is nothing better than the addition of hydrated lime to the cement to the amount of about ten per cent of its volume. In any case where the condition of impermeability is paramount, it would be well to adopt both of these expedients, irrespective of cost; and the combined expedients, if successful, would effect a true economy. The question of the economics of waterproofing concrete is treated at length in Chapter XLIII.

Increasing Fluidity of Mixture

The addition to the cement of not more than ten per cent of its volume of hydrated lime not only tends to make the finished work waterproof, but also adds greatly to the fluidity of the mixture, thus facilitating the placing of it around the reinforcing bars. Certain reliable tests have shown that the addition of lime up to fifteen per cent of the volume of cement really slightly increases the tensile strength after some three weeks; and as the addition of lime does not add materially to the expense of the concrete, it is a matter of true economy to employ it. The beneficial effect of hydrated lime is partly due to the fact that it permits a reduction in the amount of mixing water without lessening the plasticity of the mixture.

Manner and Time of Mixing

Almost all concrete nowadays is mixed by machinery, and preferably in batch mixers, although some continuous mixers have been known to give good results. The strength of the concrete is augmented as the time of mixing is increased; hence it is an economic problem for the engineer, but not for the contractor, to determine what is the best time to adopt for mixing each batch. If the time be made too short, the attainable strength and quality of the concrete are not developed; while if it be made too long, the output per mixer is reduced and the cost per cubic yard of the finished work is increased. As the contractor is paid so much per cubic yard for concrete in place, it is evident that he always loses instead of gains by increasing the time of mixing, and that the owner, up to a certain point, is a gainer by such an increase, after which he is a loser. In respect to what that limit is, most engineers differ. Contractors would like to make it thirty seconds, but are willing to concede a full minute. The author, how- ever, would set a minimum of two minutes. As loading and unloading the mixer require for the two operations, on an average, about 45 seconds, and as the time given to actual mixing is about the same, the total time needed per batch is one and a half minutes, but when the time of rotating the mixer is increased to two minutes, the total time per batch is two minutes and forty-five seconds, hence the output per mixer would be nearly halved. Nevertheless, the author believes that doubling the ordinary time of mixing will result in true economy for the owner. The authorities recognize that it is far better, when a truly-first-class job is required, to employ more mixers, even at a higher first-cost for equipment, and work them on a