Properties of concrete - Concrete Mix Design, Quality Control and Specification - page 107

5.5 DIMENSIONAL STABILITY
Dimensional stability may include undesirable degrees of thermal expansion
and also disruptive expansion due to alkali-aggregate reaction or sulfate
attack but essentially the problem is shrinkage. The major type of shrinkage
is thermal shrinkage at early age and drying shrinkage at later age, but
there are also autogenous or chemical shrinkage, carbonation shrinkage,
elastic defection, and creep under load.
Thermal shrinkage is due to the contraction of concrete as it cools from
its peak temperature during hydration to the ambient temperature based
on its coefficient of thermal expansion. This is more important in massive
elements but can also be an important source of cracking in thinner
elements, depending on restraint.
Autogenous shrinkage relates to concretes of very low w/c ratios that
shrink as a result of self-desiccation. It occurs much more rapidly than
normal drying shrinkage and produces through section shrinkage not from
the outside in. Autogenous shrinkage will be additive to thermal shrinkage
and is particularly significant in concrete with high replacement levels of
GGBS and a low w/cm ratio.
Drying shrinkage is a result of contraction of the cement paste as the
uncombined excess water evaporates. This shrinkage is restrained by the
aggregates, especially the coarse aggregates. From this it is obvious that
shrinkage will be higher if there is more water and cement and more sand.
Some coarse aggregates have an appreciable moisture movement that
will directly contribute to shrinkage but, apart from this, a higher elastic
modulus of the coarse aggregate will reduce shrinkage.
5.6 GOOD APPEARANCE
A good appearance requires that concrete be fully compacted and free from
“bug holes”. Actually the type of formwork and the mold oil used may have
a considerable effect on this aspect.
Bleed control on fair-faced concrete is important. A tendency to bleed
allows water to travel up the face of the formwork or toward any slightly
leaking joints. This can produce very unsightly results including sand
streaks and hydration staining. In its most severe form the latter can result
in black areas adjoining joints, caused by the bleed water washing the usual
gray dust coating from the cement grains, which are actually black. Since
true SCC does not bleed at all, it should be free from such defects. Air voids
formed by the release of entrapped air can be a problem, particularly when
concrete is cast against inward sloping mold faces. Controlled permeability
formliner (CPF) can be helpful in these applications to provide a pathway
for removal of air.