숏크리트로 보강된 개구부를 가지는 전단벽의 구조적 거동에 관한 실험적 연구

Abstract

철근콘크리트 구조물은 재료적, 시공적, 환경적, 구조적 요인 등으로 인하여 시간이 경과하면서 구조내력이 부족하거나 사용성이 저하하여 리모델링시 보수, 보강 등을 실시하여 안전성을 확보하여야 한다. 특히 사용성의 증가, 내력저하, 각종 하자 발생 등으로 보강을 실시해야 하는데 있어서 중요한 것은 효율적이면서 경제적인 보강이 이루어져야 한다. 특히, 세대병합 리모델링시 기존 벽체에 개구부를 갖는 경우는 필연적으로 발생한다. 따라서 본 연구에서는 세대병합을 위하여 기존 벽체에 개구부를 신설 할 경우 적정 면적비를 위한 선행 연구결과1)를 바탕으로 리모델링시 사용자의 욕구를 충족시키기 위하여 국부적으로 절단(해체)된 철근콘크리트 벽체의 구조성능을 규명하고자 한다. 더 나아가서는 손상부위를 갖는 철근콘크리트 벽체에 숏크리트를 이용한 단면증설 보강효과를 규명하고자 한다. 본 연구에서는 HPFRCC의 한 종류인 ECC를 부재(벽체)의 보강에 적용하므로써 보강효과를 규명하고 개구부를 갖는 벽체에서 효율적인 단면증설의 보강방법을 기존의 숏크리트 재료인 MDF와의 비교를 통하여 도출하고자 한다. 이상의 모든 내용은 본 논문에 총 6 장으로 구성되어 있으며, 제 1 장 서론, 제 2장 기존 연구 및 전단벽 전단 내력식, 제 3 장 실험체 계획, 제 4 장 실험체 세팅 및 각종 측정장치, 제 5 장 실험결과 분석, 제 6 장 결론 등으로 기술되어 있다. 실험결과 고인성 시멘트 복합체인 ECC로 단면증설된 WB-ECC가 손상되지 않은 실험체인 WB-Solid 보다 약 10% 높은 강도와 연성을 나타내었다. 반면, 고압 습식 스프레이인 MDF로 단면증설된 WB-MDF의 경우 WB-Solid와 동일한 내력을 보였지만, 약 30% 낮은 연성을 보였다. 아파트 리모델링시 세대간벽에 개구부가 설치 될 경우, 가장 취약해지기 쉬운 저층부(1～3층) 세대간벽의 보강방법으로 단면증설이 유효한 방법 중 하나가 될 수 있으며, 보강재료로는 일반 숏크리트 보다는 고인성시멘트복합체를 이용한 ECC 숏크리트를 이용하는 것이 유효하다고 사료된다.; Recently, it is interested that HPFRCC(High Performance Fiber Reinforced Cementitious Composites) are adopted in repair and retrofit applications, because of the characteristics relating to high tensile ductility performance, cost-sensitivity and ease of execution at construction sites. A series of three shear wall specimens was tested under constant axial stress and reversed cyclic lateral loading in order to evaluate the seismic retrofit that had been proposed for the shear wall with the opening. The opening is induced by demands of clients who want to change their living spaces. The retrofit involved the use of newly developed ECC and MDF(Macro Defect Free), both of which are sprayed through the high pressure pump, over the entire face of the wall, aimed at meeting stringent practical demands of durability with the resistance to fire and fatigue. And half-scale low-rise shear wall specimens representing the wall of an existing old shear wall dominant apartment were constructed and tested. The results, presented and discussed in this paper, indicate that the retrofit methodology can increase the displacement ductility and the strength. Especially, the strength of the specimen retrofitted by ECC was 65% higher than that of the specimen without retrofitting and two retrofit specimens did not suffer a significant loss of strength after reaching ultimate strength. The ultimate failure mode of the specimens retrofitted by ECC and MDF was found to be compression fracture in plane direction and bond failure in interface between existing and added concrete, respectively. The advantages of high composite ductility in the hardened state and the flexible processing in the fresh state will make ECC more attractive for a broad range of applications. The definition of a HPFRCC is a fiber reinforced cement based matrix material that exhibits tensile strain-hardening behavior under uniaxial tension by redistributing stresses in multiple cracks. ECC is one of the HPFRCCs that contains fibers by approximately 2% in volume and Fig. 1 shows tensile strain capacity over 3%, approximately 300 times larger than that of a normal concrete or fiber reinforced concrete(FRC). MDF is a newly developed shot concrete that is widely used in the retrofit of large structures and is renowned for its high-speed hardening it has a practical purpose. Studies on HPFRCC have rarely been done on structural members. Previous experiments focused on the material’s characteristic before and after hardened state and manufacturing skill. In present experimental program, the focus is on the structural performance of perforated RC walls retrofitted by ECC and MDF. The opening of the wall specimens simulates the door area the would induced by remodeling. Experimental Research Test Specimens. Specimens were 1/2-scale representations of a one-story wall in a Korean apartment building that was built in 1980. Based on the same reinforcement ratios as the prototype wall that was used in the study, a compressive strength of 21MPa was assumed for concrete and a yield stress of 400MPa was assumed for reinforcement in design calculations. An opening in the pierced wall is a 9001050-mm rectangular opening simulation a door area. Locations and dimensions of the opening and retrofit area are shown in Fig. 2 and 3, respectively. To obtain the strength of the wall before being perforated, ECC and MDF were casted in a thickness of 3mm over a 10cm spacing wire mesh inserted to protect it from shrinkage and craks on either side of the wall. A series of the three shear wall specimens is listed in 표 1. The material specimen of MDF and ECC were cast before being used in the retrofit and tested in 28 days past. 