Abstract:
A composite structural reinforcement is realized within the pipe through in situ stratification of at least one band of reinforcement fibers and a resin matrix. The band is arranged according to a helicoidal path along the internal side of the pipe.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This is a convention utility application derived from and corresponding to French Patent Application Serial No. 04 04833 filed May 5, 2004 having the same title, incorporated herewith by reference and for which priority is claimed.  
       BACKGROUND OF THE INVENTION  
       [0002]     In a principal aspect the present invention relates to a methods of reinforcing pipes and in particular repairing embedded pipes.  
         [0003]     These pipes belong for example to a water delivery network. In their most common form these pipes are constituted by an end to end assembly of segments of pipe made from pre-stressed concrete and having a relatively large diameter of typically 0.5 m to 6 m environ. Water flows through said pipes under a pressure of up to around twenty bars. These segments of concrete pipe can incorporate an interior metal cylinder (liner). The latter is not designed, however, to carry the stress in terms of pressure due to the fluid flowing through. This stress is carried by passive armatures (pre-fabricated cylindrical armature cage or by the pre-stressing of the concrete. The latter is realized by means of threads wound in a spiral outside of the concrete core of the pipe segment. During manufacture of this segment its concrete core is rotated about its axis to receive the thread which is checked in order to be brought under pressure. This thread is then protected against corrosion by projecting a supplementary layer of concrete or mortar over twenty to thirty millimeters.  
         [0004]      FIG. 1  shows the structure of a pre-stressed concrete pipe as used in many water delivery networks. The wall of the pipe comprises: 
        a concrete core  11  incorporating a watertight liner  12  constituted by a ductile cylinder made from thin steel (so-called embedded-cylinder pipe). The concrete of the core  11  is distributed between an internal layer  13  of a few centimeters in thickness which is situated on the internal side of the pipe and a thicker external layer  14  which in most cases is not reinforced;     pre-stressing threads wound in one or two layers around the concrete core;     protective anti-corrosion mortar  16  surrounding the threads  15  in order to passivate them;     an optional paint or resin  17  improving the anti-corrosion protection.          
         [0009]     Pipes of this type are most frequently embedded. They are exposed to a risk of corrosion according to the aggressiveness of the environment. The most common form of degradation occurs as follows: 
        migration of aggressive ions across the protective mortar  16 ;     corrosion of the pre-stressing threads  15 ;     breaking of the pre-stressing threads and local delamination;     general delamination of the external mortar  16 ;     depassivation of the threads and acceleration of corrosion.        
 
         [0015]     The process can then accelerate and lead to the breaking of the pipe. Methods of magnetic or acoustic detection allow breaks in thread to be located and the state of the pipe to be assessed. Depending upon the assessed state a decision to repair can be taken.  
         [0016]     Repair is generally carried out from the exterior. New armatures, passive or pre-stressed, are placed around the pipe in order to collar it. See for example international patent application published as WO 03/014614.  
         [0017]     Repairs can also be carried out from the inside by incorporating within the pipe a resistant core for the purpose of re-establishing the resistance lost through the breaking of the pre-stressing threads. Repairs carried out from the inside are generally more expensive. Such repairs are carried out when the excavation works are impossible or difficult.  
         [0018]     Repairs from the inside generally consist in placing a metal liner within the pipe. The interstice between the new resistant liner and the pipe is filled with grout. One difficulty is that a local error in realization can lead to infiltrations of water in the injected interstice. This water penetration can cause the concrete core of the pipe to be brought under pressure and can make the new resistant liner inefficient. This will result in a risk of a break in the pipe outside of the new liner.  
         [0019]     When repairing from the inside the reinforcements added cannot be pre-stressed. When the pipe is brought under pressure these reinforcements undergo a certain elastic deformation (increase in their diameter) which causes cracking of the concrete of the core of the pipe. The concrete breaks in traction for a very small deformation, no material having the capacity to carry the significant stress for such extension. The cracking of the concrete causes two disadvantages. It worsens the flexural strength of the pipe and hence its capacity to resist dissymmetrical thrusts of the earth. It also allows corrosion to progress from the outside environment to the inside of the pipe. It also allows corrosion to progress from the exterior to the interior of the pipe. This risks causing early corrosion of the new reinforcement if it is metal. This problem often leads to the solutions for repairs from the inside with a basis of metal liners to be dismissed.  
         [0020]     It is an object of the present invention to propose a perfected method of carrying out repairs from the inside.  
       SUMMARY OF THE INVENTION  
       [0021]     Briefly, the present invention comprises a method of reinforcing an embedded cylindrical pipe, wherein a composite structural reinforcement is applied through in situ stratification of at least one band of reinforcement fibers and a resin matrix, the band being arranged according to a substantially helicoidal path along the internal side of the pipe.  
