Dients

Still another method of determining the voids in sand is to calculate it from the weight of a cubic foot and the specific gravity of quartz. The specific gravity of a substance is its relative weight compared with water as the unit. A sub stance having a specific gravity of 2.5, for ex ample, will weigh 2.5 times as much as water, volume for volume. Now, sand is mostly broken quartz, and quartz has a specific gravity of 2.65. A cubic foot of water weighs 62.5 lbs. Therefore a cubic foot of quartz sand, if free from voids, would weigh 2.65 X62.5=165.625 lbs. Now take a cubic foot of the sand to be tested, well shaken down, and weigh it. Suppose, for example, that it weighs 112 lbs. Subtract this from 165.625 lbs., the weight of a solid cubic foot of quartz, and we find a difference of 165.625-112=53.625 lbs., which is the loss of weight due to the voids.

The percentage of voids will therefore be (53.625--:-165.625) X100=32.3 per cent.

The effects of various shapes of particles upon the percentage of voids in aggregates, is shown in Table VI. In the case of the shaken material, the shaking was carried on until no further settlement could be produced. It will be noticed that rounded particles have fewer voids than flat or angular.

Amount of Cement to Use The amount of cement to be used in mixing concrete is generally stated as a certain volume or measure, in proportion to the number of volumes of the sand and the coarser aggregate, thus, 1:2:4 indicating a mixture in which there will have to be measured out twice as much sand as cement, and four times as much gravel or stone.

To determine the proper and most econom ical amount of cement to use in order to get a maximum density and strength of concrete, is therefore simply to determine whether we shall use a 1:2:4, or a 1:3.5:7, or some other mixture.

Fortunately, here again the problem con fronting us need not present any very great difficulty. An absolute and exact determination would, indeed, be extremely difficult; but in all ordinary concrete work the ideal proportions can easily be approximated closely enough to prevent all unnecessary waste and to meet all practical demands.

The working rules and tables which have been compiled by engineers from the results of careful tests and wide observation of actual con structions, afford to all workers in concrete a ready guide to approved practice in the propor tioning of mixtures. A number of these prac tical rules and tables will be given, sufficient to enable the reader to determine the proper mix ture to use in any kind of concrete construction.

In the first place, the general principle may be laid down, that economy in the use of cement demands a careful grading of the aggregates so as to reduce voids as much as possible. If the

coarse aggregate, for example, is very uniform in size, and its particles considerably smaller than the maximum allowed, the voids can be re duced by adding coarser material. If, on the other hand, the aggregate is already almost the full allowable size, the voids can be reduced by adding finer material ranging in size from the coarse aggregate to the coarsest sand. In all aggregates, in fact, the particles should range from fine to coarse, with the coarse pre dominating, as before explained. A cement and Summer Cottage Built of Concrete on Shore of Lake Erie near Cleveland, Ohio.

Cellar walls and chimney of hollow blocks; first floor of reinforced concrete 3 inches thick, with pine flooring ; house walls cement-plas tered with pebble-dash finish ; roof of concrete 1 inch thick spread over corrugated expanded metal (trussit) supported on iron rafters and wa terproofed; porch floor of concrete on cinder foundation.

sand mixture will be vastly increased in strength by the addition of coarse aggregate. One barrel of cement, judiciously mixed with sand and gravel, may be as good as three barrels mixed with sand alone.

Having selected the sand and coarse aggre gate available for the work, and calculated the percentage of voids, the proper mixture to use can be readily determined by reference to Table VII., compiled by Mr. C. W. Boynton, which shows the proportions of cement, sand, and stone to be used for various percentages of voids. This table, in order to be on the safe side and cover all the slight errors likely to occur in de termining the voids, is based on an extra allow ance of 10 per cent in the voids of the small aggregate.

By using this table, a great waste of cement may be prevented. Mr. Boynton cites an actual instance where, on a certain sidewalk job, a mixture of 1 part cement to 4 parts aggregate was being used. The aggregate was chiefly sand, only 2 per cent of it being gravel over inch, and none of it over inch. The voids in the aggregate were about 30 per cent. Here the mixture was out of balance. Not only was there too much fine material, presenting a larger surface than the cement could cover, but the voids were larger than the cement could possibly fill. A reference to Table VII would have en abled the contractor to effect a saving of almost 40 per cent in the amount of cement used, and at the same time to deliver a far better job.