For quick and cost-efficient construction, prefab elements such as prefab floor elements are very popular. A prefab floor element known from EP 0 450 682 comprises a con- crete layer with a first pre-tensioned reinforcement member.

The floor element has a second reinforcement member, whose base is formed by two straight rods, which via a zigzag wire are connected with a third straight rod. In its longitudinal direction, the second reinforcement member has a V-shaped cross section. The two rods are placed up against the first reinforcement member and are incorporated in the concrete, whereas the third rod extends beyond the concrete and is strainable under tension. Relative to its load-bearing caun- dercity, there is a reduction in said known floor element's weight, due to the reinforcement being partly external.

It is the object of the present invention, to pro- vide an improved prefab floor element that is easy to manu- facture.

To this end the prefab floor element according to the invention is characterised in that the first reinforce- ment member is a netted first reinforcement member and the non-external portion of the second reinforcement member partly extends through openings of the netted reinforcement member coming close to the bearing surface.

The prefab floor element according to the invention is stronger or equally strong and lighter (and therefore cheaper) while having a concrete layer of the same thickness.

In the latter case fewer second reinforcement members are needed and there is more freedom with respect to the instal-

lation of pipes between the lowered ceiling and the underside of the concrete layer. The second reinforcement member, in particular the opening formed by the external portion of the reinforcement and the underside of the floor element formed, can be used for passing through conduits, and if such a floor element is used as floor element of a storey or roof, for se- curing a lowered ceiling. There is no need to pre-tension the reinforcement and this favourably affect the cost price of the prefab floor element.

The invention also relates to a method of manufac- turing a concrete prefab floor element having internal and external reinforcement, by pouring a layer of concrete on the upper side of a formwork, wherein a first reinforcement mem- ber is incorporated into the concrete layer and a second re- inforcement member is partly incorporated into the concrete layer, wherein the portion of the second reinforcement member that is not incorporated in the concrete layer comprises a rod-like element that serves for absorbing the tensile forces while the floor element is in use, and the concrete is al- lowed to set, resulting in a ready floor element that is separated from the formwork.

Such a method is known. In the known method, a floor element is manufactured on its end face by pouring concrete on a formwork. In the concrete layer a first, pre-tensioned reinforcement member is incorporated. The second reinforce- ment member comprises a base formed by two straight rods con- nected via a zigzag wire with a third straight rod. The sec- ond reinforcement member has a V-shaped cross section, and the two straight rods are placed between first reinforcement members and incorporated into the concrete, while the third rod projects therefrom. A floor element manufactured by the known method has a reduced weight. Furthermore, when using smooth formwork, a bearing side is obtained that requires no further finishing. Traditional floor elements, whose rein- forcement is entirely internal, need a relatively large por- tion of the materials used for carrying the weight of the floor element itself. In the known method, the second rein-

forcement member used is a traditional joist for concrete slabs, which is partly incorporated into the concrete slab.

It is the object of the invention to provide a sim- plified method of manufacturing a prefab-floor element.

To this end the method according to the invention is characterised in that as the first reinforcement member a netted reinforcement member is used, the portion of the sec- ond reinforcement member incorporated into the concrete layer is partly inserted through openings of the netted reinforce- ment member, and the concrete is allowed to set, resulting in the reinforced prefab floor element.

With the second reinforcement according to the in- vention a thinner floor having the same bearing capacity can be manufactured or an equally thick floor having a greater bearing capacity. The first reinforcement can be placed more closely to the underside of the floor. With the known method, the second reinforcement cannot be incorporated deeply in the concrete, and without the two rods to be incorporated in the concrete, the connection between them is poor. These two rods raise the cost price, not only because of the intrinsic costs, but also because of the numerous extra connections (welding points), while an excellent connection with the con- crete of the concrete layer is ensured. Owing to the excel- lent connection of the second reinforcement with the concrete it is even possible to use fewer second reinforcements. It has been shown, for instance, that according to the prior art a second reinforcement has to be used every 60 cm whereas, aided by the method according to the invention, placing a second reinforcement is only necessary every 1-1. 20 m. This means a further saving. In the method according to the inven- tion, the reinforcement members can be placed independently of each other (at least, provided the portions of the second reinforcement member incorporated in the concrete layer are allowed to project at least partly into the openings of the first netted reinforcement member). In practice this means that the distances are determined by the distance necessary to avoid corrosion of the reinforcement members. In the fin- ished floor, and taking into account other factors such as

