Shape modification and reinforcement of columns confined with FRP composites

Abstract

Strengthening reinforced concrete columns by using Fiber Reinforced Polymer (FRP) composites can be an effective method of retrofitting existing columns. FRP composites have a number of advantages over steel, including their high strength-to-weight ratio and excellent durability. The confinement effectiveness of FRP materials for rectangular sections can be improved by performing shape modification such that a rectangular column section is modified into a shape that does not have 90 degree comers such as an elliptical, oval or circular column. An expansive concrete can be advantageously used between the FRP material and the existing concrete in order to post-tension the FRP material circumferentially and improve confinement of the concrete. A finite element analytical model is also disclosed which model describes the stress-strain relationship for the FRP-confined columns after shape modification.

1 . A fiber reinforced polymer composite structure, comprising: 
 a) a core including an inner cement structure at least partially surrounded by an outer cement structure, said outer cement structure including a non-shrink cement or an expansive cement; and    b) a fiber reinforced polymer material at least partially surrounding said core.    
   
   
       2 . The composite structure of  claim 1 , wherein the inner cement structure has a cross-sectional shape which is different than a cross sectional shape of the core.  
   
   
       3 . The composite structure of  claim 2 , wherein the inner cement structure has a rectangular shape and the core has a circular shape.  
   
   
       4 . The composite structure of  claim 1 , wherein the outer cement structure consists essentially of an expansive cement.  
   
   
       5 . The composite structure of  claim 1 , wherein the core is post-tensioned and has a hoop stress along the fiber reinforced polymer material.  
   
   
       6 . The composite structure of  claim 1 , wherein the fiber reinforced polymer material comprises a fiber and a polymeric matrix.  
   
   
       7 . The composite structure of  claim 6 , wherein the fiber is selected from the group consisting of glass fiber, carbon fiber, aramid fiber, and combinations thereof.  
   
   
       8 . The composite structure of  claim 7 , wherein the fiber is carbon fiber.  
   
   
       9 . A method of reinforcing structural columns, comprising the steps of: 
 a) placing a fiber reinforced polymer outer shell around an existing column, said outer shell being configured to leave an open space between the existing column and the outer shell;    b) filling the open space with expansive cement or non-shrink cement.    
   
   
       10 . The method of  claim 9 , wherein the existing column has a cross-sectional shape which is different than a cross-sectional shape of the outer shell.  
   
   
       11 . The method of  claim 9 , wherein the fiber reinforced polymer outer shell comprises a fiber and a polymeric matrix.  
   
   
       12 . The method of  claim 11 , wherein the fiber is selected from the group consisting of glass fiber, carbon fiber, aramid fiber, and combinations thereof.  
   
   
       13 . The method of  claim 9 , wherein the outer shell comprises at least two pieces which are placed around the existing column to form the outer shell.  
   
   
       14 . The method of  claim 13 , wherein at least one additional layer of fiber reinforced polymer material is wrapped around the outer shell after placing the fiber reinforced polymer outer shell around the existing column.  
   
   
       15 . The method of  claim 13 , further comprising the step of splicing the at least two pieces with a vertical fiber reinforced polymer composite strip along each seam between the at least two pieces.  
   
   
       16 . The method of  claim 9 , further comprising the step of reshaping the existing column prior to placing the fiber reinforced polymer outer shell such that edges of the existing column having an angle of about 90 degrees are rounded.  
   
   
       17 . The method of  claim 9 , further comprising the steps of: 
 a) preparing a mold    b) wrapping the mold with at least one layer of fiber reinforced polymer material to form the outer shell;    c) dividing the outer shell longitudinally into at least two pieces;    
   
   
       18 . The method of  claim 9 , further comprising the step of designing the fiber reinforced outer shell prior to the step of placing the fiber reinforce outer shell around the existing column, said step of designing including: 
 a) defining a core finite element model in three dimensions corresponding to the core of the composite structure;    b) defining a jacket finite element model along an outer surface of the core finite element model;    c) defining boundary conditions for each of the core and jacket finite element models;    d) post-tensioning the jacket finite element model by applying an equivalent thermal gradient; and    e) performing finite element analysis by incremental application of a simulated load and subsequent iteration calculate force and displacement of each node within each finite element model to form a stress-strain curve; and    f) comparing the stress-strain curve to a desired performance and redefining the jacket finite element model and the boundary conditions when the stress-strain curve does not meet the desired performance.    
   
   
       19 . A method of preparing fiber reinforced polymer shells for reinforcing structural columns, comprising the steps of: 
 a) preparing a mold;    b) wrapping the mold with at least one layer of fiber reinforced polymer material to form an outer shell; and    c) dividing the outer shell longitudinally into at least two pieces.    
   
   
       20 . The method of  claim 19 , wherein the step of wrapping the mold includes a wet layup of resin coated fibers followed by curing of the resin.  
   
   
       21 . The method of  claim 19 , wherein the fiber reinforced polymer material comprises a fiber and a polymeric matrix.  
   
   
       22 . The method of  claim 21 , wherein the fiber is selected from the group consisting of glass fiber, carbon fiber, aramid fiber, and combinations thereof.  
   
   
       23 . The method of  claim 21 , wherein the fiber is in the form of a sheet or a strand.