An overview of exposed concrete classes and the requirements that must be fulfilled in planning, implementing and inspecting exposed concrete surfaces.
Exposed concrete is a term that covers concrete surfaces forming a visible part of a building’s architectural or interior design. Strictly speaking, exposed concrete surfaces are those that were in contact with the formwork shell when they were being created.
The industry’s most important orientation aid is the code of practice for exposed concrete, produced by the Deutsche Beton- und Bautechnik-Verein (DBV) and the Verein Deutscher Zementwerke (VDZ). It is aimed at architects, planners, developers, project managers, surveyors and concrete technicians. The latest version was issued in 2015.
The code defines four classes of exposed concrete:
These defined classes can be used to evaluate the quality of exposed concrete throughout planning, implementation and inspection. The code also provides consolidated information on the planning, tendering, production and evaluation of exposed concrete surfaces. As terms and requirements for implementation are clearly defined, clear communication between clients, architects, planners and contractors should be ensured, minimizing the risk of legal disputes. It is therefore advisable to make the code part of any contracts.
When using exposed concrete, a precise specification of services must be upheld alongside stipulations regarding shell skin texture, choice of material and color, concrete surface treatment, sample and reference surfaces and constructive design. In the specification of services, the planners choose the class of exposed concrete from the table in the code of practice for exposed concrete and describe the required surface characteristics, surface texture and any additional requests, such as surface coloring.
The Formwork Material Is Key
The formwork material has a decisive impact on the look and feel of exposed concrete: Formworks made from OSB or untreated wood create very different looks on exposed concrete, as the wood itself impacts the finished appearance. When producing exposed concrete, system formworks are often used, stipulating set frame sizes and the positioning of anchor holes. The set resulting look can be avoided by using a custom supporting formwork – ideally, planners will have already defined this decision back in the specification of services. Clients and developers can set the details for the formwork during the construction process using sample surfaces.
The concrete’s composition is also important for the quality of exposed concrete. Phenomenons such as demixing and bleeding must be avoided during installation and sealing. Concrete processors can fall back on a few ground rules that have proven useful in practice. These include using a sufficiently high mortar content, a cement content ≥ 300 kg/m³ and a w/c value ≤ 0.55 (can be achieved by using a liquifying additive). In order to prevent negative consequences in terms of color, no leftover water or concrete should be used. When mixing the base material, quantity deviations should be kept as minimal as possible because even small differences can be obvious on the light concrete. All concrete production should continuously stem from the same production facility using the same base materials for the full duration of the build. Different color finishes can be achieved with white cement, colored aggregate or pigments.
Creating Sample Surfaces for Comparison
Constructing and sealing the concrete monolithically can help prevent seamlines in the concrete. As already mentioned, the formwork shell impacts the design of the exposed concrete surface. Alongside wooden frames with their various types of wood and finishes, steel shells and formliners can be used to design the surfaces of exposed concrete elements. The use of formliners is also possible with the highest class of exposed concrete, guaranteeing the required quality on the finished concrete surface.
Elastic formliners like 2/81 Plafond, for example, can be used to produce completely smooth concrete surfaces. As it positively impacts the concrete’s rheology, it creates a surface that complies with the requirements for class 4 exposed concrete. Textured formliners can help to balance out smaller imperfections such as spots or color differences, ensuring a consistent appearance. After the exposed concrete is taken out of the formwork, it may be visually changed through processes such as washing, blasting or finishing by a stonemason.
A precise description of requirements in the specification of services and the provision of sample surfaces are advisable not only for planning and contractual security but also for the inspection of the exposed concrete. The code of practice for exposed concrete recommends the production of sample surfaces for comparison purposes from exposed concrete class SB2. This reference surface can be used to establish the quality, inspect the execution and check the cost. Finally, it serves as a standard upon which the client and developer agree for the final inspection. Alongside factors such as color uniformity, pore formation and formlining joints, it is also agreed that minor tolerances in texture and shade cannot be avoided when producing concrete, and exposed concrete in particular.