Patent Document

CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This is a divisional of application Ser. No. 10/357,128 filed on Feb. 3, 2003. 
     
    
     BACKGROUND OF INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The invention relates to the field of post tension systems for strengthening concrete. More particularly, the invention relates to an improved anchor and method for reducing corrosion on the wire strands of a post-tension tendon.  
         [0004]     2. Background Art  
         [0005]     Mono-strand tendons typically comprise a seven wire strand cable or tendon placed within a plastic or elastomeric sheath. A seven wire tendon is formed with six wires helically wrapped around a central core wire.  
         [0006]     Wire cable corrosion is a significant concern in post tension systems. Such corrosion occurs when water, salt and other corrosive agents contact the metallic tendon materials. Tendon failure typically occurs due to water intrusion into the interstices between the tendon and is typically concentrated at tendon ends or anchors.  
         [0007]     Such failure also occurs at portions of the tendon damaged segments caused during installation. The installation of tendons typically occurs in a rugged construction environment where the tendons can be damaged by equipment, careless handling and contact with various site hazards. When the elastomeric sheath is punctured, a water leak path contacting the wire tendon is established. The puncture must be patched to resist water intrusion between the sheath and tendon. The puncture and patch can create a discontinuity between the tendon and the sheath, and this discontinuity can impede proper installation and performance of the tendon.  
         [0008]     One conventional technique for providing extra protection in the corrosive environments is to increase the thickness of the plastic sheath covering the tendon. A plastic sheath at least forty mils thick can be formed around the tendon resist abrasion and puncture damage. Although this approach provides incremental protection against leakage, a thicker sheath does not provide redundant protection to the tendon steel.  
         [0009]     Another technique for providing extra protection in corrosive environments uses seals and grease-filled pockets for blocking water intrusion into the central tendon core. Oil or grease is pumped into the exposed tendon end to fill the interstices at the tendon ends, however this procedure does not protect the internal wire strands forming the tendon.  
         [0010]     Another technique for resisting high corrosion environments is to specially coat or otherwise treat the individual wire strand with an electrostatic fusion-bonded epoxy to a thickness between one and five mils thick. Similar wire coating techniques use galvanized wire and other corrosion resistant wires within the multiple wire cables to form a corrosion resistant tendon. Significant effort has been made to create improved corrosion resistant materials compatible with the exterior sheaths and resistant to corrosion. Corrosion resistant materials typically have an affinity to metal and are capable of displacing air and water. Additionally, such materials are relatively free from tendon attacking contaminants such as chlorides, sulfides and nitrates. However, such tendons are expensive and the effectiveness of such corrosion resistant materials may not resist corrosion after the tendon is damaged.  
         [0011]     Tendon corrosion typically occurs near the post-tension anchors because the outer sheath is removed from the wire tendon at such locations. To protect the bare wire from corrosion, protective tubes are connected to the anchor and are filled with the grease or other corrosion preventative material. This conventional practice is demonstrated by different post-tension systems. For example, U.S. Pat. No. 5,271,199 to Northern (1993) disclosed tubular members and connecting caps for attachment to an anchor. U.S. Pat. No. 5,749,185 to Sorkin (1998) disclosed split tubular members for attachment to and anchor and for installation over the tendon. U.S. Pat. No. 5,897,102 to Sorkin (1999) disclosed a tubular member having a locking surface for improving the connection to an anchor, and a cup member and extension for engagement on the other side of the anchor. U.S. Pat. No. 6,027,278 to Sorkin (2000) and U.S. Pat. No. 6,023,894 to Sorkin (2000) also disclosed a tubular member having a locking surface to improve the connection to an anchor. U.S. Pat. No. 6,098,356 to Sorkin (2000) disclosed attachable tubular members filled with corrosion resistant grease.  
         [0012]     A need exists for an improved post-tension seal for preventing fluid intrusion into the inner part of a post-tension anchor. The system should be compatible with existing installation procedures and should resist the risk of water intrusion into contact with internal tendon wires.  
       SUMMARY OF THE INVENTION  
       [0013]     The invention provides an anchor for engagement with a post-tension tendon. The anchor comprises an anchor base having an aperture oriented along a centerline for permitting insertion of the tendon therethrough, wherein the aperture has first and second surfaces each having different shape relative to said aperture centerline, and wherein the first and second surfaces continuously enlarge the size of the aperture from one side of the anchor base to another side of said anchor base. A sheath is engaged with the anchor base and includes a cylindrical extension having a contact end distal from the anchor base for contacting the tendon as the tendon is inserted through the cylindrical extension and the anchor base aperture.  
