Page 263 - Civil Engineering Project Management, Fourth Edition

Site concreting and reinforcement
Table 19.5
Percentage by weight passing BS sieve
BS410 sieve (mm) BS 882:1973 Grading of fine aggregate 243
Zone 1 Zone 2 Zone 3 Zone 4
10.0 100 100 100 100
5.0 90–100 90–100 90–100 95–100
2.36 60–95 75–100 85–100 95–100
1.18 30–70 55–90 75–100 90–100
0.60 15–34 35–59 60–79 80–100
0.30 5–20 8–30 12–40 15–50
0.15 0–10 0–10 0–10 0–15
Note: Later versions of BS 882 have substituted Grades C (coarse),
M (medium) and F (fine) for the above four zones.

was often not procurable; and Zone 4 was usually avoided if possible because
it contained too much fine material for producing the best concrete. (The
revised version of BS 882 in 1983 no longer defined four Zones for fine aggre-
gates, but substituted ‘Coarse’, ‘Medium’ and ‘Fine’ gradings which are too
wide in range to be of practical use for mix design purposes.)
In practice samples of the fine and coarse aggregates proposed to be used
should be sieved to find their typical grading. Sometimes it is found that the
coarse aggregate contains a substantial proportion of fines (below 0.5mm),
while the fine aggregate may frequently be of a uniform size. Consequently
various ratios of coarse to fine aggregate must be tried out to see which gives
the best mix. Envelopes of suitable grading curves for 20 and 40mm maximum
size aggregate are shown in Fig. 19.3. The first trial mix can adopt a ratio of fine
to coarse aggregate which, as near as possible, gives a grading approximating
to the centre of the appropriate envelope shown. Adjustment of the mix pro-
portions for subsequent trial mixes will then show whether some improve-
ment in the quality of the mix is possible. The Design of normal concrete mixes
published by the Building Research Establishment 1975 is a useful guide.

19.5 Workability of concrete and admixtures

Workability requirements for a concrete mix tend to conflict with requirements
for maximum strength, density and economy, since workability increases with
increased fines, cement, or water in a mix, but increased fines and water reduce
density and strength, while increased cement may increase shrinkage and liabil-
ity to cracking as well as adding to the cost of a mix. It is therefore necessary
to produce minimum satisfactory workability in order to keep the deleterious
effects of too much fines, cement or water to a minimum.
Workability can be measured by the well known slump test, but it is not
very accurate and is best used only for ensuring a given mix is consistent,