Experimental Investigation on Seismic Resistance of RC Shear Walls with CFRP Bars in Boundary Elements

Six identical geometry specimens with the same reinforcement details but different forms of stirrups in boundary elements were fabricated and tested. Figure 1 shows the dimensions and reinforcement details of the specimens. The boundary elements of specimens CFRPRW had rectangular stirrups and of the specimens CFRPHW had circular stirrups in boundary elements. Both rectangular and circular stirrups in boundary elements were 6 mm-diameter HPB300 steel bars with spacing of 50 mm. Eight 12 mm-diameter CFRP bars and four 8 mm-diameter steel bars were placed in each boundary element as longitudinal bars. Vertical and horizontal web reinforcement for all specimens were HRB335 steel bars with the diameter of 8 mm. The vertical web reinforcement consisted of two layers of steel bars spaced at 60 mm, and the horizontal web reinforcement also consisted of two layers of steel bars, while the spacing of which were 50 mm and 70 mm, respectively, in the region below the height of 600 mm from the base and the upper region. The high-strength and non-contract grouting material were used between the end of CFRP bars and sleeves welded on steel plate to enhance the bond between CFRP bars and concrete for the purpose of avoiding slip and ensuring the maximum performance of CFRP bars as shown in Fig. 2.

The axial load ratios of the specimens were 0.17, 0.26 and 0.33, respectively. CFRPRW1 and CFRPHW1 were tested under quasi-static reversed cyclic loading with the axial load ratio of 0.17. CFRPRW2 and CFRPHW2 were tested with the axial load ratio of 0.26. CFRPRW3 and CFRPHW3 were tested with the axial load ratio of 0.33.

Besides the six concrete shear walls reinforced with CFRP bars mentioned above, a reference specimen, RCSW, which was RC shear walls reinforced with deformed steel bars in boundary elements, was also tested under the axial load ratio of 0.17. RCSW had the same geometry and longitudinal reinforcing bars and stirrups except the longitudinal reinforcements in boundary elements compared with CFRPRW1. Besides, RCSW was tested with the test methods which was the same as CFRPRW1. The experimental program of RCSW was introduced in detail in another literature of the author (Zhao 2019).

Ready-mixed concrete was used to cast the specimens, and the concrete cubic (150 mm length) strength, f_cu, was 57.2 MPa. The concrete prismatic (300 mm high by 150 mm length) strength, f_c, was 36.8 MPa. The mechanical properties of CFRP bars and steel bars are shown in Table 1.

The test apparatus is shown in Fig. 3. The foundation beam of shear wall was anchored to the laboratory floor and restrained by four hydraulic jacks and two steel beams to eliminate horizontal slip and vertical movement. Cyclic lateral loading was applied through the MTS servo-controlled hydraulic actuator at a height of 2560 mm from the top surface of the foundation beam. A constant axial compressive load was applied by two hydraulic jacks on the top of the loading beam. And a sliding mechanism was installed between the vertical hydraulic jacks and reaction beam to ensure that the hydraulic jack can move with the specimen, leading to constant vertical loading.

Reversed cyclic lateral loading was controlled by the drift ratio, which was the ratio obtained by dividing the lateral top displacement by the shear span. The loading program was shown in Fig. 4. Two cycles were applied at each drift level with increments of 0.2% up to 2.0%. In subsequent loading, one cycle was applied at the drift levels of 2.5%, 3.0% and 3.5%, respectively.

Measuring and recording of the strains of reinforcement and concrete, as well as the displacements of specimens were carried out in the test. The strains of reinforcement and concrete were measured by strain gages with the locations as shown in Fig. 5. Strain gages were placed on CFRP bars, steel bars in the wall panel, and stirrups in the boundary elements. Figure 6 showed the locations of displacement transducers to measure the displacements of different heights of shear walls.

Figure 7a, b show the crack patterns and failure modes of RCSW and CFRPRW1, respectively. Both of them experienced a flexure-shear failure mode. The cracks propagated more adequately on the façade of CFRPRW1 compared with that of RCSW. The reason account for that phenomenon was that the CFRP bar was a kind of linear-elastic material with high tensile strength and low bond strength. Then there was no too large plastic deformation on CFRP bars at hinge region. Besides, the low bond strength allowed CFRP bars to slip in concrete. So cracks distributed along the whole height of CFRPRW1. It can also be observed that concrete spalling of RCSW was more serious than that of CFRPRW1. The conclusion can be drawn that placing CFRP bars in boundary elements can improve crack propagation and concrete spalling.

