Prefabricated prestressed concrete ceiling
The prestressed concrete prefabricated ceiling or hollow chamber prefabricated ceiling , also prestressed concrete hollow ceiling is a ceiling construction that consists of individually prefabricated prestressed concrete molded parts. The individual elements are assembled on the construction site to form a complete ceiling (full assembly construction). The construction processes are correspondingly efficient and time-saving. Due to the high degree of prefabrication, it is advisable to include pre-stressed concrete ceilings in the building planning at an early stage.
Pre-fabricated prestressed concrete ceilings span a wide area and enable flexible room design. In addition, the ventilation, heating and air conditioning of the room can be integrated into the ceiling elements.
During the production of the standard molded parts, cavities are left out inside, which reduces weight and material consumption by up to 50 percent compared to solid components . The prestressing steels are arranged in the webs that remain in between . The look of the undersides of the panels is comparable to exposed concrete or element ceilings . The board joints on the underside can be designed as visible joints or - through a special post-treatment (jointing) - without visible joints.
history
Prefabricated hollow chamber ceilings were developed in Germany in the 1930s. They are a further development of the hollow concrete floorboards, which had to be reinforced on site and cast with in-situ concrete . During the 1950s, they found widespread use, particularly in the United States. Since the 1990s, they have established themselves in many European countries due to the short construction times . In Scandinavia, for example, prestressed concrete prefabricated ceilings have a market share of around 40 percent.
Manufacturing
The production takes place in a precast concrete plant with specially set up production facilities and a special manufacturing process, through which the large spans of the pre-stressed concrete ceilings can be realized. The tension wires or tension strands are first laid in flat steel tracks up to 200 m long . Molded steel parts create the characteristic cavities. Any recesses - for example for installations - are also made at this point in time. Then the edge formwork is set up. The width is set between 1.10 and 1.40 m, with 1.20 m being the standard. To compensate for the individual structural dimensions, narrower custom-fit panels can also be produced. The statically required prestress is then applied to the strands. The number of tension strands and the pre-tensioning result from a static calculation. The preload prevents cracks when in use. At the same time, the deflection is less than with ceilings that are not prestressed.
The concreting is done with concrete of a higher strength class (usually C50 / 60) and is poured using an extruder or slip paver. After about eight hours of hardening with subsequent treatment, the production lines are sawed to their installation lengths with diamond discs and prepared for transport. The length of the molded parts can therefore be individually determined depending on the specific area of application. The same applies to the thickness, which can be between 15 and 50 centimeters.
Prestressed concrete ceilings are subject to building supervision in Germany . This means that you need a building inspection approval from the German Institute for Building Technology .
Assembly
The individually prefabricated concrete elements are brought into the installation position on the construction site by a mobile crane according to a ceiling plan. In the cross connection of the panels, excessive deflection is prevented by pouring joints with in-situ concrete . In addition, there is a concreted, circumferential and force- fit anchored ring anchor reinforcement .
Areas of application
Due to the large spans , pre-stressed concrete ceilings can be used in a wide variety of building types .
advantages
In residential construction, the rapid construction progress means that it is a cost-effective alternative to ceilings made of in-situ concrete or semi-finished parts. In addition, the high level of planning security, the low building moisture and the low-column layout are advantageous. The same applies to the construction of office buildings: with 32 centimeter high ceiling elements, designs up to 13 meters wide can be implemented. Complex partition wall connections are not required, which simplifies the interior construction.
disadvantage
The pre-tensioned plates have an upward stitch which, due to the manufacturing process, is different in the end plates of a production line, but also in the case of shorter spans. Due to the uniaxial reinforcement / tension, there are structural restrictions in contrast to the floor slab. For example, subsequent ceiling recesses may only be as wide as the hollow chambers, as otherwise the prestressing steel will be destroyed.
At the moment (August 2010) there is building inspection approval only for predominantly stationary traffic loads, i.e. forklifts , for example, are not allowed to drive on the ceilings.
Due to the lower mass compared to a ceiling made of in-situ concrete , there are disadvantages in terms of sound insulation . In order to effectively prevent the spread of structure-borne noise / impact noise, for example , in addition to the use of floating screed , sheeting made of rubber shot is laid out on the top of the wall as seamlessly as possible. These are intended to acoustically decouple the ceiling elements from the walls.
Spans
In order to determine the possible span for a specific object, the ceiling height "h" must be set in relation to the span "L" (slenderness). The guideline value h / L = 1/35 applies. H. a plate height of around 20 centimeters enables a span of around 7 meters. Depending on the use, the plate height can also be a little lower. For example, a ratio of h / L = 1/50 is used for roofs.
