Abstract:
Self-compacting concrete (SCC) is an innovative concrete that does
not require vibration for placing and compaction. It is able to flow under its
own weight, completely filling formwork and achieving full compaction, even in
the presence of congested reinforcement. The hardened concrete is dense,
homogeneous and has the same engineering properties and durability as
traditional vibrated concrete.
Self-compacting concrete offers a rapid rate of concrete
placement, with faster construction times and ease of flow around congested
reinforcement. The fluidity and segregation resistance of SCC ensures a high
level of homogeneity, minimal concrete voids and uniform concrete strength,
providing the potential for a superior level of finish and durability to the
structure. SCC is often produced with low water-cement ratio providing the
potential for high early strength, earlier demoulding and faster use of
elements and structures.
The elimination of vibrating equipment improves the environment on
and near construction and precast sites where concrete is being placed,
reducing the exposure of workers to noise and vibration.
The improved construction practice and performance, combined with
the health and safety benefits, make SCC a very attractive solution for both
precast concrete and civil engineering construction.
DEVELOPMENT OF SCC
The SCC concept was introduced into scientific world in Japan in
1986 by Professor Hajime Okamura from Tokyo University. The first prototype was
developed in 1988 by K. Ozawa from Tokyo University as a response to the growing
problems associated with concrete durability and the high demand for skilled
workers.
Engineering Properties
General
Self-compacting concrete and traditional vibrated concrete of
similar compressive strength have comparable properties and if there are
differences, these are usually covered by the safe assumptions on which the
design codes are based. However, SCC composition does differ from that of
traditional concrete so information on any small differences that may be
observed is presented in the following sections.
Durability, the capability of a concrete structure to withstand
environmental aggressive situations during its design working life without
impairing the required performance, is usually taken into account by specifying
environmental classes. This leads to limiting values of concrete composition
and minimum concrete covers to reinforcement.
In the design of concrete structures, engineers may refer to a
number of concrete properties, which are not always part of the concrete
specification. The most relevant are:
Compressive strength
Tensile strength
Modulus of elasticity
Creep
Shrinkage
Coefficient of thermal expansion
Bond to reinforcement
Shear force capacity in cold joints
Fire resistance
Where the value and/or the development of a specific concrete
property with time is critical, tests should be carried out taking into account
the exposure conditions and the dimensions of the structural member.
COMPRESSIVE STRENGTH
Self-compacting concrete with a similar water cement or cement
binder ratio will usually have a slightly higher strength compared with
traditional vibrated concrete, due to the lack of vibration giving an improved
interface between the aggregate and hardened paste. The strength development
will be similar so maturity testing will be an effective way to control the
strength development whether accelerated heating is used or not.
A number of concrete properties may be related to the concrete
compressive strength, the only concrete engineering property that is routinely
specified and tested.
Tensile Strength
Self-compacting concrete may be supplied with any specified
compressive strength class. For a given concrete strength class and maturity,
the tensile strength may be safely assumed to be the same as the one for a
normal concrete as the volume of paste (cement + fines + water) has no
significant effect on tensile strength.
In the design of reinforced concrete sections, the bending tensile
strength of the concrete is used for the evaluation of the cracking moment in
prestressed elements, for the design of reinforcement to control crack width
and spacing resulting from restrained early-age thermal contraction, for
drawing moment-curvature diagrams, for the design of unreinforced concrete
pavements and for fiber reinforced concrete.
In prestressed units the splitting tensile stresses around the
strands as well as their rate of drawn-in (slippage) in the end section when
releasing the prestressing forces are related to f`ct, the compressive strength
at release. Cracks due to splitting tensile stresses should generally be
avoided.
Creep
Creep is defined as the gradual increase in deformation (strain)
with time for a constant applied stress, also taking into account other time
dependent deformations not associated with the applied stress, i.e. shrinkage,
swelling and thermal deformation.
Creep in compression reduces the prestressing forces in
prestressed concrete elements and causes a slow transfer of load from the
concrete onto the reinforcement. Creep in tension can be beneficial in that it
in part relieves the stresses induced by other restrained movements, e.g.
drying shrinkage and thermal effects.
Creep takes place in the cement paste and it is influenced by its
porosity which is directly related to its water/cement ratio. During hydration,
the porosity of the cement paste reduces and so for a given concrete, creep
reduces as the strength increases. The type of cement is important if the age
of loading is fixed. Cements that hydrate more rapidly will have higher
strength at the age of loading, a lower stress/strength ratio and a lower
creep. As the aggregates restrain the creep of the cement paste, the higher the
volume of the aggregate and the higher the E-value of the aggregate, the lower
the creep will be.
Shrinkage
Shrinkage is the sum of the autogenous and the drying shrinkage.
Autogenous shrinkage occurs during setting and is caused by the internal
consumption of water during hydration. The volume of the hydration products is
less than the original volume of un hydrated cement and water and this
reduction in volume causes tensile stresses and results in autogenous
shrinkage.
Drying shrinkage is caused by the loss of water from the concrete
to the atmosphere. Generally this loss of water is from the cement paste, but
with a few types of aggregate the main loss of water is from the aggregate.
Drying shrinkage is relatively slow and the stresses it induces are partially
balanced by tension creep relief.
The aggregate restrains the shrinkage of the cement paste and so the
higher the volume of the aggregate and the higher the E-value of the aggregate,
the lower the drying shrinkage. A decrease in the maximum aggregate size which
results in a higher paste volume increases the drying shrinkage.
As concrete compressive strength is related to the water cement
ratio, in SCC with a low water/cement ratio drying shrinkage reduces and the
autogenous shrinkage can exceed it.
Tests performed on creep and shrinkage of different types of SCC
and a reference concrete show that
• The deformation caused by shrinkage may be higher
• The deformation caused by creep may be lower
• The value for the sum of the deformations due to shrinkage and
creep are almost similar
Due to the restrain of the presence of reinforcement in a cross
section the shrinkage strain will cause tension in concrete and compression in
the reinforcement.
Coefficient of Thermal Expansion
The coefficient of thermal expansion of concrete is the strain
produced in concrete after a unit change in temperature where the concrete is
not restrained either internally (by reinforcing bars) or externally.
The coefficient of thermal expansion of concrete varies with its
composition, age and moisture content. As the bulk of concrete comprises
aggregate, using an aggregate with a lower coefficient of thermal expansion
will reduce the coefficient of thermal expansion of the resulting concrete.
Reducing the coefficient of thermal expansion leads to a proportional reduction
in the crack control reinforcement.
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