The crack patterns and failure modes of CFRPRW1 and CFRPHW1 were shown in Fig. 7b, c, respectively. CFRPRW1 presented the typical flexible-shear failure mode. The first horizontal crack was observed near the bottom corner of the wall at the drift ratio of 0.4%, while the diagonal crack appeared after the 0.6% lateral drift, and meanwhile the yielding of outermost longitudinal steel bars occurred. As loading continued, the cover concrete near the wall toe began to spall when drift ratio reached 1.6%. And with the displacement increased, the load reached the peak value at the lateral drift of 2.5%, followed by the ultimate drift ratio of 3.0%. As shown in Fig. 7b, cracks sufficiently propagated with the distribution of the whole height of the wall panel and the spacing of approximately 100 mm. The concrete spalling region with 300 mm in height and 200 mm in depth could be observed. CFRPHW1 had the similar crack pattern and failure mode with CFRPRW1. At the lateral drift of 0.2%, a horizontal crack initiated before the 0.4% drift at which the first horizontal crack on CFRPRW1 was observed. As the lateral load increased, the diagonal crack and yielding of steel bars were observed at the same drift ratio of 0.6% with CFRPRW1, while the spalling of concrete cover at the 1.2% drift was earlier than that of CFRPRW1. The peak load and ultimate lateral drift of CFRPHW1 were of 2.5% and 3.0%, respectively, which were the same with CFRPRW1. And as shown in Fig. 7c, the height of crack propagation, the height and depth of concrete cover spalling were 1900 mm, 200 mm and 250 mm, respectively. Then it could be concluded that there were no noticeably differences in crack pattern and failure mode between CFRPRW1 and CFRPHW1.

CFRPRW2 and CFRPHW2 were tested under the 0.26 axial load ratio. The first horizontal crack appeared on the bottom section of CFRPRW2 at the drift of 0.4%. When the drift ratio reached 0.6%, the diagonal crack was observed and the outermost longitudinal steel bars yielded. The concrete spalling emerged at the drift ratio of 1.4%. Then the load increased to the peak value as the drift went to 1.8%, after which the load began to drop down and was terminated after the cycle of 3.0%. Cracks distributed through the height up to 1700 mm of the wall panel. Concrete spalling mainly occurred in the bottom area of wall panel with 250 mm height and 400 mm depth as shown in Fig. 7d. The crack pattern and failure mode of CFRPHW2 were shown in Fig. 7e. The horizontal crack and diagonal crack emerged at the lateral drift of 0.2% and 0.6%, respectively. As the lateral displacement increased, the cover concrete began to spall at the drift ratio of 1.4%. After that, CFRPHW2 reached peak load at 1.8% drift, then the load began to decline and was terminated after the cycle of 2.5%. Cracks propagated along the height of the wall panel and distributed within 1600 mm height. The concrete spalling occurred in the bottom area with 250 mm height and 550 mm depth.

CFRPRW3 and CFRPHW3 were tested under vertical load of the 0.33 axial load ratio. CFRPRW3 cracked horizontally at the 0.4% drift ratio. With the drift ratio increased to 0.6%, diagonally crack and the yielding of longitudinal steel bars appeared. Then the cover concrete began to spall when the drift ratio reached 1.4%. With the increase of displacement, CFRPRW3 reached the peak load at the 1.6% drift ratio, then the load began to decline and was terminated after the cycle of 2.5%. Cracks were distributed up to the height of 1600 mm of the wall panel as shown in Fig. 7f. The height and depth of spalling cover concrete were approximately 210 mm and 400 mm, respectively. As for the specimen of CFRPHW3, the first horizontal crack was observed at the lateral drift of 0.2%. Then the wall panel began to crack diagonally when the drift ratio reached to 0.6% and meanwhile the longitudinal steel bars yielded. The cover concrete spall at the drift ratio of 1.2%. After that, the load reached peak value at the 1.6% drift, then began to decline and was terminated after the cycle of 2.5%. Crack propagation distributed through the 1700 mm height of wall panel. The concrete spalled with the height of 300 mm and the depth of 450 mm as shown in Fig. 7g.

