Specification - Concrete Mix Design, Quality Control and Specification - page 125

the improvement over time during mix development. Chloride migration
testing can be correlated with a simple and rapid resistivity test, which
could be used during production for ongoing quality control.
6.8 CRACK WIDTH
Specification of maximum crack width is a very contentious subject. Cracking
in reinforced concrete structures is complicated. This is the reason that
CIRIA C660 defines “maximum” crack widths at the surface of the con-
crete as design “target” characteristic values with only a 5% chance of being
exceeded. Using this probabilistic approach to cracking would solve a number
of disputes regarding crack widths. Crack size is limited in water retaining
structures to enable autogenous healing and crack size is also limited in an
attempt to improve durability. We would agree with the former but ques-
tion the latter. Clearly cracks along the reinforcing such as plastic settlement
cracks are a serious durability issue regardless of their size. However, if trans-
verse cracks are important to durability, why increase their number by adding
more reinforcement to disperse them as pointed out by Beeby in the 1980s.
BS8007 advised that self-healing should seal cracks of 0.1 mm in 7 days
and 0.2 mm in 21 days. AS3735 Liquid Retaining Structures does not
nominate a limiting crack width but mentions leakage at a crack in section
7.3 Testing of Liquid Retaining Structures. This section implies a limit on
crack size at the time of testing equivalent to tightness class 2 in EN1992-3
Liquid Retaining and Containment Structures.
EN1992-3 Liquid Retaining and Containment Structures acknowledges
that cracks tend to self-heal. The recommended maximum surface crack
width is 0.2 mm, where the ratio of wall thickness to hydrostatic pressure
is less than 5, reducing to 0.05 mm, where that ratio is greater than 35.
EN1992-1-1 Design of Concrete Structures provides guidance on durability
considerations. Table 7.1N provides recommended values of w max = 0.3 mm
for exposure class XS3, which is defined in table 4.1 as corrosion induced
by seawater in tidal and splash zones. Therefore, a crack width of 0.3 mm
should be considered adequate to achieve the required durability against
aggressive conditions even if full self-healing has not occurred within
3 months. The provisions for crack size from AS3735 and EN1992-3 apply
at the time of test or at the time of filling with water if there is no test.
Mohammed and Hamada (2003) examined the corrosion of reinforce-
ment passing through a construction joint (i.e., a man-made crack). They
found that autogenous healing at the joint plane prevented corrosion except
where the joint had been treated with epoxy and latex paste. Calder and
Thomson (1988) report significant corrosion in cracks that had been fully
filled with epoxy. We would suggest that a crack width limit of 0.30 mm at
the concrete surface is appropriate in most chloride environments