Concrete is one of the most widely used building materials in the world, but it is only when its surface remains visible that it becomes a means of design expression. Exposed concrete classes are the tool that architects, planners, and contractors use to jointly determine the desired quality of this surface: how smooth, how uniform, how free of pores, and how consistent in color. Without this classification system, communication between the design team and the construction site would be left to chance, and the result would fall short of expectations or lead to costly rework. Anyone planning or constructing exposed concrete must be familiar with the classes, understand their requirements, and factor in their implications for formwork, mix design, and placement from the very beginning.

• What exposed concrete classes are and why they were developed as a classification system
• How the four classes—SB1 through SB4—are structured and what each requires
• Which surface characteristics are evaluated: pores, color, texture, formwork pattern
• What role formwork, release agents, concrete mix design, and compaction play in the final result
• How exposed concrete is properly specified in planning, both contractually and in accordance with standards
• What typical defects and flaws occur in exposed concrete and how they can be avoided
• How exposed concrete is used in architectural history and contemporary design
• What to consider regarding the care, protection, and maintenance of exposed concrete surfaces

What are exposed concrete classes? Definition, origin, and purpose

Exposed concrete refers to concrete surfaces that remain permanently visible after formwork removal and thus serve a design function. Unlike plastered, clad, or coated concrete elements, the surface itself constitutes the finished appearance. Exposed concrete classes are a classification system that categorizes these surfaces into levels of requirements based on defined characteristics. They establish a common language among designers, planners, building owners, and contractors, and make it possible to formulate quality expectations in a binding and transparent manner.

The basis for the classification system in German-speaking countries is the “Exposed Concrete” technical bulletin, published by the German Concrete and Construction Technology Association (DBV). It is not a binding law, but a recognized set of technical standards that is regularly referenced in calls for bids, specifications, and contracts. The technical guideline distinguishes four exposed concrete classes, designated SB1 through SB4, with SB1 having the lowest requirements and SB4 the highest. In parallel, other countries and the international community have their own systems, such as the British “Specification for Architectural Concrete” or the guidelines of the Austrian Concrete Association, which pursue similar objectives in terms of content but differ in terminology and level of detail.

The classification system arose from a practical necessity: exposed concrete is a material whose final result depends on a multitude of factors that are difficult to control. Formwork, release agents, concrete mix design, fresh concrete consistency, compaction energy, weather conditions, and curing all interact. Without clear agreements on what is considered acceptable, disputes arise on the construction site and in court. Exposed concrete classes solve this problem by providing measurable and assessable criteria that serve as the basis for acceptance and defect assessment.

The Four Exposed Concrete Classes SB1 through SB4: Structure and Requirements

Class SB1 represents the lowest level of requirements. It applies to concrete surfaces that remain visible but do not need to meet any specific design requirements. Typical applications include basement walls, underground structural elements, exterior retaining walls, or surfaces that are largely concealed by landscaping, furniture, or usage. Formwork marks, pores, color variations, and voids (small cavities on the concrete surface caused by trapped air) are tolerable to a certain extent. The appearance need only correspond to what results from normal, careful workmanship without special measures.

Class SB2 applies to surfaces that are visible and are intended to have a neat, uniform appearance. Requirements are specified here regarding the uniformity of the formwork pattern, the distribution of pores, and color consistency. Formwork joints must be carefully planned and executed, release agents must be applied evenly, and the concrete must be sufficiently compacted to minimize voids. SB2 is the most commonly agreed-upon class in building construction because it combines a good appearance with reasonable effort.

Class SB3 imposes higher requirements on texture, color, and pore distribution. Formwork patterns must be precise and consistent, color variations must be minimized, and the pore structure must be uniform and fine. Class SB3 typically requires special concrete mixes with an optimized fine aggregate content, carefully selected and prepared formwork materials, and experienced specialized personnel. This class is frequently found in prestigious interior spaces, facades of high-end office and cultural buildings, and in structural elements that are directly visible to users.

