Patent Publication Number: US-8534015-B2

Title: Reinforcement for concrete elements and system and method for producing reinforced concrete elements

Description:
The present invention relates to reinforcement and a reinforcement system for reinforcing concrete elements. Further, the invention relates to a method for producing such reinforcement and a method for fabricating a reinforced concrete element. The reinforcement comprises at least one elongate fibre string formed of a smaller number of single fibre filaments which together provide a fibre string. The fibre string may preferably be coated with a particle shaped material, such as sand, the sand being adhered on to the exterior surface of the string. Further, the invention relates to a method for concreting such reinforced concrete elements. 
     It is well known that concrete structures are reinforced using steel in such way that the loads and forces are transferred from the concrete to the reinforcement, aiming to obtain a structure where the tensional load and forces are taken by the reinforcement, while compressive loads and forces are taken by the concrete itself. Standard length of reinforcement bars is 12 meters and the thickness may vary between Ø6 mm to Ø48 mm. It is obvious that such steel dimensions represent a large weight and rigidity, making it difficult to handle and place the reinforcement in a structure. When placing the reinforcement of steel, the reinforcement bars must be pre-bent and then tied together in a shuttering, in order place the reinforcement in sections where tensile forces are expected. 
     Where larger lengths are to be reinforced, the reinforcement bars must overlapped each other, transferring normal stresses and tensions as shear forces through the concrete from one bar to another. Welding of the bars is also possible. Conventional steel reinforcement requires, as a general rule, a concrete coverage of at least 30 mm, while at the same time, large concentration of tensional forces are experienced in the surface edges of a concrete structure. Hence, cracks may readily appear in these areas, making it possible for water to penetrate into the concrete structure, corrosion attacking the steel reinforcement. Such attacks of corrosion increase the volume of the reinforcement beyond its original volume, producing a tensile force and possibly causing spalling. 
     It is well known to use products of carbon fibres as reinforcement, either embedded in concrete or glued to the surface of a concrete body. 
     From the applicants own WO 03/025305 A1, a method for fabricating reinforcement elements for concrete is known, the reinforcement comprising elongated, preferably continuous fibre bundles of carbon fibres, impregnated with a matrix of plastic materials, which then is cured. The fibre bundle, which comprises a very large number of single fibres, is subsequent to the impregnation and prior to curing, brought into a bath containing a particle shaped material, such as sand, which adheres to the surface of the fibre bundle without to any extent penetrating in between the various fibres. The particle shaped material is fixed to the surface during the curing process, thus forming the reinforcement element. 
     NO 138.157 shows a loop reinforcement for pre-stressed concrete structures, where the loop reinforcement comprises several resin impregnated glass fibre strings, the cross section area of each loop being increased by means of reinforcing strings of resin impregnated glass fibres which are closely connected to each loop. 
     EP 1180565 discloses a flexible reinforcement for reinforced concrete in the form of a flexible band having a high module of elasticity. The band is arranged around at lease two reinforcement bars and each end of the band is tensioned in order to form a loop around the reinforcement bars, forming a rigid connection. 
     It is known to construct concrete floating piers made up separate, independent pier elements, wherein pairs of pier elements are connected together at their corner areas. For this purpose a vertical recess or notch is arranged in each corner of each pier element together with horizontal ducts, extending from the recesses through the element wall and out at the end wall of the element. Horizontally arranged anchoring means extend between said recess at each element through said ducts in order to assemble and interconnect two pier elements. 
     Because of the recesses and the ducts, each corner is exposed to large tensile forces and loads. Hence, it is necessary to reinforce the corners and the sections surrounding the recesses heavily. 
     Said corner areas have proved to be vulnerable, however, and the concrete is crushed in spite of heavy reinforcement, when the pier elements are exposed to large loads and forces. 
     The problem to be solved is that, in addition to maintaining a high degree of tensile strength, low weight and high resistance against corrosion to ensure, good strength is maintained even at high temperatures, such as for example temperatures caused by fires of high intensity. 
