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  • In best engineering practice the engineer will produce complete bar-bending¬†schedules for use by the contractor. The engineer may not guarantee that such¬†schedules are error free and may call upon the contractor to check them. But,¬†as often as not, the contractor will fail to do this, so it is advisable for the resident¬†engineer to check the schedules so that he can forewarn the contractor of¬†any error present. In practice, few errors will be found because the advantage¬†of producing bar-bending schedules is that it applies a detailed check on the¬†validity of the reinforcement drawings supplied to the contractor.
    In some contracts the contractor is required to produce bar-bending schedules himself from the reinforcement drawings supplied under the contract.
    This is not such good practice; the engineer foregoes an opportunity to check the reinforcement drawings, and the contractor (or his reinforcement supplier) who produces the bending schedules will not necessarily be sufficiently acquainted with the design to notice some discrepancy which indicates a possible design error.
    Reinforcement is now seldom bent on site, except on sites overseas.
    Deliveries of reinforcement should be supervised by the leading steelfixer, who should check the steel against the bar schedules and direct where bars should be stocked. Bars should be delivered with identifying tags on them, but sometimes these get torn off. The leading steelfixer should not allow with  drawals from stock without his permission. If the contractor does not pay sufficient attention to this and, for example, lets various steelfixers pick what steel they think is right, the resident engineer should forewarn the contractor this is a recipe for ultimate chaos and delay.

    Properly designed and bent bars can, in the hands of a good steelfixer, be as accurately placed as formwork. Crossings of reinforcement have to be wired together so that a rigid cage is built, able to withstand concrete placing without displacement. To ensure that the correct cover is given to bars, the contractor will need to prepare many small spacer blocks of concrete of the requisite cover thickness and about 25 mm square, which are wired onto the outside of reinforcement, keeping it the required distance from the formwork to give the specified cover. All wire ties should be snipped off close to the reinforcement so that their ends do not penetrate the concrete cover and form a path for corrosion of the reinforcement. The steelfixer will need to make and position spacer bars, generally U-shaped, which keep reinforcement layers the correct distance apart in slabs and walls. He may need many of these.
    They are not included in the bar-bending schedules and the cost to the contractor of supplying and fixing them is usually included in the price for
    steelfixing. Fig. 19.5 shows some points to watch when formwork and reinforcement is being erected.
    Steel reinforcement stored on site rusts, but provided the rust is not so advanced that rust scales are formed, the rust does not appear to affect the bonding of the reinforcement to the concrete. A problem more likely to arise is the contamination of steel reinforcement with oil, grease, or bitumen. If the contractor wishes to oil or grease formwork to prevent it sticking to concrete, he should do so before the formwork is erected and not after it has been put in place. If the latter is attempted it will be almost impossible to prevent some oil or grease getting onto the reinforcement. Similarly, if contraction joints are to be bitumen painted, care must be taken not to get bitumen on bars passing through such a joint.
    The proper design and detailing of reinforcement makes a major contribution to the quality and durability of reinforced concrete. The designer must choose diameters, spacings and lengths of bars which not only meet the theoretical design requirements but which make a practical system for erection and concreting. Reinforcement to slabs must either be strong enough for the steel fixer to stand on, or spaced far enough apart for him to get a foot between bars onto the formwork below. Wall and column reinforcement must be large enough diameter that it does not tend to sag under its own weight.
    Beam reinforcement should not be so congested that it will be difficult to get concrete to surround the bars without using a mix with too high a water content.
    The designer should consider options of design available to avoid heavy¬†congestion of bars. An experienced designer who understands site erection¬†problems will make as much use as possible of the four most commonly used¬†bar diameters ‚Äď 10, 12, 20 and 25 mm. He will appreciate that a 5 m long bar¬†25 mm diameter weighs about 20 kg, so that larger diameter or longer bars can¬†be difficult for a steel fixer to handle on his own. For ease of handling, bars¬†should not exceed 6‚Äď8 m length.

