Civil Engineering Reference
In-Depth Information
does so, as the coeffi cient of thermal expansion of steel is similar to that of concrete,
it cannot place any stress on the concrete. However, if the reinforcement is in contact
with the air, as is the case for starter bars, the high thermal conductivity of steel will
cause the reinforcement to remain cooler than the surrounding concrete, at least in the
vicinity of construction joints. The concrete will then cool from a higher temperature
than the steel. This can give rise to prejudicial cracking parallel to the reinforcing bars,
particularly when large-diameter bars are used in thin concrete sections. Such cracking
is also made worse by the natural drying shrinkage of the concrete, and it is wise to
limit the diameter of reinforcing bars to one tenth of the local concrete thickness. This
limit is also important for development of sound bond between the concrete and the
steel.
3.6.4 The cracking of a concrete structure built in stages due to the
restraint by previously hardened concrete
This is probably the most widely experienced form of cracking due to the exothermic
reaction. It is most commonly encountered in walls cast onto mature concrete footings.
Most engineers will have seen the characteristic vertical cracks, usually at about 2-3 m
centres, that disfi gure these walls. As before, the concrete heats up while it is still plastic,
becomes progressively harder and cools, shortening as it does so. This shortening is
restrained by the footing, and consequently the wall cracks. The theoretical maximum
amount of cracking is given by the simple expression:
d
=
α
×
t
where
d
= total shortening;
α
= coeffi cient of expansion of concrete;
t
= temperature drop.
= 12 × 10
-6
the total potential
Thus if the temperature drop were to be 40°C and
α
shortening is 0.48 mm/m.
However, the immature concrete of a reinforced wall is capable of straining plastically
in tension, and it only cracks once this tensile strain capacity has been exhausted. Thus
the actual cracking is likely to be less than half the maximum fi gure calculated above.
The restraint to shortening reduces with height above the footing, due to the shear
fl exibility of the wall. Consequently at some height above the footing the cracks will
disappear. Just above the footing, the bond of the wall to the footing tends to spread
the tensile strain evenly along the wall, totally inhibiting cracking or creating fi ne
closely spaced cracks. Thus the characteristic crack pattern due to restrained heat of
hydration cooling is as shown in Figure 3.10.
Cracking due to this cause is particularly prejudicial to concrete that must resist
water pressure, such as cut-and-cover tunnels, because the cracks penetrate through the
full thickness of the wall and thus create leakage paths. Problems also arise for bridge
decks cast in-situ, in short segments, such as those built by the free cantilever method
(
9.3.8
). The wide top slabs that are cast against each other are in effect horizontal
walls, and tend to crack due to the restraint of the preceding segment.
The provision of horizontal reinforcement in the wall cannot stop this tendency to
crack, but the bond between the immature concrete and the steel can control the width
of the cracks. If the reinforcement consists of large-diameter bars, the bond lengths
are too great to limit effectively the crack widths. If, on the other hand, there is too
little reinforcement, the concrete will ignore it and behave as if it were unreinforced
(
3.7.2
). The optimum reinforcement consists of small bars, generally T12 or T16 at
