US RE39839 E1
An article and method for reinforcing structural members is provided. The article includes a carbon fiber material, preferably a carbon fiber mesh with a roughened surface defined by an epoxy resin. The article has lateral fibers woven into longitudinal carbon fibers that are in tension, creating a mesh. An epoxy resin is applied to the fiber mesh to form a rigidified rigified fiber mesh tape once the resin is cured. To obtain a roughened surface, a plastic sheet is removably attached to the surface of the fiber mesh. The plastic sheet is removed thereby exposing a roughened carbon fiber surface defined by the epoxy. An alternative tensile tape is also provided for reinforcing a wall structure.
1. A method for reinforcing a structural element comprising the steps of:
providing a flexible reinforcing member having a removable sheet material adhered to one side;
peeling off said removable sheet material from said flexible reinforcing member thereby exposing a roughened surface on said flexible reinforcing member;
applying a bonding agent to one of a surface of said structural element and said flexible reinforcing member;
placing said flexible reinforcing member on said structural element; and
conforming said flexible reinforcing member to resemble a surface of said structural element.
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extending along a width of the flexible reinforcing member, wherein longitudinal fibers near an edge of said flexible reinforcing member have a different modulus than longitudinal fibers in a center portion of said flexible reinforcing member.
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17. A method for reinforcing a structural element comprising the steps of:
providing a flexible reinforcing tensile tape;
applying a bonding agent to one of a surface of the structural element and said flexible reinforcing tensile tape;
placing said flexible reinforcing tensile tape on said structural element; and
conforming said flexible reinforcing tensile tape to resemble a surface of the structural element.
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This application is a continuation-in-part application of application Ser. No. 09/903,414, filed Jul. 11, 2001 which is a continuation-in-part application of application Ser. No. 09/736,853 filed on Dec. 13, 2000.
This invention relates to a product for use in reinforcing structures and a method for attaching the product to the structure and in particular to reinforce concrete walls using carbon fiber material with epoxy adhered to the carbon fiber material. The invention further includes a rigidified mesh of carbon fiber material designed for adherence to a structural element.
Walls constructed of concrete blocks are well known in the field of construction and have been extensively used for both above ground and basement walls. Such concrete walls constructed in this manner are generally capable of supporting residential and light commercial structures and are relatively inexpensive to manufacture and repair.
In order to construct a concrete wall, individual blocks are laid end to end and successive rows or courses are stacked thereon. Mortar between each adjacent block and row secures the wall together. These walls are such that they have excellent compressive strength to support structures placed upon them. However, these walls are inherently weak with respect to lateral loads and are particularly susceptible to cracking from water pressure. This inherent weakness of concrete walls is attributable to the structural characteristics of the concrete walls themselves and the mortar joints at which they are connected. Walls constructed in this manner are relatively strong in compression and are thus well suited for supporting overlying structures. However, both the concrete material and particularly the mortar joints are weak in tension, and when subjected to a tensile force, they tend to separate relatively easily.
Water penetrating deeply into the soil adjacent a basement wall causes substantial lateral movement of the expanding soil against the wall. Over a period of time, block or concrete walls develop diagonal cracks at the ends and vertical cracks near their centers. Such cracks can admit water under pressure from the surrounding soil, and, if left untreated, can progressively widen and eventually facilitate collapse of the entire structure with resultant damage to the structure supported on it. In addition to developing such cracks, concrete walls typically either bow inwardly and such bowing or tilting steadily worsens with the weight of the overflying structure. The water pressure exerts a compressive force at the outer end, therefore, basement wall cracks tend to develop on the inside of such walls.
One of the traditional methods of repairing the leaks and cracks and relieving the external pressure is to drill holes and provide for channeling of the water away on the inside. Yet another method for repairing cracks and leaks is to inject an epoxy resin into the cracks. Although these methods will prevent further water from entering the cracks they do not bind the concrete walls and prevent further cracking or bowing of the concrete walls.
Yet another means of correcting the cracks in the walls is to use fiberglass cloth with epoxy or polyester resin. Fiberglass has good tensile properties and can carry the load on the interior of the basement walls that is in tension. However, one of the major drawbacks with this method is that mixing the epoxy or polyester and wetting out the fabric is time consuming and messy.