fire safety, the netted reinforcement member is located as closely as possible to the underside. The portions of the second reinforcement member incorporated in the concrete, which face toward the bearing surface are located as closely as possible to the bearing surface. In contrast to the known method, the thickness of the concrete layer under the first, netted reinforcement member (in the finished state of the floor element), may be considerably thinner. This makes this reinforcement member more effective optionally allows it to be embodied lighter and therefor cheaper. Although the rein- forcement members will generally be made of steel, it does not exclude plastic, fibres (such as glass fibres and carbon fibres) and composites of these. Likewise, the reinforcement may have any shape, such as an I-or T-section, a spiral shape, which shapes may contribute to an improved strength or better bonding with the concrete layer. When in the present application the term bearing surface or bearing side is used, it means that after the prefab floor element has been placed, that side requires no further finishing. In the present ap- plication the term prefab floor element also encompasses a roofing floor, where the floor element serves as ceiling. The second reinforcement, in particular the openings formed by the external part of the reinforcement and the underside of the formed floor element may be used for passing through con- duits and in the case where such a floor element is used as floor element of a storey or roof, for fastening a lowered ceiling.

In accordance with an important embodiment, a rein- forcement is used as the second reinforcement, which in addi- tion to the rod-like part comprises a further part having a saw tooth or zigzag profile, wherein points of the saw teeth or zigzag project through openings of the first reinforcement member.

Such a second reinforcement member is easily manu- factured and provides excellent strength.

To improve the strength, the first netted reinforce- ment member in the finished prefab floor element is located

in the bottom 35%, preferably in the bottom 25% of the thick- ness of the concrete layer.

To improve the strength, the points of the second reinforcement member are located in the top 35%, preferably in the top 25% of the thickness of the concrete layer.

Without wishing to be tied to any theory, it is be- lieved that for an improved strength, the rod-like part and all the points of the saw tooth or zigzag profile of the fur- ther member should preferably be oriented in one plane.

To simplify the method of manufacture it is pre- ferred for the one plane to be placed perpendicularly to the plane of the formwork's upper side. Especially when larger spans are involved, it is preferred for the points of the saw tooth or zigzag profile to form a V in the longitudinal di- rection, with the points projecting, for example, alternately to the left or right. Another repeating pattern is also pos- sible, and also a W-configuration.

As the second reinforcement member it is preferred to use a reinforcement member having at least at one of its ends an end portion that extends substantially perpendicu- larly to the one plan'e of the second reinforcement.

Such an end portion facilitates easy positioning of the second reinforcement member without it being able to fall over. This renders the manufacture of the prefab floor less labour-intensive. It is also useful for anchoring the second reinforcement member in the concrete layer.

For certain applications, in particular those where the prefab floor is placed on an erected foundation or walls, it is preferred that at the ends of the two sides, the con- crete layer extend beyond the external reinforcement and the thickness of the concrete layer at those ends be adjusted to a predetermined value.

By guaranteeing a predetermined thickness, the ends of the prefab floor can be made to rest on the foundation or the top of the walls, which facilitates building.

The invention will now be elucidated by way of the detailed description below, with reference to the drawing in which

Fig. 1 shows a cross section through a finished pre- fab floor element according to the invention placed on the upper side of two walls ; Fig. 2 shows a cross section through the prefab floor element of Fig. 1 according to the line II-II ; Fig. 3 shows a detail of an end of the prefab floor element of Fig. 1 ; Fig. 4 schematically shows a cross section along the line IV-IV in Fig. 1.

Figs. 5,6 and 7 show three possibilities of con- necting the prefab floor elements according to the invention.

Referring now to Fig. 1, which shows a prefab floor element 1 according to the invention in cross section, com- prising a concrete layer 2 having an underside 3 and a upper side 4. The upper side 4 forms the bearing surface. Incorpo- rated in the concrete layer 2 are a first reinforcement mem- ber 5 of the kind well known in the art (in Fig. 2 shown to be netted), as well as a second reinforcement member 6 according to the invention. The second reinforcement member 6 comprises two kinds of elements, a rod-like element 7 and two members having a zigzag profile 8a, 8b. The pointed ends 9 of the zigzag profile of the members 8a, 8b project through the first reinforcement member 6 coming close to the upper side 4 of the concrete layer 2. As can be seen, the ends 10,11 of the concrete layer 2 of the prefab floor element 1 reinforced with the first reinforcement member 6, project beyond the rod-like element 7. The free ends lOg 11 of the prefab floor element shown in Fig. 1 is supported on two walls A, A'. By ensuring that the prefab floor element 1 is manufactured such that (at least) the ends of the concrete layer 2 have a de- fined, uniform thickness, it is possible to place a following wall (not shown) on top of the ends. In this context it should be noted that the term"defined uniform thickness" does not exclude the possibility of the ends having a profile to match the walls A, A', and augmenting the connection be- tween walls A, A', and the prefab floor element 1.

