Exposed concrete – production and curing
Formwork for exposed concrete
The formwork is decisive for the structure of an exposed concrete surface. Interesting effects can be achieved by using different formwork, as each type of formwork produces an individual surface.
In modern formwork construction, system formwork or formwork consisting mainly of system components is primarily used. The different types of formwork can be subdivided according to their use. (e.g. formwork systems for walls, ceilings, columns and 3D free-form surfaces)
The concrete formwork is decisive for the structure of an exposed concrete surface. Interesting effects can be achieved by using different formwork, as each type of formwork produces an individual surface. In modern formwork construction, system formwork or formwork consisting mainly of system components is primarily used. The different types of formwork can be subdivided according to their use. (e.g. formwork systems for walls, ceilings, columns and 3D free-form surfaces)
Formwork systems for walls
- Panel formwork
- Large-area formwork
- Round formwork
Formwork systems for slabs
- Girder slab formwork
- Panel slab formwork
- Girder grid slab formwork
- Large-area formwork
Exposed concrete refers to visible concrete surfaces that fulfil certain requirements in terms of their appearance and take on design functions. Here, the structure of a special formwork skin is specifically reflected.
Due to its flowability and fineness, the concrete can reproduce the geometry and surface of the formwork construction used particularly well. Due to this shaping ability of the fresh concrete, any shape can be produced with the help of appropriate formwork elements and formwork technology.
Exposed concrete has developed into one of the most important design tools in modern architecture precisely because of its many uses and is used for the design of almost all building tasks.
In the DBV Merkblatt Sichtbeton, 4 exposed concrete classes and the associated requirement criteria are defined. The result of this is a clearly defined requirement in terms of texture (the result of the formlining used), coarseness or fineness of pores, uniformity of colour tone, quality of construction joints, evenness of the exposed concrete surface and the creation of patterned surfaces. Clients, architects as well as planners can communicate more easily with the help of this overview and objectively agree on the quality of the fair-faced concrete. It is recommended to tender a detailed performance specification on the basis of this leaflet so that later discrepancies do not stand a chance.
The composition of the concrete’s raw materials should ensure very good workability. In order to achieve optimal exposed concrete surfaces, various criteria must be observed. First, the concrete mix should have a high mortar and cement content (≥ 300 kg/m³). The associated w/c ratio (water/cement ratio) must not exceed 0.55 and, if necessary, be supplemented by a superplasticiser. Due to the possible colour influence, no residual water and residual concrete should be used.
Especially when working on continuous structural sections, the concrete composition must remain as constant as possible. Even small variations in dosage can lead to visible differences in brightness. Therefore, the supplier plant or the concrete formulation should never be changed. To ensure a continuous working process at the concrete mixer, the transport times should be coordinated as short and consistent as possible.
Important for the optimal exposed concrete
- No segregation
- No water secretion
- Good processability
- appropriate consistency
Certain influencing factors make optimal fair-faced concrete qualities difficult and must be taken into account during planning, such as climatic conditions. Environmental conditions such as precipitation, temperature, wind or sunlight affect outdoor work. For example, concrete surfaces produced in colder or wetter conditions are darker and patchier than concretes produced in the warm or dry months (see Sealing concrete).
In the run-up to construction, all parameters that can be influenced should be taken into account and preventive measures taken accordingly to limit the imponderables to a minimum residual risk. Prefabricated concrete elements are a possible alternative. Due to the consistent manufacturing conditions and the independence from weather conditions, production in the precast plant offers a good prerequisite for a uniform surface quality of the concrete.
When choosing the formlining, there is a wide range of design options for the structure of the concrete surface. Ultimately, the surface forms the mirror image of the formwork and takes on all shapes and imprints as well as correspondingly also imperfections, scratches and nail holes.
By inserting matrices (rubber or rubber inserts), any desired concrete surface can be produced. They produce an even colour tone, as well as a reduction in shrinkage, pores and marbling. With board formwork, the suction behaviour of the formlining is decisive. A distinction is made between absorbent and non-absorbent formwork skin.
Absorbent formwork skin
- Becomes brighter after several uses
- Very strong suction capacity
- few pores
- uniform colouring
- darker concrete colour
- simple production
Non-absorbent formwork facing
- Almost smooth surfaces
- Light concrete surfaces
- Risk of marbling and pore formation
- Colour differences not avoidable
- Formwork requires very careful handling
The selection of the release agent
When selecting concrete release agents, the focus is on the interaction between formwork and concrete. There are different types of release agents. On the one hand, solvent-based or solvent-free mineral oil-based release agents and oil-in-water emulsions. As a rule, the best results are achieved when the release agent is used as sparingly as possible and the excess release agent is carefully removed.
Functions of the release agent
- Optimum release of formwork from concrete
- Perfect impression of the formwork skin surface
- Perfect impression of the formwork skin surface
- Prevention of staining and marbling
- Favouring the rise of the air bubbles
- no impairment of the adhesion of paints, plasters, adhesives, etc.
After the component has hardened, the formwork can be removed and numerous methods are available for subsequent finishing of the concrete surface.
- Fine wash / Coarse wash
- High-pressure water jets
- Grinding and polishing
Another design element for concrete surfaces is the colouring. White cement is usually mixed, which can be coloured in all colour classes by adding pigments. In contrast, coloured concretes made of grey cements do not appear as clear and bright.
A “reference surface” is a test surface by means of which the agreement on the condition of the surface is made as a binding quality for a later acceptance. The later exposed concrete must be able to prove itself in terms of pore formation as well as colour uniformity and evenness in the formwork skin area of the joints, such as ridges and offsets.
The creation of one or more test areas is recommended for exposed concrete class SB 2 and higher. In this way, the required effort can be determined and workflows optimised. In addition, these areas can be used for staff training (see concrete repair).
The golden rule is: the shorter the concrete is in the formwork, the better the concrete surface will be! But here, too, there are fixed standards regarding the duration of post-treatment.
One should protect the concrete surface from moisture after stripping the formwork, as early contact with water can lead to efflorescence (lime precipitation). Therefore, one should urgently avoid demoulding on rainy days. It is recommended to protect the concrete surface with a foil after stripping the formwork. To avoid dark spots on the surface, there must be no direct contact between the film and the concrete.
However, even with optimal preparation and taking all influencing factors into account, an absolutely flawless concrete surface cannot be produced. There are various factors that can neither be predicted nor influenced.
Are technically not flawlessly producible:
- uniform colour tone of all visible surfaces can be achieved without any problems:
- non-porous visible surfaces
- uniform pore structure
- efflorescence-free visible surfaces
- Sharp edges without minor breaks and bleeding
Discover also our solutions in the field of concrete cosmetics.