Aggregates

Occasionally one sees a mixture of cement and sand used for the concrete base in sidewalk construction, and cannot help being impressed with the fact that the user fails to appreciate tne requisite of a good concrete.

In Table VI will be found a physical analysis of a material taken from a sidewalk job, in which it was being used for the concrete in the propor tion of one part cement to four parts aggregate.

The general quality of this material was fairly good, though it will be noted that only 2 per cent of it could be considered gravel. No particles found in the sample were larger than inch. The computed voids in the sand were 33 per cent; measured by displacement, 29 per cent. The mixture of 1 cement to 4 sand, there fore, was out of balance, the cement not being sufficient to fill the voids. Not only did this vol ume of sand contain more voids than the cement could fill, but the excess of fine material de tracted from the value of the sand as it was being used, because it presented a very much'.

larger surface than the cement could possibly cover.

Size of Aggregate.

Aggregates exceeding inches in diameter should not be used. Un doubtedly there are many gravels which would give good results, though containing larger sizes; but this limit is safe and the one most often ap plied to this class of work. The lower limit, inch, which is also the upper limit for sand and stone screenings, is almost universally accepted.

Unscreened Gravel.

In many districts, un screened gravel (gravel as it comes from the bank containing both coarse and fine particles) is used. This practice should be avoided, as such material usually contains a large excess of sand, and would be much improved if screened and the proper proportions of fine and coarse particles remixed. The increased value of the remixed aggregate over the natural material would more than justify the additional expense. The case referred to in Table VI illustrates this point quite clearly.