         [0014]     In other embodiments of the invention, the cap includes a cap extension having a hollow interior for permitting passage of the tendon therethrough, and the exterior surface of the cap extension can be engagable with a pocketformer. A lock can retain the pocketformer in detachable engagement with the cap extension.  
         [0015]     In another embodiment of the invention, a post-tension anchor system comprises a post-tension tendon having a sheath and inner wire strands, an anchor base having a shaped aperture for permitting insertion of the tendon therethrough, a sheath engaged  3 ′ with the anchor base wherein said sheath includes a cylindrical extension having a contact end distal from the anchor base for contacting the tendon as the tendon is inserted through the cylindrical extension and the anchor base aperture, and a cap for sealing the tendon within the anchor base aperture. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  illustrates a mono-strand cable enclosed with a first sheath.  
         [0017]      FIG. 2  illustrates a second sheath around the first sheath.  
         [0018]      FIG. 3  illustrates a first sheath closely formed to the cable exterior surface.  
         [0019]      FIG. 4  illustrates a sectional view of an anchor base.  
         [0020]      FIG. 5  illustrates detail of a cap having different thread combinations  
         [0021]      FIG. 6  illustrates a ring cap for sealing the interior of an anchor base.  
         [0022]      FIG. 7  illustrates a cap extension attached to a cap.  
         [0023]      FIG. 8  illustrates a cap extension engaged with an anchor base.  
         [0024]      FIG. 9  illustrates one embodiment of a cap extension.  
         [0025]      FIG. 10  shows one embodiment of a cap having a removably engaged cap extension. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]     The invention provides a unique system for providing a post tension system resistant to corrosion. Each tendon typically comprises an exterior sheath surrounding at least two strands formed with a material such as carbon steel.  
         [0027]      FIG. 1  illustrates a sectional view wherein a mono-strand wire tendon  10 , formed with individual wire strands  12  about center wire  14 , is positioned within first sheath  16 . One or more wire strands  12  are helically wrapped about center wire strand  14  and form helical grooves on the exterior surface of cable  10 . Such helical grooves are cumulatively identified as shaped annulus  18  defining the space between tendon  10  and the interior cylindrical surface of first sheath  16 .  
         [0028]     Because wire strands  12  are circular in cross-section, spaces between adjacent wire strands  12  and center wire  14  are cumulatively identified as cable interior interstices  20 . As shown in  FIG. 1 , annulus  18  and interstices  20  are filled with corrosion resistant material  22 . Grease or another suitable material can be used for corrosion resistant material  22  to eliminate air pockets and to resist water intrusion into contact with wire strands  22 . By filling annulus  18  with a lubricant or corrosion resistant material  22 , the interior surface of first sheath  16  can be substantially cylindrical in one embodiment of the invention.  
         [0029]      FIGS. 2 and 3  illustrate second sheath  26  formed about first sheath  16 . Annulus  28  is formed between second sheath  26  and first sheath  16  and is filled with a lubricant  30  to facilitate sliding movement therebetween. Lubricant  30  can comprise a corrosion resistant material similar to material  22 . Grease or another lubricant is place on the outer surface of the seven strand wire tendon adjacent to the elastromeric sheath to resist corrosion created by air and water infiltration between the tendon and the sheath. In  FIG. 2  annulus  28  is substantially cylindrical. In  FIG. 3  first sheath  16  is tightly formed about the exterior surface of tendon  10  and helical grooves, filled with corrosion resistant material, are formed in the exterior surface of the first sheath  16 . This feature preferably uses a material for first sheath  16  having a thickness less than then ten mils. Conventional membranes are typically twenty-five mils thick for regular systems and forty mils thick for high corrosion resistant, encapsulated systems. By providing a slim first sheath  16  about tendon  10  to create grooves in the exterior surface of first sheath  16 , corrosion resistant material  30  can be stored in annulus  28  to resist intrusion by water of other contamination into contact with first sheath  16  or tendon  10 .  
         [0030]      FIG. 4  illustrates post-tension anchor comprising base  30  having shaped aperture  32 . Base  30  is formed with a cast metal material suitable for handling large compressive loads. Sheath  34  can be attached to base  30  in one embodiment of the invention and includes cylindrical extension  36  having a contact end  38  distal from base  30 . Contact end  38  is preferably at least four inches distal from base  32 , however shorter or longer lengths are possible within the usable scope of the invention. The inner surface of contact end  38  is preferably circular in cross-section for contacting the exterior surface of tendon  10  as tendon  10  is inserted through cylindrical extension  36  and base aperture  32 . Seal  40  can be positioned between contact end  38  and tendon  10  to restrict liquid intrusion into the inside of the cylindrical extension  36 .  
