Fig. 14.—1592 Showing result of leaky forms and poor placing. The soft cement mortar ran out, leaving areas of honeycombed surface not necessarily harmful but unsightly.
Fig. 14.—1592 Showing result of leaky forms and poor placing. The soft cement mortar ran out, leaving areas of honeycombed surface not necessarily harmful but unsightly.
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 inFigure 13. The object of the spading is to eliminate impounded air that may form pockets in the mass and to insure a smoother and moreimpervious 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 14shows the result of improper spading.
Fig. 15.—Method of forming horizontal rebate.
Fig. 15.—Method of forming horizontal rebate.
Fig. 16.—Tremie for use in placing concrete under water.
Fig. 16.—Tremie for use in placing concrete under water.
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 inFigure 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.
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.
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.
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 beforethe 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 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 11/3ounces 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.
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 sothat 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.
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.
Fig. 17.—Expansion joint showing rebate form removed and filler in place.
Fig. 17.—Expansion joint showing rebate form removed and filler in place.
Concrete expands and contracts with changes in temperature, causing cracks to appear. Contraction cracks occur in thin sections exposed to wide variations in temperature and are common in sidewalks; therefore, large stretches of concrete should not be laid without breaks or spaces to allow for the changes in size. The spaces should be filled with tar or some similar material that will yield or give when the concrete expands. A joint like that shown inFigure 17is frequently used for thick walls. A section of the wall is poured and before the next is poured the abutting end is covered with tar and paper, the thickness of the covering depending upon the length of the section and the exposure. Sidewalks and similar work, when not cast in alternate blocks, should have a one-fourth inch space left at intervals of 40 feet. The joint may be filled with tar paper or tar. Steel is used to take care of contraction in long or high walls and water-tight work. Important structures in which temperature reinforcement is necessary should be designed by one experienced in concrete design.
The subject of reinforced concrete is not within the province of this bulletin, but as openings of various widths are required in thewalls of most farm structures, a general explanation is given of the reinforcement of lintels or that portion of concrete immediately above an opening, such as a floor or window.
A lintel is a beam, and when a beam bends the lower part is stretched or pulled while the upper portion is compressed. Good concrete will stand great pressure but is not capable of resisting any great pulling or tensile stress. For this reason steel is used in the lower portion to take care of the tensile or pulling force.
It will be found generally satisfactory, where no heavy or concentrated load occurs over an opening and the span is not more than 4 feet, to place two rods three-eighths of an inch in diameter in the bottom of the lintel, so that there will be 1 inch of concrete below them. Two diagonal rods should be placed at each top corner of a window or door, as shown inFigure 18. When the opening is between 4 and 8 feet the rods should be bent up as shown inFigure 19and when between 8 and 12 feet, three one-half inch rods should be used, two of them being bent.
Barbed wire, old fencing, and scrap or rusty iron is not suitable for reinforcement. Loose rust should be cleaned off the rods and they should be free of grease and oil.
Fig. 18.—Reinforcement of openings less than 4 feet wide.Fig. 19.—Reinforcement of openings more than 4 feet wide.
Fig. 18.—Reinforcement of openings less than 4 feet wide.
Fig. 18.—Reinforcement of openings less than 4 feet wide.
Fig. 19.—Reinforcement of openings more than 4 feet wide.
Fig. 19.—Reinforcement of openings more than 4 feet wide.
Joints and imperfections in the forms are reproduced on the concrete surfaces. Patches of honeycomb and rough places are left where the mortar has run out of the forms or where the concrete has not been properly placed. Such imperfections do not necessarily affect the strength of the concrete, but they do detract from the appearance(seeFig. 14). Too of ten the finishing of the concrete work in even the more important farm buildings is neglected. With little extra trouble exposed surfaces can be given a finish which will add to the attractiveness and hence the value of the completed work. Rubbing off the form marks and pointing up depressions or holes greatly improves the appearance of the work. The rubbing may be done with a wooden float or hard-burned brick, using a little sand and water as an abrasive and a 1:2 mortar for pointing up. The surface can be worked best if the forms are removed within 24 hours or before the concrete has set too hard. After the concrete has hardened it may be necessary to use a carborundum block for rubbing.
A pleasing finish can be secured by scrubbing the surface with a stiff fiber or wire brush, using plenty of water to wash off the loosened particles. The work must be done while the surface is workable for if the concrete is too green or soft the aggregate will break out and if too hard the work can not be done effectively.
