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352 Barry’s Advanced Construction of Buildings
exposed. The top surface of the concrete is usually broken away by means of a mechanical
scabbler. This hammers the surface of the set concrete breaking away the loose surface,
leaving a clean, surface for the new concrete to form a mechanical and chemical bond.
There should be as few construction joints as practical and joints should be either vertical or
horizontal. Joints in columns are made as near as possible to beam haunching, and those in
beams at the centre or within the middle third of the span. Vertical joints are formed against
a strip board. Water bars are fixed across or cast into construction joints where there is a
need to provide a barrier to the movement of water through the joint (see Chapter 3).
Curing concrete
Concrete gradually hardens and gains strength after its initial set. For this hardening
process to proceed and the concrete to develop its maximum strength, there must be water
present in the mix. If, during the early days after the initial set, there is too rapid a loss of
water, the concrete will not develop its maximum strength. The process of preventing a
rapid loss of water is termed ‘curing concrete’. Large exposed areas of concrete such as road
surfaces are cured by covering the surface for at least a week after placing, with building
paper, plastic sheets or wet sacks to retard evaporation of water. In very dry weather, the
surface of concrete may have to be sprayed with water in addition to covering it. The formwork
around reinforced concrete is often kept in position for some days after the concrete
is placed in order to give support until the concrete has gained sufficient strength to be
self-supporting. This formwork also serves to prevent too rapid a loss of water and so helps
to cure the concrete. In very dry weather, it may be necessary to spray the formwork to
compensate for too rapid a loss of water.
Specially designed curing agents can also be used. These are chemical liquids that are
designed to be sprayed over the concrete. Once sprayed, the liquid produces a thin film
that effectively seals the water, needed for hydration, in the concrete.
Self-compacting concrete (SCC)
Self-compacting concrete (SCC) is a concrete that does not require vibration for placing
and compaction. SCC was first developed in Japan in the late 1980s and has since become
popular because it offers a rapid rate of concrete placement and hence faster construction
times. Vibration equipment is not required, helping to reduce the noise and vibration suffered
by construction workers. SCC also has the benefit of being easier to place around
closely spaced reinforcement compared with normal concrete due to its ease of flow. The
engineering properties of SCC are very similar to concrete for the same specification,
although the surface finish is usually of a higher quality.
Additions are used to improve and maintain the cohesion and segregation resistance of
SCC. The additions range from inert mineral fillers (e.g. limestone), to pozzolanic (fly ash,
silica fume) and hydraulic fillers (ground granulated blastfurnace slag). Admixtures such
as superplasticisers are an essential component of SCC, helping to bring about the water
reduction and improve fluidity. Polymer fibres may be added to improve the stability of
the SCC.
SCC is delivered ready mixed direct to site by the manufacturer and pumped to its
required position. SCC must be placed in one pour without a break in placing to maintain
its integrity. Because of the special qualities of SCC it is essential that site personnel are
trained in the specific requirements of placing SCC and that adequate supervision is in