Exposed concrete refers to concrete surfaces that are left unplastered and unveneered in order to achieve a design effect. A basic distinction is made between peeled sides and filling sides. The requirements for the appearance of exposed concrete surfaces can be met by classifying them into one of the four fair-faced concrete classes introduced in the DBV's Leaflet on Exposed Concrete. This is particularly helpful for concrete surfaces produced on site, as exposed concrete surfaces produced in the factory are generally of a higher standard.
Exposed concrete classes:
SB 1: Areas with low design requirements, such as in basements or commercial areas
SB 2: Areas with normal design requirements, such as stairwells, retaining walls
SB 3: Areas with high design requirements, such as building construction facades
SB 4: Surfaces with particularly high design requirements, such as representative areas in building construction
In the respective classes, detailed statements are also made on porosity, texture, colour uniformity, flatness, working and formwork joints.
Basic design options for exposed concrete surfaces:
Surface structure of the formwork lining:
A distinction is made between absorbent and non-absorbent formwork. Absorbent formlining (e.g. made of wood or textiles) has the advantage that excess water can escape from the concrete in the edge area. As a result, the surfaces have fewer pores, fewer cavities, but are also rougher than when using non-absorbent formwork (e.g. made of metal or plastic).
Location and type of joints:
Joint grids are automatically created by the dimensions of the formwork panels. In addition, the appearance of the façade can be designed by means of false joints by nailing strips into the formwork. The effect of a joint is also determined by its chamfer or sharpness and the width of the joint.
Subsequent surface treatment:
Shortly before hardening, washouts of the concrete surface can model the surface. After the concrete has hardened, the surface can be subsequently changed by sandblasting, but also by methods such as polishing, sawing, embossing, acidifying, grinding, etc.
Subsequent surface treatment:
Hydrophobic impregnations are primarily used to protect against weather influences and dirt. They do not normally change the appearance of the concrete.
On the other hand, glazes and coatings serve as design means. A glaze causes a change in the concrete color by adding pigments. The concrete structure is not altered by the low layer thickness, but it can, like coatings, provide a matt or glossy surface, contribute to weather resistance and graffiti protection.
Elastic coatings are primarily used to bridge cracks and close smaller pores. Due to layer thicknesses in the range of 300 μm, they conceal minor contours and unevenness of the concrete substrate.
Rigid coatings with layer thicknesses of around 80 μm prevent the entry of pollutants into the concrete and are used for colour design.
Colouring of the concrete:
Both the choice of the concrete mixture and the addition of pigments achieve a colouring of the concrete that is durable and weather-resistant. The colour spectrum ranges from white, grey and brown tones, which are primarily achieved through the recipe, to green, yellow, black, red or blue tones by means of pigmentation.
Exposed concrete requires intensive and detailed consultation between the client, architect and executing company in order to determine the desired quality. In the case of high qualities, it makes sense to produce sample areas and compare them with objects that have already been built.
Source: bauwion
