EP1987209A1

EP1987209A1 - Elements/slabs based on solid wood elements reinforced with concrete

Info

Publication number: EP1987209A1
Application number: EP07709224A
Authority: EP
Inventors: Haumann Sund
Original assignee: Combino AS
Current assignee: Combino AS
Priority date: 2006-02-10
Filing date: 2007-02-09
Publication date: 2008-11-05
Also published as: WO2007091899A1; EP1987209A4

Abstract

A construction element (10) comprising a plate formed product (1) of wooden material combined with a layer of concrete (2) is disclosed. A binding agent (5), optionally in addition to fastener means (7), is present in the stratum between the panel formed product (1) of wooden material and a cast layer of concrete (2). The binding agent (5) unites the wooden material and the concrete into complete static coaction to achieve a unitary product having high flexural rigidity. When the construction element (10) is exposed to load the neutral plane for tensile/compressive stresses within the element will always be in the wooden part so that the concrete is subjected to compression only.

Description

Elements/slabs based on solid wood elements reinforced with concrete

The present invention relates to a construction element comprising a plate formed product of wooden material combined with a top layer of concrete, and a method to fabricate the construction element. The construction element is primarily intended to be used as floor elements in buildings, without thereby being considered as a limitation of the invention. The construction element can also be described as a composite slab or floor element.

Such a plate formed product of wooden material can be an element of solid wood consisting of cross laid layers of wood lamellas, or boards, which are joined by glue or by means of mechanical connections like nails, screws and wooden dowels.

In Norway today, such elements are produced in a factory, for example with a maximum thickness of 240mm, width of 1200mm and length of 14000mm. Used as flooring, or slabs, in building the maximum span width will be approximately 6-7 meters.

Plainly sound technical, solid wood has relatively poor properties, because wood easily conducts sound, and because a slab of solid wood has low weight, i.e. about ¹A of a slab of concrete. Solutions that have been used and that provide sound technical good results are to increase the weight, either by sand, cast of concrete or by separate slabs of concrete, all carried out at the construction site. By its increased weight the concrete also adds good noise isolating properties.

However, the basic idea with the present invention is to combine a wooden element of the above mentioned kind with concrete in such a way that substantially complete static co-operation is achieved between the wooden element and the concrete in order to improve the strength properties. This implies that no relative slippage will take place in the interface between the different materials. Thus such beneficial effect can be achieved that the concrete will take up the compressive forces that appear within a slab which is supported along its edge only and is spanning over a field. As well known is, concrete has extremely high compressive strength. Correspondingly the wooden element will take up the tensile stresses acting in the transversal cross section when the slab is subjected to bending strain. Wooden material used in buildings and other constructions have precisely good tension properties. With that the positive properties of both materials are utilized in strength wise connexion.

It is further to be understood that we seek to achieve complete or 100% static cooperation between the wooden element and the concrete such that they act as if they were one single piece. However, some deviation from the desired 100% should be tolerated. "Complete static co-operation" is a normal way for a person skilled in the art how to describe a composite supporting construction. One example of such a construction is a glued-laminated wood girder that is defined as: "Load supporting wood construction in which every cross section is constructed of a number of (at least four) lamellas having substantially parallel fibre direction which by means of glue are brought to complete static co-operation".

This is achieved by a construction element of the introductory said kind which is distinguished in that the concrete layer has a thickness relative to the thickness of the prefabricated plate formed product of wooden material such that the neutral plane for the compressive/tensile stresses through the construction element at all times is located within the plate formed product of wooden material, and that a binding agent is present in the stratum between the panel formed product of wooden material and a top cast layer of concrete, said binding agent unites the wooden material and the concrete into complete static co-operation. Thus a unitary product having high flexural rigidity is achieved. Under the premises above, the thickness ratio between the plate formed product of wooden material and the layer of concrete will normally be in the order of magnitude 2:1.

With a structural element of this type the maximum span could be increased considerably, in addition to improve the sound technical properties.

The plate formed product of wood material may preferably be made of solid wood, i.e. wood laid in such a way that it is substantially void of cavities. An alternative is that the plate formed product of wood material is of wood composite / wood laminate, and preferably cross glued in layers. One type of such composite/laminate is called LVL (Laminated Veneer Lumber).