         [0031]     Cap  42  has threads  44  engaged with threadform  46  on sheath  34 . Cap  42  includes shaped end  48  configured to facilitate rotatable engagement and disengagement of cap  42  relative to sheath  34 . As illustrated, shaped end  48  can be a polygonal configuration such as a hexagonal or other shaped form suitable for engagement with a socket wrench. In other embodiments of the invention shaped end can be configured to be engagable with different drive mechanisms such as screwdrivers, wrenches, pliers and other devices. Grease  50  can be positioned within cap  42  to seal the end of tendon  10  placed therein.  
         [0032]     In one embodiment of the invention threads  44  can include a double start lead to facilitate attachment of cap  42  to sheath  34 . The double start lead can comprise threads having different sizes and pitches to provide different make-up characteristics.  FIG. 5  illustrates cap  42  and base  30  in expanded position and displays cap  42  having different threadforms  52  and  54  for selective engagement with correlating threadforms on sheath  34 . As shown in  FIG. 6 , cap  42  can also have indicator tab  56  which flares upwardly when cap  42  is fully engaged with base  30 . Such feature provides a visual indication of full engagement and an effective watertight seal between cap  42  and base  30 . As also can be seen in  FIG. 5 , the extension  36 , having seal  40  therein at the distal end  38  is formed integrally with the sheath  34 .  
         [0033]      FIG. 4  illustrates the installation of wedges  58  in contact with tendon  10  and base  30 . Wedges  58  are installed into such position after cap  42  has been removed from engagement with sheath  34  and base  30 . The invention permits wedges  58  to be installed directly against first sheath  16  or second sheath  26  of cable  10  so that wedges  58  contact wire strands  12  with minimal disruption to sheaths  16  or  26 . This feature of the invention reduces the amount of wire strands  12  requiring field repair and sealant and significantly reduces installation time and possibility of corrosion base upon failure of such field repairs. Because cap  42  is reusable, cap  42  can be reinstalled with base  30  to seal the interior of base  30 . Alternatively, another structure such as ring cap  60  can be positioned over tendon  10  to seal the interior of base  30  as shown in  FIG. 6 .  
         [0034]      FIG. 7  illustrates in exploded detail cap extension  62  integrated within cap  42 . Cap extension  62  can also comprise a separate component attached to cap  42  with snap connections, tape, threadforms, or other techniques. Cap extension  62  provides the function of extending the useful length of cap  42 , thereby permitting a longer length of tendon (not shown) to extend beyond wedges  58  within base  30  as illustrated in  FIG. 8 . Extension end  64  can be open as illustrated to permit the passage of tendon  10  therethrough or can be closed. Lock nut  66  having threadform  68  can be engaged with threadform  70  on cap extension  62  to retail a pocketformer or other apparatus or to provide a closure for the open end of extension end  64 .  
         [0035]     An example of a cap extension  62  is shown in  FIG. 10  as a separate element coupled to the cap by means of threads  73  such that the extension may be selectively engaged with the cap  42 .  
         [0036]      FIG. 9  illustrates another embodiment of cap extension  72  wherein extension tube  74  has threadform  76  and seal  78 . Lock ring  80  has threadform  82  for engagement with base  30  and for retaining extension tube  74  in a fixed position relative to base  30 . The combination of lock ring  80  and extension tube  74  significantly facilitates manufacture of extension  72 .  
         [0037]     The invention provides superior anti-corrosion protection through the entire tendon length and especially near the point of engagement with post-tension anchors. The sheath materials for tendon  10  can be selected from material classes such as nylon, polymers, metals, or other organic or inorganic or mineral or synthetic materials. An outer second sheath can be formed with a tough material resistant to punctures and stretching damage, while an interior first sheath can be formed with another material for retaining the corrosion resistant material.  
         [0038]     The configuration of base  30  permits installation and tensioning of tendon  10  without removal of sheath  16  from tendon  10  at the location of base  30 . By avoiding the disturbance of the manufactured sheath  16 , the most sensitive point of corrosion is completely eliminated. The configuration of the caps and pocket formers described in cooperation with base  30  significantly reduces labor time and cost and provides superior reliability during installation. Such reliability reduces field damage to post tension components and the possibility of corrosion resulting from such damage, and eliminates the need for costly and unreliable field repairs.  
         [0039]     Although the invention has been described in terms of certain preferred embodiments, it will become apparent to those of ordinary skill in the art that modifications and improvements can be made to the inventive concepts herein without departing from the scope of the invention. The embodiments shown herein are merely illustrative of the inventive concepts and should not be interpreted as limiting the scope of the invention

Technology Category: 4