Civil Engineering Reference
In-Depth Information
prepared for one hammer will also not necessarily apply to another. It is
probable that very few rebound hammers used for in-situ testing are in fact
regularly checked against a standard anvil, and the reliability of results may
suffer as a consequence.
The importance of specimen mass has been mentioned above; it is essential
that test specimens are either securely clamped in a heavy testing machine or
supported upon an even solid floor. Cubes or cylinders of at least 100 mm
should be used, and a minimum restraining load of 15% of the specimen
strength has been suggested for cylinders, and BS EN 12504-2 recommends
not less than 7 N/mm
2
for cubes tested with a type N hammer. Typically the
relationship between rebound number and restraining load is such that once
a sufficient load has been reached the rebound number remains reasonably
constant.
It is well established that the crushing strength of a cube tested wet is
likely to be about 10% lower than the strength of a corresponding cube
tested dry. Since rebound measurements should be taken on a dry surface, it
is recommended that wet cured cubes be dried in the laboratory atmosphere
for 24 hours before test, and it is therefore to be expected that they will yield
higher strengths than if tested wet in the standard manner.
Note that a calibration
must
be performed for each concrete that is to be
tested, before the relationship between cube strength and rebound number
can be established. The values in
Figure 1.26
accord well with the author's
experience, however.
Other near-to-surface strength tests
These include the Windsor Probe, the BRE Internal Fracture Tester, various
break-off devices and the CAPO and Lok tests used extensively in Scandinavia
and increasingly in the UK to establish concrete maturity.
Radar profiling
Over the past thirty years or so there has been an increasing usage of sub-surface
impulse radar to investigate civil engineering problems and, in particular,
concrete structures. Electromagnetic waves, typically in the frequency range
500 MHz to 1.5 GHz, will propagate through solids, with the speed and
attenuation of the signal influenced by the electrical properties of the solid
materials. The dominant physical properties are the electrical permittivity
which determines the signal velocity, and the electrical conductivity which
determines the signal attenuation. Reflections and refractions of the radar
wave will occur at interfaces between different materials and the signal
returning to the surface antenna can be interpreted to provide an evaluation
of the properties and geometry of sub-surface features.