     A further problem to be solved is to increase the production rate when producing the reinforcement as such and also for providing tailor made reinforcement solution, while reducing substantially the requirements for investments in production facilities and machinery. 
     A still further problem to be solved is to reduce the extent of and the time required for laying the reinforcement for those instances where more or less complicated tailor made reinforcements are required for various structures. 
     An object of the present invention is thus to provide a reinforcement system for concrete having improved properties, giving the structures to be cast improved strength and increased life time, and at the same time reducing the need for maintenance of the concrete structures produced. 
     A further object of the reinforcement system according to the invention is to prolong the structural load carrying capacity of the concrete structure if the concrete structure is exposed to a fire. 
     A still further object of the reinforcement system according to the invention is to provide a simple and flexible reinforcement system, making it possible to adapt and to dimension the reinforcement system to complicated structural elements. 
     A still further object of the reinforcement system is to provide a reinforcement which is simple to lay for the operator and eliminating at least partly heavy manual lifting activities. 
     The above mentioned objects are achieved by a reinforcement system and a production method as further defined in the characterizing part of the independent claims. Preferred embodiments of the invention are defined in the independent claims. 
     An essential element in the reinforcement system according to the invention is the use of closed reinforcement loops made of a plurality of continuous fibres, for example made of carbon or basalt, embedded in a matrix, wherein the loop is cured subsequent to formation of the loop and wherein the loop is coated by a layer of particles, such as for example sand. The loops are preferably elongated and may either be in the form of closed loops or elongated winds, arranged in longitudinal direction and corresponding loops or winds in a transverse direction. The semi-circular ends of loops or the winds are configured to function as an end anchoring the reinforcement. The effects of the loop reinforcement may also at least partly be achieved by providing a helical reinforcement. When such helical reinforcement is embedded in cured concrete, the helical reinforcement will function as a multi-axial reinforcement. 
     When using the reinforcement according to the invention, abrupt or sudden concentration of forces will to a much less degree appear in the region of the ends of the reinforcement. If it is necessary to “join” the reinforcement, conventional overlapping may be applied corresponding to the traditional steel reinforcement. The major difference is that the forces from one reinforcement element is transferred to the neighbouring reinforcement in that, in addition to transfer of shear strain between the reinforcement loops, a local compression zone is established in the concrete between the ends of two overlapping loops. Since concrete may resist large compressive forces, possible cracks or minute cracks in this load transfer zone will be closed by the compressive force rather than being opened up, as the case may be for conventional reinforcement. The size of such compressive forces depends on several parameters, depending inter alia on the bonding between the composite reinforcement and the surrounding concrete. 
     The reinforcement is made of a composite material, amongst other containing carbon fibres or basalt fibres. 
     The reinforcement loops according to the invention have good material properties, such as high tensile strength, low weight, and high corrosion resistance. In addition, high tensile strength is maintained even at high temperatures, such as for example during highly intensive fires. 
     Tests have shown that the reinforcement according to the invention is four times stronger than steel, while the weight is four times lower than steel. Consequently, substantial weight savings may be obtained when using the reinforcement according to the invention. 
     In addition, it should be appreciated that since the reinforcement according to the invention has a high degree of inherent resistance towards corrosion, the reinforcement may be placed close to or on the surface of the concrete element to be reinforced, thus requiring a reduced or no concrete coverage. Hence, the reinforcement may be placed where it really is needed. 
    
    
     
       The invention shall now be described in closer details, referring to the accompanying drawings, in which: 
         FIG. 1  shows schematically a vertical section through a reinforced concrete element, wherein two reinforcement loops according to the principle of the invention are shown; 
         FIG. 2  shows a view of one embodiment of a reinforcement net formed of a plurality of closed reinforcement loops; 
         FIG. 3  shows an alternative embodiment of a reinforcement net formed of a plurality of continuous reinforcement loops arranged both lengthwise and in a transverse direction; 
         FIG. 4  shows a plurality of coaxially and concentrically arranged reinforcement loops according to the invention; 
         FIG. 5  shows schematically a horizontal section through a pontoon, wherein reinforcement loops according to the invention are used for reinforcing the pontoon; 
         FIG. 6  shows schematically a vertical section through the reinforcement used in connection with the pontoon unit shown in  FIG. 5 ; 
         FIG. 7  shows schematically a vertical section through the pontoon unit shown in  FIG. 5 ; 
         FIG. 8  shows schematically the first steps in fabrication of a fibre bundle by means of a plastic material; 
         FIG. 9  shows how a loop according to the invention may be fabricated; and 
         FIG. 10  shows a vertical section through the reinforcement loop  11 , seen along the line A-A in  FIG. 9 . 
