F. K. Kong MA, MSc, PhD, CEng, FICE, FIStructE, R. H. Evans CBE, DSc, D ès Sc, DTech, PhD, CEng, FICE, FIMechE, FIStructE (auth.)-Reinforced and Prestressed Concrete-Springer US (1987)

Traditional mix design method 51
required for a target mean strength of 30 N/mm 2 • However, the durability
requirement imposes another ceiling on the w/c ratio. For most structural
concrete, it is advisable to keep the w/c ratio below, say, 0.60 irrespective
of the strength requirement; where the concrete member will in service be
exposed to freezing temperatures when wet, the w/c ratio should be kept
below, say, 0.45.
The workability required for various types of construction is given in
Table 2.6-2; Table 2.7-1 then gives the aggregate/cement ratio, which
depends on the shape, grading and maximum size of the aggregate. The
shapes of the aggregates referred to in Table 2.7-1 are (a) rounded, such
as beach gravel, (b) irregular, such as water-worn river gravels, and (c)
angular, such as crushed rocks. Corresponding to each degree of workability,
the table gives aggregate/cement ratios for four gradings of
aggregates which are shown in Fig. 2. 7-1: grading No. 1 has the lowest
proportion of fine material, while grading No. 4 has the highest proportion.
The lower the proportion of fine material, the lower is the water
content required for a given workability; in other words a higher aggregate/cement
ratio can be used. On the other hand, where the reinforcement
is congested or where it is desired to have concrete surfaces that
present a good appearance on removal of the formwork, a higher
proportion oflfine material is advantageous. Note that Table 2.7-1 refers
to aggregates of 10 mm maximum size. In general, as the maximum size of
the aggregate increases, a higher aggregate/cement ratio may be used, and
vice versa. As a very rough guide, the aggregate/cement ratios in Table
2. 7-1 may be increased by about 10% if the maximum size of the
Cl
100------~----~------~----~~----~----~~--~
90·~----~----~~----~------~----~------~-,~
801~----~----~~----~------~----~------~~~
.E 701~-----+------+-------~----~------~~~~~~~~
m
a eo
8,50
~401~-----+------+-------~~~~~~--+-~~-4~----~
c
~30
& 20~----~--~~~~~~~~t=--~~----t-----,
10~----~~~~-·~~~~----~~----~----~~--~
oL---~~~~--l_ __ L_ __ L_ __ L_~
150~m 300~m 600~m 1-18mm 2·36mm
(No.1()()) (No. 52) (No.25) (No.14) (No.7)
B S Sieve size
Fig. 2.7-1 Grading curves for 10 mm aggregate [32]
5mm
<d. in>
10mm
(fin)