GB2205596A

GB2205596A - A method of jointing concrete floor slabs

Info

Publication number: GB2205596A
Application number: GB08712984A
Authority: GB
Inventors: George Cyril Brown; Leonard Reed
Original assignee: Individual
Current assignee: Individual
Priority date: 1987-06-03
Filing date: 1987-06-03
Publication date: 1988-12-14
Anticipated expiration: 2007-06-03
Also published as: GB8712984D0; GB2205596B

Abstract

A method of laying, jointing and controlling shrinkage in concrete ground floor slabs, comprising the steps of providing a sub-base in the form of hard core layer 17 having a large aggregate surface layer 18 which is capable of absorbing moisture and forming a mechanical anchorage for the concrete layer 19. A series of tie bars 21,22, together with the rough surface layer 18 serve to maintain the slabs in close relationship by preventing shrinkage. The absence of a slip membrane beneath the concrete permits free water within the latter to be absorbed downwardly as the concrete dries out resulting in a denser and more durable floor surface with substantially no gaps between adjacent slabs. <IMAGE>

Description

A METHOD OF JOINTING CONCRETE FLOOR SLABS THIS INVENTION concerns a method of jointing and controlling shrinkage in concrete ground floor slabs. It has long been the practice to ensure that such slabs are free to shrink upon drying out, by laying the concrete on a polyethylene or other form of slip membrane. The membrane also serves, in some cases, as a damp proof course.

The effect of the slip membrane has been to permit individual concrete slabs to shrink as they dry out in order to minimise stress cracking. This shrinkage process can take place over a period of 12 months or more and can result in substantial gaps remaining between slabs. It has therefore been found necessary for contractors to return to a site a considerable time after construction has been completed, in order to repair the gaps resulting from shrinkage.

Another phenomenon experienced when concrete is laid onto a waterproof slip membrane, is that the slabs have a tendency to curl slightly, resulting in an uneven surface which must be repaired.

It is conceived that by properly controlling the tensile strength of a concrete mix it is possible to obtain adequate strength properties in a thinner concrete layer.

The thinner the layer the less free water there is within the mix and so shrinkage and cracking due to shrinkage stresses are minimised to an acceptable level. By preventing shrinkage stress cracking it is possible to omit the waterproof slip membrane and so avoid the attendant problems.

It is an object of the present invention to provide a method of constructing a concrete floor in which the need for a slip membrane is avoided.

According to the present invention there is provided a method of laying and jointing concrete floor slabs comprising the steps of providing a sub-base having a surface capable of absorbing moisture and forming a mechanical anchorage for concrete to be laid thereon to impair shrinkage thereof, laying concrete thereon with resultant seepage into interstices in the sub-base surface, and providing tie means at or adjacent to the joint between a pair of such slabs thus to maintain the slabs in close relationship by further preventing shrinkage.

An embodiment of the invention will be described, by way of example only, with reference to the accompanying drawings in which: Fig. 1 illustrates a joint between two concrete floor slabs constructed by a conventional method; and Fig. 2 is a similar view of a joint between two slabs constructed in accordance with the invention.

Referring to Fig. 1, in a conventional process for laying a concrete floor there is first provided a hard core layer 10 which is compacted and rolled to present a smooth surface which may be enhanced by the addition of a thin layer (not shown) of sand or like material. A membrane 11 of polyethylene is laid to extend across the entire surface of the hard core layer 10 and this is followed by a layer 12 of concrete. When constructing individual slabs shuttering at the joint 13 is used to retain the first of a pair of slabs until set, and it is common to include a row of spaced steel dowel bars 14 which are anchored into the first slab to extend through the shuttering to be incorporated into the next abutting slab.

To allow for extra movement in some cases the extensions of dowel bars 14 are greased and to the ends are applied a cardboard cap 15. Steel reinforcement mesh is shown at 16.

This form of construction is intended to permit shrinkage of the two slabs, whilst the dowel bar 14 serves to transfer load across the joint.