Class SB4 is the most demanding level and is specified for exposed concrete surfaces of the highest design quality. It requires not only special materials and construction methods but also intensive planning, sampling, and the creation of test surfaces even before construction begins. Color, texture, pore distribution, and formwork pattern must be virtually homogeneous across the entire surface of the structural element. Deviations that are tolerated in lower classes are considered defects in SB4. This class is labor-intensive, expensive, and requires close coordination among all parties involved. It is used in museum buildings, churches, concert halls, and other structures where the concrete surface serves as the primary design element.

Evaluation Criteria: What Is Assessed in Exposed Concrete Surfaces

The DBV Technical Bulletin on Exposed Concrete defines five main characteristics by which exposed concrete surfaces are evaluated. These characteristics form the basis for class assignment and for on-site acceptance. They are: surface flatness, formwork pattern, porosity, color, and texture. Each of these criteria can be evaluated independently of the others, and the requirements for each criterion increase with the class.

The formwork pattern describes the imprint left by the formwork on the concrete surface. It includes the arrangement and visibility of formwork joints, anchor holes, spacers, and formwork skin patterns. For SB1, irregular formwork imprints are acceptable; in SB4, formwork joints must be arranged according to a precise grid, and anchor holes must be cast symmetrically and with care. In higher classes, planning the formwork pattern is a distinct design task that must be documented in the construction drawings.

Porosity refers to the frequency, size, and distribution of pores on the concrete surface. Pores form when air bubbles in the fresh concrete cannot completely escape through compaction and become trapped at the formwork surface. A certain degree of porosity is unavoidable in exposed concrete and is tolerated to varying degrees depending on the class. The technical guideline distinguishes between individual pores, pore clusters (local accumulations of pores), and aggregate clusters (areas where the cement paste matrix is missing and the coarse aggregate is exposed). The latter are considered defects in all classes and require corrective action.

Color is one of the most difficult characteristics to control because it depends on numerous factors: type and content of cement, water-cement ratio (the ratio of water mass to cement mass in the concrete, which significantly influences consistency and strength), the application of release agents, the curing duration, and weather conditions during hardening. Color variations also arise from shading caused by uneven water accumulation behind the formwork, a phenomenon known as “clouding.” For SB3 and SB4, color consistency must be demonstrated through test panels and ensured by maintaining consistent material composition throughout the entire construction process.

Texture describes the surface structure of the concrete, which is shaped by the formwork surface. Smooth plastic or steel formwork produces a smooth concrete texture; wood formwork leaves the grain of the wood imprinted in the concrete; special textile formwork or textured formwork membranes create three-dimensional patterns. Texture is an essential design element and must be precisely specified for higher classes and verified by test panels.

Formwork, Concrete Mix Design, and Placement: The Technical Requirements

The appearance of an exposed concrete surface is largely determined by the choice of formwork. Formwork skins made of plastic or coated wood-based materials produce smooth, low-absorbency surfaces that are suitable for high-grade applications. Untreated wooden formwork absorbs water from the fresh concrete, leaving behind color variations and the wood grain as a texture. Steel and aluminum formwork are dimensionally stable and reusable, but can leave imprints if cleaned improperly or damaged. The decision regarding a formwork type is always a balance between design intent, grade, cost, and reusability.

Release agents applied to the formwork surface to facilitate formwork removal significantly affect the pore structure and color. Oily release agents can cause color stains; agents applied too thinly lead to adhesion and surface damage; agents applied too thickly create pores and color variations. For SB3 and SB4, film-forming release agents are frequently used; these form a uniform, thin layer on the formwork surface and enable reproducible results. Application must be uniform and complete, which requires careful work preparation for large formwork surfaces.

The concrete mix design for exposed concrete differs from that for concealed concrete in that it has a higher fine aggregate content, an optimized water-cement ratio, and often includes the use of admixtures such as fly ash or silica fume. A higher proportion of fine aggregate improves the workability of the fresh concrete and reduces pore formation on the formwork surface. Superplasticizers (concrete plasticizers based on polycarboxylate ethers) make it possible to increase the consistency without raising the water-cement ratio, which improves strength and durability. The consistency class of the fresh concrete must be tailored to the component geometry and the compaction options: concrete that is too stiff is difficult to compact and produces voids; concrete that is too fluid can cause segregation and bleeding.