As suggested above, the axial load ratio has relatively significant effects on the failure modes and crack patterns of specimens. When the axial load ratio increased, the crack propagation was restrained, the height of crack reduced and the concrete cover spalling became serious. While shear walls had the similar failure progression and crack propagation at the same axial load ratio, which indicates that the form of stirrup had little influence on these experimental responses at the same axial load ratio.

The hysteretic curve illustrating the lateral load–displacement response is usually used to analyze the seismic performance of shear wall since it presents structure strength, stiffness degradation and energy dissipation under cyclic loading effectively. Figure 8 shows the hysteretic curves of specimens.

The hysteretic curve of RCSW exhibited typical ductile characteristics. The lateral force reached the maximum value at small displacement level and then didn’t rise with large residual deformation until failure.

In general, the hysteresis loops of walls reinforced with CFRP bars were on rising stage and the load increased with the augment of deformation up to peak load. For shear walls with the axial load ratios of 0.17 and 0.26, there were a slightly decline after the load reached the maximum value, which revealed that shear walls exhibited stable cyclical behavior up to large deformation without any apparent deterioration of capacity. However, when the axial compression ratio increased to 0.33, an obvious decline of capacity occurred after the maximum value, and the rate of capacity degradation became faster along with the increase of lateral deformation. At the unloading stages, all hysteresis curves approached toward the force axis, which indicated that all shear walls had little residual deformations after unloading, especially when the drift was less than 1.5%, the residual deformation reached almost 0.

CFRP bars in boundary elements could enhance the lateral resistance as well as reduce the residual deformation, which satisfy well with the requirements, stable resistance and little residual deformation of resilient structures.

Figure 9 presents the skeleton curves of the specimens under different axial load ratios of 0.17, 0.26 and 0.33. The ultimate displacement and the corresponding load were plotted on the curves. According to Fig. 9a, CFRPRW1 had the peak load of 888.77 kN which was 20.1% larger than that of RCSW with the similar ultimate displacement. As for shear walls using CFRP bars, the peak load increased with the increasing axial load ratio, while the displacement at the peak load and the ultimate displacement showed the opposite tendency. At the axial load ratios of 0.17 and 0.26, the skeleton curves almost maintained the stable trend with slight decline. While the skeleton curves of specimens at the axial load ratio of 0.33 presented obvious decline branch, which was more obvious for CFRPRW3 with rectangular stirrups than CFRPHW3 with circular stirrups. Figure 9b showed the comparison of specimens with rectangular stirrups at different axial load ratios. Taking CFRPRW1 as the reference, the peak loads of CFRPRW2 and CFRPRW3 increased by 2.9% and 2.7%, respectively. And CFRPRW2 experienced the similar ultimate displacement with CFRPRW1, however, that of CFRPRW3 was 26.14% smaller. As for the specimens having circular stirrups, the peak loads of CFRPHW2 and CFRPHW3 increased by 4.7% and 8.1%, and the ultimate displacements decreased by 21.14% and 28.15% respectively compared with CFRPHW1.

The skeleton curves of the specimens with different forms of stirrups and axial load ratios were compared in Fig. 9d–f. At the axial load ratios of 0.17 and 0.26, CFRPRW1 and CFRPRW2 reached higher peak loads than CFRPHW1 and CFRPHW2. While as the axial load ratio increased to 0.33, the peak load of CFRPRW3 was similar with CFRPHW3. As for the ultimate displacement, when the axial load ratio was 0.17, CFRPRW1 and CFRPHW1 exhibited the almost same ultimate displacement indicating that the form of stirrup had no significant influence on that at this axial load ratio. As the axial load ratio reached 0.26, the ultimate displacement of CFRPRW2 with rectangular stirrups was 30.90% larger than CFRPHW2 with circular stirrups. While the ultimate displacements of the pair of specimens were similar at the 0.33 axial load ratio.

