Walls made of reinforced concrete are characterized by a high load-bearing capacity and long service life and also provide low space consumption, good sound insulation and a high heat storage capacity. Exposed concrete walls allow for a variety of forms of surface design. As a prefabricated element, reinforced concrete walls have a high degree of precision with short installation times. Since concrete consists of natural components and is extremely durable with low material consumption, walls made of reinforced concrete are also considered sustainable construction.
The exterior wall of a building forms the transition between the interior and exterior space. The high-performance material concrete offers economic and technical strengths, but also design possibilities through high-quality exposed concrete.
Not only in commercial and industrial construction, but also in residential construction, building with prefabricated or semi-prefabricated elements is becoming increasingly important. Repetitive shapes and dimensions can lead to savings through serial production in the precast plant. Early and detailed planning is essential for prefabrication in the factory, as changes are sometimes not possible or can only be achieved with great effort after the concreting process.
In-situ concrete walls are mainly used in basement areas, especially in smaller construction projects, also over terrain.
Basic advantages of exterior walls made of prefabricated reinforced concrete elements:
Stability: In the case of exterior walls made of reinforced concrete , a basic distinction is made between facades with direct vertical load transfer via the concrete panels and facades whose load transfer is carried out via a higher-level column system and in which the concrete wall panels do not fulfil a load-bearing function.
In-situ concrete walls such as precast walls are made of reinforced concrete , which the structural engineer measures according to requirements. Precast walls are usually concreted horizontally and are therefore easier to compact than upright concrete in-situ concrete walls. For example, prefabricated walls can generally be manufactured in lower wall thicknesses. In practice, however, building physics reasons such as sound insulation or fire protection requirements often speak against a reduced wall thickness.
In the case of prefabrication in the factory, the architect and structural engineer work closely with the precast plant.
Load-bearing perforated façades: With a regular and repetitive window and door arrangement, the use of prefabricated elements is economical for perforated façades. The elements are usually single-storey and up to 10 m long. The wall panels are usually designed as a single-shell prefabricated element, in which insulation and facing layer are applied on site, or as a multi-layer sandwich panel.
Ceiling tiles are often placed in recesses in the wall panel, on consoles or directly on the load-bearing wall. By grouting with the ceiling, a ring anchor can be created at the same time.
Concrete selection: In addition to the compressive strength classes (e.g. C25/30) and consistency classes (e.g. F3), the exposure classes (e.g. XF 2) must also be defined in the planning so that the components are optimally and long-term adapted to the ambient conditions depending on the area of application. A variety of factors play a role, in particular exposure to moisture or water, possible exposure to salts, abrasion and heat resistance. These specifications must be defined by the structural engineer in consultation with the client and the architect.
Thermal insulation: Due to its high density, concrete has very good thermal conductivity, so that concrete walls have almost no thermal insulation effect. Exterior walls of heated rooms must be insulated in accordance with the Energy Saving Ordinance. Only well-ventilated buildings such as parking garages or open halls can be built without insulation at all.
In terms of thermal insulation, thermal walls and sandwich facades have the advantage that the insulation no longer has to be applied on site and is also not exposed to mechanical stresses later on. With appropriate detail, a thermal bridge-free outer wall construction is possible:
Exemplary corner solutions of sandwich constructions, free of thermal bridges: The construction also influences the joint position in the façade.
Joints: A basic distinction is made between building and element joints.
Settling joints always run vertically through the entire building, including the foundations. Expansion joints play an important role, especially in the outer skin of a building and in load-bearing walls, as large temperature differences can occur on the outside and inside of the wall as a result of weather influences and direct sunlight.
In sandwich constructions, the joints can be closed to the outside by means of sealing tapes to prevent rainwater from penetrating. In the case of rear-ventilated façades, the joints should remain open or be designed in such a way that penetrating water can drain away.
Internal joints in concrete wall panels are filled, unless they are necessary building joints.
Connection between precast elements: Prefabricated reinforced concrete elements are connected to each other using prefabricated connection systems made of steel or cast steel components. These enable easy assembly of the elements on the construction site.
