Abstract:
The prestressed building structure comprises an assembly of precast elements separated by gaps occupied by an interface product, one or more prestressing sheaths having sections respectively incorporated into the precast elements, and a prestressing tendon tensioned inside the sheath. The sheath sections are respectively fitted with first and second end pieces opening out on the facing surfaces of the two precast elements. The first end piece has a flared opening. An elastic connecting sleeve is connected in a sealed manner to the second end piece. The sleeve presses against the first end piece, which compresses it longitudinally to ensure a seal between the inside of the sheath sections and the gap separating the adjacent surfaces of the two elements.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to the assembly of precast construction elements for building prestressed structures. 
         [0002]    It applies in particular, but not exclusively, to the decks of cantilevered bridges built using precast concrete elements (segments). These structures are frequently subjected to longitudinal prestressing using prestressing tendons threaded inside sheaths embedded in the concrete of several successive elements. 
         [0003]    Carrying out such prestressing is a difficult operation. The positioning of the sheath sections in the elements must be very accurate so that the prestressing tendons can be threaded without difficulty. 
         [0004]    Moreover, the sealing of the sheath at the interfaces between the elements must be ensured. This sealing is necessary to ensure the durability of the prestressing, which is subject to the risk of infiltrations at the joint between the elements. 
         [0005]    When the joint is made using the so-called “wet joint” method, an interface product such as concrete or mortar fills the gap between two adjacent elements. In this case, the seal also meets the need to prevent the interface product, or certain components of it, from entering the sheaths when it is placed between the elements, and then hindering the insertion of the tendons. 
         [0006]    Furthermore, the sheaths are often injected with a filler (cement grout, grease, wax, resin, etc.) serving in particular to protect the tendons against corrosion. This product must not escape from the sheath during injection. Some areas of the structure can have a relatively high density of sheaths, and there is no guarantee that the interface product will produce a seal between these sheaths. As a result, there is a serious risk that grout injected under pressure into a sheath will infiltrate into one or more neighbouring sheaths, in which injection then becomes very difficult or even impossible. 
         [0007]    FR-A-2 596 439 describes a linking device between sections of prestressing sheath, comprising a cylindrical sleeve engaged between the openings of two adjacent sections to ensure the continuity of the sheath, and an elastic seal surrounding the cylindrical sleeve to ensure sealing and compensate for the positioning variations and dimensional deviations of the blocks, which are assembled against each other. 
         [0008]    WO 99/043910 describes an improvement of the construction methods in which the elements are matched and then assembled in contact with each other. The matched elements are cast using the so-called “match cast” technique in which the front surface of an n th  element defines the rear side of the mould used to shape the (n+1) th  element of the series. When each element is cast, sleeves are arranged at the ends of the sheath sections that they contain. The complementary surfaces of the matched elements are then pressed against each other so that the sheath sections are arranged running on from each other to form complete sheaths. Positioning connectors are engaged in the sleeves to connect the adjacent sheath sections in a sealed manner. 
         [0009]    This last technique is well-suited to elements cast using the “match cast” technique, which ensures accurate mutual positioning of the adjacent sections of a prestressing sheath. However, its implementation can be difficult if a “wet joint” method is used. In this case, the elements are often factory cast with significant dimensional tolerances, and the gap between the adjacent surfaces of two successive elements can be several centimetres. 
       SUMMARY OF THE INVENTION 
       [0010]    The present document relates to an assembly technique for precast elements that provides an answer to the issue of sealing the sheaths when a “wet joint” method is used. 
         [0011]    It sets out a construction method for a prestressed structure having a series of precast elements. This method comprises the steps of: obtaining two successive elements in the series, each of the two elements incorporating at least one prestressing sheath section and an end piece linked to said sheath section and opening out on a surface of said element, the end piece incorporated into one of the two elements having a flared opening; connecting in a sealed manner an elastic connecting sleeve to the end piece incorporated into the other element; arranging the two successive elements relative to each other, maintaining a gap between two adjacent surfaces of the elements, the connecting sleeve being engaged in said flared opening and compressed longitudinally by the bringing together of the elements, the compression of the connecting sleeve ensuring a seal between the inside of the sheath sections and the gap between the adjacent surfaces of the elements; and placing an interface product in the gap between the adjacent surfaces of the elements. 
