SPRAYING UHPFRC FOR REINFORCEMENT AND REGENERATION OF PRE-EXISTING STRUCTURES

Abstract

A method for reinforcing a structure comprising the following steps: preparation of UHPFRC comprising a cement precursor mix, of water, a fluidizing agent and metal fibers, transporting the UHPFRC by pumping to a suitable spray nozzle, spraying the mix onto a surface of the structure by the addition of a compressed air stream in the spray nozzle.

Claims

1. A method for reinforcing a structure comprising the following steps: preparing UHPFRC comprising a cement precursor mix, of water, a fluidizing agent and metal fibers comprising at least 2% by volume of concrete, transporting the UHPFRC by pumping to a spray nozzle, spraying the mixture onto a surface of the structure by the addition of a compressed air stream in the spray nozzle.

2. The method as claimed in claim 1, wherein the spraying is performed wet at a flow rate of between 2 and 10 m3/h.

3. The method as claimed in claim 1, wherein the spraying is performed wet by the addition of compressed air with a compressed air flow rate at the nozzle or gun of between 10 000 and 17 000 l/min.

4. The method as claimed in claim 1, wherein the spraying is performed wet by the addition of compressed air with an air pressure of between 5 and 11 bar.

5. The method as claimed in claim 1, wherein the metal fibers are made of high-strength steel.

6. The method as claimed in claim 1, wherein the UHPFRC has a metal fiber content of between 2 and 3.5% by volume of concrete.

7. The method as claimed in claim 1, wherein the cement precursor mix comprises between 700 and 1000 kg of cement per m3 of mix.

8. The method as claimed in claim 1, wherein the UHPFRC has a water/cement weight ratio of less than 0.25, preferably less than 0.20.

9. The method as claimed in claim 1, wherein the fibers have a greater mean dimension, for example a length, greater than 14 mm.

10. A structure reinforced, repaired or regenerated according to the method of claim 1, wherein the structure has at least one wall spray-coated with UHPFRC.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0044] The invention will be able to be better understood from the nonlimiting exemplary embodiment described hereinbelow and on studying the attached drawing in which:

[0045] FIG. 1 represents an exemplary embodiment of a spray nozzle that can be used in the method according to the invention.

MORE DETAILED DESCRIPTION

[0046] In the context of the present nonlimiting exemplary embodiment, a UHPFRC mix is prepared beforehand having a cement content of 700 to 1000 kg/m3, a silica fume or metakaolin content of 20 to 30% by weight of cement, a metal fiber content of 2 to 3.5% by volume of concrete, a water/cement weight ratio (E/C) less than 0.25 and a shearing threshold of at least 300 Pa for a shear velocity of 0.07 s−1.

[0047] The mix concerned does not include stiffening agent.

[0048] A pumping operation is performed to pump this mix by means of a pump with pistons (twinned) at a flow rate of 2 to 10 m3 per hour in order to perform a spraying wet.

[0049] The spraying operation itself is performed by the addition of compressed air with a compressed air flow rate at the nozzle or gun of between 10 000 and 17 000 l/min and an air pressure of between 5 and 11 bar.

[0050] The nozzle used is a nozzle of 50 mm useful diameter at the input.

[0051] The spraying is done onto a surface made of concrete, of metal, of wood or even of previously prepared masonry.

[0052] No addition of stiffening agent is performed during the nozzle or gun spraying.

[0053] Thus, a coating of a layer of UHPFRC is performed with a thickness equal to at least two times the length of the greatest metal fiber.

[0054] The characteristics of this coating are assessed according to the standard NF P 18-470. A mean compression strength at 28 days of 90 to 180 MPa is measured, as is a mean tensile elastic limit at 28 days of 4 to 10 MPa, and post-cracking properties conforming to the non-brittle criterion in section 9.1 of the standard NF P18-710.

[0055] The fiber-reinforced mix is of firm, relatively dry and non-workable consistency within the meaning of the standard NF EN 206-1 and its consistency class (S1 or S2) is outside of the range covered by the standard NF P 18 470 concerning UHPFRC.