Frequently used:
Walls are often poured with other components, such as ceilings or columns. The elements are also interlocked, in particular to rule out horizontal displacements.
Exposed concrete: When planning exposed concrete surfaces, planners and builders should discuss the required appearance of the visible surfaces at an early stage. The results must be included in detail in the service description, especially in the case of high design requirements, in order to prevent disputes during acceptance. In the case of visible surfaces on cast-in-place concrete walls, this can be done primarily by classifying them into one of the four fair-faced concrete classes of the FDB's leaflets 1 and 8 for formwork surfaces. See also Lexicon article ►Exposed concrete.
In the case of exposed concrete walls produced in the factory, the quality, especially in the case of representative surfaces, can also be determined by means of sample surfaces or by reference to objects that have already been executed. In particular, the following must be determined in detail in advance:
Protection against vandalism: Exposed concrete surfaces should be protected against vandalism in endangered areas by means of a so-called graffiti protection system, as it is difficult to clean untreated concrete surfaces without damaging the surface of the concrete. This is usually done in the form of coatings or impregnations that are applied to the visible surfaces. See encyclopedia article ►Graffiti protection
windows and doors: In window and door areas, appropriate recesses must be provided in the formwork. In this case, it may make sense to insert a strip of perimeter insulation in the reveal so that there is no thermal bridge at the connection area to the window.
In the case of perforated façades, which are assembled from prefabricated elements, all recesses are already present at the time of delivery.
Installations and recesses: Electrical installations must be inserted into the formwork as empty pipes and cans before concreting in order to ensure flush-mounted installation. In the case of precast or semi-prefabricated walls, the boxes are already integrated into the component in the factory. The same applies to recesses for installation shafts, which are nailed into the formwork on the construction site in the case of in-situ concrete.
Early and detailed installation planning is therefore necessary, especially in the case of prefabrication in the precast plant.
Special forms of concrete walls: In fibre concrete , steel fibres are added to the concrete, sometimes also fibres made of glass or plastic. The fibres take over the tensile function within the material and have a positive influence on its load-bearing behaviour. The type, shape, direction and, above all, the quantity of the fibres determine the degree of static effectiveness. Under certain circumstances, this means that the reinforcement of a component made of steel meshes and bars can be completely omitted, and thus the entire reinforcement process. Fibre reinforcement can also be optimally used in precast or semi-precast walls in precast plants. Fibre concrete has only recently been included in the concrete standards, so that the usual building authority approvals will no longer be absolutely necessary in the future.
The centuries-old method of building with tamped concrete is currently being rediscovered. This is unreinforced concrete made of natural stones and cement, which is compacted in layers by long-term pushing and stamping with the feet. Building with tamped concrete requires very long installation times, as a layer height of only 15 -25 cm can be processed per day. However, if done correctly, the concrete is very durable and less susceptible to cracks.
and assembly of prefabricated elements: In advance, the access routes should be checked for suitability in order to be able to deliver the prefabricated elements, some of which are very large-format, with suitable transport vehicles. The construction crane must also be designed for the load. Before the wall elements are installed, all necessary supports and stiffening elements must be manufactured and fully load-bearing.
Ideally, the prefabricated elements are lifted directly from the vehicle into the intended position by crane via transport anchors. For interim storage, paved surfaces must be laid out in the slewing area of the crane. Repositioning always carries the risk of damage, especially in the edge area.
After the installation of the elements, the casting of the components can begin immediately.
Concrete installation (in-situ concrete): The in-situ concrete delivered or produced on site must be installed as quickly as possible. This must prevent cavities from forming in the component. This is prevented by shaking, stomping or poking. However, it should not be shaken for too long, otherwise there is a risk of segregation. This is shown by the formation of an aqueous sludge layer on the surface. Concrete must always be placed in layers and should not be brought in from a drop height of more than two metres. Concrete is affected by external conditions during setting. In extreme climatic conditions such as heat above 30°C or frost below -5°C, concrete should not be used.
Reinforcement (in-situ concrete): When installing the reinforcement, great care must be taken to ensure compliance with the required concrete covers. Otherwise, the reinforcement can corrode over the years and, in extreme cases, the structure can no longer meet its static requirements. In the case of precast walls, the reinforcement is already installed and monitored in the precast plant.