In a preferable embodiment the binding agent is a glue of the epoxy type. One type may be Mapepoxy L®, though other will surely prove to be suited by further testing. Such a combination of epoxy and concrete "glues" the concrete layer to the wood surface.

As an addition to the binding agent fastening means in the form of nails, dowels, tacks, pins, staples or similar can be used, also in combination with each other.

In addition a network in the form of reinforcement, such as mesh reinforcement, can be used together with the fastening means. Also milled out grooves extending transversally to the element, in combination with binding agent or fastening means which further secure the concrete and takes up arising shear forces when the element is loaded can be used.

As a special incident the concrete may include reinforcement in the form of embedded strength fibres, so called fibre concrete.

It is to be noted that if the construction element has a longer span that requires "centre support", the concrete will experience tension over the support and needs to be reinforced for such loading.

It is to be seen that this method can be used both as to be performed on the building site and as ready produced elements from factory.

In a further embodiment of the construction element according to the invention, the plate formed product of wood material can further include braces that are perpendicularly arranged relative to the plate plane and fixedly secured to the plate formed product. Thereby the bending stiffness of the construction element is further increased. In still another embodiment the braces can further include flange plates that are perpendicularly arranged relative to the brace plane and are fixedly secured to respective braces and the flange plates extend substantially in parallel with the plate formed product, said braces forming the web in bending stiff profiles.

In still another embodiment a larger plate can span over and be fixedly secured to respective braces, where this larger plate extends substantially in parallel with the plate formed product.

The new construction element makes the following advantages possible to achieve:

1) Better sound technical properties by increasing the weight per m², without increasing the total element thickness compared with a pure solid wood element

2) Larger span 3) Possibility to produce the elements with super height to compensate for deflection and in addition achieve pre-stress

4) Possibility to install pipes for floor heating, electricity, etc.

5) Less vulnerable against moisture during transportation and the construction period 6) Fabricated as elements such a product would be competitive, for example compared to hollow slab elements of concrete

7) Rough floor surface that withstand traffic, spill and load during the construction period

Theoretical calculations of material strengths and sound technical properties under presumption of complete static co-operation are made. The selection of materials, such as epoxy and concrete will take place out of theoretical calculations and knowledge about the properties of these materials regarding strength, shrinkage, creeping, etc. Tests and results are described on page 12.

According to the present invention also a method for fabrication of a construction element that includes a prefabricated plate formed product of wood material combined with a layer of concrete is provided, in which the invention is distinguished in that a binding agent is applied the top surface of the prefabricated plate formed product as an adherent stratum, and thereafter concrete is pour concreted as a top layer in a thickness relative to the thickness of the prefabricated plate formed product of wood material such that the neutral plane for the compressive/tensile stresses through the construction element at all times is located within the plate formed product of wood material, and the applied binding agent between the plate formed product of wood material and the concrete layer binds these together into complete static co-operation as the binding agent and concrete cure.

In a second embodiment the fabrication takes place in that fastening means first are applied to the top surface of the prefabricated plate formed product before the binding agent is applied.

As mentioned the method can take place as a combination of use of fastening means and binding agent. The construction element can, as an alternative, be produced at site, i.e. directly at the construction site. Thus it is to be understood that when the construction element is produced at the construction site, the element may have any conceivable configuration adapted to the actual building and defined by a wall structure.

Other and further objects, features and advantages will appear from the following description of some embodiments of the invention, which is given for the purpose of description, and given in context with the appended drawings where:

Fig. 1 shows in perspective view a construction element according to the invention,

Fig. 2 shows in longitudinal cross section a first embodiment of a construction element according to the invention,

Fig. 3 shows in longitudinal cross section a second embodiment of a construction element according to the invention, Fig. 4 shows in longitudinal cross section a third embodiment of a construction element according to the invention, Fig. 5 shows in longitudinal cross section a fourth embodiment of a construction element according to the invention,

Fig. 6 shows in perspective view another embodiment of a construction element according to the invention, Fig. 7 shows in perspective view another embodiment of a construction element according to the invention,

Fig. 8 shows in perspective view another embodiment of a construction element according to the invention, and

Fig. 9A-9C illustrate flooring elements having approximately equal strength and stiffness.

