Testing - Concrete Mix Design, Quality Control and Specification - page 171

into the unsaturated voids. As surface chlorides accumulate, diffusion
will then draw ions deeper into the concrete provided there is sufficient
moisture for continuous liquid pathways. Careful consideration of the ways
in which chlorides could penetrate the concrete cover in different parts of
a structure is a valuable starting point in helping solve potential durability
problems. Secondly, commercial interests often obscure the relative impor-
tance of different transport properties. Manufacturers marketing products
to enhance concrete durability will emphasise the importance of the param-
eter primarily influenced by their material. Finally, effective durability
enhancement materials or systems that change the electrochemistry of the
corrosion process, such as inhibitors, may be selected against by specifica-
tions focusing only on transport properties.
7.6.1 Compressive strength
Twenty-five years ago, Neville (1987) considered the use of concrete strength
as a basis of its acceptance as a culprit in durability problems. He stated,
“My submission is that we should have decreased our concern with strength
long since and we should have concentrated on developing practical criteria
for durable concrete which could be used in specification”. Compressive
strength on standard cubes or cylinders is routinely tested for compliance
on most projects. Thus it is not surprising that designers have tended to use
compressive strength as an indirect indicator of durability in the belief (or
hope) that strong concrete will be durable concrete. Indeed, many codes set
minimum compressive strength requirements for severe environments. AS
3600-2009 requires a minimum compressive strength of 50 MPa for tidal
or splash zones. It also recommends a 65 mm cover and 7 days continuous
curing. This minimum compressive strength requirement was developed to
ensure the required sorptivity based on the comprehensive work done by
Ho and Lewis (1984, 1988). They conducted an extensive program on the
influence of a wide range of factors on the sorptivity or capillary absorption
of concrete.
We agree that sorptivity can be an important factor in the durability of
concrete in such environments. However the conclusions regarding the rela-
tionship between strength and sorptivity (Ho and Lewis, 1988) may have
been influenced by the sample preparation procedure where the concrete
specimens were air dried for 21 days from their saturated condition in the
laboratory at 23°C and 50% RH prior to testing. As has been pointed out
by Dolch and Lovell (1987), the processes of both drying and wetting are
influenced by water to cement ratio and therefore strength. Thus higher
strength concretes would dry more slowly. As the moisture content of the
specimens at the start of the test will strongly influence their sorptivity
(Concrete Society, 1988), the sample preparation used in Ho and Lewis'
programs would tend to favor higher strength mixes.