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CONCRETE
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CONCRETE , the name given to a See also:building material consisting generally of a mixture of broken See also: The matrix most commonly used is See also:Portland cement, by far the best and See also:con- stituents strongest of them all. The subject of its manufacture and examination is a most important and interesting one, and the See also:special See also:article dealing with it should be studied (see CEMENT), Here it will only be said that before using Portland cement very careful tests should be made to ascertain its quality and con. dition. Moreover, it should be kept in a See also:damp-See also:proof See also:store for a few See also:weeks; and when taken out for use it should be mixed and placed in position as quickly as possible, because See also:rain, or even moist See also:air, spoils it by causing it to set prematurely. The See also:oldest of all the matrices is lime, and many splendid examples of its use by the See also:Romans still exist. It has been to a See also:great extent superseded by Portland cement, on account of the much greater strength of the latter, though lime concrete is still used in many places for dry foundations and small structures. To be of service the lime should be what is known as " See also:hydraulic," that is, not pure or " See also:fat," but containing some argillaceous See also:matter, and should be carefully slaked with water before being mixed with the aggregate. To ensure this being properly done, the lumps of lime should be broken up small, and enough water to slake them should be added, the lime then being allowed to See also:rest for about See also:forty-eight See also:hours, when the water changes the particles of quicklime to See also:hydrate of lime, and breaks up the hard lumps into a See also:powder. The hydrated lime, after being passed through a See also:fine See also:screen to sort out any lumps unaffected by the water, is ready for concrete making, and if not required at once should be stored in a dry See also:place. Other matrices are slag cement, a comparatively See also:recent invention, and some other natural and artificial cements which find occasional See also:advocates. Materials like See also:tar and See also:pitch are sometimes employed as a matrix; they are used hot and without water, the solidifying action being due to cooling and to evaporation of the See also:mineral See also:oils contained in them. What-ever matrix is used, it is almost invariably " diluted'.' with sand, the grains of which become coated with the finer particles of the matrix. The sand should be coarse-grained and hard. It should be See also:free from dirt—that is to say, free from See also:clay or soft mud, for instance, which prevents the cement adhering to its particles, or again from sewage matter or any substance which will chemically destroy the matrix. The grains should show no signs of decay, and by preference should be of an angular shape. The sand obtained by crushing See also:granite and hard stones is excellent. When lime is used as a matrix, certain natural earths such as pozzuolana or trans, or, failing these, powdered bricks or tiles, may be used instead of sand with great advantage. They have the See also:property of entering into chemical combination with the lime, forming a hard setting See also:compound, and increasing the hardness of the resulting concrete. The commonest aggregates are broken stone and natural See also:flint See also:gravel. Broken bricks or tiles and broken See also:furnace slag are some-times used, the essential points being that the aggregate should be hard, clean and See also:sound. Generally speaking,broken stones will be rough and angular, whereas the stones in flint gravel will be comparatively smooth and See also:round. It might be supposed, therefore,that the broken stone will necessarily be the better aggregate, but this does not always follow. Experience shows that, although spherical pebbles are to be avoided, Portland cement adheres tightly to smooth flint surfaces, and that rough stones often give a less compact concrete than smooth ones on account of the difficulty of bedding them into the matrix when laying the concrete. In mixing concrete there is always a tendency for the stones to See also:separate themselves from the sand and cement, and to See also:form " pockets " of honeycombed concrete which are neither water-tight nor strong. These are much more liable to occur when the stones -are See also:flat and angular than when they are round. See also:Modern See also:engineers favour the practice of having the stones of various sizes instead of being See also:uniform, because if these sizes are wisely proportioned the whole mixture can be made more solid, and the rough "pockets" avoided. For first-class See also:work, however, and especially in steel concrete, it is customary to reject very large stones, and to insist that all shall pass through a See also:ring a of an See also:inch in See also:diameter. The water, like all the other constituents of concrete, should be clean and free from See also:vegetable matter. At one See also:time See also:sea-water was thought to be injurious, but modern investigation