The use of carbon fiber bonded plates for structural application has been an area of study for many years. The cost of the carbon fiber material has not allowed practical applications to keep up with academic evaluations. Carbon fiber plates have been studied as an external reinforcement by the Swiss Federal Testing Laboratories and is documented in a paper written in 1995. [Meier, U,; Winsitorfer, A. Retro-fitting of Structures through external Bonding of CFRP Sheets. Non-Metallic Reinforcement of Concrete Structures, Ghent, (1995), pages 465-472]. In this case it was “used to repair a defective structure, to allow for increases in applied load, or to allow modification for changes in use.”
Another experiment was carried out in the use of carbon fiber strips to reinforce masonry structures to resist earthquakes. [STEINER, W. Strengthening of structures with CFRP strips. Advanced Composite Materials and Structures, Montreal, 1996, pp407-417.
The use of carbon fiber plates for reinforcement and repair of concrete structures is well documented and known by those skilled in the art. In recent years, this technology, which has been well documented in the literature, has, with lowered cost of carbon material, become economically feasible to apply this technology to concrete walls which are reinforced using precut strips of carbon fiber. This prevents the walls from cracking or collapsing. However, precut carbon fiber strips have to be cleaned and roughened, commonly done through sanding, to provide mechanical adhesion with the walls. The sanding process is not only time consuming, but is completely dependent on the skill of the operator sanding the surface of the strip. Sanding also may not remove oil or waxy materials and may spread such contaminants with a detrimental affect on bonding. This results in extra cost in transporting and storing the present strips.
The strips of carbon for these prior systems must also be narrow to insure that air pockets are not trapped under the carbon fiber strips. Inspection for air pockets is costly and difficult.
The strips being of high fiber density provides a very stiff strip which greatly differs in stiffness form the masonry that it is reinforcing this causes an “edge effect” stress between the reinforced and un-reinforced masonry. The stiffness variation at the edge of the strip has been shown to cause cracking at the junction for the reinforced to un-reinforced masonry due to shear at this large change in relative stiffness.
Also, the thickness of normal pultrusion or carbon plates provides a structure which is much too stiff for the bonding characteristics of the adhesive or the surface strength of the masonry structure. That is, the reinforcement is of no use if it pulls loose from the surface to which it is bonded. This loosening can be due to the reinforcement being too strong and pulling loose due to unequal thermal expansion, this condition is aggravated by the reinforcement being too stiff.
The carbon strips, which are very stiff, can come loose from the structure that is reinforcing. Generally the bonding material and the surface of the structure being reinforced is much weaker and has a significantly lower modulus of elasticity than that of the carbon fiber. This sudden loosening starts with a high stress causing a crack at an air void or other high stress area; this initial crack can propagate due to a high stress at the crack and lack of elongation in the carbon fiber material.
The use of a plate also blocks moisture flow through the masonry structure thus capturing moisture behind the plate. This moisture rich area tends to weaken the masonry and masonry to adhesive bond area. In areas of frost this moisture allows frost action to work on the bond with deleterious freezing and expansion of the moisture; this action can cause delamination.
With the limitations of the prior art in mind, it is an object of the present invention to provide an article for reinforcing a structure element to effectively resist bending or other lateral forces applied to the structure element.
Another object of the present invention is to provide an article that does not require any sanding to provide mechanical adhesion in order to attach the article to the structural element and one which will remain sufficiently clean at a job site.
It is yet another object of the present invention to provide an article that prevents air pockets from forming, that inhibits micro-crack propagation, and prevents thick glue areas from developing.
It is yet another object of the present invention to provide a method whereby an article for use in reinforcing is firmly adhered to the structural element, thereby reinforcing the structural member.
It is yet another object of the present invention to use existing materials to apply uniform pressure to firmly adhere the article to the structural element, thereby resulting in a strong reinforced structural member.
Another object of the present invention is to provide a product and method which is economical, efficient in operation, and capable of a long operating life.
