Concrete Construction Part 8

This bucket can be used to advantage where the stock piles are too far from the mixer for the derrick to reach both, the bucket being loaded and wheeled to within reach of the derrick.

[Ill.u.s.tration: Fig. 18.--Charging Bucket With Wheel and Detachable Handle.]

~TYPES OF MIXERS.~--There are two types of concrete mixing machines or concrete mixers as they are more commonly called: (1) Batch mixers and (2) continuous mixers. In mixers of the first type a charge of cement, sand, aggregate and water is put into the machine which mixes and discharges the batch before taking in another charge; charging, mixing and discharging is done in batches. In continuous mixers the cement sand, stone and water are charged into the machine in a continuous stream and the mixed concrete is discharged in another continuous stream. While all concrete mixers are either batch or continuous mixers, it is common practice because of their distinctive character to separate gravity mixers, whether batch or continuous, into a third type. In gravity mixers the concrete materials are made to mingle by falling through specially constructed troughs, or tubes, or hoppers. We shall describe mixers in this chapter as (1) batch mixers, (2) continuous mixers, and (3) gravity mixers. No attempt will be made, however, to describe all or even all the leading mixers of each type; a representative mixer or two of each type will be described, enough to give an indication of the range of practice, and the reader referred to manufacturers' literature for further information.

~Batch Mixers.~--Batch mixers are made in two princ.i.p.al forms which may be designated as tilting and non-tilting mixers. In the first form the mixer drum is tilted as one would tilt a bucket of water to discharge the batch. In non-tilting mixers the mixer drum remains in one position, the batch being discharged by special mechanism which dips it out a portion at a time. In both forms the charge is put into the mixer as a unit and kept confined as a unit during the time of mixing, which may be any period wished by the operator.

[Ill.u.s.tration: Fig. 19.--Chicago Improved Cube Concrete Mixer with Elevating Charging Hopper.]

_Chicago Improved Cube Tilting Mixer._--Figure 19 shows the improved cube mixer made by the Munic.i.p.al Engineering & Contracting Co., Chicago, Ill. The drum consists of a cubical box with rounded corners and edges.

This box has hollow gudgeons at two diagonally opposite corners and these gudgeons are open as shown to provide for charging and discharging. The box is rotated by gears meshing with a circ.u.mferential rack midway between gudgeons and another set of gears operate to tilt the mixer. The inside of the box is smooth, there being no deflectors, as its shape is such as to fold the batch repeatedly and thus accomplish the mixing.

[Ill.u.s.tration: Fig. 20.--Ransome Concrete Mixer.]

_Ransome Non-Tilting Mixer._--Figure 20 shows a representative non-tilting mixer made by the Ransome Concrete Machinery Co., Dunellen, N. J. It consists of a cylindrical drum riding on rollers and rotated by a train of gears meshing with circ.u.mferential racks on the drum. The drum has a circular opening at each end; a charging chute enters one opening and a tilting discharge chute may be thrown into or out of the opposite opening. The cylindrical sh.e.l.l of the drum is provided inside with steel plate deflectors, which plow through and pick up and drop the concrete mixture as the drum revolves. The shape and arrangement of the deflectors are such that the batch is shifted back and forth axially across the mixer. To discharge the batch the discharge chute is tilted so that its end projects into the mixer, in which position the material picked up by the deflectors drops back onto the chute and runs out. The discharge chute being independent of the mixing drum it can be thrown into and out of discharge position at will without stopping the rotation of the drum, and so can discharge any part or all of the batch at once.

The top edge of the charging chute ranges from 30 to 38 ins. in height above the top of the frame, varying with the size of the mixer.

[Ill.u.s.tration: Fig. 21.--Smith Concrete Mixer.]

_Smith Tilting Mixer._--Figure 21 shows a tilting mixer, known as the Smith mixer, made by the Contractors' Supply & Equipment Co., Chicago, Ill. The drum consists of two truncated cones with their large ends fastened together and their small ends open for receiving the charge and discharge of the batch. The drum is operated by a train of gears meshing into a rack at mid-length where the cones join. In addition there is another set of gears which tilt the drum to make the concrete flow out of the discharge end. The inside of the drum is provided with steel plate deflectors, which plow through and pick and drop the concrete mixture shifting it back and forth axially in the process.

~Continuous Mixers.~--Continuous mixers are those in which the cement, sand and stone are fed to the charging hopper in a continuous stream and the mixed concrete is discharged in another continuous stream. They are built in two princ.i.p.al forms. In one form the cement, sand and stone properly proportioned are shoveled directly into the mixing drum. In the other form these materials are dumped into separate charging hoppers and are automatically fed into the mixing drum in any relative proportions desired. One form of continuous mixer with automatic feed is described in the succeeding paragraph and another form is described in Chapter XIV. The continuous mixer without automatic feed consists simply of a trough with a rotating paddle shaft and its driving mechanism. The charging, the mixing and the discharging are done in what is virtually a succession of very small batches.

