Structural aspects of repair - Concrete Repair - page 78

Half-cell surveys help identify the areas of active corrosion on that day
and corrosion rate measurements give an instant number, but they give no
indication of the severity of reinforcement section loss. Both work best when
concrete is damp. They can help locate areas for selective opening up to
answer the structural question: 'How much loss of section and bond strength
at laps has occurred over the life of the structure?'
Selective opening up can give a better indication of current damage and the
likely extent of extra cutting out for repairs to effectively control corrosion.
Where conditions are right cathodic protection (CP) can reduce the need for
cutting out. However before applying CP the safety of the already corroded
structure must be checked.
It is an unusual repair contract which does not reveal that deterioration
is more extensive than was apparent from the initial investigation. There
is a strong case for having a trial repair contract to enable more extensive
and deeper destructive investigation and evaluation of the effectiveness of
repairs before a large-scale contract is let.
Effective repair requires cutting out to a sound substrate and to clear all
carbonated and chloride contaminated material around the reinforcement
( Figure 3.5). This cutting out inevitably further weakens the structure and
the effects of this must be evaluated before the contract is let, including the
potential consequences of more extensive cutting out, if that is found to be
necessary as work progresses.
Figure 3.5 Structurally damaging corrosion which will be substantially increased by
cutting out to clear chlorides.