Part 7 (1/2)

CHAPTER IV.

METHODS AND COST OF MAKING AND PLACING CONCRETE BY MACHINE.

The making and placing of concrete is virtually a manufacturing process.

This process as performed by manual labor is discussed in the preceding chapter; it will be discussed here as it is performed by machinery. The objects sought in using machinery for making and placing concrete are: (1) The securing of a more perfectly mixed and uniform concrete, and (2) the securing of a cheaper cost of concrete in place. As in every other manufacturing process both objects cannot be obtained to the highest degree without co-ordinate and universal efficiency throughout in plant and methods. For example, the subst.i.tution of machine mixing for hand mixing will not alone ensure cheaper concrete. If all materials are delivered to the machine in wheelbarrows and if the concrete is conveyed away in wheelbarrows, the cost of making concrete even with machine mixers is high. On the other hand, where the materials are fed from bins by gravity into the mixer and when the mixed concrete is hauled away in cars, the cost of making the concrete may be very low. Making and placing concrete by machinery involves not one but several mechanical operations working in conjunction--in a word, a concrete making plant is required.

The mechanical equipment of a concrete making plant has four duties to perform. (1) It has to transport the raw materials from the cars or boats or pits and place them in the stock piles or storage bins; (2) it has to take the raw materials from stock and charge them to the mixer; (3) it has to mix the raw materials into concrete and discharge the mixture into transportable vehicles; and (4) it has to transport these vehicles from the mixer to the work and discharge them. As all these operations are interrelated component parts of one great process, it is plain why one operation cannot lag without causing all the other operations to slow up.

The mechanical devices which may be used for each of these operations are various, and they may be combined in various ways to make the complete train of machinery necessary to the complete process. In this chapter we shall describe the character and qualities of each type of devices separately. The practicable ways of combining them to form a complete concrete making plant are best ill.u.s.trated by descriptions and records of work of actual plants, and such descriptions and records for each cla.s.s of structure considered in this book are given in the following chapters and may be found by consulting the index. In describing the various machines and devices we have made one cla.s.sification for those used in handling raw materials and mixed concrete, for the reason that nearly all of them are suitable for either purpose.

~UNLOADING WITH GRAB BUCKETS.~--The orange-peel or clam-sh.e.l.l bucket is an excellent device for unloading sand or stone from cars or barges. The cost of unloading, including cleaning up the portions not reached by the bucket, is not more than from 2 to 5 cts. per cu. yd. A grab bucket of either of these types can be applied to any derrick. In unloading broken stone from barges at Ossining, N. Y., a Hayward clam-sh.e.l.l on a stiff-leg derrick unloaded 100 cu. yds. of broken stone per day from barge into wagons, with one engineman and one helper. In addition to the bucket work there was 24 hours' labor cleaning on each 500-cu. yd. barge load. The labor cost of unloading a 500-cu. yd. barge was as follows:

Per Cu. Yd.

One engineman, at $2.50 2.5 cts.

One helper, at $1.50 1.5 cts.

Labor cleaning, at $1.50 0.7 cts.

-------- Total cost per cubic yard 4.7 cts.

~INCLINES.~--Inclines to reach the tops of mixer and storage bins and the level of concrete work can be operated on about the following grades: For teams hauling wagons or cars, 2 per cent. maximum grade. A single heavy team will haul a 5-cu. yd. car, with ordinary bearings, weighing 2 tons empty and 12 tons loaded, with ease on a 1 per cent. grade, and with some difficulty on a 2 per cent. grade. A locomotive will handle cars on a grade of from 4 to 5 per cent. For team haulage 20-lb.

rails may be used, and for locomotives 30-lb. rails. Grades steeper than about 5 per cent. require cable haulage.

~TRESTLE AND CAR PLANTS.~--Trestle and car plants for handling both concrete materials and mixed concrete have a wide range of application and numerous examples of such plants are described in succeeding chapters, and are noted in the index at the end of the book. The following estimates of the cost of a trestle and car plant are given by Mr. Wm. G. Fargo. The work is a.s.sumed to cover an area of 100200 ft.

and to have three-fourths of its bulk below the economical elevation of the mixer, which stands within 50 ft. of the near side of the work. If the work is under 3,000 cu. yds. in bulk and there is a reasonable time limit for completion one mixer of 200 cu. yds. capacity per 10-hour day is a.s.sumed to be sufficient. The items of car plant cost will be about as follows:

150 ft. trestle, at $1.50 $225 5 split switches with spring bridles, at $18 90 2 iron turntables, at $30 60 3-2/3 cu. yd. steel cars with roller bearings 190 ------ Total $565

The trestle a.s.sumed is double 24-in. gage track, 6 ft. on centers; stringers 68 ins.22 to 24 ft.; ties 26 ins., 2 ft. on centers; running boards 212 ins. for each track, and 12-lb. rails; trestle legs, average length 30 ft., of green poles at 5 cts. per foot. This outfit with repairs and renewals amounting to 10 per cent., is considered good for five season's work and the timber work for several jobs if not too far apart. The yearly rental on the basis of five seasons' work would be $124.30, or $1 per working day for a season of five months. Three cars delivering cu. yd. batches can deliver 200 cu. yds. of concrete, an average of 100 ft. from the mixer in 10 hours. Five men, including a man tending switches and turntable and one man to help dump, can operate the plant. With wages at $1.75 per day the labor cost of handling 200 cu.

yds. of concrete would be 4-1/8 cts. per cu. yd.