finds no objection to it except on the See also:score of See also:appearance, efflorescence being more likely to occur when it is used. Sometimes in massive concrete structures large and heavy stones as big as a See also:man can lift are buried in the concrete after it is laid in position but while it is still wet. The stones should be hard and clean, and care must be taken that they are completely surrounded. Such concrete is known as See also:rubble concrete. In proportioning the quantities of matrix to aggregate the ideal to be aimed at is to get a concrete in which the voids or air-spaces shall be as small as possible; and as the lime or cement tlona~r. is usually by far the most expensive See also:item, it is desir- able to use as little of it as is consistent with strength. When natural flint gravel containing both stones and sand is used, it is usual to mix so much gravel with so much lime or cement. The proportions in practice generally run from 3 to I for very strong work, down to 12 to r for unimportant work. Some engineers have the sand separated from the stones by screens or See also:sieves and then remixed in definite proportions. When stones and sand are obtained from different See also:sources, their relative proportions have to be decided upon. A See also:common way of doing this is first to choose a proportion of sand to cement, which will probably vary from r to r up to 4 to r. It then remains to determine what proportion of stones should be added. For this purpose a large can, whose See also:volume is known, is filled loosely with stones, and the volume of the voids between them is determined by measuring how much water the can will hold in addition to the stones. It is then assumed that the quasltity of sand and cement should be equal to the voids. Moreover, the volume of sand and cement together is generally assumed to be equal to that of the sand alone, as the cement to a large extent fills up voids in the sand. For example, suppose it is resolved to use 2 parts of sand to r of cement, and suppose that experiment shows that in a pailful of stones two-fifths of the volume consists of voids, then 2 parts of sand (or sand with cement) will fill voids in 5 parts of stones, and the proportion of cement, sand, stones becomes 1:2:5. There are several weak points in this reasoning, and a more accurate way of determining the best proportions is to try different mixtures of cement, stones and sand, filling them into different pails of the same size, and then ascertaining, by weighing the pails, which mixture is the densest. In determining the amount of water to be added, several things must be considered. The amount required to combine chemically with the cement is about 16% by See also:weight, but in practice much more than this is used, because of loss by evapora- tion, and the difficulty of ensuring that the water shall be uni- formly distributed. If the situation is cool, the stone hard,' and the concrete carefully rammed directly it is laid down and kept moist with damp cloths, only just sufficient to moisten the whole mass is required. On the other See also:hand, water should be given generously in hot See also:weather, also when absorbent stone is used or when the concrete is not rammed. In these cases the concrete should be allowed to take all it can, but an excess of water which would flow away, carrying the cement with it, should be avoided. The thorough mixing of the constituents is a most important item in the See also:production of See also:good concrete. Its See also:object is to distri- See also:bute all the materials evenly throughout the mass, and it is performed in many different ways, both by hand and by See also:machine. The relative values of hand and machine work are often discussed. Roughly it may be said that where a large mass of concrete is to be mixed at one or two places a good machine will be of great advantage. On the other hand, where the mixing See also:platform has to be constantly shifted, hand mixing is the more convenient way. In hand mixing it is usual to measure out from See also:gauge boxes the sand, stones and cement or lime in a heap on a wooden platform. Then they are turned once or twice in their dry See also:state by men with shovels. Next water is carefully added, and the mixture again turned, when it is ready for depositing. For important work and especially for thin structures the number of turnings should be increased. Many types of mixing See also:machines are obtainable; the favourite type is one in which the materials are placed in a large iron See also:box which is made to rotate, thus tumbling the matrix and aggregate over each other again and again. Another simple apparatus is a large See also:vertical See also:pipe or shoot in which sloping baffle plates or shelves are placed at intervals. The materials are fed in at the See also:top of the shoot and fall from shelf to shelf, the mixing being effected by the various shocks thus given. When mixed the concrete is carried at once to the position required, and if the matrix is See also:quick-setting Portland cement this operation must not be delayed. One of the few drawbacks of concrete is that, unlike See also:brickwork or See also:masonry, it has nearly always to be deposited within moulds