Referring now to Fig. 2, in accordance with the invention, a method of laying a concrete floor consists of initially providing a hard core layer 17 having a rough surface 18 which may be provided by a surface layer of large aggregate, for example, stone chippings of between 20mm and lOOmm. The layer 18 may be considered as a slip resistant absorbtion layer as will be described.

Concrete whose properties are carefully controlled to maximise its tensile strength is laid as a layer 19, with or without a shrinkage and crack control mesh 20 of steel or plastics.

The avoidance of a waterproof slip membrane permits much of the free water in the concrete to be absorbed by the layers 17 and 18, whilst there is seepage of the concrete into the interstices in the layer 18 to form and anchorage for the slab 19.

At the joint between two slabs there is provided a specially designed tie member 21 which consists of a row of spaced U-shaped dowel bars 22 of mild steel rod having a diameter of up to 30mm. Alternatively, the upstanding legs of each bar 22 may be inclined inwardly or outwardly, or arcuate. The bars 22 are welded to a similarly shaped strip of reinforcement mesh 23, and are spaced apart by a distance of up to lO0Omm. The width of strip 21 is in the region of 400mm and for manufacturing convenience each strip is approximately 2 metres in length.

By packing beneath the strip 21 and appropriate shuttering above the latter it can be included within the concrete slab layer 19 at each joint, and is capable of transferring an applied load across the joint as in the case of dowel bars 14 in the conventional system illustrated in Fig. 1 whilst its shape ensures that it ties the slabs together in close relationship by further preventing shrinkage as the concrete dries out.

The combination of the roughened and absorbent surface of the sub-floor materials, and the tie members 21 ensure that the concrete is prevented from shrinking in an uncontrolled manner, any shrinkage being minimised and evenly distributed across the extent of a slab. Thus the requirement for a slip membrane is avoided and the joints between the slabs remain tight requiring no further attention as the concrete dries out.

A further advantage of a floor laid in accordance with the present invention when compared with conventional methods incorporating waterproof membranes is that excessive water bleeding to the surface when it is prevented from bleeding downwardly by a waterproof barrier can result in a soft and dusty floor surface. In contrast, by providing an absorbent sub-base with no waterproof membrane, much of the free water may bleed downwardly resulting in a denser and more durable floor surface and a concrete of higher overall strength.

Claims

1. A method of laying and jointing concrete floor slabs, comprising the steps of providing a sub-base having a surface capable of absorbing moisture and forming a mechanical anchorage for concrete to be laid thereon to impair shrinkage thereof, laying concrete thereon with resultant seepage into interstices in the sub-base surface, and providing tie means at or adjacent to the joint between a pair of such slabs thus to maintain the slabs in close relationship by further preventing shrinkage.

2. A method according to Claim 1, wherein said subbase is provided in the form of a hard core layer having a rough surface.

3. A method according to Claim 1 or Claim 2, wherein said sub-base has a surface of large aggregate whose particle size is beteween 20mm and lO0mm.

4. A method according to Claim or Claim 2, wherein said sub-base consists of a first hard core layer and a surface layer of large aggregate of between 20mm and lOOmm.

5. A method according to any preceding claim, wherein the properties of said concrete are carefully controlled to maximise its tensile strength.

6. A method according to any preceding claim, in which said concrete has embedded therein a shrinkage and crack control members or material.

7. A method according to any preceding claim, wherein said tie means comprises a row of spaced substantially Ushaped dowel bars extending across the joint and welded to a channel sectioned strip of reinforcement mesh extending along the joint.

8. A method according to Claim 7, wherein said dowel bars are spaced apart along said strip by a distance of 1000mm or less.

9. A method according to Claim 7, wherein said dowel bars are of mild steel rod having a diameter of 30mm or less.

10. A method according to Claim 7, wherein the upstanding legs of said U-shaped dowel bars are inclined inwardly or outwardly, or are arcuate.

11. A method according to any preceding claim, wherein concrete is packed beneath said tie means and shuttered above the latter.

12. A method according to any preceding claim, wherein the sub-base contains no waterproof membrane thus to permit free water within the concrete to bleed downwardly.

13. A method of laying and jointing concrete floor slabs, substantially as hereinbefore described.