This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol5", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
The question as to the correct quantity of water has called forth much unnecessary controversy and unconsidered faddism. Mixtures are very generally known as "wet" and "dry," some preferring to use the concrete in an almost sloppy condition, while others add the least possible amount of water. The very terms " wet " and "dry," indeed, suggest excess in either direction.
All concrete, no matter how mixed, will always contain cavities, and the best mixing will be that which reduces the amount of these cavities to the minimum.
If a "dry" mixture be used, a greater amount of mixing and turning over will be necessary to thoroughly coat the aggregate with the mortar, while there is every possibility that part of the cement will not receive sufficient water for its proper setting. The mixture, however, in this state is easily rammed, and will shrink much in the process.
If a " wet" mixture be used the fluidity of the mass will result in a fair degree of consolidation without the employment of ramming; and indeed, if the mixture be very wet, ramming will be impossible. The excess water distributed throughout the mass must leave cavities on drying out; but while the voids in concrete mixed dry may take the form of actual cavities, those in concrete mixed wet will be distributed throughout the cement, which will be left of a porous nature, and although the material is devoid of any large cavities the strength is certainly much reduced.
There is little doubt that concrete made with only just sufficient water to thoroughly wet all the cement will, if mixed with great care and well rammed, produce the strongest concrete; but the necessary degree of care cannot be ensured in actual work.
It seems reasonable, then, to use sufficient water to leave no doubt as to the thorough wetting of the cement, which will allow efficient mixing to be easily accomplished, and at the same time will not produce a concrete too wet to be thoroughly rammed. Any water in excess of this may be considered as deleterious, to the production of the strongest concrete.
A shingle or similar aggregate, besides not permitting such perfect adherence with the cement as will broken stone, at the same time, if fairly wet, becomes too fluid to be efficiently rammed. In certain cases it is not possible to thoroughly ram concrete on account of its position in the work, and in such cases a wet and fluid mixture will give the best result.
In armoured concrete, as regards the thorough protection of the surface of the rods, a moderately wet mixture is desirable to ensure a cement coating; while, on the other hand, closer adherence" between metal and concrete is obtained when excess of water has not been used.
The properties of concrete, its resistance to various forms of stress, its elasticity and expansion, must all depend upon the proportions of the concrete and the nature of the component materials, the amount of water, the extent of mixing and ramming, the expedition with which the work is effected, the absence of vibration during setting, and the age of the finished concrete. However, before calculations can be entered upon to find the necessary dimensions of parts it is first necessary to know something of the behaviour of concrete under stress, and to fix values on which the calculations may be based. For important works it will be advisable to make tests with the materials to be used, mixed and treated in the same manner as they will ordinarily be treated in the work; for very different results may be obtained if the concrete to form the test pieces be mixed in small quantities in a laboratory.
For concrete mixed in the proportions of 1 2 4, carefully mixed with good materials and hard stone aggregate, the crushing strength may be taken as roughly 1 ton per square inch at the end of 1 month or 6 weeks after mixing. The strength increases rapidly at first, and more and more slowly as the age of the concrete increases. At the end of six months the crushing strength will probably be half as much again as that indicated above. After concrete has been made for about a year the strength increases very slowly but indefinitely.
The age at which the concrete may be called upon to carry its full load will generally be uncertain, but for safety it may be assumed that this will take place after 4 to 6 weeks, and the crushing resistance may be taken as 2240 lbs. per square inch.
The strength of various mixtures is found to vary much as does the proportion of the cement in the mortar. Thus if the proportion of cement and sand in the concrete be 1: 1, - i.e. 1 of cement in 2 of mortar, the crushing resistance of the concrete will probably be nearly twice as great as if the mortar is composed of 1 of cement to 3 of sand (or 1 of cement in 4 of mortar).
In deciding upon the factor of safety for steelwork it was shown that the elastic limit, which is roughly half the ultimate strength, must never be reached, and that this value was halved for safety, thus producing a factor of safety of 4 on the ultimate resistance. In the case of concrete the elastic limit cannot be clearly defined as in the case of steel, although attempts have been made to fix its value; but even if such a point exists the advantage gained in deciding upon it will be largely nullified by the use of a further large and arbitrary factor. In using a heterogeneous material such as concrete it is obvious that a larger factor should be used than when dealing with steel. It is the usual practice, then, to employ a factor of safety of 5 or 6.