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So many factors enter into the making of good concrete, that a hard and fast rule applicable to all cases cannot be made, but in general it may be said 30 sec. or even 1 min. of mixing is inadequate. It is far better, when it is desired to do a thoroughly first-class job, to employ more mixers even at a higher first cost for equipment and work

FIG. 42. Charging hopper mounted on mixer frame.

them on a longer schedule, than it is to attempt with one mixer to get out concrete on a rapid-fire schedule. The latter method often brings a chain of unfortunate consequences, for not only is the concrete inadequately mixed and the cement insufficiently used, but also excess water is nearly always added in order to make the mass free-working and to diminish the labor of mixing.

16c. Loading the Mixer.-There are many time economies that may be effected in loading the charge of materials into the mixer. Various types of loading mechanism have been designed to meet different conditions of service and the time cycle of each is different. A study of each type will show its adaptability to particular needs.

Charging Hoppers.-Where a charging hopper mounted on the mixer frame can be used, as in Fig. 42, the limitation to charging time is dependent upon the design of this hopper,

upon the slope of its sides and upon the size of opening from hopper to drum. Inasmuch as this type of charging device is usually loaded by gravity from superposed measuring hoppers, like considerations must be taken into account in their design; and always there must be promptitude in releasing of gates, etc. In some very large operations, pneumatic opening devices have been installed with an interlocking system, so that a sequence of operations is carried out with almost perfect regularity and great efficiency.

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Power Loaders. Side

loaders or power loaders are often attached to mixers in order to give the advantages of low loading, as well as those of relatively high discharge of mixed materials. The general type of mechanism employed is shown in Fig. 43. The type of loading hopper or skip varies with different manufacturers, some hoppers having a raised back, requiring a slight incline for wheelbarrows that must be dumped into the hopper, while others permit running wheelbarrows directly on to the hopper back itself. Through a friction clutch, the power loader is elevated by the same motive power which drives the mixer drum. Inasmuch as it is required to hoist such loading skips to a considerable height before materials will run from them into the mixer drum, it is essential that sufficient power be provided to hoist this skip rapidly, as otherwise an undue amount of time will be consumed in this elevating operation. The mixers of different manufac

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ture vary widely as to speed of hoisting; and it will generally be found that the more expensive mixers have a better and more rapid hoisting mechanism, in addition to their other economies, than have the cheaper types of mixing machines.

Low-charging Mixers.-Low-charging mixers (see Fig. 38), particularly in smaller units, have of recent years been meeting with favor. In such mixers the opening at the charging end is relatively larger than in other types of

drum mixers and blading about this opening on the interior of the drum is so disposed as to draw the materials within the drum from a relatively small hopper of low height into which they are charged by wheelbarrows. With such mixers an inclined runway platform of 2% to 3 ft. in height is required. Their advantages, therefore, consist in a simplification of charging and the absence of hoisting mechanisms rather than in any particular efficiency of mixing operation. Furthermore, these machines are relatively low in price and a number of small units, gasoline or electric motor-driven, are often very advantageous when distributed about the work. From a standpoint of thorough mixing and flexibility of operation, there is much to recommend this practice, inasmuch as the needs of one part of the work can be supplied without reference to other parts or causing an overdraft on any one machine with consequent speeding up of operrations as is the case when all parts of the work are demanding concrete at the same time from a single, centralized plant.

16d. Measuring Materials. It is often taken for granted that measurement of materials for a concrete batch is of little or no importance and that it can be accomplished in almost any way. It is probable that the average mix varies at least 50% in its proportions from those desired, and for this reason alone it is not to be wondered that much concrete found on every hand is so variable in quality.

Materials should be measured either in bottomless boxes placed on wheelbarrows, or like devices, or else in a barrow pan permitting of struck measurement. A measuring barrow of known capacity permitting struck measurement is shown in Fig. 44. At the same time these barrows are adapted by reason of their balance, to the conveyance of considerable quantities of material at one time. Measuring hoppers of known capacity, if carefully filled, can be made to function quite accurately; but where they are not struck, or where there is pronounced variation in the moisture content of the sand, the quantities of materials obtained per batch will be found surprisingly variable.

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17. Transporting and Placing Concrete.-Providing means for transporting mixed concrete and for placing it properly in forms, both in first cost and in ultimate effect, rank equal in importance

FIG. 44.-Measuring barrow.

with the operations of proportioning, and of mixing raw materials. In mixed concrete, not only are the raw materials to be handled and oftentimes conveyed to considerable distances, but in addition this must be done at low unit cost and in such a manner and so expeditiously as to protect the mixed mass from injury..

The means usually adopted for the conveyance and placing of concrete are some sort of bucket or else open spouts or chutes through which the concrete flows by gravity after being hoisted from the mixer in a tower, or else in barrows or carts. The particular means adopted in any case, will depend upon the size of the operation, upon the physical conditions attendant, and upon the financial limitations to plant imposed by commercial consideration.

17a. Barrows. As affecting perhaps the great bulk of concrete used today, it will be proper to first consider the use of barrows. This method involves less original plant outlay than the others before enumerated. In many instances, the cost of installation of an elaborate plant would cover not only the cost of the barrows themselves, but a great part of the entire cost of distribution of the concrete by other means.

The ordinary wheelbarrow having a flat pan is not well adapted to the distribution of concrete. In such a barrow a man can handle about 12 to 2 cu. ft. of mixed concrete. This load he can wheel about 25 ft. every 3 min., the objection to the pan wheelbarrow being that the man's working rate is necessarily cut down by the care which is required to keep the materials from slopping over the sides. Furthermore by the design of the barrow a large proportion of the weight of the load is on the man's arms, rather than on the wheel. Deep pan barrows have been designed to overcome this difficulty, but have not wholly accomplished the desired end.

