Formwork shells have a major impact on the look and quality of any exposed concrete surface.
Formwork shells keep the liquid concrete in shape as it is being poured. It is generally removed after hardening. Which shell is right for a construction project depends on the function and type of building. The choice of formwork system and shell can also impact the look of the finished concrete surface.
In principle, formwork shells must be precise, well-sealed, clean and stable. This is the only way to ensure that it will stay put under the pressure of the poured concrete, that no concrete leaks out, that the concrete elements have the right dimensions and that the visible surface is free from defects.
Formwork shells may be made from one of three materials:
Wooden formwork shells are the most traditional option in the construction industry. Depending on the wood used, the finished concrete may have a raw or smooth surface. Wooden formwork shells can be used like a negative mold on the concrete to emboss a certain wooden pattern into the surface. When producing wooden formwork shells, the boards must be very densely arranged in order to avoid visible gaps in the finished concrete. Discoloration in the concrete can be prevented by sealing the wood with varnish. Unsanded, raw shell boards create a rustic concrete surface, while planed boards or plywood sheets create a smooth concrete finish. But even planed or burnt boards do not leave an extremely smooth surface: knots in the wood also appear on the concrete.
Steel formwork shells can be used when concrete surfaces must be completely smooth. Unlike wooden formwork shells, which generally cannot be reused, steel formwork can be used for multiple batches. It is therefore more expensive than wooden shells. What’s more, more deformation of the formwork panels is to be expected.
Plastic formwork is relatively inexpensive and can be reused several times. Shells made from fiberglass-reinforced polyester are particularly resilient to wear and tear. For the best possible results, plastic shells must be watered before pouring. Since the smooth plastic surfaces are not absorbent, water might be collecting at the surface.
Creative freedom with formliners
While wooden shells can be used to emboss various wooden textures into the concrete and steel or plastic shells ensure a smooth finish, formliners give you even more freedom when creatively designing exposed concrete. Elastic textured formliners by RECKLI can implement any design on exposed concrete surfaces and are reusable up to 100 times. Architects and planners can choose from over 200 textures in the RECKLI SELECT series to implement natural looks such as wood and rock, ripples and waves, Oriental-inspired and fantasy patterns. With formliners from the VISUALS series, graphics, photos and even text can be imprinted into the concrete. And formliners from the RECKLI UNIQUE series are custom-made to the client’s own needs and designs.
To realize the desired look in exposed concrete, architects and planners should weigh all the various shell options and establish their requirements for their chosen shell early on in the planning phase.
Choosing a system that works with shell anchors has a massive impact on the look and function of the finished exposed concrete surface. Shell anchors lock the shell to the shuttering to prevent the pressure of the freshly poured concrete from creating spills out of the formwork. DIN 18216 precisely stipulates requirements for the use of shell anchors.
After the shell is removed, the shell anchors leave a hole. The diameter and look of these anchor holes vary depending on the producer of the particular shell system. The holes are sealed with plugs that may be made from polyurethane, aluminum or concrete. Alternatively, the holes may be plastered over.
Weighing up absorbent and non-absorbent shells
Untreated boards on wooden formwork are absorbent and therefore soak up water from the concrete immediately next to the wood as they hydrate. Therefore, absorbent shells should be watered 6 to 12 hours before use. This prevents them from soaking up too much water from the fresh concrete.
The boards take in more water during their first use than during subsequent uses. It is therefore advisable to never use new shell boards together with older boards in formwork shells: the difference in absorbency can impact the look – especially the color – of the dry concrete surfaces and may lead to irregular results.
When using untreated wood for formwork shells, substances within the wood may be released and cause dusting on the concrete surface. To prevent this, workers may apply cement slurry to the formwork shell.
Plastic-coated wooden shells with especially treated wood, steel shells and plastic shells are all non-absorbent shells. RECKLI formliners are also non-absorbent. Excess water collects and allows the production of very smooth surfaces that comply with the highest class of exposed concrete.