Reference is first made to figure 1 and a construction element 10 according to the invention will now be described. The construction element 10 is of a composite construction mainly consisting of wood material and concrete. The wood material is in the form of a plate formed element or product 1 which is lying as a supporting layer. Such a plate formed product 1 can be a wood element of solid wood consisting of cross laid layers of wood lamellas, or boards Ia, that are joined by glue or by means of mechanical connections like nails, screws and dowels (not shown).

One alternative is that the plate formed product 1 is of wood composite/wood laminate (not shown). One type of wood laminate is named LVL (Laminated Veneer Lumber).

One layer of concrete 2 is provided on the plate formed product 1. In one embodiment the construction element 10 may have respective complementary ledges 3, 4 along the edges thereof. Thus construction elements 10 that are laid adjacent to each other also will overlap each other and create a splice that connect the elements 10 together at the same time as a unitary, smooth surface is created.

Reference is now made to figure 2 that illustrates in closer detail a first embodiment of the internal structure of a construction element 1OA. Components, parts or elements that repeat in the figures are given the same reference number in all figures. The plate formed element 1 of wood material is lying lowermost, the layer with concrete 2 uppermost and a stratum with binding agent 5 is located between them. The stratum with binding agent 5 binds the wood material and the concrete together to substantially complete static co-operation, preferably fully static co-operation. With that the desired unitary, bending stiff product is achieved. The binding agent can be a glue of the type epoxy. One alternative may be Mapepoxy L®, though others will surely prove suitable by further tests. Such a combination of epoxy and concrete "glues" the concrete layer to the wood surface.

As it appears from the figure it is also cast a reinforcing mesh 6 into the concrete. However, it is to be understood that such mesh reinforcement, even if it will be used in some applications, is not mandatory. As an alternative, or an addition, the concrete may include armouring in the form of embedded strength fibres, often called fibre concrete.

Reference is now made to figure 3 that illustrates a cross section through a second embodiment of a construction element 1OB. As before, the plate formed element 1 is lying lowermost and the layer with concrete 2 uppermost. Instead of a stratum with binding agent 5, fastening means 7 is now used between the layers 1 and 2. As an alternative a combination of fastening means 7 and binding agent 5 can be used to bind the wood material and the concrete together to substantially complete static co- operation, preferably fully static co-operation. With that the desired unitary, bending stiff product is achieved. The fastening means 7 may assume different forms that are used alone or in combination and examples to be mentioned are nails 7', dowels, pins, staples 7", pins 7'" or similar.

As it appears from the figure, also here it is shown cast in of a reinforcing mesh 6 into the concrete. However, it is to be understood that such mesh reinforcement, even if it will be used in some applications, is not mandatory. As an alternative, or an addition, the concrete may include armouring in the form of embedded strength fibres, often called fibre concrete.

Reference is now made to figure 4 that shows a cross section through a third embodiment of a construction element 1OC. As before, the plate formed element 1 is lying lowermost and the layer with concrete 2 uppermost. Instead of, or in combination with fastening means 7 between the layers 1 and 2, it is now arranged a number of grooves 8 in the plate formed element 1. As an alternative a combination of grooves 8, fastening means 7 and binding agent 5 can be used to bind the wood material and concrete together to substantially complete static co-operation, preferably fully static co-operation. With that the desired unitary, bending stiff product is achieved. The grooves 8 can preferably be arranged so that they extend transversally of the direction for the main loads within the construction element 1OC.

Reference is now made to figure 5 that shows a cross section through a fourth embodiment of a construction element 10D. As before, the plate formed element 1 is lying lowermost and the layer with concrete 2 uppermost. As in the embodiment according to figure 4 it is now arranged a number of grooves 8 in the plate formed element 1. A number of armouring bails 9 is connected to the mesh reinforcement 6 and extend down into the grooves 8 for co-operation with these. The armouring bails 9 act instead of, or in combination with fastening means 7 between the layers 1 and 2. As an alternative a combination of grooves 8, armouring bails 9, fastening means 7 and binding agent 5 can be used to bind the wood material and concrete together to substantially complete static co-operation, preferably fully static co-operation. With that the desired unitary, bending stiff product is achieved.

