CF Anchors

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CF Anchors

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  • SEISMIC STRENGTHENING OF REINFORCED CONCRETE WALLS

    BY SR-CF SYSTEM

    - Methods and Effects of Shear Strengtheningby Carbon Fiber Sheets and CF-anchors -

    Yasuo Jinno Hideo TsukagoshiShimizu Corporation

    JAPAN

    Keywords: strengthening, reinforced concrete wall, carbon fiber sheet, CF-anchor, shear

    1 INTRODUCTION

    The SR-CF system[1] is a method for retrofitting existing reinforced concrete buildings againstearthquakes by laminating carbon fiber sheets. The method improves the shear strengths ofindependent columns[2], columns with side walls[3],[4], beams[5], and walls[6]. Strengthening of reinforced concrete structures with carbon fiber sheets is the best method forretrofitting existing buildings since it features small and light materials, little noise and vibration, shortworking periods, and no welding works. Carbon fiber sheets, very strong against tensile forces, arelittle adhesive to concrete surfaces and prone to peeling from concrete when a force is applied. Forthis reason, the method has been effective as long as the peeling-off is prevented, such as bywrapping a carbon fiber sheet around an independent column and forming a hoop of carbon fiber. On the other hand, the SR-CF system is effective to retrofit columns with side walls, beams, andwalls, around which hoops of carbon fiber sheets are difficult to form. The SR-CF system uses specialdevices called the CF-anchors to join the carbon fiber sheets which are separated by side walls and tofix carbon fiber sheets to reinforced concrete building frames. The use of the CF-anchor is the mostcharacteristic in this system. This paper describes the shear strengthening of walls by the SR-CF system and shear tests on thereinforced walls, and then proposes methods for assessing the shear resistance.

    Fixing to the peripheral frame

    Carbon fiber sheetAdhering to the carbon fiber sheet

    Penetrating-type CF-anchor

    Carbon fiber sheet

    Adhering to the carbon fiber sheet

    Penetrating the wall

    Fixing-type CF-anchor

    Strengthening a beam

    Strengthening an independent column

    Strengthening a column with a side wallStrengthening a wall

    Fig 1 Outline of the SR-CF System

    109

    Seismic design of concrete structuresSession 6

  • 2 METHODS FOR STRENGTHENING A WALL

    2.1 Outline of CF-anchors Carbon fiber strands used in the SR-CF systemare strings of 2 to 3 mm in diameter consisting of24,000 (called the "24K" strings) or 12,000 (calledthe "12K" strings) carbon fibers, each of which isseveral microns in diameter. The CF-anchor is abundle of such carbon fiber strands, and isimmersed into epoxy resin and hardened beforeuse as in carbon fiber sheets. The CF-anchors may be classified into twocategories by the usage. One is penetrating type,and another the fixing type. The penetratinganchors are used for the shear strengthening ofcolumns with side walls. A bundle of carbon fiberstrands is passed through a hole drilled at the sidewall. The ends of the bundle are spread like a fanand glued to the carbon fiber sheet pasted on thecolumn. The bundle joins the two ends of thecarbon fiber sheet, which was separated by theside wall. Consequently, it is made possible toenvelop the column by the sheet. The fixing-type CF-anchors are used for theshear strengthening of walls. An end of the CF-anchor bundle is spread like a fan and glued to acarbon fiber sheet. The other end is inserted into ahole drilled on the concrete wall and is fixed withinjected epoxy resin. The anchors fix the edges ofa carbon fiber sheet on a concrete wall.

    2.2 Methods for strengthening walls The SR-CF system strengthens walls by thecarbon fiber sheet diagonally glued on the surfaceof the wall with its edges fixed to the peripheralcolumn, beam, and floor using CF-anchors. Thismethod gives the carbon fiber sheet theperformance of a tensile brace, and increases theshear resistance of the wall. The process of strengthening a wall using CF-anchors is shown in Figure 2.(1) Smoothen the surface of the concrete wall byremoving soft section. Drill holes for installing CF-anchors on the peripheral frame (side columns,beam, and the floor).(2) Apply primers on the surface of concrete.(3) Laminate carbon fiber sheets on the wallsurface. The fibers of the sheets should bediagonally installed. Apply sheets so that the fibersof each sheet are diagonal along the oppositeangle to each other. The two carbon fiber sheetsare counted as one layer. Laminate the necessarynumbers of layers.

