A ductility-type seismic retrofitting technique is proposed for reinforced concrete (RC) columns. This involves arranging high-strength steel prestressing bars (PC bars) on the four faces of an RC column as external hoops. Hence, active confinement due to the tensile forces in the PC bars as well as passive confinement and transverse reinforcement can be expected. With regard to the active and passive confinements, previous authors have shown that the compressive strength and ductility of confined concrete are greatly improved and have proposed a stress–strain relation for confined concrete. The aim of the present study is to experimentally investigate the shear strength and shear resistance mechanism of RC columns retrofitted by this technique.
The retrofitting details of the test specimens are shown in Table 1. The elevation and cross-section are shown in Fig. 1. To investigate the shear resistance mechanism (truss or arch mechanism), two types of specimen are considered in this study. One is a retrofitted RC column with no bond force between the concrete and the embedded longitudinal reinforcement, thereby generating the arch mechanism. The other is a retrofitted RC column with bonded rebars to evaluate the truss mechanism, which relies heavily on the bond resistance between the rebars and concrete. The column specimens used in this test had a square cross-sectional dimension of 250×250 mm2, a height of either 500 mm or 750 mm, and a shear span-to-depth ratio (M/VD) of either 1.0 or 1.5. The longitudinal reinforcement ratio (pg) of the retrofitted specimens with bonded rebars was either 3.67% (8-D19) or 5.51% (12-D19) and that of the specimens with unbonded rebars was 1.36% (12-D10). The shear reinforcement ratio (pw) of the column specimens was 0.08%. The test parameters of the column specimens are the initial tension strain of the PC bars, their spacing, the axial force ratio, shear span-to-depth ratio, and the bonded/unbonded nature of the rebars.
In Chapter 3, the following main points are discussed. (1) The initial tension force of the PC bars enhances the shear strength of the truss mechanism. (2) Applying a lateral confinement pressure to an unbounded RC column, the shear strength of the arch mechanism can be increased. (3) For the bonded specimens, the measured gradient of the compressive diagonal force of the truss mechanism, which is one of the internal forces of an analogous truss comprising PC bars that act as tension members, the bond force of rebars, and the diagonal concrete force was nearly 45°. (4) The shear strength of the arch mechanism increases with increasing axial force. (5) In the unbonded specimens, the slope of the line of thrust and the depth of the compression zone of the arch mechanism correlate with the lateral confinement pressure and the vertical axial load. (6) In the bonded specimens, the depth of the compression zone of the arch mechanism had no correlation with the lateral confinement pressure and the vertical axial load.
Chapter 4 describes the comparison between the experimental shear strength and calculated results based on the modified Arakawa mean and Minami equations shown in Figs. 17 and 18. These figures show that the test results are in agreement with the modified Arakawa mean equation, and that the modified Minami equation can reasonably be applied to assess the shear strength considering the truss and arch mechanisms.
