Part 2

Wood for forms must be of a kind that is easily worked and that will retain its shape when exposed to the weather. White pine is the best wood, but is seldom used because of its cost. Spruce, yellow pine, and fir are satisfactory woods for forms and are best, used partially green or unseasoned.

The edges of boards should be surfaced, tongued and, grooved, or beveled in order to obtain a tight form, so that the soft mortar will not ooze out. A better surface* is secured if the boards are dressed on one side and are free of loose knots or other imperfections.

As forms must be removed, they should be so planned that they can be taken down without destroying the lumber, especially if the boards are used for sheathing or again for forms. Therefore the nailing of the boards to the support should be only sufficient to keep them in place until the concrete has hardened. Greasing the surface next to the concrete with crude oil, soap solution, or linseed oil will prevent the concrete from adhering and facilitate removal.

METAL FORMS.

Metal forms can be used to advantage when the work involved is to be repeated many times. If it is known or if it is probable that the forms may have to be altered, the relative costs of wood and metal forms should be carefully determined.

Metal forms of various types and designs may be purchased. Although the first cost may be high, yet their use may lower the total cost when the work is such as to warrant it.

Circular forms may be built as shown in Figure 3. The sheathing is generally of wood 4 to 6 inches wide, or sheet metal, and, if of wood, is laid perpendicular to the battens. In forms of small diameter, sheet metal sheathing is necessary if a smooth surface is desired, as the 4-inch boards can not be made to conform to a true circle. The radius used for cutting the battens of the inner circle should be the thickness of the sheathing less than the inside radius of the structure and the same amount greater than the outer radius for the outside battens.

REMOVAL OF FORMS.

The period of time after which forms may be removed varies according to conditions. Rich and dry mixtures set quickly, and warm weather tends to hasten the setting of concrete. The character of the structural member and the loadings also must be considered.

Thus, an unloaded wall 12 inches or more thick may be stripped of forms in from 1 to 3 days, while the forms of thinner walls should remain in place from 2 to 5 days. Slab forms and the sides of beam and girder forms may be removed in from 6 to 14 days if the span is not over 7 feet. The bottoms of beam and girder forms, even though of a span less than 7 feet, should remain in place and braced form 10 to 14 days and even longer. Experience is the best guide to the time of removal, but if there is any doubt ample time should be allowed, especially in cold weather.

BUILDING AND SETTING FORMS.

Concrete, while plastic, exerts a great pressure on the confining walls, necessitating rigid tying and bracing of the forms to keep them from bulging out of alignment. The effect of the bulging of a form is corrected only at a considerable expense; hence it is advisable to pour the concrete to a depth of not more than 2-1/2 or 3 feet, allowing it to set or harden before pouring more.

The form most used in concrete construction is that for a straight wall. The methods of building such a form apply in general to the forms for most structural work, though modifications may be necessary to meet particular conditions.

The straight wall form may be built continuous (Figs. 4 and 5), or in panels of a size convenient to handle, and from stock lengths of lumber (Fig. 6). Generally the face boards are placed horizontally and secured to studs or posts. The face boards may be 1 or 2 inches thick and from 6 to 10 inches wide, preference being given to the narrower widths, which are less liable to cup or warp. The thickness depends upon the spacing of the studs, the number of times the forms are to be used, and the depth of pouring. Ordinary sheathing, if the joints are made tight, is satisfactory for foundations of dwellings, etc., and the studs, if 2 by 4 inches, should be spaced 18 inches on centers. The studs for a long, high form had best be 4 by 4 inches or 2 by 6 inches, spaced from 2 to 3 feet center to center. The studs of the inside and outside forms must be tied together to prevent spreading; this is conveniently done with No. 10 wire, as shown in Figure 4, or with one-half or three-quarter inch bolts, which is the more expensive method. Bolts should be greased to facilitate removal. Temporary spacers of wood, 1 by 2 inches, of a length equal to the thickness of the wall, should be used to prevent drawing the forms together when the wire or bolt is tightened. They should be spaced at the ties, but need not be at every wire, and are knocked out and removed as the concreting progresses.

The ties should be spaced on each stud about 2-1/2 feet vertically. If more than 3 feet of concrete is poured at one time the ties should be closer together, vertically, at the bottom of each pouring. The thickness of the wall does not affect the number of ties. On removing the forms the wires should be clipped close to the face of the concrete and punched back, unless the surface is to be stuccoed. If a pit hole is caused by punching back the wire it should be pointed up with mortar, which then should be rubbed to make it blend with the general surface.

MIXING.

PREPARATION OF PLANT.

