Wood and concrete are common composite materials, especially in the renovation of wooden beam ceilings. But they are also used in new buildings and increasingly also as prefabricated elements. The specific building material properties of wood and concrete in terms of sound, fire and heat insulation can be combined well, especially in the case of floor ceilings. In composite technology, both materials are used optimally: wood, which can also absorb tensile loads well, acts in the lower slab area, while concrete is used in the compressive upper area. The connection of wood and concrete is made by means of suitable composite materials, so that the ceiling can be regarded statically as a component and receives its sufficient rigidity via the concrete slab.
A supporting structure consisting of a wooden component that is connected to a concrete slab in a shear-resistant manner is called a wood-concrete composite structure. The timber component can be both a conventional wooden beam layer made of solid wood or glulam, but also a so-called board stacking plate, which consists of wooden boards glued together on edge and serves as a flat layer for the concrete layer.
Timber composite ceilings are structurally treated as wooden ceilings and are therefore designed and dimensioned according to Eurocode 5. DIN 1052, which was relevant before the introduction of the Eurocode regulations, has since been withdrawn. However, since not all contents have been incorporated into the European standards, DIN 1052-10 was created.
In addition, there is still no set of rules in timber-concrete composite construction that is specifically designed for this construction technique, as is the case in steel composite construction, for example.
Renovation of wooden beam ceilings: Wooden beam ceilings in old buildings are often no longer able to meet modern requirements in terms of load-bearing capacity, deflection, sound insulation and fire protection. Thanks to the additional concrete slab and the bond with the old beam layer, the existing ceiling can be upgraded and kept fit for use with relatively little effort. This is referred to as a so-called ceiling upgrade. Especially in the area of monument protection, this is a way to preserve stucco ceilings, friezes, etc.
The old beam layer will be exposed and, as far as still suitable, left in the existing structure. The original board layer can also often be retained and thus acts as a lost formwork for the concrete layer.
Depending on the choice of fastener, this is applied directly to the beams or glued into the wood. This is usually done through the board layer, as far as the approval of the lanyard allows it. A PE film is inserted between the wood and concrete as an intermediate layer to prevent moisture from entering the wood. After the reinforcement has been laid, the concreting process can begin.
If the room height may not or should not be reduced by an additional layer of concrete for the composite ceiling in the specific case of renovation, it is possible to place the concrete layer between the rafters. The bond does not take place on the load-bearing beam layer, but attaches to it laterally via composite means. The original ceiling thickness is retained.
Ceilings in wood-concrete composite construction in new buildings: While the use of a wood-concrete composite construction is often obvious in the case of renovation, it is often more difficult to weigh up the advantages and disadvantages in new buildings. In the following, the wood-concrete composite ceiling is compared with a conventional timber beam ceiling and a conventional in-situ concrete ceiling :
Advantages over a classic timber beam ceiling:
Advantages over a classic in-situ concrete slab:
ceiling soffit possibleStructural engineering: Since wood-concrete composite slabs are a relatively new construction technique, the static calculation of the slab, but also the selection and number of suitable composite materials is usually prepared in close cooperation between the structural engineer and the manufacturing plant. Especially when it comes to prefabricated slab elements, ceiling manufacturers offer the complete preparation of the structural design of the slab. For the architect, pre-dimensioning programs on the websites of the major ceiling and composite manufacturers are helpful.
Fire protection: Compared to a pure timber construction, a ceiling in wood-concrete composite construction has great advantages. On the one hand, the concrete layer blocks smoke gases in both directions, and on the other hand, in the event of a fire, the wooden beams are protected from at least one side by the adjacent concrete layer against excessive heating, which has a positive effect on the fire resistance period. Conversely, wooden beams and slabs protect the concrete layer, which prevents spalling.
In addition, in composite construction , both components, i.e. wood and concrete together, ensure the load transfer, so that when the wood cross-section is reduced due to fire, the concrete layer can briefly take over load shares of the wood. The composite materials are protected inside the component.