    Bond laps have to be allowed for and should be at places which are convenient for the erection of formwork and for concreting. Starter bars in floor slabs are nearly always necessary for bonding to the reinforcement in walls.
    The length of their vertical arm should not be longer than is necessary to provide adequate bond length and support the wall reinforcement so they present minimum impedance for slab concreting. If the designer wishes to use   hooked bars, he should make sure that the thickness of slab or wall in which they are to be placed is sufficient to accommodate such hooks.

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  • The skill required by carpenters to make and erect formwork for concrete is¬†seldom fully appreciated. The formwork must remain ‚Äėtrue to line and level‚Äô¬†despite substantial loading from the wet concrete. Column and wall faces¬†have to be strictly vertical, and beam soffits strictly level, or any departure¬†will be easily visible by eye. Formwork for concrete which is to remain¬†exposed to view has to be planned and built as carefully as if it were a permanent¬†feature of the building. Many methods have been tried to make the¬†appearance of exposed concrete attractive: but any of them can be ruined by¬†honeycombing, a bad construction joint, or by subsequent weathering revealing¬†that one pour of concrete has not been identical with adjacent pours, or¬†that the amount of vibration used in compacting one panel has been different¬†from that used in others. If concrete has to remain exposed to public view,¬†then the resident engineer should endeavour to agree with the contractor¬†what is the most suitable method for achieving the finish required if the specification¬†or drawings do not give exact guidance on the matter. The problem¬†is that if, through lack of detailed attention, a ‚Äėmishap‚Äô on the exposed surface¬†is revealed when the formwork is struck, it is virtually impossible to rectify it.
    Sometimes rendering the whole surface is the only acceptable remedy.

    Where concrete will not remain exposed to view, minor discrepancies can be¬†accepted. ‚ÄėFins‚Äô of concrete caused by the mix leaking through butt joints in the¬†formwork should be knocked off. Shallow honeycombing should be chiselled¬†out, and a chase cut along any defective construction joint. The cut-out area or¬†chase should be washed, brushed with a thick cement grout, and then filled¬†with a dryish mortar mix. This rectifying work should be done as soon as possible¬†so the mortar mix has a better chance of bonding to the ‚Äėgreen‚Äô concrete.
    Shrinkage cracking of concrete is a common experience. The shrinkage of¬†concrete due to drying is of the order of 0.2‚Äď0.5mm/m for the first 28 days.¬†Subsequently concrete may expand slightly when wet and shrink on drying.
    The coefficient of temperature expansion or contraction is very much smaller, of the order of 0.007mm/m per degree centigrade of change. Rich concrete mixtures tend to shrink more than lean mixes. The use of large aggregate, such as 40 mm instead of 20 mm, helps to minimize shrinkage. To avoid cracking of concrete due to shrinkage, wall lengths of concrete should be limited to about 9 m if restrained at the base or ends. Heavy foundations to a wall should not be allowed to stand and dry out for a long period before the wall is erected, because the wall concrete bonding to the base may be unable to shrink without cracking. Concrete is more elastic than is commonly appreciated, for example the unrestrained top of a 300 mm diameter reinforced concrete column 4m high can be made to oscillate through nearly 1 cm by push of the hand.

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  • The resident engineer must agree with the contractor where construction¬†joints should be placed; but he should not require them to be placed in¬†impracticable positions and must allow for the manner in which formwork¬†must necessarily be erected. There are positions for construction joints which¬†are ‚Äėtraditional‚Äô even though the position may not seem to be the most desirable¬†from a structural point of view. For instance a construction joint usually¬†has to occur at the base of a wall even though it cantilevers from a base slab,¬†which is a point of maximum tensile stress in one face of the wall concrete.
    This joint is best sited 150 mm above the base slab so as to give a firm fixing for the wall shutters and the best possibility of achieving a sound joint. In water-retaining work it is important to keep the number of construction joints to a minimum.
    The bonding of one layer of concrete to a previous layer is usually accomplished¬†by cleaning the surface of the old concrete with a high pressure water¬†jet, and placing a layer at least 2 cm thick of mortar on the exposed surface¬†immediately before the new concrete is placed. Sometimes a proprietary¬†bonding mortar is used, especially when refilling cut-out portions of defective¬†concrete. Wire brushing of the old surface is not so effective as water jetting,¬†is laborious, and can seldom be properly done when reinforcement passes¬†through a joint. Aproblem frequently encountered is that of finding debris on¬†a construction joint at the bottom of erected formwork. Such debris must be ¬†removed before the mortar layer and new concrete is placed. Usually it is the¬†job of the resident engineer‚Äôs inspector to inspect formwork and the cleanliness¬†of construction joints before permission is given to the contractor to start¬†concreting. If the contractor runs ‚ÄėQuality Assurance‚Äô one of his staff should¬†act as inspector of formwork, but this does not relieve the resident engineer of¬†his need to inspect on behalf of the engineer.

