Structures

Form Follows Formwork

Designers tend not to think about formwork. By its nature, it is “temporary works” and the permanent-works designers leave it to the contractor to sort out. It is, however, very useful for the permanent-works designers to have an appreciation of what is available, and to understand how their decisions affect the formwork and its design.

This article covers standard systems for falsework and the options available for formwork. Formwork is the part of the system against which the concrete is cast. Normally it is made of ply, steel or GRP, but other materials can be used. Falsework is the temporary scaffolding that holds the formwork in place. We normally call the system as a whole “formwork”. Most falsework is prefabricated and modular, and normally hired from the formwork suppliers.

Together the formwork and falsework represent approximately half the cost of an in-situ concrete frame. This makes it more important, in terms of cost and programme, than either the type of concrete used or the quantity of reinforcement. For a reinforced-concrete bridge, formwork can represent up to 90% of the cost. Therefore, to achieve an economic solution for a concrete frame, the designers should think about the formwork required, and make it as simple and straightforward as possible.

All formwork should be designed to take the pressures from the wet concrete, no matter which type of finish is required. For basic and ordinary finishes (see National Structural Concrete Specification v4) no further requirements need to be considered. For plain and special finishes, “good quality” formwork is specified and the arrangement of the panels and tie bolts should be in a regular pattern. The maximum step in the concrete between formwork panels is 3mm.

Design of the formwork system

The design for the formwork is normally done by the specialist temporary-works designer to British Standard BS 5975, “Code of practice for temporary works procedures and the permissible stress design of falsework”. The formwork needs to take the full weight of the wet concrete before the concrete sets. Typically this is taken as 26kN/m3. Normally, walls are formed with two-sided formwork so that the pressure from the concrete can be taken by ties through the concrete wall.

The pressures on the wall formwork are typically 60-80kN/m2 and can easily exceed 100kN/m2. If self-compacting concrete is used, the pressures tend to increase as the wet concrete is designed to flow, rather than just slump. The ties through the concrete wall can typically take 95kN at working loads so spacing of the ties is critical and usually set out by the formwork designer, or fixed in the type of formwork system employed.

Ties should not be spaced too far apart or too far from the end of the formwork, or it will deflect too much under the weight of the wet concrete. For plain or special concrete finishes, the location of the ties in relation to the formwork panels should be considered, as it is always visible. Tie-bolt holes are frequently filled, but the outline is always evident.

Where the wall is one-sided – ie cast against another wall, such as a lining wall cast against a contiguous piled wall – then the pressures have to be taken by significantly more extensive falsework, typically in the form of A-frames. These need to be anchored into the permanent structure or into the ground to prevent movement.

Systems

Innovation in formwork has had a huge impact on the speed of construction of concrete-framed buildings and structures. Modern formwork systems have continued to evolve to improve health and safety on site as well as cost and programme benefits. The elements in the falsework used for the systems are reused many times in order to keep environmental impact to a minimum.

Table formwork is a large pre-assembled formwork and falsework unit, often forming a complete bay of suspended floor slab. It offers mobility and quick installation for construction projects with regular plan layouts or long, repetitive structures, so is highly suitable for flat slabs. In tableform systems, a series of individual falsework components, including primary beams and props, are connected to form a complete table, with plan area of up to about 100m2.

The formwork can be mounted on castor wheels or trolley units, allowing it to be moved horizontally. It relies on there being sufficient room for the tableform to swing out after the concrete has been struck, to be replaced on the slab above. The area below the tableform must be kept free of personnel during the lift, and these health and safety implications need to be taken into consideration when using tableforms.

Horizontal panel systems are now more common than tableform systems. These lightweight modular systems are engineered to be robust, easy to handle and capable of dealing with both regular and irregular formwork areas, with the number of different components minimised for quick installation. The system comprises props and panels, with the heaviest component capable of being handled by two operatives. This frees up the crane for other uses.

The column formwork systems now available are normally modular in nature and allow quick assembly and erection on-site while minimising labour and crane time. They are available in steel, aluminium and even cardboard (not reusable, but recycled) with a variety of internal face surfaces depending on the concrete finish required. Innovations have led to adjustable, reusable column forms that can be clamped on-site to give different column sizes.

Crane-lifted wall panel systems are commonly used on building sites to form vertical elements and usually consist of a steel frame with plywood, steel, plastic or composite facing material. The systems are normally modular in nature, and assembly times and labour costs are considerably lower than traditional formwork methods with far fewer components required. Panel systems are extremely flexible and can be used for constructing standard concrete walls, perimeter basement walls and columns, and in conjunction with jumpform climbing systems.

