Abstract:
A stay-in-place composite form provides a strong and durable concrete structure. The form includes a composite shell having an inner wall surface defining an enclosure into which concrete may be poured and allowed to harden. The composite shell may be made of one or several layers of fabric having a resin matrix impregnated therein. The concrete hardens to form a concrete core within the enclosure and a liner is affixed to the inner wall surface of the composite shell to protect the composite shell from alkalinity in the concrete core. The liner includes at least one sheet of a water-impermeable material to protect the concrete core from water and other corrosive elements. The fabric layers are selected such that the fibers elongate as the concrete is poured into the enclosure due to a weight of the concrete and partially shrink back to compensate for shrinkage of the concrete as the concrete dries to form the concrete core. Such stay-in-place composite form can be used in prefabricated form to strengthen new constructions.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     This invention relates generally to concrete support structures and in particular, to stay-in-place forms (i.e., composite shells) for forming concrete support structures. 
     2. Description of the Related Art 
     Concrete columns are commonly used as upright supports for superstructures. Bridge supports, freeway overpass supports, building structural supports and parking structure supports are just a few of the many uses for concrete columns. Other concrete support members such as beams, walls, slabs, girders, struts, braces, etc. are employed to impart strength and stability to a large variety of structures. These concrete support structures exist in a wide variety of shapes. Typically, these concrete support structures have circular, square or rectangular cross-sections. However, numerous other cross-sectional shapes have been used including regular polygonal shapes and irregular cross-sections. The size of the concrete support structures also varies greatly depending upon the intended use. Concrete columns with diameters on the order of 2 to 20 feet and lengths of well over 50 feet are commonly used as bridge or overpass supports. 
     Conventionally, some concrete columns have been constructed by filling a cylindrical form having a network of rebar mounted therein with a concrete composition, allowing the composition to cure, and removing the form. 
     Also, in the past, elongate paper fiber tubes have been used to form concrete columns. The tubes are made by spirally winding several layers of strong fiber paper. The spirally wound paper is laminated along its seams with a special adhesive. The outside of the tube can be coated with hot wax for protection against adverse weather conditions. Concrete is poured into the tube and allowed to harden so as to form a column. After hardening, the tube is stripped away from the concrete column and scrapped. 
     Rather than paper tubes, reusable steel or wood forms can also be used. Concrete is poured into these forms and allowed to harden. After hardening, the form is removed from the concrete structure and can be used again. 
     All of these conventional concrete support structures are subject to deterioration of their long-term durability and integrity. Permeability of the exposed concrete by water can cause the concrete to deteriorate over time. When moisture is trapped in the concrete and freezes, cracks typically form in the concrete structural members. In addition, some of these conventional concrete support structures are located in earthquake prone areas but do not have adequate metal reinforcement or structural design to withstand high degrees of asymmetric loading. 
     More recently, composites have been used to repair and retrofit columns, beams, walls, tanks, chimneys and other structural elements. However, a need exists to use composites in a prefabricated form to strengthen new constructions, protect internal reinforcing steel, provide fiber reinforcement outside of a concrete layer, to provide better appearance features, and to solve the above problems. 
     SUMMARY OF INVENTION 
     A stay-in-place composite form in accordance with the present invention provides increased strength and durability to concrete support structures. The stay-in-place form can be used in prefabricated form or can be fabricated at the construction site, to strengthen new constructions. 
     The stay-in-place form includes a composite shell made up of fibrous fabric layers impregnated with a resin matrix. The composite shell has an inner wall surface defining an enclosure into which concrete may be poured and allowed to harden to form a concrete core. As the concrete is poured into the enclosure, the fibers in the fabric material elongate due to the weight of the concrete. Then, as the concrete dries, the fibers partially shrink back to compensate for shrinkage of the concrete. 
