Concrete future - Concrete Mix Design, Quality Control and Specification - page 278

more data on issues where overdesign may be occurring would also provide
a technical basis for more sustainable construction without exposing the
engineer to additional risk. We would urge the concrete industry to collect
and publish as much data as possible on the in situ performance of concrete
with appropriate cross-references to initial compliance testing.
13.1.4 Specifications
In the third edition of this topic, Ken Day expressed the hope that the
practice of specifying minimum cement contents and requiring mixes to be
submitted and not subsequently varied would have finally died out by the
publication of that edition of his topic. However, these practices are still
very much alive in 2012. Other prescriptive requirements of specifications,
such as aggregate grading, maximum supplementary cementitious replace-
ment levels, placement temperatures, and workability, tend to stifle mix
optimisation and are an impediment to sustainability. They also often lead
to unintended detrimental effects of concrete performance.
Designers of concrete structures and infrastructure should specify the
properties they have assumed in their design, including strength, movement,
and durability. However, few specifiers are also concrete technologists and
many specifications are a blend of sometimes contradictory prescriptive
and performance requirements. The performance requirements often just
added onto previous specifications.
Existing codes accept that concrete strength follows a normal distribu-
tion and should be considered in terms of mean strength and standard
deviation rather than an absolute limit. However, when cubes or cylinders
are lower than specified strength, the engineer often requires an investiga-
tion by coring rather by analysing the results to determine whether the low
result constitutes a genuine downturn or an isolated statistical aberration.
To avoid the inconvenience and cost of coring or other testing, producers
may choose to overdesign their concrete mixes, significantly reducing the
sustainability of the concrete. Unnecessary testing of in situ concrete is
an impediment to sustainability. Day has long advocated a penalty system
where concrete that is “contractually” deficient results in a nominal cost
to the producer. Clearly where compliance testing suggests “structurally”
deficient concrete, an appropriate investigation would be required.
There are two basic requirements of a concrete control system. One
should provide an accurate assessment of quality and the other should facil-
itate intervention as quickly as possible to restore the required quality in the
event of any downturn. Accordingly, the specification must ensure that mix
design and quality control are controlled by the concrete producer. Any
external party cannot require corrective action based on as little evidence as
a properly motivated producer will require. The large range of admixtures
and supplementary cementitious materials now available makes external