Mix design - Concrete Mix Design, Quality Control and Specification - page 194

value is between 150 and 180 liters per cubic metre of the concrete, with
a lower value being chosen for higher performance concrete. The desired
workability will be obtained by increased admixture dosage rather than a
higher water content.
The powder content will be composed of cement plus a selected percentage
of one or more of fly ash, ground-granulated blast-furnace slag (GGBS), rice
husk ash, silica fume, metakaolin, and finely ground limestone. Entrained
air may also be required for frost resistance or to assist in workability. The
choice between these materials will be strongly influenced by relative cost
and availability (which will differ very markedly in different parts of the
world), and by the perceived or specified need for durability and for strength
at various ages. In many, but not all, parts of the world, fly ash (PFA) or
limestone will be the most economical solution, but GGBS will give greater
durability and silica fume higher strength (at all ages). Perhaps surprisingly,
superfine limestone also gives good early strength. Reliability of supply of
consistent material and of prompt advice from the supplier of any variation
will also be an important consideration for the concrete producer.
The aforementioned process may be regarded as simple or complex
depending on the range of materials economically available and the
perceived competence of the material suppliers. It is likely that a continued
program of trial mixes will be worthwhile (largely on a production scale
with the concrete supplied for a less critical use). The resulting mix may
even be suitable for self-compacting concrete (the use of which is likely to
increase substantially in the future) with an increased dose of high-range
water-reducing admixture (HRWRA). This can be checked by filling an
upside-down slump cone with it and lifting the slump cone. To be self-
compacting the concrete must flow out to a radius of at least 550 mm.
Although flowing outward it is important to observe the leading edge of the
flow, this must be blunt, containing coarse aggregate. Even a slight “halo”
of preceding fluid paste without aggregate indicates that the mix has insuf-
ficient fines to provide the necessary cohesion.
8.1 MIX SUITABILITY FACTOR (MSF)/SPECIFIC
SURFACE THEORY (FROM THE THIRD EDITION)
The basic concept of specific surface (SS) mix design is extremely simple but
requires modification to work effectively. The simple basis is that a given
degree of workability will require an appropriate specific surface to avoid
segregation, the higher the workability, the higher the required specific sur-
face. Knowing the individual specific surfaces of the coarse aggregate and the
fine aggregate, the required sand percentage can be calculated (see Table 8.1).
It is well known that a finer sand will have a higher water requirement
than the same amount of a coarser sand, but specific surface theory says