     In one embodiment of the present invention, the percentage of elongation of the resin matrix is greater than the percentage of elongation of the fibers. Typically, the percentage of elongation of the fibers and resin matrix prevents a gap from forming between the concrete core and the composite shell when the concrete shrinks. 
     A liner made of a water-impermeable material is affixed to the inner wall surface of the composite shell to protect the composite shell from alkalinity or other chemical products in the concrete core. This liner is in direct contact with an outer surface of the concrete core and either completely or partially surrounds the concrete core. 
     In one embodiment of the present invention, the stay-in-place form is manufactured using a rigid collapsible tubular member. The exterior surface of the tubular member is wrapped with the liner and then the fabric layers impregnated with resin are applied to the liner. Once the fabric layers cure, the tube is collapsed and removed from beneath the liner. What remains is a hollow stay-in-place composite form. 
     In yet another embodiment of the present invention, the stay-in-place form is manufactured using a mandrel. In such embodiment, the liner is applied to an exterior surface of the mandrel and then the fabric layers impregnated with resin are applied to the liner. Once the fabric layers cure, the liner and harden fabric layers are separated from the mandrel. Again, what remains is a hollow stay-in-place composite form. 
     In still another embodiment of the present invention, the collapsible tube or the mandrel is rotated about an axis while the fabric layer and the resin matrix is applied to the liner. Such rotation maintains the form of the tube and composite shell, and ensures that the resin is uniformly distributed. The rotation of the tube or mandrel continues until the resin impregnated fabric layers are fully cured. 
     These and other features and advantages of the present invention will become apparent by reference to the following detailed description and accompanying drawings which set forth several illustrative embodiments in which the principles of the invention are utilized. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective longitudinal view illustrating the stay-in-place form in accordance with the present invention; 
     FIG. 2 is a perspective longitudinal view illustrating a fully reinforced support structure using the stay-in-place form of the present invention; 
     FIG. 3 is a detailed sectional view of an exemplary reinforced composite material in accordance with the present invention; 
     FIG. 4 is a detailed sectional view of an alternative exemplary reinforced composite material in accordance with the present invention; 
     FIG. 5 depicts a weave pattern which is the same as the weave pattern shown in FIG. 4 except that the yarns are stitch bonded together; 
     FIG. 6 is a detailed partial section of the face of an external surface of composite shell covered with multiple fabric layers; 
     FIG. 7 is a perspective view of a protective liner; 
     FIG. 8 is a cross-sectional inner view of an alternate embodiment of the stay-in-place-form in accordance with the present invention; 
     FIG. 9 is a cross-sectional inner view of a second alternate embodiment of the stay-in-place-form in accordance with the present invention; 
     FIG. 10 is a cross-sectional inner view of a third alternate embodiment of the stay-in-place-form place-form in accordance with the present invention; 
     FIGS. 11A and 11B are a perspective longitudinal view and a cross-sectional inner view, respectively, illustrating a fourth alternate embodiment of the stay-in-place form in accordance with the present invention; 
     FIGS. 12A-12J are perspective views illustrating the steps of manufacturing a precast stay-in-place form constructed in accordance with the present invention; 
     FIG. 13 is a demonstrative representation depicting the impregnation of a fabric layer prior to application to the tubular form in accordance with the present invention; and 
     FIG. 14 is a perspective view illustrating application of a liner to a mandrel in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Stay-In-Place Form 
     Referring to FIG. 1, a perspective view of a stay-in-place form  100  for use as a support structure, such as a column or beam, is shown. Although stay-in-place form  100  is illustrated as an elongate tubular structure in FIG. 1, it will be appreciated that stay-in-place form  100  may be any desired shape, such as rectangular or octagonal. Stay-in-place form  100  includes an exterior composite shell  101  and a liner  103  secured to the inner surface of composite shell  101 . In this way, stay-in-place form  100  provides a hollow closed form into which a slurry of concrete or cement material  105  is placed. Slurry  105  fills stay-in-place form  100  and hardens to form a concrete core  205  of a fully reinforced support structure  200 , illustrated in FIG.  2 . 
