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Fig. 13: Profiles of reinforcing bars

Fig. 13: Profiles of reinforcing bars ments, the arrangement and spacing of reinforcing bars and meshes also has to take account of optimum compaction; a poker vibrator must be able to pass through the cage of reinforcement.

Great attention must be paid to ensuring that the reinforcement has adequate concrete cover. Almost all damage to reinforced concrete structures can be attributed to insufficient concrete cover and not settlement or a lack of reinforcement. Sections with inadequate concrete cover are potential weak spots and invite corrosion of the reinforcing bars. The oxide crystals of the rust require more volume than the steel, and the ensuing bursting action results in the concrete cover cracking, thus allowing further corroding influences (moisture, air) even easier access to the steel, which can, in the end, impair the load-carrying capacity of the member. The concrete cover, i.e. the distance between the concrete surface (or the surface of the formwork) and the nearest reinforcing bars, depends on various factors but should not be less than 3 cm.

Fig. 15: Steelfixers at work

Formwork

In order to achieve the desired final form, concrete is cast in formwork.

Concrete cast in formwork on the building site is known as in situ concrete. The concrete cast in a factory, to produce prefabricated components, is known as precast concrete.

The building of formwork for concrete sometimes calls for excellent carpentry skills. The f ormwork material itself must be of sufficient strength and must represent a stable assembly propped and stiffened so that it remains dimensionally accurate (no distortion) during placing and compaction of the concrete.

All butt and construction joints must be sealed with appropriate materials, and the f ormwork must be leak-proof on all sides to prevent cement paste from escaping during compaction.

Formwork for concrete surfaces that are to remain exposed in the finished building can make use of a number of materials depending on the type of surface required, e.g. timber boards, wood-based panels, sheet steel; even fibre- cement, corrugated sheet metal, glass, rubber or plastic inlays are used on occasions.

Timber formwork

Boards

In Switzerland the timber boards used for f ormwork are mainly indigenous species such as spruce or pine. The selection and assembly of the boards presumes a certain level of knowledge and experience. Boards of the same age having the same density and same resin content will exhibit similar absorption behaviour; boards with a high or low resin content can be seen to behave differently as soon as the release agent (oil, wax emulsion) is applied. Concrete surfaces cast against new, highly absorbent boards will have a lighter colour than those cast against old or reused boards.

Format: The dimensions are governed by the possibilities for solid timber. The boards should not distort when in contact with water or moisture. Max. width: approx. 30 cm; max. length: approx. 500-600 cm; customary width: 10-15 cm; customary length: up to 300 cm.

Panels

Compared with timber boards, f ormwork panels made from wood-based materials have considerable advantages. They are lighter in weight and can be assembled faster (50-70% of the erection costs can be saved when using panels instead of boards). In addition, they last longer because the synthetic resin lacquer which is normally used to coat such panels detaches more readily from the concrete when striking the f ormwork. Format: Formwork panels are available in the most diverse sizes with the maximum dimensions depending on the conditions on site. In Switzerland the formats 50 x 200 cm and 50 x 250 cm, for example, are widely used.

Modular formwork, table forms,, wall forms Industry can now supply a highly varied range of formwork systems that enable large areas to be set up and taken down quickly: modular elements for walls, floor formwork with appropriate propping, self-supporting climbing and sliding formwork, etc.

In order to combine the economic advantages of modular f ormwork with the aesthetic qualities of other types of formwork, modular formwork is these days often used merely to support "traditional" boards and panels.

Steel formwork

Forms made from sheet steel are used both for in situ and precast concrete. The higher capital cost of such formwork is usually offset by the high number of reuses possible.

Fig. 17: Steel wall forms

Formwork surfaces

The formwork material (timber, wood-based panel, plywood, hardboard, fibre-cement, steel, plastic, etc.) and its surface finish (rough, planed, smooth, plastic-coated, etc.) determine the surface texture of the exposed concrete.

The smoothness or roughness of the f ormwork can influence the shade of the exposed concrete surface. For instance, completely smooth f ormwork results in an exposed concrete surface with a lighter colour than one produced with rough formwork.

Release agents

These are oil, wax, paste and emulsion products applied to the contact faces between the formwork material and the concrete to enable easier separation of formwork and concrete surface - without damage - when striking the forms. In addition, they help to create a consistent surface finish on the concrete and protect the formwork material, helping to ensure that it can be reused.

The suitability of a particular release agent depends on the material of the formwork (timber, plywood, hardboard, fibre-cement, steel, plastic, etc.).

Placing and compacting the concrete

Good-quality exposed concrete surfaces call for a completely homogeneous, dense concrete structure. The wet concrete must be placed in the concrete without undergoing any changes, i.e. segregation, and then evenly compacted in situ.

Compacting

The purpose of compacting the concrete is not only to ensure that the formwork is completely filled, but rather to dissipate trapped pockets of air, distribute the cement paste evenly and ensure that the aggregates are densely packed without any voids. In addition, compaction guarantees that the concrete forms a dense boundary layer at the surface and thus fully surrounds the reinforcement.

Methods of compaction Punning: with rods or bars

Tapping forms: for low formwork heights Vibrating: standard method on building sites immersion (poker) vibrators are immersed in the wet concrete external vibrators vibrate the form-work from outside Tamping: in the past the customary method of compaction

Fig. 18: Compacting the concrete

Vibrating

A poker vibrator should be quickly immersed to the necessary depth and then pulled out slowly so that the concrete flows together again behind the tip of the poker.

Vibrators should not be used to spread the concrete because this can lead to segregation. If segregation does occur during compaction, the result is clearly recognisable differences in the structure of the concrete, possibly even honeycombing on the surface.

The depth of concrete placed in one operation should be limited. The weight of the wet concrete can be so great that pockets of air cannot escape to the surface.

Fig. 18: Compacting the concrete

Curing

The hardening, or setting, of the concrete is not the result of it drying out. If we allow concrete to dry out too quickly, this leads to shrinkage cracks because the tensile strength is too low. And if we sprinkle the concrete with water, efflorescence (lime deposits on the surface) will almost certainly be the outcome. The answer is to allow the concrete to retain its own moisture for as long as possible, which is best achieved by covering it with waterproof sheeting. These must be positioned as close to the concrete surface as possible but without touching it because otherwise they may cause blemishes.

Such methods are labour-intensive but indispensable for exposed - especially fair-face - concrete surfaces.

Level of concrete before compaction

Level of concrete before compaction

Figs 19 and 20: Compacting with a rod (punning) (left) and a poker vibrator (right)

Compaction procedure

Concrete already compactec

Figs 19 and 20: Compacting with a rod (punning) (left) and a poker vibrator (right)

Compaction procedure

Construction joints

When working with in situ concrete, joints between earlier and later pours are almost inevitable. The strength of the formwork required to resist the pressure of the wet concrete also places a limit on the quantity of concrete that can be economically placed in one operation. Concreting operations must therefore be planned in stages and separated by joints.

The location and form of these construction joints are determined by the architect and the structural engineer together. Given the fact that it is impossible to conceal such joints, it is advisable to plan them very carefully.

If new concrete is to be cast against a existing concrete surface (a construction j oint), the concrete surface at the point of contact must be thoroughly roughened and cleaned, and prior to pouring the wet concrete wetted as well. And if such a construction j oint must be watertight, it is advisable to use a richer mix at the junction with the existing concrete or to coat it first with a layer of cement mortar. It is also possible to add a retarder to the last section prior to the construction j oint so that the concrete at the intended j oint position does not set immediately and the following concrete can then be cast against this "still wet" concrete.

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