Artistic effects can be secured by picking or tooling the surface with a bush hammer, toothed chisel, or pick. For such treatment the concrete should be two or three weeks old to prevent breaking out the aggregate. Other finishes may be obtained by etching with acid to expose selected colored aggregates and by the application of stucco. The limitations of the bulletin do not permit of a discussion of these more elaborate treatments.
Concrete is practically fireproof in that it can not be consumed by fire, but unless properly made and of the right materials it will disintegrate, at least on the surface.
To resist fire concrete should be mixed fairly rich, say, 1:11/2:3, or 1:2:4 and special care should be taken to grade the sand and gravel to secure a dense mixture.
The aggregates should be selected with a view to their fire-resisting properties. The sand should be siliceous and the larger aggregate should not disintegrate when heated; hence, marble, granite, limestone, materials containing quartz, and some gravels are unsuitable. Cinders are specially valuable, due to their non-conductivity, but can not be used where strength is required. Trap rock will resist destruction by heat and produce a strong concrete. Blast furnace slag is very good for this purpose.
Fireplaces and chimneys of dwellings[2]may be constructed of ordinary concrete but the back, jambs, and inner hearth, which are directly exposed to the heat of the fire, should be made of specially prepared concrete as described above or should be lined with firebrick, although concrete made with broken hard-burned brick or terra cotta has been used successfully. If suitable large-sized aggregate is not available a mixture of one part cement and three parts sand may be used.
[2]See Farmers' Bulletin No. 1230, Chimneys and Fireplaces, U. S. Department of Agriculture.
[2]See Farmers' Bulletin No. 1230, Chimneys and Fireplaces, U. S. Department of Agriculture.
Practical water-tightness in concrete may be secured by using a fairly rich mixture properly proportioned. Foreign ingredients, membrane and surface coatings, or other means need not be used, except where poor workmanship is likely or where considerable damage and inconvenience may result in case of leakage. Under such circumstances the membrane treatment used in addition to a properly proportioned concrete, while the most expensive method of waterproofing, probably will give the most reliable results. This treatment consists of layers of burlap or tar paper cemented to the surface and together with tar or asphalt. Where the membrane is subject to injury it is sometimes protected by a coating of cement mortar or brick backing.
First-class workmanship and special attention to details are required to secure water-tightness. The essential requisite is that the voids be filled. A lean mixture may be made more impervious by using hydrated lime which tends to fill the voids and makes the concrete flow easily. A little more cement in the mixture would serve the same purpose. The lime should not be in excess of 10 per cent of the weight of the cement and under no circumstances should unslaked lime be used.
The materials for water-tight concrete must be well graded, so as to obtain a maximum density; that is, enough sand must be used to fill the spaces between the gravel or stone and enough cement to fill the spaces between the grains of sand. A 1:2:4 concrete will prove practically impermeable in ordinary construction, but if a head or pressure of water is to be resisted a 1:2:3 or richer mixture may be necessary. The consistency is very important. A sluggishly flowing consistency is best, for if the concrete is too wet the mortar may flow away from the stone, leaving leaky places and, if too dry, the mass may prove porous. The proportions and consistency must be accurately maintained for each batch and the concrete must be exceptionally well mixed.
It is necessary to exercise great care in the placing of the concrete. Where practicable, the structure or object should be poured in one operation to avoid leaky joints, but when this is not possible precautions should be taken to secure a tight joint between concreteof different ages. The surface of concrete which has set must be cleaned of dirt and scum down to the true concrete. This surface then should be well whetted and painted immediately with a creamy mixture of cement and water before placing the new concrete. A good plan, when discontinuing work on structures intended to hold liquids, is to embed a 6 or 8 inch strip of tin or thin sheet metal to half its width in the concrete so that the other half will project into the new concrete.
A wall thick enough to resist the stresses put upon it will generally resist percolation of water, but 6 inches may be considered as a minimum.
Contraction and expansion must be controlled to avoid the occurrence of leaks. To guard against cracks due to unequal settlement or other causes, most concrete designed for water-tightness should be reinforced. In some mass work, special contraction joints, as described on page 23 may be necessary. Rules for the use of reinforcement and contraction joints can not be given, as the requirements in each case vary with the conditions to be met.
October 21, 1929
This bulletin is a contribution from
U. S. GOVERNMENT PRINTING OFFICE: 1929
Transcriber NotesAll illustrations were moved so as to not split paragraphs.
Transcriber Notes
All illustrations were moved so as to not split paragraphs.