         [0012]    The elastic connecting sleeve separates in a sealed manner the inside of the prestressing sheath from the gap between the two elements, which must be filled with concrete or another interface product. The arrangement allows for a seal to be produced without accessing the connecting area, which is prevented by the narrowness of the gap between the elements. However, this gap has a significant thickness and its dimensions are not accurately guaranteed given the manufacturing tolerances of the elements and possible inaccuracies on assembly. There can also be misalignments between the sheath sections and end pieces relative to their theoretical positioning in the elements. The gradual flaring of at least one of the end pieces and the elasticity of the connecting sleeves that extend between the two end pieces allows for them to be deformed in such a way as to compensate for the various deviations and inaccuracies linked to the manufacturing and assembly of the elements. 
         [0013]    The gap between the adjacent surfaces of the elements can for example have a thickness of between 3 and 6 centimetres. A typical order of magnitude for the longitudinal compression capacity of the connecting sleeve is a capacity greater than one centimetre. Moreover, a typical order of magnitude for the misalignment between the two end pieces permitted by the connecting sleeve is in an angular range greater than one degree. 
         [0014]    Another aspect of the invention relates to a building structure comprising: an assembly of at least two precast elements having two respective facing surfaces separated by a gap occupied by an interface product; at least one prestressing sheath having two sections respectively incorporated into the precast elements; and a prestressing tendon tensioned inside the sheath. The sheath sections are respectively fitted with first and second end pieces opening out on the facing surfaces of the two precast elements, the first end piece having a flared opening. An elastic connecting sleeve connected in a sealed manner to the second end piece is pressed against the first end piece, which compresses it longitudinally to provide a seal between the inside of the sheath sections and the gap separating said surfaces. 
         [0015]    A further aspect of the invention relates to a connection system for prestressing sheath sections, comprising: first and second end pieces, each having a rear side capable of being connected to a sheath section incorporated into a respective precast construction element and a front side to open out on a surface of said element, the front side of the first end piece having a flared opening; and an elastic connecting sleeve having one side capable of being connected in a sealed manner to the second end piece and an opposite side capable of cooperating with the first end piece, the connecting sleeve having a longitudinal compression capacity and a transverse deformation capacity in order to be compressed when said surfaces of the elements are brought together, whilst permitting an offset and misalignment between the two end pieces, the compression being capable of providing a seal between the inside of the sheath sections and a gap separating said surfaces. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    Further features and advantages of the present invention will become apparent from the following description of non-limitative embodiment examples, with reference to the attached drawings. 
           [0017]      FIG. 1  is a perspective view of a precast segment to which the method according to the invention can be applied. 
           [0018]      FIG. 2  is a diagram showing the juxtaposition of two adjacent segments equipped with prestressing sheath sections and forming part of a series of precast elements. 
           [0019]      FIG. 3  is a block diagram of a connection system according to one embodiment of the invention. 
           [0020]      FIG. 4  is a diagram of a female end piece of the system in this embodiment. 
           [0021]      FIG. 5  is a diagram showing a possible arrangement of a prestressing sheath section near the end surface of an element. 
           [0022]      FIGS. 6 and 7  are respectively profile and front diagrammatic views of a variant embodiment of a female end piece of the connection system. 
           [0023]      FIG. 8  is a longitudinal cross-sectional view of another embodiment of the connection system according to the invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0024]    The invention is described below in its non-limitative application to the cantilever construction of a precast segment bridge. 
         [0025]    Such a segment  1  is shown in  FIG. 1 . The element  1  has the general form of a caisson delimited at the bottom by a base  2 , laterally by two symmetrically sloping walls  3 , and at the top by a deck  4  extended laterally beyond the walls  3  to define the width of the bridge. 
         [0026]    In the longitudinal direction, the element  1  is delimited by substantially parallel rear  7  and front  6  surfaces. The rear surface  7  is intended to face the front surface, of complementary shape, of the previous element installed on the structure under construction (for the first element installed on a pier of the bridge, the complementary surface belongs to the pier). Similarly, the front surface  6  of the element  1  is intended to face the rear surface of the next element to be installed. The complementary shaped surfaces of the adjacent elements can possibly be provided with bosses  8  facilitating the relative positioning of the elements when they are brought together. 
         [0027]    The element  1  (or  1 A,  1 B in  FIG. 2 ) comprises a number of longitudinal sheath sections  10  ( 10 A,  10 B in  FIG. 2 ) intended to receive prestressing tendons  15 . The prestressing tendons  15  are anchored onto the structure at their ends by means of appropriate anchors. Some of these anchors  11  can possibly be arranged on sheaves  12  provided inside the caisson shape of the element. The sheath sections  10  open out on the rear surface  7  and/or the front surface  6  of the element. The continuity and sealing of each prestressing sheath  10  must be ensured at the interfaces between the elements. To this end, a connection system is used, embodiments of which are described below with reference to  FIGS. 3 to 8 . 
