Structural aspects of repair - Concrete Repair - page 77

the area to ensure that the core goes between reinforcing bars, not through
one. The core size should be minimised to that which is essential for tests,
typically 50 mm or 70 mm.
Once the reinforcement layout and visual signs of corrosion at spall
locations are apparent, a sensible estimate, guided by selective opening up,
needs to be made of the extent of corroding bars which are not yet spalling
the cover. Corrosion of a bar near the surface will cause a spall, but if the bar
depth is more than twice its diameter there will be a very severe loss of section
before a spall can develop. With closely spaced deep bars delamination of an
area can develop, rather than individual spalls above bars.
One needs to consider the likely condition of steel in hidden concrete
behind cladding and in joints where carbonation and/or chloride ingress
will have developed. Periodic moisture from condensation, driving rain and/
or due to failing sealants can then accelerate corrosion. Some cladding has
hidden fixings of mild steel, which will corrode, or galvanised steel which
last only a little longer or of stainless steel which is good, as long as there is
no bimetallic corrosion.
When concrete is saturated, which limits oxygen availability, and there is
a high chloride concentration, corrosion develops as 'black rust' without the
expansion to produce spalling. This makes it difficult to detect. Structurally
'black rust' is important as it tends to cut bars locally, ( Figure 3.4), especially
the bends on the shear links and column stirrups which rapidly reduces the
beam or column strength.
Figure 3.4 Local pitting corrosion of links in a half joint, from chlorides in saturated
concrete.