Compaction of fresh concrete using internal vibrators (immersion vibrators) must be carried out with particular care in exposed concrete. Vibration too close to the formwork can drive voids to the surface; insufficient compaction leaves air pockets. Experienced concrete workers develop a feel for the correct distance and duration of vibration. For very narrow cross-sections or heavily reinforced components, the use of external vibrators or vibrating tables may be advisable. Curing—that is, protecting the fresh concrete surface from drying out too quickly by covering, moistening, or applying curing agents—affects strength development and color consistency and is mandatory for all exposed concrete classes.

Planning, Bidding, and Contractual Agreements for Exposed Concrete

The correct specification of exposed concrete classes in planning and bidding is one of the most common sources of error in the construction process. If only “exposed concrete” is specified without indicating a class, there is no basis for a verifiable acceptance. It is equally problematic to agree on a high class without establishing the necessary prerequisites: sufficient planning time, test areas, qualified contractors, and a realistic budget.

The DBV technical bulletin recommends explicitly specifying the class for each exposed concrete component and additionally detailing the relevant characteristics individually. It is possible to agree on different requirement levels for various characteristics—for example, SB3 for color and porosity, but SB2 for the formwork pattern. This differentiated agreement requires expertise but enables a precise and fair definition of quality. Sample surfaces, which are produced and approved before the main construction work begins, are indispensable for SB3 and SB4: They serve as a binding reference for the entire subsequent construction process and protect both contracting parties.

The acceptance of exposed concrete surfaces takes place after formwork removal under defined conditions: sufficient daylight or equivalent artificial light, a viewing distance of typically three meters, and a viewing angle that does not produce oblique lighting conditions. Criticism based on oblique lighting—that is, the assessment of a surface under low-angle light—is expressly prohibited for exposed concrete because it reveals irregularities that would not be perceptible under normal viewing conditions. This regulation is often the subject of disputes in practice and must therefore be clearly stipulated in the contract.

Exposed Concrete in Architecture: History, Present, and Design Potential

Exposed concrete as a deliberate design element has its roots in the early 20th century, when architects began to emphasize—rather than conceal—the sculptural and tectonic qualities of concrete. Le Corbusier coined the term “béton brut” (raw concrete) and made the untreated concrete surface a defining feature of his work with buildings such as the Unité d’Habitation in Marseille and the Sainte-Marie de La Tourette monastery. Brutalism, a postwar architectural movement based on this approach, made exposed concrete the hallmark of an entire era. Buildings such as the Barbican Centre in London and Alvar Aalto’s Wolfsburg Cultural Center demonstrate how expressively and diversely exposed concrete can be used.

In contemporary architecture, exposed concrete has not lost its significance as a means of expression but has continued to evolve. Architects such as Tadao Ando have refined exposed concrete into a precision art form, in which flawless, silky-smooth surfaces with precisely placed anchor holes and perfect color consistency create a meditative quality. Ando’s Church of Light in Osaka and the Modern Art Museum in Fort Worth are examples of exposed concrete of the highest quality, in which the surface itself becomes an architectural element. In contrast, other architects deliberately emphasize the roughness and imperfection of exposed concrete to highlight the material’s authenticity and the presence of craftsmanship.

The design range of exposed concrete extends from smooth, monochromatic surfaces to textured, colored, or post-treated surfaces. Washed concrete, in which the cement paste is removed by washing or blasting to reveal the aggregate, creates a lively, grainy texture. Sandblasted concrete takes on a matte, velvety surface. Ground or polished concrete reveals the aggregate pattern in cross-section and is reminiscent of terrazzo. Concrete colored with pigment additives allows for a wide color spectrum that goes far beyond the usual gray. All of these variants are compatible with the classification system, as long as the requirements for the respective characteristics are clearly defined.

Common Defects, Flaws, and How to Avoid Them

Exposed concrete does not forgive mistakes. What remains hidden in plastered or clad structural elements becomes clearly visible in exposed concrete. The most common defects are gravel pockets, color variations, clouding, irregular pore distribution, and cracks. Gravel pockets result from insufficient compaction, concrete that is too stiff, or segregation during placement. They are not only aesthetically displeasing but can also compromise the durability of the structural element because the lack of a cement paste matrix leaves the reinforcement unprotected.