     Composite shell  101  is formed of a resin-impregnated composite reinforcement layer  107 , as illustrated in FIG.  1 . Composite reinforcement layer  107  is in direct contact with the outer surface of liner  103  and may be made of a single layer of fabric, although typically reinforcement layer  107  is made up of multiple layers of fabric. In the exemplary embodiment illustrated in FIG. 1, composite reinforcement layer  107  is made of seven fabric layers  109 - 115 . Each of fabric layers  109 - 115  has first and second parallel selvedges. For example, the first and second selvedges for fabric layer  109  are shown at  109 A and  109 B, respectively. The first and second selvedges for fabric layer  110  are shown at  110 A and  110 B, respectively. In an exemplary embodiment, the width of the fabric between the selvedges may be from twelve to one hundred inches wide. Fabric layers  109 - 115  may include a single fabric layer or they may be laminates made up of two or more layers of fabric. 
     An exemplary fabric is shown in FIG.  3 . The fabric is preferably a plain woven fabric having warp yarns  301  and fill yarns  303 . The warp yarns  301  and fill yarns  303  may be made from the same fibers or they may be different. The fabric may be comprised of, for example, glass, carbon, boron, graphite, polyaramid, boron, Kevlar, silica, quartz, ceramic, polyethylene, aramid, or other fibers. A wide variety of types of weaves and fiber orientations may be used in the fabric. Where a single layer of fabric is used, it will often be desirable to use weft cloth containing both horizontal and vertical fibers. For example, composite reinforcement layer  107  may include vertical, horizontal and off-axis fibers which can minimize or eliminate the need for steel reinforcement in support structure  200 . Where multiple layers of fabric are used, it will often be desirable to alternate the orientation of the fibers to provide maximum strength along multiple axes. Typically, fibers oriented along the longitudinal axis provide stiffness of composite shell  101  whereas fibers oriented along the horizontal axis provide strength in the hoop direction or along the circumference of composite shell  101 . Such strengthening in the hoop direction prevents buckling of the longitudinal fibers and restricts the movement of concrete core  205  of support structure  200  in FIG.  2 . 
     Referring again to FIG. 3, the warp yarns  301  are preferably made from glass. The fill yarns  303  are preferably a combination of glass fibers  305  and polyaramid fibers  307 . The diameters of the glass and polyaramid fibers preferably range from about 3 microns to about 30 microns. It is preferred that each glass yarn include between about 200 to 8,000 fibers. The fabric is preferably a plain woven fabric, but may also be a 2 to 8 harness satin weave. The number of warp yarns per inch is preferably between about 5 to 20. The preferred number of fill yarns per inch is preferably between about 0.5 and 5.0. The warp yarns extend substantially parallel to the selvedge  309  with the fill yarns extending substantially perpendicular to the selvedge  309  and substantially parallel to the axis of the stay-in-place form  100 . This particular fabric weave configuration provides reinforcement in both longitudinal and axial directions. This configuration is believed to be effective in reinforcing the stay-in-place form  100  against asymmetric loads experienced by the support structure  200  of FIG. 2, during an earthquake. 
     A preferred alternate fabric pattern is shown in FIG.  4 . In this fabric pattern, plus bias angle yarns  401  extend at an angle of between about 20 to 70 degrees relative to the selvedge  403  of the fabric. The preferred angle is 45 degrees relative to the selvedge  403 . The plus bias angle yams  401  are preferably made from yarn material the same as described in connection with the fabric shown in FIG.  3 . Minus bias angle yarns  405  extend at an angle of between about −20 to −70 degrees relative to the selvedge  403 . The minus bias angle yarns  405  are preferably substantially perpendicular to the plus bias angle yarns  401 . The bias yams  401  and  403  are preferably composed of the same yarn material. The number of yarns per inch for both the plus and minus bias angle is preferably between about 5 and 30 with about 10 yarns per inch being particularly preferred. 