    In liquid-retaining structures resilient plastic waterstops are usually provided at contraction joints. Fixing half their width in the stop-end shuttering to a narrow reinforced concrete wall often leaves a congested space for the concrete which must therefore be most carefully vibrated in place to ensure that the waterstop is bedded in sound concrete. If the concrete face of the joint is to be bitumen painted before the next wall section is built, bitumen must not get on the waterstop.
    Floor joint grooves need cleaning out by water jetting, then surface drying as much as possible with an air blower before the priming compound supplied by the manufacturer of the joint filler is applied to the groove faces. It is essential that this primer is not omitted, and the filler must be pushed down to the bottom of the groove. Joint grooves are normally filled after the concrete has been allowed to dry out for 2 or 3 weeks when most shrinkage on drying should have taken place (see Section 19.11).
    Leaks from liquid retaining concrete structures are most likely to occur from opening up of wall joints due to wall movement, especially at the corners of rectangular tanks; and puncturing of the floor joint filler under liquid pressure where the filler has not been solidly filled to the base of the groove.

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  • Specifications often contain clauses dealing with the transport of concrete,¬†requiring re-mixing after transport beyond a certain limit, limiting the height¬†through which concrete can be dropped, and requiring no concrete be placed¬†when more than a certain time has elapsed since mixing. In practice, problems¬†of this sort seldom prove significant. Sometimes it may be necessary to insist¬†that a contractor uses a closed chute to discharge concrete through a height in¬†order to prevent segregation. Also it may be desirable to ensure mixed concrete¬†is not left unplaced for over-long. A requirement often found in specifications¬†is that concrete must not be placed after it reaches its ‚Äėinitial set‚Äô which, for¬†ordinary Portland cement concrete may take place 1‚Äď2 h after mixing, dependent¬†on temperature, etc. However, a hardening on the outside due to surface¬†drying can occur after about half-hour‚Äôs standing, especially in hot weather.
    If this concrete is ‚Äėknocked up again‚Äô and shows it can be satisfactorily placed¬†it need not be rejected. On the other hand, if a delay is so lengthy that the¬†concrete hardens into lumps, such concrete must be discharged to waste.
    Pumped concrete usually poses more problems for the contractor than it¬†does for the resident engineer, since only well graded mixes relatively rich in¬†cement are pumpable. Usually several mortar batches must be sent through¬†the pipeline to ‚Äėlubricate it‚Äô before the first batch of concrete is pumped¬†through, and pumping must thereafter be continuous. It is not easy to pump concrete¬†more than 300‚Äď400 m. If a stoppage of the flow of concrete occurs for any ¬†reason, the contractor has to take swift action to prevent concrete solidifying¬†in the pipeline. Compressed air is used to force the final concrete batch through¬†the line, followed by water to clean the pipes. Plasticizers are frequently used¬†in pumped concrete; these increase its workability without requiring increased¬†cement or water. There are a wide variety based on different chemicals; BS¬†5075:1982 gives their main characteristics, but they should not be permitted¬†by the resident engineer except to the extent allowed in the specification or¬†sanctioned by the engineer.

    Concrete can also be blown through a delivery pipe using a blower or¬†compressed air. One batch at a time is blown through. The end of the delivery¬†pipe must be directed into the area to be concreted, not against formwork¬†which may be dislodged by the force of the ejected concrete. Proper warnings¬†must be given to personnel before each ‚Äėshot‚Äô because aggregate can rebound¬†and be dangerous, especially when blowing concrete into closed spaces such¬†as the soffit to a tunnel lining.
    The skip method of placing concrete is widely used. Skips can be either bottom-opening, or tip-over. In either case there can be a considerable bounce and sway of the skip when the concrete is discharged. The work should always be under the charge of an experienced ganger who keeps a continuous watch over the safety of his men.