    (1) Smoothen the surface of the wall Drill holes on the peripheral columns, beam, and floor for inserting CF-anchors

    (2) Apply primers on the surface of the wall

    (3) Adhere carbon fiber sheets on the surface of the wall

    CF-anchors

    (4) Bundle CF strands to prepare a CF-anchor(5) Insert the CF-anchor that is immersed with epoxy resin into a hole

    (6) Spread the CF strands and adhere to the carbon fiber sheet

    Fig.2 Process of strengthening walls

    Proceedings of the 1st fib Congress

    110

  • (4) Prepare CF-anchors by bundling the sameamount of carbon fiber strands as those containedin the carbon fiber sheets. (For example, a carbonfiber sheet of 300 g/m2 in fiber area weightcontains carbon fibers that are almost equivalentto 19 "24K" strands per a width of 100 mm. To fixtwo layers of such carbon fiber sheets byspreading the end of a CF-anchor to a width of200 mm, the CF-anchor should be prepared bybundling 19 x (200 / 100) x 2 = 76 strands.)(5) Immerse the upper halves of the CF-anchorsinto epoxy resin, and insert the ends into the holesdrilled on the columns and beam.(6) Apply epoxy resin on the edges of the carbonfiber sheets, spread the remaining halves of theCF-anchors like fans, and paste the anchors to thesheets by immersing epoxy resin. The central lineof each fan should be along the direction of fibersof a carbon fiber sheet. Since the carbon fibersheets are diagonally applied along the twodifferent opposite angles, divide the CF-anchorsinto two groups from the center of the beam, andspread and paste the anchors to the direction ofthe opposite corner. The CF-anchors for fixing thesheets to a column are similarly fixed by changingthe directions at the center of the column.

    3 TEST PROGRAM

    The bending shear resistance of wall specimens,that were reinforced by the SR-CF system, wastested to examine the shear strengthening effectof the SR-CF system on existing reinforcedconcrete walls. Table 1 shows the specimens, and Table 2shows the properties of the materials used. Theexperimental parameter was the method forstrengthening walls. Series 2 tests wereconducted by changing the number of carbon fibersheet layers, the angle of installing carbon fibersheets, and the kind of carbon fiber sheets. InSeries 1 tests, the authors also examined wallspecimens on which the carbon fiber sheets wereinstalled horizontally and vertically. The carbonfiber sheets used in the experiments were mainlyPAN sheets and partly carbon fiber sheets of largeYoung s modulus. Figure 3 shows the rebar arrangement and themethods for strengthening the walls, and theloading configuration. Each specimen was a wallwith columns at both sides. In Series 1 tests, thethickness of the walls at the sections to be testedwas 100 mm, the internal height was 1,200 mm,

    300

    590 kN590 kN

    A CF-anchor is embedded for a depth of 150 mm, and is adhered to the sheets for a length of 200 mm.

    R=20

    Axial load of 470 kN

    Lateral load

    300 1800 300

    2 layers of CF sheetsWallColumn

    Finishing mortar ( WM-series)

    1 layer of CF sheets

    Adhering length=200mm

    Longitudinal reinforcement of columns : 8-19 Hoops of columns : 4-@100Wall reinforcement :4-@200 double for both vertical and horizontal directions

    300

    CF-anchors

    Wall:1 layer of CF sheet for both the face and back surfaces

    Columns:2 layers of CF sheets

    R=20

    Lateral load Lateral load

    300 2700 300

    300

    CF-anchorsCF sheets installed on the wall

    2 layers of CF sheetsCF-anchors

    CF sheets are installed along the two opposite angles (the figure shows installation of 2 layers for each angle).

    Fixing CF-anchor

    Penetrating CF-anchor

    Angle of CF sheet

    Series 1 (for WM-D-CI)

    Series 2 (for No.2)

    2010020

    100

    100

    300100

    1200

    900

    470 kN

    Lateral load

    Longitudinal reinforcement of columns : 8-19Hoops of columns : 4-@100Wall reinforcement :4-@150 double for both vertical and horizontal directions

    WallColumn

    200

    2 layers of CF sheets

    Fig.3 Configuration of specimens and strengthening methods

    111

    Seismic design of concrete structuresSession 6

  • Table 2 Mechanical properties of materials

    (a) Concrete (b) Rebar

    (c) Carbon fiber sheet

    (d) Carbon fiber strand

    Fiber weight(g/m )

    PAN type 300300

    Tensile strength(MPa)34802640Pitch type

    2Design thickness

    (mm)0.1670.142

    Type of CF sheet

    PAN type (24K)

    Tensile strength(MPa)4500

    Density(kg/m )

    1800

    Type of CF strand3

    Size amount(%)0.2

    Compressive strength(MPa)

    Series 1No.1No.2No.3No.4

    27.925.626.324.728.6

    Series 2

    No.5 30.5No.6 26.0No.7 29.5No.8 30.5

    Tensile strength(MPa)2.382.492.522.192.652.302.432.852.86

    Yield strength(MPa)

    Series 1

    19

    419

    474330438344

    Series 2

    Tensile strength(MPa)

    519450498

    539

    4

    .

    and the internal length was 1,800 mm. The side column was 300 mm x 300 mm in section. The Series2 specimens were walls of 100 mm in thickness, 900 mm in internal height, and 2,700 mm in internallength with side columns of 300 mm x 300 mm in s