Post-treatment of cast-in-place concrete: The drying process of concrete is called hydration. This leads to drying out and hardening of the concrete component. Concrete components must be treated by suitable measures during the setting period. Otherwise, the concrete will set unevenly quickly as a result of solar radiation or wind, so that cracks can occur. After 28 days, the concrete component is fully hardened and the hydration is complete. Thorough and careful follow-up treatment is expressly required in DIN 1045-2. The following measures are available for the post-treatment of in-situ concrete components:
The type and duration of the post-treatment are regulated in DIN 1045-3. The Cement Leaflet B8, published by the Association of German Cement Works, is also helpful in this respect.
Stripping: According to DIN 1045-3, cast-in-place concrete components may be formed if the concrete has hardened sufficiently. However, the standard does not define any guideline values for stripping periods. So here you have to fall back on empirical values. In addition, excessive loads on the fresh concrete components must be avoided initially. As a rule, reinforced concrete walls should remain in the formwork for at least a week. Further information can be found in the DBV leaflet "Concrete formwork and stripping deadlines".
Acceptance of exposed concrete surfaces: An exposed concrete surface should be assessed at the usual viewing distance of the user and under normal lighting conditions. The overall impression is decisive here. If this does not meet expectations, individual criteria are examined. These include structural differences, marbling, cloud formation, limestone plumes, efflorescence, pore frequency, bulging, edge breaks, emerging probation or spacers. Slight deviations from the agreed appearance must be tolerated and are in some cases typical of fair-faced concrete technology. For example, pores in the surfaces, hairline cracks and irregularities in the shade cannot be completely ruled out technically.
Note: DIN 1045-1: 2008-08 was withdrawn on 31.12.2010 because this standard contradicts the European design standard (DIN EN 1992-1-1:2011-01 and DIN EN 1992-1-1/NA:2011-01).
DIN 1045-2, Structures made of concrete, reinforced concrete and prestressed concrete - Part 2: Concrete - Specification, properties, production and conformity - Rules of application for DIN EN 206-1
DIN 1045-3, Structures made of concrete, reinforced concrete and prestressed concrete - Part 3: Construction - Rules of application for DIN EN 13670
DIN 1045-4, Structures made of concrete, reinforced concrete and prestressed concrete - Part 4: Supplementary rules for the manufacture and conformity of precast elements
DIN EN 1992-1-1, Eurocode 2: Design and construction of reinforced concrete and prestressed concrete structures - Part 1-1: General design rules and rules for building construction
DIN EN 1992-1-1/NA, National Annex - Nationally defined parameters - Eurocode 2: Design and construction of reinforced concrete and prestressed concrete structures - Part 1-1: General design rules and rules for building construction
DIN EN 1992-1-1/NA/A1, National Annex - Nationally defined parameters - Eurocode 2: Design and construction of reinforced concrete and prestressed concrete structures - Part 1-1: General design rules and rules for the construction of reinforced concrete and prestressed concrete structures Building construction, Amendment A1
DIN EN 1992-1-2, Eurocode 2: Design and construction of reinforced concrete and prestressed concrete structures - Part 1-2: General rules - Structural design for fire
DIN EN 1992-1-2/NA, National Annex - Nationally defined parameters - Eurocode 2: Design and construction of reinforced concrete and prestressed concrete structures - Part 1-2: General rules - Structural design for fires
DIN EN 13670, Execution of concrete structures
Cement leaflet B8, Technical information on the post-treatment of concrete components, Publisher: Verein Deutscher Zementwerke
Leaflet "Exposed concrete", Planning, tendering, contract drafting, execution and acceptance, Publisher: Deutscher Beton- und Bautechnik-Verein
Leaflet "Concrete formwork and stripping deadlines", Publisher: Deutscher Beton- und Bautechnik-Verein
►InformationsZentrum Beton GmbH
►Qualitätsgemeinschaft SYSPRO, Doppelwände und Thermowände
►Fachvereinigung Deutscher Betonfertigteilbau e.V.
Source: bauwion