[Ill.u.s.tration: Fig. 22.--Eureka Automatic Feed Continuous Mixer.]

_Eureka Automatic Feed Mixer._--Figure 22 shows the construction of the continuous mixer built by the Eureka Machine Co., Lansing, Mich. The cement bin and feeder is the small one in the foreground. There is a pocketed cylinder revolving between concave plates, opening into the hopper above, from which the pockets in the feeder are filled, and discharging directly into the mixing trough below. Back of this is shown the feeder for sand or gravel up to 2-in. screen size. This is a pocketed cylinder similar to that used in the cement feeder, except that it is larger, and instead of being provided on the discharge side with a concave plate, is surmounted by a roller, held by springs. This serves to cut off the excessive flow of material, but provides sufficient flexibility to allow the rough coa.r.s.e material to be fed through the machine without its catching. The feeder for crushed stone is a similar construction on larger lines, to handle material up to 3-in. size. These several feeders can be set to give any desired mixture. On any material fit to be used in concrete, they will measure with an error of less than 5 per cent., an agitator being provided in the sand bin to prevent damp sand from bridging over the feeder, and preventing its action. The mixer consists of a trough, with a square shaft, on which are mounted 37 mixing paddles, which are slipped on in rotation, so as to form practically a continuous conveyor, but as each paddle is distinct, and is shaped like the mold board of a plow, the material, as it pa.s.ses from one to the next, is turned over and stirred. Water is sprayed into the ma.s.s at the center of the trough. The result is a dry mix, followed by a wet mix. The mixing trough is made of heavy gage steel, well reinforced, and practically indestructible. To take care of the discharge of material while changing wheelbarrows, a hood is provided on the discharge end of the machine, which can be lowered, and will hold about a wheelbarrow load.

~Gravity Mixers.~--Gravity mixers are constructed in two general forms.

The first form is a trough whose bottom or sides or both are provided with pegs, deflectors or other devices for giving the material a zig-zag motion as it flows down the trough. The second form consists of a series of hoppers set one above the other so that the batch is spilled from one into the next and is thus mixed.

The chief advantage claimed for gravity mixers is that no power is required to operate them. This is obviously so only in the sense that gravity mixers have no power-operated moving mechanism, and the fact should not be overestimated. The cost of power used in the actual performance of mixing is a very small item. The distance between feed and discharge levels is always greater for gravity mixers than for machine mixers, and the power required to raise the concrete materials the excess height may easily be greater than the power required to operate a machine mixer. On the other hand the simplicity of the gravity mixer insures low maintenance costs.

_Gilbreth Trough Mixer._--Figure 23 shows the construction of one of the best known makes of gravity mixers of the trough form. In operation the cement, sand and stone in the proper proportions are spread in superimposed layers on a shoveling board at hopper level and are then shoveled as evenly as possible into the hopper. From the hopper the materials flow down the trough, receiving the water about half way down, and are mixed by being cut and turned by the pins and deflectors. The trough of the mixer is about 10 ft. long.

[Ill.u.s.tration: Fig. 23.--Gilbreth Gravity Mixer, Trough Form.]

[Ill.u.s.tration: Fig. 24.--Hains Gravity Mixer, Fixed Hopper Form.]

_Hains Gravity Mixer._--The form of gravity mixer made by the Hains Concrete Mixer Co., Washington, D. C., is shown by Figs. 24 and 25. The charge pa.s.ses through the hoppers in succession. Considering first the stationary plant, shown by Fig. 24, the four hoppers at the top have a combined capacity of one of the lower hoppers. Each top hopper is charged with cement, sand and stone in the order named and in the proper proportions. Water is then dashed over the tops of the filled hoppers and they are dumped simultaneously into the hopper next below. This hopper is then discharged into the next and so on to the bottom.

Meanwhile the four top hoppers have been charged with materials for another batch. It will be observed that (1) the concrete is mixed in separate batches and (2) the ingredients making a batch are accurately proportioned and begin to be mixed for the whole batch at once. The best arrangement is to have the top of the hopper tower carry sand and stone bins which chute directly into the top hoppers. In the telescopic mixer shown by Fig. 25 the purpose has been to provide a mixer which, hung from a derrick or cableway, will receive a charge of raw materials at stock pile and deliver a batch of mixed concrete to the work, the operation of mixing being performed during the hoist to the work. By providing two mixers so that one can be charged while the other is being hoisted continuous operation is secured. The following are records of operation of stationary gravity mixers of this type.

[Ill.u.s.tration: Fig. 25.--Hains Gravity Mixer, Telescoping Hopper Form.]

