On the surface: Formwork for visual concrete
Exposed concrete finishes can be both beautiful and sustainable. But their success relies on designers understanding the many different ways of creating formwork, writes Hannah Fothergill
When it comes to high-quality visual concrete, the type, control and accuracy of formwork is often the differentiating factor. There is an old industry adage that good quality concrete is the result of good quality joinery. While contemporary formwork systems are more likely to consist of a kit of parts with reusable layers and elements, bespoke, individually crafted timber moulds are also still possible. The solution chosen by a contractor will depend to a large extent upon the scale and form of the concrete being poured, but may also be dictated by aesthetic decisions such as tie bolt locations and joints. It is therefore useful for architects and engineers to understand the processes and the many different ways of creating formwork to align their design aspirations with achievable outcomes.
In the National Structural Concrete Specification (NSCS), the surface finish of concrete is divided into four categories: basic, ordinary, plain and special. Visual concrete tends to be plain – where visual quality is of some importance – or special, where visual quality is of high importance, and architectural formed finishes and worked finishes are required. Architects and structural engineers do not typically design or choose the formwork system – this is the responsibility of the specialist concrete contractor. However, for visual concrete, where there is a desire to control the sheen or texture of the concrete surface, type of tie bolt holes, joint lines and other details, it is useful to understand the available formwork options and their impact on the concrete. The desired visual concrete results should be clearly described in tender documents, using benchmarks, and design intent drawings, informed by advice from the contractor and supply chain. This is especially important for complex geometries or other unusual features. The contractor’s detailed proposals should be reviewed and agreed before construction begins.
FORMWORK FACING MATERIALS
The formwork facing material is a key area to consider, as this is the surface that is in direct contact with the concrete. The type of facing board may dictate the overall choice of formwork solution, and potential materials include plywood, steel, polycarbonate, timber or special ply boards. Impervious materials such as steel and phenolic-faced ply create a more reflective surface sheen and can give a slightly mottled tonal appearance to the concrete, with more blow holes. Permeable materials, on the other hand, tend to provide a matt finish and fewer blow holes. Some standardised systems offer a choice of facing material, others are integral and more commonly used for basic and ordinary concrete finishes.
Where specific surface finishes or controlled joint lines are required, a form-liner can be added to the surface of the formwork system, providing more design freedom. Form-liners typically
provide little support, but may add some useful stiffness and tolerance control to the formwork.
For curved forms or textured surfaces, the choice of formwork facing material to enclose the concrete will also be dictated by the ability of the material to be bent or made into the design form.
Common types of formwork facing material include:
- Steel. This creates a shiny finish, as it is non-absorbent. Concrete is prone to more blow holes and mottling. Steel is extremely durable and can be reused many times on different projects.
- Phenolic board / high-density overlay board (HDO). This creates a shiny finish and leaves tiny pinholes as it is non- absorbent. HDO is very durable and can be re-used multiple times.
- Paper-faced board / medium-density overlay board (MDO). This creates a matt finish. MDO has medium durability and can be reused, but not as many times as HDO.
- High-quality birch-faced or other ply. The timber grain is lightly expressed in the surface of the concrete, with a matt finish. Reuse is limited.
- Timber. Nowadays, if the visual aim is a classic boardmarked finish, similar to that seen in the National Theatre, this is typically achieved by adding timber boards as a form-liner.
- Polyurethane elastomer. Commonly referred to as flexible form-liners, these reusable form-liners are most commonly used for architectural precast but are also available for in-situ concrete, and can be a cost-effective way to create a textured appearance over a large area. Thousands of standardised patterns are available, and it is possible to create bespoke designs too.
- Glass-reinforced plastic (GRP). This is a good option for repetitive ribbed and coffered slabs with bespoke profiles and where the visual quality of the concrete is important. GRP gives a shiny finish which needs to be rubbed down with post-finishing if a matt concrete finish is desired. Alternatively, the mould maker can sometimes make the GRP face slightly textured, but this should be discussed early on in the design and procurement process.
- Coated expanded polystyrene (EPS). This is available as both off-the-shelf standardised units and bespoke machined shapes. It is usually coated in a durable material to allow reuse, although the number of reuses and as-struck concrete texture depends on the coating type. Commonly used coatings include ABS, correx, fibreglass and polyurea.
- Be imaginative! You can cast against pretty much anything as long as you can strike the formwork. Everything from fabric to dimpled drainage boards and corrugated metal have been used to create textural effects in concrete. Typical formwork systems for columns include:
- Cardboard column formers. Prefabricated cardboard column formers are typically used for rounded columns, either with a spiral or vertical seam. They tend to be single use and are lined internally with plastic, which results in a shiny as-struck finish.
- Steel column formers. Prefabricated steel formers are highly durable and can be reused multiple times on various projects. Commonly used on infrastructure projects, steel column formers give a shiny as struck concrete finish.
OTHER DESIGN CONSIDERATIONS
Important or tricky details should be trialled with full-scale mock-ups and requested in the tender documents. If this is not possible within budget, it is advisable to carry out tests in areas that are not so visually important, such as back-of-house plant rooms and basements.
Tie bolt holes
The layout of tie bolts is another factor that can affect the contractor’s choice of formwork system. Tie bolts are an efficient, modern solution for connecting the two sides of vertical formwork together while holding them the correct distance apart. Without them, large temporary supports and props would be needed around the formwork, taking up space and material on site.
The final positioning of tie bolt holes will be subject to detailed design by the contractor, informed by tolerance and construction requirements and the formwork system selected. If the layout and number is important aesthetically, this should be made clear by the design team on their design intent drawings. Some systems require tie bolts in fixed locations, where they are needed structurally – for example, they are often spaced more closely at wall ends or door openings. Designers can opt to add dummy tie bolts for visual effect, but this needs to be understood and controlled through discussion with the contractor.
