Civil Engineering Reference
In-Depth Information
3.5.5 Cracking of sections where bending theory does not apply
The code formulae are not applicable to areas of reinforced concrete structures that
do not work in bending. The designer is left with little guidance. Some elements of the
author's personal experience may be of use.
The cracking of concrete is not a linear function of the stress in the steel. In the
author's experience, if this stress remains below some critical fi gure, the concrete does
not appear to crack, even though its theoretical tensile stress is several times higher
than its tensile strength.
One example of this is a jacking system designed by the author. Hexagonal reinforced
concrete jacking heads were used to lift a heavy load. The jacking heads rested on a
central hydraulic jack, and six prestressing grade bars suspended the load from the head,
Figure 3.8. The reinforcement for the jacking head was designed as hexagonal hoops,
resisting six radial concrete struts. As the stress in the steel would be constant all round
the head, it was feared that the concrete may crack unacceptably, and consequently
the working stress in the steel was limited to 150 MPa. The theoretical concrete stress
adjacent to the steel may be found by dividing the steel stress by the modular ratio,
and would be some 25 MPa under the effect of short-term loads, approximately six
times its tensile strength. The strain in the steel, neglecting its bond to the concrete,
would be 150 / 200,000 = 7.5 × 10
-4
, or 0.75 mm/m. The bond to the concrete
would reduce this strain slightly. If the strain in the steel is assumed to be reduced by
20 per cent, it would become 0.6 mm/m. Hence one would expect to see cracks in the
concrete totalling 0.6 mm/m. However, no cracks in the concrete were discernible to
the naked eye. Neville [7] states that cracks fi ner than 0.13 mm are not visible without
magnifi cation. Either the concrete had cracked but the cracks were too small to see,
and hence to worry about, or the concrete had yielded in tension without cracking.
Similar experiences elsewhere have convinced the author that limiting the working
stress of the reinforcing steel to about 150 MPa generally results in an apparently
crack-free concrete, while if the steel stress does not exceed 250 MPa, cracking will
remain within tolerable limits.
3.6 The exothermic reaction
3.6.1 General
As cement sets, it heats up. The temperature rise of the setting concrete depends on
the cement content, on the fi neness of grinding of the cement which governs the speed
of the chemical reaction, on the type of cement and on the thickness of the member.
Thick sections of concrete with a high content of fi nely ground cement can attain
setting temperatures of up to 90°C. If cement is delivered to site still hot from the
furnace of the cement works, temperatures of the setting concrete even in quite thin
sections may be unexpectedly high.
The time taken for the temperature to rise to its peak and then to cool down
depends principally on the thickness of the structural element, Figure 3.9. In general
terms, as the concrete is heating up and expanding it is still plastic. By the time it starts
to cool and shrink, it has hardened, with rapidly increasing strength and modulus of
elasticity. Consequently the heating phase does not cause the concrete to be stressed
signifi cantly in compression. On the other hand, any restraint to the shrinkage of the
