Cement is usually purchased for delivery in sacks under a specification which defines a sack as containing 0.9 cu. ft. of cement and weighing 95 lb. While occasionally a sack should be weighed as a check, it is customary to use a sack as 0.9 cu. ft. and measure the quantity of sand to correspond with a certain number of sacks of cement. The measuring devices should be arranged so that the sand can be rapidly and accurately measured and delivered (with the cement) to the mixer with the least labor. The mixer should be a batch mixer of which many satisfactory types are on the market. The water should also be measured in order to secure uniformity in the consistency of output. It is useless to specify the exact amount of water as it will naturally vary with weather, climate, condition of sand and somewhat with the use to which the mortar is put. Two or three trials will determine the amount of water for a proper consistency. A gage should be set in the measuring tank in such a way that it may be readily adjusted for varying conditions. The mortar should be thinner than in the laboratory practice of making briquettes but should be stiffer than is used in ordinary brick work, as it must properly bed heavy stone. It should be stiff enough for the heaviest work on the dam, and for use in filling in with spalls it may be readily thinned on the dam by the addition of water.
Until within the last few years it was universally considered that mortar should be used before it had begun to set, and on any work where there seemed to be a chance that mortar might remain unused for any length of time it was customary to specify that mortar remaining unused for twenty to thirty minutes after being mixed should be thrown away. Included with other experiments carried on in connection with the Wachusett dam, was a set designed to show whether or not such a specification was necessary. Quick-and slow-setting mortar of both Portland and natural cements, with various periods of from thirty minutes to two hours elapsed time between mixing and using were made into briquettes and tested at various periods after making. Some of the tests were made on mortar worked (i.e., tempered) continuously during the time between mixing and using, and some were simply allowed to stand and were tempered just before using. All the experiments, for the Portland and slow-setting natural cement showed no loss and often a gain in strength. Only the quick-setting natural cement showed some loss.
These results were so conclusive that during the construction of the dam 2-yd. batches of mortar were mixed and used under such conditions that two hours often elapsed before the last of the batch was used. If it had been found that thirty minutes was the advisable limit of time in which to allow a batch to be used, a radical difference in the method of mixing and using the mortar would have been involved, and it would probably have been necessary to resort to the method of sending the ingredients on dry and adding the water on the dam.
Better concrete can be made from gravel than from crushed stone, but it is almost invariably the case on a large masonry dam that it is much cheaper to use crushed stone. The run of the crusher may be used without screening, and if it contains enough very fine stone some slight allowance may be made in the amount of accompanying sand. However, it will be found that it is better to have rather fine aggregate and that 2-in. maximum size is better than 3 in. The reasons in favor of fine aggregate and plenty of sand are that the resulting concrete is more fluid, and that it can run and enter the places it must better than a harsher mixture; also when once in place many more spalls can be put into the concrete. Into a 1 - 2 1/2 - 4 concrete of proper consistency and with 2-in. maximum aggregate, many more spalls can be put than are necessary to equalize (for cement per cu. yd. of masonry) a 1 - 2 1/2 - 5 concrete with 3-in. maximum aggregate and containing all the spalls practicable for that mixture. The concrete should be used soon after being mixed, not to avoid initial set but because it must be mixed quite wet; so wet that if allowed to stand it will unmix to a certain extent, i.e., the aggregate will tend to settle and the water to come to the surface. This should be avoided as far as possible.