or , framing which give it the required shape, and Moulds. which are removed after it is set. Indeed, the trouble and expense of these moulds sometimes prohibit its use. It is essential that they shall be strong and stiff, so as not to yield at all from the pressure of the wet concrete. The moulds for the See also:face of a See also:wall consist • generally of wooden shutters, leaning against upright timbers which are secured by See also:horizontal or raking struts to See also:firm ground, or to anything that will See also:bear the weight. If a smooth and neat face is wanted other precautions must be taken. The shutters must be planed, and coated with a mixture of See also:soap and oil, so as to come away easily after the concrete is set. Moreover, when depositing the concrete, a See also:shovel or other See also:tool must be worked between the wet concrete and the shutter. This draws sand and water to the face and prevents the rough stones from showing themselves. Sometimes rough concrete is rendered over with a See also:plaster of cement and sand after the shutters have been removed, but this is liable to See also:peel off and should be avoided. The method of depositing depends on the situation. If for important walls, or for small scantlings such as steel concrete generally involves, the concrete should be deposited in quite small quantities and very carefully rammed ag osltinto position. If for massive walls, it is usual to tip it out in large quantities from a See also:barrow or See also:wagon, and simply spread it in layers about a See also:foot thick. Depositing concrete under water for breakwatersand See also:bridge foundations requires special skill and special appliances. It is usually done in one of three ways:—(a) By moulding the concrete ashore into large blocks, which, when sufficiently hard, are lowered through the water into position by a See also:crane or similar machine with the aid of See also:divers. The most notable instance of this type of construction was at the See also:port of See also:Dublin, where Mr B. B. Stoney made blocks no less than 35o tons in weight. Each See also:block formed a piece of the See also:quay wall 12 ft. long and 27 ft. high, being made on See also:shore and then deposited in position by floating sheers of special See also:design. (b) By moulding the concrete into what are called " bag-blocks." In this See also:system the concrete is filled into bags, which are at once lowered through the water like the blocks. But in this See also:case the concrete being still wet can adapt itself more or less to the shape of the adjoining bags, and strong rough walls can be built in this way. Sometimes the bags are made of enormous size, as at See also:Aberdeen See also:breakwater, where the contents of each bag weighed 5o tons. The See also:canvas was laid in a hopper See also:barge and there filled with the concrete and sewn up. The enormous bag was then dropped through a See also:door in the bottom of the barge upon the breakwater See also:foundation. (c) By depositing the wet concrete through the water between temporary upright timber frames which form the two faces of the wall. In this case very great care has to be taken to prevent the cement from being washed away from the other constituents when passing through the water. Indeed, this is See also:bound to happen more or less, but it is guarded against by lowering the concrete slowly in a special box, the bottom of which is opened as it reaches the ground on which the concrete is to be laid. This method can only be carried out in still water, and where strong and tight framing can be built which will prevent the concrete from escaping. For small work the box can be replaced by a canvas bag secured by a special tripping noose which can be loosened when the bag has reached the ground. The concrete escapes from the bag, which is then See also:drawn up and refilled. Concrete may be compared with other building materials like masonry or timber from various points of view, such as strength. strength, durability, convenience of building, See also:fire- resistance, appearance and cost. Its strength varies within very wide limits according to the quality and proportions of the constituents, and the skill shown in mixing and placing them. To give a rough See also:idea, however, it may be said that its safe crushing load would be about 4 cwt. per sq. in. for lime concrete, and 1 to 5 cwt. for Portland cement concrete. The safe tensile strength of Portland cement concrete would be some-thing like one-tenth of its compressive strength, and might be far less. On this account it is usual to neglect the tensile strength of concrete in designing structures, and to arrange the material in such a way that tensile stresses are avoided. Hence slabs or beams of long span should not be built of See also:plain concrete, though when reinforced with steel it is admirably adapted for these purposes. In regard to durability good Portland cement concrete is one of the most durable materials known. Neither hot, See also:cold, nor Durability. wet weather has practically any effect whatever upon it. See also:Frost will not injure it after it has once set, though it is essential to guard it from frost during the operations of mixing and depositing. The