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)

50 Properties of structural concrete
and economy. Mix design methods are useful as guides in the initial
selection of these proportions, but it must be strongly emphasized that the
final proportions to be adopted should be established by actual trials and
adjustments on site.
It can be said that all practical mix design methods are based on the
following two simple observations:
(a)
(b)
The free w/c ratio is the single most important factor that influences the
strength of the concrete.
The water content is the single most important factor that influences the
workability of the fresh concrete mix.
Note that in calculating the w/c ratio in (a) above, only the weight of the
free water is used. The total water in the concrete mix consists of the water
absorbed by the aggregate and the free water, which is the total water less
the absorbed water and is available for the hydration and the lubrication of
the mix. Similarly, the water content in (b) is expressed as the weight of the
free water per unit volume of concrete. The water content required for a
specified workability depends on the maximum size, the shape, grading
and surface texture of the aggregate but is relatively independent of the
cement content (i.e. the weight of cement per unit volume of concrete).
Mix design methods have been proposed by the Road Research
Laboratory [6, 32], the American Concrete Institute [33, 34] and, more
recently, the Department of the Environment [35]. Two of these methods
are described below.
2. 7(a) Traditional mix design method
For several decades since the 1940s, mix design in the UK has been much
influenced by a paper authored by Dr A. R. Collins which was published in
Concrete and Constructional Engineering in October 1939 and which later
became Road Note No.4 [6]. According to the Road Note No.4 Method, a
w/c ratio is first chosen to satisfy the requirements of strength and
durability; the aggregate/cement ratio is then chosen to satisfy the
workability requirement. Note that specifying the w/c ratio and the
aggregate/cement ratio is equivalent to specifying the w/c ratio and the
water content; of the three variables, only two are independent.
Suppose it is required to design a mix to have a certain average strength
at 28 days. This average strength is called the target mean strength and is
statistically related to the required characteristic strength, as will be
explained in Section 2.8. For the time being it is sufficient to note that the
target mean strength will be some value which exceeds the characteristic
strength by a suitable margin, called the current margin. Now, for properly
compacted concretes, the strength depends primarily on the w/c ratio; in
fact quantitative relationships are given in Fig. 2.5-2. Of course these
relationships are no more than average values, and when trial mixes are
actually carried out, the relationship may well be found to be somewhat
different. Nevertheless, Fig. 2.5-2 provides a useful starting point. For
~xample, with ordinary Portland cement, a w/c ratio of about 0.58 is