The energy dissipation capacity of the shear walls was evaluated using the equivalent viscous damping coefficient (EVDC) and cumulative energy dissipation (CED) based on the Chinese Specification for Seismic Test of Buildings (2015). Figure 10a shows the EVDC-drift relationships of RCSW and CFRPRW1. RCSW had larger EVDC compared with CFRPRW1 because of the plastic deformation of deformed steel bars. Then the conclusion that ordinary RC shear walls had well energy dissipation capacity than CFRP reinforcing walls can be drawn. As one can see in Fig. 10b, c, EVDC of the shear walls reinforced with CFRP bars at different axial load ratios are compared. There was an inverted triangle trend of the EVDC-drift relation curve. In general, EVDC reached the minimum value at 1.4% drift and then increased to the maximum value until failure. And EVDC was in positive correlation with the axial load ratio at a given drift. Figure 10d–f compared EVDC of the shear walls with two kinds of stirrups. It can be observed that EVDC decreased and then increased at the axial load ratios of 0.17 and 0.26, and EVDC of the shear walls using circular stirrups were larger than those of the specimens using rectangular stirrups at relatively large lateral deformation level. While the shear walls with different stirrups had the approximate same EVDC at the axial load ratio of 0.33.

The CED of shear walls were shown in Table 2. On one hand, the influences of the axial load ratio on CED were related to the form of stirrup. For shear walls with rectangular stirrups, the CED of CFRPRW2, which was similar with CFRPRW3, increased by 23.86% compared with that of CFRPRW1. As for specimens confined by circular stirrups, no noticeable difference of CED among CFRPHW1, CFRPHW2 and CFRPHW3 could be observed. On the other hand, the influences of the form of stirrup on CED also concerned with the axial load ratio. When the axial load ratio was 0.17, the form of stirrup had little effect on CED. While as the axial compression ratio reached 0.26 and 0.33, compared with specimens having circular stirrups, the CED of specimens with rectangular stirrups were 21.76% and 26.57% larger, respectively.

Displacement transducers DH0–DH8 recorded the lateral displacements at the 600 mm, 1200 mm, 1600 mm, 2000 mm, 2560 mm height respectively from the bottom of the shear walls. As shown in Fig. 11, the measured lateral displacement curves obtained through DH0 ~ DH8 at the drifts of 0.6%, 1.0%, 1.6%, 2.0%, 2.5% and 3.0% evidence the flexural mode exhibited during the test for all specimens. At the onset of loading, the lateral displacement increased linearly along the height, then as the loading continued, lateral displacement curves began to incline to the displacement axis and the displacements of upper segments of specimens increased faster to present nonlinear variation along the height of wall due to the occurrence of plastic deformation.

Figure 12a displayed the strain–displacement curves of CFRP bars in the six shear walls. It could be concluded that the strains of CFRP bars varied cyclically with the drifts when shear walls were tested under the reversed cyclic lateral loading. And the strains represented the linear-varied trend at an angle of 45° taking the origin of coordinates as center roughly. When the drift reached 0, the strains of CFRP bars also almost recovered to 0, however, there were residual compression strains at the axial load ratio of 0.33. The phenomenon revealed that CFRP bars could present linear elasticity and offer resilience for shear walls to reduce residual deformations even at the large axial load ratio during the reversed cyclic lateral loading test. Besides, the resilience properties of CFRP bars were affected by the axial load ratio regardless of the forms of stirrups in boundary element.

Figure 12b showed the strains of the stirrups in the boundary elements at the height of 100 mm from the bottom of the shear walls. The strain of rectangular stirrup was quite higher than that of the circular stirrup at the same lateral displacement and axial load ratio, which meant that the performance and constraining function of the rectangular stirrup were much better. Furthermore, the strains of stirrups increased with the axial compression ratio and reached yielding at the axial compression ratios of 0.26 and 0.33.

To compare the strains between CFRP bars and steel bars in the shear walls, Fig. 12c depicted the strains of CFRP and steel bars at the height of 100 mm from the bottom of the shear walls. The CFRP bars in the shear walls remained good linear elasticity under reversed cyclic load, though the steel bars yielded at the large drift resulting in obvious plastic deformation. The linear elasticity of CFRP bar could produce self-centering ability for the shear walls, while the plastic deformation of steel bars contributed to the energy dissipation.

