Specification - Concrete Mix Design, Quality Control and Specification - page 121

repairs and litigation where the specification will come under scrutiny.
The problem of prescriptive specification of rheology can also occur
with self-consolidating concrete (SCC) where overzealous specifiers can
require very high workability parameters that can lead to segregation. We
would suggest that the specification require that the contractor or premix
company confirm that the rheology of the concrete is satisfactory for
the proposed placement procedure and the mix developed complies with
the performance parameters. Assessment that the rheology of the mix is
acceptable on site should be based on the supplier's proposed value and
established tolerances.
There is a tendency to want to specify a w/c ratio, since this is the best
overall criterion of concrete quality. There are three reasons not to do this.
One is that strength is much easier to use as a control. Another is that if
there is some factor causing a departure from the anticipated w/c versus
strength relationship (such as bond to coarse aggregate), then strength is
the better guide. Third, the necessary average quality will be dependent
on the degree of control and producers able to achieve lower variability
must be allowed to profit by it. The use of tests such as the AASHTO
T318 microwave test to check fresh water content is an excellent tool for
producers to control the water content of their concrete, but we question its
value as a compliance test to confirm w/c ratio, although the Port Authority
of New York and New Jersey (PNY&NJ) seems to have had success with
using the procedure for this purpose.
6.7 DURABILITY
Premature deterioration of reinforced concrete due to chloride-induced
corrosion of reinforcement 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. 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 specifications to ensure the durability of reinforced concrete.
International codes provide prescriptive solutions to increase the required
concrete quality and cover thickness to improve chloride resistance.
Following is a brief summary of the durability guidance given in various
codes and standards for concrete exposed to a marine environment.
Australian Standard 4997 “Guidelines for the design of maritime
structures” classifies the splash zone as C2 and recommends mini-
mum concrete strength of 40 MPa with a minimum cover of 75 mm to
achieve a design life of only 25 years. The notes list a number of addi-
tional measures to achieve a longer design life. AS 3600 (Concrete