In building a dock at Baltimore, Md., a plant consisting of two large hoppers and four charging hoppers with sand and stone bins above was used. One man at each large conical hopper tending the gates and two men charging the four pyramidal hoppers composed the mixer gang. A scow load of sand and another of stone were moored alongside the work and a clam-sh.e.l.l bucket dredge loaded the material from these barges into the mixer bins. Each batch was 25 cu. ft. of 1-2-5 concrete rammed in place.

The men at the upper hoppers would empty a sack of cement in each, and then by opening gates in the bottom of the bins above, allow the necessary amounts of sand and stone to flow in, marks having been previously made on the sides of the hoppers to show the correct proportion of each of the ingredients. The amount of water found by experience to be necessary, would then be dashed into the hoppers, and the charges allowed to run into the first cone hopper below. Refilling would begin at the top while the men were caring for the first charge in the lower hoppers. The process was thus continuous. The concrete was chuted directly into place from the bottom hopper. The record of output was 110 batches per 10-hour day. Wages of common labor were $1.50 per day. The labor cost per cubic yard of concrete in place was 35 cts.

In constructing the Cedar Grove reservoir at Newark, N. J., a Hains mixer made the following records of output:

Cu. yds.

Best output per 10-hour day 403 Average daily output for best month 302 Average daily output for whole job 225

The stone, sand and cement were all raised by bucket elevators to the top of the high wooden tower that supported the bins and mixer. There were 10 men operating the mixer so that (exclusive of power, interest and depreciation) the labor cost of mixing averaged only 7 cts. per cu.

yd.; during one month it was as low as 5 cts. per cu. yd. This does not include delivering the materials to the men at the mixer, nor does it include conveying the concrete away and placing it. The work was done by contract.

~OUTPUT OF MIXERS.~--With a good mixer the output depends upon the methods of conveying the materials to and from the mixer. Most makers of mixers publish capacities of their machines in batches or cubic yards output per hour; these figures may generally be taken as stating nearly the maximum output possible. Considering batch mixers, as being the type most commonly used, it may be a.s.sumed that where the work is well organized and no delay occurs in delivering the materials to the mixer that a batch every 2 minutes, or 300 batches in 10 hours, will be averaged, and there are a few records of a batch every 1 minutes.

To ill.u.s.trate to how great an extent the output of a mixer depends on the methods adopted in handling the materials to and from the mixer we compare two actual cases that came under the authors' observation. The mixers used were of the same size and make. In one case the stone was shoveled into the charging hopper by four men and the sand and cement were delivered in barrows by four other men; six men took the concrete away in wheelbarrows. The output of the mixer was one batch every 5 minutes, or 120 batches, or 60 cu. yds., in 10 hours. In the other case the sand and the stone were chuted directly into the charging hopper from overhead bins and the mixer discharged into one-batch buckets on cars. The output of the mixer was one batch every 2 minutes, or 300 batches in 10 hours. In the first case the capacity of the mixer was limited by the ability of a gang of workable size to get the raw materials to and the mixed concrete away from the mixer. In the second case the capacity was limited only by the amount of mixing deemed necessary.

While the necessity of rapid charging of a mixer to secure its best output is generally realized it is often forgotten that the rapidity of discharge is also a factor of importance. The size of the conveyor by which the concrete is removed affects the time of discharge. By timing a string of wheelbarrows in line the authors have found that it takes about 7 seconds to fill each barrow; as a rule slight delays will increase this time to 10 seconds. With a load of 1 cu. ft. per barrow it requires 13 barrow loads to take away a cu. yd. batch. This makes the time of discharging a batch 130 seconds, or say 2 minutes. The same mixer discharging into a batch size bucket will discharge in 15 to 20 seconds, saving at least 1 minutes in discharging each batch.

~MIXER EFFICIENCY.~--Various attempts have been made to rate the efficiency of concrete mixers. In all cases a percentage basis of comparison has been adopted; arbitrary values are a.s.signed to the several functions of a mixer, such as 40 per cent. for perfect mixing, 10 per cent. for time of mixing and 25 per cent. for control of water, the total being 100 per cent., and each mixer a.n.a.lyzed and given a rating according as it is considered to approach the full value of any function. Such percentage ratings are unscientific and misleading; they present definite figures for what are mere arbitrary determinations. The values a.s.signed to the several functions are purely arbitrary in the first place, and in the second place the decision as to how near those values any mixer approaches are matters of personal judgment.

_The most efficient mixer is the one that gives the maximum product of standard quality at the least cost for production._

This rule recognizes the fact that in practical construction different standards of quality are accepted for different kinds of work. No engineer demands, for example, the same quality of mixture for a pavement base that he does for a reinforced concrete girder. If mixer A turns out concrete of a quality suitable for pavement base cheaper than does mixer B, then it is the more efficient mixer for the purpose, even though mixer B will make the superior quality of concrete required for a reinforced girder while mixer A will not. This method of determining efficiency holds accurate for any standard of quality that may be demanded.

