Civil Engineering Reference
In-Depth Information
Figure 3.11
Singapore Central Expressway (Photo: Robert Benaim)
for the soffi t of the top slab is trapped between the webs and must include a fl exible
component or be released to allow the slab to shorten as it cools.
3.6.7 Measures to limit heat-of-hydration cracking
The problem for the designer is that cracking due to this cause is not reliably
predictable. The immature concrete can absorb signifi cant tensile strains without
cracking. Relatively trivial factors may push the concrete over its limit, and trigger
cracking. Benaim designed the twin-cell cut-and-cover tunnel of the Singapore Central
Expressway, Figure 3.11, which was many hundred metres long, with walls and slabs
generally 1 m thick or more. The roof slabs of both cells were cast simultaneously,
but whereas one cell was virtually uncracked, the other exhibited signifi cant restraint
cracking. The only difference between the two cells was the degree of shading from
the sun offered by the sheet piling lining the excavation!
The designer has the choice between:
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specifying considerable tonnages of reinforcement as a precaution to control
cracking that may not occur;
modifying the concrete mix to reduce the peak temperature reached;
cooling the concrete during mixing;
cooling the concrete after casting;
adding admixtures which seal the cracks;
in a repetitive structure, adopting an observational technique, and adding
reinforcement as necessary to control the observed crack pattern.
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