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Structural Concrete Frames 353
place to monitor the pour. Guidance is provided by manufacturers into the rate of placing
and measures to take if a problem occurs while the SCC is being placed, such as a stoppage
in the flow of the SCC.
SCC can also be used in the manufacture of precast concrete products, especially when
a high-quality surface appearance is required.
Deformation of concrete
Hardened concrete will suffer deformation due to:
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Elastic deformation that occurs instantaneously and is dependent on applied stress
Drying shrinkage that occurs over a long period and is independent of the stress in
concrete
Creep, which occurs over a long period and is dependent on stress in concrete
Expansion and contraction due to changes in temperature and moisture
Alkali–silica reaction (ASR)
Elastic deformation
Under the stress of dead and applied loads of a building, hardened concrete deforms elastically.
Vertical elements such as columns and walls are compressed and shorten in height,
and horizontal elements such as beams and floors lengthen due to bending. These comparatively
small deformations, which are related to the strength of the concrete, are predictable
and allowance is made in design.
Drying shrinkage
The drying shrinkage of concrete is affected principally by the amount of water in concrete
at the time of mixing and to a lesser extent by the cement content of the concrete. It can
also be affected by a porous aggregate losing water. Drying shrinkage is restrained by the
amount of reinforcement in concrete. The rate of shrinkage is affected by the humidity and
temperature of the surrounding air, the rate of airflow over the surface and the proportion
of surface area to volume of concrete. Where concrete dries in the open air in summer,
small masses of concrete will suffer about a half of the total drying shrinkage a month after
placing, and large masses about a half of the total shrinkage a year after placing. Shrinkage
will not generally affect the strength or stability of a concrete structure but is sufficient to
require the need for movement joints where solid materials such as brick and block are
built up to the concrete frame.
Creep
Under sustained load, concrete deforms as a result of the mobility of absorbed water within
the cement gel under the action of sustained stress. From the point of view of design, creep
may be considered as an irrecoverable deformation that occurs with time at an everdecreasing
rate under the action of sustained load. Creep deformation continues over very
long periods of time to the extent that measurable deformation can occur 30 years after
concrete has been placed. The factors that affect creep of concrete are the concrete mix,
relative humidity and temperature, size of member and applied stress.
Most aggregates used in dense concrete are inert and do not suffer creep deformation
under load. The hardened cement water paste surrounding the particles of aggregate is