Civil Engineering Project Management (4th Edition)

250 Civil Engineering Project Management
discharges from the silo. It is better if concrete batches are made up per bag or
(more usually) per 2 No. 50 kg bags of cement, in which case only variations in
the weight of aggregate affect the mix; but this method is only possible for relatively
modest concrete outputs, not when large pours are required. The cement
content of a mix cannot be directly tested; hence the importance of keeping
watch on the batching plant accuracy. It would not be unreasonable for the
resident engineer to ask the contractor to conduct regular tests at suitable times
on the accuracy of the batching plant. A responsible contractor will realize that
it is better to ensure his plant is accurate, than to face the difficulty of finding
that concrete placed is below the required strength.
Occasionally on small sites or overseas, volume batching of concrete is
used. The weight per unit volume of aggregates has to be obtained by weighing
the amount required to loosely fill a measured container. Suitable wooden
gauge boxes for aggregate, sand and cement then have to be made up for
a given mix. Average weights of Portland cement are 1280 kg/m 3 (80 lb/ft 3 )
loose, or 1440 kg/m 3 (90 lb/ft 3 ) when shaken. If hand mixing is adopted, fairly
large gauge boxes with no bottom can be used, since they are placed on a
mixing platform, filled and lifted off. They would usually be sized for 1 bag
(50 kg) of cement. The bulking of the sand according to its moisture content
has to be allowed for.
19.9 Conveyance and placing of concrete
Specifications often contain clauses dealing with the transport of concrete,
requiring re-mixing after transport beyond a certain limit, limiting the height
through which concrete can be dropped, and requiring no concrete be placed
when more than a certain time has elapsed since mixing. In practice, problems
of this sort seldom prove significant. Sometimes it may be necessary to insist
that a contractor uses a closed chute to discharge concrete through a height in
order to prevent segregation. Also it may be desirable to ensure mixed concrete
is not left unplaced for over-long. A requirement often found in specifications
is that concrete must not be placed after it reaches its ‘initial set’ which, for
ordinary Portland cement concrete may take place 1–2 h after mixing, dependent
on temperature, etc. However, a hardening on the outside due to surface
drying can occur after about half-hour’s standing, especially in hot weather.
If this concrete is ‘knocked up again’ and shows it can be satisfactorily placed
it need not be rejected. On the other hand, if a delay is so lengthy that the
concrete hardens into lumps, such concrete must be discharged to waste.
Pumped concrete usually poses more problems for the contractor than it
does for the resident engineer, since only well graded mixes relatively rich in
cement are pumpable. Usually several mortar batches must be sent through
the pipeline to ‘lubricate it’ before the first batch of concrete is pumped
through, and pumping must thereafter be continuous. It is not easy to pump concrete
more than 300–400 m. If a stoppage of the flow of concrete occurs for any