In this way, a ceiling in wood-concrete composite construction with appropriate reinforcement, sufficient concrete cover and including the screed as an overall component can meet common fire resistance requirements.
Sound insulation: The protection goals in terms of noise protection must be determined in advance between the planner and the client, as several sets of regulations exist side by side. Especially in buildings with several units, high requirements apply according to the state of the art, which in particular concern the sound transmission between the units, i.e. also the transmission via false ceilings. DIN 4109 regulates the absolute minimum standard, which is considered outdated today. Planners should use the increased values according to DIN 4109 Supplement 2 or VDI Guideline 4100 as the absolute minimum standard. The Dega Recommendation 103 of the German Society for Acoustics can also be helpful in this respect.
The protection against airborne noise increases with an increase in the area-related mass of the ceiling tile, which is determined by the thickness and bulk density of a component. In principle, concrete as a heavy building material provides very good conditions for the containment of airborne noise. For example, ceilings of this construction method with a high proportion of concrete have advantages in terms of their sound insulation effect. Compared to the pure wooden beam ceiling, the composite ceiling is less susceptible to vibrations. By integrating the ceiling into the wall, the transmission of secondary sound paths is also reduced.
An improvement in impact sound insulation is hardly achieved by increasing the area-related mass. A double-shell construction is far more effective in this respect. Floating screed is particularly effective as a second shell. It is acoustically decoupled from the ceiling and wall construction by impact sound insulation and edge insulation strips, see bauwion page ► 400 | Construction site screeds.
Thermal insulation: While concrete has almost no thermal insulation effect due to its good thermal conductivity, solid wood with a thermal conductivity in the range of 0.13 W/mK has a good thermal insulation effect. In the case of beam ceilings, the space between the beams can also be used as a room for insulation materials.
Thermal insulation can be dispensed with in well-ventilated rooms above and below the ceiling (e.g. in buildings open to the outside) and in ceilings that only separate heated rooms from each other.
Ceilings in composite construction that separate heated interiors from outdoor spaces (e.g. passageways, flat roofs) must be insulated in accordance with the Energy Saving Ordinance (EnEV). Ceilings against rooms that are included in the calculation as unheated or are not within the system boundary of the heated building volume (e.g. unheated basement or attic) must also be insulated, otherwise condensation cannot be ruled out.
Concrete selection: The compressive strength class of the concrete must be determined in the planning. For ceilings in wood-concrete composite construction, this is usually C20/25 or higher. The consistency class (e.g. F3) and exposure class (e.g. XF 2) are also defined, the latter especially when a component comes into contact with the outside air. These specifications must be made by the structural engineer in consultation with the client and the architect.
Composite: The choice of composite or fastener depends primarily on the type of slab construction, as well as on the static requirement in terms of bending stiffness. The composite determines whether the ceiling construction works in its function as a composite component, it is responsible for ensuring that the two materials wood and concrete do not shift against each other in the event of load. Most composite materials must be approved by the building authorities.
In principle, elongated, pin-shaped composites such as nails, composite screws and anchors or the like are suitable for beam ceilings . They are usually designed to penetrate a wooden formwork above the beam.
Pin-shaped composites can only be used to a limited extent for board stacking ceilings , as the respective thickness of the boards contained is generally not sufficient to maintain the necessary edge spacing. Flat steel locks, notches, incisions in the wood or staggered board heights within the panel are particularly suitable here.
Shear connectors, e.g. the HBV shear connectors from TiComTec, which ensure an almost rigid bond, can be used in both board stack ceilings and beam ceilings.
See also bauwion encyclopedia article ►Wood-concrete composite construction, composite materials.
Installations: Beam gaps are the easiest way to integrate installation cables into the ceiling. However, empty conduits, e.g. for electrics, can also be laid in the cast-in-place concrete layer if the layer thickness allows the required covering. In the case of point-shaped composite agents, but also in the case of elongated shear connectors, which are usually not used over the entire length, installation cables can be laid between them. However, this type of pipe laying should be coordinated with the structural engineer.
