Civil Engineering Reference
In-Depth Information
Cement content
Coarse aggregate type
b Member characteristics
Mass
Compaction
Surface type
Age, rate of hardening and curing type
Surface carbonation
Moisture condition
Stress state and temperature.
Since each of these factors may affect the readings obtained, any attempts
to compare or estimate concrete strength will be valid only if they are all
standardised for the concrete under test and for the calibration specimens.
These influences have different magnitudes. Hammer orientation will also
influence measured values although correction factors can be used to allow
for this effect and the Proceq Silver Schmidt has a built-in correction system
for orientation.
For a detailed review of factors influencing the evaluation of surface
hardness by the Schmidt hammer method see 'Testing of Concrete in
Structures' (Bungey et al., 2006). One of the most important factors is surface
carbonation. Concrete exposed to the atmosphere will normally form a hard
carbonated skin, whose thickness will depend upon the exposure conditions
and age. It may exceed 20 mm for old concrete although it is unlikely to be
significant at ages of less than three months. The depth of carbonation can
easily be determined by the phenolphthalein spray method. In extreme cases
it is known that the overestimate of strength from this cause may be up to
50% and is thus of great importance. When significant carbonation is known
to exist, the surface layer ceases to be representative of the concrete within
an element
Calibration
Clearly, the influences of the variables described above are so great that it
is very unlikely that a general calibration curve relating rebound number
to strength, as provided by the equipment manufacturers, will be of any
practical value. The same applies to the use of computer data processing
to give strength predictions based on results from the electronic rebound
hammer, unless the conversions are based on case-specific data. Strength
calibration must be based on the particular mix under investigation, and the
mould surface, curing and age of laboratory specimens should correspond
as closely as possible to the in-place concrete. It is essential that correct
functioning of the rebound hammer is checked regularly using a standard
steel anvil of known mass. This is necessary because wear may change the
spring and internal friction characteristics of the equipment. Calibrations
