Part 3

CONTRACTION AND EXPANSION JOINTS.

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 in Figure 17 is 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.

LINTELS.

The subject of reinforced concrete is not within the province of this bulletin, but as openings of various widths are required in the walls 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 in Figure 18. When the opening is between 4 and 8 feet the rods should be bent up as shown in Figure 19 and 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.

SURFACE FINISH.

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 (see Fig. 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 EXPOSED TO FIRE.

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, 1:1-1/2:3, 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.

WATER-TIGHT CONCRETE.

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 concrete of 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.

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ORGANIZATION OF THE UNITED STATES DEPARTMENT OF AGRICULTURE

October 21, 1929

_Secretary of Agriculture_ Arthur M. Hyde. _Assistant Secretary_ R. W. Dunlap. _Director of Scientific Work_ A. F. Woods. _Director of Regulatory Work_ Walter G. Campbell. _Director of Extension Work_ C. W. Warburton. _Director of Personnel and Business W. W. Stockberger. Administration_ _Director of Information_ M. S. Eisenhower. _Solicitor_ R. W. Williams. _Weather Bureau_ Charles F. Marvin, _Chief_. _Bureau of Animal Industry_ John R. Mohler, _Chief_. _Bureau of Dairy Industry_ O. E. Reed, _Chief_. _Bureau of Plant Industry_ William A. Taylor, _Chief_. _Forest Service_ R. Y. Stuart, _Chief_. _Bureau of Chemistry and Soils_ H. G. Knight, _Chief_. _Bureau of Entomology_ C. L. Marlatt, _Chief_. _Bureau of Biological Survey_ Paul G. Redington, _Chief_. _Bureau of Public Roads_ Thomas H. MacDonald, _Chief_. _Bureau of Agricultural Economics_ Nils A. Olsen, _Chief_. _Bureau of Home Economics_ Louise Stanley, _Chief_. _Plant Quarantine and Control C. L. Marlatt, _Chief_. Administration_ _Grain Futures Administration_ J. W. T. Duvel, _Chief_. _Food, Drug, and Insecticide Walter G. Campbell, _Director of Administration_ Regulatory Work, in Charge_. _Office of Experiment Stations_ E. W. Allen, _Chief_. _Office of Cooperative Extension Work_ C. B. Smith, _Chief_. _Library_ Claribel R. Barnett, _Librarian_.

This bulletin is a contribution from

_Bureau of Public Roads_ Thomas H. MacDonald, _Chief_. _Division of Agricultural Engineering_ S. H. McCrory, _in Charge_.

U. S. GOVERNMENT PRINTING OFFICE: 1929

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Transcriber Notes

All illustrations were moved so as to not split paragraphs.