The Use of Reinforcing and Boxing in Preparing Concrete

New Zealand Journal of Agriculture, Volume 79, Issue 6, 15 December 1949, Page 571

The Use of Reinforcing and Boxing in Preparing Concrete

TN the first article of this series, which was published in last month’s issue of the “Journal,” H. W. T. Eggers, Engineer, Department of Agriculture, Wellington, dealt with the properties and methods of working concrete. This article describes reinforcing and forms, and the third article, to be published in next month’s issue, will describe concrete bricks, pipes, and posts.

THE greatest strength of concrete is compressive, and if any other stressing is to be given a concrete structure, this stressing must be provided for by the use of steel reinforcing. From this it will be realised that as reinforcing is used for a special purpose, to gain the maximum strength with the greatest economy, the sizing and position of reinforcing must be arranged in such a manner that the size is suited to the degree of stress and the position to the line of stress. Reinforced concrete is therefore a combination of concrete and steel acting as a unit as a result of the adhesion between the two materials. Beams of plain concrete fail by tension under very low stresses, but if reinforced by the embedment of steel in their tensile portion, they may be stressed to the compressive working limit of concrete.. Reinforced-concrete structures are practically monolithic, more rigid than steel, and substantially fireproof. Reinforced concrete is used in parts of the structure in which tensile and compressive stresses exist, such as beams and slabs, and also in members subject to secondary bending stresses such

as columns and struts. Reinforcing is also used to prevent cracks caused by changes of temperature and shrinkage, as in walls. The use of reinforcing placed without regard to the work it is supposed to perform is a waste of material and labour, because unless the reinforcing is placed along the lines of stress, the structure is no stronger for its presence. If, on the other hand, the sizing and positioning of reinforcing are designed in correct relation to the stresses involved, the quantity of reinforced concrete required for any given loading will be considerably less than any unreinforced concrete used for the same loading. Reinforcing Steel Reinforcing steel is generally ordinary mild steel supplied in the shape of bars, plain round bars being most extensively used. A number of deformed bars (that is, bars with irregular surfaces) have been designed to produce mechanical bond and greater adhesion between steel and concrete. For all ordinary work plain bars may be used with safety, though deformed bars are of advantage in resisting

temperature stresses. Fabric reinforcing, such as triangular mesh, expanded metal, welded wire, etc., is adapted in certain cases for slabs, walls, or partitions, or moulded articles such as watering troughs. Usually, however, bars are more economical. All steel used for reinforcing should stand bending cold to an angle of 180 degrees around a diameter equal to that of the piece tested, without fracturing the skin at the bend. It is of the utmost importance ./ that all steel is clean and free from rust arid that all scale is removed before fixing in the work. In the work all cross rods must be •sufficiently tied to the longitudinal rods so that the reinforcing forms a rigid mat which will not be displaced by the placing of the concrete. Softiron wire is the best material for binding rods together. Splices in rods should be lapped at least 40 diameters and securely bound, and all ends should be hooked, except on bars used solely for the purpose of taking temperature stresses. Fabrics should be lapped for not less than 15in. in the direction of the longitudinal wires, and the side lap should not be less than 3in. Bending and Fixing Reinforcement As all steel must be accurately placed and supported in its correct position in the forms, it is very necessary that bending and cranking are done accurately. Bars must be cut to length and bent to fit their respective

positions in such a manner that they are easily assembled. Even if they can be assembled, wrongly-bent bars will not occupy their correct positions and are therefore not capable of providing the necessary resistance moments. They may also reduce the coverage of concrete if too close to the forms, causing the concrete to spall off. When a reinforced structure is designed accurate drawings of all reinforcing are prepared, and reinforcing must be bent exactly as shown in the drawing, as the designers have studied the methods of positioning as well as its strength characteristics. Reinforcing can be ordered already cut and bent according to drawings, but if this work is done on the site, accuracy is essential. The simplest equipment for bending light bars consists of a number of steel dowels fitting into holes in a baulk of timber supported so that its upper surface is 3ft. to 3ft. 6in. above ground level (see diagram. above). A length of gas or water pipe which slips over the free end of the . bar is used for pulling the bars round the dowels to make a bend. Simple bar-bending machines are also obtainable and give good results. The dimensions of tails and hooks on reinforcing bars are of little importance and any excess in. length may be taken up at these points. In bending bars with a crank near each end it is advisable to start by making these bends first and using any surplus length of bar in the end hooks. Forms of Shuttering Timber Concrete is worked in a plastic state and can be moulded to any shape required. The moulds, or sections from which the moulds are built, are known as boxing, forms, or shuttering, and consist of either timber or metal. Timber is by far the commonest material from which concrete forms are constructed, but sheet metal is used for special applications where a required profile is more easily obtained by bending a sheet of steel.

