Civil Engineering Project Management (4th Edition)
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Site concreting and reinforcement 245<br />
construction site this reduction of workability is not usually noticeable since<br />
more than 10 min usually passes before the concrete is placed. However, if<br />
samples for workability are taken, a time lapse of 10 min should be allowed<br />
before they are tested.<br />
Admixtures to concrete are sometimes proposed by the contractor for<br />
approval, such as plasticizers or air-entraining agents to improve workability,<br />
or an accelerator to assist the contractor strike formwork early. If the specification<br />
does not define which admixtures or special cement can be used, the resident<br />
engineer should not agree to any such proposal from the contractor but<br />
pass it to the engineer for decision. There are many admixtures on the market,<br />
each having its own characteristics, some of which can be disadvantageous.<br />
For instance the use of the accelerator calcium chloride is not permitted for<br />
steel reinforced concrete under BS 8110 because it increases the risk of corrosion<br />
of the steel. Rapid hardening cement can cause a high concrete temperature<br />
leading to shrinkage and cracking; and air entraining agents reduce the density<br />
and strength of concrete. This does not mean that no admixtures should be<br />
permitted, but that the complex reactions they can cause make it necessary to<br />
call in specialist advice to ensure their safe use in any particular case. Very high<br />
strength concrete of up to 100 N/mm 2 strength or more, as used in high rise<br />
buildings, etc. is obtained primarily by use of a very low water/cement ratio. As<br />
a consequence adequate workability has to be achieved by use of an admixture.<br />
The specification must state precisely what is required.<br />
Special aggregates for concrete usually comprise light-weight materials;<br />
mostly used only for particular building purposes (e.g. screeds for thermal or<br />
fire insulation, etc.), or used in precast concrete products.<br />
19.6 Practical points in producing good concrete<br />
Provided certain simple rules are followed good concrete can be achieved by<br />
methods varying from the ‘bucket and spade’ hand-labour method to use of<br />
the most sophisticated weigh-batching and mixing plant. The following shows<br />
the principal matters that should receive the resident engineer’s attention.<br />
First, choose good aggregates. The best guide is to use well-known local<br />
aggregates that have been and are being used satisfactorily on other jobs<br />
elsewhere. A reputable supplier will be able to name many jobs where his<br />
aggregate has been used, and the resident engineer will not be over-cautious<br />
if he visits one or two of these where the concrete is exposed to view. When<br />
the aggregates are being delivered on the job (not just the first few loads, but<br />
the loads when the supply has really got going), random loads as delivered<br />
should be examined. Handfuls of aggregate should be taken up and examined<br />
in detail, looking for small balls of clay, soft spongy stones, flaky stones,<br />
pieces of brick, soft shale, crumbly bits of sandstone, and whether clay or dirt<br />
is left on the hands after returning the handful. If the engineer finds more than<br />
one or two pieces of weak stone, or more than a single small piece of clay from