232 PLAIN CONCRETE 



using run of crusher the very finest dust should be washed 

 or screened out as it tends to coat the large pieces and 

 to prevent the cement from adhering to them. 



Selection of Aggregates. Usually the character of the 

 aggregates used in mixing concrete depends on the avail- 

 ability of the supply. Where there is much choice in the 

 selection of the aggregates those which are hardest and 

 which break with a cubical fracture will make the best con- 

 crete although rounded pebbles are considered by some 

 engineers to possess great advantages. 



The size of the aggregates has much to do with the quality 

 and strength of the concrete. It can, however, be stated 

 as a general proposition that the larger the stones the 

 stronger will be the concrete. This fact is shown by the 

 accompanying table, which gives the results of tests made 

 at the Watertown Arsenal in 1898. It is interesting to note 

 that the concrete becomes heavier per cubic foot, or, in other 

 words, more dense, the larger the stone used. All these 

 tests were made with concrete manufactured in the pro- 

 portion of 1 part of cement, 1 part of sand, and 3 parts of 

 broken stones, or a 1-1-3 (1 to 1 to 3) mixture, as it is usually 

 expressed. The figures on cinder concrete in the table are 

 added simply to give a comparison of weights, for it will 

 be noted that the cinder concrete is older than the other 

 concretes and therefore stronger in proportion. 



Aggregates that consist of stone of varying sizes are best 

 for making concrete, owing to the fact that they pack closer. 

 It is well however, to screen all the fine particles, such as 

 }-in. sizes, and use them with the sand, as otherwise they 

 will not mix properly with the cement. 



Broken trap rock is the best aggregate for concrete work, 

 the next in value is broken granite, while the third in order 

 of merit is good clean gravel. In fact, these three aggregates 

 can be classified together, and the item of cost only should 

 influence the selection of any one of them. Marble, lime- 

 stone, and slag make good aggregates, in the order named, 

 but marble and limestone are objectionable if the concrete 

 is to be used as a fireproofing. The poorest aggregates are 

 sandstone, slate, and shale. The sandstone is inefficient 



