Page 269 - Civil Engineering Project Management, Fourth Edition

249
Site concreting and reinforcement
container with freshly mixed and compacted concrete. An adequately dense
concrete cannot be made with badly graded aggregate or with an excess of
water.
If mixing takes place on site the accuracy of the weigh-batching plant
should be checked regularly. Actual errors found on a typical hand-operated
weight batcher were:
• zero error on scale: up to 15kg
• 20mm stone: 78–106 per cent of required value
• sand: 97–125 per cent of required value
• cement (ex silo): 80–110 per cent of required value
Allowance in a mix has to be made for the weight of the moisture content of
the sand which can be very variable when stocked in the open. Fig. 19.4 shows
the relationship between the bulking factor and moisture content. Some
devices are available for measuring the moisture content of a sand, but meas-
uring the moisture in every batch is not a practical proposition. Instead, typical
samples of sand from the stockpile under varying weather conditions can be
weighed, then dried and weighed again. This gives a guide as to the weights of
fine aggregate to be used under ‘dry’, ‘moist’, or ‘wet’ conditions. The moisture
content of the coarse aggregate is not usually checked as it has little effect on
the weight of the material.
Checking the cement content of the mix is particularly important if the cement
is held in a silo. Serious under-weights of cement can occur due to machine
faults with ‘automatic’ weighing equipment as well as with operator-controlled

Fine sand
30

Coarse
Bulking as 20 sand
per cent of
dry volume

10

2 4 6 8 10 12
Per cent moisture content (by weight)

Fig. 19.4. Bulking of sand according to moisture content