FR2886319A1 - Closing plate for e.g. excavation on roadway, has external shells, and wire core made of porous material having specified density, where material is chosen among balsa wood, polyisocynate foam and polyvinyl chloride - Google Patents

Abstract

The plate has two external shells (2, 3), and a wire core (8) made of a porous material having a specified density, where the material is chosen among baltekSB(RTM: balsa wood), tarecpir(RTM: polyisocynate foam) and Klegecell(RTM: polyvinyl chloride). Each shell is constituted by stacking two layers (7), where each layer is constituted by stacking a layer of a glass yarn layer and a tissue layer of a glass fiber. The shells and core are integrated between them by an impregnation with a norsodyne(RTM: polyester resin). An independent claim is also included for a method for fabricating a plate.

Description

The present invention relates to a closure plate for a cavity

provided in a roadway, having a composite sandwich-type laminate structure.

Currently, the protective plates are made of steel. They allow temporary cover of excavations, present on site, to allow the passage of vehicles and pedestrians.

They are heavy and must therefore be handled using machinery. They can therefore easily cause accidents.

Light protection plates exist and allow easy handling, but they do not allow the passage of vehicles, in particular trucks, because they are not strong enough (FR 2 420 605).

Composite structures of the sandwich type are known, but they have rarely been used in the road sector. Those which are described for this type of application have a much too complex structure and their manufacturing process is therefore complicated and expensive, for example due to the presence of reinforcing partitions in the core (EP 0 147 050).

Surprisingly, the inventors of the present invention have found that it is possible to easily manufacture a protective plate of composite materials which is light and capable of retaining the strength of the steel plates to withstand the passage of vehicles temporarily during construction sites. and in particular supporting an axle of 13 T. The present invention therefore relates to a closure plate for a cavity formed in a roadway, having a layered structure and comprising two outer skins and a central core, characterized in that the core is made of porous materials having a density less than 160 kg / m3, advantageously less than 155 kg / m3 and a compressive strength, applied perpendicular to the longitudinal plane, greater than 0.4 MPa, advantageously between 0.4 and 14 MPa , advantageously chosen from balsa, polyurethane foam and polyvinyl chloride foam, in that each skin consists of a stack of 'at least two bilayers, each bilayer being constituted by a stack of a layer of glass mat and of a woven layer of glass fibers with substantially perpendicular intersection and in that the two skins and the core are joined together by impregnation with a polyester resin.

The skins and the core therefore extend in a substantially longitudinal direction. Advantageously, the plate according to the present invention is symmetrical with respect to a vertical median plane.

Advantageously, in at least one of its cross sections of the plate, the thickness of at least one of the skins of the plate according to the present invention decreases from the center towards the periphery of the plate.

Advantageously, at least two of the bilayers, advantageously three or four, of the skin of decreasing thickness, have a different dimension, that is to say even more advantageously a length and a width different from the other bilayers. Thus, in a particular embodiment of the invention, the skin contains at least 2, advantageously 3 or 4 series of bilayers of different dimensions, advantageously of different length and width.

Advantageously, the two skins are integral with one another over at least part of the periphery of the plate, advantageously over the entire periphery of the plate. Even more advantageously, this joining is carried out by means of a layer of glass mast, advantageously of 450 g / m2.

In a particular embodiment, the outer surface of one of the skins is substantially planar.

In another particular embodiment, the present invention has a stepped pyramidal shape, advantageously with a channel or rectangular base. Advantageously, the plate according to the present invention has at least two degrees, even more advantageously four.

In a third embodiment, the present invention has a double pyramidal shape with a caned or rectangular base, each pyramid being connected by the base and extending on either side of this base. Advantageously, the plate according to the present invention has at least two degrees in each pyramid, even more advantageously four.

Advantageously, the periphery of the plate intended to serve as points of support on the roadway, the part of the plate comprising the core being intended to cover the cavity. Advantageously, in the case of plates with a square base, intended to cover substantially channeled cavities, called excavations, the peripheral part of the plate located on the four sides is intended to serve as support points on the roadway.

In the case of plates with a rectangular base intended to cover substantially rectangular cavities, called trenches, the peripheral part of the plate located on only two of the sides, advantageously the lengths, is intended to serve as points of support on the roadway.

In an advantageous embodiment, the plate comprises means for anchoring in the roadway, advantageously located in the periphery of the plate, advantageously in the part where the two skins are integral with one another.

Advantageously, these anchoring means are screwing means. In particular, the plate has cutouts, advantageously circular and of conical shape, 8 in number, allowing the passage of fixing screws.

The plate is therefore composed of two materials joined together by means of a resin: a core of porous material and two skins of fibers and glass mat.

The materials used to manufacture the plate are therefore the following: the core is made of porous materials having a density of less than 160 kg / m3, advantageously less than 155 kg / m3, and a compressive strength, applied perpendicular to the longitudinal plane, greater than 0.4 MPa, advantageously between 0.4 and 14 MPa.

Advantageously, the core is chosen from polyurethane foam, polyvinyl chloride foam and balsa, advantageously it is balsa. Advantageously, the density of the core is between 70 and 155 kg / m 3. Advantageously, the polyurethane foam has a density of between 70 and 85 kg / m3, in particular between 77 and 83 kg / m3, advantageously of around 80 kg / m3. Advantageously, the polyurethane foam has not been produced using HCFCs.

Advantageously, the polyurethane foam is a polyisocyanurate polyurethane foam, advantageously it is TARECPIR 80 from the company 20 Tarec Insulation, and has the characteristics indicated in the following table 1: Characteristics Standard Unit Value Density ASTM D 1622 kg / m3 80 3 Resistance to ASTM D1621 or DIN kPa 750 100 compression 53421 kPa 600 100 // Tensile strength ASTM D1623 or DIN kPa 850 150 // 53430 kPa 700 150 Cell content ASTM D 2856 Proc A%> = 90 closed Thermal conductivity ASTM C581 or ISO 8301 mW / mK 29 2 Friability ASTM C 421% 15 10 Water absorption ISO 2896 or ASTM D 2842 max% + 3 Vapor permeability ASTM E96 or ISO R 1663 g / m2 24H 5 3 d water (23 C 85% RH) Coeff. expansion 10E-06mm / mmK 40 60 linear Temperature C (max) + 120 C (min) - 200 Dimensional stability ASTM D 2126 or ISO 2796 max% 1 length and width (24 H 93 C) mm 0.5 thickness ( 24 H -30 C) (48 H 70 C 95% RH) Advantageously, the density of the balsa is between 140 and 160 kg / m3, advantageously between 150 and 155 kg / m3, even more advantageously around 151 kg / m3.

In an advantageous embodiment, the balsa used is standing timber. Advantageously, this is balsa Baltek SB 100 from Baltek, the characteristics of which are given in table 2 below: Typical properties of Standard Unit SB.100 Baltek SB Nominal density ASTM C 271 kg / m3 151 apparent Resistance to ASTM C 365 MPa 12, 67 compression psi 1837 perpendicular to plane Compressive modulus ASTM C 365 MPa 3021 perpendicular to plane Psi 566661 Tensile strength ASTM C 297 MPa 13.00 perpendicular to plane Psi 1886 Tensile modulus ASTM C 297 MPa 3518 perpendicular at Psi plane 510 178 Resistance to ASTM C 273 MPa 2.94 shear Psi 427 Shear modulus ASTM C 273 MPa 157 psi 22829 Thermal conductivity ASTM C 177 W / mK 0.066 at room temperature Advantageously, polyvinyl chloride foam has a lower density at 155 kg / m3, advantageously between 120 and 140 kg / m3, even more advantageously around 130 kg / m3.

In an advantageous embodiment, the polyvinyl chloride foam is a Klegecell resin from the company DIAB, advantageously Klegecell of quality R, advantageously Klegecell R130, the characteristics of which are given in Table 3 below: KLEGECELL R130 ( average properties) Characteristics Standard Unit Value Density iso 845 Kg / m3 130 Resistance to iso 844 MPa 2.75 compression 1 Compression modulus 1 ASTMD 1621 B73 MPa 250 Tensile strength // iso 1926 MPa 3.7 Tensile modulus / / iso 1926 MPa 128 Shear strength iso 1922 MPa 1.85 Shear modulus iso 1922 MPa 53.5 Shear elongation iso 1922% 18 Thermal conductivity at ASTM C518 mW / (mk) 32 10 C Water absorption at ASTM C272 kg / m2 0.03 1 week at 40 C Steam permeability iso 1663 g / m2 x 2411 2.0 water at 23 C Coefficient of expansion ASTM D696 1 / C 3.10 "5 linear between -30 C and + 20 C Temperature DIN 53 424 C 95 Operating temperature C 70 max - 200 min The skins consist of at least two bilayers, each bilayer consisting of a stack of a layer of glass mast and of a woven layer of glass fibers with substantially perpendicular intersections.

Advantageously, the mat layer and the woven layer of the bilayer are fixed together by a mechanical connection, advantageously by stitching.

Advantageously, the characteristics of the woven layer of glass fibers and of the mast layer are as follows: É Characteristics of the woven layer of glass fiber (long uni-directional fibers): Young's modulus in X: 26 Gpa - Young's modulus in Y: 4 Gpa - Poisson's ratio: 0.3 - Stress at break: 250 MPa É Characteristics of the mat layer of glass (mixed short fibers): - Young's modulus in X and Y: 8 Gpa - Poisson's ratio: 0.3 - Stress at break: 100 MPa Advantageously, the bilayer according to the present invention is in Rovimat sold by the company Chomarat, advantageously in Rovimat 800 T2 / 300, the characteristics and components of which are combined. in Table 4 below: The resin used to impregnate the assembly and to join the skins and the core together is a polyester resin, advantageously an isophthalic unsaturated polyester resin. Advantageously, this resin is pre-accelerated.

Advantageously, this is the Norsodyne resin from Cray Valley, and even more advantageously, Norsodyne S 70361 TAI, advantageously having the characteristics shown in the following table 5: Weight Indicative thickness Mechanical resistance Indicative chain Weft 1098 g / m2 6.5% 1.40 mm 1770 N / cm 1770 N / cm

SUPPORT COMPONENTS

Material GLASS E Weight 790 g / m2 5% Weave NATTÉ MAT Material GLASS E Fiber length 50 mm Weight 300 g / m2 10%

BINDING

Materials SYNTHETIC Weight 8 g / m2 10% Density at 20 C 1, 10 g / cm3 Brookfield Viscosity RVT at 25 C 6 dPa.s M3V50 Dry extract 52% Reactivity Method R 13 bis Test temperature 25 C Catalytic system 1, 5% PMEC 50 Gel time 28 min Peak time 40 min Peak temperature 195 C The properties (average values) of unreinforced hardened Norsodyne S 70361 TA1 resin are shown in the following table 6: Mechanical properties Traction according to ISO standard 527 Breaking stress 60 Mpa Elongation at break 1.6% Bending according to standard ISO 178 Breaking stress 100 Mpa Modulus of elasticity 3900 Mpa Thermomechanical properties HDT according to standard ISO 75-2 A 100 C In an advantageous embodiment , the two skins of the plate according to the present invention have the same thickness.

Even more advantageously, each skin of the plate according to the present invention contains the same number of bilayers, advantageously the same number of bilayers of the same size, that is to say advantageously of the same length and width.

Advantageously, each bilayer has the same thickness.

Advantageously, the glass fiber / resin weight ratio is 50/50 in each skin.

Advantageously, each skin contains between 6 and 12 bilayers, advantageously between 8 and 9 bilayers.

Advantageously, each bilayer has a thickness of between 1, 2 and 1, 5 mm. Advantageously, each bilayer has a thickness of approximately 1.3 mm including resin.

In a particular embodiment, advantageously for plates with a square base intended to cover square cavities, the ratio between the thickness of the core and the sum of those of the two skins is between 0.15 and 1.5, advantageously between 0.35 and 0.8, advantageously between 0.5 and 0.65.

In a particular embodiment for plates with a square base intended to cover square cavities, the core has a thickness of between 10 and 14 mm, advantageously between 10 and 13 mm. Advantageously, the core has a thickness of between 12 and 13 mm.

In another particular embodiment, advantageously for plates with a rectangular base intended to cover trenches, the core has a thickness of between 25 and 50 mm. Advantageously, in this case, the ratio between the thickness of the core and the sum of that of the two skins is between 0.5 and 4, advantageously between 0.7 and 3.5, again so more advantageous between 0.8 and 3, advantageously between 1 and 2.5.

In a particular embodiment, the plate according to the present invention has a weight less than or equal to 50 kg, advantageously between 30 and 41 kg. Advantageously, the plate has a breaking strength of 35 tons.

The present invention further relates to a method of manufacturing a plate according to the present invention comprising the steps of successively stacking the various bilayers and the core in a mold, of applying the polyester resin and of compacting under vacuum.

Advantageously, during the manufacture of the plate, for each skin the bilayers are stacked from the outside to the inside of the plate, starting with the bilayers of larger dimensions before the bilayers of smaller dimensions.

Advantageously, the method according to the present invention is of the contact molding and vacuum compacting type.

Advantageously, the method comprises the following steps: (a) -Position of the bilayer, (b) -Application of resin by roller, (c) Repeating steps (a) and (b) until all the bilayers of the first skin are put in place, (d) - Placement of the core, (e) - Placement of the bilayer, (f) Application of resin by roller (g) Reiteration of steps (d ) and (e) until all bilayers of the second skin are in place.

There is no waiting time to respect between the different stages.

Then, vacuuming of the voids is carried out to densify the plate. The resin curing time is approximately 1 hour.

The bilayers and the core can also be superimposed without the application of resin, the latter being introduced into the plate at the end by vacuum suction.

The present invention further relates to the use of a plate according to the present invention for closing off a service cavity formed in a roadway. Advantageously, this cavity has a square (excavation) or rectangular (trench) section.

It further relates to the use of a plate according to the present invention to temporarily cover excavations or trenches and allow the passage of pedestrians or vehicles.

The invention will be better understood and other aims, advantages and characteristics thereof will emerge more clearly from the description which follows and which is made with reference to the appended drawings shown non-limiting examples of embodiment of the invention and in which : - Figure 1 shows a schematic top view of a first plate according to the present invention with a square base, - Figure 2 shows a schematic side view taken along arrow II of Figure 1 of the first plate according to invention, - Figure 3 shows a schematic sectional view along the vertical plane III-III of Figure 1 of the first plate according to the invention during its manufacture before resin impregnation and compression, - Figure 4 shows a photo of the plate of Figure 1, - Figure 5 shows a schematic sectional view along a vertical median plane of a second plate according to the invention with a square base, - Figure 6 shows a view similar to FIG. 3 during the manufacture of the second plate according to the invention before resin impregnation and compression, - FIG. 7 represents a top view of a third plate according to the invention with a rectangular base, - FIG. 8 represents a schematic side view taken along arrow VIII of figure 7 of the third plate according to the invention, - figure 9 shows a schematic side view taken along arrow IX of figure 7 of the third plate according to the invention .

Figures 1, 2, 3 and 4 show a first plate according to the invention. This plate 1 has a pyramidal shape with a square base. It has four degrees. The degrees are present on the upper skin 2 while the lower skin 3 is flat. The plate 1 is intended to be placed horizontally on trenches or excavations, the flat surface 3 being intended to be in contact with the ground. Plate 1 is symmetrical with respect to its vertical median planes. Its vertical mid-plane AA (figure 2) defines two parts which are symmetrical with respect to each other 4 and 5 and each comprise four degrees on the upper skin 2. Each degree has a square section and is symmetrical with respect to its vertical median planes. The first degree constitutes the periphery 6 of the plate 1 in which the two skins 2, 3 are integral. It has a thickness greater than that of each of the other three degrees. This first degree 6 is intended to serve as a support on the roadway. Each skin 2, 3 contains six bilayers 7, the length and width of which correspond to the length and width of the final plate 1. The core 8 only separates the skins 2, 3 at the level of the center of the plate 1, that is to say at the level of the three upper degrees and is therefore not present in the periphery 6 of the plate, that is to say the first degree. Each of these degrees is due to a bilayer 7 of a different length and width 9, 10, 11. The connection between the two skins 2, 3 is achieved by means of a layer of glass mast 12 During its manufacture (FIG. 3), the bilayers 5 are stacked in such a way that: in the bottom of the mold are first placed the bilayers of higher dimensions 13 then the bilayers of lower dimensions 11, 10, 9, that is to say of smaller and smaller dimensions. This makes it possible to manufacture the first skin 3.

Then, the core 8 and the glass mast 12 are stacked. Finally, the second skin is produced by stacking the bilayers 7, from the bilayer of smaller dimensions to the bilayers of larger dimensions 9, 10, 11 and 13.

After compression and making the holes for anchoring and handling, the plate according to Figure 4 is obtained.

Figures 5 and 6 show a second plate 1 according to the invention. This plate is identical to the first plate except for the following differences: This plate 1 has the shape of a double pyramid with a symmetrical degree linked by the base with a square section and each pyramid extends respectively upwards and downwards. Its horizontal median plane BB defines two parts 14 and 15 of the plate 1 which are symmetrical with respect to one another (FIG. 5).

Each degree with square section is symmetrical with respect to its vertical median planes and to its horizontal median plane. Each skin 2 and 3 has four symmetrical degrees. The stacking of the bilayers 7 in each skin 2, 3 is carried out in the reverse order of that of the first plate according to the invention. At the bottom of the mold, to make the first skin 3, the smaller-sized bilayers 9, 10, 11 are first stacked before the larger-sized bilayers 13 and then after placing the core 8 and the mast. glass 12, in order to produce the second skin 2, the bilayers of larger dimensions 13 are stacked before the bilayers of smaller dimensions 11, 10, 9 (FIG. 6).

Figures 7, 8 and 9 show a third plate (1) according to the invention. This plate is identical to the first plate except for the following differences: It has a rectangular base. It therefore has a pyramidal shape with a rectangular base. The side view along its width (according to arrow VIII of FIG. 7) has four degrees (FIG. 8). The side view along its length (that is to say along the arrow IX of FIG. 7) shows three degrees (FIG. 9), the last degree in the center having a thickness twice as great as the second degree.

Each degree has a rectangular section and is symmetrical about its vertical median planes. The vertical median plane CC of FIG. 9 thus defines two other parts 16 and 17 of the plate 1 which are symmetrical with respect to one another. Only the parts 18 and 19 of the periphery 6 are intended to serve as a support on the roadway.

The following examples are given by way of non-limiting indication: Example 1 Production of a plate according to the present invention with a square base and a stepped pyramidal structure.

A 1.20m by 1.20m plate to cover a 0.80m by 0.80m pit was made (Figures 1 to 3 and 5).

The following materials are used: For the skins: 9 bilayers per skin in ROVIMAT 800T2 / 300 (manufacturer: 10 Auguste CHOMARAT) approximately 1.3 mm thick per layer (resin included).