     It is preferred that the fabric weave patterns be held securely in place relative to each other. This is preferably accomplished by stitch bonding the yarns together as shown in FIG.  5 . An alternate method of holding the yarns in place is by the use of adhesive or leno weaving processes, both of which are well known to those skilled in the art. In FIG. 5, exemplary yarns used to provide the stitch bonding are shown in phantom at  501 . The process by which the yarns are stitch bonded together is conventional and will not be described in detail. The smaller yarns used to provide the stitch bonding may be made from the same materials as the principal yarns or from any other suitable material commonly used to stitch bond fabric yarns together. The fabric shown in FIG. 3 may be stitch bonded. Also, if desired, unidirectional fabric which is stitch bonded may be used in accordance with the present invention. 
     In FIG. 6, a portion of a composite reinforcement layer surrounding a concrete column is shown generally at  601 . The composite reinforcement layer  601  includes an interior fabric layer  603  which is the same as the fabric layer shown in FIG.  5 . In addition, an exterior fabric layer  605  is provided which is the same as the fabric layer shown in FIG.  3 . This dual fabric layer composite reinforcement  601  provides added structural strength when desired. 
     In another embodiment, the composite reinforcement layer  107  of FIG. 1 may have an inner layer of longitudinal axial fibers and an outer layer of circumferential hoop fibers. For example, the multilayer reinforcement material  107  may include a first reinforcement layer including two fabric layers of glass or carbon fibers in a longitudinal direction and a second high strength composite reinforcement layer including three layers of glass or carbon fibers in the hoop direction. In another embodiment, the high strength composite reinforcement layers have spiral layers. These fabric layers not only provide the structural integrity of the composite shell  101 , but also provide significant reinforcement against externally applied forces. 
     All of the fabric layers  109 - 115  must be impregnated with a resin in order to function properly in accordance with the present invention. Suitable resins for use in accordance with the present invention include polyester, epoxy, polyamide, bismaleimide, vinylester, urethanes and polyurea. Other impregnating resins may be utilized provided that they have the same degree of strength and toughness provided by the previously listed resins. Epoxy based resin systems are preferred. It is also preferred that the fiber and resin matrix are waterproof. 
     Referring again to FIG. 1, when slurry  105  is poured into stay-in-place form, the weight of slurry  105  elongates or stretches the fibers in reinforcement layer  107  causing these fibers to be stressed. Thus, liner  103 , reinforcement layer  107 , and the resin impregnated into reinforcement layer  107  are selected to permit elongation of the fibers when slurry  105  is poured into stay-in-place form  100 . In particular, the resin must be flexible enough to allow for such post-tensioning of the fibers. Having been elongated during the pouring of concrete  105 , the fibers are stressed, which strengthens the fibers and allows for reduced thickness of stay-in-place form  100 . These fibers will then partially shrink back or relax to compensate for concrete shrinkage as concrete slurry  105  dries. As a result, the final percent of elongation of the resin should be greater than percent of elongation of the fibers so that the reinforcement layer  107  does not crack from stress caused by the weight of the concrete. For example, in one embodiment the glass fibers have 2% elongation and the epoxy has 3-4% elongation. The percent of elongation of the resin should be balanced with the percent of elongation of the fibers so that there is some stress on the fibers from the weight of the concrete, but not so much so that there is cracking. With such a balance, the fibers are able to shrink back to compensate for concrete shrinkage once slurry  105  hardens without leaving any gaps between concrete core  205  and liner  103  of support structure  200 , illustrated in FIG.  2 . 
