Civil Engineering Reference
In-Depth Information
Figure 1.5
shows a car park with early signs of failure due to punching
shear. This causes radial cracking around column positions as the deck flexes
about the support. At later stages, an annular crack appears, which can
indicate the onset of failure. Pipers Row car park in Wolverhampton (
Figure
1.6)
collapsed partly due to this phenomenon.
Corrosion and corrosion damage in concrete
The main causes of corrosion of steel in concrete are chloride attack and
carbonation. Damage due to the latter is illustrated in
Figure 1.7.
These
two mechanisms are unusual in that they do not attack the integrity of the
concrete. Instead, aggressive chemical species pass through the pores in the
concrete and attack or depassivate the passive oxide layer. Carbon dioxide
and the chloride ions are very unusual in penetrating the concrete without
significantly damaging it. The author has, however, come across clauses in
repair bills requesting 'remove all weak, carbonated concrete'. Carbonated
concrete is not necessarily weak: indeed carbonation hardens the concrete.
It is only an issue where there is steel reinforcement within the carbonated
zone. Of course the concrete might be carbonated because it is weak, then
strength may be an issue for the structure.
The corrosion of steel reinforcement which is used in concrete is caused
by electrochemical reactions. During corrosion, a part of the steel surface
becomes anodic (corroding) and metal dissolution occurs, with small
charged particles, called ions, going into solution. Further along the bar a