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  • until some days after the concrete has been placed. If weak concrete appears to have been placed in a structure a difficult situation arises. The resident engineer can ask for the offending concrete to be demolished and re-built but this may pose such difficulty and delay that the decision ought not to be made on site without first discussing the problem with the engineer. The action taken depends upon how far the strength of the concrete falls short of the required strength, the load-bearing function of the under-strength concrete, and whether some alternative exists which does not involve breaking out the faulty concrete.
    Frequent site checks of concrete quality can help to avoid such problems.
    Section 19.6 has already indicated that the water content of a mix can easily be judged by eye; and if the quality of the aggregate stocks held on site is kept under reasonable supervision, defects arising from aggregate quality or water content are unlikely to arise. Thus it is to the batching plant, and more particularly
    to the cement content, that checks should be directed.
    One of the simplest on-the-spot tests which can be conducted is the density of freshly made concrete. This should be at least 2350 kg/m3 (147 lb/ ft3) for a C20 mix and 2390 kg/m3 (149 lb/ ft3) for a C30 mix on the assumption that the relative density of the aggregate is 2.65. The trial concrete mixes, however, should have revealed the typical densities expected for various grades of mix. The density can be obtained by filling and weighing an 0.015m3 (0.5 ft3) container with freshly mixed and compacted concrete. An adequately dense concrete cannot be made with badly graded aggregate or with an excess of water.
    If mixing takes place on site the accuracy of the weigh-batching plant should be checked regularly. Actual errors found on a typical hand-operated weight batcher were:
    ‚ÄĘ zero error on scale: up to 15 kg
    ‚ÄĘ 20 mm stone: 78‚Äď106 per cent of required value
    ‚ÄĘ sand: 97‚Äď125 per cent of required value
    ‚ÄĘ cement (ex silo): 80‚Äď110 per cent of required value
    Allowance in a mix has to be made for the weight of the moisture content of the sand which can be very variable when stocked in the open. Fig. 19.4 shows the relationship between the bulking factor and moisture content. Some devices are available for measuring the moisture content of a sand, but measuring the moisture in every batch is not a practical proposition. Instead, typical samples of sand from the stockpile under varying weather conditions can be weighed, then dried and weighed again. This gives a guide as to the weights of fine aggregate to be used under ‚Äėdry‚Äô, ‚Äėmoist‚Äô, or ‚Äėwet‚Äô conditions. The moisture content of the coarse aggregate is not usually checked as it has little effect on the weight of the material.
    Checking the cement content of the mix is particularly important if the cement is held in a silo. Serious under-weights of cement can occur due to machine faults with ‚Äėautomatic‚Äô weighing equipment as well as with operator-controlled

    discharges from the silo. It is better if concrete batches are made up per bag or (more usually) per 2 No. 50 kg bags of cement, in which case only variations in the weight of aggregate affect the mix; but this method is only possible for relatively
    modest concrete outputs, not when large pours are required. The cement content of a mix cannot be directly tested; hence the importance of keeping watch on the batching plant accuracy. It would not be unreasonable for the resident engineer to ask the contractor to conduct regular tests at suitable times on the accuracy of the batching plant. Aresponsible contractor will realize that it is better to ensure his plant is accurate, than to face the difficulty of finding that concrete placed is below the required strength.
    Occasionally on small sites or overseas, volume batching of concrete is used. The weight per unit volume of aggregates has to be obtained by weighing the amount required to loosely fill a measured container. Suitable wooden gauge boxes for aggregate, sand and cement then have to be made up for a given mix. Average weights of Portland cement are 1280 kg/m3 (80 lb/ ft3) loose, or 1440 kg/m3 (90 lb/ ft3) when shaken. If hand mixing is adopted, fairly large gauge boxes with no bottom can be used, since they are placed on a mixing platform, filled and lifted off. They would usually be sized for 1 bag (50 kg) of cement. The bulking of the sand according to its moisture content has to be allowed for.

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