Civil Engineering Reference
In-Depth Information
Carbonation-induced reinforcement corrosion
Conventional testing with phenolphthalein will be sufficient in most cases
to measure carbonation depth in Portland cement concretes. However,
with HAC concrete and in some Portland cement concretes such as those
containing SRPC, petrographic examination is the definitive means for
measuring carbonation depth. It is important to establish whether or not
there is evidence for moisture penetration since corrosion will not proceed
in dry conditions. Carbonation and moisture-induced corrosion is generally
associated with obvious expansion and spalling - particularly where
reinforcement is situated close to concrete surfaces or intersected by cracks
initiated by other causes such as drying shrinkage or AAR.
Alkali-aggregate reaction (AAR)
It is important to establish the aggregate type and whether or not AAR
is associated with deleterious expansion.
Figure 2.5
illustrates the typical
appearance of concrete affected by AAR in a thin section.
SEM/EDXA can provide valuable information about the chemical
composition of gel deposits that can be of assistance in assessing whether
or not AAR is likely to be on-going. Core expansion testing can be used
to measure the potential for future expansion. It is important that repair
materials selected do not act as a source of alkalis and that the repair material
contains aggregates that are of low reactivity (Concrete Society, 1999)
with respect to AAR. Core sampling of concrete structures for the possible
presence of AAR should sample the concrete at depth and should be at least
150 mm deep. The cores should be washed clean and immediately wrapped
in several layers of cling film before dispatch to the laboratory. Petrographic
examination is the definitive method for detecting AAR.
alkali-silica gel.
