1. Introduction
Yamakawa et al. proposed an emergency retrofit method using stressed external hoops with respect to the R/C column suffered shear damage and discussed its effects. The aim of this paper is to identify the bond splitting strength of main bars after the emergency retrofit, τbru, as a part of the development of this emergency retrofit method.
2. Test Specimens
The total number of test specimens is 29, where the cross-sectional dimension is 250 mm×250 mm. The main test variables are damage level, diameter of main bar, db, number of main bars, n, compressive strength of concrete, σB, external hoop reinforcement ratio, pwpc, and initial lateral pressure, σr (see Fig. 1 and Table 1).
3. Test Method
First, a cantilever type monotonic tensile test was conducted to introduce a damage to the test specimen, where load was applied until the maximum splitting crack width reaches target for respective test specimen. After the damage introduction loading, external hoops were installed as the emergency retrofit (see Fig. 1). And then, main bars were pulled until failure. Displacement transducers measure slip displacements at free end for each main bar (see Fig. 3).
4. Test Results
In the loading after emergency retrofit, all test specimens failed in side split failure mode (see Fig. 5). The larger the bond slip displacement at the end of damage introduction loading, scd, the smaller τbru (see Figs. 4 (a), (b), (c)). Also, the larger σr, the larger τbru (see Figs. 4 (d), (e), (f)).
5. Discussions on Bond Splitting Strength after Emergency Retrofit
The test results showed that scd is adequate as a damage index that indicates deterioration in bond capacity caused by the earthquake (see Fig. 8). Based on this, Equation (7) was derived as a formula for calclating τbru. However, since scd cannot be evaluated directly in an actual column member, it needs to be evaluated based on any measurable physical quantity. In this study, we evaluated scd indirectly based on crack area and crack width, respectively, and examined performance of those evaluations (see Figs. 14, 15, 16 and 17).
6. Conclusions
1) scd was found to have a close correlation with τbru. This suggests that scd can serve as a damage index.
2) If the bond splitting strength in case when stressed external hoops are installed without initial damage, τbu, is taken as a standard, there was no significant difference in the decrease rate of τbru/τbu due to an increase of scd despite of difference in db, σB, and pwpc. On the other hand, the decrease rate is different with σr. Based on this fact, Equation (7) was derived as a formula for calculating τbru. Equation (7) provided the bond strength of 1/0.834 to 1/1.28 times the experimental value.
3) If scd was evaluated indirectly with the area of bond splitting crack, Equation (7) provided the bond strength of 1/0.727 to 1/1.61 times the experimental value. Also, if scd was evaluated with the maximum width of bond splitting cracks which can be obtained more easily, Equation (7) provided the strength of 1/0.700 to 1/1.68 times the experimental value.
