WO1989009864A1

WO1989009864A1 - A formwork panel

Info

Publication number: WO1989009864A1
Application number: PCT/GB1989/000383
Authority: WO
Inventors: Thomas Alexander Harrison
Original assignee: British Cement Association
Priority date: 1988-04-14
Filing date: 1989-04-14
Publication date: 1989-10-19
Also published as: GB8808832D0

Abstract

A formwork panel is provided for use in moulding concrete in the building and civil engineering industries. The panel comprises a composite sheet (1) and a porous liner (2) attached to the sheet uniformly over substantially the whole surface of the sheet. The sheet and liner are permeable/porous to allow drainage of water from the concrete. The sheet (1) is preferably of wood-chip and binder and the liner (2) is preferably non-fibrous.

Description

- I -

A FORMWORK PANEL

This invention relates to a formwork panel (sometimes called a form or formwork sheeting) and a method for making such a panel.

In the erection of buildings and civil engineering structures which employ concrete, the concrete is moulded in formwork which is erected on-site and typically comprises hardboard, chipboard, plywood, GRC, GRP or steel panels rigidly mounted on a wooden steel or aluminium framework, enclosing the space in which the concrete is to be placed.

It is a general aim in this field to try to achieve a durable concrete structure having a smooth and strong surface which is free from air bubbles or blow holes. The surface properties of the concrete control its visible appearance and its durability to weathering and chemical attack from the atmosphere.

Recently, efforts have been directed towards improving the surface properties of concrete, particularly its density and surface regularity, by using a lining of a fibrous polymeric material sheet. The lining is used in combination with a geo-textile grid mounted on timber wallings. Such a structure gives improved drainage over conventional plywood or steel panels, but is only generally suitable where the concrete is to be faced with another material, because of high deflections between the wallings, under the pressure of the fresh concrete. Fibrous sheets of this nature are generally not able to be re-used. Furthermore, the on-site erection of a formwork web and a lining is labour-intensive. The Japanese company Kumagai Gumi Company Limited have developed a combination of a form panel and a liner intended to increase the density of the concrete towards the surface. The liner is a textile and the form panel is plywood, steel or fibre-reinforced plastic having tiny holes drilled therethrough, having a diameter of 3 to 6 millimetres. It is claimed that excess water and entrapped air are bled out of the concrete, through the textile liner and through the holes in the form panels. It is, however, very expensive to drill holes in plywood, steel or glass reinforced plastic. Furthermore, a matrix of holes drilled through the form panel can lead to uneven drainage of water, and hence uneven surface properties in the resulting concrete.

It is an aim of the present invention to provide an improved formwork panel which is cheap, re-usable, easy to erect and allows for the reduction of pore water pressure in the surface zones of fresh concrete and provides good surface regularity of the cast concrete with other desired surface properties.

According to the present invention, there is provided a formwork panel for concrete, comprising: a composite sheet of structure-providing particles bound by a binder, the particles being sufficiently loosely packed as to provide permeable channels through the sheet; and a water permeable liner having pores which are sufficiently large as to allow the passage of water therethrough, but not so large as to admit normal particles of cement; the channels and pores of the sheet and liner being evenly distributed over substantially the whole area of the sheet and liner respectively and the liner being attached to the backing sheet uniformly over substantially the whole surface thereof. References to structure-providing particles are to be understood to extend to fibres.

The liner is preferably non-fibrous, and more preferably made of polymeric material. By use of such a liner, greater re-usability of the panel can be achieved, over and above known textile and fibrous liners.

The average pore size of the liner is preferably less than about 20 micrometres and more preferably less than about 10 micrometres.

It is found that a liner having an average pore size up to at least the more preferred maximum can substantially exclude BS12, BS146, BS6588 and EN197 cements. These cements have particle sizes down to about 0.1 micrometres, with typically 5% of particles being less than 2 micrometres in diameter. When wetted, the particles flock together and the larger flocculations so formed are unable to enter the pores of the liner. 0.1 micrometres is a preferred lower limit on the average pore size. Ultimately the minimum pore size is the size of a water molecule.

The composite sheet may be made of wood-chips. These wood-chips preferably have a length of between about 5 mm and about 30mm and more preferably between about 10mm and about 20 mm. The preferred average length-to-width ratio of the chips is from about 15 to about 50.

According to the present invention, there is also provided a method of manufacture of a formwork panel for concrete, comprising the steps of coating structure- providing particles with a binder, forming the particles into a sheet, in which the particles are sufficently loosely packed as to provide permeable channels through the sheet, the channels being evenly disributed over substantially the whole area of the sheet, and adhering a porous liner to the sheet by means of the binder. the binder having pores sufficiently large as to allow the passage of water therethrough, but not so large as to admit normal cement when wet, the pores of the sheet being evenly distributed over substantially the whole area thereof.

In this way, the binder which binds the structure-providing particles together also fixes the liner to the sheet. This leads to a very simple method of manufacture, whereby the liner can be rolled onto the sheet, as the sheet is formed and before the binder sets, whereby the binder, when set, fixes the liner to the surface of the sheet.

The binder also has the function of reducing water absorption into the wood chips. Alternatively, the chips may be coated with a material to reduce water absorption prior to the application of binder.

A preferred embodiment of the invention will now be described, by way of example, with reference to the drawing, which shows a cross sectional view of a panel according to the invention.

In the Figure, there is shown a chip-board sheet 1, which is 20 mm thick and comprises wood-chips of from 10 to 20 mm length, having a typical length-to-width ratio of 30. The wood-chips are bound together by a resin or glue binder, but are not so compressed that the voids between the wood-chips are blocked by binder, but rather the chips are sufficently loosely pressed together for the voids to form continuous channels through the sheet. Attached to the surface of sheet 1, there is a liner 2. The liner 2 is a sheet of polymeric material such as Corovin (trade mar ) , which is porous. The pores of the sheet are typically a few microns in width. Other materials may be used which, in air, allow the passage of water vapour but prevent the passage of liquid water, only allowing the passage of liquid water in the presence of a pressure head. The liner 2 is attached to the sheet 1 uniformly across the interface therebetween by means of the resin which binds together the chips of the sheet 1. The sheet 1 is not so saturated with binder that the binder spreads across the liner 2, to block the pores therein, but rather the chips have a sufficiently thin layer of resin coating them that the liner 2 adheres to the chips at isolated points, thereby avoiding blockage of the pores in the liner. An alternative method of attaching the liner 2 to the sheet 1 is by coating the surface of the sheet 1 with a matrix of lines of dots of adhesive, and then rolling the liner 2 onto the sheet 1. In this latter embodiment, the pores of the liner 2 will be blocked by the adhesive, but only over a small proportion of the total area of the liner.

A method of manufacture of the panel thus described is as follows. The chips of wood which form the sheet 1 are allowed to fall down a tower, into which is sprayed the binder. In this way, the chips are coated with a thin even coating of binder. At the bottom of the tower, the chips are fed onto a roller or conveyor and passed between two pressing rollers. Between these pressing rollers, the chips are pressed together into a sheet. The sheet passes between two further rollers, around one of which passes the liner 2. In this way, that roller feeds the liner 2 onto the surface of the formwork sheet 1. This feeding of the liner onto the sheet is carried out at a time at which the binder of the sheet has not yet set. The liner therefore adheres to the surface of the sheet. The binder is then cured, or allowed to cure, in a manner standard in the art of chip-board manufacture.

The manner of use of the formwork panel is illustrated in the Figure. In the figure, the panel (generally indicated at 3) is acting as a former for a body of concrete 4. The concrete is fresh and therefore exerts a pressure (the pore water pressure plus a component of the effective stress), indicated by arrow A. A vibrator is inserted into the concrete 4 and the concrete is vibrated, as is normal in the art, allowing the concrete to settle and the pore water pressure to increase. This overcomes surface tension forces and the pore water seeps through the liner 2 and through the channels in the sheet 1 along the paths indicated by the arrows B. This seepage of water is able to occur uniformly across the panel 3. In this way the excess pore water is quickly drained away, particle contact is established in the concrete and the formwork pressure rapidly falls.

With the invention, the whole stiffening process of the concrete is speeded-up. Moreover the rapid and uniform drainage of the water from the surface leads to a lower water-cement ratio and an increased surface density of the concrete, with a smooth finish, leading to improved surface properties.

The liner 2 does not allow particles of cement to pass therethrough, nor even to enter the pores therof, and is thereby re-useable with a minimum degree of cleaning. Because the liner 2 is adhered to the sheet 1 uniformly across the whole area of the panel, the panel can be erected and dismantled as a whole and does not thereby require any labour over and above that required for a normal formwork panel.

Corovin as employed for liner 2 in the example above is a polyolefin sheeting available from Corovin GmbH, Peine, Federal Republic of Germany.

Claims

1. A formwork panel for concrete, comprising: a composite sheet of structure-providing particles bound by a binder, the particles being sufficiently loosely packed so as to provide permeable channels through the sheet; and a porous liner having pores which are sufficiently large as to allow the passage of water therethrough, but not so large as to admit normal cement when wet; the channels and pores of the sheet and liner being evenly distributed over substantially the whole area of the sheet and liner respectively and the liner being attached to the sheet uniformly over substantially the whole surface thereof.

2. A panel according to claim 1, wherein the liner is of non-fibrous material.

3. A panel according to claim 2, wherein the liner is of polymeric material.

4. A panel according to any one of claims 1 to 3, wherein the liner has pores having an approximate average pore size of less than about 20 micrometres.

5. A panel according to any one of claims 1 to 4 wherein the composite sheet comprises wood-chips.

6. A method of manufacture of a formwork panel for concrete, comprising the steps of coating structure-providing particles with a binder, forming the particles into a sheet, in which the particles are sufficiently loosely packed as to provide permeable channels through the sheet, the channels being evenly distributed over substantially the whole area of the sheet, and adhering a porous liner to the sheet by means of the binder, the liner having pores sufficiently large as to allow the passage of water therethrough, but not so large as to admit normal particles of cement, the pores of the sheet being evenly distributed over substantially the whole area thereof.