The zigzag elements 8a, 8b are connected with the rod-like element 7 by means of, for example, welding. The

rod-like element 7 is a rod-like element 7 that is under strain of tension. It is preferably manufactured from the usual reinforcing steel and a suitable diameter for a prefab floor 1 with a span of 5.4 m is 16 mm, wherein the second re- inforcement members are spaced approximately 1.2 m from each other. Instead of two zigzag elements 8a, 8b, it is also pos- sible to use one single continuous zigzag element 8 (not shown). However, near the middle of the rod-like element 7, the zigzag elements 8a, 8b contribute little to the strength of the prefab floor and may be omitted.

Fig. 2 shows the openings 12 of the netted first re- inforcement member 5, through which the pointed ends 9 (seen in Fig. 1) project. The second reinforcement member 6, and more in particular one end of each of the elements with zig- zag profile 8a, 8b, is provided with end portions 13a, 13b, which fulfil two functions. When manufacturing the prefab floor element 1, they help to prevent the second reinforce- ment members 6 from falling over. When using the prefab floor 1, the end portions 13a, 13b help to absorb and transmit forces to the second reinforcement member 6. To this end it is preferred to use an auxiliary element 14, which helps to distribute the forces over the netted reinforcement member 5.

As will be discussed later, floor elements 1 can be connected with one another. To augment the interconnection between adjacent floor elements 1, it is possible to use strengthening elements 15 may be used.

Fig. 3 shows an end 10 of the prefab floor element 1 depicted in Fig. 1, wherein as usual the reference numerals correspond with the above-described elements.

The schematic cross section along line IV-IV of Fig.

1 shown in Fig. 4 depicts the concrete layer 2, the first netted reinforcement member 5 and the second reinforcement member 6. The points 9 of the zigzag profile 8a and the rod- like element 7 oriented towards the upper side 4, lie in one plane. The second reinforcement member 6 is placed perpen- dicularly on the first reinforcement member 5. The first re- inforcement member 5 extends parallel to the upper side 4 of the concrete layer 2.

The prefab floor elements 1 according to the inven- tion can be placed to abut against each other so as to form a larger floor element (or ceiling element). Fig. 5 (for the sake of simplicity the reinforcement according to the inven- tion is not shown) depicts a simple embodiment, in which the floor element 1 tapers slightly towards the bearing surface (which during manufacture may also aid separation), and whereby a V-shaped groove is formed when two prefab floor elements 1, 1'are placed so as to abut against each other.

In practice this will be filled with, for example, mortar B.

The prefab floor elements 1, 1'may also have sides that overlap, as shown in Fig. 6.

For a very strong connection between the floor ele- ments 1, 1', a strip of metal 16, 16'may during manufacture of the concrete layer 2 be embedded in the longitudinal sides of the concrete layer 2, which strips 16,16'are connected with the netted reinforcement member 5, 5'by reinforcement means 15, 15'. As explained above, the longitudinal sides form a V-shaped groove, into which a wire 17 is placed. The wire 17 is welded to the strips 16,16'after which the V- shaped groove is filled with mortar, rubber or the like.

Although the invention described concerns a prefab floor element comprising a concrete layer, a different mate- rial such as plastic or glass may also be used. Likewise, the reinforcement or parts thereof may consist of materials other than metal, such as (super) fibres, the person skilled in the art is familiar with the various types (Twarong Dyneema, glass fibre, carbon fibre, etc.). To the ordinary person skilled in the art it will be obvious that reference numerals with an accent correspond with those without accent but apply to another floor element. generally, the prefab floor element will be a rectangular prefab floor element. The rod-like ele- ment of a second reinforcement member will in general extend substantially perpendicularly to the ends of the floor ele- ment. If it is desirable to keep the second reinforcement at a specific distance from the (final) top surface of the pre- fab floor element, it is possible to use spacers, such as two rings fastened to the bent end of the second reinforcement.