Advice to specifiers - Concrete Mix Design, Quality Control and Specification - page 2

of a control system. One is that it should provide a precise judgement of
quality and the other that it should react as quickly as possible to restore
the required quality in the event of any downturn. Any attempt to combine
these two into a single requirement is almost certain to achieve neither.
What needs to be learned is that mix design and quality control must
be in the hands of the concrete producer. This has always been the case
because any external supervisor cannot require corrective action based
on as little evidence as a properly motivated producer will require. It is
now even more the case because a large variety of admixtures and cement
replacement supplementary cementitious materials is available, and a pro-
ducer needs to have conducted trials to establish which materials and which
suppliers of materials will best enable him or her to consistently produce
the most economical satisfactory compliant concrete for the project at
hand. It is important that feedback and cooperation be established between
the concrete producer and his or her material suppliers, and it is generally
undesirable that such links be unnecessarily discarded, along with current
performance history, by requiring the use of unfamiliar materials. However,
those few specifiers who do have expert knowledge beyond that of most
producers should certainly make it available as advice but preferably with
an alternative performance option.
In the United States the “P2P” battle still rages (P2P, prescription to per-
formance as a specification basis) and has not yet influenced official ACI
practice, even though it is now 10 years since Command Alkon purchased
Ken Day's ConAd system and licensed it to hundreds of producers in the
United States and around the world.
In the United Kingdom and Europe the current system is nearer to
Australian practice but is even more solidly entrenched. The problems there,
as presented in Day's paper at the International Federation for Structural
Concrete (FIB) 2008, are a limited ability to include multiple groups in a
multigrade analysis, a failure to use multivariable CUSUMs to link cause
and effect, and postponing problem detection to an analysis of a substantial
number of 28-day results.
A genuine difficulty with performance specifications is the difficulty in
specifying durability. Opponents of performance specifications see this as a
justification for prescribing some mix features.
1.2 SPECIFYING DURABILITY
The common practice to specify a minimum cementitious content to achieve
“durability” is misguided. First, Buenfeld and Okundi (1998) showed that,
at a given water to cement ratio (w/c), the higher binder content actually
increased chloride ion ingress in concrete. This is hardly surprising when
transport processes occur primarily through the paste fraction of the