammer consists of a steel cylinder in which a spring-controlled hammer mass can slide (see Figure 1). The plunger retracts against a spring when pressed against the concrete surface and this spring is automatically released when fully compressed, causing the hammer mass to impact through the plunger against the concrete surface. The distance the spring-controlled mass rebounds can be read through a window in the side of the casing.
The instrument comes in a protective case which includes a carborundum stone for preparation of the concrete surface.
The calibration of the instrument should be checked periodically on a solid steel section. If any change in the reading occurs, then the instrument should be sent back to the manufacturer for re calibration.
METHOD OF TESTING
Select a test area of not greater than 300mm square. This should preferably have a smooth surface finish, although a trowelled surface is acceptable. Rough surfaces should be prepared using the carborundum stone provided with the instrument. Any surface water should be wiped from the concrete.
To prevent bias of results, readings should be taken on a grid basis with at least 5 and preferably 10 readings in each general area. No two readings should be taken in exactly the same spot.
To take the reading press the plunger strongly and steadily against the concrete at right angles to the surface until the spring loaded mass is triggered from the locked position. After the impact read the scale index whilst the hammer is still in the test position.
Record and use all readings in analysis (including abnormally high or low readings) unless there is good reason to doubt the validity of a particular reading.
To obtain a correlation between strength and rebound number, tests can be undertaken in the vicinity of recovered cores. In this case Schmidt hammer tests should be made on the concrete insitu and cores subsequently taken for compressive strength tests. It is necessary to test a number of specimens which encompass the likely range of strengths in the structure.
ANALYSIS OF RESULTS
When using the Schmidt hammer to check the uniformity of the concrete in different areas of a structure it will be generally sufficient to calculate the means and standard deviation of results at each area. Contour plots will help to identify problem areas.
The direction of testing affects the readings; a correction must be applied to the rebound number (R) where the instrument is not used on a vertical surface.
Correction for inclination
An approximation for the strength of gravel aggregate concrete can be obtained from Figure 2.
Where a reliable measure of the concrete strength is required it is essential to establish the relationship between the rebound number and the concrete strength measured on recovered cores.
The accuracy of the concrete strength determined using the Schmidt hammer will depend on the influences of the factors listed in section 7 on the validity of the correlation test results.
The mean of sets of readings in individual areas is likely to be accurate to within ± 5% of the true value if 10 readings are taken (± 15/ n if n is less than 10).
An indication of the expected range of strength of the concrete tested can be obtained from Figure 1 where Wmax and Wmin are the maximum and minimum strengths, Wm is the mean strength assessed from the rebound number and < is the dispersion obtained from the graph.
The relationship between rebound number and compressive strength is affected by a number of factors of which the most significant are:
- Coarse aggregate type
- Mass and rigidity of the specimen member being tested; flexible members give a low reading
- Surface type and compaction
- Age; old dry concrete gives a high reading
- Surface carbonation; carbonated concrete gives a high reading
- Moisture condition of the concrete; wet concrete gives a low reading
- Characteristics of the Schmidt hammer including spring stiffness and internal friction
- Low strength concrete; damage by the impact results in a low reading
The top of a concrete pour is not representative of the concrete quality. Therefore, where possible, testing should be carried out at mid-height of side faces or on the soffit of the structure.
- Impact plunger
- Housing compl
- Rider with guide rod
- Pushbutton compl.
- Hammer guide bar
- Two-part ring
- Rear cover
- Compression spring
- Hammer mass
- Retaining spring
- Impact spring
- Guide sleeve
- Felt washer
- Plexiglass window
- Trip screw
- Lock nut
- Pawl spring