Color variations and mottling are the most common causes of disputes during acceptance inspections. They result from uneven application of release agents, water accumulation behind the formwork, varying curing times across different concrete placement sections, or the use of cement from different batches. Avoiding these defects requires consistent material uniformity throughout the entire construction process: the same cement from the same delivery, the same mix design, the same compaction energy, and the same curing. Any deviation leaves a visible trace.

Cracks in exposed concrete surfaces are a particularly sensitive issue because they can have both aesthetic and structural implications. Shrinkage cracks, which result from the drying out of the concrete, must be minimized in exposed concrete through adequate curing and an optimized mix design with a reduced water-cement ratio. Separation cracks, which arise from constraint stresses in statically indeterminate systems, must be controlled through structural measures such as expansion joints or reinforcement designs. The acceptability of cracks is regulated in the relevant standards, particularly DIN EN 1992 (Eurocode 2) for reinforced concrete construction, and depends on the exposure class and service requirements.

Protection, Care, and Maintenance of Exposed Concrete Surfaces

Exposed concrete is durable but not maintenance-free. Untreated exposed concrete surfaces in outdoor areas are exposed to weathering, which over time can lead to soiling, carbonation, efflorescence, and biological growth. Carbonation is a chemical process in which carbon dioxide from the air reacts with the calcium hydroxide in the cement paste to form calcium carbonate. This process is inevitable, progresses from the surface inward, and does not compromise durability if the reinforcement is adequately covered by concrete. However, it changes the color of the surface and, in thin structural elements, can endanger the reinforcement over the long term.

Hydrophobic impregnations (water-repellent treatments that penetrate the concrete pores without sealing the surface) are a common means of protecting exposed concrete surfaces in outdoor areas. They reduce water absorption, slow carbonation, and make it harder for dirt to adhere, without significantly altering the appearance. Sealants and coatings offer stronger protection but alter the surface’s appearance and vapor permeability and require renewal after a few years. The choice of the right protection system depends on the exposure, the concrete quality, and the design requirements.

Cleaning exposed concrete surfaces requires care. High-pressure washers can damage the cement skin and alter the pore structure; aggressive cleaning agents can corrode the surface and cause efflorescence. For most types of soiling, cleaning with clear water and soft brushes is sufficient. Stubborn stains caused by oil, rust, or biological growth require specific cleaning agents that are compatible with the chemistry of the concrete. Repair work on exposed concrete surfaces—such as filling gravel pockets or evening out color differences—is technically demanding and should only be performed by professionals with experience in exposed concrete repair, because any rework carries the risk of visible deviations from the original condition.

Exposed Concrete Classes as the Foundation for Quality and Trust in the Construction Process

Exposed concrete classes are far more than a bureaucratic classification. They form the foundation of a shared understanding of quality that unites architects, engineers, contractors, and building owners. Agreeing on an exposed concrete class not only establishes a specific appearance but also defines the effort, methods, and responsibilities necessary to achieve that appearance. This clarity protects all parties involved and creates the conditions for exposed concrete to become what it is in the best examples of architectural history: a material of unparalleled presence and honesty.

Choosing the right class is both a design and an economic decision. Specifying SB4 for a basement wall would be a waste of resources; specifying SB1 for a representative entrance hall would be a disappointment. The art lies in realistically defining the requirements, carefully preparing the execution, and aligning the expectations of all parties involved through test panels and clear communication. Exposed concrete rewards this care with surfaces that last for decades and develop a patina over time that no other material can replicate.

The classification system in the DBV technical guideline is a living document that reflects current practice and is continually being refined. New materials such as white cement, colored concretes, and high-performance concretes with very dense textures expand the design possibilities and place new demands on the classification system. Anyone planning to use exposed concrete should be familiar with the current technical guideline, avoid applying the classes mechanically, and instead interpret them within the context of the specific project, while viewing the execution as an integral part of the design process from the very beginning. After all, exposed concrete is not created in the office but on the construction site, and its quality depends on how well the design and execution complement each other.