     Liner  103  is received to the inner wall surface of hollow composite shell  101 . A perspective view of liner  103  is illustrated in FIG.  7 . As shown, liner  103  is flexible so that it will conform to the inner wall surface of composite shell  101  regardless of the shape of the shell  101 . Referring again to FIG. 2, liner  103  is formed of a water-resistant and impermeable material to protect concrete core  205  from moisture and corrosive materials, as well as to protect the composite shell  101  from the alkalinity in concrete core  205 . Liner  103  can be fabricated from plastic or rubber materials such as polystyrene, vinyl, polyethylene, chlorosulfonated polyethylene, such as HYPALON, synthetic rubber, such as NEOPRENE, EPDM (ethylene-propylene-diene terpolymer), rubber, or other resistive materials. 
     The thickness of liner  103  should be sufficient to prevent damage when slurry  105  is poured into stay-in-place form  100 . For example, if liner  103  is too thin, the weight of the slurry  105  may tear liner  103  as it is poured into stay-in-place form  100 . In an exemplary embodiment, the thickness of liner  103  is between {fraction (1/64)} and ¼ of an inch. 
     Stay-in-place form  100  is filled with slurry  105  which hardens within stay-in-place form  100  to form a concrete core  205  of structural member  200  shown in FIG. 2, such as a column or beam. Stay-in-place form  100  is not removed from concrete core  205 , but rather remains in place to increase the shear strength and longevity of support structure  200  over that of conventional support structures. 
     One way to increase the structural integrity of concrete structural member  200 , illustrated in FIG. 2, is to attach reinforcing bars to the inner surface of stay-in-place form  100 . FIG. 8 illustrates an alternate embodiment of the present invention, in which a cross-section of stay-in-place form  800  is shown with reinforcing bars  801 ,  809 . Stay-in-place form  800  has the same outer composite shell  101  and liner  103 , but also has reinforcing bars  801 ,  809  such as steel or composite reinforcing bars, secured to the inner surface of stay-in-place form  800  to provide further reinforcement. 
     As shown in FIG. 8, anchors or stiffener tabs  803  are received by grooves  805  and are distributed about the inner wall surface of stay-in-place form  800 . These anchors  803  extend horizontally from the inner wall surface of composite shell  101 , through liner  103 , and terminate within the enclosure of stay-in-place form  800 . In one embodiment, anchors  803  terminate in clamps  807  that are used to hold vertically extending reinforcing bars  801 . With such configuration, reinforcing bars  801  can be pre-installed at the factory or snapped into clamps  807  at the construction site. In an alternate embodiment, vertically extending reinforcement bars  809  are integrally formed with anchor  805 . 
     As shown in FIG. 8, vertically extending reinforcing bars  801 ,  809  may extend a partial length of composite shell  101 . Alternatively, referring to the cross-section view of stay-in-place form  900  illustrated in FIG. 9, vertically extending bars  901 ,  903  may extend along a substantial length of composite shell  101 . Also, referring to the cross-section view of stay-in-place form  10  illustrated in FIG. 10, reinforcing bars  1001  may extend across the enclosure within stay-in-place form. It also will be appreciated that although reinforcing bars are illustrated as vertically and horizontally reinforcement bars in FIGS. 8-10, reinforcement bars can be situated in other positions, such as diagonally or circumferentially. 
     Stay-in-place forms  100  and  800 , illustrated in FIGS. 1 and 8 respectively, have been disclosed as complete tubular or columnar enclosures. However, stay-in-place forms may also be partial enclosures. FIG. 11A illustrates a perspective view of a stay-in-place form  1100  that has a horizontally extending hollow rectangular channel shape. Stay-in-place form  800  includes a horizontally extending hollow channel composite shell  1101  and a liner  1103  secured to the inner surface of composite shell  1101 . In this way, stay-in-place form  1100  provides a channel form into which a slurry of concrete or cement material  105  is placed, which upon hardening, creates a fully reinforced support structure. With this configuration, stay-in-place form  1100  only partially surrounds a concrete core and may be used, for example, to construct beams. Since the upper portion of the channel shaped stay-in-place form  1100  is open, the beam can easily connect to another support structure (not shown). 
