Patent Publication Number: US-2002007604-A1

Title: Intermediate anchorage for concrete structures

Description:
TECHNICAL FIELD OF THE INVENTION  
       [0001] The present invention relates to intermediate anchorages for the unbonded post-tensioning tendons used to reinforce concrete slabs and other concrete structures.  
       BACKGROUND OF THE INVENTION  
       [0002] Concrete slabs and other structures are used to form floors, walls, and other elements of buildings. Each such concrete structure is typically reinforced by placing an unbonded post-tensioning tendon in a form (usually a wood form) prior to pouring of the concrete and then tensioning the tendon after the concrete has been poured into the form and has set. This reinforcement permits the concrete structure to bear greater loads than would otherwise be the case.  
       [0003] The tendon is composed of a greased high strength steel cable contained within a sheathing. Additionally, the tendon is usually provided with two terminal anchors, one at each end of the concrete structure that is to be reinforced. The tendon is installed in the form and the concrete is poured and allowed to set. The tendon is then tensioned, and the tension on the tendon is locked off at the terminal anchors. In corrosive environments such as parking garages, corrosion protection is used to protect the tendon and the anchors. Accordingly, it is known to provide sleeves and seals behind the terminal anchors and caps and seals on the frontsides of the terminal anchors.  
       [0004] When concrete slabs and other structures are so large that they require two or more pours, intermediate anchors between the terminal anchors are added to the tendon at the construction joint between the different pours. In order to install an intermediate anchor, the intermediate anchor is positioned at the construction joint and the first pour of concrete is made. After this concrete has set, sheathing is stripped from the tendon on the frontside of the intermediate anchor so as to expose a long portion of the cable. A jack is applied so that it grips this exposed portion with wedges (tensioning grippers) and is operated to tension the tendon. A wedge is then installed in the intermediate anchor so that it grips the cable where the sheathing has been stripped away to thereby hold the tension. The length of sheathing that is stripped away is typically equal to the length of the jack and, therefore, a long length of the cable is exposed.  
       [0005] As in the case of the terminal anchors, corrosion is also a problem where intermediate anchors are used because, even though the intermediate anchor is buried in concrete, concrete is porous allowing water to carry corrosive chemicals such as salt to the portion of the tendon that has been stripped of sheathing. Therefore, the tendon at the intermediate anchor must also be provided with corrosion protection.  
       [0006] This corrosion protection is traditionally provided by covering the stripped portion of the tendon on the backside of the intermediate anchor with a split plastic tube or a long solid plastic tube with split seals. Split seals are also provided on the frontside of the intermediate anchor in an attempt to connect back to the sheathing. However, these traditional corrosion protection systems do not provide adequate corrosion protection.  
       [0007] The present invention is directed to a sealing arrangement and method for use in connection with an intermediate anchor that overcomes one or more of the problems of the prior art.  
       SUMMARY OF THE INVENTION  
       [0008] In accordance with one aspect of the present invention, a method comprises the following: (i) placing an intermediate anchor having a wedge hole at a concrete construction joint; (ii) inserting a tendon through the intermediate anchor leaving a sheathing of the tendon substantially intact; (iii) making a single cut circumferentially around the sheathing inside the wedge hole; (iv) tensioning the sheathed tendon following setting of concrete poured on a bearing side of the concrete construction joint, whereby the tensioning causes the sheathing to pull away from the single cut so as to thereby create an exposed portion of the tendon; and, (v) gripping the exposed portion with a wedge to hold the tension.  
       [0009] In accordance with another aspect of the present invention, an intermediate anchor system for a tendon comprises an intermediate anchor and backside and frontside seals. The intermediate anchor has a backside, a frontside, and a wedge hole arranged to receive a wedge. The backside and frontside seals are at the backside and the frontside, respectively, of the intermediate anchor. The backside and frontside seals are arranged to seal an exposed portion of the tendon within the wedge hole, and the exposed portion of the tendon is confined to the wedge hole.  