     
    
    
       FIG. 1  shows schematically a vertical section through a concrete element  10 , schematically shown as a rectangular beam, seen from above. As indicated, the beam is schematically reinforced by means of two reinforcement loops  11 . A plurality of reinforcement loops  11  may be used, but from a clarity point of view, only two reinforcement loops  11  are shown in the Figure. It should be appreciated, however, that a large number of reinforcement loops  11  may be used, dependent upon the forces and loads which the concrete element from a design point of view must be dimensioned for. The reinforcement loops  11  may be arranged in any preferred plane, including the horizontal and the vertical plane. As indicated in  FIG. 1 , the reinforcement loops  11  are arranged in the horizontal plane, one end of one loop overlapping the other, forming a closed cylindrical room  12  between themselves. The opposite end of each reinforcement loop  11  forms a closed semi-circle  14 . 
     When the concrete element is subjected to tensile loads, for example as indicated by the arrows in  FIG. 1 , the two overlapping ends of the reinforcement loops  11 , will together form the closed cylindrical room  12 , exposing the concrete inside said room  12  for compression and hence, functioning as an end anchor causing a local pre-stressing compression. The ends of the loops  11  function thus as an end anchor for the reinforcement, while at the same time the straight parts of the loops  11  functioning as conventional reinforcement. 
     It should be appreciated that the loops  11  according to the embodiment shown may for example be formed of a small number of single fibre filaments which may be interconnected by means of a matrix in order to form a fibre string, coated with a particle shaped material on the exterior of the string. The particle shaped material may for example be sand. 
     The strings  11  may for example have a height of 1-5 cm, while the thickness may for example be 1-2 mm. The elongated loop  11  may be formed by repeatedly winding said fibre string in order to form the closed loops  11 . 
     The loops  11  may be configured in such way that their ends for example may have the form of semi-circles or semi-ovals. 
       FIG. 2  shows an alternative embodiment of reinforcement according to the invention. Also this embodiment is shown in relation to a concrete slab  10 , and like the embodiment shown in  FIG. 1 , only one layer of reinforcement is shown. The embodiment comprises a plurality of closed loops  11  arranged in succession after each other, interconnected at least at their ends by means of elongated fibre strings  15 , thus forming a reinforcement net or a reinforcement mat. Said elongated fibre strings  15  may either be in the form of straight strings, or in the form of loops positioned perpendicular with respect to the loops  11 . Such net or mat may for example be used as reinforcement for concrete floor, concrete walls or the like. 
     A reinforcement embodiment as shown in the Figures may for example be used as reinforcement for concrete columns. 
       FIG. 3  shows a third embodiment of a reinforcement mat, where the loops  11  are in the form of transverse winds  16  which are interconnected by a plurality of elongated winds  17 . The fibre strings forming the winds  16 , 17  may for example have dimensions as specified above in respect to  FIG. 1 . 
     As indicated in  FIG. 3 , two of the loops  16 ′ may be laid so that their end is extending out of the concrete element  10 . The loops  16 ′ may for example be used for attaching the concrete element  10  to an adjacent concrete element (not shown). In such case, the loops may for example be placed in a corresponding recess in the adjacent concrete element, whereupon the two concrete elements may be inter-concreted in situ. It should be appreciated that the number of loops  16 ′ which are extending out the concrete element  10  may be one or several without deviating from the inventive concept. 
