Mix design - Concrete Mix Design, Quality Control and Specification - page 199

8.2.2 Upper limit of fineness
The fine limit for a sand is reached when a further reduction in sand pro-
portion will leave insufficient mortar (sand plus cement paste) to provide
adequate lubrication to the coarse aggregate. With a very fine sand it is
possible to get quite close to using a cubic foot of coarse aggregate by loose
volume in a cubic foot of concrete, and the shape and grading of the coarse
aggregate makes a substantial difference to where the limit is. The limit
will certainly be close; however, when the coarse aggregate approaches
60% by solid volume of the total concrete. Again from the other point of
view, the problem is likely to arise with sands of FM around 1.5 (with a
high cement content) to 1.8 (with a low cement content) or, in ConAd SS
terms, in excess of 90 with any cement content. It is also possible that a
high cement to sand ratio is intrinsically undesirable in the same way that
a heavily oversanded mix is undesirable (e.g., higher shrinkage). A sand
weight less than the weight of cementitious materials should be viewed with
suspicion and avoided if possible.
8.2.3 Coping with extreme sand gradings
The important point is rarely the establishment of the exact limit, rather it is
the fact that within these quite wide limits, grading is not the problem that
most typical specifications would suggest. It is of course necessary to accu-
rately determine what proportion of sand should be used in each particular
case and this is the main strength of the method of mix design evolved by Day.
An example of the coarse limit was encountered in Indonesia. The local
sand on occasions had less than 3% passing a 300 micron sieve. Its fineness
modulus was only of the order of 3.0, which did not seem an excessively
high figure. However, its specific surface of 40 to 42 was clearly exces-
sively low. Increasing the proportion of this sand did not solve the problem,
which was excessive bleeding. Eventually a choice had to be made between
a proportion of finer sand, even though not locally available and so very
expensive, and the use of additional cement purely for bleeding suppres-
sion. Another alternative would have been air entrainment, but this was
rejected, again due to nonavailability locally and also because the produc-
tion personnel were unfamiliar with it and had no test experience or equip-
ment. There have been very coarse sands in Singapore and in Australia
requiring 48% to 55% of sand, but these have all occurred when relatively
high cement contents were required. In an extreme case, where the sand is
very coarse and only a low strength and therefore a low cement content is
required, the following possibilities should be considered:
1. Use of a small proportion of a second fine sand (even if quite expensive).
2. Use of a small proportion of crusher fines with a high “fines” content.