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2.2. Strengthening schemes
As shown in Fig. 3, the U form, 100 mm wide carbon fiber strips wrapped in transverse direction was spaced 200 mm on center apart, with 150 mm anchorage length on the top surface. The two carbon fiber strips pasted in longitudinal direction on bottom surface were 150 mm wide and spaced 250 mm on center apart. The retrofit schemes of four specimens are shown in Table 1.
2.3. Loading scheme
This quasi-static test followed load-deformation mixed control loading scheme. Load control was followed before cracking. At each load step torque was applied one cycle in positive and negative direction with the constant torque bending ration (T/M = 0.375). After concrete cracked,deformation control was followed and stage loads were applied according to the integral multiple of crack twist angle (hcr) until specimen yielding, usually three cycles.After yielding of box beam, stage loads were applied according to the integral multiple of yielding twist angle
(hY), usually three cycles, until the specimen lost its capacity to carry more load in one loading direction. Specimen failure was determined as this point.
3. Test result
3.1. Test process
The failure patterns of all the four specimens were bending torsional failure. With the increase of torque moment and vertical load on the 3/4 point of the beam, vertical crack due to tensile stress occurred at first at the bottom on shear torque added side of a certain cycle.When torque moment was applied in another direction, vertical crack appeared at the bottom on shear torque
added side of this moment. Before specimen yielding,the concrete cracks were narrow and developed to diagonal direction due to shear forces. They could be closed when unloading. The cracks in positive direction could not be observed when torque moment applied in negative direction. After specimen yielding concrete cracks developed quickly and could not be closed completely when unloading. Crossed netlike cracks formed on the beam surfaces. With the increase of times of cycle, diagonal cracks on the two adjacent sides opened throughout the surfaces. At the failure point obvious bending deformation and extensive crossed netlike cracks at 35_–50_ along the length of the beam can be observed and the concrete at the shear torque counteract side of a certain cycle were crushed (as shown in Fig. 4). By comparing the crack developments of strengthened specimens (B-6, B-7, B-8) with that of the unstrengthened specimen (B-5), it can be obtained that cracks of strengthened beams distribute more evenly with smaller width and develop more slowly. This is because that cracks were not allowed to widen due to the restraint provided by the fiber.
3.2. Torsional capacity and deformation capacity
The crack torque, crack twist angle, yield torque, yield twist angle, ultimate torque, ultimate twist angle and ductility factor for all the tested beams are listed in Table 2.CFS strengthening has few effects on the crack torque,yield torque and torsional rigidity before concrete cracking.
However, it is more effective in improving the torsional strength. Applying one layer U-shape transversal wrapped CFS strips (B-6) resulted in 31.22% increase of ultimate torque (It is the even value of the increase of positive and negative torque moment). Wrapping one layer U-shape transversal CFS strips and one layer longitudinal CFS strips on bottom surface of box beam (B-7) improved the torsional moment capacity by up to 32.5%. Strengthened with two layer U-shape transversal CFS strips and one layer longitudinal CFS strips on bottom surface of box beam (B-8) improved the torsional moment capacity by up to 48.57%.It can be also concluded in Table 2 that CFS strengthening can obviously improve the deformation capacities of box beams. Compared with the reference beam B-5, the ultimate twist angle of specimen B-6 is improved by 20.3%. That of B-7 is improved by 24.6% and that of B-8 is improved by 56.1%. It can be concluded that deformation capacity is improved with the amount of CFS and transversal CFS strips have greater retrofit effect than longitudinal CFS on bottom surface. It must be noticed that ductility factor of strengthened beam is less than unstrengthened beam (Ductility factor is defined as the ratio of ultimate twist angle to yielding twist angle of one specimen).However, the ductility factors of strengthened beams are still 3.0, which still satisfy with the requirements of aseismic code [7].