Broken Stone

The per cent of voids in broken stone varies with the hardness of the stone, the form of the fragments, and the relative proportions of the several sizes present. The last element is the most important. If broken stone passing a 31-inch ring and not a finch screen be separated into three sizes, any one size will give from 52 to 54 per cent of voids loose, while equal parts of any two of the three sizes will give 48 to 50 per cent, and a mixture in which the volume of the smallest size is equal to the sum of the other two gives a trifle less than 48 per cent. Notice, however, that unscreened crushed stone has only 32 to 35 per cent voids—see lines 7 and 11 of Table 20. This is a very excellent reason for not screening the broken stone to be Used in making concrete.

A mass of pebbles retained between the same screens as a corre sponding mass of broken stone has only about three fourths as many voids as the stone.

The weight of crushed stone varies with the amount of compacting it has received, whether by being dropped into the bin or car, or by being shaken during transportation. There

are not much definite data on this subject. In one test a mass of crushed trap dropped about 8 feet into a bin had weights as follows: * finch trap and under 2 648 lb. per cu. yd.

finch to 1 f inch trap

2 432 " " " " 1 f inch to 3-inch trap 2 526 it " " it Crushed limestone is occasionally sold f.o.b. at the quarry as low as 35 to 40 cents per ton (about 42 to 48 cents per cu. yd.), and frequently as low as 45 to 50 cents per ton (54 to 60 cents per cu. yd.). The cost of crushed trap f.o.b. at the quarries in New Jersey for several years previous to 1900, was 40 to 50 cents per ton (about 50 to 62 cents per cu. yd.); but in that year it was increased nearly 50 per cent. In Massachusetts, the cost of broken trap on cars at the end of the railroad transportation, varies from $1.10 to $1.60 per ton (about $1.32 to $1.93 per cu. yd.). In Boston, the cost of crushed granite delivered on the streets is $1.65 to $1.90 per ton. In Montreal, syenite delivered on the street costs an average of $1.15 to $1.20 per ton.