표 2, 3. show the properties of MDF, ECC and fiber. Test Setup. A picture of the test setup is shown in Fig. 4. Two actuator were mounted vertically, one on either side of the wall and applied with 630kN of constant axial compression during testing. This corresponded to approximately 10% of the computed concentric capacity of the walls which has the lateral load was applied at the top of the wall by a 2000kN hydraulic actuator mounted between the specimen and a reaction wall in the displacement control mode. Guide beams and ball-zigs were used to minimize out-of-plane movement and simulate the diaphragm effect of the slab. Discussion of Results Failure Patterns. For WB-MDF, crack propagation on the web and interface of the wall had compression failure at the base which occurred at a drift of 0.9%. For WB-ECC, the high-tensile-fiber contained in ECC controlled cracking effectively, cracks on the web of the wall were hardly noticeable until a drift of 0.9%. At a drift of 1%, crack propagation at the interface of the ECC and a compression failure at the base caused the final failure. The two strengthened specimens failed after crack propagation at the interface but what there was a noticed difference that the interface crack in the WB-ECC propagates toward the wall center unlikly the crack in WB-MDF propagates vertically. This indicates that the ECC material attaches better to the wall face than the MDF material. Stiffness and ductility. Initial stiffness of the MDF(118kN/mm) and ECC(136kN/mm) are 23.5% and 42.3% higher than that of the WB-0.23(95.6kN/mm). Despite identical casting amounts, initial stiffness of the WB-ECC is 15% higher than that of WB-MDF. This can be attributed to the high tensile strength of the fiber that delays initial hairline-width cracks. The definition of ductility ratio(μ) is a maximum displacement divided by the displacement at yield. And yield displacement is defined as the X component of the point which has the 0.75Pu as Y component lying on the backbone curve. As 표 4. shows, the ductility ratio of WB-0.23, MDF and ECC are 1.45, 1.6 and 2.4, respectively. The ductility of WB-ECC is much higher than WB-0.23 and WB-ECC by 65% and 50%, respectively. Strength Considerations. The strength of WB-MDF and WB-ECC were 48% and 67% higher than that of the original specimen(WB-0.23). This is clearly shown in Fig. 5(b) and the post-peak resistance of the wall was dramatically reduced (a sudden reduction in strength) in WB-MDF. This sudden drop is mainly attributed to the absence of the ductile material that can resist on fractures over high deflection and the partial destruction of the bond between the reinforcement and concrete surface. Strength degradation rates after maximum strength was reached in WB-MDF, ECC were 9.4% and 5%, respectively. It should be noted that WB-ECC was able to sustain more loading cycles and with little strength degradation than WB-MDF. Shear Deformation. Fig. 7 shows lateral deflection attributed to shear deformation measured around the opening of each specimen at specified drift. The Lateral deflection of original specimen (WB-0.23) is higher than the others by approximately 57%. Especially, the deflection of WB-ECC is lower than that of the WB-MDF, mainly because the fiber bridges over shear cracks and resist on shear deformation effectively. Energy Dissipation. Cumulative energy dissipated is shown Fig. 8. The energy dissipation of the three walls shows differences after cycle 12(at drift of 0.6%) because of the difference in yield point. Fig. 8 reveals that MDF and ECC dissipates 24.5% and 31.7% of it’s total energy at cycle 18(at a drift of 0.9%), respectively. Summary 1) For the strengthened specimen, partial failure occurred in the interface between wall and reinforcement before the diagonal shear failure. However, the interface crack in WB-ECC propagates towards the wall’s center while the crack in WB-MDF propagates vertically. This is due to the ECC material attaching better on the wall face than the MDF material. 2) Ultimate strength of the MDF(786kN) and ECC(888kN) are48% and 67% higher than that of the WB-0.23(555kN). 3) Initial stiffness of strengthened specimens is much higher than that of the original specimen. However, the stiffness reduction rate of the strengthened specimens, after the point of yielding of WB-0.23, was similar to that of the original specimen(WB-0.23). 4) WB-ECC was able to sustain more loading cycles and with smaller strength degradation than WB-MDF.