Residual deformation and residual crack width were two important parameters to assess the resilient performance of shear wall specimens, both of which had great influence on the reparability of shear walls after earthquake. Figure 13 presented the relative curves of peak and residual displacement after unloading at each drift level. The comparison of residual displacement–displacement relationship of walls using ordinary deformed steel bars and CFRP bars are shown in Fig. 13a. CFRPRW1 exhibited little residual displacement during the whole test. While the residual displacement of RCSW increased sharply to large value with the increase of the displacement. According to the result, Using CFRP bars can reduce the residual displacement from 20.97 mm to 11.10 mm after unloading from the displacement of 54 mm. The residual displacement–displacement curves of walls reinforced with CFRP bars are shown in Fig. 13b, at the incipient stage of loading, the residual displacement curves of the six specimens were similar and there was an upward trend for residual displacement with the increase of peak displacement. In subsequent stages of loading, the residual displacements were on rise, especially when the axial load ratio reached 0.33, the slope of residual displacement-peak displacement curve became steep obviously. The maximum residual displacement of CFRPRW1 was 11.10 mm and the corresponding peak displacement was 53.83 mm, which meant the deformation recovered 79.83% after unloading. The maximum residual displacements of CFRPHW1, CFRPRW2, CFRPHW2, CFRPRW3 and CFRPHW3 were 6.03 mm, 5.51 mm, 7.31 mm, 6.83 mm and 8.86 mm, respectively. And the corresponding peak displacements were 50.21 mm, 50.07 mm, 41.73 mm, 39.85 mm and 41.92 mm. The deformations of those specimens recovered 87.99%, 89.00%, 82.48%, 82.86% and 78.66%, respectively. The conclusion could be drawn that placing CFRP bars as longitudinal tensile material could make the lateral deformation recover a lot after the reversed cyclic loading.

Figure 14 illustrated the variation curves with drift ratios of the maximum crack width and residual crack width after unloading. According to Fig. 14a, the maximum crack width of RCSW increased sharply with the drift and eventually reached to 2.1 mm which was 24% larger than CFRPRW1 at the drift ratio of 2.5%. Besides, the ordinary RC shear wall RCSW had residual crack width of 1.0 mm at 2.5% drift, while the residual crack width of CFRPRW1 was only 0.63 mm. Figure 14c, d present the crack widths of CFRPRW. In the beginning of loading, the variation tendency of maximum crack widths was similar for the six specimens, and the residual crack width was almost 0 within 0.6% drift. As the drift increased, the maximum crack widths increased inversely with the axial load ratio, while the slopes of residual crack width variation curves became steep for all specimens with the increasing axial load ratio. When the drift ratio was 2.5%, the maximum crack width of CFRPRW1 was 1.70 mm, and the corresponding residual crack width after unloading was 0.63 mm, which meant the crack width reduced by 62.94%. The other five specimens CFRPHW1, CFRPRW2, CFRPHW2, CFRPRW3, CFRPHW3 had the maximum crack widths of 1.64 mm, 1.00 mm, 2.56 mm, 0.59 mm and 0.36 mm, respectively. And their corresponding residual crack widths were 0.39 mm, 0.03 mm, 0.36 mm, 0.11 mm and 0.03 mm. It could be obtained that the crack widths reduced by 76.22%, 97.00%, 85.90%, 78.43% and 91.67% respectively after unloading. The conclusion could be drawn that the crack widths of all shear walls reduced a lot after the reversed cyclic loading.

The analysis above concluded that all CFRP bars reinforcing specimens had little deformation and crack width after unloading, which showed great resilience of deformation and crack.

Figure 15 shows stiffness degradation curves of specimens. RCSW exhibits larger initial stiffness than CFRPRW1 until the displacement of 20 mm, after which RCSW and CFRPRW1 had almost similar stiffness degradation. The reason account for that phenomenon is that the Young’s modulus of steel bar is larger than that of CFRP bars, so ordinary RC shear wall had large initial stiffness. As for shear walls reinforced with CFRP bars, all specimens had similar stiffness degradation tendency. The stiffness degrades rapidly until the displacement of 6 mm. Then the stiffness degradation become tardy with the increase of displacement. The effects of axial load ratio on stiffness degradation can be concluded that stiffness degradation become slow as axial load ratio increases. Stirrup type also influences stiffness degradation. Shear walls with rectangular stirrups had rapid stiffness degradation than that with circular stirrups at the axial load ratios of 0.17 and 0.26. When the axial load ratio reached 0.33, there were no significant differences between these two stirrup types.