Page 273 - Civil Engineering Project Management, Fourth Edition

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Site concreting and reinforcement
Where concrete will not remain exposed to view, minor discrepancies can be
accepted. ‘Fins’ of concrete caused by the mix leaking through butt joints in the
formwork should be knocked off. Shallow honeycombing should be chiselled
out, and a chase cut along any defective construction joint. The cut-out area or
chase should be washed, brushed with a thick cement grout, and then filled
with a dryish mortar mix. This rectifying work should be done as soon as pos-
sible so the mortar mix has a better chance of bonding to the ‘green’ concrete.
Shrinkage cracking of concrete is a common experience. The shrinkage of
concrete due to drying is of the order of 0.2–0.5mm/m for the first 28 days.
Subsequently concrete may expand slightly when wet and shrink on drying.
The coefficient of temperature expansion or contraction is very much smaller,
of the order of 0.007mm/m per degree centigrade of change. Rich concrete
mixtures tend to shrink more than lean mixes. The use of large aggregate, such
as 40mm instead of 20mm, helps to minimize shrinkage. To avoid cracking of
concrete due to shrinkage, wall lengths of concrete should be limited to about
9m if restrained at the base or ends. Heavy foundations to a wall should not
be allowed to stand and dry out for a long period before the wall is erected,
because the wall concrete bonding to the base may be unable to shrink without
cracking. Concrete is more elastic than is commonly appreciated, for example
the unrestrained top of a 300mm diameter reinforced concrete column 4m
high can be made to oscillate through nearly 1cm by push of the hand.

19.12 Handling and fixing steel reinforcement

In best engineering practice the engineer will produce complete bar-bending
schedules for use by the contractor. The engineer may not guarantee that such
schedules are error free and may call upon the contractor to check them. But,
as often as not, the contractor will fail to do this, so it is advisable for the resi-
dent engineer to check the schedules so that he can forewarn the contractor of
any error present. In practice, few errors will be found because the advantage
of producing bar-bending schedules is that it applies a detailed check on the
validity of the reinforcement drawings supplied to the contractor.
In some contracts the contractor is required to produce bar-bending sched-
ules himself from the reinforcement drawings supplied under the contract.
This is not such good practice; the engineer foregoes an opportunity to check
the reinforcement drawings, and the contractor (or his reinforcement sup-
plier) who produces the bending schedules will not necessarily be sufficiently
acquainted with the design to notice some discrepancy which indicates a pos-
sible design error.
Reinforcement is now seldom bent on site, except on sites overseas.
Deliveries of reinforcement should be supervised by the leading steelfixer,
who should check the steel against the bar schedules and direct where bars
should be stocked. Bars should be delivered with identifying tags on them,
but sometimes these get torn off. The leading steelfixer should not allow with-