Some contractors use sectional steel shuttering which can be bolted together and forms . a very convenient means of erecting shuttering for straight-run work such as walls. The most suitable timber for use- as boxing is pine, as it is relatively cheap and, being soft, is easily worked. It is also available in all sizes and is easily obtained. Freedom from knots and coarse grain is desirable, as these will show on the finished concrete; for this reason white pine (kahikatea) is one of the best timbers to use for mouldings or in any situation where an extrasmooth finish is required. However, white pine is too expensive and has too little strength for form timber generally. Partially-seasoned timber is the best for form construction, because if the timber is too dry, it will tend to swell from absorption of moisture, and green timber will tend to dry out and shrink in hot weather, causing fins and ridges on the concrete. Timber may be rough or dressed. It may be dressed in various ways, such as on all four sides, on one side and one edge, on one side and two edges, etc. Generally it is best to use timber dressed on all four sides, as it will then be of more uniform size and is more easily adaptable for different purposes. Wood of any size should be dressed to uniform thickness so that the pieces will match up; this is particularly important with sheathing, because otherwise the joints will require to be planed down. Sheathing 1 to 2in. thick for straight runs such as walls may be tongue and grooved, square, or have a bevelled edge. Tongue and groove gives the best results, and a bevelled edge is good if the wood is very dry, because when built up it will not buckle so easily when swelling. Thickness of timber will depend on the available supply and the load to be carried, but more often on the supply, as any normal size can be used to advantage by adjusting the spacing of the supports. For all gen-

eral purposes lin. timber dressed to 13/16in. will be found the most useful, planks 6in. wide being the most handy, except for a large area of sheathing where 12in. planks will require less labour to erect. The lengths of timber ordered for boxing should, where it can be specified, be of such a size that they can be used to the best advantage with the least waste. Attention to this point may save an appreciable amount of timber if the work is extensive. Sheathing can be ordered in random lengths as it generally has to be cut up, and short lengths can always be worked in. ' Where exact dimensions have to be met joists, studs, posts, beam bottoms, etc., should be ordered to the nearest commercial length to the height or span required. A span of, say, sft. 6in. should be ordered in 12ft. lengths for the least waste. Care in specifying the lengths is important; otherwise there will be a lot of short ends and a surprising percentage of waste. As timber is a costly item in reinforced concrete construction it should be ordered and used with care. Nails Common wire-cut steel nails are generally used. Double-headed nails, if they can be obtained, are an advantage, as they lean be withdrawn easily. Wire and Bolts In vertical sections such as columns and walls a horizontal pressure caused by the hydrostatic head of the wet concrete will act on the shutters. As this pressure is exerted equally in all directions, it can be guarded against by tying the boxing supports with wire or bolts. No. 9 black, annealed wire gives the best service for ordinary work. ■ Steel or galvanised-iron wire should not be used, as it is brittle, hard to handle, and too springy. Bolts with washers and nuts are used in heavier wall construction. If they are to be drawn after use, they should be well greased or fitted with sleeves.

