Mix design - Concrete Mix Design, Quality Control and Specification - page 198

than 7 mm should be avoided if possible, especially if any tendency to
segregation has been noticed.
8.2 LIMITATION OF FINENESS AND
COARSENESS OF SAND GRADING
A wide range of sand fineness can be accommodated by appropriate adjust-
ment of sand percentage to give a desired combined aggregate specific sur-
face, but there are limits.
8.2.1 Upper limit of coarseness
A sand reaches the upper limit of coarseness when there is insufficient paste
(cement, water, and entrained air) in the mortar to provide adequate lubri-
cation. This occurs not so much due to the coarser sand requiring more
paste per unit quantity of sand, but rather because more sand must be used
to provide the desired surface area if it is coarser. If the sand quantity is
not increased, the overall mix will be too harsh, and will segregate unless
of very low slump. If it is increased beyond the limit, the water requirement
rises to provide the required total paste volume required. Strength will be
reduced, the concrete will almost certainly bleed severely, and workability
will suffer in a different way, that is, it will have unsatisfactory mortar
quality rather than an inadequate amount of mortar. A comprehensive
mathematical treatment of this problem is given by Dewar (1999), but here
we will deal only with a few rules of thumb. What is important is that users
should recognise the problem when they encounter it. As noted earlier, this
will not occur at a particular sand percentage for all mixes but will depend
on several other factors. Some rules of thumb to indicate when the problem
should be considered are
1. Sand percentages in the range of 50% of total aggregates (in low
cement mixes) to 65% (in high cement mixes) (very rough guide).
2. Solid volume of sand exceeding about 5 times the solid volume of
cementitious material. With normal sand and cement this can be
taken as a sand to cement ratio of about 4 by weight. When fly ash or
very heavy or light sands are involved, the volume figure applies. This
guide is still not invariably accurate because the limit is affected by
the particle shape and grading of both the sand and coarse aggregate
and by the use of air entrainment.
3. From a different viewpoint, the problem may arise when the FM (fine-
ness modulus) of the sand exceeds 3.0 in low cement content mixes or
3.5 in high cement content mixes. In ConAd specific surface terms the
danger signals may be around 40 for high cement contents and 45 for
low cement contents.