         [0022]     When the pipe is brought back into service the pressure of the liquid tightens the fibers of the band which, if it has appropriate dimensions, will be able to carry most of the radial stress. The composite structure avoids the risks of corrosion encountered with solutions using metal liners. The helicoidal layout allows the in situ stratification in situ of the composite to be carried out which facilitates implementation and avoids the problems of watertight connection, the band covering itself with each turn.  
         [0023]     The band can be mainly composed in particular of carbon fibers with well-established qualities as reinforcement fibers.  
         [0024]     The pipes in question generally comprise a concrete core incorporating a metal liner, possibly with one or several pre-stressing threads wound around the concrete core and encased by a protective mortar.  
         [0025]     The porosity of the composite reinforcement, even if it is very well realized, is low but remains relatively high in comparison with that of metal liners. In order to avoid the resulting disadvantages one possibility is to withdraw the concrete of the core located on the internal side of the liner in a repair section to adhere the composite structural reinforcement directly to the liner. The interstitial spaces between the liner and the composite reinforcement are thus avoided, the same being are likely to be penetrated by water if the composite reinforcement is not perfectly watertight. A situation should in fact be avoided wherein the water comes into contact with the liner by connecting the latter to the internal pressure of the pipe because this would render inefficient the new resistant covering having the role of taking this pressure to relieve the embrittled structure previously put in place.  
         [0026]     In a further embodiment the composite structural reinforcement is adhered to the concrete of the core located on the internal side of the liner in the repair section. This simplifies implementation by avoiding the need to withdraw the concrete of the internal side over a great length. If defects in watertightness of the composite reinforcement are feared an advantageous embodiment consists in piercing drainage holes through the liner in the repair section before placing the composite structural reinforcement. These drainage holes cannot run from the concrete core and this limits contamination of the liner through external agents. If a small quantity of water manages to infiltrate through the composite reinforcement it is drained outside of the liner in such a way that the latter is not brought under pressure. In order to bring about watertightness at the ends of the repair section the concrete of the core situated on the internal side of the liner is withdrawn locally here and the composite structural reinforcement is adhered directly to the liner. The ends of the repair section are aligned preferably with joints between consecutive cylindrical sections assembled to form the pipe.  
         [0027]     In an advantageous embodiment, holes are pierced in the core of the pipe which cross the liner and extend in the concrete of the core situated on the external side of the liner and armatures are sealed in said holes before placing the composite structural reinforcement. These armatures connect the new cutting reinforcement to the core of the pipe through the liner and thus provide resistance to the negative moments generated in the pipe when it is brought back under pressure. This results in the cracking process of the concrete being controlled, avoiding wide cracks which would allow aggressive external agents to penetrate as far as the liner.  
         [0028]     An advantageous embodiment consists in arranging a layer of compressible material between the internal side of the pipe and the composite structural reinforcement. This material, for example a foam or felt, absorbs in crushing the deformations of the reinforcement due to the extension of the fibers of the band when the pipe is brought under pressure. It thus avoids connecting the service pressure to the liner.  
         [0029]     A further aspect of the present invention is connected to underground works, including a reinforced cylindrical pipe in carrying out a method as described above.  
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0030]     In the detailed description which follows, reference will be made to the drawing comprised of the following figures:  
         [0031]      FIG. 1  is a previously described, is a sectional view of the wall of a pipe to which the invention can apply.  
         [0032]      FIG. 2  is a perspective view of a pipe repaired according to the invention.  
         [0033]     FIGS.  3  to  5  are sectional views of walls of pipes repaired according to several embodiments of the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0034]     Referring to the figures, a composite reinforcement  20  is arranged inside a damaged underground pipe. This reinforcement is composed of a band of fibers  21  wound according to a helicoidal path along the internal side of the pipe, as illustrated by  FIG. 2  (in which the ground material surrounding the pipe is not shown).  
         [0035]     The pipes in question are preferably embedded cylinder pipes as described previously with reference to  FIG. 1 . Their diameter is sufficient to allow the intervention of a human operator inside, for example 1.5 to 4.5 m. The service pressure in the pipe can be up to around 0.5 MPa.  
         [0036]     The reinforcement fibers of the band  21  are made from a resistant material (resistance to breaking typically greater than 1500 MPa) and are of a high elastic module (typically between 100 and 400 GPa). Carbon fibers will generally be used for this (resistance to breaking in the range of 3000 MPa). By way of example the bands  21  have a width of 600 mm and they are composed of carbon fibers oriented in the sense of a chain with a small proportion of glass fibers arranged in the sense of a frame in order to obtain a coherent tissue.  