The finishing of tie bolt holes must also be considered. A hole will be left in the concrete by the tie bolt cone. This needs to be filled or finished to cover the tie bolt sleeve, which is left
in place. The finish can be expressed or blended in, either by filling on site or with precast cone heads. It is nearly impossible to make tie bolt holes disappear altogether, no matter what finishing method is used.
Joints between sheets of formwork facing material or form- liner will also always be visible. Even with the tightest joints, there will be a small amount of moisture seepage between the boards and some variation in position is to be expected. For a “plain” concrete finish, a 3mm tolerance is permitted between formwork boards within the NSCS v4 specification. Tolerances can be made tighter for “special” concrete finishes, but the NSCS states that these must be achievable. The formwork should be grout-tight and carefully jointed to eliminate grout loss and minimise the inevitable darkened line at the joint from moisture loss.
Board sizes, setting out and joint types are often refined during a trial process, but it is important to establish the design intent pre-tender. When setting out joints it is best to use standard board sizes, usually 1,200 x 2,440mm for flat panel applications. Ply boards of 18mm thickness will usually be sufficient, but this depends on the height of the pour and other site factors, so should be discussed with the contractor as part of its detail design process.
The design team should decide whether they would prefer joints to be installed flush or expressed, and how the boards are to be fixed. Fixing from behind is preferable as this avoids damaging the board facing and does not show in the finished concrete. It is best to avoid using cut-edge boards as they are always more difficult to seal on site. If possible test details of joints with the contractor on a full- scale mock-up.
Traditionally, corner details of concrete were nearly always chamfered. Contemporary formwork systems do enable sharper corners, but these can be prone to damage during the striking of the formwork and by follow-on trades, particularly if corner angles are acute / less than 90 degrees. Repairs are not impossible, but do require colour and texture matching and can add cost.
Coffered and ribbed slabs often have slightly rounded edges or angled sides, known as draft angles. These make it easier to place the concrete and strike the formwork, minimising damage to the concrete and allowing the formwork to be reused.
Release agents are applied to the surface of the formwork to enable it to be removed easily and without damaging the concrete. They work by preventing a bond between the concrete and the form face. There are numerous types of release agent, and concrete contractors are likely to have their own preferences. At the tender stage, it is usually sufficient for the design team to simply state that the release agent should be selected to suit the concrete mix and formwork facing material, and applied correctly in accordance with the manufacturer’s guidance.
The intended release agent should be included in a trial panel to test that it is appropriate before use. If the release agent is incorrectly or unevenly applied, staining can occur on the finished concrete.
It is important not to forget about unformed faces too. These are the surfaces of the concrete that are not poured against the form face or form-liner. Unformed faces are a common detail on stairs and handrails and can either be post-finished by hand or covered with capping details.
COMPLEX FORMS AND INNOVATIONS
Formwork manufacture continues to evolve, and is now at an exciting juncture combining worlds of craft, technology and science. The epoxy resin coated EPS formwork elements supplied by Cordek to form the wonderfully curvaceous bowls of the Folkstone Urban Sports Park ((CQ279, summer 2022) were produced using complex 3D computer modelling and millimetre-precise CNC 5-axis routing; whereas the elaborate and cathedralesque curves of the V&A Dundee (CQ265, autumn 2018) were constructed using bespoke pieces of hand-built timber formwork, supplied by PERI. In both projects, the accuracy and complexity of the 3D curves and unusual geometries were key, albeit achieved through very different methods.
Whether mass-produced, bespoke, machined by robots or built by hand, formwork manufacturers across the board are using 3D modelling and augmented reality to enable architects, engineers and contractors to visualise the spatial relationship between formwork systems and their surroundings, thereby aiding early design and site coordination.
A great deal of innovation and research is also focused on material efficiency and sustainability, which in turn is creating new languages of architecture. The capabilities of digital engineering software paired with advancements in 3D printing technologies for formwork presented the opportunity to create ultra-slender structural slabs, placing concrete only where needed, at DBT’s admirable Smart Slab project in Zurich. The importance of material efficiency is by no means limited to the concrete itself. Designers and contractors are increasingly appreciating formwork as a valued resource within the circular economy of construction rather than a disposable by-product.
Projects such as Pear Tree House by Edgley Design, Hugh Broughton’s IstructE headquarters and Laboratorio Espresso by DO-Architecture have imaginatively and successfully reused timber formwork boards to create beautiful interior fit-outs and external fencing. Innovations in fabric formwork are also opening the door to super-lightweight and easy-to-transport materially efficient formwork. Structure Mode’s school project in Cambodia is a great example of this, with fabric offering a quick and simple solution that also engaged seamstresses and tailors from the local community with the project.
Texture and pattern are seeing something of a renaissance in concrete as a means of creating sensory interest and architectural character but perhaps most encouragingly to promote and support biodiversity. The Mumbles Seahive project designed by Cubex Industries in partnership with Swansea University is exemplary of this. The design team used 11 standard patterns and two bespoke seaweed and oyster shell patterns from flexible form-liner supplier Reckli to create a series of ultra-high-performance concrete panels, which were then applied to the Mumbles sea defences. After just three months, colonisation of the panels by barnacles and other sea life had already begun – a genuine bringing together of craft, technology and science in the art of concrete formwork
Photos: Roman Keller, Hufton + Crow, PERI Ltd, James Brittain, Hufton+Crow; Hollaway Studio, Edgley Design, Daniel Hopkinson, Timothy Soar, John Wood Photowork, Arup