Before starting to mix, annoyance and money may be saved by planning the location of the mixing plant with regard to convenience in depositing the concrete in the forms and ease of access to the materials. Often the board can be located so that by moving it once or twice the bulk of the concrete may be shoveled directly into the forms. It is more economical to wheel material a distance of from 10 to 25 feet than to carry it in shovels. Eight feet is about as far as it is profitable to shovel.

When material is to be wheeled, runways of planks should be provided, because more material can be handled in a given time, and the wear and tear on men and equipment is not so great. The planks used in the runways should be thick enough to sustain the weight passing over them and should be 10 to 12 inches wide to permit foot room. They should be anchored securely and made rigid, as springy or loose boards retard progress of the work. Smooth joints in the planking will prevent bumping and stumbling.

NUMBER OF MEN.

The number of men required is determined by the amount of concrete to be placed in a given time, the method of mixing, and the size of the batch; that is, the number of bags of cement mixed at one time. The amount of concrete one man can mix by hand in a day depends upon the experience of the man, the layout of the work, and other duties required of him. One man should average 1-1/2 to 1-3/4 cubic yards of concrete in eight hours, including mixing and wheeling not more than 50 feet.

The gang for a one-bag batch may consist of 3 men, but a larger number make a more efficient force, for when the concrete is mixed by hand the men can take turns at the various tasks and will not tire so easily. The assigning of tasks so that each man's time fits into that of the others requires considerable study and is one of the chief factors making for loss or profit.

MACHINE MIXING.

Good concrete can be mixed by hand or machine. The quantity of concrete work in prospect is the factor that determines the more economical method. A small amount (say 100 to 200 cubic yards) does not warrant the purchase of a machine, but it is often feasible and economical to hire a machine from a neighbor or contractor if the quantity of concrete to be placed is more than 15 cubic yards.

A mixer should be purchased only after careful consideration of the amount and character of the work to be done and the conditions affecting its use.

The two types of mixers most used are the batch mixer, which mixes and dumps a definite quantity, and the continuous, which discharges a constant stream of concrete. The continuous type is not adapted to farm work unless the concrete can be handled as fast as it is mixed, thus permitting the machine to work continuously.

There are numerous types and various sizes of batch mixers. A one-bag batch machine is most suitable for general work, though there are smaller mixers that may prove handy. Some of the smallest sizes are operated by hand, but the medium and large sizes are power operated. Mixers can be had with or without the power plant attached and may be stationary or on wheels, which facilitate moving to different sites. Engines used for sawing wood, the larger ones used for pumping water, and tractors furnish sufficient power to operate an average mixer. Figure 7 shows a homemade mixer built of discarded farm implement parts and operated by the farm engine.

Directions for operating a mixer are generally furnished with the machine. The tendency is to use too much water in mixing concrete in a machine. The consistency of the mixture should be as described under the heading "Consistency" on page 8. The mixing should be continued for at least a minute after the drum has been charged, but a better mixture is secured if two minutes are allowed. At the end of each day's work the machine should be thoroughly washed, and when not in use it should be well greased and covered.

HAND MIXING.

Hand mixing is the more economical on the farm unless a large amount of work is to be done at one time. Few tools need be purchased, and, as a rule, only farm help need be employed. The following tools will be needed in mixing and placing plain concrete: Two or more square-end short-handled shovels, 1 heavy garden rake, 1 sprinkling can or bucket (if a hose is not available), 1 52-gallon barrel, 2 wheelbarrows with metal trays, 1 sand screen (Fig. 8), 1 tamper (Fig. 9), 1 wood float or trowel (Fig. 10), measuring boxes (Fig. 11), mixing board (Fig. 12), 1 spader (Fig. 13). The number of shovels and wheelbarrows needed will depend upon the size of the batch, number of men mixing, and the layout of the work. Long-handled pointed shovels will be found more convenient at the sand and gravel piles.

A bottomless box is necessary for convenient and accurate measurement of the sand and gravel. Where wheelbarrow measurement of materials is practiced, as in charging a mixer, the capacity of the wheelbarrow should be determined by use of a measuring box. The box may be made as illustrated in Figure 11, from boards 12 inches wide. The dimensions in Table 2 are of boxes for use in measuring quantities for mixtures of various proportions, assuming that one bag of cement is used in a batch. If two bags are used in a batch the boxes should be filled twice.

Table 2.--_Inside dimensions of measuring boxes for various proportions._

[1-bag batch, box 12 inches deep.]

Proportion. Box for sand. Box for gravel. ----------- --------------- ---------------- _Feet._ _Feet._ 1:1:2 1 by 1 1 by 2 1:1-1/2:3 1 by 1-1/2 1 by 3 1:2:4 1 by 2 2 by 2 1:2-1/2:5 1-1/4 by 2 2 by 2-1/2 1:3:5 1-1/2 by 2 2 by 2-1/2 1:3:6 1-1/2 by 2 2 by 3

A tight platform should be provided similar to that illustrated in Figure 12 upon which to mix the concrete. For mixing 1 or 2 bag batches a platform 9 by 10 feet will serve.