same praise cannot, how-ever, be given to lime concrete. Even though the best hydraulic lime be used it is See also:wise to confine it to places where it is not exposed to the air, or to See also:running water, and indeed for important structures the use of lime should be avoided. Good Portland cement is so much stronger than any lime that there are few situations where it is not cheaper as well as better to use the former, because, although cement is the more expensive matrix, a smaller proportion of it will suffice for use. Lime should never be used in work exposed to sea-water, or to water containing chemicals of any kind. Portland cement concrete, on the other hand, may be used without fear in sea-water, provided that certain reasonable precautions are taken. Considerable alarm was created about the See also:year 1887 by the failure of two or three large structures of Portland cement concrete exposed to sea-water, both in See also:England and other countries. The matter was carefully investigated, and it was found that the sulphate of See also:magnesia in the sea-water has a decomposing action on Portland cements, especially those which contain a large proportion of lime or even of alumina. Indeed, no Portland cement is free from the liability to be decomposed by sea-water, and on a moderate See also:scale this action is always going on more or less. But to ensure the permanence of structures in sea-water the great object is to choose a cement containing as little lime and alumina as possible, and free from sulphates such as See also:gypsum; and more important still to proportion the sand and stones in the concrete' in such a way that the structure is practically non-porous. If this is done there is really nothing to fear. On the other hand, if the concrete is rough and porous the sea-water will gradually eat into the See also:heart of the structure, especially in a case like a See also:dam, where the water, being higher on one See also:side than the other, constantly forces its way through the rough material, and decomposes the Portland cement it contains. As regards its convenience for building purposes it may be said roughly that in " mass " work concrete is vastly more Conven- convenient than any other material. But concrete is ience and hampered by the fact that the See also:surface always has to appear- be formed by means of wooden or other framing, and ante. in the case of thin walls or floors this framing becomes a serious item, involving expense and delay. In appearance concrete can rarely if ever See also:rival stone or brickwork. It is true that it can be moulded to any desired shape, but See also:mouldings in concrete generally give the appearance of being unsatisfactory imitations of stone. Moreover, its See also:colour is not pleasing. These defects will no doubt be overcome as concrete grows in popularity as a building material and its aesthetic treatment is better understood. Concrete pavings are being used in buildings offirst importance, the aggregate being very carefully selected, and in many cases the whole mixture coloured by the use of See also:pigments. Care must be taken in their selection, however, as certain colouring matters such as red See also:lead are destructive to the cement. One of the great objections to the appearance of concrete is the fact that soon after its erection irregular cracks invariably appear on its surface. These cracks are probably due to shrinkage while setting, aggravated by changes in temperature. They occur no less in structures of masonry and brickwork, but in these cases they generally follow the See also:joints, and are almost imperceptible. In the case of a smooth concrete face there are no joints to follow, and the cracks become an ugly feature. They are sometimes regulated by forming artificial " joints " in the structure by embedding strips of See also:wood or See also:sheet iron at See also:regular intervals, thus forming " lines of weakness," at which the cracks therefore take place. A pleasing " rough " appearance can be given to concrete by brushing it over soon after it has set with a stiff See also:brush dipped in water or dilute See also:acid. Or, if hard, its surface can be picked all over with a See also:bush See also:hammer. At one time Portland cement concrete was considered to be lacking in fireproof qualities, but now it is regarded as one of the best fire-resisting materials known. Although experi- R ments on this matter are badly needed, there is little to fe fi doubt that good steel concrete is very nearly indestruc- tible to fire. by fire. The matrix should be Portland cement, and the nature of the aggregate is important. Cinders have been and are still much favoured for this purpose. The See also:reason for this preference lies in the fact that being porous and full of air, they are a good non-conductor. But they are weak, and modern experience goes to show that a strong concrete is the best, and that probably materials like broken clamp bricks or burnt clay, which are porous and yet strong, are far better than cinders as a fireproof aggregate. See also:Limestone should be avoided, as it soon splits under See also:heat. The steel reinforcement is of immense importance in fireproof work, because, if properly designed, it