Page 31 - Concrete-Technology-Interacted_Book_Prof-MIK_Neat

PART 2

Concrete repairs

7.1 Patch repairs

Before approaching concrete repairs, consideration must first be given to the cause of the
problem. This is fundamental to the success or failure of the repair, and a lack of adequate
attention at this point can jeopardize the whole job.

If the problem has been diagnosed as being due to carbonation-induced or chloride-
induced corrosion of the reinforcement, patch repairs may be used, although with the
latter, there are precautions needed to ensure a successful repair. This chapter deals with
cracking and spalling due to reinforcement corrosion only. If the problem has been
diagnosed as due to ASR or any of the other mechanisms of failure, the repair may need
to be specifically designed for that particular contract and structure, and it is impossible
to generalize about the approach which might be taken.

7.1.1 Patch repairing carbonation-induced corrosion

It must first be understood that carbonation is a variable process and the carbonation front
will probably not be uniform over the structure. Regular testing to determine the cover to
the steel reinforcement and the penetration of carbonation, by the phenolphthalein spray
method, will be required.

Criteria employed in the past have been to establish which areas of concrete fall into
the following categories:

(a) Visibly spalled concrete with exposed steel reinforcement.
(b) Areas which sound hollow when tapped lightly with a club hammer (these may often

be found around actual spalled areas showing the problem is worse than is visually
apparent).
(c) Areas where the cover to the steel reinforcement is less than 10 mm.
(d) Areas where the carbonation front has encroached to within 5 mm of the reinforcement
(this necessitates carbonation testing at least every 2 metres or so).
(e) Areas of honeycombed concrete.
(f) Areas where the half cell potential values numerically exceed –200 mV (copper/
copper sulfate). Using this criterion, however, caution must be exercised as half cell
potentials only show areas of active corrosion. In the summer, whole areas of corrosion
may shut down as the concrete dries out. Detection of carbonation-induced corrosion
is not as reliable as for chloride-induced corrosion using the half cell method. If the
above criteria have been followed, half cell potential testing is not strictly necessary
but can be a useful additional aid to diagnosis.

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