DIRECTIONS FOR HAND MIXING.

The mixing board should be located in convenient relation to the supply of materials and the work and should be level. The sand box is placed on the board, about 2 feet from one of the longer sides, and filled level with sand; the box is then lifted away and the sand spread in a 3 or 4 inch layer. The cement is spread as evenly as possible on' top of the sand. Two men with shovels, standing on opposite sides of the pile, turn the sand and cement in such a way that the materials axe thoroughly mixed. In turning the material it should not be simply dumped off the shovel, but should be shaken off the ends and sides, so that the two constituents will be mixed as they fall. The mass should be turned two or three times, or until it is of uniform color and there are no streaks of either sand or cement. A man with a hoe or rake may assist by raking the top over as the two men turn. When the sand-cement mixture is of a uniform color it should be spread out carefully in a layer and the gravel box placed on top. The box is filled with gravel and then removed, the gravel being spread over the sand-cement mixture. The mass is soaked with about one-half the quantity of water to be used, care being taken not to wash away any of the cement. The materials then should be turned over in much the same manner as was the sand-cement, except that instead of shaking them off the end of the shovel the whole load should be dumped and dragged back toward the mixer with the square end of the shovel. The wet gravel picks up the sand and cement as it rolls over when dragged back. The mixing should be continued until the mass is uniform, water being added to the dry spots during the mixing until the desired consistency is obtained.

Experience counts considerably in mixing concrete with the least amount of labor; ordinarily three or four turnings are required to mix the materials thoroughly. After the final turning the concrete should be shoveled into a compact pile and then is ready for placing in the forms.

PLACING.

PLACING CONCRETE.

The mixed concrete should be deposited in the forms within from 20 to 30 minutes from the time the water is added to the cement, as it begins to set or harden after this time. To disturb the concrete after the set has begun is risky, as it will lose some of its strength, the extent of the injury depending upon the seriousness of the disturbance.

Concrete which has set before it can be placed in the forms should not be tempered or softened with water, but should be discarded.

To prevent delay in placing, all forms should be examined before the mixing is begun to see that they are properly braced, that all chips or loose particles are removed, that the surface of concrete which has set has been properly roughened and wetted to assure a bond, as described on page 20, and that all reinforcement, bolts, inserts, etc., are properly located and secured.

At the lunch' period, or at the end of a day's work, the mixing board and equipment should be thoroughly washed, for if this is not done many pounds of heavy concrete are needlessly carried around by the men and the addition of a pound in the weight of tools will lower the efficiency of the workers. Moreover, it will save time and wear and tear of equipment incident to cutting the surplus concrete away with a cold chisel.

In depositing concrete in the forms care should be taken that the materials do not separate.

If the mixing is done close to the place of depositing, the concrete may be shoveled into the forms directly or through a chute. If it is necessary to lift or transport the concrete, buckets and wheelbarrows are convenient containers. The concrete should be deposited in horizontal layers, preferably not over 6 inches thick, and a spade or paddle should be worked up and down against the forms to push the coarse material away from the surface, as illustrated in Figure 13. The object of the spading is to eliminate impounded air that may form pockets in the mass and to insure a smoother and more impervious surface. In addition to being spaded, stiff concrete should be rammed until water flushes to the surface. Tapping the forms with a hammer is a very effective way of securing a smooth surface. Figure 14 shows the result of improper spading.

Fresh concrete will riot bond readily to concrete that has hardened and a seam may be formed that will permit water to trickle through. When bonding fresh concrete to that which has been in place for a short time it is usually sufficient to roughen the hardened surface with a pick or by other means so as to expose the gravel or stone, and to clean off all loose particles. The hardened concrete should be soaked with water, the excess water removed, and the surface then given a coat of grout (a mixture of cement and water) of the consistency of cream just before the new concrete is deposited.

When pouring of a wall is to be discontinued for some time, provision for the bonding of future work should be made. This may be done by placing short steel dowels in the concrete when it is poured, or a rebated joint or groove may be made, as shown in Figure 15. In bonding a new wall to old concrete, holes should be drilled for the dowels, which should be grouted in, and the old surface should be roughened, cleaned, and wetted; or a groove may be cut in the old wall to receive the new concrete.

PLACING UNDER WATER.

