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354 Barry’s Advanced Construction of Buildings
subject to creep deformation under stress due to movements of absorbed water. The relative
volume of cement gel to aggregate therefore affects deformation due to creep. Changing
from a 1 : 1 : 2 to a 1 : 2 : 4 cement, fine and coarse aggregate mix increases the volume of
aggregate from 60% to 75%, yet causes a reduction in creep by as much as 50%.
Temperature, relative humidity and the size of members have an effect on the hydration
of cement and migration of water around the cement gel towards the surface of concrete.
In general, creep is greater the lower the humidity and increases with a rise in temperature.
Small section members of concrete will lose water more rapidly than large members and
will suffer greater creep deformation during the period of initial drying. The effect of creep
deformation has the most serious effect through stress loss in prestressed concrete, deflection
increase in large-span beams, buckling of slender columns and buckling of cladding
in tall buildings.
Alkali–silica reaction
The chemical reaction of high silica-content aggregate with alkaline cement causes a gel to
form, which expands and causes concrete to crack. The expansion, cracking and damage
to concrete are often most severe where there is an external source of water in large quantities.
Foundations, motorway bridges and concrete subject to heavy condensation have
suffered severe damage through ASR. The expansion caused by the gel formed by the reaction
is not uniform in time or location. The reaction may develop slowly in some structures
yet very rapidly in others and may affect one part of a structure but not another. Changes
in the method of manufacture of cement, which has produced a cement with higher alkalinity,
are thought to be one of the causes of some noted failures. To minimise the effect of ASR,
it is recommended that cement-rich mixes and high silica-content aggregates be avoided.
6.3 Reinforcement
Concrete is strong in resisting compressive stress but comparatively weak in resisting tensile
stress. The tensile strength of concrete is between one-tenth and one-twentieth of its compressive
strength. Steel, which has good tensile strength, is cast into reinforced concrete
members in the position or positions where maximum tensile stress occurs. To determine
where tensile and compressive stresses occur in a structural member, it is convenient to
consider the behaviour of an elastic material under stress. A bar of rubber laid across (not
fixed) two supports will bend under load. The top surface will shorten and become compressed
under stress, while the bottom surface becomes stretched under tensile stress, as
illustrated in Figure 6.1.
A member that is supported so that the supports do not restrain bending while under
load is termed ‘simply supported’. From Figure 6.1 it will be seen that maximum stretching due
to tension occurs at the outwardly curved underside of the rubber bar. If the bar were of
concrete, it would seem logical to cast steel reinforcement in the underside of the bar. In that
position, the steel would be exposed to the surrounding air and it would rust and gradually
lose strength. Further, if a fire occurred in the building, near to the beam, the steel might
lose so much strength as to impair its reinforcing effect and the beam would collapse. It is
usual practice, therefore, to cast the steel reinforcement into concrete so that there is at
least 15 mm of concrete cover between the reinforcement and the surface of the concrete.