       FIG. 4  shows schematically a fourth embodiment of the invention, where the reinforcement loops  11 - 11 ″ are placed concentric with respect to each other. The reinforcement loop  11  has the longest length, the reinforcement loop  11 ′ being somewhat shorter, while the reinforcement loop  11 ″ has the shortest length. According to such embodiment, it is possible, by means of the loops  11 - 11 ″, to place the major part of the reinforcement in sections where the need of a reinforcement cross-section is largest. The concrete element shown in  FIG. 4  may for example be a beam supported at each end. According to this solution, the bending moments may be largest at the middle portion of the beam and consequently, this portion requires the heaviest reinforcement. Such embodiment results in the most optimal use of the material volumes. 
       FIGS. 5 and 6  show an example of the use of the reinforcement loops  11  according to the invention, used in relation to one possible embodiment, where each end of the loops  11  are wound around a cylindrical tube  18 . According to the embodiment shown in  FIGS. 5 and 6 , the concrete structure forms a part of a floating pier  20  of the type comprising several elements which are tied together, intended to form for example a long, modularized floating pier or the like.  FIG. 5  shows a horizontal section through the floating element  20 , while  FIG. 6  shows a part where only the cylindrical tubes  18  and the reinforcement loops are shown. According to this embodiment the cylindrical tubes  18  are formed of cylindrical steel tubes, positioned at the corners of the floating body  20 . It should be appreciated, however, that the cylinders  18  also may be made of materials other than steel, such as other types of metal or plastic materials. As for the previously shown embodiments, the reinforcement loops  11  are wound around pairs of adjacent cylindrical tubes  18 , both in longitudinal direction and in transverse direction of the floating body  20 .  FIGS. 5 and 6  show only those loops  11  which are wound in the longitudinal direction of the floating body  20 . 
     In order to facilitate interconnection of two adjacent floating bodies  20 , or tying an element to a shore anchor point  22 , each of the corners, in relation to the cylindrical bodies  18 , is provided with recesses  21 . Correspondingly, the cylindrical bodies  18  are provided with an opening and a flange  24  provided with a hole, forming a supporting surface for a tie rod  23  or the like, for inter-connecting or tying together one floating body with another floating body or to the anchor point on shore. The tie rod  23  may be attached inside the cylindrical body  18  by means of an anchor plate  25  so that the tie rod may be tightened up. As shown in  FIG. 5 , only one such tie rod  23  is shown. It should be appreciated, however, that that such tie rod  23  may be employed in respect to each of the cylindrical bodies  18  in order to fix the floating body to shore anchors  22  or for tying two adjacent neighbouring floating bodies  20  together. The arrow P indicates the direction of the pulling force, acting on the floating body  20  at the corner. 
     It should be appreciated that the attachment and the tie-in of the tie rod may be done in any way known to a person skilled in the art. 
       FIG. 7  shows a vertical section through the floating body  20  shown in  FIG. 5 , where the reinforcement loops  11  and two cylindrical bodies  18  are shown. As shown, the reinforcement, together with the cylindrical bodies, are arranged in the upper half of the buoyancy body. 
       FIGS. 8 and 9  shows schematically a possible way to fabricate the fibres forming part of the reinforcement and showing a way to fabricate the loops. In the first part of the production line, as illustrated in  FIG. 8 , a larger number of continuous single fibres or filaments  26  are drawn or pulled from a corresponding number of filament or fibre spools or reels R 1 . The fibres  26  are firstly collected and fed down into a bath of a floating plastic materials or a matrix  27 , in order to become impregnated. The collected fibre bundle  29  may preferably be pulled by means of driven rolls, such as the ones identified by the reference numbers R 2  and R 3 . The impregnated fibre bundle is the pulled over a roller R 4 , pulling the bundle out of the bath, possibly by pre-tensioning the bundle, which may be obtained by a pulling means  28  comprising a pair of rollers. These rollers  28  may also function as a means for squeezing out the possible surplus of uncured plastic materials or matrix which the fibre bundle is impregnated with. From the rollers  28  the impregnated fibre bundle  29  is pull for example for winding around a drum shaped body as indicated in  FIG. 9 . 
