Abstract:
An apparatus and method for reducing corrosion in post-tension construction is described. An anchor is engagable with a post-tension tendon and comprises an anchor base and sheath engaged with the anchor base, and a cap for sealing the portion of the tendon within the anchor. The sheath can include an extension having a contact end distal from the anchor base for contacting the tendon as the tendon is inserted through the extension and the anchor base aperture. The cap can extend completely through the anchor base for connection to the anchor base of a sheath or sheath extension attached to the base. A pocketformer is attachable to the sheath for generating a void in concrete.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    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.  
           [0002]    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.  
           [0003]    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.  
           [0004]    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.  
           [0005]    One conventional technique for providing extra protection in 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.  
           [0006]    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.  
           [0007]    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.  
           [0008]    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 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.  
           [0009]    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  
         [0010]    The invention provides an anchor and pocketformer for engagement with a post-tension tendon. The apparatus comprises an anchor base having a shaped aperture for permitting insertion of the tendon therethrough, a sheath engaged 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 pocketformer detachably engagable with the sheath.  
           [0011]    In different embodiments of the invention, the pocketformer can comprise a spindle and a pocketformer body engagable with the spindle. Either the spindle or the pocketformer can be attachable to the sheath, and the spindle can extend through the anchor base to provide a continuous path for insertion of the tendon therethrough. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 illustrates a mono-strand cable enclosed with a first sheath.  
         [0013]    [0013]FIG. 2 illustrates a second sheath.  
         [0014]    [0014]FIG. 3 illustrates a first sheath closely formed to the cable exterior surface.  
         [0015]    [0015]FIG. 4 illustrates an exploded view of a base, spindle, pocketformer and retainer cap.  
         [0016]    [0016]FIG. 5 illustrates a cap and spindle directly attachable to a base sheath.  
         [0017]    [0017]FIG. 6 illustrates a pocketformer integrated with a spindle.  
         [0018]    [0018]FIG. 7 illustrates a sheath cutter. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]    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.  
         [0020]    [0020]FIG. 1 illustrates a sectional view wherein 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 .  
         [0021]    Because wire strands  12  are circular in cross-seciton, 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.  
         [0022]    [0022]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 elastomeric 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 first sheath  16 . This feature preferably uses a material for first sheath  16  having a thickness less than 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  which is capable of fitting tightly 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 .  
         [0023]    [0023]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  is attached to base  30  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  30 , 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 cylindrical extension  36 .  
         [0024]    [0024]FIG. 4 illustrates one embodiment of the invention in expanded form wherein extension  36  includes threadform  42  proximate to base  30 . Spindle  44  is attachable to threadform  42  with threadform  48  formed on a first end of spindle  44 . By inserting spindle  44  completely through anchor base  30 , a continuous path is created for insertion of tendon  10  therethrough.  
         [0025]    Spindle  44  can be substantially shaped as a cylinder having hollow interior  50  for receiving tendon  10  therethrough, however other shapes can be used to accomplish the function described herein. A second end of spindle  44  has threadform  52  for connection to cap  54 . Cap  54  can provide the function of locking pocketformer  56  onto spindle  44  and can have aperture  58  therethrough for permitting withdrawal of tendon  10  therethrough. Threadform  60  provides rotatable engagement with threadform  52 . In another embodiment of the invention cap  54  can be closed to seal the interior of spindle  44  from entry of contaminants into hollow interior  50 .  
         [0026]    In the inventive embodiment shown in FIG. 4, a locked connection between extension  36  and spindle  44  is accomplished without requiring threads or other connector within base  30 . This feature of the invention saves time in the field by permitting quick installation and detachment while eliminating the need for expensive milling of threads into the metallic components of base  30 . This feature of the invention also permits factory assembly of corresponding components before such components are shipped to the field for installation.  
         [0027]    Seal end  62  of pocketformer  56  can be shaped to provide a tight fit with sheath  32 . Preferably such fit can be configured so that engagement of cap  54  urges pocketformer  56  into a fluid tight seal with sheath  32 . Alternatively, a seal (not shown) can be inserted therebetween.  
         [0028]    [0028]FIG. 5 illustrates another embodiment of the invention wherein spindle  64  has an enlarged first end  66  having threadform  68  for rotational engagement with threadform  70  in sheath  32  as shown in FIG. 4. Cylindrical body  72  of spindle  64  includes threadform  74  for engagement with cap  54  to secure pocketformer  56  as described for FIG. 4. This embodiment of the invention provides for spindle  64  to be attached directly to sheath  32  without modifying the configuration of body  30 .  
         [0029]    In another embodiment of the invention as shown in FIG. 6, spindle and pocketformer can be integrated into a single component shown as pocketformer  76  having threadform  78  for rotatable engagement with sheath  32 , spindle section  80  having aperture  82  for permitting passage of tendon  10  therethrough, and threadform  84  on an exterior surface of spindle section  80  for engagement with sealing cap  54 . Cap  54  can selectively provide a seal for closing aperture  82  from fluid intrusion. Alternatively, threadform  84  can provide a connection for an extension tube (not shown) similar to extension  36  extending to a location distal from base  30 .  
         [0030]    Referring to FIG. 4, spindle  44  is capable of extending through base  30  because of the unique formation of shaped aperture  32  therethrough. In one embodiment of the invention as illustrated, shaped aperture  32  can comprise an aperture having a compound surface having at least two different surfaces with different shapes or angles relative to the longitudinal axis illustrated. Surface  86  comprises a truncated conical surface at an angle two degrees from the longitudinal axis. Although such angle is two degrees, the angle can be changed to range between two and five degrees within the scope of the invention. Surface  88  comprises a truncated conical surface seven degrees from the longitudinal axis or centerline, which is the standard angle used in the industry from wedges. The combination of multiple surfaces  86  and  88  permits a larger aperture size to be created through anchor base  30 , thereby permitting the insertion of spindle  44  therethrough. Such configuration continuously enlarges the size of the aperture, thereby preventing restrictions which might impede insertion of tendon  10  therethrough.  
         [0031]    [0031]FIG. 7 illustrates another embodiment of the invention wherein sheath cutter  90  is integrated within anchor base  30  for the purpose of stripping either sheath  16  or sheath  26  or both (if present). By locating cutter  90  in such position, the outer sheath of tendon  10  is automatically stripped as tendon  10  is inserted through base  30 . This feature of the invention dramatically saves installation time and results in a cleaner sheath cut than typically possible in field installations. Various configurations of such cutter are possible, permitting the partial or complete removal of sheath material from the end or middle section of tendon  10 .  
         [0032]    The invention provides superior anti-corrosion protection through the entire tendon length, and especially around the point of engagement with post-tension anchors. The sheath materials 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.  
         [0033]    The configuration of base  32  permits installation and tensioning of tendon  10  without removal of sheath  16  from tendon  10  at the location of base  32 . By avoiding substantial 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  32  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.  
         [0034]    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.