Each skin comprises 6 bilayers of dimensions (Length x Width) 1.20 mx 1.20 m, one bilayer of dimensions (Length x Width) 800 mm x 800 mm, one bilayer of dimensions (Length x width) 600 mm x 600 mm and a bilayer of dimensions (Length x Width) 400 mm x 400 mm. The stacking of these bilayers from the outside of the plate towards the inside, that is to say towards the core, is as follows: first the 6 layers of 1200 x 1200 mm, then the layer of 800 x 800 mm, then the 600 x 600 mm layer and finally the 400 x 400 mm layer (figure 3).

At the ends, where the core is absent, addition between the two skins of a layer of 450 g / m2 glass mast (M705), the characteristics of which are as follows: The M705 mast consists of base threads cut in 50 mm long glass fibers, assembled in the form of a mat by means of an emulsion binder. Length of base yarn cut: 50 mm, Tex of base yarn: 40, Emulsion binder content: 3.2% by weight (450 g / m2).

For the core: polyurethane foam (TARECPIR 80) with a density of about 80 kg / m3, a thickness of 12 mm and a dimension of 800 mm x 800 mm.

For the resin: unsaturated isophthalic polyester thermosetting resin (Norsodyne S 70361 TAI); The 12 plies of the bilayer (periphery of the plate) contain eight circular cutouts for the passage of the fixing screws, as well as four oblong cutouts for handling (FIG. 4).

The total mass of the plate obtained (Figure 4) is 38 kg.

This plate is made by contact molding with vacuum densification in the mold shown in Figure 4.

The process is therefore the following: The stacking is carried out as indicated in FIG. 3 in a mold: the 6 bilayers of dimensions 1200 mm × 1200 mm are first stacked. The bilayer of dimensions 800 mm × 800 mm, then of dimensions of 600 mm × 600 mm, then of dimensions of 400 mm × 400 mm, is then stacked. After the installation of each bilayer, resin is applied by roller.

Then the core and the glass mast layer are applied to the ends. Finally, for the realization of the upper skin, we apply the bilayer of dimensions 400 x 400 mm, then the bilayer of dimensions 600 x 600 mm, then the bilayer of dimensions 800 x 800 mm, then the 6 bilayers of dimensions 1200 x 1200 mm. After the installation of each bilayer, resin is applied by roller.

The mold is closed and compacted under vacuum by vacuuming up the voids. The resin curing time is approximately 1 hour. A 38 kg plate is thus obtained (FIG. 4), the skin which was in contact with the bottom of the mold is flat, the whole of the plate having a pyramidal structure with steps with a square base (FIGS. 1 and 2). This plate therefore has a thickness at the center of 35.5 mm and a thickness at the periphery of 17.5 mm. The second and third degrees are respectively 30 and 33 mm thick.

Example 2: Production of a plate according to the present invention with a square base and a double step pyramidal structure A plate of 1.20 mx 1.20 m to cover an excavation of 0.80 mx 0.80 m was manufactured (figures 5 and 6). The materials and the process used are the same as those of Example 1 except for the following differences: - each skin comprises 5 bilayers of dimensions 1.20 mx 1.20 m instead of 6. - each bilayer has a thickness of about 1.5 mm.

- the thickness of the core is 10 mm.

- the stacking in the mold is carried out starting with the bilayer of 400 x 400 mm, then the bilayer of 600 x 600 mm, then the bilayer of 800 x 800 mm, then the 5 bilayers of 1200 x 1200 mm, then the 'core and on the periphery, the glass mast layer, then the 5 bilayers of 1200 x 1200 mm, then the bilayer of 800 x 800 mm, then the bilayer of 600 x 600 mm, then the bilayer of 400 x 400 mm (figure 6). Before compression, the thickness of the periphery is 15 mm and the thickness of the center is 35 mm.

After compression, a plate is obtained having a double pyramidal structure with a square base, the thickness of which on the periphery is 15 mm and in the center of 35 mm and being intended to be placed on excavations 800 mm wide and 800 mm length (figure 5).

The plate has a total mass of 31 kg.

Example 3: Production of a plate according to the present invention with a stepped pyramidal structure The method and the materials are identical to those of Example 1. The only difference is that the core is made of balsa (Baltek SB100) of thickness 12.7 mm. A 36 kg plate is obtained having the structure as shown in Figures 1 to 3 and 5.

Example 4 Production of a plate according to the present invention with a rectangular base and a stepped pyramidal structure A plate of dimensions 1200 × 800 mm to cover a trench of width 800 mm was made (FIGS. 7 to 9). The materials are identical to those indicated in Example 3, as is the manufacturing process. The dimensions of the bilayers are as follows: 1200 x 800 mm for the largest, 800 x 100 mm, 600 x 80 mm and 400 x 60 mm. The thickness of the balsa core (Baltek SB100) is 50 mm. We obtain a plate of 45 kg.

Example 5: bending fracture test in static mode on the plate according to Example 3.

The test conditions are as follows: The on-site support conditions were reproduced for the test: The periphery 6 of the plate rests on a frame with internal dimensions 80 cm x 80 cm. This frame is covered with pieces of rubber in order to avoid local puncturing of the plate.

Several progressive loads were applied to the plate, until failure.

A 300mm x 200mm steel plate was used to apply the force. A rubber pad is present between the steel plate and the plate according to the present invention in order to simulate the support of a tire.

At the same time as the force and position sensors (recording at 10 Hz), 5 strain gauges were glued on the plate according to example 3 (recording at 1 Hz) including: 2 gauges facing each other, on each side of the plate according to Example 3 (n 4 and 5) and 3 gauges at 120 in the center of the plate according to Example 3 (n 1,2,3).

A plot of the displacement force curve as a function of the deflection makes it possible to measure the rigidity of the plate (and in particular the losses of rigidity) during the successive loadings.

Note that the arrow mentioned above includes the crushing of the rubber pad between the backing plate and the plate according to Example 3 (about 6 mm at 85 kN).

The following Table 7 collates the results regarding the measured deflection (in mm assuming the cushion collapses by 6 mm constantly during loading) versus the applied load (in kN).

Plate deflection load (kN) (mm) 16 67 87 24 111 42 152 51 209 59 262 311 69 346 We can notice that the plate according to example 3 remains intact during the first 3 loads (up to 11 tonnes) because the load rises can be superimposed and the hysteresis of the load rises are very low.

At 11 tonnes, there was a first minor break inside the plate according to Example 3 (probably the detachment of the upper skin), which causes a consequent loss of rigidity during the 4th load increase.

Then, with each increase in the load, we can see a loss of rigidity of the plate according to example 3, and this until failure at about 35 tons. After rupture of the plate according to Example 3 at 35 T, there is a delamination between the lower skin and the upper skin of the plate according to Example 3 and a crack from the center to the edge of the plate according to example 3.

It should be noted that even after this rupture, the plate according to Example 3 could easily be loaded up to 6.5 tonnes, resulting in an obviously significant deflection (measured manually at 36 mm).

It should be noted that the loading and support conditions of this test present a very unfavorable case for determining the maximum breaking load, this maximum load being in reality undoubtedly greater: - The supports are perfectly rigid by compared to the plate according to example 3 (which is not the case with the roadway) -The loading was done with a steel plate which was also rigid, which has the effect, even with an elastomeric cushion , to distribute the load only on the edges of this steel plate.

The first information given by the gauges concerns the gauge 5 (upper face of the plate according to Example 3, load application side) which works in compression, the plate according to Example 3 therefore has a behavior under bending load.

From the 4th loading, it should be noted that the symmetry is no longer guaranteed between gauge 4 and gauge 5 (after cracking at 111 kN). This means that there was shearing and detachment at the interface of the core and the upper skin of the plate according to Example 3.

It is possible to calculate the stress level at the center of the plate using the three gauges 1, 2 & 3, in particular at 8 tonnes of loading (correlation with the calculation): At 8 tonnes, l = 7750 def; 2 = 3670 [Kief; 3 = 3382 def, which gives us a stress state of 119 MPa in the direction of loading and 63 MPa in the transverse direction (for a measured deflection of 13 mm).

Knowing that the conditions are a little different (bearing surface of 150 x 260 mm), the calculations give us a maximum stress of 90 MPa (27%) for a deflection of 15 mm (+ 15%). In addition, the calculation presents a more favorable case because the load is distributed uniformly (as in the case of a real tire).

Thus, the plate according to Example 3 has a maximum load before breaking of 35 tonnes in static. After breaking the plate according to Example 3, loading up to 6.5 tonnes is still possible.

Claims (19)

1. Plate (1) for closing off a cavity formed in a roadway, having a layered structure and comprising two outer skins (2, 3) and a central core (8), characterized in that the core is in. porous materials having a density of less than 160 kg / m3, advantageously less than 155 kg / m3 and strength. compression, applied perpendicular to the longitudinal plane, greater than 0.4 MPa, advantageously between 0.4 and 14 MPa, advantageously chosen from balsa, polyurethane foam and polyvinyl chloride foam, in that each skin (2, 3) consists of a stack of at least two bilayers (7), each bilayer consisting of a stack of a layer of glass mat and a woven layer of glass fibers with substantially perpendicular intersection and in that the two skins (2, 3) and the core (8) are joined together by impregnation with a polyester resin. 2. Plate (1) according to claim 1 characterized in that it is symmetrical with respect to a vertical median plane (AA). 3. Plate (1) according to claim 1 or 2 characterized in that in at least one of its cross sections, the thickness of at least one of the skins (2) decreases from the center to the periphery (6) of the plate. . 4. Plate (1) according to claim 3 characterized in that at least two of the bilayers (9, 10) preferably four (9, 10, 11, 13), of the skin (2) of decreasing thickness, have a different dimension. 5. Plate (1) according to any one of the preceding claims characterized in that the two skins (2, 3) are integral with one another on at least part of the periphery (4) of the plate ( 1). 6. Plate (1) according to any one of the preceding claims characterized in that the outer surface of one of the skins (3) is substantially planar. 7. Plate (1) according to any one of the preceding claims characterized in that it has a pyramidal shape degree, square or rectangular base. 8. Plate (1) according to any one of the preceding claims characterized in that it further comprises means for anchoring in the roadway, advantageously located in the periphery (6) of the plate (1), advantageously in the part where the two skins (2, 3) are integral with one another. 9. Plate (1) according to any one of the preceding claims, characterized in that the mat layer and the woven layer of the bilayer (7) are fixed together by a mechanical connection, advantageously by stitching. 10. Plate (1) according to any one of the preceding claims characterized in that the two skins (2, 3) have the same thickness. 11. Plate (1) according to any one of the preceding claims, characterized in that each skin (2, 3) contains the same number of bilayers (7), advantageously the same number of bilayers of the same size. 12. Plate (1) according to any one of the preceding claims, characterized in that each bilayer (7) has the same thickness. 13. Plate (1) according to any one of the preceding claims, characterized in that its weight is less than 50 kg. 14. Plate (1) according to any one of the preceding claims, characterized in that its breaking strength is 35 tonnes. 15. A method of manufacturing a plate (1) according to any one of the preceding claims, characterized in that it comprises the steps of successive stacking of the various bilayers (7) and of the core (8) in a mold. , polyester resin application and vacuum compaction. 16. The method of claim 15 characterized in that for each skin (2, 3) the bilayers (7) are stacked from the outside to the inside of the plate 1 starting with the bilayers of larger dimensions (13). before the smaller bilayers (11, 10, 9). 17. A method according to any one of claims 15 or 16 characterized in that it is of the contact molding and vacuum compacting type. 18. Use of a plate (1) according to any one of claims 1 to 14 for sealing a service cavity formed in a roadway. 19. Use of a plate (1) according to any one of claims 1 to 14 to temporarily cover excavations or trenches and allow the passage of pedestrians or vehicles.