Aggregates for concrete - Concrete Mix Design, Quality Control and Specification - page 39

alternatives in the current volume but, in view of the widespread use of
fineness modulus, some attention should be given to it.
Table 3.2 is given in two of Popovic's topics (1982, 1992) and is derived
from Walker and Bartel (1947). This table provides an optimum value
for the fineness modulus of the combined coarse and fine aggregates.
Table 3.2 is valid for natural sand and rounded gravel having voids of 35%.
Subtract 0.1 from the tabulated values for each 5% increase in voids. For
air entrained concretes, add 0.1 to the tabulated values. The values are for
25 to 50 mm slump concrete; subtract 0.25 for 100 mm slump and for zero
slump add 0.25.
The following equation, also from Popovics (1982), gives the water
required to provide a 100 mm slump in units of pound per cubic yard
(lb/cu yd) (divide by 1.685 to convert to liters per cubic metre).
Water requirement = c {0.1 + 0.032[(2 m - 60) 2 + 6570]/( c - 100)}
where
m = fineness modulus of combined aggregates
c = cement content in lb/cu yd (= kg/m 3 × 1.685)
Murdock (1960) and Hughes (1954) also introduce a term for angularity of
grains. This clearly influences water requirement but cannot conveniently
be used to give an adjustment to these values (see next section).
The concept of specific surface mix design is that an appropriate specific
surface for the overall grading be selected allowing for the intended use.
High specific surface improves the cohesion and resistance to segregation
but at the expense of increased water demand. For most applications, the
value is set at the lowest level that will provide a nonsegregating mix, as
this gives the best economy in cement. Low workability, high-strength con-
crete (e.g., for precast products with heavy vibration) is resistant to seg-
regation even with low specific surface, which reduces the water demand
even beyond that which follows directly from acceptance of a stiffer cement
paste. Greater workability requires more water or admixture and also a
higher specific surface in order to prevent segregation. Day's mix suitability
factor (MSF) summarises this concept, as shown in Chapter 8, Table 8.1.
Based on the application and recommended MSF, the sand percentage
is then calculated to provide the required specific surface. The method has
produced usable concrete mixes with natural sand percentages varying
from 15% to 55% of total aggregates in particular circumstances, but 25%
to 50% of sand is a fairly safe range.
The grading zones do not overlap because the 0.6 mm sieve is taken
as the criterion. However looking at the SS values or even the FM values
(Table 3.3), it is clear that the properties of the natural sands in different
zones are likely to overlap. This can be avoided by defining a Zone 1 sand