COST OF AGGREGATE.—The locality in which the work is done determines the cost of the aggregate. Concerns producing broken stone or screened and washed gravel for concrete are to be found within shipping distance in most sections of the country so that these materials may be purchased in any amount desired. The cost will then be the market price of the material f. o. b. cars at plant plus the freight rates and the cost of unloading and haulage to the stock piles. If the contractor uses a local stone or gravel the aggregate cost will be, for stone the costs of quarrying and crushing and transportation, and, for gravel, the cost of excavation, screening, washing and transportation.
SCREENED OR CRUSHER-RUN STONE FOR CONCRETE.—Formerly engineers almost universally demanded that broken stone for concrete should have all the finer particles screened out. This practice has been modified to some considerable extent in recent years by using all the crusher product both coarse and fine, or, as it is commonly expressed, by using run-of-crusher stone. The comparative merits of screened and crusher-run stone for concrete work are questions of comparative economy and convenience. The fine stone dust and chips produced in crushing stone are not, as was once thought, deleterious; they simply take the place of so much of the sand which would, were the stone screened, be required to balance the sand and stone mixture. It is seldom that the proportion of chips and dust produced in crushing stone is large enough to replace the sand constituent entirely; some sand has nearly always to be added to run-of-crusher stone and it is in determining the amount of this addition that uncertainty lies. The proportions of dust and chips in crushed stone vary with the kind of stone and with the kind of crusher used. Furthermore, when run-of-crusher stone is chuted from the crusher into a bin or pile the screenings and the coarse stones segregate. Examination of a crusher-run stone pile will show a cone-shaped heart of fine material enclosed by a shell of coarser stone, consequently when this pile of stone is taken from to make concrete a uniform mixture of fine and coarse particles is not secured, the material taken from the outside of the pile will be mostly coarse and that from the inside mostly fine. This segregation combined with the natural variation in the crusher product makes the task of adding sand and producing a balanced sand and stone mixture one of extreme uncertainty and some difficulty unless considerable expenditure is made in testing and reproportioning. When the product of the crusher is screened the task of proportioning the sand to the stone is a straightforward operation, and the screened out chips and dust can be used as a portion of the sand if desired. The only saving, then, in using crusher-run stone direct is the very small one of not having to screen out the fine material. The conclusion must be that the economy of unscreened stone for concrete is a very doubtful quantity, and that the risk of irregularity in unscreened stone mixtures is a serious one. The engineer's specifications will generally determine for the contractor whether he is to use screened or crusher-run stone, but these same specifications will not guarantee the regularity of the resulting concrete mixture; this will be the contractor's burden and if the engineer's inspection is rigid and the crusher-run product runs uneven for the reasons given above it will be a burden of considerable expense. The contractor will do well to know his product or to know his man before bidding less or even as little on crusher-run as on screened stone concrete.
COST OF QUARRYING AND CRUSHING STONE.—The following examples of the cost of quarrying and crushing stone are fairly representative of the conditions which would prevail on ordinary contract work. In quarrying and crushing New Jersey trap rock with gyratory crushers the following was the cost of producing 200 cu. yds. per day:
| Per day. | Per cu. yd. | |
| 3 drillers at $2.75 | $ 8.25 | $0.041 |
| 3 helpers at $1.75 | 5.25 | 0.026 |
| 10 men barring out and sledging | 15.00 | 0.075 |
| 14 men loading carts | 21.00 | 0.105 |
| 4 cart horses | 6.00 | 0.030 |
| 2 cart drivers | 3.00 | 0.015 |
| 2 men dumping carts and feeding crusher | 3.00 | 0.015 |
| 1 fireman for drill boiler | 2.50 | 0.013 |
| 1 engineman for crusher | 3.00 | 0.015 |
| 1 blacksmith | 3.00 | 0.015 |
| 1 blacksmith helper | 2.00 | 0.010 |
| 1 foreman | 5.00 | 0.025 |
| 2 tons coal at $3.50 | 7.00 | 0.035 |
| 150 lbs. 40% dynamite at 15 cts. | 22.50 | 0.113 |
| ——— | ——— | |
| Total | $106.50 | $0.533 |
The quarry face worked was 12 to 18 ft., and the stone was crushed to 2-in. size. Owing to the seamy character of the rock it was broken by blasting into comparatively small pieces requiring very little sledging. The stone was loaded into one-horse dump carts, the driver taking one cart to the crusher while the other was being loaded. The haul was 100 ft. The carts were dumped into an inclined chute leading to a No. 5 Gates crusher. The stone was elevated by a bucket elevator and screened. All stone larger than 2 ins. was returned through a chute to a No. 3 Gates crusher for recrushing. The cost given above does not include interest, depreciation, and repairs; these items would add about $8 to $10 more per day or 4 to 5 cts. per cubic yard.
In quarrying limestone, where the face of the quarry was only 5 to 6 ft. high, and where the amount of stripping was small, one steam drill was used. This drill received its steam from the same boiler that supplied the crusher engine. The drill averaged 60 ft. of hole drilled per 10-hr. day, but was poorly handled and frequently laid off for repairs. The cost of quarrying and crushing was as follows:
Quarry.
| 1 driller | $ 2.50 |
| 1 helper | 1.50 |
| 1 man stripping | 1.50 |
| 4 men quarrying | 6.00 |
| 1 blacksmith | 2.50 |
| ⅛ ton coal at $3 | 1.00 |
| Repairs to drill | .60 |
| Hose, drill steel and interest on plant | .90 |
| 24 lbs. dynamite | 3.60 |
| ——— | |
| Total | $20.10 |
Crusher.
| 1 engineman | $ 2.50 |
| 2 men feeding crusher | 3.50 |
| 6 men wheeling | 9.00 |
| 1 bin man | 1.50 |
| 1 general foreman | 3.00 |
| ⅓ ton coal at $3 | 1.00 |
| 1 gallon oil | .25 |
| Repairs to crusher | 1.00 |
| Repairs to engine and boiler | 1.00 |
| Interest on plant | 1.00 |
| ——— | |
| Total | $23.75 |