36 Mix design
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 the author. A recent 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 40–42 was clearly excessively 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 suppression. Another alternative would have been air entrainment but this was rejected, again due to non-availability locally, but also because the production personnel were unfamiliar with it and had no test experience or equipment. There have been very coarse sands in Singapore and in Australia requiring 48–55% 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 2 3
4 5
Use of a small proportion of a second fine sand (even if quite expensive). Use of a small proportion of crusher fines with a high ‘fines’ content. Use of fly-ash, which has 37% greater volume than an equal weight of cement (if in an area where fly-ash is inexpensive, more might be used than strictly necessary for strength). Use of air entrainment (as valuable, volume for volume, as cement for this purpose). If no alternative is less expensive, the use of more cement than necessary on strength grounds would certainly solve the problem since it both reduces the sand percentage required for a given MSF and provides more paste to fill the sand voids.
Extreme testing of the fine limit has also occurred. In 1956 (Day, 1959) a case was encountered where the sand percentage calculated by the author’s system came to 15% (virtually all the sand passed the 300 micron [No. 50 ASTM] sieve). It proved possible to obtain a 1/4 (7 mm) single sized crushed rock and the concrete was made with 10% of this material and 15% of sand (the balance being 75% of an almost single sized 20 mm [3/4 inch] crushed rock). During the early development of the system (in the early 1950’s in England) sand percentages of 22–23% were used but, although the sand was purchased as ‘plastering sand’ rather than ‘concreting sand’, this was an example of the use of a very low ‘MSF’ on earth dry concrete rather than the use of a very fine sand. It should always be possible to use a proportion of crushed fines (choosing a coarse variety) when the natural sand is too fine for use alone. However the particle shape of the crushed fines will increase water requirement, and therefore increase cement requirement, at least somewhat.