Properties of concrete - Concrete Mix Design, Quality Control and Specification - page 88

steel. Roman concrete was not reinforced and this is a major reason for its
survival for centuries. However, as we shall see, there can be durability
problems with unreinforced concrete aside from structural issues.
The major factor in the corrosion of reinforcing steel is the thickness and
quality of the concrete cover. In theory, without adequate cover, concrete
cannot protect the reinforcement. However excessive cover means that the
surface concrete is essentially unreinforced and can crack due to thermal
stresses or shrinkage, sometimes with the reinforcing cage acting as a crack
inducer. Therefore, specifications that call for 100 mm cover or more may
result in no effective cover whatsoever! The good durability of spun pipes,
ferrocement, and steel fibre reinforced concrete with limited or negligible
cover to the reinforcement highlights that corrosion is a complicated subject
and there can be exceptions to the general rules.
With reasonable cover, the next factor is the penetrability of the con-
crete. “Penetrability” is a collective term describing the transport prop-
erties of concrete, including absorption, permeability, and diffusion.
Basically steel will not rust unless water and oxygen can reach it and
it has been depassivated by chloride ions or carbonation. Since it is the
alkalinity of cement that provides passivation of the steel, and since a
lower water to cement ratio (w/c) tends to reduce penetrability, it used
to be thought that a high Portland cement content was the appropriate
way to achieve durability. The substitution of a proportion of fly ash
or blast-furnace slag for some of the cement was considered to reduce
durability. It is now realised that substitution of cementitious materi-
als generally reduces penetrability and is an important positive factor in
reducing corrosion. However, good curing can be even more important
with concrete containing fly ash or slag to be effective. There are also a
wide range of admixtures and technologies that can be used to reduce
corrosion of reinforced concrete.
An exception to the general rule regarding corrosion can be carbonation.
Carbonation occurs when penetrating carbon dioxide dissolves in the
pore water and reacts with calcium hydroxide reducing the alkalinity of
the matrix. The phenomenon is sensitive to the internal relative humidity
of the concrete. The conditions that lead to higher rates of carbonation
where the internal relative humidity is between 50% and 70% would not
be expected to cause significant corrosion in the event of the carbonation
front reaching the reinforcing steel due to the lack of moisture and higher
resistivity. The propagation phase of the reinforcement corrosion due to
carbonation could be 100 years. Accordingly a 100-year design life could
be assured even if the concrete was carbonated before removing the form-
work! In situations where the concrete can carbonate and then have access
to moisture can have more rapid rates of corrosion. Vehicular tunnels with
high carbon dioxide concentration at the internal surface and moisture
availability to the reinforcing due to wick action may require extra care