Background Various reinforcing grids and meshes are known in civil engineering for use in constructions made of earth, debris, concrete, asphalt and other materials. For example, sloping earthworks are often built in association with road and railroad constructions and bridge approaches and the like. It is desirable to be able to construct such earthworks with steep slope gradients having high stability. Mesh or grid structures are placed on the fill as the slope is constructed, layer by layer, in order to stabilize and reinforce the slope. Plastic mesh structures are also used to reinforce soil for other applications and to reinforce concrete and asphalt.

U. S. Patent 4,374,798 Mercer discloses a plastic mesh structure formed by stretching in either one direction or in two perpen- dicular directions a sheet having a rectangular pattern of holes to orient the strands between the holes. The orientation of the polymer molecules that occurs during such stretching increases the tensile strength of the sheet in the direction of the stretching and reduces the tendency of the

polymers to deform under long term sustained loading, i. e. to creep.

Products of this type are available commercially and are commonly referred to as geogrid.

Prior art biaxially stretched geogrid is made by mounting a punched plastic sheet into a gantry and exerting tension in a longitudinal direction in order to stretch the sheet. The sheet is then stretched a second time by means of gripping and pulling tools in a direction transverse to the longitudinal direction. It has been found that it is difficult to achieve good results with the second stretching because once the sheet has been stretched longitudinally, a subsequent transverse stretching can leave unstretched or poorly stretched areas which can lead to product failure under load.

In use, load and stress on a sheet can occur in any direction in the plane of the grid, not only in the one or two directions of strand orientation of a uniaxially or biaxially stretched sheet. It would be desirable to provide a sheet that is pre-stressed in all directions in the plane of the sheet, so the sheet can better withstand load in all directions.

It would further be desirable to effect a uniform stretching throughout the sheet, without leaving unstretched areas. It would also be desirable to effect such pre-stressing in a single manufacturing step, rather than requiring multiple steps.

Summary of Invention

The invention provides a pre-stressed plastic or metal sheet structure that is pre-stressed in all directions along the plane of the sheet, and a process for making such structure.

The process comprises the steps of providing a plastic or metal sheet, pressing a plurality of spaced male mold members into the sheet to form bulges in it and thereby pre-stress it in all directions in the plane of the sheet around the bulges, and then removing the bulges, or a part of the bulges, to form holes in the sheet. The bulges can be cut off at a distance from the plane of the sheet to leave rims around the holes projecting outside the plane of the sheet.

Bulging of the sheet effects pre-stressing in a substantially radial configuration extending outward around each bulge in the plane of the sheet. Bulges are sufficiently closely spaced to each other that all areas of the sheet are pre-stressed.

The pressing step is preferably accomplished by means of a tool comprising a first part having a plurality of male mold elements and a second part having a plurality of mating openings or concavities to receive the male mold elements. The sheet is pressed between the first and second parts of the tool to form the bulges in it.

The first and second parts of the tool may be mating rollers that come together at a nip, so that the sheet can be fed between the rollers and pressed in a continuous process, or they may be plates bearing the mold elements. The male mold elements may be integral to such

rollers or plates or they may be individually moveable, for example by pistons or other mechanical means, or by hydraulic or pneumatic means.

The first and second parts of the tool may each have both male mold elements and openings or concavities.

The bulges formed in the sheet, and accordingly the rims formed from the bulges, may be made on one side of the sheet, or they may be made on both sides, for example in an alternating arrangement.

The structure produced by the foregoing process according to the invention is a pre-stressed plastic or metal sheet structure comprising a sheet having a plurality of spaced holes, the sheet being pre-stressed in all directions about the bulges along the plane of the sheet.

Preferably, the sheet includes rims around each of the holes, formed by cutting off only part of each bulge, leaving a rim. The sheet may have rims projecting all on one side of the sheet or have some rims projecting on one side and some on the opposite side. The rims serve the function of helping to anchor the sheet in loose soil particles and other aggregates when the sheet is used for reinforcement. Particles from successive layers on both sides of the sheet are held within the rims, causing greater bonding between the successive layers of aggregate. Preferably the bulges on the sheet are cut off parallel to the plane of the sheet so the rims are of uniform height around the holes.

Sheets according to the invention are intended for use in a number of discrete civil engineering applications. One use is for reinforcing back-fill in the construction of retaining wall systems, bridge

abutments and similar structures. Another use is for reinforcing slopes in the construction of earthwork structures. In such uses, the earth or back-fill is built in layers, with the sheet structure being laid between layers, according to methods well known in the prior art for the use of geogrid. Sheets according to the invention can be used in permanent structures due to their superior strength, in contrast to many prior art geogrids, which can be used only in temporary earthwork structures. A further use is for reinforcing concrete, for example in the making of pre- fabricated concrete and concrete pavement. In such cases, the sheet structure is substituted for rebar or other metal members commonly used in the reinforcement of concrete. A further use for reinforcing asphalt, for example in the construction of roads, runways and tennis courts.

Again, the sheet structure is applied in the same manner that known geogrid products are used for such purposes. A further use for the plastic sheet structure is for reinforcing sub-bases in the construction of embankments, foundations and loading platforms. The sheet structure is also useful for repair and maintenance in any of these fields of applica- tion, for example for repairing cracks and damaged areas in existing earthworks, roadways, etc.

The process of the present invention is much simpler to carry out than the prior art process for making biaxially stretched geogrid, as it requires only a single operation of forming bulges in the sheet in order to pre-stress it, and further achieves the pre-stressing in all directions about each of the bulges, in the plane of the sheet. This multi-directional pre-stressing results in a considerable improvement in the strength and durability of the product, as compared to prior art geogrid. In prior art

products, the polymer chains in the plastic material are ordered parallel to the one direction or two directions of stretching, and, as mentioned above, sequential two-directional stretching can cause irregularities and leave poorly stretched areas in the sheet. In use, however, load can be applied to the sheet in any direction, not merely the one or two directions in which it was stretched. In the process according to the invention, the pressing and bulging causes molecular chains in the sheet to be oriented in all directions about each bulge, in the plane of the sheet. The product therefore has a more uniform restraining force resisting elongation under load. The sheet produced in accordance with the present process shows a better and more uniform restraining force than prior art geogrids, and can be used in permanent structures.

Brief Description of Drawings In drawings which show preferred embodiments of the invention, Fig. 1 is a perspective view of the upper tool element; Fig. 2 is a perspective view of the lower tool element; Fig. 3 is a cross-sectional view of the upper and lower tool elements with the plastic sheet therebetween; Fig. 4 and Fig. 5 are cross-sectional views which illustrate steps during the fabrication of the sheet structure;

Fig. 6 is a schematic view of an apparatus for carrying out the process of the invention; Fig. 7 is a perspective view of the sheet after the bulges have been formed therein; Fig. 8 is a perspective view of the sheet after the bulges have been removed therefrom; Fig. 9 (a) and 9 (b) are perspective views of an upper and lower tool element respectively of a second embodiment of the invention; Fig. 10 is a cross-sectional view of the upper and lower tool element, in the second embodiment of the invention, with the sheet therebetween; Fig. 11 is a perspective view of a plastic sheet after the formation of the bulges, according to the second embodiment of the invention; and Fig. 12 is a perspective view of the sheet of Fig. 11 after removal of the bulges.

Description In a preferred embodiment, the sheet structure is plastic and the apparatus used to carry out the process of making the structure is

adopted for working a plastic sheet. Referring first to Figure 1, there is shown an upper tool element 1 having male mold elements 2 thereon spaced apart from one another in rows and columns. The male mold elements 2 preferably have a generally square base with rounded corners, smooth sides and a rounded top. Fig. 2 shows lower tool element 3 which has an array of spaced holes 4 which function as female mold elements. The holes 4 are generally square in shape and are sized to receive the male mold elements 2 of the upper tool element plus the bulges of the plastic sheet formed on the male mold elements 2, as discussed below. Holes 4 in the lower tool element are concentrically aligned with the male mold elements 2 in the upper tool element. The upper and lower tool elements are steel structures of a sufficient strength for the intended purpose. They may be in the form of substantially flat plates as illustrated in Figures 1 and 2, or may be in the form of mating cylindrical rollers, whereby a plastic sheet may be fed through the nip between the two rollers for continuous production.

The male mold elements may be circular in cross-section, rather than generally square, or of other selected shapes. In such case the female mold elements would have a corresponding mating shape.

The upper and lower tool elements are shown in the drawings having only a small number of mold elements for convenience of illustration, but it is contemplated that in use they would have large numbers of elements to permit processing of large sheets, or a continuous roll, of plastic. Sheets of one to two meters wide are preferred. For use in back-fill reinforcement and in concrete and asphalt reinforcement, one

meter width is preferred; for sub-base reinforcement, two meter width is preferred. It is apparent that sheets of any practical and desirable width and length can be made by the process.

