POETLAND CEMENT CONCEETE EOADS. 5 



The coarse aggregate, whether crushed stone, gravel, or slag, should 

 possess at least as great resistance to wear as the mortar which fills 

 the voids of the aggregate. Any sound stone or gravel, moderately 

 hard and tough, will meet this requirement, but in general the harder 

 and tougher the coarse aggregate, the greater will be the resistance 

 to wear offered by the concrete. The best available stone should 

 always be used. 



The difficulties experienced in securing coarse aggregate of satis- 

 factory quality are frequently caused by a lack of proper facilities 

 for preparing the natural materials available. Very few gravel pits 

 furnish a gravel suitable for use in concrete pavements without wash- 

 ing ; and properly equipped washing plants are both difficult and ex- 

 pensive to construct. On the other hand, a great many stone quarries 

 contain pockets of clay or inferior stone which should not be per- 

 mitted in the aggregate, and it is sometimes very difficult to remove 

 the objectionable materials while the stone is being crushed and 

 screened. It is also frequently difficult to screen out the dust formed 

 in crushing some varieties of stone. These difficulties can be largely 

 overcome by obtaining the coarse aggregate from commercial sources 

 that are properly equipped to supply clean, well-graded aggregates. 



The coarse aggregate should be free from shale, slate, coal, ocher, 

 or other materials which easily disintegrate and should meet the 

 following requirements: 



Stone : French coefficient of wear, not less than 7. 



Gravel : When subjected to the abrasion test as described in Bulletin 555, 

 United States Department of Agriculture, page 30, the loss by abrasion should 

 not be more than 12 per cent. 



Slag: The slag should be an approved blast-furnace product, weighing not 

 less than SO pounds per cubic foot. 



A well-graded coarse aggregate is necessary in order that the per- 

 centage of voids may be as small as practicable. The grading of the 

 coarse aggregate is usually accomplished by specifying the various 

 percentages of material which will pass or be retained on screens 

 with circular openings of different sizes. The maximum size of 

 aggregate used varies according to the practice of various States and 

 the character of materials available. The maximum size most com- 

 monly specified is 2-| inches. A v\^ell-graded coarse aggregate should 

 meet the following requirements : 



Per cent. 



Passing 2-inch screen 100 



Passing a 2-inch screen and retained on a 1-inch screen 25-60 



Passing i-inch screen, not more than 10 



WATER. 



Water used in mixing concrete should be practically free from oil, 

 acid, alkali, or organic matter and reasonably clear. Brackish water 