It is another object of the present invention to provide a method for adhering a breathable strip and adhesive to allow moisture migration through the bonded reinforcement.
It is another object of the present invention to provide a method for applying a reinforcement structure that is wider than a strip to minimize the “edge effect” stresses at the boundary between the reinforced and un-reinforced structure.
It is also an object to provide reinforcement through spacing and choice of the fiber type to provide a better match between the reinforcement and the structure. This provides a strip which does not overpower the bonding material.
Another object is to taper the edges of the sheet to reduce stress concentrations along the edges of the strip.
Another object is to provide a reinforcement that, if it fails, will fail in a progressive manner thus providing a visual warning that failure is occurring.
Another object is to provide a method of applying reinforcements to the wall which hold the reinforcement in place until the bonding is completed for a neat, strong and workmanlike reinforcement.
Another object is to provide a method of manufacturing a grid where the lateral fibers are woven or wrapped to the longitudinal fibers thus trying the fibers together to allow the stiff longitudinal fibers to carry the load and the elastic lateral fibers to deform to spread the loading more evenly across the glue and supported structure.
In accordance with the preferred embodiment of the present invention, an article (a reinforcing member) and method for reinforcing structural elements, such as concrete walls, support beams and the like, are provided.
The article in accordance with the present invention comprises a carbon fiber strip with an exposed roughened surface. In order to make the article, epoxy resin is applied to carbon fibers. The epoxy resin is allowed to permeate the thickness of the carbon fibers. Due to the uneven surface of the carbon fibers, a thin layer of epoxy forms on a top or exterior of the carbon fiber. A plastic cover sheet is then placed on top of the carbon fiber. Carbon fiber material with the epoxy and the plastic fiber is then subjected to high heat and pressure to cure the epoxy thereby forming the carbon fiber into a rigid sheet, with an adhered cover sheet, that can be cut into strips. At the job site, the cover sheet is readily removed and the resultant sheet of carbon fiber will have a roughened surface defined by the epoxy resin where it had adhered to the cover sheet. The cover sheet keeps out greases and oils that sanding may not remove.
In an alternate embodiment of the present invention, the article comprises a rigidified carbon fiber mesh tape. The mesh tape is comprised of a number of carbon fibers woven together to form a rigidified matrix. When bonded to a structural element, the bonding agent flows through the mesh, eliminating air pockets and thick glue areas. Moreover, propagation of micro-cracks formed between fibers is limited to the spaces between fibers and cannot propagate along the mesh tape.
The mesh tape when used with a breathable adhesive will not trap moisture behind the reinforcement.
The mesh tape since air entrapment is not an issue allows a wide sheet to be used. The wider sheet minimizes the structural changes between the reinforced and un-reinforced transition in the structure.
The edges of the mesh tape can have wider fiber spacing to feather the stiffness of the tape so not to cause a large stress concentration at the junction of the reinforced structure to the un-reinforced structure.
The method of adhering the article to a structural element comprises the steps of applying a second epoxy resin to the structural element; adhering the article to the second epoxy resin; allowing the second resin to cure while applying pressure to the article and structural element. Pressure may be applied through use of an overlying plastic sheet, the edges of which are sealed with the help of an adhesive to the structural element. A uniform pressure is applied with the help of an external vacuum pump and the vacuum is applied until the epoxy is cured and the article is firmly fixed to the structural element.
The following description of the preferred embodiment is merely exemplary in nature, and is in no way intended to limit the invention, or its application, or uses.
Referring now to the drawings, shown in
To obtain the strip 10, a layer of carbon fiber 13 having a first surface 16 and a second surface 18, is provided. The first surface 16 and the second surface 18 of the carbon fiber 13 define a thickness 20. The fibers (not shown specifically in the drawings) of the carbon fiber 13 are generally axially oriented with respect to the strip 10 for good tensile strength as is well known.