       FIG. 9  shows an impregnated, but not yet cured fibre bundle  29  which is wound around two elongated cylindrical drums  30 . The drums  30  may be interconnected by means of one or more arms  31  which at their middle point may be supported by a shaft  32  which is parallel with the axis of the drum. By rotating the interconnected drums  30  around its axis  32 , impregnated but yet not cured fibre bundles  29  are wound onto each other, forming a loop shaped reinforcement  11 . 
       FIG. 10  shows a section through the fibre bundle  29 , seen along the line A-A in  FIG. 9 . The fibre bundle  29  is wound on the drum body  30 , 31 , 32 , so that the fibre loop  11  is given a more or less circular cross section, as shown in  FIG. 10 . Alternatively, the fibre bundle  29  may be wound onto the drum so that the cross section becomes more or less oval. 
     When winding of a loop  11  is completed to the desired shape and dimension, the exterior of the loop may be coated with a particle shaped material, such as sand, and thereupon the loop is cured in a suitable manner. It should be appreciated that the particle shaped material shall adhere only to the external surface of the bundle, so that the fibres inside the bundle  29  are not exposed to sharp particle surfaces. The purpose of the particle shaped material coated on the exterior of the loops  11  is to secure proper bonding between the concrete and the fibre bundle when concreted. 
     In case the reinforcement shall have a different shape, such as for example elongated loops which wind to and fro, then the method for manufacturing the impregnated, but yet not cured fibre bundle  29  will correspond to the method described in respect to  FIG. 9 . The fibre bundle  29  is then wound around a specifically developed template, giving the required reinforcement shape, whereupon a particle shaped material is applied to the uncured surface of the fibre bundle  29  prior to curing in any suitable way. 
     The fibre material used in the fibre bundle  29  may according to the present invention be formed for example of a material with a very high melting point, for example exceeding 1000° C., while the impregnating material or the matrix may for example be made of a plastic material, such as thermo plastics. Carbon or basalt may be a suitable material for the fibre filaments  26 . 
     A substantial advantage of using fibre materials of this type is that a major part of the reinforcing effect will be maintained even if the concrete structure is exposed to very temperatures, for example caused by a fire. Even if the impregnating material/matrix is melted or burned away, which may occur at a temperature around 200° C., the continuous fibre bundle will still be positioned inside its “concrete corridor”, more or less free of oxygen. Since oxygen is not present, materials such as carbon and basalt or similar type of materials, may withstand very high temperatures, such as 1000° C. or more. 
     If the reinforcement loop is made of a thick fibre bundle, wound few times around the loop, such a fibre bundle will be pulled out of its “corridor” after the fire. If the reinforcement loop according to the present invention is made of thinner fibre bundles, wound around the loop a very large number of times, the loop will able to withstand substantial tension even when the impregnating material/matrix has evaporated away. 
     Unless otherwise explicitly specified in the text, it should be appreciated that the term loop also shall include winds or helixes, formed of the fibre strings or bundles according to the invention. 
     Although cylindrical bodies are described above, it should be appreciated that the term “cylindrical bodies” includes a body where the surfaces, around which the fibre reinforcement is wound, are curved. The part of the cylindrical body which is not intended to be in contact with the fibre reinforcement may have any suitable shape. It should further be appreciated that the cylindrical body either may be solid and compact or may be hollow without deviating from the inventive idea. 
     Further, it should be appreciated that that the fibre loops may range from thick and long to short and thin. In combination or separate, the long and thick loops may take the tensile forces, while use of a large number of short loops may prevent, or at least reduce, spalling of the concrete caused by quick increase in temperature in case of fires. This may be due to the fact that a single loop will function, even if the heat from the fire has carbonized or evaporated away the matrix. 
     Further, its should be appreciated that although the loops are oval, they may still have a more or less rounded shape. 
     Small loops according to the invention are suitable for use in respect to gunite, and the loops may also prevent formation of cracking and minute cracks in the concrete.