Civil Engineering Reference
In-Depth Information
3.2.1 Cutting out to a sound substrate
Analysis of repair failures shows that bruising of the substrate concrete is
often a cause of low strength and delamination. A low pull-off test value
only gives an indication of a problem, not a diagnosis. Ensuring that the
coring for the test goes one diameter beyond the repair/concrete interface
and removing this deep section, enables both sides of the pull-off fracture to
be examined and a petrographic examination of a longitudinal section to be
made. The tensile strength of reasonable concrete is typically 3 to 4 MPa, but
variable in the pull-off test. When concrete is bruised by a jack hammer there
is a weak surface of cracked material, (
Figure 3.6),
and this is where the pull-
off failure often occurs. For structural repair, high-pressure water jetting can
give an excellent surface. Where the underlying concrete is inherently weak
from a poor mix, a stronger repair will simply pull off and this limits the
structural effectiveness.
3.2.2 Removal of carbonated or chloride-contaminated concrete
The removal of carbonated or chloride-contaminated concrete needs to be
considered in depth and for the surrounding area. An inherent characteristic
of concrete and of the conditions of exposure, which strongly influence
chloride ingress as well as carbonation, is their high variability. Combined
with this is the variability of cover depth. It is important to take sufficient
samples to get a true distribution. Spot samples can be very misleading.
The durability surveys will have given an indication of the overall patterns
of corrosion. This should differentiate localised problems with shallow bars
and carbonation of 15 mm maximum from those areas where carbonation
depths are similar to the average reinforcement bar depth (cover + half
diameter). It will also have identified if chlorides are contributing to
