우리나라의 내진설계기준은 1988년에 6층이상의 건축물에 처음 시행되었으며, 그 이후 3층이상의 건축물에 점차적으로 적용되었다. 그러나, 1988년 이전에 지어졌거나 설계기준에 적용되지 않은 건축물들은 여전히 지진에 취약하다. 최근 안전성 확보 및 기존 건축물의 내구성을 위한 적절한 보강재료 및 공법에 관한 연구가 활발히 진행되고 있다. FRP 보강공법은 경량성, 높은 인장강도 및 내구성, 내부식성 등의 장점을 가지고 있어 다양하게 적용되고 있지만 환경적 요인에 의해 강도 저하가 발생할 수 있으며, 높은 내구성에 비하여 취성적인 파괴가 발생할 수 있다는 단점이 있다. 따라서 본 연구에서는 기존의 FRP 섬유보다 우수한 연신율 및 화학적 저항성, 마모 및 마멸에 대한 높은 저항성, 높은 인장강도, 낮은 밀도를 가진 UHMWPE섬유를 철근콘크리트 보에 적용하여 성능을 분석·평가하였다. 일반적인 철근콘크리트 보의 주요한 파괴형태인 휨파괴 및 전단파괴를 고려하여 연성적인 휨설계를 하였으며, 전단 경간비를 2.5로 설정하여 휨·전단파괴를 유도하였다. 주된 랩핑방법인 U자 자켓팅을 통하여 기존의 탄소섬유, UHMWPE섬유 1겹,2겹의 보강변수에 따른 파괴양상, 강도 및 연성을 확인하기 위하여 정적가력실험을 실시하였다. 탄소섬유, UHMWPE섬유 1겹, UHMWPE섬유 2겹을 보강한 실험체는 무보강 실험체의 최대하중인 241.1kN에 대비하여 각각 2.38% 감소, 4.47%, 3.86%의 강도가 향상되었으며, 연성비는 각각 6.0, 7.2, 7.0, 5.9로 나타났으며, 무보강 실험체에 대비하여 각각 19.24%, 16.09% 향상, 2.82% 감소되는 것을 분석하였다. UHMWPE섬유 1겹, 2겹을 보강한 경우 강도적 측면에서 탄소섬유보다 우수한 강도증진효과가 나타났으며, 연성적 측면에서는 UHMWPE섬유 1겹 보강과 탄소섬유 보강은 유사한 연성적인 거동을 나타냈으나, UHMWPE섬유 2겹을 보강한 경우 유효단면적 손실로 인하여 계면박리현상으로 취성적인 거동을 나타냈다. 위 결과를 고려할 때, UHMWPE섬유 시트로 보강된 RC보는 연성과 우수한 강도 증진효과를 나타낼 것이라 판단된다.
The seismic design code in Korea was introduced first in 1988 for newly constructed buildings higher than six stories and has been gradually applied to three-story buildings since then. However, many of existing buildings which were built before 1988 or those lower than application of seismic standard are still vulnerable to earthquake. Many recent research studies to reduce seismic vulnerability of existing structures and improve their performance during earthquakes have focused on seismic retrofit using fiber reinforced plastic (FRP) sheets wrapped around the structural member. However, FRP retrofitting method has been known to cause strength reduction and brittle failure of members due to various environmental factors. This study presents an experimental investigation on the performance of RC beams strengthened with Ultra-High-Molecular-Weight Polyethylene(UHMWPE) fiber sheet. Compared to conventional fiber, UHMWPE has high tensile strength, high elongation, chemical resistance, high resistance into abrasion, and low density. A total of four specimens with a shear-span ratio of 2.5 were designed and constructed by considering flexural-shear failure mode. The first specimen (NRF) was a normal beam without any retrofit. The second specimen (CRF) was strengthened by the conventional carbon fiber sheet. The third and fourth specimens (URF-1 and URF-2) were retrofitted with UHMWPE fiber of 1 and 2 layers, respectively. The FRP sheet was wrapped around three sides of the member, where an integral slab makes it impractical to completely wrap the member and both of flexural and shear strengths can be improved. The specimens were experimentally investigated through static tests in order to assess the structural behavior of the retrofitted RC beams. Experimental results showed that the maximum force of the specimen NRF was 241.1kN. Compared to the specimen NRF, the maximum force of the specimen CRF was reduced by 2.38%, while those of specimens URF-1 and URF-2 increased by 4.47% and 3.86%, respectively. The obtained experimental data were used to estimate the displacement ductility of each specimen. The displacement ductility ratio of the specimen NRF was estimated as 6.0. The ductility ratios of the retrofitted specimens, CRF, URF-1, and URF-2 were found to be about 7.2, 7.0, and 5.9, respectively. Compared to the specimen NRF, the ductility of specimens CRF and URF-1 increased by 19.24% and 16.09%, respectively. Thus, the specimens retrofitted with carbon and UHMWPE fiber showed ductile behavior. However, the ductility of the specimen URF-2 was reduced by 2.82%. This could be inferred that the Interface debonding phenomenon between concrete and FRP sheet due to loss of effective section area causes brittle behavior of the specimen URF-2. Therefore, considering observations from these tests described above, it is clearly shown that the retrofitting method using UHMWPE fiber sheet can improve the strength and ductility of RC beam.