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)

54 Properties of structural concrete
aggregate is 20 mm; they should be increased by about 30% if the
maximum size is 40 mm; more accurate values are given in References 6
and 32. Also, Table 2.7-1 refers only to the four gradings in Fig. 2.7-1. In
practice, the aggregates available may not have natural gradings closely
resembling any of these four. It is possible to combine two or more
aggregates to give a grading approximating to the one required; the
procedure is quite simple and is explained in References 6 and 36. Or trial
mixes may be prepared with the available aggregates and adjustments
made by systematic trial and error; indeed the results of the trial mixes may
well show that the grading of an aggregate as supplied gives quite
satisfactory results.
Example 2. 7-1
Given the following data, design a mix if the target mean strength is
40 N/mm 2 at 28 days.
SOLUTION
Cement:
Aggregate:
Type of construction:
Condition of exposure:
ordinary Portland
irregular, maximum size 10 mm, grading
similar to No. 2 in Fig. 2.7-1
normal reinforced concrete work using
high-frequency vibrator
exposed to climate of Great Britain
From Fig. 2.5-2 the required w/c ratio is, say, 0.48. For exposure to
climatic conditions in Great Britain, any w/c ratio below, say, 0.6 or 0.55
would usually be satisfactory (but see Section 2.5(e) on Durability).
From Table 2.6-2 a low workability is sufficient. Table 2.7-1 shows
that, for ail irregular aggregate with low workability, the aggregate/ cement
ratio for grading No. 2 is 3.9 for w/c ratio = 0.45 and 4.6 for w/c ratio =
0.50. By linear interpolation, that for a w/c ratio of 0.48 is
3.9 + ~:~~=~:!~X (4.6- 3.9) == 4.3
Ans. wlc ratio = 0.48; aggregate/cement ratio= 4.3.
2.7(b) DoE mix design method
The Department of the Environment's Design of Normal Concrete Mixes
[35], published in November 1975, originated from the long-established
Road Note No. 4 [6] referred to in Section 2.7(a). The DoE mix design
method [35] is intended to replace the traditional method based on Road
Note No. 4, but the principal objectives remain unchanged: to obtain a
preliminary estimate of the mix proportions as a basis to make trial mixes
to arrive at the final mix proportions that satisfy the strength, workability
and durability requirements.
Compared with the Road Note No. 4 method the DoE mix design
method has several new features: