Architectural assembly forming an electromagnetic radiation shielding

Abstract

The invention relates to an architectural assembly consisting of wall elements made of concrete containing conductive particles with a conductive mesh forming an electromagnetic radiation shielding. Said wall elements consist of a panel 1 made of concrete containing conductive particles without a conductive mesh, at least one face of which is provided with a skin 2, 3 comprising a conductive mesh, with meshes being less than 3030 mm in dimensions.

Claims

1. An architectural assembly comprising: wall elements having a panel made of concrete containing conductive particles, the panel having at least one face on a surface coated with at least one skin comprising a conductive mesh forming a broadband electromagnetic radiation shielding, with each unit of the conductive mesh being less than 3030 mm in dimensions.

2. The architectural assembly according to claim 1, wherein said conductive mesh is fastened to at least one of the surfaces of said panel made of concrete containing conductive particles.

3. The architectural assembly according to claim 1, wherein said concrete contains from 0.5 to 2% by weight of conductive particles.

4. The architectural assembly according to claim 1, wherein the concrete has a resistivity of less than 300 Ohmmeter.

5. The architectural assembly according to claim 1, wherein the concrete contains conductive particles having a length from 5 to 15 mm and a section from 0.1 to 0.5 mm.

6. The architectural assembly according to claim 1, wherein the panel comprises openings that are surrounded with a metal conductive sheath.

7. The architectural assembly according to claim 6, wherein said metal conductive sheath is made integral with the wall using screws engaged in tapped holes to provide a mechanical and electrical connection with the conductive mesh and the concrete containing conductive particles.

8. The architectural assembly according to claim 1, wherein said wall elements are connected together by means of expansion joints made of a conductive material, made integral with the peripheral areas of the two elements connected by concrete screws or nails to provide a mechanical and electrical connection with the mesh and concrete containing conductive particles.

9. A method for building an architectural assembly comprising: providing wall elements made of concrete containing conductive particles; forming a broadband electromagnetic radiation shielding with a panel coated with at least one skin comprising a conductive mesh, with each unit of the mesh being less than 3030 mm in dimensions; and placing the conductive mesh on at least one face of the wall element made of concrete containing conductive particles in the absence of the conductive mesh being within the concrete containing conductive particles.

10. The method for building an architectural assembly according to claim 9, wherein said skin consists of the conductive mesh embedded in a finishing material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will be best understood when reading the following detailed description of a non-restrictive exemplary embodiment, while referring to the appended drawings, wherein:

(2) FIG. 1 shows a sectional view of a panel element according to the invention,

(3) FIG. 2 shows the attenuation curves computed by absorption of a 0.50 m thick concrete. (A) curve: plain concrete, (B) curve: concrete added with steel fibres

(4) FIG. 3 shows the attenuation curves of a single layer of a steel grid, welded at (A) 50 mm grid, (B) 5 mm grid intersections

(5) FIG. 4 shows the attenuation curves obtained when combining a layer of a 5 mm steel grid and a wall made of 0.50 m fibrous concrete

(6) FIG. 5 shows a sectional view of the opening provided in one panel

(7) FIG. 6 shows a sectional view of the connection between two panels.

(8) The object of the present invention is the building of a construction limiting the propagation of electromagnetic waves in a large frequency band, from the inside outwards and/or from the outside inwards.

DETAILED DESCRIPTION

(9) FIG. 1 shows a sectional view of an exemplary embodiment of a panel element according to the invention.

(10) It consists of a panel 1 made of concrete containing conductive particles. In the example disclosed, such particles consist of metallic fibres, 13 mm long and 0.2 mm in section. Concentration amounts to 20 kg per square meter of concrete. The panel 1 is 35 cm thick. It contains no electromagnetic shielding metallic mesh. This does not exclude the presence of metallic reinforcements or reinforcement bars for making the reinforced concrete. On the contrary, the panel 1 comprises no mesh, having small-sized meshes, with sides of less than 70 mm.

(11) In the example disclosed, the panel 1 is coated with an internal skin 2 and an external skin 3. The thickness of such skin is 20 mm in the example disclosed.

(12) The thickness of the panel 1 is above 80% of the total thickness of the panel element, with each skin 2, 3 having a thickness of less than 10% of the thickness of the panel 1.

(13) Such skins 2, 3 contain a conductive mesh.

(14) Mesh means a bidimensional assembly of conductors having an electrical continuity of conduction at the intersections of the interleaved conductors. Such mesh may also consist of a metallic gauze fixed by rivets or staples on the panel made of concrete containing conductive particles.