       [0010] In accordance with still another aspect of the present invention, an intermediate anchor system comprises a tendon, an intermediate anchor, a wedge, a backside seal, and a frontside seal. The tendon has a greased cable within a sheathing, the tendon has an exposed portion, and the exposed portion has no sheathing. The intermediate anchor having a backside, a frontside, and a wedge hole arranged to receive a wedge. The sheathed tendon extends through the intermediate anchor so that the exposed portion is within the wedge hole. The wedge is within the wedge hole and is clamped to the exposed portion of the sheathed tendon. The backside seal engages the sheathed tendon at the backside of the intermediate anchor. The frontside seal engages the sheathed tendon at the frontside of the intermediate anchor. The backside and frontside seals seal the exposed portion of the sheathed tendon.  
       [0011] In accordance with yet another aspect of the present invention, a method comprises the following: (i) placing an intermediate anchor having a wedge hole at a concrete construction joint; (ii) inserting a tendon through the intermediate anchor leaving a sheathing of the tendon substantially intact; (iii) sealing the intermediate anchor on a bearing side of the concrete construction joint; (iv) making a cut circumferentially around the sheathing in the wedge hole; (v) tensioning the sheathed tendon following setting of concrete poured on the bearing side of the concrete construction joint, thereby creating an exposed portion of the tendon; (vi) gripping the exposed portion with a wedge to hold the tension; and, (vii) sealing the intermediate anchor on a stressing side of the concrete construction joint.  
       [0012] In accordance with a further aspect of the present invention, an intermediate anchor is provided to anchor a tendon in concrete. The tendon has an outside diameter. The intermediate anchor has an O-ring to provide a seal between the intermediate anchor and the tendon, and the O-ring has an inside diameter. The inside diameter of the O-ring is sufficiently larger than the outside diameter of the tendon in order to permit the O-ring to move freely over the tendon during installation. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
     [0013] These and other features and advantages of the present invention will become more apparent from a detailed consideration of the invention when taken in conjunction with the single FIGURE of the drawing which is a partially sectioned side view of a tendon and an intermediate anchor system according to at least one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION  
     [0014] An intermediate anchor system  10  according to one embodiment of the present invention is shown in the drawing. The intermediate anchor system  10  includes an anchor main body  12  (usually a ductile iron casting) and a cover  14  (usually a plastic coating) together forming an encapsulated intermediate anchor  16 . The cover  14  has a neck  18  on a backside of the encapsulated intermediate anchor  16 . The neck  18  receives a neck  20  of an adaptor  22 . The neck  20  of the adaptor  22  has an external rib  24  that is formed around its perimeter and that mates with an internal recess  26  of the neck  18  so that the adaptor  22  is positively coupled to the encapsulated intermediate anchor  16  of the intermediate anchor system  10 . The adaptor  22  also has an internally threaded portion  28  that receives a backside O-ring seal  30  and an externally threaded backside bushing  32 .  
     [0015] The encapsulated intermediate anchor  16 , the adaptor  22 , the backside O-ring seal  30 , and the externally threaded backside bushing  32  are located on a bearing side  34  of a construction joint  36 . Accordingly, during installation of the intermediate anchor system  10 , the encapsulated intermediate anchor  16  is suitably attached to a form  37  such as by nailing it to the form  37  at the construction joint  36 . A tendon  38  is inserted through the externally threaded backside bushing  32 , the backside O-ring seal  30 , the adaptor  22 , and the encapsulated intermediate anchor  16 , and is placed in the form  37  so that, when the concrete is poured into the form  37  during a first pour on the bearing side  34 , the tendon  38  will have its proper location. The tendon  38  is typically composed of a plastic sheathed high strength steel cable that is greased inside of the plastic sheathing.  
     [0016] The adaptor  22  is affixed to the encapsulated intermediate anchor  16  by inserting the neck  20  into the neck  18  until the external rib  24  snaps into the recess internal recess  26  of the neck  18 . The externally threaded backside bushing  32  is then threaded into the adaptor  22  until the backside O-ring seal  30  is forced into sealing engagement with the tendon  38  and the adaptor  22 . Accordingly, the backside O-ring seal  30  together with the adaptor  22  and the tendon  38  seals the encapsulated intermediate anchor  16  on the bearing side  34  of the construction joint  36 . A secondary O-ring  30   a  provides a seal between the adapter  22  and the neck  18  of the encapsulated intermediate anchor  16 .  
     [0017] An open cap  40  is applied to a frontside of the encapsulated intermediate anchor  16  and has a rib  42  around an external perimeter of the open cap  40 . The encapsulated intermediate anchor  16  has a complementary rib  44  around an internal perimeter thereof. The open cap  40  has an internally threaded portion  46  that threadably engages an externally threaded frontside bushing  48 . The open cap  40 , the externally threaded frontside bushing  48 , and an O-ring  50  are placed on the tendon  38 , but are not engaged to one another until tensioning of the tendon  38  on the bearing side  34  is complete. To permit the O-ring  50  to move easily over the tendon  38 , the inner diameter of the O-ring  50  is at least somewhat larger than the outer diameter of the tendon  38 . This feature is important because intermediate anchors are applied to tendons off-site and the tendons with their corresponding intermediate anchors mounted thereon are then moved to the site for final installation. Therefore, it is frequently necessary to move intermediate anchors over long runs of their tendons which could damage the O-rings if the O-rings cannot move freely over their tendons.  
     [0018] Concrete may then be poured on the bearing side  34  of the construction joint  36 . A pocket former may be applied to the encapsulated intermediate anchor  16  prior to pouring of the concrete in order to prevent concrete from flowing into a wedge hole  52  of the encapsulated intermediate anchor  16 . After pouring, this pocket former may then be removed.  
     [0019] Once the concrete sets on the bearing side  34 , a circumferential cut is made around the sheathing of the tendon  38  within the wedge hole  52  of the encapsulated intermediate anchor  16 . The tendon  38  is then tensioned by placing the nose of a hydraulic jack against the encapsulated intermediate anchor  16  on a stressing side  54  of the construction joint  36 . A gripper wedge  56  is placed on the tendon  38  at the opposing end of the hydraulic jack so that the gripper wedge  56  penetrates the sheathing of the tendon  38  and engages the steel cable within the sheathing. The tendon  38  is tensioned with the gripper wedge  56  by use of the jack.  
     [0020] During tensioning, the steel cable of the tendon  38  stretches. Because the sheathing is bonded to the cured concrete on the bearing side  34 , the sheathing of the tendon  38  in the wedge hole  52  will open as the steel cable stretches, thereby creating an exposed portion  58  in the sheathing. After a proper amount of tensioning has been applied to the tendon  38 , a permanent wedge  60  surrounding the exposed portion  58  of the tendon  38  is placed into the wedge hole  52  of the encapsulated intermediate anchor  16 . When the jack is released, force is transferred from the gripper wedge  56  to the permanent wedge  60  so as to lock off the tension in the tendon  38  on the bearing side  34  of the construction joint  36 .  
     [0021] The open cap  40  is then attached to the encapsulated intermediate anchor  16  so that the rib  42  is captured by the corresponding rib  44  to thereby lock the open cap  40  onto the encapsulated intermediate anchor  16  and to thereby compress a gasket  61  to form a seal between the open cap  40  and the encapsulated intermediate anchor  16 . The externally threaded frontside bushing  48  is threaded into the internally threaded portion  46  until the frontside O-ring seal  50  is biased against the tendon  38  and against the internally threaded portion  46  of the open cap  40  to form a seal around the tendon  38  to thereby seal the encapsulated intermediate anchor  16  on the stressing side  54  of the construction joint  36 .  
     [0022] Also, the gripper wedge  56  penetrates the sheathing during tensioning of the tendon  38 , thereby damaging the sheathing at that location. This damaged portion of the sheathing may be repaired such as by taping so as to restore the integrity of the sheathing.  
     [0023] Accordingly, the exposed portion  58  of the tendon  38  is sealed by the encapsulated intermediate anchor  16 , the adaptor  22 , the backside O-ring seal  30 , and the externally threaded backside bushing  32  on the bearing side  34  of the construction joint  36 , and by the encapsulated intermediate anchor  16 , the open cap  40 , the frontside O-ring seal  50 , and the externally threaded frontside bushing  48  on the stressing side  54  of the construction joint  36 . Thereafter, concrete may be poured into the form  37  during a second pour on the stressing side  54 .  
     [0024] Certain modifications of the present invention will occur to those practicing in the art of the present invention. Accordingly, the description of the present invention is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention. The details may be varied substantially without departing from the spirit of the invention, and the exclusive use of all modifications which are within the scope of the appended claims is reserved.