Page 256 - Civil Engineering Project Management, Fourth Edition

19
Site concreting

and reinforcement

19.1 Development of concrete practice

Although many contractors now use ‘ready-mix’ concrete where it is conveni-
ent and economic, there are still many projects on which concrete is mixed on
site. On remote sites and sites overseas there may be no ready-mix suppliers.
If large pours are required it may be more economic for a contractor to pro-
duce concrete on site. In other cases a contractor may fear a ready-mix sup-
plier would not be able to cope with variations in his site requirements. Traffic
holdups can cause delay to delivery lorries, and create ‘bunching’ of deliveries.
No contractor likes to see a partially completed pour of concrete moving
towards its initial set when no further ready-mix lorries arrive. Concrete pro-
duced on site is under his control, he can start concreting as soon as formwork
is ready, and can stop concreting in an organized manner if some difficulty
arises in placing. Such problems are the contractor’s but if he proposes to use
ready-mix, the resident engineer should check that the supplier can meet the
specified requirements for concrete. Some ready-mix suppliers, for instance,
may not supply a 40mm (1.5in.) size coarse aggregate mix.
On the majority of construction sites the concrete used is made of natural
aggregates such as gravel, sand and crushed rock mixed with water and
ordinary Portland cement to BS12:1978.
For many years, concrete mixes were used as specified in BS CP114:1948,
The structural use of reinforced concrete in buildings. This contained a table of
recommended standard concrete mixes by weight, which is reproduced in Table
19.1 because it sets out familiar mixes still widely used for unsophisticated
concrete work. In 1981, BS 5328, Methods for specifying concrete was produced.
This was later re-issued in four parts – BS 5328:1997 Part 1, Guide to specifying
concrete; Part 2, Methods for specifying concrete mixes. (Parts 3 and 4 were pro-
duced in 1990, the former dealing with producing and transporting concrete,
and the latter with sampling and testing concrete.)