enables the concrete to hold together and do its work even when it has been cracked by fire and water. On the other hand, the concrete, being a non-conductor, preserves the steel from being softened and See also:twisted by excessive temperature. Only very See also:general remarks can be made on the subject of cost, as this item varies greatly in different situations and with the See also:market See also:price of the materials used. But in England cost. it may be said that for massive work such as big walls and foundations concrete is nearly always cheaper than brickwork or masonry. On the other hand, for reasons already given, thin walls, such as See also:house walls, will cost more in concrete. Steel concrete is even more difficult to generalize about, as its use is comparatively new, but even in the matter of first cost it is proving a serious rival to timber and to See also:plate steel work, in floors, bridges and tanks, and to brickwork and plain concrete in structures such as culverts and retaining walls, towers and domes. ' Artificial Stones.—T here are many varieties of concrete known as " artificial stones " which can now be bought ready moulded into the form of paving slabs, wall blocks and pipes: they are both pleasing in appearance and very durable, being carefully made by skilled workmen. Granolithic, globe granite and synthetic stone are examples of these. Some, such as See also:victoria stone, imperial stone and others, are hardened and rendered non-porous after manufacture by See also:immersion in a See also:solution of silicate of soda. Others, like See also:Ford's silicate of lime-stone, are practically lime mortars of excellent quality, which can be carved and cut like a See also:sandstone of fine quality. Steel Concrete.—The introduction of steel concrete (also known as ferroconcrete, armoured concrete, or reinforced concrete) is generally attributed to See also:Joseph Monier, a See also:French gardener, who about the year 1868 was anxious to build some concrete water basins. In See also:order to reduce the thickness of the walls and See also:floor he conceived the idea of strengthening them by building in a network of iron rods. As a matter of fact other inventors were at work before Monier, but he deserves much See also:credit for having pushed his invention with vigour, and for having popularized the use of this invaluable combination. The important point of his idea was that it combined steel and concrete in such a way that the best qualities of each material were brought into See also:play. Concrete is readily procured and easily moulded into shape. It has considerable compressive or crushing strength, but is somewhat deficient in shearing strength, and distinctly weak in tensile or pulling strength. Steel, on the other hand, is easily procurable in simple forms such as long bars, and is exceedingly strong. But it is difficult and expensive to work up into various forms. Concrete has been avoided for making beams, slabs and thin walls, just because its deficiency in tensile strength _doomed it to failure in such structures. But if a concrete slab be " reinforced " with a network of small steel rods on its under surface where the tensile stresses occur (see fig. r) its strength will be enormously increased. Thus the one point of weakness in the concrete slab is overcome by the addition of steel in its simplest form, and both materials are used to their best advantage. The scientific and See also:practical value of this idea was soon seized upon by various inventors and others, and the number of patented systems of combining steel with concrete is constantly increasing. Many of them are but slight modifications of the older systems,
and no See also:attempt will be made here to describe them in full. In
England it is customary to allow the patentee of one or other
system to furnish his
own designs, but this is
as much because he has
gained the experience
needed for success as
because of any special
virtue in this or that
system. The See also:majority of
these systems have
emahated from See also:France,
where steel concrete is
largely used. See also:America
and See also:Germany adopted
them readily, and in
England some very large
structures have been
erected with this material.
The concrete itself
should always be the very
See also:Section through Intersection. best quality, and Portland
account of its superiority
to all others. The aggregate should be the best obtainable and of different sizes, the stones being freshly crushed and screened to pass through a s in. ring. Very special care should be taken so to proportion the sand as to make a perfectly impervious mixture. The proportions generally used are 4 to I and 5 to r in the case of gravel concrete, or I:2: 4 or I:22: 6 in the case of broken stone concrete. But, generally speaking, in steel concrete the cost of the cement is but a small item of the whole expense, and it is See also:worth while to be generous with it. If It is used in piles or structures where it is likely to be bruised the proportion of cement should be increased. The mixing and
laying should all be done very thoroughly; the concrete should be rammed in position, and any old surface of concrete which has to be covered should be cleaned and coated with fresh cement.
