This method statement describes the method to be adopted for assessing the
surface concrete quality using a rebound hammer (often referred to as a Schmidt
hammer after its inventor).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.

EQUIPMENT DETAILS

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.

INSTRUMENT CALIBRATION

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.

Rebound NumberCorrection for inclinationUpwardsDownwards+90º
+45º**-45º****-90º**20304050

60

-5.4-4.7-3.9-3.1

-2.3

-3.5-3.1-2.6-2.1

-1.6

+2.5+2.3+2.0+1.6

+1.3

+3.4+3.1+2.7+2.2+1.7An 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.

ACCURACY

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.

PROBLEM AREAS

The relationship between rebound number and compressive strength is affected by
a number of factors of which the most significant are:

  1. Coarse aggregate type
  2. Mass and rigidity of the specimen member being tested; flexible members give
    a low reading
  3. Surface type and compaction
  4. Age; old dry concrete gives a high reading
  5. Surface carbonation; carbonated concrete gives a high reading
  6. Moisture condition of the concrete; wet concrete gives a low reading
  7. Characteristics of the Schmidt hammer including spring stiffness and
    internal friction
  8. 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.

  1. Impact plunger
  2. Housing compl
  3. Rider with guide rod
  4. Pushbutton compl.
  5. Hammer guide bar
  6. Disk
  7. Cap
  8. Two-part ring
  9. Rear cover
  10. Compression spring
  11. Pawl
  12. Hammer mass
  13. Retaining spring
  14. Impact spring
  15. Guide sleeve
  16. Felt washer
  17. Plexiglass window
  18. Trip screw
  19. Lock nut
  20. Pin
  21. Pawl spring


**


** Figure 2Relationship Between Rebound Number and Cube Compressive Strength