Background Art When casting concrete in a form, the form is made up of form boards which usually have standard dimensions but which can also be sawn out to other dimensions as desired. In the joint between neighbouring form boards, studs are usually arranged to hold the form boards to- gether by means of nail joints or the like. When the form is completed, flowing concrete can be poured into the form. A great problem in such casting of concrete in a form is the air which in the form of bubbles can remain in the solidifying concrete and make it porous, fre- quently with bubbles in the surface. According to prior art technique, this problem is obviated by means of an air-permeable protective fabric of the type defined above, which is cut out to the desired dimensions and which in most cases is fixed to the inside of the com- pleted form. The protective fabric also protects the form from any spilled concrete which is difficult to remove from the form board but which does not adhere to the protective fabric.

By the prior art protective fabrics often being fixed by tacks or nails or by means of a stapler to the inside of the form, several drawbacks arise. On the one hand it is difficult to stretch the fabric to a suffi- cient degree and, on the other hand, ugly marks are made

in the concrete surface by the nail or tack by means of which the fabric was attached to the form. Moreover, a plurality of holes are made in the protective fabric by the tack or nail, which makes it impossible to reuse the fabric since there is a risk of concrete leaking through the damaged portions of the fabric. Concrete may also leak close to the edge of the form board, where the fabric ends.

A known solution to prevent concrete from leaking is described in patent specification EP 0,429,752 A1.

According to this specification, an air-permeable pro- tective fabric is folded round the edges of the form boards to be clamped to the back of the form boards by means of stretching devices. Further, a grid is prefer- ably placed between the protective fabric and the form boards, which imparts to the cured concrete a patterned, but unusually pore-free surface. It is also stated that experiments without a grid have produced a comparable result.

In practice, it has been found that the solution suggested by the EP patent specification, involving the intermediate grid which causes a patterned surface, is not used and that the solution without this grid is un- suitable since the protective fabric tends to adhere as the concrete cures. Consequently, the stripping must take place quickly and the leaving of the fabric, which is indicated as a possible alternative, and the subsequent watering thereof after stripping only result in the pro- tective fabric adhering to the concrete surface. As a result, the fabric is rapidly destroyed since fibres are torn away from the fabric when removing it from the cured concrete, and owing to the torn-off fibres which remain in the concrete surface, the concrete surface will not achieve its maximum tightness and smoothness.

Description of the Invention Therefore the object of the present invention is to provide a protective fabric of the type mentioned by way

of introduction, which is foldable round and fixable to the back of a form board and is very durable and easy to remove, clean and reuse and which always gives a very smooth and tight concrete surface.

According to the present invention, this object is achieved by said protective fabric being a hygroscopic fibre cloth, which is made of fibres of a plastic mate- rial, such as polyester or polypropylene, which by hot rolling has been given a certain, limited degree of stiffness and pits in the side facing away from the con- crete, the protective fabric having a thickness which is smaller than or equal to 1.5 mm, preferably smaller than or equal to 1.2 mm, and most advantageously a thickness in the range 0.8-1.2 mm, thereby being sufficiently pliable and flexible to be folded round the edges of the form board and be stretched to pliably and closely abut against the front, edges and back of the form board.

The invention is based on the knowledge that to achieve a smooth and tight concrete surface, from which the protective fabric comes off easily, it is most im- portant that the concrete be watered during curing since, in case of deficiency of water, the surface layer becomes porous and brittle during curing. According to the inven- tion, this is achieved by the hygroscopic cloth material and, in particular, the pits formed in this material in the initial phase of the curing storing excess water so as then to recirculate it to the surface layer of the concrete in the final phase of the curing. Thus, the surface layer of the concrete will be extremely tight and hard, the protective fabric comes off very easily when stripping and the protective fabric will be very easy to clean since sufficient water has been supplied to all concrete particles so as to cure in the surface layer and not penetrate into and adhere to the protective fabric.

According to a preferred embodiment of the inven- tion, the pits are truncated, the truncated top of the

cone facing the concrete. The advantage of this type of pits is that they are very easy to manufacture by hot rolling using a roll which has pins projecting from its circumferential surface.

If desired, the protective fabric according to the invention can also be reinforced, for example, by in- serting reinforcing wire. This may be advantageous when large wall surfaces are to be cast or in case of high outdoor temperatures.

Finally, the protective fabric according to the present invention is preferably fixed to the back of the form board, i. e. the side of the form board facing away from the concrete, with the aid of suitable fixing means, such as nails or tacks. In other words, special stretching devices are not necessary for the protective fabric since, owing to the hot rolling, such rigidity has been imparted to the protective fabric that no extreme stretching is required.

Description of the Figures Fig. 1 is a cross-section of a preferred embodiment of a protective fabric for carrying out the method ac- cording to the invention, arranged between a form board and a quantity of flowing concrete.

Fig. 2 illustrates on a larger scale a portion of the protective fabric shown in Fig. 1 in cross-section.

Preferred Embodiment Fig. 1 shows a form board 1 for a form (not shown) for casting of concrete 2. The form board 1 consists of a flat board of, for example, plywood or steel. The form board 1 has a front 3 to be directed towards the con- crete 2 and an opposite side 5, below referred to as the back, to be directed away from the concrete 2. According to the invention, an air-permeable protective fabric 4, which is made of a hygroscopic fibre cloth of a type that will be described in more detail, is arranged to abut against the large face of the front 3 of a form board be- fore this is built into the desired form. The fabric 4

has a larger surface area than the large face of the front 3 of the form board, so that edge portions 4a of the protective fabric 4 get outside the front 3, prefer- ably so that such edge portions 4a form along the entire circumference of the front 3. The edge portions 4a are folded round the edges of the form board 1, as shown in Fig. 1, while the edge portions 4a are stretched (in a conventional manner, not shown in detail) on the back of the form board, so that the fabric 4 is made to abut in a pliable and tight manner, against the front 3, edges and back 5 of the form board 1. The fabric is fixed to the back 5 of the form board 1 in a manner which is conven- tional per se with the aid of schematically illustrated fixing means 6, such as tacks or nails, by using a sta- pler of the like.

After thus having provided each of the required form boards with a protective fabric, the desired form can be built by conventional joining of the form boards.

As shown in Fig. 2, in which the protective fabric 4 is illustrated on a larger scale, the protective fabric 4 has a plurality of pits 7 in the form of truncated cones.

The truncated top 8 of the cone is adapted to be directed towards the solidifying concrete.

The protective fabric 4 functions as follows.

As flowing concrete 2 is poured into the form, the concrete presses the protective fabric 4 more firmly against the form board 1 while at the same time air from the concrete leaks out through the protective fabric.

Thanks to the pressure of the concrete against the pro- tective fabric 4, the leaking air will be pressed further along the side of the protective fabric 4 facing away from the concrete, i. e. between the protective fabric 4 and the front 3 of the form board 1, so that the air thus migrates into the open air after having reached the end of the form. Simultaneously, the hygroscopic protective fabric and especially the pits 7 formed therein, in an initial curing phase, take up excess water which then in

a final curing phase is recirculated to the surface layer of the concrete, which is thus watered.

According to the invention, the protective fabric 4 is made of a hygroscopic fibre cloth, which is made of fibres of a plastic material, such as polyester or poly- propylene. The protective fabric 4 is sufficiently soft, pliable and flexible to be folded round the edges of the form board 1. The pliability thus increases by the pro- tective fabric 4 being very thin, having a thickness of < 1.5 mm, preferably < 1. mm, and most advantageously a thickness in the range 0.8-1.2 mm. To prevent it from being baggy, the fabric has been given a certain, limited degree of stiffness by its two sides being hot rolled in a manner known per se. This stiffness results in the pro- tective fabric 4 not having the usually baggy form of thin cloth, which would otherwise make the fabric rise and bubble when contacting the flowing concrete. In addi- tion, the hot rolling operation contributes to the fi- bres, especially in the side of the protective fabric 4 which is intended to face the concrete 2, always remain- ing in the surface layer of the protective fabric 4 and not being capable of rising. This means that they cannot adhere to the concrete 2 and that the protective fabric 4 is always easy to remove from the cured concrete 2.

During hot rolling, about 1 mm2 large pits 7 are suitably embossed close to each other in the side of the protective fabric 4 which is intended to face away from the concrete 2, said pits 7 having a depth which essen- tially corresponds to the thickness of the fabric and contributing to the fabric, in cooperation with the pos- teriorly situated form board 1, being capable of tempo- rarily storing about 1.5 1 of water per square metre.

Besides, above all for reasons of manufacture, the pits 7 have an essentially truncated conical shape, the trun- cated top 8 facing the concrete 2.

The fabric material can, according to the invention, consist of, for example, non-woven material of propylene,

for example a fabric material which is sold by the company Fiberweb under the trade name I 300 NOW Spund Bond.

The fabric can optionally be reinforced, for example by means of inserted reinforcing wire or the like. The reinforcement can be, for example, fibre glass reinforce- ment, thereby preventing the fabric from stretching be- cause of heat in the warm season.