Concrete future - Concrete Mix Design, Quality Control and Specification - page 279

intervention even more difficult. A competent concrete producer has to con-
duct trials to establish which products, and which suppliers of materials,
will best enable him to consistently produce the most economical compliant
concrete for a particular project. He should be encouraged to do so by the
specification. All parties to the project will benefit from a competent and
motivated concrete supplier with consistent supply that complies with the
specification requirements. Specifications for nonstrength properties can
be more complicated and this is often used as a justification for prescribing
some mix features, sometimes significantly reducing the sustainability of
the concrete or the ability of the supplier to innovate.
Premature deterioration of reinforced concrete is a global problem
that costs billions of dollars annually. In severe environments, concrete
structures have often failed to achieve their required service life without
major maintenance, which is unsustainable. As more specifications now
require a minimum design life of 100 years or more for major projects
and infrastructure, there is even more demand for appropriate specifica-
tions to ensure concrete durability. International codes provide prescriptive
solutions to increase the required concrete quality and cover thickness to
improve chloride resistance. The common practice to specify a minimum
cementitious content to achieve “durability” is an impediment to sustain-
ability. First, Buenfeld and Okundi (1998) showed that, at a given water
to cementitious materials ratio (w/cm), the higher binder content actually
increased chloride ion ingress in concrete. Similar results were found by
Dhir et al. (2004). This is hardly surprising when transport processes occur
primarily through the paste fraction of the concrete. Second, an unneces-
sarily high cementitious content may lead to increased cracking due to ther-
mal stresses and shrinkage, which could reduce durability. Unnecessarily
wasting cementitious materials also increases the environmental impact of
the concrete. Another unintended consequence of minimum cementitious
content requirements in specifications is that it creates a competitive dis-
advantage for the more competent concrete suppliers who have invested
in effective quality control systems to be able to reduce variability and
cementitious content.
One difficulty in specifying durability performance is the absence of a
generally accepted comprehensive test at a reasonably early age. An increas-
ing number of specifications require compliance testing of transport prop-
erties during construction in an attempt to improve the expected durability
of reinforced concrete structures. However, the required performance for
the different specified parameters to achieve the desired durability has often
not been established. Unlike compressive strength, there is little informa-
tion available on the expected variation in the results to calculate an appro-
priate characteristic value.
In the case of chloride-induced corrosion, performance requirements may
include diffusion, migration, resistivity or water transport measurements,