     Referring now to FIG. 11B, a cross-sectional view of stay-in-place form  1100  along line A—A is illustrated. As shown in FIG. 11B, stay-in-place form  1100  includes reinforcement bars  1105  that extend across the width of the channel-shaped composite shell  1101 , to provide additional reinforcement. In addition, stay-in-place form  1100  also includes built-in connectors  1107 , which may be made of various materials such as fiber composite, steel, etc., formed into composite shell  1101  to connect the completed beam with another support structure, such as a column, foundation or other beam. Stay-in-place form  1100  may also include anchors at the edges or other areas of composite shell  1101  to further reinforce the completed support structure. In all of these embodiments, reinforcement bars  1105  and anchors  1107  are designed to withstand the stresses of concrete slurry  105  that is to be poured into the enclosure. 
     Stay-in-place forms  100 ,  800 ,  900 ,  1000 ,  1100  can be used as a cast-in-place structural member where the construction of the stay-in-place form is done at or near a construction site. Alternatively, stay-in-place forms  100 ,  800 ,  900 ,  1000 ,  1100  can be used as precast members, where construction of the stay-in-place form is done in a factory and is then shipped to the construction site. 
     Method of Manufacturing Stay-In-Place Form 
     FIGS. 12A-12J illustrate the sequence of steps employed to fabricate stay-in-place form  100  using a reusable form  1201  such as that illustrated in FIG.  12 A. Care should be taken in selecting the shape of reusable form  1201 , as the shape of reusable form  1201  will determine the shape of resulting stay-in-place form  100 . In the embodiment illustrated in FIG. 12A, reusable form  1201  is a tubular form. In this FIG. 12A a perspective view of tubular form  1201  is shown. In an exemplary embodiment, tubular form  1201  is fabricated from a fiber paper which is formed by spirally winding and laminating the fiber paper together with a special adhesive along seams  1203 . Although, tubular form  1201  is fabricated from fiber paper, it will be appreciated that tubular form  1201  can be fabricated from other types of material so long as tubular form  1201  is rigid and collapsible. 
     A small slit or groove  1205  is cut into the inner surface of tubular form  1201 , as illustrated in FIG.  12 B. Referring now to FIGS. 12C and 12D, a cross-sectional view of tubular form  1201  is shown along line B—B. As shown in FIG. 12C, a tool  1207  such as a steel blade, is able to grasp the small slit  1205 . This enables a portion of tubular form  1201  to be pulled inward as illustrated in FIG. 12D, thereby reducing the diameter of tubular form  1201 . The importance of this collapsing of tubular form  1201  will be explained later in the specification. 
     FIG. 12E illustrates a perspective view of tubular form  1201  lying on its side. Water bags  1208 , illustrated with phantom lines, may be placed inside tubular form  1201  to maintain the shape of tubular form  1201  during the fabrication process of stay-in-place form  100 . It will be appreciated that although water bags  1208  are illustrated to maintain the shape of tubular form  1201 , it will be appreciated that other devices, such as mechanically expandable wood or steel, placed at the ends of tubular form  1201 , can be used for the same purpose. 
     Once water bags  1208  have been inserted into tubular form  1201 , liner  103  is applied to tubular form  1201 . FIG. 12F, illustrates a top plan view of liner  103  being applied to the outer surface of tubular form  1201 . Liner  103  is wrapped tightly around tubular form  1201  such that the lateral edges of liner  103  overlap and are held together with an adhesive material such as tape or glue. In some instances it is desirable to prevent at least one end of liner  103  from slipping relative to tubular form  1201 . In such instances, liner  103  may be adhered to tubular form  1201 , such as by applying tape, glue or some other adhesive material to liner  103 , tubular form  1201  or both. 
