efficient and not-too-expensive apparatus for protecting the finished concrete from injury by frost or heat. Doing all these things scientifically and systematically will often enable a contractor to make money out of a job on which otherwise there would have been a loss.

Amongst the economic problems that arise in the manufacture of concrete, the principal ones are the following:

A. Best proportions of materials.

B. Reduction of voids in the aggregate.

C. Using a mixture of gravel and sand without screening.

D. Waterproofing.

E. Increasing fluidity of mixture.

F. Manner and time of mixing.

G. Amount of mixing water.

H. Use of large stones in mass.

I. Vibration and jigging of newly placed concrete.

J. Age of cement.

K. Protection of fresh concrete.

Each of these economic questions will be taken up in the above order and discussed in detail.

Best Proportions of Materials

Without knowing in advance the kinds and characteristics of the aggregates which the successful bidder will employ, it is impracticable for an engineer to specify the best possible proportions for concrete, hence he is either compelled to name three or four standard mixtures or else to specify certain maximum limits of the materials in the aggregate and an amount of cement proportionate to the volume of voids therein.

The principle to be adopted in specifying the proportions for concrete is to use a little more than enough cement to fill the voids in the sand and a little more than enough of the resulting mortar to fill the voids in the broken stone or gravel. If the mixing were perfect, there might not be good reason for these excesses of cement and mortar; but, of course, it never is. The author is satisfied with an excess of ten per cent for each case, or with an excess of ten per cent of cement above the volume of voids in the aggregate.

The most common proportions specified are those given in the following table:

Table 39a