n The formwork should be sufficiently rigid to prevent undue deflection during the placing of the concrete.
n It should be of sufficient strength to carry the working loads and the weight or pressure of the wet concrete, and to withstand incidental loading and vibration of the concrete.
n It should be set to line and level within the specified tolerances and include any camber that may be required.
n Joints should be sufficiently tight to prevent loss of water or cement paste from the concrete, which can have a serious effect on the appearance of the finished concrete.
n The size of panels or units should permit safe and easy handling using the equipment available on site. The design should permit an orderly and simple method of erection and striking.
n The arrangements of panels should be such that they are not 'trapped' during striking, and it should be possible to strike side forms from beams without disturbing the soffit formwork.
Types of formwork
Over the years the number of materials used for formwork has grown considerably, although traditional methods using materials such as timber are still used. Formwork facing materials include timber, plain and resin-faced plywood, steel, alloy, concrete, glass fibre reinforced plastics (GRP), glass fibre reinforced cement (GRC), hardboard and expanded polystyrene. In addition, form liners of rubber, thermoplastics or other sheet materials, including permeable liners, to produce controlled permeability formwork (CPF) systems may be used (see Influence of formwork on page 49). Some liners are re-usable but others can only be used once.
Many systems of proprietary formwork are available, and large jobs often make use of special formwork designed as a system for that job (Figure 29). Precast concrete, profiled steel decking units, GRC panels or other materials may be used as permanent formwork; the supplier's instructions regarding bearings, supports and ties must be carefully observed.
The system of false work supporting the formwork must be designed to withstand the loads imposed on it. Tubular steel scaffolding and adjustable proprietary steel props are the most common forms of support, although heavy-duty shoring and specially designed supporting systems are often required. When adjustable steel props are used, they must be installed so that they are vertical and loaded axially, and the hardened steel pins provided by the manufacturers must be used.
Slip forms are occasionally used for walls, lift shafts and building cores, silos, towers, chimneys and shaft linings. This type of form is moved almost continuously, usually by means of hydraulic jacks, leaving concrete of the required shape and dimensions behind. Slip forming saves time by eliminating the task of striking and resetting formwork and by allowing continuous concreting, but it is not normally an economic solution for vertical structures less than about 15 m high. The design and operation of slip forms require considerable experience and are usually undertaken only by specialist subcontractors.
Design of formwork
Formwork should be designed to withstand all expected loads. These include the self-weight, weight of reinforcement, weight of wet concrete, construction and wind loads, all incidental loads caused by placing and compacting the concrete, and the horizontal pressure of the wet concrete against vertical formwork. Detailed information about these loadings is given in BS 5975, CIRIA Report 108 and the Concrete Society publication, CS030.
Particular care is needed to provide an adequate number of form ties where these are used to link together the opposite panels of a wall form. Whereas slightly inadequate design of other elements of the formwork may lead to large deflections or leakage, the failure of form ties can more easily cause a dangerous collapse. Failure in ties may also occur when they are over-tightened or put into bending rather than simple tension.
The strength of formwork, although very important, is often secondary to its stiffness, which must be sufficient to prevent it deflecting significantly under load, otherwise the resulting concrete surface will show the deformation. When using plywood, it is important to recognize that stiffness parallel to the face grain is less than the stiffness at right-angles to it.
Formwork must be watertight, because small leaks lead to unsightly stains on the concrete surface and large leaks can cause honeycombing. The use of foamed plastic sealing strip or moisture curing gunned silicone rubber provides effective means of sealing joints. Joints may also be sealed by adhesive tape, but it must be accepted that such joints will be apparent on the finished surface. Such patterning is generally acceptable in public spaces such as car parks if it is formalized or set uniformly with the bays of the structure.
Surface treatment
Where the appearance of the concrete is of importance, it is vital that care is taken with the surface of the form. All marks on the form, such as vibrating poker 'burns', as well as varying properties in the form-face material, for example uneven water absorbency in timber, will show on the finished concrete. Loose wire and other debris should be cleaned out of forms prior to concreting; this is usually done with a compressed air hose. It is particularly important that all steel particles are removed as they will rust and spoil the final appearance of the concrete.
To permit easy striking of the formwork and to reduce the incidence of blowholes, the surface of the form must be coated with a release agent prior to concreting. There are various types of release agent, the merits of which are summarized in Table 16. The most useful are chemical release agents, neat oils with surfactants, and mould cream emulsions. Release agents should be applied to give a very light film. A common fault is the use of too much. If it is thin, application by an airless spray is recommended. Thicker oils may be applied by brush or cloth and spread as far as possible, all excess being removed with a cloth. The use of barrier paints produces a hardwearing surface and may extend the life of timber or plywood forms. If paint is not used, three coats of mould oil should be applied before the form is used for the first time. Some barrier paints are not suitable for use with certain tropical hardwoods, and the manufacturer's advice should be sought on this point. To avoid contamination of reinforcement, the release agent should be applied before the forms are erected, but then it may be necessary to protect the forms from the weather.
Table : Types of release agent.
Release agent type
|
Comments
|
Chemical release
agent
|
Recommended for all types of formwork. Suitable for high-quality finishes. Based on light,
volatile oils that usually
dry on the surface of the
form to leave a thin coating which is resistant to washing off by rain. The dried coating gives a safer surface to walk on than an oily film, and
the release agent does not then transfer from
operatives' footwear onto reinforcement. Rate of
coverage is greater than for conventional oils.
More expensive for a given volume but can be
economical if used sparingly.
|
Mould cream
emulsion
|
Widely used release
agent recommended for all
types of formwork except steel. Especially
recommended for absorbent forms such as timber. Suitable for high-quality finishes. Mix
thoroughly before application and use as supplied without further
dilution. Avoid using
emulsions when there is a risk of freezing. Storage life may be limited.
May be spray-
applied with care.
|
Neat oil with surfactant
|
A useful general-purpose release agent for all
types of formwork, including steel. Over-
application may result in staining of the
concrete. Oil film may be affected
by heavy rain.
|
Unpainted timber forms become progressively less absorbent as the pores of the wood become filled with cement paste during use. This affects the appearance of the finished work - an abrupt colour change will be seen on the concrete - so new and old materials should not be used alongside each other. Similarly, patches of new material in old formwork will produce noticeable colour changes.
In a similar way, forms made from painted timber and various types of plastics having a glazed or glossy surface produce an appearance that changes with the number of uses. In this case the surface glaze is reduced by the first use, and subsequent uses produce a less highly polished surface on the concrete. Some plastic-faced plywoods give a similar effect. The first few uses of these materials can sometimes produce a very hard, dense, and almost black surface to the concrete. This is probably caused by slight movement of the form at a critical time during the hardening process; normally it occurs only with impervious form faces. Similar discolouration of concrete placed against a steel form can usually be attributed to the presence of mill scale on the steel.
Striking of formwork
The period which should elapse before the formwork is struck will vary from job to job and will depend on the concrete used, the weather and the exposure of the site, any subsequent treatment to be given to the concrete, the method of curing and other factors. Formwork must not be removed until the concrete is strong enough to be self-supporting and able to carry imposed loads. Thus, the time of striking should be related to the strength of the concrete, and obviously soffit forms to beams and slabs must be left in place longer than is necessary for the side forms.
Subject to the requirements of the specification and where no other information is available, the periods given in Table 14 under Strength development may be taken as a general guide for the removal of formwork. It should be noted that these periods relate to concrete made with Portland cement CEM I 42,5N; shorter periods may apply where a more rapid-hardening cement is used, but considerably longer periods may be required where the concrete contains ggbs or pfa.
When using Table 14, if the surface temperature cannot be obtained, air temperatures may be used. Alternatively, the tables of striking times published in CIRIA Report 1 36 can be used. These take into account the grade of the concrete, the cement type, the dimensions of the section, the type of formwork, the temperature of the concrete at the time of placing and the mean air temperature. Shorter formwork striking times are achievable by measuring the strength development of the in-situ concrete.
Soffit formwork may be struck when the in-situ strength of the concrete is 10 N/mm2 or twice the stress to which it will be subjected, whichever is the greater. The in-situ strength can be assessed by pull-out tests (see page 60) or from cubes cured, as far as possible, under the same conditions as the in-situ concrete or by temperature-matched curing by which test cubes are immersed in water whose temperature is made to copy that of the structure. Proprietary quick-strip systems permit the removal of soffit formwork without disturbing the propping.
When formwork to beam sides, walls and columns is struck at early ages the concrete will still be 'green' and easily damaged, so extra care is required to avoid damage to arrises and other features; this is particularly important during cold weather. Striking must be carried out with care, to avoid damage to arrises and projections, and it may be necessary to protect some of the work from damage immediately after removing the forms.
Curing should start as soon as the formwork has been removed and, if necessary, the concrete should be insulated as a protection against low temperatures. Timber formwork is a good insulator in its own right, so in winter it is particularly important to avoid thermal shock to the warm concrete when timber or insulated steel forms are removed and the concrete is exposed to the cold air. If the formwork is not required elsewhere, it may be convenient to leave it in place until the concrete has cooled from its high early temperature. The formwork must be removed slowly as the sudden removal of supports is equivalent to a shock load on the partly hardened concrete. Careful removal is also less likely to damage the formwork itself.
Care of formwork
Formwork frequently accounts for over a third of the cost of the finished concrete, so it should be handled with care. The life of forms can be extended considerably by careful treatment, thus decreasing the overall cost of the job. Rough treatment may make timber and plywood forms useless after one pour, whereas eight or more uses may be obtained by following good site practice.
All formwork should be cleaned as soon as it has been struck. Timber and plywood forms are best cleaned with a stiff brush to remove dust and grout; stubborn bits of grout can be removed using a wooden scraper. With GRP and other plastics, a brush and wet cloth are all that should be needed. The use of steel scrapers should be restricted to steel formwork.
Steel forms should be lightly oiled to prevent rusting if they are not going to be re-used immediately; similarly, timber and untreated plywood should be given a coat of release agent for protection.
Any formwork surface defects such as depressions, splits, nail holes or other unwanted holes should be repaired and made good.
Formwork should be properly stored and protected. Panels and plywood sheets are best stored horizontally on a flat base, clear of the ground, so that they lie flat without twisting, and should be stacked face-to-face to protect the surfaces. Large panels are usually best stored vertically in specially made racks. Stored formwork should be protected from the sun and weather by tarpaulins or plastic sheeting.