Specification - Concrete Mix Design, Quality Control and Specification - page 128

only practicable on large contracts for which aggregates are being specially
produced. Even so some variation is inevitable, and it is difficult both to
require rigid compliance with specified proportions and to provide for
variation. This path leads to full acceptance of total responsibility for con-
crete quality by the supervising authority, which is undesirable for many
reasons (from needing to take over control of incoming materials quality
to facing claims by the contractor that any defects in the finished product
are due to matters beyond his control). The Australian government airfield
construction branch used such techniques in the 1980s. Excellent concrete
resulted, and it was considered by those in charge that the high cost was
justified by the importance of the work.
The preferable course is to specify as closely as possible the properties
required of the concrete and require the contractor to set out in full detail
exactly how he proposes to provide them, including his specification limits
on incoming materials and within what limits and to what accuracy he
proposes to adjust the mix. This clearly gives the contractor absolute free-
dom to propose the most economical and practicable way of providing
concrete of the required properties. It is much easier to detect any unsatis-
factory features of such a proposal than it is to so specify a mix that it could
not possibly have any unsatisfactory features.
Once the contractor's proposals have been accepted by the supervisor,
they become the specification. Insistence on conformance to this specifica-
tion is easier since the contractor, having proposed it himself, cannot claim
it to be unrealistic in any way, and there can be no surprise “loopholes”
in the original specification. Of course, in the authors' opinion, even this
type of individual attention to mix regulation by a purchaser would only
be justified on very large projects, usually those with a dedicated supplying
plant.
6.12 SHOULD MIXES BE SUBMITTED?
An important question is whether mixes should be submitted for approval,
and if so, approval by whom. It seems reasonable that a purchaser should
be entitled to know what is in the concrete he is purchasing. The purpose
of such a submission should be to ensure that the mix has no objection-
able features. These might include admixtures containing calcium chloride,
air-entraining agents known to give an excessive bubble size, potentially
reactive aggregates, and aggregates known to have high moisture move-
ment or to cause popouts in exposed surfaces. The list is not extensive and
a list of materials rather than mix proportions might meet the need, how-
ever, to be effective assessment needs to be by a qualified and experienced
concrete technologist.