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.

Description

RELATED APPLICATIONS [0001] This application claims benefit of U.S. Provisional Patent Application No. 60 / 610,265 filed on Sep. 15, 2004, and U.S. Provisional Patent Application No. 60 / 640,545 filed on Dec. 30, 2004, each of which are incorporated herein by reference.BACKGROUND OF THE INVENTION [0002] In recent years, fiber reinforced polymer (FRP) composites have emerged as an alternative to traditional materials for strengthening and rehabilitation of structures. The light weight of FRP, high-strength to weight ratio, corrosion resistance, and high efficiency of construction are among many of the advantages which encourage civil engineers to use this material. FRP composites have been used in the retrofit of bridge columns due to insufficient capacity or displacement ductility. FRP jackets can provide lateral confinement to the concrete columns that can substantially enhance their compressive strength and ultimate axial strain. One of the most significant problems which concerns c...

AI Technical Summary

Benefits of technology

[0008] It has been recognized that development of improved materials and methods for strengthening structural columns which avoid many of the above deficiencies would be a significant advancement in the industry. Accordingly, the present invention provides fiber reinforced polymer (FRP) composite structures which include a core and a fiber reinforced polymer material at least partially surrounding the core. The core includes an inner cement structure at least partially surrounded by an outer cement structure. The outer cement structure comprises or consists essentially of expansive cement or non-shrink cement. Typically, this is the result of applying the principles of the present invention to reinforce existing structures. Frequently, the inner cement structure of non-expansive cement can include steel reinforcements. However, the materials and methods of the present invention can significantly reduce or even eliminate the need for steel reinforcement in cement structures.

[0009] In another aspect of the present invention, the inner cement structure can have a cross-sectional shape which is different than a cross-sectional shape of the core. This results in a composite structure having a non-uniform gap thickness or non-uniform thickness of the non-shrink or expansive portion of the core. Thus, a pre-existing structure having, for example, a rectangular cross-section can be modified into a structure having a circular, oval or elliptical cross-section. Modification of the cross-sectional shape can have multiple advantages. The elimination of corners can reduce stress concentration and early failure of the FRP jacket. Typically, the FRP shell is cured before the grout is poured in the space between the existing column and the FRP shell. Therefore, the effect on the FRP shell is a post-tensioning, and the effect on the existing column is radial compression. For example, the FRP materials and post-tensioning of the FRP jacket can provide improved mechanical properties as described in more detail below. Additionally, elliptical, oval and circular shapes can provide a greater degree of strength under asymmetric loads than comparable rectangular configurations.

[0010] In accordance with an embodiment of the present invention, the FRP jacket can be post-tensioned and have a hoop stress along the FRP material. In one aspect, post-tensioning of the FRP jacket can be readily accomplished by using expansive concrete. The post-tensioning induced in the present invention can be in the form of tensile stress along the FRP fibers, i.e. circumferential rather than axial.

[0011] In yet another aspect of the present invention, the FRP material can include a fiber and a polymeric matrix. Typical fibers can include, but are not limited to, glass fiber, carbon fiber, aramid fiber, and combinations thereof. Glass and carbon fibers tend to be cost effective and provide good mechanical properties. Aramid fibers are light, durable and are known to have high tenacity. The selection of the fiber can be based on factors such as cost, strength, rigidity, and long-term stability. Additionally, each type of fiber offers different performance characteristics and suitability for various applications. For example, aramids may come in low, high, and very high modulus configurations. Carbon fibers are also available with a large range of moduli; with upper limits nearly four times that of steel. Of the several glass fiber types, glass-based FRP reinforcement is least expensive and generally uses either E-glass or S-glass fibers. The fiber material for use in FRP can be provided as sheets which can be cut to a desired size or as lengths of fiber which can be wrapped and / or laid as desired to form a particular shape.

Problems solved by technology

This results in a composite structure having a non-uniform gap thickness or non-uniform thickness of the non-shrink or expansive portion of the core.

Images