CHAPTER V.

METHODS AND COST OF DEPOSITING CONCRETE UNDER WATER AND OF SUBAQUEOUS GROUTING.

Mixed concrete if emptied loose and allowed to sink through water is destroyed; the cement paste is washed away and the sand and stone settle onto the bottom more or less segregated and practically without cementing value. In fact, if concrete is deposited with the utmost care in closed buckets and there is any current to speak of a considerable portion of cement is certain to wash out of the deposited ma.s.s. Even in almost still water some of the cement will rise to the surface and appear as a sort of milky sc.u.m, commonly called _laitance_. Placing concrete under water, therefore, involves the distinctive task of providing means to prevent the washing action of the water. It is also distinguished from work done in air by the fact that it cannot be compacted by ramming, but the main problem is that of preventing wash during and after placing.

~DEPOSITING IN CLOSED BUCKETS.~--Special buckets for depositing concrete under water are made by several manufacturers of concrete buckets. These buckets vary in detail but are all similar in having doors to close the concrete away from the water and, generally, in being bottom dumping.

The bucket shown by Fig. 26 was designed by Mr. John F. O'Rourke, and is built by the c.o.c.kburn Barrow & Machine Co., of Jersey City, N. J. This bucket was used in depositing the concrete for the City Island Bridge foundations described in Chapter XII and also in a number of other works. It consists of a nearly cubical sh.e.l.l of steel open at top and bottom, and having heavy timbers rivetted around the bottom edges. The open top has two flat flap doors. Two similar doors hinged about midway of the sides close to form a V-shaped hopper bottom inside the sh.e.l.l and serve when open, to close the openings in the sides of the sh.e.l.l. In loading the bucket the bottom doors are drawn inward and upward by the chains and held by a temporary key. The loaded bucket is then lifted by the bail and the key removed, since when suspended the pull on the bail holds the chains taut and the doors closed. As soon as the bucket rests on the bottom the pull of the concrete on the doors slides the bail down and the doors swing downward and back discharging the concrete. The timbers around the bottom edges keep the bucket from sinking into the deposited concrete, and the doors and sh.e.l.l exclude all water from the batch until it is finally in place.

[Ill.u.s.tration: Fig. 26.--O'Rourke Bucket fur Depositing Concrete Under Water.]

The subaqueous concrete bucket shown by Figs. 27 and 28 is made by the Cyclopean Iron Works Co., Jersey City, N. J. Fig. 27 shows the bucket suspended full ready for lowering; the cover is closed and latched and the bail is held vertical by the tag line catch A. Other points to be noted are the eccentric pivoting of the bail, the latch unlocking lever and roller B and C, and the stop D. In the position shown the bucket is lowered through the water and when at the proper depth just above bottom the tag line is given a sharp pull, uncatching the bail.

The body of the bucket turns bottom side up, revolving on the bail pivots, and just as the revolution is completed the bail engages the roller C on the latch unlocking lever and swings the lever enough to unlatch the top and allow it to swing down as shown by Fig. 28 and release the concrete. The stop D keeps the body of the bucket from swinging beyond the vertical in dumping.

[Ill.u.s.tration: Fig. 27.--Cyclopean Bucket for Depositing Concrete Under Water (Closed Position).]

[Ill.u.s.tration: Fig. 28.--Cyclopean Bucket for Depositing Concrete Under Water (Open Position).]

Figures 29 and 30 show the subaqueous concrete bucket made by the G. L.

Stuebner Iron Works, Long Island City, N. Y., essentially the same bucket, omitting the cover and with a peaked bail, is used for work in air. For subaqueous work the safety hooks A are lifted from the angles B and wired to the bail in the position shown by the dotted lines, and a tag line is attached to the handle bar C. The bucket being filled and the cover placed is lowered through the water to the bottom and then discharged by a pull on the tag line.

~DEPOSITING IN BAGS.~--Two methods of depositing concrete in bags are available to the engineer; one method is to employ a bag of heavy tight woven material, from which the concrete is emptied at the bottom, the bag serving like the buckets previously described simply as means of conveyance, and the other method is to use bags of paper or loose woven gunnysack which are left in the work, the idea being that the paper will soften or the cement will ooze out through the openings in the cloth sufficiently to bond the separate bagfuls into a practically solid ma.s.s.

[Ill.u.s.tration: Fig. 29.--Stuebner Bucket for Depositing Concrete Under Water (Closed Position).]

[Ill.u.s.tration: Fig. 30.--Stuebner Bucket for Depositing Concrete Under Water (Open Position).]

[Ill.u.s.tration: Fig. 31.--Bag for Depositing Concrete Under Water.]