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Professor dr. hab. Sergei N. Leonovich E-mail: snleonovich@yandex.ru Organization of Building works FORMWORK SYSTEMS
WALL FORMWORK • Centering A type of falsework used as a temporary support to masonry arches. • FalseworkThe term applied to timber, scaffolding or other temporary structural framing to support the forms, shutters or moulds for concrete work or masonry arches. Falsework can be of either sawn timber or steel sections, scaffold tube and couplings being an option for the latter. The structure must have sufficient strength to prevent deformation, deflection and collapse under the full load of wet concrete, operatives and equipment. Falsework must remain in position until the permanent structure is sufficiently strong to support itself and ongoing work. • Moulds These are generally associated with the factory manufacture of precast concrete units such as blocks, lintels and feature work of artificial or reconstructed stonework, e.g. sills. Moulds can be of planed timber, steel or plastics. Plastic materials are often used as a smooth lining or facing to timber. • Shuttering A term generally restricted to the use of planed timber, plywood sheeting, steel and other materials used for supporting and shaping the finished concrete on site. Timber products will require mould oil treatment to the surface exposed to concrete, to prevent cement adhering to the timber and subsequently spalling when the shuttering is struck.
Concrete (materials 28%; labour 12%) = 40%. • Reinforcement (materials 18%; labour 7%) = 25%. • Formwork (materials 15%; labour 20%) = 35%.
Low cost Only the amount of money necessary that will produce the required form to be expended. • Strength Careful selection of formwork materials and falsework to obtain the most economic balance in terms of quantity used and continuing site activity around the assembled formwork. • Finish Selection of method, materials and, if necessary, linings to produce the desired result direct from the formwork. Applied finishes are usually specified, and therefore method is the only real factor over which the builder would have any economic control. • Assembly Consideration must be given to the use of patent systems and mechanical handling plant. • Material Advantages of using either timber, steel or glass-reinforced plastics should be considered; generally, timber’s light weight and adaptability make it The most favoured material, but steel and glass-reinforced plastics will provide more uses than timber, although they cannot be repaired as easily. • Design Within the confines of the architectural and/or structural design, formwork should be as repetitive and adaptable as possible. Timber forms have Limited reuse, with exposed surfaces suffering the most damage. Five or six reuses are usual, whereas steel and plastics can be reused indefinitely if cared for. These are pressed or moulded into fixed shapes, having specific application to repetitive uses such as columns and beams in multi-storey buildings. • Joints These should be tight enough to prevent grout leakage. Sealing of joints can be achieved with compressible plastic tape between components or with mastic or silicone sealant applied to junctions of adjacent pieces of formwork.
SLIDING FORMWORK The basic components of slip formwork are: • Side forms These need to be strongly braced, and are loadbearing of timber and/or steel construction. Steel forms are heavier than timber, and more difficult to assemble and repair, but they have lower frictional loading, are easier to clean and have better durability. Timber forms are lighter, have better flexibility, are easier to repair and are generally favoured. The side forms must be adequately stiffened with horizontal walings and vertical puncheons to resist the lateral pressure of concrete and transfer the loads of working platforms to the supporting yokes. • Yokes These assist in supporting the suspended working platforms andtransferthe platform and side form loads to the jacking rods. Yokes are usually made of framed steelwork suitably braced and designed to provide the necessary bearings for the working platforms. • Working platforms Three working levels are usually provided. The first is situated above the yokes at a height of about 2.000 m above the top of the wall forms for the use of the steel fixers. The second level is a platform over the entire inner floor area at a level coinciding with the top of the wall forms, and is usedbythe concrete gang for storage of materials and to carry levelling instruments and jacking control equipment. • Hydraulic jacks The jacks used are usually specified by their loadbearing capacities, such as 3 tonnes or 6 tonnes, and consist of two clamps operated by a piston. The clamps operate on a jacking rod of 25 to 50 mm diameter according to the design load, and are installed in banks operated from a central control togivean all-round consistent rate of climb.
PERMANENT FORMWORK Incertaincircumstancesformworkisleftpermanentlyinplacebecauseofthedifficultyand/orcostofremovingitoncetheconcretehasbeencast. A typicalexampleofthesecircumstanceswouldbewhen a beamandslabraftfoundationwithshallowupstandbeamsandanin-situslabhavebeenconstructed. Apartfromthecostaspect, considerationmustbegiventoanynuisancethatsuchanarrangementcouldcauseinthefinishedstructure, suchasthelikelihoodoffungalorinsectattackandthepossibleriskoffire. Permanentformworkcanalsobe a meansofutilisingthefacingmaterialasbothformworkandoutercladding, especiallyintheconstructionofin-situreinforcedconcretewalls. Theexternalfaceorcladdingissupportedbytheconventionalinternalfaceformwork, whichcanincertaincircumstancesovercometheexternalstruttingorsupportproblemsoftenencounteredwithhigh-risestructures.
PATENT FORMWORK Intryingtodesignorformulatetheidealsystemforformworkthefollowingmustbeconsidered: • Strength To carry the concrete and working loads. • Lightness without strength reduction To enable maximum-size units to be employed. • Durability without prohibitive costs To gain maximum usage of materials. • Good and accurate finish straight from the formwork To reduce thecostlylabour element of making good and patching, which in itself is a difficult operation to accomplish without it being obvious that this kind of treatment was found necessary. • Erection and dismantling times. • Ability to employ unskilled or semi-skilled labour.
Patentorformworksystemshavebeendevisedtosatisfymostoftheaboverequirementsbythestandardisationofformsandbyeasymethodsofsecuringandbracingthepositionedformwork.Themajorcomponentofanyformworksystemistheunitpanel, whichshouldfulfilthefollowingrequirements: • available in a wide variety of sizes based on a standard module, usually multiples and submultiples of 300 mm; • manufactured from durable materials; • covered with a facing material that is durable and capable of producing the desired finish; • interchangeable so that they can be used for beams, columns and slabs; • formed so that they can be easily connected together to form large unit panels; • lightweight so that individual unit panels can be handled without mechanical aid; • designed so that the whole formwork can be assembled and dismantled easily by unskilled or semi-skilled labour; • capable of being adapted so that non-standard width inserts of traditional formwork materials can be included where lengths or widths are not exact multiples of the unit panels.
TABLE FORMWORK The basic requirements for a system of table formwork are: • a means of adjustment for aligning and levelling the forms; • adequate means of lowering the forms so that they can be dropped clear of the newly cast slab; generally the provision for lowering the forms can also be used for final levelling purposes; • means of manoeuvring the forms clear of the structure to a point where they can be attached to the crane for final extraction, lifting and repositioning ready to receive the next concrete pour operation; • a means of providing a working platform at the external edge of the slab to eliminate the need for an independent scaffold, which would be obstructive to the system.