The reinforcement mostly consists of mild steel and sometimes of wrought iron: steel, however, is stronger. and generally cheaper, so that in See also:English practice it holds the See also: It should be mild and is usually specified to have a breaking (tensile) strength of 28 to 32 tons per sq. in., with an See also:elongation of at least 20% in 8 in. Any See also:bar should be capable of being See also:bent cold to the shape of the letterUwithout breaking it. The steel is generally used in the form of long bars of circular section. At first it was feared that such bars would have a tendency to slip through the concrete in which they were em-bedded, but experiments have shown that if the bar is not painted but has a natural rusty surface a very considerable See also:adhesion between the concrete and steel —as much as 2 cwt. per sq. in. of contact surface—may be relied upon. Many devices are used, however, to ensure the adhesion between concrete and bar being perfect. (I) In the Hennebique system of construction the bars are flattened at the end and split to form a " See also:fish tail." (2) In the Ransome system round bars are rejected in favour of square bars, which have been twisted in a See also:lathe in "See also:barley Fm. 3.-Hennebique System. See also:sugar " See also:fashion. (3) In the Habrick system a flat bar similarly twisted is used. (4) In the Thacher system a flat bar with projections like See also:rivet heads is specially rolled for this purpose. (5) In the See also:Kahn system a square bar with " branches " is used. (6) In the "See also:expanded See also:metal" system no bars are used, but instead a strong steel netting is manufactured in large sheets by special machinery. It is nude by cutting a See also:series of long slots at regular intervals in a plain steel plate, which is then forcibly stretched out sideways until the slots become See also:diamond-shaped openings, and a trellis work of steel without any joints is the result (fig. 2). The structures in which steel concrete is used may be analysed as consisting essentially of (I) walls, (2) columns, (3) piles, (4) beams, (5) slabs, (6) See also:arches. The designs differ considerably according to which of these purposes the structure is to fulfil. The effect of reinforcing walls with steel is that they can be made much thinner. The steel reinforcement is generally applied in the form of vertical rods built in the wall at intervals, with lighter horizontal rods which See also:cross the vertical ones, and thus form a network of steel which is buried in the concrete. These rods assist in taking the weight, and the whole network binds the concrete together and prevents it from FIG. 4. cracking under a heavy load. The vertical Hennebique System. rods should not be quite in the See also:middle of the wall but near the inner and See also:outer faces alternately. Care must be taken, however, that all the rods are covered by at least an Expanded Metal. I .- - < _:,..: d:=r =_-a _ :- _.._a =r_.ra _- a<..== =: i-era-.= =_' =:r_< 0: _ 4—1 , .1 2" See also:Tube for Pitching See also:Pile See also:Longitudinal Section. Steel Wins Rings inch of concrete to preserve them from damage by See also:rust or fire. problem arises how best to arrange the steel so as to assist the In the Cottancin system the concrete is replaced by bricks concrete in bearing them. To meet tensile stresses the steel is pierced with holes through which the vertical rods are threaded; nearly always inserted in the form of bars running along the See also:beam. the horizontal tie-rods are also used, but these do not merely See also:Figs. 6 to 9 show how they are arranged for different loading. cross the vertical ones, but are See also:woven in and out of them. In each case the object is to place the bars as nearly as possible Columns have generally to bear a heavier weight than walls, where the tensile stresses occur. In cases where all the stresses and have to be correspondingly stronger. They have usually are heavy, that portion of the beam which is under See also:compression been made square with a vertical steel See also:rod at each corner. To is similarly reinforced, though with smaller bars (figs. ro and r r). prevent these rods from spreading apart they must be tied together But as these tension and. at frequent intervals. compression bars are 1 In some systems this is generally placed near the ' done by loops of stout under and upper surface 17 See also:wire connecting each of the beam they are of FIG.6. rod to its See also:neighbour, little use in helping to and placed one above resist the shearing the other about every stresses which are great- \~ ro in. up the See also:column est at its neutral See also:axis. FIG. 13. (figs. 3 and 4). In other (See BRIDGES.) These systems a stout wire is shearing stresses in a heavily loaded beam would cause it to split horizontally at or near the