         [0028]    After the positioning of an element  1 B with a connection system installed at the joints of the sheath sections  10 A,  10 B, an interface product  16 , which will generally be concrete, is injected into the gap between the element  1 B and the previous element  1 A in the series. This gap typically has a thickness of between 3 and 6 centimetres. The sealing of the sheath is important to prevent components of the interface concrete  16  entering the sheath  10 , which would hinder the subsequent threading of the tendons  15 . 
         [0029]    Once the interface concrete has set, the next element is assembled. If one (or several) prestressing sheaths  10  has its (their) last section in the element that has just been installed, the threading, anchoring and tensioning of a prestressing tendon  15  in this sheath can take place, possibly after having checked the seal using a pneumatic device. Threading can be carried out using conventional techniques. After tensioning, filler, generally cement grout, is injected into the sheath  10  to protect the metal of the tendon  15  against corrosion. The sealing of the sheath is important to prevent grout injected in a fluid state from escaping at the interfaces between the elements. 
         [0030]    The successive elements  1 ,  1 A,  1 B of the series are prefabricated from cast concrete. In the embodiment shown in  FIG. 3 , the rear surface  7  of element  1 B is facing the front surface  6  of the previous element  1 A in the series. At the interface, the sheath sections  10 A,  10 B embedded in the concrete of elements  1 A,  1 B are respectively provided with two end pieces  20 A,  20 B also incorporated into the concrete of the element and made for example from a rigid plastic material. In the example shown, a male end piece  20 A has its rear side connected to the sheath section  10 A incorporated into the element  1 A already in place on the structure, whilst a female end piece  20 B has its rear side connected to the sheath section  10 B of the new element  1 B on its rear surface  7 . The end pieces  20 A,  20 B are connected to the sheath sections  10 A,  10 B in a sealed manner, and are placed in the mould used to produce the elements  1 A,  1 B. In general, the end pieces  20 A,  20 B do not extend beyond the end surface  6 ,  7  of the element, for reasons of ease of casting. They can be positioned in the mould using studs positioned at the appropriate places on the inside surfaces of the walls of the moulds. After form removal, the front sides of the end pieces  20 A,  20 B open out on the surfaces of the elements, which will be placed facing each other when the bridge deck is assembled. 
         [0031]    In addition to the end pieces  20 A,  20 B, the connection system shown in  FIG. 3  comprises an elastic connecting sheath  21  made from an elastomer material. To enhance the elasticity of the sleeve  21  and its deformability in both an axial direction and transverse to the direction X of the sheath at the interface between the elements, the sleeve can be shaped like a bellows, as shown in  FIG. 3 . 
         [0032]    The sleeve  21  is connected in a sealed manner to the male end piece  20 A of the connection system. This connection is for example achieved by clipping or by screwing the rear side of the sleeve  21  in the male end piece  20 A. It takes place after the forms have been removed from the element  1 A. The front side of the connecting sleeve  21  cooperates with the female end piece  20 B of the facing element. The sleeve  21  and the end pieces  20 A,  20 B are sized so that the sleeve  21  is compressed axially when the two elements  1 A,  1 B are brought together on assembly. 
         [0033]    The female end piece  20 B has an opening  22  that flares gradually as shown in  FIGS. 3 and 4 . This flare  22  facilitates the insertion of the connection sleeve  21  without it being necessary to manipulate it when the elements  1 A,  1 B are brought together. 
         [0034]    The elasticity of the sleeve  21  allows for tolerances to be permitted in the accuracy of the production of the concrete elements  1 A,  1 B, which tolerances are usually several centimetres. The sleeve  21  should therefore have a longitudinal compression capacity greater than 1 cm. 
         [0035]    Furthermore, it is very difficult to accurately guarantee the positioning of the sheath sections  10 A,  10 B parallel to the surfaces  6 ,  7  of the elements, as well as their orientation relative to these surfaces. The capacity of the sleeve  21  to deform transversely at the joint plane between the elements  1 A,  1 B also allows for these inaccuracies to be absorbed. The misalignment between the end pieces  20 A,  20 B of the sheath sections that the sleeve  21  can compensate for is greater than 1 degree and can even be around 10 degrees or more. 
         [0036]    The gradual flaring of the opening of the female end piece  20 B can be of frusto-conical shape, as shown in  FIG. 4 , with a half-cone angle α sufficient to facilitate the approach of the elastic connecting sleeve  21 . The flare  22  allows for the end of the connecting sleeve  21  to be conveyed to a recess  23  provided at the bottom of the female end piece when the two elements are brought together. The front end of the sleeve  21  can be shaped so that it nests firmly in the recess  23  in order to ensure, by clipping, a sealed connection under the action of the return force exerted due to the elasticity of the compressed sleeve. The flare  22  can also contribute to deforming the sleeve  21  if the two sheath sections are not exactly aligned. For a frusto-conical flare  22  of length L, the cone must have a sufficient opening at its base for the sleeve  21  to enter fully into the cone during the bringing together of the two elements  1 A,  1 B. The flare  22  must then compensate for: the effect of any local gradient β of the sheath relative to the joint plane between the elements  1 A,  1 B (see  FIG. 5 ); the effect of the positioning inaccuracy of the two end pieces  10 A,  10 B in relation to each other (offset Δ parallel to the joint plane); the fact that the sleeve  21  is unfolded, i.e. not compressed, by a length D on bringing together. 
         [0037]    Under these conditions, the minimum opening angle α of the cone verifies: 
         [0000]    
       