[0056] This does not prevent the fiber-reinforced mix from being pumped over a distance of more than 100 m by means of a piston pump and sprayed with compressed air at very high flow rate with a small nozzle diameter of 50 mm, making it possible to achieve yields of 2 to 10 m3/h without the formation of urchins, or the segregation of the mix during the pumping and the spraying.

[0057] Because of the length of the pipes that can be up to a hundred or so meters, the reduced diameter at the nozzle output of the order of 50 mm, the rheology of the UHPFRC having a high shear threshold and viscosity, the risk of formation of urchins and of heating of the product in the pipes, it was necessary to choose a piston pump rather than a rotor pump so as to prioritize the working pressure over the flow rate.

[0058] For this, the Applicant selected a pump with pistons or cylinders (twinned) with low flow rate, i.e. of between 2 and 10 m3 per hour, with high working pressure, i.e. of 80 bar at least, with large piston diameter, i.e. of 200 mm, having a great chamber length, i.e. of 570 mm at least, and having a large output diameter, for example of the order of 150 mm.

[0059] The pump is equipped with a so-called “S” valve associated with a balancing mechanism. The power of the activation cylinders (transverse cylinders ensuring the balance) of the S valve and their hydraulic control system were adapted to prevent or limit the flow rate fluctuations.

[0060] This redimensioning makes it possible to rapidly shear the section of fiber-reinforced concrete (UHPFRC) having a high strength in order to minimize the flow rate fluctuations while remaining within the working range of the cylinders.

[0061] This adaptation makes it possible to obtain a flow rate that is even and suited to the operation of spraying a fiber-reinforced mix with high viscosity.

[0062] The spraying can be done manually or by means of a robot. The spraying can be performed in one layer or in several successive layers without exceeding a delay of three hours between successive layers, notably in order to avoid the formation of cold joints.

[0063] FIG. 1 shows a nozzle 1 suited to spraying of UHPFRC. The nozzle is mounted on an end of a UHPFRC intake pipe 10. It comprises a triple-input intake 11 which allows the mixing of the UHPFRC originating from the pipe 10 with a compressed air stream. The compressed air stream can be single or dual, and originates from a coupling of the intake to one and/or the other compressed air supply 12, 13. The triple-input intake 11 is linked to a spray gun 15. The link between the triple-input intake 11 and the spray gun 15 can be of any kind and for example consist of snap-fitting or screwing means. Advantageously, the link is provided with a fusible safety bushing 16 allowing the spray nozzle or gun 15 to be detached in the event of a pressure greater than a certain threshold, for example when the spray gun is plugged.

[0064] The fusible safety bushing makes it possible to prevent damage to the nozzle 1. It can be provided with a bypass to UHPFRC containing means, not represented.

[0065] The spray gun 15 has a substantially circular section and a dimension less than or equal to that of the UHPFRC intake pipe 10. If the section of the spray gun 15 is chosen to be less than that of the UHPFRC intake pipe 10, there is a consequential increase in the pressure in the spray gun which can allow for a better spraying of the concrete.

[0066] For example, the pipe 10 can have a circular section of 50 mm diameter and the spray gun 15 can have a circular section of a diameter of between 40 and 50 mm.

[0067] The length of the spray gun is advantageously between 300 and 400 mm for reasons of maneuverability of the gun and for pressure-containing purposes.

[0068] Advantageously, the UHPFRC intake pipe 10 is flexible. It can consist of a material that may or may not be elastically deformable.

[0069] Conversely, the spray gun 15 advantageously consists of a rigid material.

[0070] The spray gun 15 and the UHPFRC intake pipe 10 are required to be handled, so they are advantageously thermally insulated, in order to be able to spray in total safety when the mix heats up.

[0071] It is understood that the embodiments described are nonlimiting and that improvements can be made to the invention without departing from the scope thereof.

[0072] Unless stipulated otherwise, the word “or” is equivalent to and/or. Likewise, the word “one” is equivalent to “at least one” unless stipulated otherwise.