Patent Application
20160130813
This invention relates to the strengthening of wall-like reinforced concrete (RC) columns using fiber reinforced polymer material surrounding the columns after shape modification employing a plurality of segments.
The use of reinforced concrete columns of various shapes for carrying vertical loads is well known and widely practiced. However, strengthening and/or rehabilitation of such columns is often required to eliminate structural problems resulting from unusual loading, exposure to aggressive environment, aging, inadequate design and/or poor construction. Upgrading of the columns with fiber reinforced polymer (FRP) sheets is a well-known technique for repair and rehabilitation by placing the fibers mainly transverse to the longitudinal axis of the columns.
It has been recognized that the effectiveness of the FRP confinement in delaying and limiting unstable crack propagation depends to a large degree on the stiffness of the FRP jacket. However, it is also well known that the use of FRP confining systems for columns with a rectangular cross section is less effective than on a circular cross section due to a part of the cross sections remaining unconfined and that the effectiveness depends on the sharpness of corners. In addition, it is well known that the confinement provided by FRP to the confined concrete in a rectangular shaped column is reduced as the aspect ratio increases. In fact, the benefit of FRP wrapping in terms of ductility should be neglected if the aspect ratio is more than 1.5.
A U.S. Patent Publication No. 2006/0070338 in the name of Pantelides et al. discloses Shape Modification and Reinforcement of Columns Confined with FRP Composites. As disclosed, 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 corners 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 describes the stress-strain relationship for the FRP-confined columns after shape modification.
Notwithstanding the above, it is presently believed that there is a need and a potential commercial market for an improved shaped modification of wall-like rectangular reinforced concrete columns in accordance with the present invention.
In essence, the present invention contemplates shape modification of wall-like rectangular columns for improved confinement provided by multiple circular segment bulb sections. The process of strengthening requires a simple framework in the form of a generally semicircular PVC pipes which are cut to required shapes in the form of segments of a circle. The cement grout is added to the space between the pipes and the column. After the hardening of cement grout, the PVC pipes are removed and an FRP sheet is affixed to the modified column shape i.e. column plus segments.
An important aspect of the present invention resides in the use of vertical steel strips attached to the column by steel studs. The steel studs pass through the column through passages made in the column and FRP sheets as well as the steel strips. The column strengthening is achieved by increasing the area of the cross section and more importantly by confinement of a column by FRP materials. In addition, the vertical steel strips with steel studs also contribute to column strengthening.
To be more specific, a fiber reinforced composite structure comprises or consists of the following.
A reinforced concrete column having a wall-like rectangular cross section with an aspect ratio (length of the wall to thickness of the wall) of greater than 1.5:1, a plurality of arcuate cement segments disposed on each side and the ends of the wall-like rectangular column and a plurality of passages extending through the column and one or more segments.
The invention further comprises or consists of a plurality of vertical steel strips with one of the steel strips between adjacent arcuate segments on each side of the wall-like rectangular column.
In addition, the invention includes a plurality of steel rod like shear studs.
A further embodiment of the invention contemplates a method for strengthening wall-like rectangular reinforced concrete columns by shape modification using reinforced polymer materials.
The method in accordance with the present invention comprises or consists of the following steps.
In a first step, a reinforced rectangular wall-like concrete column is surrounded by a plurality of arcuate segments formed by a plastic pipe or the like placed against the column and filled with grout. The arcuate segments may have a semicircular shape with a plurality of segments on each side of the column and with a series of at least two generally vertical segments on each side of the wall-like column aligned with an equal number of segments on an opposite side of the wall-like column.
The spaces between the semicircular plastic form and the rectangular column are filled with grout and after hardening the plastic forms (pipes) are removed. In a preferred embodiment of the invention, there is a relatively small space between adjacent segments on each side of the wall-like column that are filled with grout and connect or join adjacent segments together.
A plurality of passages extend through the rectangular wall-like columns, the arcuate segments or connections join the adjacent segment together as well as through a number of steel strips.
In addition, a plurality of steel strips are provided with openings aligned with the passageways and placed with the openings therein aligned with the passageway and with the steel strips vertically aligned between the arcuate segments. A plurality of steel bolts are used to maintain the steel strips and arcuate segments together under compression.
Sheets of fiber reinforced polymer material is then wrapped around the wall-like columns and arcuate segments with the fibers under tension to thereby reinforce the wall-like rectangular column.
It should be recognized that the FRP material can include a fiber and polymeric matrix. Typically fibers can include but are not limited to glass fiber, carbon fiber and aramid fiber or 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 section of the fibers can be based on factors such as costs, 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 length of fiber which can be wrapped and/or laid as desired to form a particular shape.
The polymeric resins used as the matrix for the fibers are usually thermosetting resins. Most available FRP materials are provided with polymeric resins such as polyesters, vinylesters or epoxies although other polymeric materials can also be used.
The invention will now be described in connection with the accompanying drawings wherein like reference numerals have been used to identify like parts.
The steps involved in the process of strengthening a rectangular wall-like reinforced concrete column comprise or consist of
The commonly adopted method for the strengthening of wall-like rectangular RC columns using FRP confinement through shape modification involves the conversion of column cross-sections to generally elliptical shapes with a plurality of arc shapes on opposite sides of the column and oval shapes on each end. The confinement provided by FRP depends on the column offset used in shape modification. The more is the offset, more will be the confinement but there may be a limit to it due to the restriction on column width. Moreover, confinement provided by FRP to the elliptical or oval cross-sections is not uniform.
For rectangular columns of cross-section a×b with a being the longer side (i.e. a>b), the diameter of end circular bulbs, D, may be taken as (b+2c) where c is the maximum permissible column offset from the longer side of column cross-section (i.e. the thickness of cement grout). The value of c may however vary from 25 mm to b/2. For some cases, the column offset from shorter side of the column cross-section, which is usually not a major concern, may be more than the value of c. The diameter of middle circular bulb/bulbs may be kept the same as the end circular bulbs and the size of circular segment may be based on the space available after accommodating the end circular bulbs. For keeping the same magnitude of maximum column offset for each circular bulb, the diameter of middle circular bulb/bulbs may be varied. Alternatively, the diameter of bulbs. D may be decided using the following relation:
where n=number of bulbs; u=clear distance between adjoining mortar bulbs which is slightly more than the width of vertical steel strip between the adjoining bulbs due to the thickness of FRP sheet (u=βb). The value of maximum column offset, c, can be determined using: c=(D−b)/2. The above formula is based on the assumption of the same diameter of each bulb and the same magnitude of maximum column offset for each bulb. The number of bulbs may be taken to be approximately equal to the aspect ratio of column. The number of bulbs may however be lower or higher than the value of aspect ratio, α (α=a/b). It is worth mentioning here that, in general, the column offset may be reduced by increasing the number of bulbs. The value of u may vary from 25 to 100 mm. The use of the above formula for the preliminary proportioning of circular bulbs for the strengthening of wall-like columns is explained for several column cross sections in Tables 1 and 2. The column shown in
The concrete mix (or cement grout) may be designed according to the design and construction requirements.
Though the patent is especially useful for wall like columns but the invention may be used for all rectangular column cross sections due to which Tables 1 and 2 cover a wide range of aspect ratios of column cross-sections.
While the invention has been described in connection with its accompanying drawings, it should be recognized that changes and modifications may be made therein without departing from the scope of the appended claims.