         
           
             
               
                 
                   
                     tan 
                      
                     
                         
                     
                      
                     α 
                   
                   ≥ 
                   
                     
                       tan 
                        
                       
                           
                       
                        
                       β 
                     
                     + 
                     
                       Δ 
                       
                         L 
                         × 
                         cos 
                          
                         
                             
                         
                          
                         β 
                       
                     
                     + 
                     
                       
                         D 
                         × 
                         tan 
                          
                         
                             
                         
                          
                         β 
                       
                       L 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0038]    Moreover, the opening of the cone can facilitate the sliding of the sleeve  21  towards the recess  23  despite the friction of the sleeve on the female end piece  20 B, whatever the mutual positioning defect of the end pieces. If the friction is defined by a cone with a half cone angle φ, another condition on the maximum opening of the frusto-conical flare  22  is: 
         [0000]      α&lt;90°−β−φ 
         [0039]    The frusto-conical shape with a circular cone for the flare  22  has the advantage of being simple to produce. It also allows for the avoidance of any ambiguity in the direction of placing the female end piece  20 B in the formwork, and therefore of any risk of error. 
         [0040]    In certain cases however, the opening of the cone at the end surface  6 ,  7  of the element can have relatively large dimensions, which can be problematic, particularly when several neighbouring prestressing sheaths have to cross the gap between the two elements. Moreover, if the sheaths are embedded in a relatively narrow concrete part, such as a segment web, the width of the cone can become significant relative to the total width of the part and lead to a weakening of the structure. 
         [0041]    In the minimum angle condition (1), it can be observed that only the term 
         [0000]    
       
