The mixture of 1 : 3 : 5 used to be the standard for concrete in large masses placed in the dry, but there are certain sands of such uniform grain-size that one portion of cement to three of sand will not fill all the voids therein; and, although the mortar produced will be ample in amount to fill all the voids in the stone or gravel, the mass resulting will be permeable by water, and, therefore, not first-class concrete.

The proportion of 1 : 2 1/2 : 5 is better than that of 1 : 3 : 5, because the amount of cement is always more than enough to fill the voids in the sand, and hat of the mortar is more than enough to fill the voids in the stone or gravel. It is a perfectly safe mixture but not an economic one. It is being rather widely specified today for mass concrete placed in the dry; and for small jobs it is a good one to adopt therefor.

The proportion of 1 : 2 : 4 is the common one for reinforced-concrete; and the stone or gravel should not be very coarse, because otherwise the concrete would not flow properly between the reinforcing bars, and voids might result. In view of the importance of having reinforced-concrete as perfect as possible, and of the fact that the mortar should take a firm grip on the reinforcing bars, it is not advisable to use for this purpose any concrete less rich in cement than this mixture.

The proportion of 1 : 2 : 3 with fine broken stone or gravel is the author's standard for concrete to be deposited through water by trémie or trip-bucket. It contains an excess of cement to provide for the contin- gency that, in spite of all precautions, there may be a slight flow of water through some portion of the concrete. The author has often had occasion to examine concrete of this mixture placed through water, and has invariably found it to be perfectly satisfactory, in fact, just as good as the less-rich concrete placed in the dry. It has been stated by good authority that this proportion makes much better concrete for reinforced work than does the standard proportion of 1 : 2 : 4; hence it might prove economic to adopt the richer mixture therefor, but it would first be necessary to educate the profession to the advisability of the innovation.

The only excuse for adopting in a specification a general clause for a 1 : 3 : 6 mixture is to save expense in a structure where the total cost has to be held down to an absolute minimum, in order to meet a limited appropriation; and then it should usually be confined to locations below ground where no frost can reach. It would be legitimate to adopt it for large anchorages where great mass is required, in which case it might have to be faced with richer concrete and employed only in localities where the climate is mild. But when special care is taken in the grading of the aggregate, a 1 : 3 : 6 proportion can safely be employed, because in graded sand the 1 to 3 proportion will fill all voids in the mortar, and the latter will fill all voids in the stone, thus producing satisfactory concrete, providing, of course, that the mixing be thoroughly done.

The 1 : 3 : 6 mixture does not always make good concrete, and the author would hesitate a long time before deciding to adopt it on any of