Referring in particular to
An alternate embodiment of the invention, shown in
In other words, the carbon fibers are layered on top and below the longitudinal fibers 102, providing a fiber mesh tape 100 with straight longitudinal fibers. The straight fibers are thus ideally oriented to carry tension loading. The lateral fibers may ideally be made of another fiber that is very elastic and can be woven into the stiffer longitudinal fibers. A Kevlar® type lateral fiber is ideal since it has very high elongation this elongation spreads the loading more evenly across the glue and the reinforced structure. Also a partial or local failure will not propagate, as with a plate, since the longitudinal fibers will stretch and reallocate the load by allowing adjacent longitudinal fibers to share the loading. This prevents a rapid crack propagation found in solid plates with a catastrophic or sudden failure. The longitudinal fiber should be kept straight since a woven fiber will tend to straighten when loaded which will put stress on the bonding agent and will tend to not be as stiff. A reduction of manufacturing cost and a reduction in lateral stiffness may be achieved by having lateral fibers only on one side. Note the lateral fibers can be woven to the longitudinal fibers using a fiber wrapping pattern to provide better attachment between the lateral and longitudinal fibers. Also this fiber can be chosen for adhesion with the chosen agent for bonding the structure.
The longitudinal fibers 102 and lateral fibers 104 may be of any cross-sectional shape, such as flat (ribbon like), rectangular, oval or round. In the preferred embodiment, the lateral fibers 104 have a flat cross-section, as seen in
As shown in
Generally speaking, the longitudinal fibers 102 and lateral fibers 104 may be spaced anywhere from over 1 inch apart to less than 1/32 inches apart so long as the spacing is sufficient to allow adhesive to flow between the fibers 102, 104, as will be discussed in detail herein. The longitudinal fibers 102 can vary in spacing to provide a tapered (or feathered) stiffness at the edge of the strip; this tapered stiffness provides a gradual change in stress between the reinforced and un-reinforced structure thus minimizing stress concentrations in the reinforced structure at the edge of the reinforcement strip. Alternatively, longitudinal fibers having tapered thicknesses along the edge of the strip or a different modulus can be used to feather the stiffness of the strip at the edges in order to minimize or reduce the stress concentrations at the edge of the reinforcement strip. The rigidified fiber mesh tape 100 has a roughened surface 28 exposed or produced upon removal of a cover sheet 24, as will be discussed in detail herein. In the preferred embodiment, the fibers 102, 104 are made of pre-cured carbon rods. The pre-cured carbon rods are positioned in a planar configuration and woven together with fibers 104. Alternatively, a fiber filament can be wound around the fiber strands 102, 104 to keep the tows from spreading out and closing the windows. In a preferred embodiment, filaments are wound helically along a length of the longitudinal fibers. Moreover, longitudinal fibers 102 and lateral fibers 104 may be of different materials to provide desired properties of rigidified mesh tape 100. For example, either fibers 102 or 104 may be formed from a fibrous material having high axial bending to allow conformance to non-planar surfaces such as columns, tubes, or other curved surfaces. Additionally, the fibers 102, 104 may be constructed of a material having desirable chemical or mechanical properties to optimize adhesion to the structural element 12.
In the alternate embodiment as shown in
As mentioned above, to provide a strong bond between the reinforcing strip 10 or rigified fiber mesh tape 100 and the reinforced structural element 12, it is important to have the surface of the strip 10 or rigified fiber mesh tape 100 clean and roughed. In order to keep the surface clean and provide a roughened surface, over the layer of epoxy resin 22, on the surface 16 (and optionally on the surface 18), is applied a flexible cover sheet 24 of impermeable textile, nylon, or plastic material. The side of the cover sheet 24 in contact with the epoxy resin 22 preferably exhibits a texture, such as a woven texture surface. The carbon fiber 13 or rigidified fiber mesh tape 100, with the epoxy resin 22 and the cover sheet 24 applied, are subject to high temperature and pressure, via known techniques, allowing the epoxy resin 22 to cure. Once the epoxy resin 22 has cured, the result is a rigid carbon fiber sheet 14 or rigidified fiber mesh tape 100 having a removable cover sheet 24 covering one or both surfaces thereof. This rigid carbon fiber sheet 14 or rigidified fiber mesh tape 100 may then be cut or sawn into the desired sizes forming the strip 10. In this form, the strip 10 or rigidified fiber mesh tape 100 can be stored and/or shipped to a job site for use. With the rigidified fiber mesh tape 100, the resin applied during the manufacture of the open fabric tends to fill the window between the mesh. When the textured cover sheet is removed, these windows remain adhered to the cover sheet and leave the openings clear. Thus, the cover sheet provides both a roughened surface, but also open windows.
At the job site during shipping and handling, the cover sheet 24 prevents dirt, hand oil, grease, and other debris, oils, and chemical from coming into contact with the carbon fiber sheet 14 or rigidified fiber mesh tape 100. The cover sheet 24 also protects the bonding surface from mechanical abrasion or polishing. Immediately prior to use, the cover sheet 24 is removed, or more accurately peeled away, from the surface 16 of the carbon fiber sheet 14 or rigidified fiber mesh tape 100 leaving exposed a clean roughened surface 28. This process saves time in the field as compared to prior methods which require solvent cleaning and abrasion steps. This roughened surface 28 is a result of at least two factors, individually or in combination. First, the roughened surface 26 of the cover sheet 24 causes an impression to be formed in the epoxy resin 22 on the surface 16 as it cures. Second, as the cover sheet 24 is removed from the carbon fiber sheet 14, some of the epoxy resin 22 remains adhered to the plastic sheet 24 and breaks away from the carbon fiber sheet 14 or rigidified fiber mesh tape 100.
As illustrated in the drawings, the strips 10 or rigidified fiber mesh tape 100 are pre-cut and provided in suitable lengths for their intended use. However, it is possible to use large sheets and/or cut them to the required sizes before adhering them to reinforce a structural element 12.
As shown in
In order to attach the strip 10 or rigidified fiber mesh tape 100 to the structural element 12 to reinforce the same, a second epoxy resin 50 is applied to the structural element 12. The second epoxy resin 50 may be of the same kind or different kind from the first epoxy resin 22 such as a water permeable adhesive to allow the structure to breathe. An example of a water permeable adhesive is a urethane-type adhesive. The carbon fiber sheet 14 or rigidified fiber mesh tape 100, with the plastic sheet 24 removed and the roughened surface 28 exposed, is placed against the front face 30 of the structural member 12 where the second epoxy resin 50 has been applied, such that the roughened surface 28 is contacting the second epoxy resin 50.
In order to ensure that the carbon fiber sheet 14 or rigidified fiber mesh tape 100 firmly adheres to the structural element 12, sheet 14 or fiber mesh tape 100 should be held in place until the epoxy resin 50 cures. This is achieved in the method of the present invention by covering the sheet 14 or rigidified fiber mesh tape 100 with a sheet of air impermeable material 54, such as a plastic sheet, and by positioning between the impermeable material 54 and the carbon fiber sheet 14 or rigidified fiber mesh tape 100 a material 52 which will allow for uniform evacuation of air from between the impermeable material 54 and the structural element 12. In the preferred embodiment, the material 52 is a commercially available bubble wrap, such as that extensively used in the packaging industry. Alternatively, a permeable textile, fabric or other material which will facilitate even evacuation over the sheet 14 or rigified fiber mesh tape 100, may also be used. In the preferred embodiment, any commercially available plastic sheet may be used as the impermeable material 54. The edges of the impermeable material 54 are then sealed to the structural element 12 through adhesives or adhesive strips 56.
With the impermeable material 54 so mounted to the structural element 12 over the carbon fiber sheet 14 or rigidified fiber mesh tape 100, air is evacuated through use of a vacuum pump 58 coupled via a vacuum line and fitted to the impermeable material 54. Due to the presence of the material 52, when a vacuum is applied, a uniform pressure is applied over the entire sheet 14 or rigidified fiber mesh tape 100. The vacuum is applied until the second epoxy resin 50 cures and the sheet 14 or rigidified fiber mesh tape 100 firmly adheres to the structural element 12. Under this method, the normal curing time with common epoxies is expected to be about 3-4 hours, after which impermeable material 54 and material 52 are removed. The carbon fiber sheet 14 or rigidified fiber mesh tape 100 will thereafter be firmly attached to the structural element 12 providing the desired reinforcement thereto.