Ultimately, the specific range decreases with the increasing traffic load . A live load of 2.0 kN / m 2 is estimated for residential buildings . Correspondingly, spans of up to 7.5 meters are possible in this application area with a 20-centimeter panel. In industrial construction, however, an expected traffic load of 10 kN / m 2 is the basis of the calculation, so that spans of 5 meters are possible. This 10 kN / m 2 is also the maximum permissible traffic load for a pre-stressed concrete floor. In individual cases, traffic loads of up to 12.5 kN / m 2 are possible with panel heights of over 25 centimeters .
The table opposite provides an overview of the traffic load of individual building types with the corresponding spans.
More functions
ventilation
Prefabricated prestressed concrete ceilings offer the possibility of integrating a ventilation system for the building interiors directly into the ceiling cross-section. The hollow chambers of the ceiling elements act as ventilation ducts by equipping them with a special coating at the factory. Plate valves or slot outlets in the ceiling underside ensure the connection between the room and the ventilation duct inside the hollow slab. The ventilation outlets are also made during manufacture.
Because of the large cross-section of the hollow chambers, a ventilation ceiling keeps air velocities and pressure loss low. This reduces energy consumption and flow noise. In addition, the height of the room can be used to the full as there are no disruptive components such as ventilation pipes. With a higher number of floors, entire floors can be obtained in this way.
Air conditioning
The cooling and heating of interiors can also be controlled via a pre-stressed concrete ceiling, which is referred to as thermal component activation . The manufacturer equips the ceiling elements with a pipe system through which tempered water is passed. When the water is cool, the concrete absorbs the room heat and thus ensures a lower, homogeneous temperature control without unpleasant drafts. If water flows through the pipes that is warmer than room temperature, the opposite effect occurs. The concrete heats up and releases the heat evenly into the room.
Since concrete has a high thermal storage capacity, a long reaction time corresponding to the mass must be taken into account when planning the systems. Particularly when the component cools down, longer shutdown times with increasing mass must be observed. In the case of solid, thermally effective ceilings, the system is opposed to fully solid concrete ceilings with thermal component activation (TBA) on the sluggish side and air -conditioned ceilings with significantly shorter reaction / decay times on the nimble side. The subsequently installed heating / cooling ceilings form a separate group . You have i. d. Usually the shortest response times. The greater inertia of the TBA tends to help reduce the peak loads on building services systems, especially cooling units, but without being able to cover these peaks themselves. On the other hand, nimble air-conditioning ceilings can also cover peak loads. The controllability is to be seen as an advantage here, but at the price of the lower storage mass, which leads to higher loads on the system side at peak times. In practice, the respective disadvantages are usually counteracted by supplementary measures, such as additional quick heating support (e.g. radiators, warm air heating systems with combined ventilation, etc.) or additional storage masses integrated by the cooling circuit (e.g. basement or underground car park floors, buffer storage, etc.) .) balanced. Heating / cooling ceilings require relatively little energy during operation. For example, in the extension of the Ministry of Food, Agriculture and Health in Berlin , this construction method achieved savings of around 40 percent. For the sole heating of buildings with air-conditioned ceilings, nimble systems are preferred. The combination with a modern heat pump is a sensible application.
Thermal insulation
The standard molded parts of prestressed concrete prefabricated ceilings can be equipped with an additional insulating layer made of expanded polystyrene (EPS) at the factory. The insulation thickness can be up to 250 millimeters, which corresponds to a heat transfer coefficient of 0.14 W / (m 2 · K).
Thermally insulated pre-fabricated concrete ceilings are mainly used as a floor slab for the building foundation. But they are also suitable as a ceiling over crawl spaces or unheated basement rooms. In combination with a climate-controlled ceiling construction, they enable economical building concepts with a high level of energy efficiency.
literature
- Andrej Albert, Heiko Denk, Martin Mertens, Andreas Nitsch: Prestressed concrete - basics and application examples. Werner Verlag (2007), ISBN 3-8041-1090-8
- Günter Rombach: Prestressed concrete construction. Verlag Ernst & Sohn (2003), ISBN 3-433-02535-5
- Josef Hegger, Stephan Görtz: Transverse load-bearing capacity of reinforced and prestressed concrete beams made from normal and high-performance concrete. (2007), ISBN 978-3-410-65757-6
- Jens Peter Grunert: On the load-bearing behavior of prestressed precast beams made of steel fiber reinforced concrete without reinforcement. (2006), ISBN 978-3-89288-176-6
- Jörg Moersch: On the hydrogen-induced stress corrosion cracking of high-strength prestressing steels. Investigations into the durability of prestressed concrete components. (2005), ISBN 978-3-410-65763-7