Just as concrete can be moulded into any conceivable shape, formwork can also be provided in any shape. The one proviso is that the form has to be able to be demoulded. Intricate shapes can be made, but re-entrant corners and other difficult to demould shapes should be avoided. Formwork suppliers can supply 3D forms for nonstandard cases to site which can then be bolted together. There are often a range of ways in which a particular shape can be created, depending on the balance of requirements for reusability, cost and finish. For formwork options for casting ribbed slabs, see CQ 268.

Formwork finishes

Where visual concrete is specified, the type of formwork facing material is critical to the appearance of the concrete. Generally, the less absorbent the formwork face, the shinier the finished surface of the concrete. So steel formwork will tend to give a shiny surface and MDO (medium density overlay) ply will give a matt finish. Table 1 summarises the finishes available from the different types of formwork.

This is not an exhaustive list and other options are available. Formwork linings, often using the materials mentioned, can also be used together with a proprietary falsework system to provide the desired surface finish. The choice of formwork depends on the finish required and the number of times the formwork can be reused.

Steel formwork can be reused thousands of times if maintained and cleaned properly, but costs significantly more than the ply formworks available. Contractors will tend to choose formwork based on the required finish, programme, repeatability and cost to come up with the most appropriate and cost-effective solution.

Future formwork

Formwork design continues to evolve and to be an area of innovation, with suppliers starting to use 3D visualisation to check it on site and to ensure coordination between the formwork and any inserts or cast-in brackets, in collaboration with the BIM model. One recent development has been Mace’s jump factory (CQ 269), which allows hybrid – that is, precast combined with in-situ – concrete work to be carried out under factory conditions inside a movable enclosure. This is lifted up with the building as it progresses, and can allow the contractor to achieve speeds of one storey every 55 hours, which includes the installation of prefabricated pods.

Formwork is an important part of the design and construction of concrete, too frequently overlooked. But it is well worth developing an appreciation of the ways in which formwork can add to the design concept, and suppliers are very willing to explain the possibilities and provide advice to help designers get the best out of concrete.

Table 1: Types of Formwork Finish

Formwork finish Description
Softwood plywood Plywood consists of a number of plies of timber laid orthogonally to the adjacent plies and glued together.
WBP grade should be used for concrete work. Softwoods are not ideal because tannins can be released,
staining the concrete, and they have a limited number of uses before the surface becomes unsuitable.
Natural birch
plywood

Birch is a hardwood and ideal for forming visual concrete surfaces. Birch can be used throughout the
plywood or as veneer to a softwood plywood. A high-quality face finish should be chosen for visual concrete.
Waterproofing is required and an acrylic lacquer is recommended.
Phenolic film faced
(PFF) plywood
Plywood can be faced with a phenolic film to maximise the number of reuses, hide the wood grain and
defects, and prevent timber staining the concrete. Phenolin is a fully cured and impervious resin hot-pressed
onto the face of the plywood to give a dark brown, waterproof finish. A shiny smooth concrete finish is
obtained.
Medium density
overlay (MDO)
An overlay of paper saturated in resin is used with plywood to minimise tannins released from the timber.
MDO is less prone to temporary surface ripple than PFF. MDOs generally give a satin finish to the concrete,
although some products do offer a matt finish.
High density
overlay (HDO)

This has a higher resin content than MDO and thus gives a better surface finish, at a cost. Tannin migration
is prevented, which makes it more suitable for situations in which the ply is in contact with the concrete for
longer periods, for example slabs rather than walls. HDOs give a shiny finish to the concrete.

PFF orientated
strand board (OSB)

An alternative to PFF plywood. OSB is an engineered timber product made from strips or strands of timber. It
is orientated orthogonally and bonded with a resin.
Steel Steel can be used to give a high-quality finish and is most cost-effective through multiple reuses. It is
currently used for column shutters and tunnel-form construction. The face of the steel must be treated with
care, as any marks and imperfections will be imprinted on the concrete surface. Sand-blasting must not be
used to clean the shutter, as it will pit the surface.
Cardboard (EPS
and plastic lined)
Impermeable cardboard is often used for circular column formwork. It can be used for visual concrete but
tends to be difficult to strike and is disposed of after each use. However, the cardboard former can be used as
temporary protection to the column.
Polypropylene,
plastic composite
Alternative materials have been developed, avoiding the problems of swelling, staining and water
damage associated with timber-based systems. Manufacturers claim long life and a high-quality finish
can be achieved with the right workmanship. The provision of “seamless” formwork may be possible in

Photos Richard Chivers, Hunnebeck, Peri

Top to bottom The formwork for Kengo Kuma’s V&A Dundee comprised 1,200 unique CNC-cut shutters, created by Peri from plywood-faced timber with a controlled-permeability liner. These were supported on a vast system of steel falsework; Hunnebeck’s TOPEC modular slab formwork in use at Helical Bar’s Fulham Wharf development in London; The in-situ slab at PAD Studio’s Canoe Lake tennis pavilion in Portsmouth was poured in one go using phenolic-faced ply formwork.