Concrete can be placed under still water if proper precautions are taken. It should never be placed, while soft, in running water unless a form or cofferdam is used, as the cement will be washed out. When concrete is to be placed under water a form of tube or chute, known as a tremie (Fig. 16), may be used advantageously. The tube should be of sheet metal, about 8 inches in diameter, with a hopper on top, and means should be provided for quickly raising and lowering it without jolts, so that the concrete will feed out at the bottom without breaking the seal. The lower end of the tube should rest on the bottom or on the concrete as it is built up and a continuous flow of concrete, mixed somewhat soft so that it will flow easily, should be maintained.

Scum or laitance is likely to form on concrete when placed under water, and unless all of the concrete is! poured in one operation and brought to a little above the water surface, seams or planes of weakness will occur.

CARE OF CONCRETE.

After the concrete has been poured, care should be taken that it does not dry out too quickly, and in hot weather it must be protected from the sun. Exposed surfaces and objects made of dry concrete should be sprayed thoroughly with water twice or oftener each day for a week or 10 days. Sometimes surfaces are shielded with canvas, paper, boards, or layers of moist sand.

PROTECTION FROM FREEZING WEATHER.

CONCRETING IN FREEZING WEATHER.

If suitable methods are used, good concrete work can be done in cold weather, but with more difficulty and at somewhat greater cost than when the weather is warm. Ordinarily it is best not to attempt to do concrete work during freezing weather. However, the extra cost at times may be warranted by urgent need of the structure or the fact that other farm work is not so pressing during the winter and the concrete work may be carried on without seriously interfering with regular farm operations.

Concrete must be protected from alternate freezing and thawing until it has set. Cold retards the setting and hardening of concrete; therefore, even though the temperature is not at the freezing point, the concrete should be protected and special care taken not to subject it to loads. The forms should be kept in place until there is no doubt that the concrete has properly hardened. Hot water should be poured on the concrete to make sure that apparent hardness is real and not due to a frozen condition. Just before the concrete is placed all ice and frost should be removed from the forms and reinforcement, if used, by warming the surfaces with steam or by other means.

Concrete that has been frozen once may, with proper care, attain its ultimate strength, but should it freeze a second time the chances of saving the work are very slight. Exposed surfaces are apt to scale or pit if the concrete is allowed to freeze before it is thoroughly hardened.

Pleating the materials, protecting the green concrete, and the use of salt are precautions generally taken to prevent freezing.

THE USE OF SALT.

The use of salt is objectionable, as it forms a white efflorescence on exterior surfaces and is liable to corrode the steel in reinforced concrete work. The quantity of salt required varies with the temperature, but it should not exceed 10 per cent of the weight of the water used in mixing. A 10 per cent solution is eight-tenths (approximately 13 ounces) of a pound of salt per gallon of water and will prevent freezing at a temperature of 22° F. Lower temperatures would require a greater proportion of salt, which would impair the strength of the concrete, and hence is not practicable.

A rule, frequently advocated, for varying the percentage of salt is to use 1-1/3 ounces per gallon of water for each degree Fahrenheit below freezing. Since it is impossible to foretell the exact drop in temperature, the exact quantity of salt can not be predetermined, so that provision should be made for several degrees lower than anticipated. The salt should be dissolved in the mixing water, and in order that the proportion be correct the amount of water required for each batch should be determined by trial and this quantity used throughout the work.

THE USE OF HEAT.

Perhaps the most satisfactory method of preventing freezing of concrete is to heat the materials and to inclose or cover the completed work for a few days or until most of the water has disappeared and sufficient strength has developed. In extreme weather protection may be needed for five or six days. When the weather is cold but not freezing, heating the materials will be sufficient. If a freeze is expected the concrete work should be protected by wood inclosures, paper, or canvas, over which, if the surface is horizontal, may be spread a 6 or 8 inch layer of straw. Manure should not be used to protect fresh concrete, since the acids in it are destructive and cause unsightly stains. Splits or other openings in coverings may admit cold, which may freeze parts of the work. As the temperature drops (to about 20° F.) it will be necessary to arrange the covering so that live steam can be turned in between it and the concrete or that heat may be supplied from stoves or salamanders.

Mass work, except in very cold weather, will not require as careful protection as thin sections and, as a rule, the forms are sufficient if the exposed parts are covered.

HEATING MATERIALS.

The water can be heated sufficiently for use in concrete (approximately 150° F.) in kettles on stoves or by steam from a boiler.

A metal smokestack placed horizontally with a fire in one end makes an efficient heater for the sand and gravel. The materials are piled over the stack, but not so high that their weight will crush the pipe. Small quantities of sand and gravel may be heated on top of metal plate with a fire under it. If a small boiler is available it may be economical to use steam for heating the sand and gravel. Steam is effective when forced from nozzles into the piles or circulated through perforated pipes placed under the material. Covering the piles with canvas or other material will retain much of the heat.