(15) It may also be integrated in a finishing material or a layer of concrete placed on the panel 1.

(16) The performances of a shielded volume are measured through the ratio between the field in the concerned area, without shielding, and the field remaining when shielding is provided. I.e., in decibels:
Eff. (dB)=20.Math.log [E1 (Volt/m)/E2 (Volt/m)]

(17) Reference is more simply made to attenuation between the outside field and the inside field, although such expressions are not rigorous.

(18) To obtain such attenuation, the wall must be made of a conductive material, with a nature and a thickness suitable for the expressed needs.

(19) Concrete with the usual cement/sand proportions, is a poor conductor. When thick enough, though, the skin effect results in the electromagnetic waves above a certain frequency going less and less deep into the wall thickness: concrete starts to absorb them. FIG. 2 shows the attenuation curves computed by absorption of a 0.50 m thick concrete. (A) curve: plain concrete, (B) curve, addition of steel fibres for a 0.50 m thick homogeneous concrete wall (resistivity 300 .Math.m).

(20) If concrete contains steel fibres in a suitable proportion, resistivity may be 30 .Math.m. With a low concentration conductive filler, concrete, which was nearly radio wave transparent up to 100 MHz starts providing significant attenuation (B curve, 15 dB) at the same frequency and becomes more and more efficient beyond.

(21) The only conductive filler does not make it possible, however, to give satisfactory performances at low frequencies, of less than 100 MHz, which correspond to radio transmission of ultra-short or metric waves.

(22) Unlike concrete, a metallic mesh opposes a very conductive surface to the incident electromagnetic wave. The latter is strongly reflected because of the sudden change in impedance, like optical reflection. When frequency increases, though, the efficiency of the grid decreases since the length of the incident field wavelength decreases and gets closer to the mesh dimension. When such mesh is equivalent to a half wavelength, the grid acts as a perfectly tuned aerial and lets the whole field through. Beyond, it will remain nearly transparent (FIG. 2).

(23) The invention is based on the general principle of a combination of fibrous concrete with a conductive grid.

(24) FIGS. 3 and 4 show the attenuation rate of one single layer of steel grid, welded at the 50 mm grid (A), 5 mm grid (B) intersections (FIG. 3) and the attenuation rate obtained by combining a layer of 5 mm steel grid, and a 0.50 m fibrous concrete wall (FIG. 4).

(25) When associating at best the properties of HF absorbing fibrous concrete, and those of a low frequency-efficient mesh, which gradually loses this advantage when frequency increases, premises can be built, the walls of which will sufficiently attenuate the electromagnetic fields in a very wide frequency range. FIG. 3 shows that combining a 5 mm mesh with 0.50 mm fibrous concrete guarantees at least 60 dB, from 1 MHz up to >10 GHz.

(26) In the left part of the curve, the mesh mainly provides attenuation, and filled concrete takes over from 150 MHz.

(27) In the solutions of the prior art, where a grid is embedded in concrete, the performance is inadequate. The invention, which consists in forming, on a panel filled with conductive particles without any conductive mesh, and in positioning a conductive mesh on such panel, makes it possible to significantly improve such performances.

(28) If the grid is embedded in the mesh thickness, the mismatch between the incident field and the impedance of the mesh is less favourable. Calculation shows that fibrous concrete with a resistivity of 30 .Math.m starts participating in the shielding around 50-100 MHz; it has then reached its characteristic impedance Zc of about 100. If the mesh is embedded, the field which hits it is no longer a field in the air (with Zc=377) but a field in a slightly conductive medium. This results in the reflection loss of the field on the mesh no longer being proportional to Zc (air)/Zmesh, but to Zc (concrete)/Z(mesh). This less sharp transition causes a loss of a 4 factor in the expected attenuation by the mesh.

(29) On the contrary, if, as provided by the invention, the mesh is on a surface, the first air-mesh interface is significantly mismatched and the reflection loss is maximum. Such advantage is particularly crucial if the sources of the radiated field desired to be attenuated are close to (for instance less than a few meters away from) the wall. Now this is exactly the case when the shielding of the premises must provide anti-compromise protection (a protection against electromagnetic spying). The attenuation of a mesh in a close magnetic field is then no longer as good as relative to distant sources. It is thus essential, in such applications, to maintain the best possible performances of the mesh.