         [0037]     The composite reinforcement  20  is placed in the following way during repair works carried out on a section of the pipe after emptying: 
        if necessary, the internal side of the repair section is prepared mechanically and/or chemically (cleaning, sanding, degreasing, . . . );     this side is coated with a thin film of resin in the liquid state;     the band of carbon fibers  21  is applied, possibly pre-impregnated with resin, to the resin film still in liquid state, causing it to follow its helicoidal path along the internal side of the pipe. By way of example it can be provided that the advancing of the band over a winding turn corresponds to around half the width of the band and this ensures two thicknesses of fibers on the surface of the pipe;     this band is processed in order to cause it to penetrate into the resin and impregnate it totally;     and the resin is polymerized.        
 
         [0043]     The resin used can have an epoxide base. The resin can be heatset and provided to polymerize at ambient temperature. It is for example an epoxy resin, bi-components of the type mentioned EP-A-0 799 951.  
         [0044]     In the embodiment shown in  FIG. 3  the preliminary preparation stage of the internal surface of the pipe includes the removal of the internal layer of concrete  13  in the repair section. This exposes the liner  12  to which the composite reinforcement  20  is directly applied. The interstices between the composite reinforcement  20  and the liner  12  are eliminated by reason of the very good adherence of the composite to steel.  
         [0045]     In the embodiment shown in  FIG. 4  the internal layer of concrete  13  is maintained in the repair section except at its ends where the liner  12  is exposed. The band of fibers  21  is placed according to the helicoidal path between the two ends of the section. Before placing it a layer  25  of material which can be deformed under compression is installed on the internal side of the pipe. This layer  25  is realized for example in a polyurethane foam or felt over a thickness of one to a few centimeters. Its role is to absorb the dilatation of the reinforcement which is produced through extension of the carbon fibers when the pipe is brought under pressure in avoiding this pressure being transmitted to the core  11  of the pipe. The problems of cracking of the concrete can thus be eliminated.  
         [0046]     At the ends of the repair section the deformable layer  25  is interrupted so that the composite reinforcement  20  adheres directly to the liner  12 . This closes in a watertight way the volume occupied by the residual layer of concrete  13  and the deformable layer  25  and thus avoids the internal pressure establishing itself in this volume. Sufficient watertightness will typically be obtained with an adhered reinforcement/liner interface extending over a length of a few centimeters. As shown in  FIG. 4  the interruption area of the concrete layer  13  can be managed so as to give it a beveled profile (for example over a length of one to twenty or thirty centimeters according to the thickness of the layer  13 ) in order that the reinforcement changes its degree of softness.  
         [0047]     The ends of the repair section where the new reinforcement  20  adheres directly to the liner  12  are advantageously provided to the right of the joints between the consecutive segments of pipe which are generally spigot joints. Owing to the good adherence of the composite to steel this arrangement ensures good protection in the proximity of the joints where corrosion is often more likely to establish itself than in the area of the segments.  
         [0048]     The non-zero porosity of the composite reinforcement can possibly allow water to infiltrate in the volume taken up by the residual layer of concrete  13  and the deformable layer  25  but in very small quantities if the reinforcement is carefully applied. In order to evacuate this water towards the exterior before the pressure can establish itself in this volume a judicious arrangement consists in realizing drains through the old liner  12 . These drains consist of holes  28  pierced in the concrete of the core  11  before applying the reinforcement  20 . The holes  28  cross the layers  13  and  12  and extend over a certain depth in the layer of concrete  14 . It is preferable not to cause them to run from this layer  13  in order to avoid sectioning the pre-stressing threads  15  with the piercing tool. The drains are realized for example with a hole  28  every 1 or 2 m 2 . The micro-cracking of the concrete of the layer  14  allows the infiltrated water to be evacuated to the exterior beyond the drains  28 .  
         [0049]     In the embodiment shown in  FIG. 5 , there is no attempt to prevent the cracking of the concrete when the pipe is brought under pressure. The composite reinforcement  20  is adhered directly to the layer of concrete  13 . In order to control the cracking which occurs by reason of the extension of the bands of fibers a cutting connection with the concrete core is realized through the liner  12 . This connection can be realized with the aid of armatures in the form of bars extending radially in the concrete core  11  and arranged at regular intervals on the repaired surface. Before applying the reinforcement  20  holes are pierced in the core of the pipe and armatures  31 - 33  are engaged. The armatures  31 - 33  are sealed in the holes, for example with the aid of the same resin as that used to constitute the matrix of the reinforcement  20 .  
         [0050]     These armatures  31 ,  33  can consist of carbon fiber core drivers coated with adherence resin and threaded in the holes. Their overshooting ends are spread in a fan over the surface before being covered by the reinforcement (see EP-A-0 837 201) in order to connect the concrete core  11  to the composite reinforcement  20  and to the metal liner  12 . Some of these armatures ( 33  in  FIG. 5 ) can remain confined to the superficial concrete layer  13  in order to connect it to the composite reinforcement  20 .  
         [0051]     In an alternative embodiment the armatures  32  consist of steel bars driven into the previously pierced holes. These bars allow the concrete core  11  to be connected to the liner  12 .