Now a method for use during fabrication of a construction element 10 of the above said kind will be described. A prefabricated plate formed product 1 of wood material is basically used as a base or supporting layer. The plate formed product 1 forms the bottom in a mould for casting of a concrete layer 2. Before casting of concrete takes place a binding agent is applied to the top surface of the prefabricated plate formed product 1. Thereafter the concrete is poured out as a top layer. The applied binding agent between the plate formed product of wood material and concrete layer binds these together to a unitary bending stiff product as the binding agent and the concrete are hardened. The binding agent shall also act as an adherent stratum to complete static co- operation between the hardened concrete layer 2 and the plate formed product 1.

In a second embodiment the fabrication takes place in that one or more different fastening means 7 firstly are applied to the top surface of the prefabricated plate formed product 1 before the binding agent is applied. Thereafter the concrete is poured out and binds the plate formed product 1 of wood material and concrete layer 2 together to a unitary bending stiff product as the concrete is curing and the fastening means 7 are cast fixedly within the concrete layer 2.

With reference to figure 6, another embodiment of a construction element 2OA according to the invention is illustrated. The construction element 2OA comprises one of the variants of the construction element 10- 1OD shown and described in connection with the figures 1 to 5. What is characteristic for this embodiment is that the plate formed product 1 of wood material further comprises braces 11 arranged perpendicular to the plate plane and are fixedly secured to the plate formed product 1.

With reference to figure 7 still another embodiment of a construction element 2OB according to the invention is illustrated. The construction element 2OB comprises one of the variants of the construction element 10-1 OD shown and described in connection with the figures 1 to 5. What is characteristic for this embodiment is that the braces 11 further comprises flange plates 12 that are perpendicularly arranged relative to the brace plane and are fixedly secured to respective braces 11. The flange plates extend substantially in parallel with the plate formed product 1 , and the braces form web in flexural rigid profiles of the I-beam type.

With reference to figure 8 still another embodiment of a construction element 2OC according to the invention is illustrated. The construction element 2OC comprises one of the variants of the construction element 10- 1OD shown and described in connection with the figures 1 to 5. What is characteristic for this embodiment is that a larger plate 13 spans over and is fixedly secured to respective braces 11. This larger plate 13 extends substantially in parallel with the plate formed product 1 and increases the bending stiffness in the construction element 2OC considerably.

Still another embodiment of the construction element according to the embodiments shown in the figures 6-8 will be that one replaces the plate formed product 1 with separate plates (not shown) that are placed between the braces 11 and approximately flush with the upper edge of the braces 11. With that they will form the bottom of a mould for the pour concreted layer 2.

Below a supplemental explanation of what is achieved with the present invention will follow, and is attempted to be schematically illustrated in the figures 9A-9C. It is also outlined further presumptions and aspects to be regarded.

Fig. 9 A shows a section of a floor in the form of the plate formed product 1 or element. The plate formed product 1 is, as before, fabricated of solid wood constructed as a cross laminate. Such a plate formed product 1 of solid wood can for example have a thickness of 200mm as figure 9A suggests. The product has full strength wise load carrying capacity and minimum deflection and is in this respect completely satisfactory.

However, they have, as previously mentioned, very poor sound isolating properties. This can be experienced between floors in residential houses as particularly unpleasant. As one possibility how to solve this noise problem, a layer of 60mm concrete 2 can be pour concreted on top, as illustrated in figure 9C. Thus satisfactory sound isolation is achieved. However, in order to carry this layer 2, the thickness of the plate formed product 1 of wooden material needs to be increased with 20mm, as shown in figure 9C, to achieve the same load carrying capacity and the minimum deflection as before. This provides a total construction height of 280mm, as indicated in figure 9C.

Figure 9C thus illustrates the traditional way of thinking. The object creating the basis for the present invention was as follows. Is it possible to have the concrete layer 2 to take part of the strength wise load carrying of the flooring and in such a way that the total construction height of 200mm is substantially maintained? The solution is illustrated in figure 9B. The plate formed product 1 of solid wood is reduced in thickness by 60mm corresponding to the layer of pour concreted concrete 2. By simultaneously perform gluing between the plate formed element 1 of solid wood and the concrete 2, with to that suitable glue 5, complete static co-operation between the wood and the concrete is exactly achieved. This provides great strength wise difference compared to when concrete is concreted "loosely" on top of the solid wood.

