Other things being equal the strength varies with the amount of cement put into the concrete. For the same amount of cement and the same consistency of the mixture, the strength increases with increased density of concrete (that is, with decreased voids), and the effort should be made so to proportion the fine and coarse aggregates as to produce the densest concrete (least voids) with the aggregates available. For the same consistency, the strength then will vary with the ratio of the amount of cement to the amount of the voids.
So far as the mixing water is concerned, the greatest strength in the concrete will be attained at a rather dry mix; that which produces the least volume of concrete. The addition of more water results in a concrete of less strength; 40 per cent more water may give a concrete of less than half the normal strength. The reduction in strength is then very marked for the wetter mixes, and the water content used is a feature of considerable importance in the design of concrete mixtures.
Permeability is affected by the same elements as strength, but the size and discontinuity of the pores have a greater influence.
Workability is an important quality; in some respects it will have to be obtained at the expense of strength. Increasing the amount of mixing water increases the workability of the mixtures, with a resulting decrease in strength which may have to be accepted or else overcome by increasing the cement in the mix.
An excess of water is often used unnecessarily through ignorance of the injurious results. A high proportion of coarse aggregate, up to a certain limit, will give concrete of high strength, but the mixture will be harsh-working and not easy to place. Lower proportions of coarse aggregate will give greater workability and better uniformity of product, the latter being an important matter. It is apparent that the degree of workability of the mixture needed will depend upon the nature of the construction—for a pavement where the concrete will receive substantial tamping or working the water content may be much less than that which may need to be used in placing concrete around reinforcement in narrow members, or where little tamping or spading can be done. The nature of the work will affect the standard of consistency to be specified.