~CABLEWAYS.~--Cableways arranged to span the work and if the area is wide to travel across the work at right angles to the span will handle concrete, concrete materials, forms, steel and supplies with great economy. They are particularly suitable for bridge and dam work, filter and reservoir work, building foundations and low buildings. The arrangement of a cableway plant for bridge work is described in Chapter XVII. A cableway of 800 ft. clear span on fixed towers 45 ft. high will cost complete from $4,500 to $5,000, and will handle 200 cu. yds. of concrete per 10-hour day. To put the cableway on traveling towers will cost about $1,000 more. In constructing the Pittsburg filtration work four traveling cableways from 250 to 600 ft. span were used. The towers were from 50 to 60 ft. in height and each traveled on a 5-rail track.

The cableways were self-propelling. With conditions favorable each cableway delivered 300 cu. yds. of concrete per day. A cableway plant for heavy fortification work is described in Chapter XI.

~BELT CONVEYORS.~--Belt conveyors may be used successfully for handling both concrete materials and mixed concrete. For handling wet concrete the slope must be quite flat, and the belt must be provided with some means of cleaning off the sticky mortar paste. In several cases rotating brushes stationed at the end of the belt, where it turns over the tail pulley, have worked successfully; these brushes sweep the belt clean.

Except for the cleaning device the ordinary arrangement of belt conveyor for dry materials serves for concrete.

In constructing a large gas works at Astoria, Long Island, near New York city, belt conveyors were used to handle both the sand, gravel and cement bags and the mixed concrete. The belt for handling sand and gravel is shown by Fig. 13. A derrick operating a clam-sh.e.l.l unloaded the sand and gravel into a small hopper, discharging into dump cars operated by a ”d.i.n.ky” up an incline, pa.s.sing over sand and gravel storage bins. A 20-in. belt conveyor ran horizontally 105 ft. under the bins, then up an incline of 3.4 ft. in 125 ft. to feeding hoppers over the mixers. This conveyor received alternately sand and gravel by chute from the storage bins and bags of cement loaded by hand, and carried them to the feeding bins and mixer platform. The speed of the belt was 350 ft. per minute, and it required 6 h.p. to operate it when carrying 100 tons per hour. The mixing was done in two Smith mixers, which turned out 70 cu. yds. or 35 cu. yds. each per hour. The mixed concrete was delivered onto a 50-ft. 24-in. belt conveyor traveling at a speed of 400 ft. per minute and dumping through a chute into cars. Only 1 h.p. was required to run the concrete conveyor. A rotating brush was used to keep the belt clean at the dumping end. It will be noted that only a small amount of power is required for operation.

[Ill.u.s.tration: Fig. 13.--Belt Conveyor Transporting Sand and Gravel.]

~CHUTES.~--Chutes of wood or iron are among the simplest and most efficient means of moving the cement, sand and stone and the mixed concrete when the ground levels permit such devices.

Bags of cement if given a start in casting will slide down a steel or very smooth wooden chute with a slope of 1 ft. in 5 or 6 ft. A wooden trough 12 ins. deep and 24 ins. wide with boards dressed on the inside may be used. When the inclination is steep and the fall is great, some device is necessary to diminish the velocity of descent; the following is an example of such a device which was successfully employed in a chute of the above dimensions, 400 ft. long and having a drop of 110 ft.

This chute had a maximum inclination of 45 and its lower end curved to a horizontal tangent, running into the storehouse. Near the bottom of the chute a horizontal strip was nailed across the upper edges and to it was nailed the upper end of a 20 ft., 112-in. board, the lower end of which rested on the bottom of the chute. Several pieces of timber spiked to the upper side loaded the lower end of this board. The cement bag in descending wedged itself into the angle between the chute and the board and lifted the latter, the spring of the board and the weight at the lower end offering enough resistance to cut down the velocity. After the chute had been in use for some time and had worn smooth it was found necessary to add two more brakes to check the bags.

Broken stone will slide down a steel or steel lined chute with a slope of 1 in 3 or 4 ft. if given a start in casting. Damp sand will not slide down a chute with a slope of 1 in 1.

A wet cement grout will flow down a smooth plank chute, with a slope of 1 in 4 ft., and wet concrete will move on the same slope; comparatively dry concrete requires a slope of nearly 1 in 1, or 45, to secure free movement. Mr. W. J. Douglas gives the following examples of conveying concrete by chute, prefaced by the statement that his experience indicates that concrete can thus be conveyed considerable distances without material injury if proper precautions are taken.