Here it is particularly important that the concrete 2 is subjected to compression stresses only. As known the concrete has poor ability to take up tension, but has excellent properties when it comes to compression stresses and compressive strength. For the now proposed construction element 10, it is vital that the neutral plane for the tensile/compressive stresses needs to be located within the plate formed product 1 of solid wood, so that the concrete 2 continuously is subjected to compression forces only. Since the concrete has such a high compressive strength and is completely incompressible, the gluing between the layers will transform vertically acting forces against the floor to horizontally acting compression forces through the concrete layer 2. And the concrete layer 2 thus acts as if it is completely rigid regarding possible motions. Thus a so called flexural rigid product is achieved.

Due to the difference in gravity between the wood material and the concrete, it is further to be understood that in order to achieve that the concrete layer 2 only can be subjected to compressive forces, the plate formed product 1 of wood material always need^' to have a larger thickness than the thickness of the layer of concrete 2, and in such a thickness ratio that the neutral plane for the compressive/tensile stresses through the construction element at all times is located within the plate formed product 1 of wood material. The specific gravity for concrete and wooden material is in the order of magnitude 2000 kg/m and 500 kg/m respectively.

In practice this means that the concrete layer 2 usually needs to have a thickness less than half the thickness of the prefabricated plate formed product 1 of wood material such that the neutral plane for compressive/tensile stresses through the construction element 10 at all times is located within the plate formed product 1 of wood material.

Wood elements that are to be combined with concrete in fully static co-operation need to have minor dimensional changes only, i.e. be close to dimension stable during varying moisture of wood and temperature. A wood element being most possible stable is a presumption to avoid loss/creeping issues in this type composite construction element, also called a composite floor.

Wood shrinks and swells very differently depending on the fibre direction. A board or a plank shrinks 20-25 times more in the lateral direction than in the longitudinal direction.

When the strata in a solid wood element is bound completely together in layers transversally to the fibre direction (for example by means of glue), the shrinkage (and swelling) will in large degree follow the shrinkage of the wood in the longitudinal direction. Thus a stable element with minor and close to equal shrinking properties in width and length is achieved. This effect is well known from products as lamella parquet, plywood and some kinds of LVL and will also be true for cross glued solid wood.

Tests and results

Sound

A number of sound tests are conducted by "Norges Byggforskningsinstitutt" (NBI) in laboratory in Oslo. The elements that were tested were built up by cross glued solid wood in the dimension 180 x 1200 x 3600mm. Two elements having width of 1200mm and one having width of 600mm were interconnected into a flooring element having the measures 3000 x 3600mm.

Initially the plain plate formed product 1 of wood material (solid wood floor element) having thickness of 180mm was tested to ascertain the values for step sound and airborne sound. Thereafter the element was added on by several alternatives of sound insulation, and measurements were taken for the different alternatives in order to find out the best and most cost effective solution that would satisfy the sound requirements for floors between apartments.

All these measurements would then form basis for the comparison with subsequent measuring of the construction element 10.

The plate formed product 1 was then "glued on" 60mm concrete 2 by the so named "wet-on- wet" method. After a curing period new sound measuring of the construction element took place, firstly without additional insulation, in order to be able to compare the value that was measured for the plain plate formed product 1.

The results corresponded very well to the theoretic calculations that were made in advance. The improvement for air sound became 8 dB and when we know that 1OdB is approximately a halving of the sound level, this is very good.

Also measurements with alternative additional structures were made to be able to compare with the corresponding for the plain plate formed product 1.

Further laboratory measurements was considered unnecessary by NBI since the theoretic calculations and the laboratory measurements agreed so well with each other. The sound project is terminated with complete report from NBI.