176. Concrete Carts.-Two-wheel concrete carts (Fig. 45) are better adapted to this work than wheelbarrows, both because they can carry a larger load and also because this

The usual two

load is balanced on the wheels themselves with little or no strain on the man. wheel concrete car is of 6-cu. ft. capacity in which about 41⁄2 cu. ft. of mixed concrete can be carried by one man.

In this comparison there are, however, certain cost offsets to be made. Wheelbarrows require less scaffolding than do the heavier and wider carts, so that the cost of this runway must be carefully estimated. When runways must be elevated, the showing becomes more favorable for carts, as bents or supports for wheelbarrows must be practically of the same size and strength as those for carts. Turnouts and gangways must in both cases be of ample width so that there may not be congestion in the passing of full and empty carts going to and returning from the forms.

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FIG. 45.-Concrete buggy.

tion of distribution in carts.

17c. Buckets.-There is a great variety in types of buckets adapted to the distribution of concrete. Some of these buckets are straight-side skips, as in Fig. 10, p. 842 adapted to dump by overturning. Others are bottom dumping buckets operated by a man at the form; and these bottom dumping buckets may be of various patterns, adapted to some particular use. An example of this sort of bucket is shown in Fig. 11, p. 842, in which the bottom is so constructed as to form a long narrow opening, actuated through a powerful lever mechanism. A great variety of these devices is on the market and the needs of each particular situation must be studied and met by as specialized a product for that use as financial considerations will permit.

17d. Spouts or Chutes.-The handling of concrete through spouts or chutes at the present time is in more extensive use than any of the foregoing methods of distribution, with the possible excepThe economic features of spouting are attractive. To raise concrete vertically in a tower by means of a skip bucket and engine located at the central mixer plant, then distributing by gravity through channels which can be arranged in convenient sections to cover any area with a radius from 10 to 300 ft. from the base of the tower, appeals strongly both to engineering and to business sense. Further, the ease of handling by gravity is usually greater and the time cost per cubic yard for placing is usually less than in transferring the same quantity of material in hand-barrows, in cableway buckets, or in cars. Yet in spite of its many good points, the convenience of spouting has brought about many abuses.

In all spouting installations, care must be taken to have the chutes at a working inclination. Furthermore, it is important to maintain a uniform pitch throughout the entire line, in order that the flow may be thorough and uninterrupted and not subject to slackening at one part and accelerated flow in another. The pitch also must be greater when the material is to be carried to a considerable distance than when it is to be carried only a short distance, for as the distance increases, the friction of the concrete in a chute tends to overcome its initial momentum. Whereas, therefore, a wet concrete will flow 50 ft. with the pitch of 1 in 6 it becomes necessary to increase this pitch to 1 in 4 for a distributing distance of 100 ft., while a distance of 300 or 400 ft. will require a pitch of 1 in 3. The slopes as above described are based upon chute rigidly supported having uniform pitch throughout; and it would be even better to increase this pitch in order that concretes of a drier consistency may be used.

17e. Sections Used in Spouting. It is desirable that concrete spouting be arranged in a series of units which may be assembled in various combinations. Continuousline spouting should be changeable to swivel-head, or swivel-head to continuous-line, as the conditions of the work require, it being necessary, of course, to have in stock a supply of the necessary units. This interchangeability is of great value in service, for spouts wear at the head and foot of each unit of length. By reversing a trough section, end for end, when showing heavy wear at one point, a new, unworn surface may be put at point of greatest wear.

A standard trough section is made of No. 14 gage steel, forming a trough 81⁄2 in. deep by 10 in. wide on top. The bottom is curved to practically a semicircle of 4-in. radius, the upper

part of the sides being straight and tangent to the curve. Each section is punched with standard spacing, arranged for connecting all of the various attachments.

The hopper head attached at one end for receiving the concrete from the bin, or from an upper trough section, forms one point of support of the next trough section. At the other end is the splash hood. By fastening the hopper head to the trough section at one end, and the splash hood at the other, we have the complete trough section, Fig. 46. These 24 X 24-in. hopper heads, as well as the splash hoods, can be bolted to either end of any standard trough section.

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Standard trough sections are joined for continuous-line spouting by bolting together their angle-iron yokes or flanges and bolting on the compression plate part. Thus, several sections are joined together, with a hopper head at one end of the entire group, and a splash hood at the other end.

Fig. 47 shows the swivel-hook used in making the flexible joint between successive trough sections for swivelhead spouting and shows one of these joints, in which the upper line of spouting is supported by a fall and tackle attached to the bail on the splash hood; while the lower line is supported by the swivel-hook, connecting the lower hopper head with the splash hood of the upper line. The swivel-hook is kept clear of the path of the concrete.

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In some cases it is desirable to have a flexible joint in continuous-line spouting. In this case the two sections are put together in a different manner, Fig. 48, where both the hopper head and the splash hood are dispensed with. The hanger plate is here used in conjunction with a special yoke, after one of the angle-iron yokes has been removed. This allows a slight movement sideways, without requiring the attachments for the swivel-head operation. Various types of spouting have been tried, ranging from round pipe to rectangular troughs. Best results have been secured from the use of 5-in. pipes, or 10-in. open troughs, the latter having the preference for flat slopes, and the former where there is necessity for varying pitch, with a likelihood of steeper pitch than named above.

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