Treatment of Shuttering For , work where the concrete is not to be plastered all shuttering coming in contact with the concrete should be well oiled or greased to allow easy stripping and to prevent the concrete adhering to and coming away with the wood. If the concrete is to be plastered, the plaster will adhere better if the surface is roughened; alternatively, a retarding liquid may be used on the shuttering. Special nonstaining oil can be obtained for this purpose, though soft soap and water are quite satisfactory. Loads The load to be carried by shuttering is the weight of the wet concrete, the shutters themselves, and a live load which allows for impact, wheeling over the shutters, etc., and is therefore a construction load. The weight of the shutters can be neglected, as it is small compared with the other loads. In casting reinforced slabs such as flat roofs, floors above ground, or beams, calculations of loading on the shuttering may be simplified by taking the weight of concrete as 1441 b. per cubic foot. It is then necessary only to multiply the thickness of the floor by 12 to get .the weight per square foot, or to multiply the depth of a beam by its width to get the weight per lineal foot. For example, a sin. slab will weigh 601 b. per square foot, and a beam lOin. wide by 18in. deep will weigh 1801 b. per lineal foot. Incline slabs such as may be required as an approach to an elevated floor will cause an overturning movement at the top of the posts supporting the shuttering, and this must be countered by adequate bracing.

The construction live load is generally taken at 751 b. per square foot, and will exist only during concreting and then for short periods. The piling of material on freshly-poured concrete is dangerous as the shuttering may not have been designed to take the added load. If it is known beforehand that material must be placed on the concrete the day after it is poured, the shutters should be made extra stiff to carry the extra loading with only slight deflection. Pressure The horizontal pressure exerted in vertical sections through the hydrostatic head of wet concrete is the pressure which causes most of the bulging and collapse of shutters. The pressure on the shuttering will depend on the rate of filling and the temperature. The faster the shutters are filled and the lower the temperature the greater will be the pressure, because the concrete does not set as' quickly and thus relieve the pressure. If a wall were poured so slowly that each layer set before the next layer was poured, by the time the- shuttering was full the pressure at the bottom would be no greater than at the top. This is the principle employed with sectional shuttering, which is raised at about the same rate as the concrete sets, so that each layer supports the layer above. As the outward pressure depends mainly on the rate of pouring, column sides will be under greater pressure than wall sides, because they fill faster. Vertical sections should always

STAGES IN ERECTING SHUTTERING

be poured as slowly as is consistent with economy and in layers about 12in. thick. Column boxes should never be filled to the top without a break; instead each batch of concrete should be distributed over several columns. Concrete in heavy walls and piers in which large stones or “plums” can be embedded will always exert less pressure on the shutters than when the stones are omitted, because true hydrostatic pressure will not exist. , The consistency of the concrete also affects the pressure, which increases with the increase of the percentage of water in the mix. Construction of Shuttering As shuttering is the mould in which the plastic concrete is cast to the shape required the inside of that mould must be the exact profile of the finished concrete. Any roughness or irregularity in the mould will show on the finished concrete. Shuttering for walls or vertical sections must be set true in relation to the vertical; if one or both sides of a wall are battered, the angle of batter will be relative to the vertical. The procedure in erecting, shuttering for walls is shown in the diagrams on the preceding page. If the founda-

tions have been dug and footings run with the vertical reinforcing rods cast in (preparation of the foundations and footings for walls will be dealt with in a later article in this series), the shuttering for the walls is built on vertical supports of 4in. by 2in. timbers at least 12in. longer than the finished height of the wall above the footings. Nail the bottom two planks firmly to the supports, placing one at each end and spacing intermediate supports at about 18in. to 2ft. centres, depending on the height of the wall. If the planks are not sufficiently long to span the full length of the wall, the first planks must lap one end support lin. only to permit other planks to be butted to the same support. Make sure all plank ends are square and nailed squarely to the supports. Erect this section of shuttering on the footings in correct line and position, and support it rigidly in a vertical position with braces from the top of every second support nailed to stakes driven into the ground. The other side shuttering can now be erected, using the same method and keeping the supports opposite each other. This side does not need any bracing, being tied at the top by a piece of timber nailed across opposite

CtVbRAL modern machines That are trailer drawn derive their power from the tractor engine through a power take-off shaft. Most implement manufacturers provide shields to cover these shafts, but in many instances drivers neglect '.to place these shields in position when hitching tractors to the implements. Failure to do so leaves a driver liable to an accident such as this one caught by ,the camera. The coat tail became caught in the universal joint .of the shaft as the driver dismounted from his machine, and a serious accident might have resulted had not prompt action been taken by an assistant to short out the motor. Never dismount from a tractor without disengaging the power take-off shaft and always cover revolving shafts and axles with the shields provided. —C. J. CROSBIE, Farm Machinery Instructor, Department of Agriculture, Christchurch.