Strength and rigidity (stiffness)

The construction element 10 was disassembled after the sound tests were terminated and the two largest elements having width of 1200mm were tested regarding strength and rigidity in respective turns. The element was subjected to two line loadings placed in accordance with International regulations for this kind of testing and within 5 minutes the load was increased to about 400 kN before rupture took place. The results were very edifying, inter alia the "gluing" of the concrete indicated no weaknesses and the rupture pattern was as for a plain solid wood element, i.e. that shear rupture within the lateral lamellas close to the neutral axis or plane was the weakest spot.

It is not ready written end report for this test, but already now it can be stated that theory and practise agree.

The proportion between the solid wood and concrete was here such that the neutral axis was located underneath the concrete layer so that tensile stresses within the not reinforced concrete did not appear. "The glue joint" indicated no weaknesses in for example elongation or displacement between concrete and wood.

It was also exiting to ascertain that these full scale tests agreed so well with the minor laboratory tests that were made previously, with regard to increased E-module (rigidity). The E-module for the construction element 10 was here proven to be approx. 40 % higher than for a plain solid wood element. This means that in addition to better sound technical properties, the construction element 10 will prove better against vibrations and the span width may be increased compared with a plain solid wood floor element.

The tests that are conducted until now indicates that the theoretic calculations that formed basis for the invention agrees well with the laboratory tests. In addition it can be stated that glue and the gluing method that is used enables complete static co-operation between wood and concrete.

Claims

P a t e n t c l a i m s

1.

A construction element (10) comprising a plate formed product (1) of wood material combined with a top layer of concrete, characterized in that the concrete layer (2) has a thickness relative to the thickness of the prefabricated plate formed product of wooden material such that the neutral plane for the compressive/tensile stresses through the construction element at all times is located within the plate formed product of wooden material, and that a binding agent (5) is present in the stratum between the panel formed product (1) of wooden material and the top cast layer of concrete (2), said binding agent (5) unites the wooden material and the concrete into complete static co-operation.

2.

The construction element according to claim 1 , characterized in that the thickness ratio between the plate formed product (1) of wooden material and the layer of concrete (2) is approximately 2:1.

3.

The construction element according to claim 1 or 2, characterized in that the plate formed product (1) of wooden material is of solid wood.

4.

The construction element according to any of the claims 1 — 3, characterized in that the plate formed product (1) of wooden material is of wood laminate, preferably cross glued.

5.

The construction element according to any of the claims 1 - 4, characterized in that the binding agent (5) is an epoxy.

6.

The construction element according to any of the claims 1 - 5, characterized in that the element comprises fixation means in the form of milled out grooves (8), nails (7'), staples (7"), dowels (7'"), etc. or a combination of one or more of these.

7.

The construction element according to claim 5, characterized in that the fixation means comprises a mesh in the form of a reinforcement (6).

8.

The construction element according to any of the claims 1 — 7, characterized in that the concrete comprises reinforcement in the form of embedded strength fibers, often called fiber concrete.

9.

The construction element according to any of the claims 1 — 8, characterized in that the plate formed product (1) of wooden material further comprises braces perpendicularly arranged relative to the plate plane and fixedly secured to the plate formed product.

10.

The construction element according to claim 9, characterized in that the braces further comprises flange plates perpendicularly arranged relative to the brace plane that are fixedly secured to respective braces and the flange plates extend substantially in parallel with the plate formed product, said braces form the web in flexural rigid profiles.

11.

The construction element according to claim 9, characterized in that a larger plate spans over and is fixedly secured to respective braces, said larger plate extending substantially in parallel with the plate formed product.

12.

A method for fabrication of a construction element that includes a prefabricated plate formed product of wood material combined with a layer of concrete, characterized in that a binding agent is applied the top surface of the prefabricated plate formed product as an adherent stratum, and thereafter concrete is pour concreted as a top layer in a thickness relative to the thickness of the prefabricated plate formed product of wood material such that the neutral plane for the compressive/tensile stresses through the construction element at all times is located within the plate formed product of wood material, said applied binding agent between the plate formed product of wood material and the concrete layer binds these together into complete static co-operation as the binding agent and concrete cure.

13.

The method according to claim 12, characterized in that fastening means first are applied to the top surface of the prefabricated plate formed product before the binding agent is applied.

14.

The method according to claim 11, 12 or 13, characterized in that the construction element is fabricated at the site, i.e. directly at the construction site.