In a preferred embodiment, the male mold elements have a diameter of about 20-70 mm and a height of up to about 20 mm; the thickness of the upper tool element (between the mold elements) is about 10-20 mm and the thickness of the lower tool element is about 6-8 The preferred starting material for the process according to the invention is a plastic sheet 5. A wide range of possible thicknesses of sheet 5 may be used, with the preferred range of thickness being about 1-3 mm. The plastic may be chosen from a range of materials, including polystyrene and polyvinyl chloride and semi-crystalline plastic such as polypropylene, polyester, polyethylene and polyamide.

Copolymers and plastics with reinforcing fibres mixed therein may also be used. Preferably, polyethylene, polypropylene, or high density polyethylene, is used.

The process preferably includes the step of heating the plastic sheet prior to molding. Polypropylene is preferably heated to about 90- 130° C and high density polyethylene to about 60-110° C.

Referring to Fig. 3, there is shown in cross-section sheet 5 positioned between the upper tool element 1 and lower tool element 3, ready for the pressing operation. The sheet 5 is supported by the lower

tool element 3. The upper tool element 1 is pressed downward by suitable hydraulic, pneumatic or mechanical means known in the art, so that male mold elements 2 are pushed through holes 4 in the lower tool element 3, until the lower surface 10 of the upper tool element between the male mold elements, contacts the plastic sheet 5 on the upper surface of the lower tool element 3, forming bulges in the plastic sheet 5 over the male mold elements 2, and extending through the holes 4. Fig. 4 illustrates the position of the tool elements after the upper tool element has been pressed downward, forming bulges 6 through holes 4, and is slightly raised therefrom. Fig. 5 illustrates the position of the tool elements a little later, when the upper tool element is further raised, fully removing male mold elements 2 from the bulges 6.

Figure 7 illustrates the plastic sheet 5 after the formation of bulges 6. Once this intermediate product has been formed, the bulges 6 are cut off, leaving holes 7 in the plastic sheet. The cutting operation is illustrated in Fig. 5, in which knife 11 is applied laterally along the lower surface 12 of the lower tool element 3, to slice off the bulges 6 from the sheet 5. The bulges are cut at a selected distance normal to the plane of the sheet 5, leaving a rim 14 around each hole, projecting beyond the surface of the sheet 5. The height of the rims 14 is determined by, and is equal to, the thickness of the lower tool element 3, as the plane of the sheet lies along the top surface of lower tool element 5 and the bulges are cut along its lower surface. Lower tool elements of different thickness can be provided in order to produce rims of different heights. The resulting sheet, i. e. the final product of the process, shown in Fig. 8, is then removed from the lower tool element. The rims 14 may be any of

various heights, depending upon the application to which the product is to be put. Preferred heights for most applications are about 6-8 mm A second embodiment of the invention is shown in Figs. 9- 12. Here, the product has rims protruding on both sides of the sheet.

The upper tool element 24, shown in Fig. 9 (a), and lower tool element 16, shown in Fig. 9 (b), each have male mold elements 20 and female mold elements 22 in alternating positions in rows and columns on the face of the tool elements. The mold elements 20,22 are sized and positioned so that male mold elements 20 of one tool element are received within female mold elements 22 of the other tool element. To carry out the pre-stressing process, the upper tool element 24 and lower tool element 16 are aligned as shown in Figure 10 so that the male mold elements 20 of each tool element are centered with corresponding female mold elements 22 of the other tool element. The plastic sheet 15 is positioned therebetween and the upper tool element 24 is pressed downward so that male mold elements 20 of each tool element press the plastic sheet into female mold elements 22 of the other tool element and form bulges therein.

The bulged sheet produced by this pressing process is shown in Figure 11, and has bulges 32 which project above and below the plane of the sheet 15, in an alternating array. The bulges are then cut off the sheet by two cutting tools, one positioned above the plane of the sheet and one below, leaving alternating upwardly and downwardly projecting circumferential rims 33,35 respectively about the holes 37 formed by the cutting process. This product is shown in Fig. 12. Alternatively, the

bulges can be removed completely to the surface of the plastic sheet 15, leaving rimless holes.

It will be apparent that the upper and lower tool elements 24, 16 can take the form of either cylindrical rollers or plates.

Figure 6 is a schematic illustration of an apparatus for carrying out the pre-stressing process of the invention. Plastic sheet 5 is unwound from roll 9 and fed into press 42, which contains the pressing tool elements in plate or roller form. The sheet then passes into cutting section 44 in which the bulges are cut off the sheet. The pre-stressed sheet 5, with holes 7, exits from section 44 of the apparatus and can be cut into sections or collected on a windup roll. The apparatus may optionally include heating elements in or upstream from press 42 in order to heat the plastic sheet.

As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.