강섬유로 보강된 초고강도 콘크리트 휨재의 연성설계

Abstract

본 연구의 주요 목적은 현재 개발이 가속화되고 있는 초고성능 콘크리트의 일종인 섬유보강 초고강도 콘크리트의 표준적 사용을 위한 설계방안을 마련하는 데에 있다. 콘크리트의 고강도화는 구조물의 안전성, 내구성, 신뢰성, 비용절감 효과를 위한 실용적인 방안의 하나로 지속적인 연구가 이루어지고 있으며 그에 따른 사용량도 꾸준히 증가하고 있는 추세이다. 그러나 최근의 재료기술의 발달로 인해 콘크리트가 보유한 압축강도의 증진이 급속도로 빠르게 진행됨에 따라 구조설계기준에 적용할 정도의 적절한 기반 자료가 부족하여 구조물에의 적용에 많은 제한이 따르고 있다. 특히 초고강도콘크리트의 폭발적 압괴는 구조물에의 적용을 꺼리게 하는 첫 번째 조건이 되고 있다. 또한, 기존의 콘크리트가 표준 재료를 통해 제작되고 있는 것과는 달리 초고강도 콘크리트는 다양한 기초재료의 개발에 의해 다양한 배합비로 제작되고 있다. 이는 다양한 역학적 특성의 큰 변동성이 나타남을 의미하며 이에 따라 초고강도 콘크리트의 표준 특성을 정의하지 못하고 있는 추세이다. 이러한 문제점이 드러남에 따라 현행설계기준에 따른 표준적 설계방안이 아닌 대체적 설계방안이 각국에서 수립되고 있다. 대체적 설계 방안은 재료의 특성을 정의하고 이를 검증하는 시험 절차를 별도로 요구하고 있으며, 시험 결과를 직접적으로 설계에 반영하도록 하는 방법론을 취하고 있다. 콘크리트 구조물에 대한 각국의 현행 기준들은 각 부재별, 용도별 실험 결과에 기반한 계수 및 제한 사항에 크게 영향을 받는 바, 대체적 설계안이 최소한의 안전성을 보유하고 있는지에 대한 문제는 잠재적으로 존재하게 된다. 이러한 이유로 100MPa을 초과하는 초고강도 콘크리트는 20세기 후반 개발되었음에도 불구하고 전세계적으로 그 사용량이 많지 않은 상황이다. 본 논문에서는 초고강도 콘크리트의 사용을 위한 강도 및 연성능력의 확보 방안에 대해 제안하였다. 설계방법은 현행 구조설계기준과의 연속성을 보장하기 위해 설계 개념을 통일하여 구축하였으며, 보통강도 콘크리트와 초고강도 콘크리트의 설계에 대한 괴리를 줄이기 위한 방안을 모색하였다. 초고강도 콘크리트 기본적인 구조설계 방안을 제시하기 위해 재료 시험과 부재 실험이 연속적으로 수행되었으며 이를 통한 해석적 연구절차가 진행되었다. 초고강도콘크리트의 폭발적 파괴거동을 방지하기 위해 강섬유를 혼입하였으며 강섬유의 유무에 따른 초고강도 콘크리트의 거동에 때한 분석을 수행하였다. 표준실린더 시험체를 사용한 재료시험 결과를 통해 감섬유의 효과를 확인하였으며, 보통강도 콘크리트부터 100MPa을 초과하는 압축강도를 보유한 초고강도 콘크리트에 대한 기존 시험 결과를 수집하여 압축응력-변형률 관계를 제안하였다. 압축측에서의 폭발적 파괴를 방지하기 위한 강섬유에 의한 거동의 변화도 모델로 구축하였으며, 휨인성 시험을 통한 인장응력-변형률 관계에 대한 도출 방안을 구축하였다. 재료시험을 통한 압축 및 인장 거동의 단순화를 통해 간략화된 휨강도 산정방안을 도출하였으며 전단경간비 6인 실험체를 제작하여 휨강도 모델의 적합성을 검증하였다. 제안된 초고강도 콘크리트의 범위를 포함할 수 있는 응력-변형률 관계를 사용한 초고강도 콘크리트 부재 단면의 단면해석을 수행하였다. 단면해석은 본 연구에서 수행된 실험 및 기존에 수행된 섬유보강 고강도 실험체들을 통해 적합성이 검증되었다. 철근콘크리트 부재의 주요 검토 대상 중 하나인 연성 능력에 대한 분석이 단면해석을 통해 수행되었다. 현행구조설계기준은 초고강도 콘크리트 부재의 연성을 보통강도 콘크리트가 보유한 연성만큼 확보해주지 못함을 확인하였다. 대체설계에서 사용될 수 있도록 요구되는 연성비를 고려할 수 있는 초고강도 콘크리트가 사용된 휨부재의 최대철근비 제한 방안을 도출하였다. 초고강도 콘크리트 휨부재의 휨파괴를 통한 연성을 보장하기 위해 전단성능에 대한 검토가 수행되었다. 보강된 섬유가 스터럽을 대신하는 컨셉을 통해 스터럽으로 보강되지 않은 초고강도 콘크리트 부재에 대한 실험이 수행되었으며 특정 혼입량 이상이 사용될 경우 최소전단철근량을 대신할 수 있음을 확인할 수 있었다. 섬유보강 초고강도 콘크리트의 전단설계를 위한 전단강도 모델이 아치액션과 빔액션의 도입을 통해 구성되었으며 기존 연구 결과를 통해 전단강도 산정 모델의 적합성을 검증하였다. 도출된 전단강도 산정 모델을 휨강도 산정 모델과 접합하여 콘크리트의 압축강도-전단스팬비-섬유의 보강량-주철근비에 따른, 초고강도 콘크리트 휨 부재가 연성 거동을 할 수 있도록 설계방안을 수립하여 섬유보강 초고강도 콘크리트 부재의 설계에 대한 하나의 대안이 되도록 연구가 진행되었다. |The main purpose of this study is to develop the design method of the ultra high-strength fiber reinforced concrete. Strengthening the concrete, that is one of the practical ways for the structure of the safety, durability, reliability, cost savings, have been studied constantly. Accordingly, the usage of high-strength concrete is increasing steadily. However, as the enhancement of the compressive strength of the concrete rapidly due to the fast development of materials technology, many restrictions on the application of high-strength concrete in building because of lack of background data. In particular, The explosive crushing of ultra high-strength concrete has been the first cause to be reluctant to apply ultra-high-strength concrete on the structure. In addition, ultra-high strength concrete is produced with various mixing proportion by the development of a variety of raw materials. It cause thee variation of material properties. In this study, strength and ductility assurance method is proposed for using UHSC. The proposed design method was established to unify the design concept in order to ensure the continuity of the current structural design