Page 265 - Civil Engineering Project Management, Fourth Edition

construction site this reduction of workability is not usually noticeable since
more than 10min usually passes before the concrete is placed. However, if
samples for workability are taken, a time lapse of 10min should be allowed
before they are tested. Site concreting and reinforcement 245
Admixtures to concrete are sometimes proposed by the contractor for
approval, such as plasticizers or air-entraining agents to improve workability,
or an accelerator to assist the contractor strike formwork early. If the specifica-
tion does not define which admixtures or special cement can be used, the resi-
dent engineer should not agree to any such proposal from the contractor but
pass it to the engineer for decision. There are many admixtures on the market,
each having its own characteristics, some of which can be disadvantageous.
For instance the use of the accelerator calcium chloride is not permitted for
steel reinforced concrete under BS 8110 because it increases the risk of corrosion
of the steel. Rapid hardening cement can cause a high concrete temperature
leading to shrinkage and cracking; and air entraining agents reduce the density
and strength of concrete. This does not mean that no admixtures should be
permitted, but that the complex reactions they can cause make it necessary to
call in specialist advice to ensure their safe use in any particular case. Very high
2
strength concrete of up to 100N/mm strength or more, as used in high rise
buildings, etc. is obtained primarily by use of a very low water/cement ratio. As
a consequence adequate workability has to be achieved by use of an admixture.
The specification must state precisely what is required.
Special aggregates for concrete usually comprise light-weight materials;
mostly used only for particular building purposes (e.g. screeds for thermal or
fire insulation, etc.), or used in precast concrete products.

19.6 Practical points in producing good concrete

Provided certain simple rules are followed good concrete can be achieved by
methods varying from the ‘bucket and spade’ hand-labour method to use of
the most sophisticated weigh-batching and mixing plant. The following shows
the principal matters that should receive the resident engineer’s attention.
First, choose good aggregates. The best guide is to use well-known local
aggregates that have been and are being used satisfactorily on other jobs
elsewhere. A reputable supplier will be able to name many jobs where his
aggregate has been used, and the resident engineer will not be over-cautious
if he visits one or two of these where the concrete is exposed to view. When
the aggregates are being delivered on the job (not just the first few loads, but
the loads when the supply has really got going), random loads as delivered
should be examined. Handfuls of aggregate should be taken up and exam-
ined in detail, looking for small balls of clay, soft spongy stones, flaky stones,
pieces of brick, soft shale, crumbly bits of sandstone, and whether clay or dirt
is left on the hands after returning the handful. If the engineer finds more than
one or two pieces of weak stone, or more than a single small piece of clay from