Today, modern reinforced concrete technology is a highly developed and high-performance construction technology. Concrete is used in a wide variety of qualities for various requirements. The composition of cement, aggregate, water, admixtures and air can achieve a wide variety of properties. Prefabricated and semi-prefabricated elements such as double-walled elements are playing an increasingly important role in current construction activity. The elements prefabricated in the factory lead to significantly shorter construction times and increase precision.
In addition to conventional in-situ concrete walls, element walls are increasingly being used for the construction of reinforced concrete walls, as they require a shorter construction time, can be built without formwork and have a high surface quality that can remain visible depending on the requirements. A special form of element walls is the thermal wall, which is manufactured at the factory with core insulation within the two precast shells.
element walls and in-situ concrete walls in comparison: Element walls (double walls) can be erected in a significantly shorter construction time compared to conventional in-situ concrete walls. In addition, no formwork is required on site, as the approx. 6 cm thick precast shells function as lost formwork. The joints between the elements of the double wall are filled, as are smaller cavities. After that, the wall is ready to paint. The prerequisite is careful planning and the necessary lead time for the production of the semi-finished elements.
In terms of pure construction costs, element walls are somewhat more expensive than in-situ concrete walls. From the following points of view, element walls are nevertheless more economical than in-situ concrete walls:
Stability: In-situ concrete walls such as element walls are always made of reinforced concrete, which the structural engineer measures according to requirements. In addition to the loads from the structure, walls in contact with the ground are also subject to the pressure from the existing soil. There is no difference in performance between element and in-situ concrete walls.
Concrete selection: In addition to the compressive strength class (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 here, such as exposure to moisture or water, possible exposure to salts, abrasion, heat resistance and much more. These specifications must be defined by the structural engineer in consultation with the client and the architect.
Sealing: Components in contact with the ground must prevent the penetration of moisture and standing water. All basement wall systems made of reinforced concrete can achieve this equally by being designed as a so-called white or black tank. A black tank represents an additional (usually bituminous) waterproofing layer on the outside of the reinforced concrete wall (► 105 | Waterproofing – Black Tub). In the case of a white tank, the waterproofing function is performed by the reinforced concrete wall itself, and for this purpose, among other things, the reinforcement guide, the concrete quality and the joint formation are planned in such a way that external sealing is no longer necessary (► 106 | Waterproofing – White Tub). However, the WU guideline responsible for this only regulates the transport of moisture in liquid form. A possible vapour diffusion is not excluded according to the standard, even if this is currently controversial in the professional world. For this reason, a black tub is usually used in high-quality basement rooms, which also prevents vapour diffusion to the inside.
Thermal insulation: Due to its high density, concrete has a very good thermal conductivity, so that concrete walls have almost no heat-insulating effect. Walls in contact with the ground to heated rooms must be insulated in accordance with the Energy Saving Ordinance. Rooms that are included in the calculation as unheated or are not within the system boundary of the heated building volume should also be insulated at least to a small extent, otherwise condensation on the room side cannot be ruled out. Only well-ventilated rooms such as underground garages can be built without any insulation at all. In terms of thermal insulation, the thermal wall offers the advantages that the insulation no longer has to be applied on site and that it is also not exposed to mechanical stresses. However, the transition point to other components such as the ceiling or floor slab cannot be created without a thermal bridge, which can be unproblematic in many cases, but must be avoided for the construction of highly thermally insulated houses.
Formwork: In the planning, it must be taken into account that the formwork for cast-in-place concrete walls must be set up in a perpendicular and alignment-oriented manner. For this purpose, a floor slab overhang of at least 10 to 15 cm should be provided. In justified exceptional cases, such as when transitioning to another section of the building, a floor slab overhang can be dispensed with, but a temporary support surface for the formwork elements must then be created. Double walls can also be placed flush with the outside without protrusion on the floor slab. They also have advantages where it would not be practical to erect formwork for in-situ concrete walls, e.g. adjacent to existing buildings.
Windows/doors: When installing basement windows, special construction elements are often used that are inserted into the formwork before concreting, so that no recess has to be created. If, in contrast, conventional windows or doors are installed, recesses must be built into the formwork. In this case, it may make sense to insert a strip of perimeter insulation in the reveal (see detail) so that there is no thermal bridge at the connection area to the window.
Radon radiation: A basement made of reinforced concrete significantly reduces the input of natural radon radiation from the ground. Radon is a colourless, odourless and tasteless radioactive noble gas that occurs everywhere in nature, albeit locally in very different concentrations. According to a letter from the Bavarian State Agency for Environmental Protection, radon and the associated decay products are the second most common cause of lung cancer after smoking.
Installations: Electrical installations can be nailed into the formwork as empty pipes and cans before concreting, so that flush-mounted installation is also possible in the basement. In the case of panel walls, the cans are already installed in the semi-finished part in the factory.
concrete paving: 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: 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: The drying process of the 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. Helpful in this context is the Cement Leaflet B8, published by the Association of German Cement Works (see below).
Stripping: According to DIN 1045-3, cast-in-place concrete components may be formed if the concrete has hardened sufficiently. However, no guideline values for stripping deadlines have been defined. So here you have to fall back on empirical values. In addition, excessive loads on the fresh concrete components must be avoided initially. Stripping too early can lead to cracks forming when backfilling the excavation pit due to the external pressure and vibrations. As a rule, basement walls should remain in the formwork for at least a week.
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
► Qualitätsgemeinschaft Doppelwand Bayern
► InformationsZentum Beton (IZB)
► Quality Association SYSPRO Double Walls and Thermal Walls
Source: bauwion
