Seismic Retrofitting of Corroded Reinforced Concrete Columns Using Carbon Composites

ACI Structural Journal, May/Jun 2007 by Aquino, Wilkins, Hawkins, Neil M

Carbon fiber-reinforced composite band schemes

The number of carbon composite layers used was determined by following the procedure developed by Shkurti,13 and reported in Hawkins et al.,12 for upgrading columns with inadequate length lap-splices. Because different CFRP band widths and spacing were used in the various specimens, the thickness of the bands was calculated according to the procedure presented in Reference 13 to provide the same level of confinement. Therefore, the structural responses of all the retrofitted specimens were expected to be similar. Details for the calculation of the CFRP composite thickness are given in the following. A more detailed rationale for the mechanics behind the design approach outlined herein can be found in Reference 13. The band-width layouts used are shown in Table 1 and the CFRP system properties are given in Table 2.

First, a crack width is calculated for the shearing plane between the dowel bars and the column bars. This crack width is calculated as

... (1)

where D' is the diameter measured to the centerline of the dowel bars and ε^sub d^ is the limit strain in the CFRP jacket or bands. This limit strain is to be taken as 1/3 of the ultimate strain of the jacket, but not greater than 0.003.12 For this work, a limiting strain equal to 0.003 was used because this number was smaller than 1/3 of the ultimate strain (0.0155) of the CFRP. Next, the maximum shear strength v^sub cim^ that can be developed in the shearing plane between the dowel bars and the column bars is calculated from the crack width as

... (2)

where a is the maximum aggregate size (19 mm [3/4 in.] for this work) and f'^sub c^ is the concrete compressive strength. To develop the full tensile strength of the dowel bars, the shear stress that must act on the shearing plane is calculated as

... (3)

where A^sub b^ is the area of a dowel bar, f^sub s^ is the tensile strength of the steel, d^sub b1^ is the diameter of a dowel bar, d^sub b2^ is the diameter of a column bar, c is the clear cover to the surface of the dowel bars, n is the number of dowel bars, and L^sub s^ is the splice length. The compressive stress f^sub ci^ that must act normal to the shearing plane to develop vci can be calculated from14

... (4)

The compressive stress f^sub ci^ should not be taken to be less than 0.24 MPa (35 psi). The thickness t of the CFRP composite is then calculated as

... (5)

where D is the diameter of the column, E^sub c^ is the elastic modulus of the CFRP composite, and L^sub c^ is total length of CFRP covering the lap splice length. If the CFRP band width is denoted by B and n^sub c^ is the number of bands over the splice length, the total length of CFRP can be computed as

L^sub c^ = n^sub c^ x B (6)

The number of CFRP layers n^sub layers^ can then be determined as

... (7)

where t^sub sheet^ is the thickness of one dry carbon fiber sheet. This thickness t^sub sheet^ was equal to 0.1638 mm (0.0066 in.) (0.63 mm [0.0252 in.] × 0.26) in the specimens used for this work. The number of bands, band width, and thickness for the retrofitted specimens are given in Table 2.