centre. To prevent this many ingenious devices have been introduced. (I) Perhaps one of the most efficient is a See also:diagonal bracing of steel wire passing to and fro between the upper and See also:lower bars and firmly secured to each by lapping or otherwise (fig. I2); this See also:device is used in the Coignet and other French systems. (2) In the Hennebique system (which has found great favour in England) vertical bands or " stirrups," as they are generally called, of hoop steel are used (fig. 13). They are of U shape, and passing round the tension bars extend to the top of the beam (figs. 14 and 3). They are exceedingly thin, but being buried in concrete no danger of their perishing from rust is to be feared. (3) In the Boussiron system a similar See also:stirrup is used, but instead of being vertical the two parts are spread so that each is slightly inclined. (4) In the Coularon system, the stirrups are inclined as in fig..15, and consist of rods, the ends of which are hooked over the tension and compression bars. (5) In the Kahn system the stirrups are similarly arranged, but instead of being merely secured to the tension bar, they form an integral part of it like branches on a See also:stem, the bar being rolled to a special section to admit of this. (6) In many systems such as the " expanded metal " system, the tension and compression rods together with the stirrups are all abandoned in favour of FIG. 15. a single rolled steel See also:joist of I section, buried in concrete (see fig. 16). Probably the weight of steel used in this way is excessive, but the joists are cheap, readily procurable and easy to handle. Floor slabs may be regarded as wide and shallow beams, and the remarks made about the stresses in the one apply to the other also; accordingly, the various devices which are used for strengthening beams recur in the slabs. But in a thin slab, with its comparatively small span and See also:light load, the concrete is generally strong enough to bear the shearing stresses unaided, and the reinforcement is devoted to assisting it where the tensile stresses occur. For this purpose many designers simply E'V®YA I1 JAYAVAVLVA See also:wound continuously in a See also:spiral form round the four rods. Modern investigation goes to prove that the latter is theoretically the more economical way of using the steel, as the spiral binding wire acts like the binding of a wire See also:gun, and prevents the concrete which it encloses from bursting even under very great loads. That steel concrete can be used for piles is perhaps the most astonishing feature in this invention. The fact that a compara- tively brittle material like concrete can be subjected not only to heavy loads but also e- to the See also:jar and vibra- tion from the blows of a heavy pile See also:ram makes it appear as if its nature and See also:pro- perties had been changed by the steel reinforcement. In a sense this is un- doubtedly the case. A. G. Considere's ex- periments have shown that concrete when reinforced is capable of being stretched, without fracture, about twenty times as much as plain concrete. Most of the piles driven in Great See also:Britain have been made on the Hennebique system with four or six longitudinal steel rods tied together by stirrups or loops at frequent intervals. Piles made on the See also:Williams system have a steel rolled joist of I section buried in the heart of the pile, and round it a series of steel wire hoops at regular intervals (fig. 5). Whatever system is used, care must be taken not >- • _ - - = s?S se,- - - ' -''•. to See also:batter the See also:head of the pile to pieces with the heavy ram. To prevent this an iron " See also:helmet " containing a lining of sawdust is fitted over Wthe head of the pile. The sawdust adapts it-self to the rough shape of the concrete, and deadens the See also:blow to some extent. But it is in the design of steel concrete beams that the greatest ingenuity has been shown, and almost every patentee of a " system " has some new device for arranging the steel reinforcement to the best advantage. Concrete by itself, though strong in compression, can offer but little resistance to tensile and shearing stresses, and as these stresses always occur in beams the use the modification of the Monier system, consisting' of a horizontal network of crossed steel rods buried in the concrete. " Expanded metal " too is admirably adapted for the purpose (fig. r). In the Matrai system thin wires are used instead of rods, and are securely fastened to rolled steel joists, which form the beams on which the slabs rest; moreover, the wires instead of being stretched tight from side to side of the slab are allowed to sag as much as the thickness of the concrete will allow. Additional information and CommentsThere are no comments yet for this article.
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