Patent Application
20080141614
Concrete and other masonry or cementitious materials have high compressive strength but relatively low tensile strength. Thus, when concrete is employed as a structural material, it is conventional to incorporate reinforcing members to enhance the tensile strength of the structure. The reinforcing members are usually comprised of a rigid rod or bar, such as a steel rod or bar. Such reinforcing members are typically referred to as “rebar”. Unfortunately, steel and other metals are susceptible to oxidation. In addition, such materials are quite rigid prior to use so that the placement of such reinforcing members can be difficult and time-intensive. As a result, conventional metal rebar must be cut into pieces and joined in order to form a “criss-cross” or other desired pattern.
One possible solution is to use glass fiber formulations as structural rebar in conjunction with a thermoplastic resin. For example, U.S. Pat. No. 6,048,598 to Bryan, III et al. discloses a twisted rope rebar having individual fibers bound to each other by a thermosetting resin. U.S. Pat. No. 5,580,642 to Okamoto et al. discloses a reinforcing member comprised of reinforcing fibers and thermoplastic fibers. U.S. Pat. Nos. 5,593,536 and 5,626,700 to Kaiser disclose an apparatus for forming reinforcing structural rebar including a combination of pultrusion and SMC (sheet molding compound). The modified pultrusion produces a rebar having a core of thermoset resin reinforcing material and an outer sheet molding compound. U.S. Pat. No. 5,077,113 to Kakihara et al. proposes an inner filament bundle layer spirally wound around a fiber-reinforced core, a plurality of intermediate filament bundles oriented axially along the core, and an outer filament bundle spirally wound around the core and the other bundles. U.S. Pat. No. 4,620,401 to L'Esperance et al. proposes a fiber reinforced thermosetting resin core and a plurality of continuous fibers helically wound around the core and impregnated with the thermosetting resin. The Jackson U.S. Pat. No. 2,425,883 discloses a rod or bar formed of fine glass fibers with a phenolic resin cured under heat.
Despite these advances, there remains a need to provide an improved structural rebar that overcomes the disadvantages and complexities of the prior art.
The present invention provides an improved composite reinforcement bar or rebar structure. The rebar structure is generally formed by continuous fibers embedded in a thermoplastic resin matrix to form a reinforcement bar. The bar is flattened to achieve a cross sectional aspect ratio greater than one to one. The bar is then twisted in a substantially helical manner. In one embodiment of the rebar structure, the bar has a substantially elliptical cross sectional shape with a cross sectional aspect ratio of about two to one and a twist pitch of about 30 centimeters. The matrix may be a thermoplastic resin such as polypropylene, and the fibers may be formed of glass. The thermoplastic resin matrix allows the matrix to be softened by the application of heat to thereby bend or flex the bar to desired shapes. The capability of being conveniently bent is also aided by the cross sectional shape and aspect ratio and by the twist applied to the bar. Once bent to a desired shape, the bar is allowed to cool and re-harden to a substantially rigid state.
Objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention.
The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof.
As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which may be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure.
Referring now to the drawings in more detail, the reference numeral 1 generally designates a flexible fiber reinforced composite reinforcement bar or rebar structure embodying the present invention. The rebar structure 1 generally includes a plurality of reinforcement fibers 2 (
The fibers move through a guides 16 which might consist of guide pins and tensioners, depending upon the final size of the end product. The guides 16, apart from guiding the path of the fibers 2, helps increase the surface area of the within a matrix impregnation chamber 18. The illustrated process 10 includes a dryer 20 into which thermoplastic resin 3 is fed. A heater component 22 heats the thermoplastic resin to a plastic state. A screw “pump” 24 forces the heated resin into the impregnation chamber 18.
The impregnation chamber 18, an important component of the process 10, includes two parts. In a first part 26, the fibers 2 come into contact with the thermoplastic polymer 3 pumped into the impregnation chamber 18. The design of the chamber 18 enables creation of high shear zones for the thermoplastic polymer 3 that results in significant reduction of the viscosity thereof. This reduction of the viscosity tremendously improves the impregnation of the high viscous polymeric material 3 into the fibers 2. In a second part 28 of the impregnation chamber 18, the impregnated fibers 2 are converged into a consolidated impregnated rebar 30. Depending upon the final shape required, the consolidated rebar 30 is given its final shape while it is still hot.
Once the rebar 30 with its final shape leaves the impregnation chamber 18, it goes through a cooler system 32. The design of the cooler system depends upon the final form of the product. For thermoplastic rebar 30, the cooler system 32 might be in the form of a long tube with water sprinklers (not shown) attached along its length. The sprinklers would be used to spray water on the thermoplastic rebar 30 to cool its surface.
The impregnated rebar 30 next moves through the puller 36. The puller 36 pulls the impregnated rebar 30 though the entire device throughout the manufacturing process 10. Finally, the impregnated rebar enters a cutter station 38, which cuts the final product to its required length.
One embodiment thermoplastic rebar 30 consists of E-glass, or electrical grade glass, as the fiber reinforcement 2 and polypropylene as the thermoplastic matrix 3. The fiber volume ratio is approximately 45% of the total volume of the rebar 30, a representative value for typical long fiber thermoplastic processes. A thermoplastic rebar design optimization was performed using ABAQUS™ finite element analysis software (Dassault Systemes Societe Anonyme France, www.simulia.com). An optimal profile for the rebar 30 was found to be an elliptical cross sectional shape having an aspect ratio of about 2:1, with specific dimensions varying for different rebar sizes. In one embodiment, the rebar 30 has a major axis of about 0.75 inch (19.05 mm) and a minor axis of about 0.375 inch (9.53 mm). It is foreseen that the rebar 30 could alternatively have other flattened shapes which are not specifically elliptical. Further, the optimal profile also includes a twist pitch of 30 centimeters (cm) or about one twist per 12 inches of rebar 30. Alternatively, the twist pitch may fall within a range of about 6 to 24 inches (15.24 to 60.96 cm). An example profile is illustrated below in
A thermoplastic matrix 3 was chosen over thermoset because a thermoplastic material has the potential for being bendable in the field. One embodiment of the rebar structure incorporates a polypropylene resin as the thermoplastic matrix 3. However, it is foreseen that other thermoplastic resins could be advantageously employed for use in some applications and environments. Bending the rebar 30 may require onsite heating, which will reduce the stresses resulting from the applied bending force. The heating is preferably not of a temperature which would actually melt the thermoplastic material 3, but only to temporarily soften the rebar 30 for bending. The heating temperature may range from about 150 to 200F (65.6 to 93.3° C.).
A rebar structure 1 having an elliptical cross-section with bends along the major axis appears to meet the demands of being bendable in the field. The elliptical shape minimizes transverse stress, while twists allow ease of bending without having to align the rebar. The twist pitch represents the resolution of bend length; that is, if the pitch is 30 cm, the rebar can only be bent every 30 cm. It was determined that increasing the twists in the rebar 30 (that is, decreasing the twist pitch) increases stress and strain values. Of the many twist pitches considered during analysis, the profile which showed the least longitudinal stress was the pitch 30 cm. Further, rebar was found to be optimally bendable in the horizontal to normal plane of the cross section, that is, about the major axis.
Various aspect ratios were also considered during analysis. It was found that increasing the aspect ratio reduced the longitudinal stress, but increased the transverse stress. Increasing the aspect ratio also increased the likelihood for buckling. An aspect ratio of 2:1 was identified as the optimal design parameter, and is illustrated in
In summary, an optimized embodiment of the thermoplastic rebar structure 1 meeting the criteria of bendability in the field yet not requiring alignment included a polypropylene matrix 3 with E-glass fibers 2 at a 45% fiber volume ratio, a substantially elliptical profile with an aspect ratio of about 2:1, and a twist pitch of about 30 centimeters.
It is to be understood that while certain forms of the present invention have been illustrated and described herein, it is not to be limited to the specific forms or arrangement of parts described and shown.
This application claims priority under 35 U.S.C. 119(e) and 37 C.F.R. 1.78(a)(4) based upon copending U.S. Provisional Application, Ser. No. 60/874,828 for FLEXIBLE REBAR, filed Dec. 14, 2006, which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60874828 | Dec 2006 | US |
