{Contributed by P. R. Strong)

Materials

Cement, in binding the aggregate together, may perhaps be considered to be the most important ingredient of concrete, and none but the best slow-setting Portland cement should be used. The quality should be undoubted, and every consignment should be tested by all the tests allowed for in the specification. The quality of the cement may well be specified to conform with the specification of the Engineering Standards Committee.

The aggregate should have strength at least as great as that which the cement alone will eventually possess. Aggregate containing pieces varying up to those of moderate size make the strongest concrete, but for armoured concrete the size must be small on account of the thinness of the members, and in order that the reinforcement may be thoroughly surrounded. Thus for beams the aggregate should be capable of passing a 1 or 1 1/4-inch ring in all directions, while for floor slabs this should again be reduced to 1/2 inch. The properties of various aggregates as regards fire resistance has already been discussed. Aggregates of sharp hard nature, such as broken stone of good quality, make the strongest concrete, while the strength of coke-breeze concrete may be only one-half to a quarter of that similarly formed with broken stone, and may be even less than a quarter, so that this material is evidently unsuitable for pillars or beams or wherever heavy loads arc to be carried.

Sand, which must be hard and sharp, should preferably be of coarse quality - that is to say, it should be of the nature of grit.

Water may possibly contain chemical impurities which will deleteriously affect the setting of the cement, and unless the water is of known quality it should be analysed.

All materials should be thoroughly clean, for dirt of fine nature will prevent the proper adhesion of cement to aggregate, or of cement to cement. All aggregate and sand, unless thoroughly clean, should be washed, and dirty water should on no account be used.

Proportions

It is of the first importance that the mixing of concrete should be in correct proportions, and this part of the work being in the hands of labourers, the exact proportions of all ingredients, cement, stone, sand, and water should be laid down, and the use of accurate measuring boxes should be insisted upon.

The strength of concrete depends largely upon the amount of really hard aggregate that can be put into a given volume. There is little doubt but that the strongest concrete could be made with material varying evenly in size from pieces of moderate size down to the size of the grit grains, the spaces between the larger portions being filled by those of smaller size, and the whole being cemented together with cement. That is to say, the concrete would consist of two ingredients only, namely, aggregate of varying size and cement. This condition is not easily met in practice, so that the mixture is generally regarded as consisting of aggregate of moderate size embedded in a mortar of cement and sand; and on account of the usual method of proportioning the materials by volume, the latter method may appear on the face of it to produce an equally strong or even stronger material, but this will in reality be obtained with an increased proportion of cement, as will be shown.

It is evident that, in order to form a solid mass, the quantity of cement and sand should be just sufficient to fill the interstices caused by the coarseness of the aggregate. The extent of these interstices may be found by filling a receptacle of known capacity with the aggregate, by then ramming it, and by measuring the quantity of water required to bring its surface up to the level of the top of the aggregate. If the aggregate be composed of coke-breeze, broken brick, or other porous material, it must be thoroughly soaked before making the experiment. The volume of the voids in proportion to the volume of the aggregate, as ordinarily measured, is thus found, and this quantity, increased by about 10 per cent. of the total volume to ensure the thorough flushing of the work, gives the necessary amount of mortar. This volume is constant for the particular aggregate, no matter what strength is required: the strength of the concrete must be regulated by varying the strength of the mortar.

For example, if the extent of the voids in a particular aggregate when rammed is found to be 45 per cent. of its original volume, adding 10 per cent., the proportion of mortar necessary becomes 55 per cent. As cement and sand when mixed with water will decrease in bulk by approximately 20 per cent., their amounts must be increased by a quarter, and the volume of mortar will be 55 + 55/4 = 69 per cent. Then if the mortar is to be composed in the proportion of 1:2, the volume of aggregate required with these three parts of mortar =100x3/69 = 4.35; that is to say, the proportions should be 1 2 4 1/3.

1:2:4 is a very usual proportion with ordinary aggregates, and has been found by experiment to yield high compressive resistance.

Considering an aggregate of the nature assumed above, as long as the mortar is mixed in the proportions 1 2, a variation in the volume of the mortar, whether in the way of increase or decrease, will diminish the resistance of the concrete. Supposing that it be desired to form a stronger concrete with mortar mixed with cement and sand in equal proportions, the aggregate being the same as was assumed above, the volume of the mortar must remain the same, and the proportions will become 1 1/2: 1 1/2: 4 1/3

If stone and sand be intimately mixed the resulting mixture will be found to take little if any greater space than that taken up by the stone alone. Thus if a concrete of the proportions 1 2 4 be taken and the sand and stone be mixed, the proportion will become one of cement to four of a mixture of sand and stone. Therefore if the aggregate consist of a natural mixture in these proportions, the concrete mixed in the proportions of one part of cement and six of this mixture will have considerably less cement in proportion to the mass of finished concrete than would be the case if the sand and stone were measured separately. Thus when an aggregate itself contains sand it is impossible to proportion the ingredients accurately, and for this reason the apparently extravagant process of screening out the sand is often resorted to, and is the only way by which the proper proportions can be continuously and mechanically measured.

There exists much confusion as to the proportions intended by the expression 1 6, some reading this as 1 of cement to 2 of sand and 4 of stone, while others will read it as 1 of cement to 6 of stone, the sand being added to the cement in the proper proportion. To obviate all confusion the measure of each ingredient should be specified as 1 2 4; while it should be laid down that measuring boxes shall be used for each ingredient.