Wood-concrete composite ceilings, other variants:
A box ceiling is a variant of a beam ceiling in wood-concrete composite construction. The beam layer is closed on the underside with a multi-layer panel press-glued. It is also part of the overall statics of the component due to its load-bearing capacity under tension. The advantage lies in the achievement of larger spans.
In a cassette or hollow box girder ceiling in wood-concrete composite construction, individual insulation panels are placed between the composites on the wooden panel as a variant of a ceiling with a board stacking board , which reduce the weight of the ceiling and enable high spans of up to over 15 m.
Acoustic panels can also be used as a support board for a wood-concrete composite construction. The special profiling on the underside and the absorber layers permanently integrated into the panel make the composite construction a fully-fledged acoustic ceiling.
As a variant of a ceiling with a board stacking plate, the board can be partially dissolved, saving weight and material. Horizontal "board stack girders" remain, to which the reinforced concrete slab is connected via shear connectors. The ceiling soffit also has acoustic advantages over a ceiling with a classic board stacking board due to its dissolved surface. High spans of up to over 15 m can be realized in this way.
Installation of reinforcement: In the case of wood-concrete composite slabs, it is customary to reinforce the concrete layer at least with a reinforcement mesh and, if necessary, with additional reinforcement cross-sections. Exact details are specified in the reinforcement plan.
When installing the reinforcement on site, 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.
Concrete paving: Before the concreting process, all wooden components should be separated from the concrete layer with an intermediate layer in the form of a PE film, impregnation, cardboard or similar in order to prevent or minimise moisture ingress during the concreting process. Care must be taken to ensure sufficient overlap or bonding or welding.
In addition, all structures whose concrete layer is laid on site must be supported during concreting. Information about the number and position of the yokes can be obtained from the structural engineer or the manufacturing plant.
Concrete for ceilings delivered or produced on site must in principle be installed as quickly as possible. This must prevent cavities from forming in the component. This is prevented by shaking, stomping or poking. However, if this is done for too long, 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. It is influenced by external conditions during setting. In extreme climatic conditions such as heat (over 30°C) or frost (below -5°C), concrete may not be poured without suitable additional measures.
Post-treatment of 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 usually fully hardened and 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 (see Standards and Literature), is also helpful in this context.
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 1052-10, Production and execution of timber structures - Part 10: Supplementary provisions
DIN 4102-4, Fire behaviour of building materials and components - Part 4: Composition and application of classified building materials, components and special components
DIN 4109, sound insulation in building construction; Requirements and verifications
DIN 4109 Supplement 2, Sound insulation in building construction; Instructions for planning and execution; proposals for increased sound insulation; Recommendations for sound insulation in one's own home or work area
DIN 20000-1, Application of building products in buildings - Part 1: Wood-based materials
DIN EN 206, Concrete - Specification, properties, production and conformity
DIN EN 1995-1-1, Eurocode 5: Design and construction of timber buildings - Part 1-1: General - General rules and regulations for building construction
DIN EN 1995-1-1/A2, Eurocode 5: Design and Construction of Timber Structures - Part 1-1: General - General Rules and Rules for Building Construction
DIN EN 1995-1-1/NA, National Annex - Nationally Determined Parameters - Eurocode 5: Design and Construction of Timber Structures - Part 1-1: General - General Rules and Rules for Building Construction
DIN EN 1995-1-2, Eurocode 5: Design and construction of timber structures - Part 1-2: General rules - Structural design for fire
DIN EN 1995-1-2/NA, National Annex - Nationally determined parameters - Eurocode 5: Design and construction of timber structures - Part 1-2: General rules - Structural design for fire
DIN EN 13501-1, Classification of construction products and construction methods with regard to their fire behaviour, Part 1: Classification with the results of the tests on the fire behaviour of construction products
►DBV leaflet "Concrete formwork and stripping periods
"►Cement Leaflet B8, Technical Notes on the Post-Treatment of Concrete Components, Publisher: Verein Deutscher Zementwerke
Source: bauwion