standards. And this study looking for ways to reduce the gap in the design of normal strength concrete and ultra high-strength concrete. In order to present the basic design method of UHSC, materials and member experiments have been performed continuously. And the analytical study was performed based on these test results. Steel fiber is applied into UHSC in order to prevent the explosive failure behavior of UHSC. The behavior of UHSC due to the presence of steel fiber is analyzed. The effect of fiber was check on by the test of the standard cylider specimen. And the relationship between compressive strength and strain was proposed by collecting the existing test results ranged from normal strength concrete to ultra high strength concrete exceeding 100MPa. The material model of ultra high-strength fiber reinforced concrete was established. The methodology of deducting the relationship between tensile stress and strain from the flexural toughness test was established Simplified methods for calculating flexural strength was developed by simplifying the tensile and compressive behaviour through the material test. And The flexural model was evaluated by the flexural test specimen which had the shear span ratio of 6. Section analysis of UHSC was performed using the relationship between stress and strain which can include proposed UHSC strength range. The results of section analysis were verified with the tests of UHSFRC specimens performed in this study and the existing. The ductility capacity, one of the significant factor of RC beam, was analyzed by the section analysis. The UHSC member designed by current design code has lower ductility compared to the normal strength concrete member. For this reason, the limitation method of maximum reinforcement ratio was proposed for using in the alternative design method which can consider the ductility ratio in flexural member. In order to ensure the ductility of UHSC flexural member by inducing flexural failure, the shear strength of UHSC was examined. The UHSC specimens which were not reinforced with the stirrups due to the concept that steel fiber can replace the stirrup were tested in this study. it is verified that steel fiber can be replaced the amount of minimum shear reinforcement if steel fiber was applied over the specific amount. The shear strength model of UHSC was proposed which was composed of Arch action and Beam action. Based on the previous test results, the suitability of this proposed model was verified. The design method of UHSC was established so that UHSC member was behaved ductile by integrating the calculation model of UHSC shear strength and the calculation model of UHSC flexural strength based on compressive strength, shear span ratio, the amount of fiber and longitudinal reinforcement ratio. This study was conducted to be an alternative for the design of UHSFRC member.; The main purpose of this study is to develop the design method of the ultra high-strength fiber reinforced concrete. Strengthening the concrete, that is one of the practical ways for the structure of the safety, durability, reliability, cost savings, have been studied constantly. Accordingly, the usage of high-strength concrete is increasing steadily. However, as the enhancement of the compressive strength of the concrete rapidly due to the fast development of materials technology, many restrictions on the application of high-strength concrete in building because of lack of