(30) When the mesh is embedded in concrete, casting a wall in several operations, or assembling several prefabricated panels then rises the delicate issue of the edge-to-edge positioning, without any electrical discontinuity, of the grid elements, on the 4 sides. Any discontinuity (for instance staples here and there) ruins the efficiency of the mesh by creating a long slot, and thus leakage. On the contrary, laying the mesh on a surface with a continuous metal-metal contact is easily provided by lapped panels, like wall paper.

(31) To obtain intelligent concrete, the characteristics of which can be adjusted beforehand to obtain given performances, the invention consists in acting on three easily controlled parameters: concrete thickness, proportion of conductive particles, nature of such particles, dimension of the meshes in the mesh.

(32) Laying the mesh on a surface facilitates the adaptation of through elements: ventilation ducts, cable bushings and fluid conduits, door jambs and armor panes, etc. . . .

(33) When building, processing wall elements is not enough. The transmission of electromagnetic waves through openings, or panel junctions also has to be limited. FIG. 5 represents a schematic view of one opening.

(34) Openings more particularly relate to doors, windows, passages for conduits, cables and fluid pipes, ventilation ducts, etc.

(35) The closing element 4, for instance a bushing panel, is attached to the periphery of the opening provided in the panel.

(36) A conductive sheath 5, 6 is positioned on the edge of each panel element. Such panel 5, 6 consists of a metal sheet with a thickness greater than or equal to 0.5 millimeters.

(37) The walls made of concrete containing conductive particles 1 are provided with a hole wherein a metal peg 7, 8 is accommodated.

(38) The screws 9, 10 ensure the electrical and mechanical connection of the closing element 4 via the pegs 7, 8 with the sheath 5, 6 and the mesh 2. A conductive joint 11, 12 is inserted between the sheath 5, 6 and the closing element 4.

(39) FIG. 6 shows a sectional view of a junction between two panel elements. A metal strap 20 consisting of a thin conductive sheet having an omega-shaped section is attached by concrete screws or nails 21, 22 and steel bands 23, 24.

(40) Additionally, such metal strap 20 enables the shielding to follow the variations in the spacing of the expansion joint between the two panel elements.

(41) In the case of oblique angles of incidence, the electromagnetic wave may meet only a small concrete thickness, or even no thickness at all, at the openings in the concrete panel+mesh.

(42) As the conductive mesh cannot provide high frequency protection alone, the opening edge must be equipped with a conductive coating having an attenuation at least equal to that of the selected filled concrete thickness. Such sheath may be made of sheet metal, metal strap, metal cloth or conductive paint; a 90 return must provide continuity with the mesh.

(43) The wall characteristics are as follows: Resistivity of fibrous concrete30 .Math.m (an order of magnitude of the Alluets type mix) Dielectric constant: 10 i.e. a wall characteristic impedance120, constant for 100 MHz Steel-mesh size: 55 mm mesh

(44) Target performances (application of CEI 61000 Standard entitled Electromagnetic Compatibility (CEM)): Effects of the electromagnetic pulses at high altitude (IEM-HA): 50 kV/m peak. Rt: 2.5 ns, duration 50%: 25 ns High Intensity Radiation Field (HIRF): up to 10 kV/m peak, 30 MHz at 5 GHz attenuation 50 dB on the whole 0.1 MHz spectrum at at least 5 GHz

(45) With such an attenuation, the residual fields will be: IEMN160V/m peak pulse HIRF33V/m

(46) With such an attenuation, the residual fields are: Effects of the electromagnetic pulses at high altitude (IEM-HA)160V/m peak pulse HIRF33V/m

(47) Such values entail no risk of serious damage or failure to equipment which would at least comply with the CEM European directive, in the Industrial Severity category.

(48) Results

(49) TABLE-US-00001 F (MHz) 0.1 1 10 30 100 300 1,000 3,000 Concrete skin depth of 8.5 2.7 0.85 0.5 0.27 0.16 0.085 0.05 Absorption for 0.50 m (dB) 0.5 1.6 5.1 8.7 16 27 51 87 Absorption for 1 m (dB) 1 3.2 10 17.4 32 54 102 174 5 5 mm mesh (dB) 96 76 56 46 36 26 16 6 Total attenuation (dB) with a 96.5 77.5 61 54.7 52 53 67 93 0.50 m wall: with a 1 m wall 97 79 66 63.4 68 80 118 180

(50) The wall attenuation will combine with that of the secondary elements (doors, ventilation, etc.) which also have to comply with the >50 dB requirement.

(51) Positioning the grid on the surface instead of embedding it within concrete wins 6 dB thanks to a better field/grid impedance mismatch.