     Once liner  103  has been wrapped around tubular form  1201 , a composite reinforcement layer  107 , as illustrated in FIG. 1, is applied to the exposed outer surface of liner  103 , as illustrated in FIG.  12 G. As explained above in reference to reinforcement layer  107 , such reinforcement layer may be applied in a variety of different patterns and may be made up of multiple layers of fabric. In the exemplary embodiment illustrated in FIG. 1, composite reinforcement layer  107  is made up of fabric layers  109 - 115 . All of the fabric layers  109 - 115  must be impregnated with a resin in order to function properly in accordance with the present invention. Preferably, the resin is impregnated into the fabric prior to application to the exterior surface of liner  103 . However, if desired, the resin may be impregnated into the fabric after the fabric is wrapped around the liner. 
     As illustrated in FIGS. 12G-12H, fabric layers  109 - 115  are resin impregnated prior to application to liner  103  so that the final fabric layers  109 - 115  are provided within a resin matrix. For example, referring to FIG. 13, a fabric  1301  is shown being unwound from roll  1303  and dipped in resin  1305  for impregnation prior to application to liner  103 . Once a sufficient length of fabric  1301  has been impregnated with resin  1305 , the impregnated fabric layer is cut from roll  1303  and is applied to the exterior surface of liner  103 , as shown in FIGS. 12G-12H. The length of impregnated fabric is chosen to provide either one wrapping or multiple wrappings of liner  103 . Once in place, the resin impregnated fabric layer is allowed to cure to form the composite reinforcement layer  107 . 
     In an alternate embodiment, fabric layers  109 - 115  are impregnated with resin after being wrapped around liner  103 . In either embodiment, it is preferable that tubular form  1201  be rotated around an axis B in a direction indicated by arrow A, as shown in FIG. 12G, while the fabric layers are wrapped around liner  103 . Such rotation maintains the form of tubular form  1201  and ensures that the resin is uniformly distributed. Tubular form  1201  may be suspended or rotated on a platform while this rotation takes place. The rotation of tubular form  1201  continues until the resin impregnated fabric layers are fully cured. 
     Curing of the resins is carried out in accordance with well known procedures which will vary depending upon the particular resin matrix used. The various catalysts, curing agents and additives which are typically employed with such resin systems may be used. The amount of resin which is impregnated into the fabric is preferably sufficient to saturate the fabric. 
     Once the fabric layers are fully cured, tubular form  1201  is pulled out from liner  103 . One technique for removing tubular form  1201  is to use a release tool  1207 , such as a steel blade, as illustrated in FIGS. 12C-12D. Release tool  1207  is inserted into slit  1205  as illustrated in FIG.  12 C. Pulling on release tool  1207 , causes a portion of tubular form  1201  to be pulled inward and away from liner  103 , thereby reducing the diameter of the form  1201 , as shown in FIGS.  12 D. FIGS. 12I-12J further illustrate the collapsing of tubular form  1201 . FIG. 12I illustrates a cross-sectional view along line B of liner  103  and composite reinforcement layer  107  wrapped around tubular form  1201  as shown in FIG.  12 G. FIG.12J illustrates a top plan view of tubular form  1201  being collapsed inward and away from liner  103 . Using this technique, tubular form  1201  can be collapsed and pulled out from beneath liner  103 . Once tubular form  1201  is pulled out, the resulting structure is stay-in-place form  100 , illustrated in FIG.  1 . 
     In an alternate embodiment, stay-in-place form  100  is formed using a mandrel, as illustrated in FIG.  14 . In such an embodiment, mandrel  1401  serves as a core around which liner  103  is wrapped, as illustrated in FIG.  14 . Composite reinforcement layer  107  impregnated with the resin is then continuously wrapped around liner  103  until a desired thickness is obtained, as illustrated in FIGS. 12G and 12H. Once the fibers are cured, liner  103  and the hardened shell formed from composite reinforcement layer  107  are slipped off mandrel  1401 . In either embodiment, the resulting structure is stay-in-place form  100 . 
     Various other modifications and alterations in the structure and method of operation of this invention will be apparent to those skilled in the art without departing from the scope and spirit of this invention. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments.