24-08-2012, 02:23 PM
The Use of Glass Fiber Reinforced Polymer Reinforcement in Concrete Beams
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Abstract
The American Concrete Institute runs a biannual competition to develop an optimized design of a beam reinforced with Fiber Reinforced Polymer bars and mesh. While this competition did not take place this year, the rules from the 2007 competition were used to provide constraints for an investigation of reinforcement layouts within concrete beams. Maintaining a constant quality of workmanship was extremely difficult, leading to results that differ quite widely from the finite element model created, though the relative strengths of the beams in the models was as expected based on an ANSYS model. Overall it appears that simply reinforced beams were most successful.
Introduction
Over the last half century much research has been done on the use of alternative reinforcement methods for concrete. The most important of these materials developed are fiber reinforced polymers (FRP). While not yet widely used in industry, these materials are important to construction in highly corrosive environments. As a result of this importance, recent research has focused on the flexural and shear capacity of beams reinforced using FRP. The American Concrete Institute (ACI) promotes this research by encouraging engineering students to gain experience working on simple FRP reinforced structures. One form of this encouragement is through the use of a competition with the goal of optimizing for load-weight ratio a beam reinforced with FRP bars and grid.
The most recent competition was conducted in 2007 as the ACI FRP Composites competition. An equivalent competition was not conducted this year. Despite the lack of a competition, the principles involved in optimizing a beam design are still significant. These principles include determination of beam dimensions, concrete mix design, and reinforcement layout. Within the rules of the competition all of these are to be determined by the competitor, though some parameters are provided. This makes the rules provided by ACI an ideal way to explore concrete design reinforced with FRP.
Carbon Fiber Grid-
Most information about carbon fiber grid comes from the manufacturer. ACI has not yet provided a report, as the grid is not used widely except in decorative concrete. The manufacturer chosen by the competition is TechFab, LLC, who provided their 1 in by 1in C3000 grid. The features of the mesh that they emphasize in their literature are the ability of the mesh to minimize crack width and the ease of using it relative to welded wire mesh. They also provide a value for the tensile strength of the carbon grid as being 3300 pounds per foot. As this is low compared to the bars, it seemed most likely that the role of the mesh would be as shear reinforcement due to the relative ease of shaping the mesh. While the mesh is suggested as beam reinforcement, the literature does not say how it should be used. One main aspect of the project was to determine the grid’s usefulness in creating beams.
ANSYS-
ANSYS is a finite element analysis software. Finite element analysis is a numerical method for solving complex problems by breaking up the solid body into smaller elements that can be solved separately. For modeling concrete, ANSYS provides an element, Solid65, which replicates the behavior of concrete by adding cracking and crushing abilities to a simple solid element. The element also takes into account the non-linear material properties of concrete as well as the non-linearity of large deflections. It also allows for the modeling of reinforcement smeared within the elements. This
capability was used to model the mesh, but it was decided that the bars should be best modeled as axial load elements. A.F. Barbosa and G.O. Ribeiro recommended this substitution in the paper “Analysis of Reinforced Concrete Structures Using ANSYS Nonlinear Concrete Model”. The elements chosen were LINK8 elements according to Verification Problem No. 146 provided by ANSYS. The model for the beams was created by writing a code in the ANSYS Parametric Design Language as opposed to using the GUI. Further discussion of the elements used in the model is included below.