In the rigidified fiber mesh tape embodiment of the invention, the second epoxy resin 50 will flow through the spaces between the longitudinal fibers 102 and the lateral fibers 104 such that no thick glue areas are formed. Moreover, as pressure is applied, air will flow through the mesh holes between the fibers 102, 104 preventing air pocket formation. When a plate is utilized according to other methods and is glued to the structural element, the plate and glue can trap air between the carbon fiber plate and the structural element. Air voids are stress risers and a source of micro-cracks. These micro-cracks, once started, may follow the plate surface and can lead to delamination of the plate. Similarly, thick glue areas may form between the carbon fiber plate and the structural element. Since the glue itself fails at a lower force than does the carbon fiber plate or the structural element, these thick glue areas create weak points in the reinforcement.
An alternate method of applying the strip 10 or fiber mesh tape 100 of the structural element 12 is provided wherein the cover sheet 24 is removed from the strip to expose the roughened surface 28 on a side of the strip 10. A generous layer of epoxy resin 50 is applied along either the structural element 12 or the strip 10, or both. The strip 10 is placed against the front face 30 of the structural element 12 while spreading epoxy resin 50 around the periphery of the strip 10 and onto the structural element 12.
To ensure that the strip 10 or tape 100 firmly adheres to the structural element 12, the strip 10 is desirably held in place until the epoxy resin 50 is fully cured. This is accomplished by placing an impermeable material 54, such as a plastic sheet over the strip 10 and a portion of the front face 30 of the structural element 12. The impermeable material 54 is held in place over the strip 10 by the epoxy resin 50 around the periphery of the strip 10 due to the mechanical adhesion of the epoxy resin 50.
Once the impermeable material 54 is positioned generally over the strip 10, the epoxy resin 50 is manually dispersed to provide proper distribution and bonding to the structural element 12. In a preferred method, a deformable member such as a squeegee is manually traced over the surface of the impermeable material to disperse the epoxy resin 50 between the strip 10 and the structural element 12. As the epoxy resin 50 is dispersed, epoxy resin 50 is spread past the periphery of the strip and contacts the structural element 12 and the impermeable material 54. The mechanical adhesion of the epoxy resin 50 couples the impermeable material 54 to the structural element 12 while the epoxy resin 50 is curing. Also, the mechanical adhesion of the epoxy resin 50 holds the strip 10 to the structural element 12 during curing. By this method, no additional support is needed to hold the impermeable material 54 in place while the epoxy resin 50 is curing.
Another alternate embodiment of the invention, shown in
With continued reference to the structure of tensile tape 120, a first epoxy resin 136 is applied to either the first surface 130 or the second surface 132 of the tensile tape 120. In the preferred embodiment as illustrated in the drawings, the epoxy resin is applied to the first surface 130. Since the carbon fibers 128 when stacked into the desired shape of the tensile tape 120 inherently includes gaps or voids between them, the epoxy resin permeates the entire thickness 134 of the carbon fiber 128 and a thin layer or at least some of the epoxy resin 136 remains on the surface of the carbon fiber 128. In the preferred embodiment, the epoxy resin 136 has low viscosity such that when applied to the tensile tape 120 it permeates the carbon fibers 128. The first epoxy resin 136 is applied to either surface 130 or second surface 132 of the tensile tape 120. In the preferred embodiment as illustrated in the drawings, the epoxy resin is applied to the first surface 130. Since the carbon fibers 128 when stacked in the desired shape of the tensile tape 120 inherently includes areas between the fibers that are resin rich. This resin rich window will stick to the peel ply and be torn out of the tape when the peel ply is removed leaving an open window. The portion of the resin that permeates the longitudinal and lateral fibers remains after the peel ply is torn off. The epoxy resin 136 has a low viscosity such that when applied to the carbon and/or other fiber material the glue fully permeates the fibers.
The axial flexibility of the carbon fibers 128 and the relatively small thickness 134 of the tensile tape 120 allow conformance to surfaces having various shapes or contours. As shown in
The carbon tensile tape 120 is shown in