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Forms.-Form work as applied to buildings will be treated in detail in Part II, Chapter XIX. Since the method of cutting walls, core drilling holes and constructing forms and their removal are very much the same for different types of structures, very little needs to be said under this heading.
Forms should be substantial and unyielding, and built so that the concrete will conform to the dimensions shown on the designer's plans, and they should also be tight so as to prevent the leakage of mortar. Forms may be either continuous or sectional, or a combination of both, depending upon the economy of the work. The concrete in any given section should be allowed to harden for 36 hours before the forms are removed and, in freezing weather, extra care must be taken to make sure that the concrete has had sufficient time to become thoroughly set" Material once used for forms should be cleaned before being used again.
The lumber used for concrete forms should have a nominal thickness of at least it in. before surfacing and should be of a good quality of Douglas fir or Southern long-leaf yellow pine. The lumber for face work should be dressed on one side and on both edges to a uniform thickness and width. The lumber for backing and other rough work may be un-surfaced and of an inferior grade of
Design of a cantilever retaining wall to be 18 feet high above the ground and to support an earth bank whose surface has an upward It to 1 slope from the top of wall. The face of the wall is to be placed on a property and the same weights for the earth and concrete as in the designs given for illustration J and take 3 short tons per square foot as the safe bearing power of the soil~ The coefficient of friction of the concrete upon the earth foundation is 0040. Assume plain round rods of medium steel. Employ the working stresses recommended by the Joint Committee for a 2000-lb. concrete and design by the equivalent fluid-pressure method, using 25 lb. per cubic foot as the equivalent fluid weight. Take a length of base which will bring the resultant pressure within the middle third of the base and assume the thickness of the footing at 30 in. Use tables of Vol. 1. When finding the pressure on the vertical plane passing through the rear end of concrete footing, treat the wall (for this consideration only) as acted upon by a surcharge (instead of the sloping backfill) equal to from! To the actual depth of earth above the top of wall at this plane is more than or less than 6 feet. Design the wall of the preceding problem using Rankin's formula for earth pressure. Take the angle of internal friction of the earth filling at 45°.
Design a steel reinforced retaining wall to be 30 ft. high above the ground and to support a level bank of earth. The wall is also to sustain an additional 5-ft. surcharge. Place the vertical slab so as to give a minimum amount of material in the wall. Space the center of the huge hand hued beam only 8 ft. at center. All other data is to be taken the same as in Problem 2 concerning the home improvement.
This treatise will deal principally with reinforced-concrete buildings of the skeleton type of construction; in other words, it will be assumed in the discussions which follow (unless otherwise stated) that the student has in mind the usual type of concrete building (Fig. 41), in which there is an exterior as well as an interior frame of reinforced concrete and in which the walls (if any) are simply light curtain or panel walls of either concrete brick, built at some convenient time after the adjacent frame or work is completed. Outside bearing walls with few openings are sometimes employed in reinforced-concrete construction, but it is in the former type of structure that the present-day builder
is particularly interested) because it is the easier to build. and the more economical. Sometimes only the basement walls of buildings are of the bearing type. In a common form of such construction, described in Art. 53, the basement wall between any two consecutive piers is made sufficiently deep to act as a beam and to distribute the load over the entire bay from column to column. The skeleton type of construction is not always apparent in completed buildings (Fig. 42), especially at first glance, since, from an architectural standpoint, it sometimes becomes necessary to cover the exterior columns and walls with brick, terra-cotta, marble, limestone, or other material. The methods of tying a brick or stone facing to the concrete will receive attentionˇ in Art. 55
Reinforced-concrete slabs are quite often employed in the floors and roofs of steel-frame buildings. The different ways of placing the concrete slab with reference to the steel frame and the manner of covering the beams and girders for fire protection will be described in Art. 20. At the frame of reinforced concrete with the ordinary brick bearing wall structures which are common to every community. The methods of design given in this text will apply to every part of this type of construction except the simple details of the brick walls.
General Types of Concrete Cutting -There are four general types of cutting of reinforced-concrete floors: (1) monolithic beam and girder construction, (2) flat slab construction, (3) unit construction, and (4) steel frame construction with concrete slabs. In the fourth type mentioned, the beams and girders are usually covered with concrete for fire protection.
Monolithic Beam and Girder Construction.-The monolithic beam and girder floors may be divided into two groups the first including solid concrete floors, and the second what is known as terra-cotta hollow-tile floors. The arrangement of beams and girders in the solid type has been considered in Art. 58 of Volume I, and much has been said of this type as to methods of design. This is by far the older of the two types, but the hollow tile is now used quite extensively for light buildings such as modern store buildings and office structures.
Fig. 43 shows a typical one-way hollow-tile slab and Fig. 44 a two-way tile construction. No cross-beams are employed in the one-way type except the small ribs of the floor slab formed between the rows of hollow tile. In the two-way type, crossbeams are placed at the columns. The tiles are placed directly upon the forms with the reinforcing rods in the spaces between them, and the concrete is filled in between the tiles and poured over the top to form the floor. The ribs form a series 01 comparatively light T-beams side by side with flanges usually two or more inches in thickness. The main beams or girders are also of T-shape. The flanges of these beams or girders are usually of the same thickness as the floor slab, but lighter tiles are sometimes used near the stem, in which case the flange becomes thinner than when the tiles are entirely omitted at this part of the floor. The function of the tiles is simply to create a void in the concrete and thus to decrease the dead weight of slab, and they do not enter into the calculations for strength of floor.
Either hard-burned or semi-porous tile may be used in reinforced-concrete floor construction. Hard-burned tile, due to its density, has a higher crushing strength and will, therefore, undergo a greater stress without any sign of failure, but it does not seem to be as good a fire-resisting material as the semi-porous.
The commercial sizes of tiles are usually 12 in. by 12 in. in plan and vary in depth from 4 in. to 16 in. The depth of a tile concrete floor should be designed so as to allow for these commercial sizes. The standard sizes manufactured by the Tiles are likely to vary t in. from the dimensions specified so that the plans should show the full thickness of the floor and the minimum amount of concrete topping. If the tiles are small, due to shrinkage in burning, the thickness of floor should be made up in concrete.
Unless the tile is thoroughly sprinkled before the floor is poured, slight depressions will pour over the ribs. This is because the hollow tile absorbs the moisture in the concrete of the top coat, causing it to set more quickly than the rib with its greater body of concrete and greater shrinkage. Sprinkling of the tile should be insisted upon, especially in hot weather.
Hollow-tile floors are generally plastered on' the underside as it is only in the roughest kind of work that this is not done. The surface of the tiles should be deeply scored so that the plaster will bind firmly constitute the beams and cross-braces of the floor.
(( Other systems approach the practical value of this one in some degree, as for instance that one where hollow tiles are used in. place at the third annual convention of the Engineering Society of repeated use of a reasonable number of cells, the cost of the tile would probably exceed the proportionate cost of wood cells. The tiles serve
suggests the arrangement of many beams of small size, each support... lug a small floor space. To this there arises the objection of increased cost. Beams of long span and narrow width suggest cross bracing, and again the comparison with wood construction suffers as regards expense. The good elements of wood construction are, however, precisely those of advantage in concrete. Wood beams are of long span, slight dimensions, and are set closely together. Each beam is braced supports a considerable area. Such a beam is usually calculated to do the work intended, with a good margin of safety, but suppose the casting of the beam proves to be faulty, or in some manner the beam suffers slight damage. At once the entire area is threatened. This is accomplished to its neighbor, by ultimately cross bridging itself. Under the beams all sorts of pipes and utilities are extended, and a flat ceiling is placed beneath. Upon the beams the floor is laid. What could be more reasonable and convenient? Should a pipe fail it requires only the removal of the ceiling for the necessary readjustments are readily made. Should a beam be slightly defective, or suffer injury, the beams adjacent will support for ordinary use more satisfactory than the factory type of construction. In competition with other systems it has been found less expensive large area of the floor. None of these "The system employed at the University of Wisconsin has been advantages are to be had in the preplaced type of reinforced-concrete floor construction. (By the preplaced, Mr. Peabody means the ordinary type of beam-and-girder construction dealt with in Volume found practical, economical, and effective, and for buildings intended where the requirements of the specification for the steel and other elements forming a necessary part of good construction have been conserved.) This has been put to proof by obtaining alternate propositions on the same work, imposing only the requirements that the steel shall be strained not to exceed a certain amount that spaces shall be "By the system described, spans from 12 to 26 ft. have been successfully cast at the University during the past two years. It is proposed to cast beams of 28-ft. span this year. These will require joists 8 in. wide on the bottom, 14 in. deep and 10 in. wide on top. With a spacing of 3 ft. 4 in. from center to center of joists the amount of concrete employed does not seem excessive: The cross braces in the floor, of the wood cells. Where but one building is to be constructed but the cost of the cells might excel the cost of the tile. Where the building at the top, due to the flaring sides of the molds gives increased resistance to crushing, while the' narrow bottom, enclosing the steel, is sufficient and economical is of several stories, or in buildings of large enough area so that one part can be constructed before another, thus permitting the due to the intervals between the lengths of cells, have not been considered in the estimated strength of the floor. Their value in holding the Commission's annual report for the fiscal year ending 2010 shows the method of constructing and reinforcing the concrete diamond saw of a retaining wall at Buffalo. New York. Fig. 40 shows the finished wall. The following is quoted from a paper read by Mr. Arthur provided for pipes, utilities etc., and that the ceilings shall be flat. "These Strips are for attaching the ordinary ceilings to this type of concrete construction, these small strips to distribute the load upon the floor." The increased width of the beams the joints against lateral strain is unquestionable and they serve also and their weight adds to the dead load of the floor construction. "There is room for trouble in constructions where a certain beam no useful purpose except as forms between which the beams are cast, nails, thus affording a ready means of applying the furring strips for the ceiling. These in turn give the needed spaces for pipes, etc. Upon the furring strips wire lath or plaster board is nailed and the plastering is applied. The most convenient size of cells appears to be 2 ft, 8 in. wide X 6 ft. 6 in. long. Beyond this size they cannot be so easily handled. Some cells were made 8 ft. long and a few 5 ft. At times it is necessary to build special cells to finish out a span of wood are cast into the bottom of each joist and secured by bent its repair. Should it be necessary to change the dimensions of rooms, the load, or if necessary, a section can be replaced without removing a Each of these accidents made a break about a foot square, leaving the steel fabric but little damaged, and the repair of the floor very easy. Drilling through the floor for extending steam risers or setting anchor bolts shows the strength to be ample if not excessive.
Centering the present depth of 1ˇ1 in. for the cells, there is of course all economical limit to spans. Deeper cells would bring a new set of calculated values, however, so that there is no reason why long spans should not be used. JI
Fig. 46 is an actual view of the floor construction employed at the University of Wisconsin. Note the wood cells in the foreground ready to be removed. Note also the wood strips in the bottoms of the beams. "The floor sheet is two inches thick including the sand finish, seems fragile: Experience, however, shows that except for the mechanical" Variation in strength of floors was made by spacing the cells farther apart, making the concrete joists wider for the heavier floor. The percentage of steel was then increased according to rule. The joists were sometimes left exposed over laboratories, and made a very presentable appearance difficulty of casting, the floor could be thinner. Floors have been broken during construction, but by blows which would break other floors considered amply strong. On one instance a scaffold plank fell about 16 ft., striking on end. At another time a piece of sandstone 3 ft. long, weighing about 450 lb. fell the .same distance upon the floor.
difficulty of casting, the floor could be thinner. Floors have been broken during construction, but by blows which would break other floors considered amply strong. On one instance a scaffold plank fell about 16 ft., striking on end. At another time a piece of sandstone 3 ft. long, weighing about 450 lb. fell the .same distance upon the floor. We are turning this into a concrete "foundation" cutting web site.
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Concrete Foundations Do it Yourself
Reinforced Concrete is concrete that has been additionally strengthened by having embedded into it some metal, usually steel reinforcing bar also known as rebar. Steel reinforcing bar will almost double concrete's tinsel strength. The component aggregate materials should separately possess certain properties, if satisfactory strength and durability are to be obtained in the structures having both of these materials used in combination. The properties of each concrete material being utilized, and those properties in particular should be emphasized which have the most to do with the safe and economic designing of concrete foundations.
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Boston, Massachusetts - Concrete used in reinforced concrete foundations should be strong, of uniform quality, free from voids, and thoroughly sound. These qualities are required even more than in massive concrete foundations, as the sections in reinforced concrete structures are comparatively small and the stability of a given concrete foundation depends upon the strength and durability of every part used in the poring, reinforcing and forming.
Concrete Basement Walls - The proportions commonly used in American practice or standards may vary from about 1:1 1/2:3 to 1:3:6, using either crushed stone or gravel as aggregate. The rich mixture is usually required in structural foundations subjected to high stresses or where exceptional water tightness is desired. On the other hand, the use of a 1:3:6 concrete requires careful grading of the materials to produce satisfactory results, even for ordinary flat concrete work.
Concrete Forming MA - The aggregate employed in reinforced concrete construction should be of high grade; only Portland cement should be used, and the brand selected should conform to the specifications of the American Society for Testing Materials- for these specifications are now accepted as the American standard.
Concrete Footings - The sand employed should not contain any clay, vegetable loam, sticks, and organic matter and should be of hard, dense, tough material. Siliceous quartz sands are the best, although sands from any durable rock will suffice.
Waterproof Basements Free from Mold or Mold Free Basements
Sharp sand was formally a requirement in all residential concrete foundation construction, but this property is by no means essential. To be sure, by the use of sharp sand there is a slight tendency toward a concrete of greater crushing or tinsel strength than when sand of rounded grains is used, but this influence on the result is of less importance than the size of grain, or granular-metric composition. Moreover, the sharper the sand used-the relative sizes of the grains remaining the same-the greater the percentage of voids, and consequently the greater the amount of cement required to produce a given density. (The term density is here used to express the ratio of the volume of the solid particles to the total volume of the concrete.) It is now generally conceded that the requirement of sharpness of sand should be omitted from concrete specifications.
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Pressure Tests of mortar and concrete show that: strength and water-tightness increase with density, and so the best sand as to size is one which will produce the smallest volume of mortar of standard consistency when mixed with the given cement in the required proportions. To put it somewhat differently,-the best sand for strength, for water-tightness, and also for is one which is so graded from fine to coarse so that the percentage of voids in the resulting mortar is reduced to a minimum. Such sand has a very coarse appearance as the amount of fine material required is very small.
It has been found that the densest mixture or concrete occurs with particles of different sizes and also that the least density occurs when the grains are all of the same size. Coarse and fine sands are thus inferior to graded sands for the use in concrete, but of the two extremes the coarse sand is preferable. The reason for this is due to the fact that the coarse sand has a less total grain surface in a unit volume, even when the sands considered contain portion of solid matter and voids. Less total grain surface means less cement and water to coat the grains, and less labor required in mixing the concrete. The additional amount of cement and water required in the case of the fine sand reduces the density of the resulting mortar and likewise its strength.
The finer the sand, the more nearly uniform the size of the grains, and consequently the greater the proportion of voids. Fine sand is seldom satisfactory and should not be used unless coarse sand is simply not available. Even in such cases, tests of strength should be made with the idea of determining what extra cost may be justified in securing a coarser material for the mixture of concrete.
A screen with 1/4-in, openings is generally employed for separating out large material from sand. Specifications should limit the maximum amount of loam or clay to be allowed in any given concrete mixture. Loam should never be permitted, but clay to the amount of 5 to 10 per cent, if evenly divided, is often beneficial in lean concrete. In rich concrete the strength and density is decreased by even slight additions of clay; but in lean concretes the clay helps to fill the voids of the sand, and causes the cementing material to coat the grains better and to bind them together more strongly.
Broken stone screenings have a small percentage of voids and, when free from clay, usually make excellent sand for use in concrete. These screenings ordinarily give a stronger concrete than natural sand but are likely to contain an undue amount of dust, especially when obtained from soft stone; in such a case the mass should be screened before being used in mixing mortar or concrete. Gravel screenings also constitute a good material in place of sand. All material passing a 1/4-in, screen is generally considered as sand, or fine aggregate; while all material larger than this size is classed as coarse aggregate.
Stone - For the coarse aggregate, either crushed stone or gravel is generally used. Any stone is suitable which is clean and durable and which has sufficient strength to prevent the strength of the concrete from being limited by the strength of the stone. Trap, granite, limestone, and the more compact sandstone are generally employed. Aggregates containing soft, flat, or elongated particles should never be used.
All that has been said concerning voids in sand applies with equal force to the coarse aggregate. Screens varying by a quarter of an inch from 1/4 inch up are desirable, but a very useful analysis may be made with fewer screens. A uniform size of stone filled with mortar does not make as dense or as strong a concrete as one in which the coarse aggregate is well graded-that is, where the small stones partly fill the larger interstices. A straight line on a mechanical-analysis diagram indicates a uniform grading of size.
A general rule is that the larger the stone, the stronger and denser the concrete. Experience has shown that for reinforced concrete that the maximum size should not be more than about 1 inch to 1 1/2 inches, in order for the concrete to fit itself closely around the reinforcing metal. Subsequently, the smaller the stone and the greater the surface to be coated, means the greater the amount of cement required.
Most gravel is sufficiently durable for the use in concrete as an aggregate. The gravel should be at least reasonably clean, although a quantity of finely divided clay equal to 5 to 10 per cent of the gravel may add to the strength of the concrete, if the cement paste does not entirely fill the voids. The presence of clay requires very thorough mixing. When gravel is used, it should be screened to separate the sand and then be remixed in order that the proportions may be definite to result in a quality concrete mixture.
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