         
           
             
               Δ 
                
               
                   
               
             
             
               L 
               × 
               cos 
                
               
                   
               
                
               β 
             
           
         
       
     
         [0000]    relating to the positioning tolerance of the end pieces relative to each other is omnidirectional. The other two terms relating to the gradient of the sheath and the extension of the sleeve only operate in an a priori known direction, namely the direction of minimum angle between the sheath section  10 B and the joint plane. This direction of minimum angle is the direction in which the angle β is shown in  FIG. 5 . 
         [0042]    Under these conditions, it can be prudent to provide an anisotropic flare of the female end piece, as shown in  FIGS. 6 and 7 . In this embodiment, the front side of the female end piece  30 B has one half provided with a frusto-conical circular flare (lower part of  FIGS. 6 and 7 ), with a half-cone angle α′ of the order of 
         [0000]    
       
         
           
             Arc 
              
             
                 
             
              
             
               tan 
                
               
                 ( 
                 
                   Δ 
                   
                     L 
                     × 
                     cos 
                      
                     
                         
                     
                      
                     
                       β 
                       max 
                     
                   
                 
                 ) 
               
             
           
         
       
     
         [0000]    where β max  is the maximum gradient of the sheath relative to the joint plane. The other half of the front side of the female end piece  30 B (upper part of  FIGS. 6 and 7 ) has a flare in the shape of a cone with an elliptical base, the half cone angle α of the cone on the major axis of the ellipse verifying condition (1), α then being of the order of 
         [0000]    
       
         
           
             Arc 
              
             
                 
             
              
             
               
                 tan 
                  
                 
                   ( 
                   
                     
                       tan 
                        
                       
                           
                       
                        
                       
                         β 
                         max 
                       
                     
                     + 
                     
                       Δ 
                       
                         L 
                         × 
                         cos 
                          
                         
                             
                         
                          
                         
                           β 
                           max 
                         
                       
                     
                     + 
                     
                       
                         D 
                         × 
                         tan 
                          
                         
                             
                         
                          
                         
                           β 
                           max 
                         
                       
                       L 
                     
                   
                   ) 
                 
               
               . 
             
           
         
       
     
         [0043]    For the assembly of such a female end piece  30 B on its sheath section  10 B in the mould for the concrete element, the end piece is oriented so that it presents its maximum flare, that is, the major axis of the elliptical shape, in the direction of minimum angle between the sheath section and the joint plane. 
         [0044]    Under these conditions, the performance of the connection system can be optimum while limiting the extension of the end piece  30 B on the surface of the element  1 B in directions other than the direction in which it is genuinely necessary. 
         [0045]    The invention is not limited to the embodiments described above. In particular, the female end piece is not necessarily incorporated into the new element that is being assembled: it can also be on the previously installed element. In another embodiment, the connecting sleeve  21  can be in one piece with the male end piece connected to the sheath section of one of the elements. 
         [0046]    In yet another embodiment, such as that shown in  FIG. 8 , the end pieces  40 A,  40 B incorporated into the two adjacent precast elements  1 A,  1 B are made up of identical parts, which allows for their production cost to be minimised and avoids confusion during the casting of the concrete elements. In this example, each end piece  40 A,  40 B has an internally threaded recess  43  capable of receiving in a sealed manner the threaded rear side of the elastic connecting sleeve  41 . Beyond this recess  43 , the end piece  40 A,  40 B ends in a frusto-conical flare  42  as previously described. Still in the example in  FIG. 8 , the elastic connecting sleeve  41  has a generally M-shaped profile forming a bellows that permits both longitudinal compression and transverse offset. The arm of the M located on the front side of the sleeve  41  presses, when the elements are brought together, against the flared opening  42  of the end piece  40 A incorporated into the other element. The sealing results from the contact area between the front part of the sleeve and the frusto-conical opening  42 . In  FIG. 8 , the sleeve  41  is shown at rest by dashed lines, and in its compressed position by solid lines. It will be noted that the compression of the sleeve gives rise to almost no encroachment on the inner section of the sheath, where the prestressing tendons will be threaded.