Concrete Construction Part 2

The labor cost of erecting bins and installing a 916 jaw crusher, elevator, etc., averages about $75, including hauling the plant two or three miles, and dismantling the plant when work is finished.

The following is a record of the cost of crushing stone and cobbles on four jobs at Newton, Ma.s.s., in 1891. On jobs A and B the stone was quarried and crushed; on jobs C and D cobblestones were crushed. A 915-in. Farrel-Marsondon crusher was used, stone being fed in by two laborers. A rotary screen having , 1 and 2-in. openings delivered the stone into bins having four compartments, the last receiving the "tailings" which had failed to pa.s.s through the screen. The broken stone was measured in carts as they left the bin, but several cart loads were weighed, giving the following weights per cubic foot of broken stone:

-----------Size.--------------

-in. 1-in. 2-ins. Tailings.

lbs. lbs. lbs. lbs.

Greenish trap rock, "A" 95.8 84.3 88.3 91.0 Conglomerate, "B" 101.0 87.7 94.4 ....

Cobblestones, "C" and "D" 102.5 98.0 99.6 ....

A one-horse cart held 26 to 28 cu. ft. (average 1 cu. yd.) of broken stone; a two-horse cart, 40 to 42 cu. ft., at the crusher.

---------------------Job.-------------

A. B. C. D.

Hours run 412 144 101 198 Short tons per hour 9.0 11.2 15.7 12.1 Cu. yds. per hour 7.7 8.9 11.8 9.0 Per cent of tailings 31.8 29.3 17.5 20.5 Per cent of 2-in. stone 51.3 51.9 57.0 55.1 Per cent of 1-in. stone 10.2 .... .... ....

Per cent of -in. stone or dust 6.7 18.8 25.5 23.4

---------------------Job.-------------

A. B. C. D.

Explosives, coal for drill and repairs $0.084 $0.018 .... ....

Labor steam drilling 0.092 .... .... ....

Labor hand drilling .... 0.249 .... ....

Sharpening tools 0.069 0.023 .... ....

Sledging stone for crusher 0.279 0.420 .... ....

Loading carts 0.098 0.127 .... $0.144 Carting to crusher 0.072 0.062 $0.314 0.098 Feeding crusher 0.053 0.053 0.033 0.065 Engineer of crusher 0.031 0.038 0.029 0.036 Coal for crusher 0.079 0.050 0.047 0.044 Repairs to crusher 0.041 .... .... 0.011 Moving portable crusher .... 0.023 .... 0.019 Watchman ($1.75 a day) .... 0.053 0.022 0.030 ------ ------ ------ ------ Total cost per cu. yd. $0.898 $1.116 $0.445 $0.447 Total cost per short ton 0.745 0.885 0.330 0.372

Note.--"A" was trap rock; "B" was conglomerate rock; "C" and "D" were trap and granite cobblestones. Common laborers on jobs "A" and "D" were paid $1.75 per 9-hr. day; on jobs "B" and "C,"

$1.50 per 9-hr. day; two-horse cart and driver, $5 per day; blacksmith, $2.50; engineer on crusher, $2 on job "A," $2.25 on "B," $2.00 on "C," $2.50 on "D"; steam driller received $3, and helper $1.75 a day; foreman, $3 a day. Coal was $5.25 per short ton. Forcite powder, 11-1/3 cts. per lb.

For a full discussion of quarrying and crushing methods and costs and for descriptions of crushing machinery and plants the reader is referred to "Rock Excavation; Methods and Cost," by Halbert P. Gillette.

~SCREENING AND WASHING GRAVEL.~--Handwork is resorted to in screening gravel only when the amount to be screened is small and when it is simply required to separate the fine sand without sorting the coa.r.s.er material into sizes. The gravel is shoveled against a portable inclined screen through which the sand drops while the pebbles slide down and acc.u.mulate at the bottom. The cost of screening by hand is the cost of shoveling the gravel against the screen divided by the number of cubic yards of saved material. In screening gravel for sand the richer the gravel is in fine material the cheaper will be the cost per cubic yard for screening; on the contrary in screening gravel for the pebbles the less sand there is in the gravel the cheaper will be the cost per cubic yard for screening. The cost of shoveling divided by the number of cubic yards shoveled is the cost of screening only when both the sand and the coa.r.s.er material are saved. Tests made in the pit will enable the contractor to estimate how many cubic yards of gravel must be shoveled to get a cubic yard of sand or pebbles. An energetic man will shovel about 25 cu. yds. of gravel against a screen per 10-hour day and keep the screened material cleared away, providing no carrying is necessary.

A mechanical arrangement capable of handling a considerably larger yardage of material is shown by Fig. 8. Two men and a team are required.

The team is attached to the sc.r.a.per by means of the rope pa.s.sing through the pulley at the top of the incline. The sc.r.a.per is loaded in the usual manner, hauled up the incline until its wheels are stopped by blocks and then the team is backed up to slacken the rope and permit the sc.r.a.per to tip and dump its load. The trip holding the sc.r.a.per while dumping is operated from the ground. The sc.r.a.per load falls onto an inclined screen which takes out the sand and delivers the pebbles into the wagon.

By erecting bins to catch the sand and pebbles this same arrangement could be made continuous in operation.

[Ill.u.s.tration: Fig. 8.--Device for Excavating and Screening Gravel and Loading Wagons.]

[Ill.u.s.tration: Fig. 9.--Gravel Washing Plant of 120 to 130 Cu. Yds., Per Hour Capacity.]

In commercial gravel mining, the gravel is usually sorted into several sizes and generally it is washed as well as screened. Where the pebbles run into larger sizes a crushing plant is also usually installed to reduce the large stones. Works producing several hundred cubic yards of screened and washed gravel per day require a plant of larger size and greater cost than even a very large piece of concrete work will warrant, so that only general mention will be made here of such plants. The commercial sizes of gravel are usually 2-in., 1-in., -in. and -in., down to sand. No very detailed costs of producing gravel by these commercial plants are available. At the plant of the Lake Sh.o.r.e & Michigan Southern Ry., where gravel is screened and washed for ballast, the gravel is pa.s.sed over a 2-in., a -in., a -in. and a 1/8-in. screen in turn and the fine sand is saved. About 2,000 tons are handled per day; the washed gravel, 2-in. to 1/8-in. sizes, represents from 40 to 65 per cent. of the raw gravel and costs from 23 to 30 cts. per cu. yd., for excavation, screening and washing. The drawings of Fig. 9 show a gravel washing plant having a capacity of 120 to 130 cu. yds. per hour, operated by the Stewart-Peck Sand Co., of Kansas City, Mo. Where washing alone is necessary a plant of one or two washer units like those here shown could be installed without excessive cost by a contractor at any point where water is available. Each washer unit consists of two hexagonal troughs 18 ins. in diameter and 18 ft. long. A shaft carrying blades set spirally is rotated in each trough to agitate the gravel and force it along; each trough also has a fall of 6 ins. toward its receiving end. The two troughs are inclosed in a tank or box and above and between them is a 5-in. pipe having -in. holes 3 ins. apart so arranged that the streams are directed into the troughs. The water and dirt pa.s.s off at the lower end of the troughs while the gravel is fed by the screws into a chute discharging into a bucket elevator, which in turn feeds into a storage bin. The gravel to be washed runs from 2 ins.

to 1/8-in. in size; it is excavated by steam shovel and loaded into 1 cu. yd. dump cars, three of which are hauled by a mule to the washers, where the load is dumped into the troughs. The plant having a capacity of 120 to 130 cu. yds. per hour cost $25,000, including pump and an 8-in. pipe line a mile long. A 100-hp. engine operates the plant, and 20 men are needed for all purposes. This plant produces washed gravel at a profit for 40 cts. per cu. yd.

CHAPTER II.

THEORY AND PRACTICE OF PROPORTIONING CONCRETE.

American engineers proportion concrete mixtures by measure, thus a 1-3-5 concrete is one composed of 1 volume of cement, 3 volumes of sand and 5 volumes of aggregate. In Continental Europe concrete is commonly proportioned by weight and there have been prominent advocates of this practice among American engineers. It is not evident how such a change in prevailing American practice would be of practical advantage. Aside from the fact that it is seldom convenient to weigh the ingredients of each batch, sand, stone and gravel are by no means constant in specific gravity, so that the greater exactness of proportioning by weight is not apparent. In this volume only incidental attention is given to gravimetric methods of proportioning concrete.

~VOIDS.~--Both the sand and the aggregates employed for concrete contain voids. The amount of this void s.p.a.ce depends upon a number of conditions. As the task of proportioning concrete consists in so proportioning the several materials that all void s.p.a.ces are filled with finer material the conditions influencing the proportion of voids in sand and aggregates must be known.

~Voids in Sand.~--The two conditions exerting the greatest influence on the proportion of voids in sand are the presence of moisture and the size of the grains of which the sand is composed.

TABLE I.--SHOWING EFFECT OF ADDITIONS OF DIFFERENT PERCENTAGES OF MOISTURE ON VOLUME OF SAND.

Per cent of water in sand 0 0.5 1 2 3 5 10 Weight per cu. yd. of fine Lbs. Lbs. Lbs. Lbs. Lbs. Lbs. Lbs.

sand and water 3,457 2,206 2,085 2,044 2,037 2,035 2,133 Weight per cu. yd. of coa.r.s.e sand and water 2,551 2,466 2,380 2,122 2,058 2,070 2,200

The volume of sand is greatly affected by the presence of varying percentages of moisture in the sand. A dry loose sand that has 45 per cent. voids if mixed with 5 per cent. by weight of water will swell, unless tamped, to such an extent that its voids may be 57 per cent. The same sand if saturated with water until it becomes a thin paste may show only 37 per cent. voids after the sand has settled. Table I shows the results of tests made by Feret, the French experimenter. Two kinds of sand were used, a very fine sand and a coa.r.s.e sand. They were measured in a box that held 2 cu. ft. and was 8 ins. deep, the sand being shoveled into the box but not tamped or shaken. After measuring and weighing the dry sand 0.5 per cent. by weight of water was added and the sand was mixed and shoveled back into the box again and then weighed.

These operations were repeated with varying percentages of water up to 10 per cent. It will be noted that the weight of mixed water and sand is given; to ascertain the exact weight of dry sand in any mixture, divide the weight given in the table by 100 per cent. plus the given tabular per cent.; thus the weight of dry, fine sand in a 5 per cent. mixture is 2,035 1.5 = 1,98 lbs. per cu. yd. The voids in the dry sand were 45 per cent. and in the sand with 5 per cent. moisture they were 56.7 per cent. Pouring water onto loose, dry sand compacts it. By mixing fine sand and water to a thin paste and allowing it to settle, it was found that the sand occupied 11 per cent. less s.p.a.ce than when measured dry.

The voids in fine sand, having a specific gravity of 2.65, were determined by measurement in a quart measure and found to be as follows:

Sand not packed, per cent. voids 44 Sand shaken to refusal, per cent. voids 35 Sand saturated with water, per cent. voids 37

Another series of tests made by Mr. H. P. Boardman, using Chicago sand having 34 to 40 per cent. voids, showed the following results:

Water added, per cent. 2 4 6 8 10 Resulting per cent. increase 17.6 22 19.5 16.6 15.6

Mr. Wm. B. Fuller found by tests that a dry sand, having 34 per cent.

voids, shrunk 9.6 per cent. in volume upon thorough tamping until it had 27 per cent. voids. The same sand moistened with 6 per cent. water and loose had 44 per cent. voids, which was reduced to 31 per cent. by ramming. The same sand saturated with water had 33 per cent. voids and by thorough ramming its volume was reduced 8 per cent. until the sand had only 26 per cent. voids. Further experiments might be quoted and will be found recorded in several general treatises on concrete, but these are enough to demonstrate conclusively that any theory of the quant.i.ty of cement in mortar to be correct must take into account the effect of moisture on the voids in sand.

The effect of the size and the shape of the component grains on the amount of voids in sand is considerable. Feret's experiments are conclusive on these points, and they alone will be followed here. Taking for convenience three sizes of sand Feret mixed them in all the varying proportions possible with a total of 10 parts; there were 66 mixtures.

The sizes used were: Large (L), sand composed of grains pa.s.sing a sieve of 5 meshes per linear inch and retained on a sieve of 15 meshes per linear inch; medium (M>), sand pa.s.sing a sieve of 15 meshes and retained on a sieve of 50 meshes per linear inch, and fine (F), sand pa.s.sing a 50-mesh sieve. With a dry sand whose grains have a specific gravity of 2.65, the weight of a cubic yard of either the fine, or the medium, or the large size, was 2,190 lbs., which is equivalent to 51 per cent. voids. The greatest weight of mixture, 2,840 lbs. per cu. yd., was an L_{6}M_{0}F_{4} mixture, that is, one composed of six parts large, no parts medium and 4 parts fine; this mixture was the densest of the 66 mixtures made, having 36 per cent. voids. It will be noted that the common opinion that the densest mixture is obtained by a mixture of gradually increasing sizes of grains is incorrect; there must be enough difference in the size of the grains to provide voids so large that the smaller grains will enter them and not wedge the larger grains apart.

Turning now to the shape of the grains, the tests showed that rounded grains give less voids than angular grains. Using sand having a composition of L_{5}M_{3}F_{2} Feret got the following results:

--Per cent. Voids-- Kind of Grains. Shaken. Unshaken.

Natural sand, rounded grains 25.6 35.9 Crushed quartzite, angular grains 27.4 42.1 Crushed sh.e.l.ls, flat grains 31.8 44.3 Residue of quartzite, flat grains 34.6 47.5

The sand was shaken until no further settlement occurred. It is plain from these data on the effect of size and shape of grains on voids why it is that discrepancies exist in the published data on voids in dry sand. An idea of the wide variation in the granulometric composition of different sands is given by Table II. Table III shows the voids as determined for sands from different localities in the United States.

TABLE II.--SHOWING GRANULOMETRIC COMPOSITIONS OF DIFFERENT SANDS.

Held by a Sieve. A B C E No. 10 35.3% No. 20 32.1 12.8% 4.2% 11% No. 30 14.6 49.0 12.5 14 No. 40 ... ... 44.4 ...

No. 50 9.6 29.3 ... 53 No. 100 4.9 5.7 ... ...

No. 200 2.0 2.3 ... ...

----- ----- ----- ----- Voids 33% 39% 41.7% 31%