background data. In particular, The explosive crushing of ultra high-strength concrete has been the first cause to be reluctant to apply ultra-high-strength concrete on the structure. In addition, ultra-high strength concrete is produced with various mixing proportion by the development of a variety of raw materials. It cause thee variation of material properties. In this study, strength and ductility assurance method is proposed for using UHSC. The proposed design method was established to unify the design concept in order to ensure the continuity of the current structural design standards. And this study looking for ways to reduce the gap in the design of normal strength concrete and ultra high-strength concrete. In order to present the basic design method of UHSC, materials and member experiments have been performed continuously. And the analytical study was performed based on these test results. Steel fiber is applied into UHSC in order to prevent the explosive failure behavior of UHSC. The behavior of UHSC due to the presence of steel fiber is analyzed. The effect of fiber was check on by the test of the standard cylider specimen. And the relationship between compressive strength and strain was proposed by collecting the existing test results ranged from normal strength concrete to ultra high strength concrete exceeding 100MPa. The material model of ultra high-strength fiber reinforced concrete was established. The methodology of deducting the relationship between tensile stress and strain from the flexural toughness test was established Simplified methods for calculating flexural strength was developed by simplifying the tensile and compressive behaviour through the material test. And The flexural model was evaluated by the flexural test specimen which had the shear span ratio of 6. Section analysis of UHSC was performed using the relationship between stress and strain which can include proposed UHSC strength range. The results of section analysis were verified with the tests of UHSFRC specimens performed in this study and the existing. The ductility capacity, one of the significant factor of RC beam, was analyzed by the section analysis. The UHSC member designed by current design code has lower ductility compared to the normal strength concrete member. For this reason, the limitation method of maximum reinforcement ratio was proposed for using in the alternative design method which can consider the ductility ratio in flexural member. In order to ensure the ductility of UHSC flexural member by inducing flexural failure, the shear strength of UHSC was examined. The UHSC specimens which were not reinforced with the stirrups due to the concept that steel fiber can replace the stirrup were tested in this study. it is verified that steel fiber can be replaced the amount of minimum shear reinforcement if steel fiber was applied over the specific amount. The shear strength model of UHSC was proposed which was composed of Arch action and Beam action. Based on the previous test results, the suitability of this proposed model was verified. The design method of UHSC was established so that UHSC member was behaved ductile by integrating the calculation model of UHSC shear strength and the calculation model of UHSC flexural strength based on compressive strength, shear span ratio, the amount of fiber and longitudinal reinforcement ratio. This study was conducted to be an alternative for the design of UHSFRC member.