[국내논문]스터럽 절단 탄소섬유판 표면매립공법의 휨 보강 성능 평가 Assessment of Flexural Strengthening Behavior Using the Stirrup-Cutting Near Surface Mounted(CNSM) CFRP strip원문보기
최근 탄소섬유판 매립공법의 콘크리트 구조물의 보강에 적용되고 있다. 그러나 탄소섬유판 표면매립공법은 피복부 콘크리트의 강도 부족과 높이 부족 등으로 인하여 그 적용의 제한이 발생하기도 한다. 본 연구에서는 이와 같은 이유로 인하여 전단 스터럽을 절단하고 탄소섬유판을 주철근의 위치에 표면매립 보강하는 공법에 대하여 고찰하였다. 일반적인 표면매립공법과 스터럽을 절단하고 표면매립공법을 적용한 보에 대한 휨 실험을 수행하였으며, 결과를 서로 비교하였다. 탄소섬유판의 길이를 실험변수로 하였다. 실험결과에 따르면, 전단 스터럽을 절단한 보강 보의 휨거동은 일반적인 표면매립공법이 적용된 실험체의 거동과 유사한 전형적인 휨 거동을 나타내었으며, 스터럽의 절단으로 인한 구조거동상의 문제는 발생하지 않았다. 따라서, 일반적인 현장 여건에 의하여 탄소섬유판의 적용이 곤란한 경우에는 스터럽을 절단하는 본 공법의 적용이 가능한 것으로 판단된다.
최근 탄소섬유판 매립공법의 콘크리트 구조물의 보강에 적용되고 있다. 그러나 탄소섬유판 표면매립공법은 피복부 콘크리트의 강도 부족과 높이 부족 등으로 인하여 그 적용의 제한이 발생하기도 한다. 본 연구에서는 이와 같은 이유로 인하여 전단 스터럽을 절단하고 탄소섬유판을 주철근의 위치에 표면매립 보강하는 공법에 대하여 고찰하였다. 일반적인 표면매립공법과 스터럽을 절단하고 표면매립공법을 적용한 보에 대한 휨 실험을 수행하였으며, 결과를 서로 비교하였다. 탄소섬유판의 길이를 실험변수로 하였다. 실험결과에 따르면, 전단 스터럽을 절단한 보강 보의 휨거동은 일반적인 표면매립공법이 적용된 실험체의 거동과 유사한 전형적인 휨 거동을 나타내었으며, 스터럽의 절단으로 인한 구조거동상의 문제는 발생하지 않았다. 따라서, 일반적인 현장 여건에 의하여 탄소섬유판의 적용이 곤란한 경우에는 스터럽을 절단하는 본 공법의 적용이 가능한 것으로 판단된다.
Recently, the near surface mounted (NSM) FRP strengthening technique has been actively applied to deteriorated concrete structures for rehabilitation purposes. However, the use of this conventional NSM technique could be restricted due to the insufficient height or strength of the concrete cover. In...
Recently, the near surface mounted (NSM) FRP strengthening technique has been actively applied to deteriorated concrete structures for rehabilitation purposes. However, the use of this conventional NSM technique could be restricted due to the insufficient height or strength of the concrete cover. In this study, the stirrup-Cutting Near Surface Mounted(CNSM) technique was considered as an alternative, whereby NSM strips are placed at a deeper level, namely at the level of the main steel reinforcement. A flexural test of a concrete beam strengthened with CNSM technique was performed and the results were then compared to those for a concrete beam strengthened by the conventional NSM technique. The embedment length of the CFRP strips was varied in order to increase the effect of the anchoring depth of the NSM and CNSM CFRP strips in the beam specimens. From the results of the test, the beam with the CNSM CFRP strip showed typical structural behavior similar to that of the beam with the NSM CFRP strip. Moreover, there was no apparent structural degradation resulting from the stirrup partial-cutting. Consequently, the CNSM strengthening technique can be suitably utilized for extensively damaged concrete structures where it is difficult to apply the conventional NSM technique.
Recently, the near surface mounted (NSM) FRP strengthening technique has been actively applied to deteriorated concrete structures for rehabilitation purposes. However, the use of this conventional NSM technique could be restricted due to the insufficient height or strength of the concrete cover. In this study, the stirrup-Cutting Near Surface Mounted(CNSM) technique was considered as an alternative, whereby NSM strips are placed at a deeper level, namely at the level of the main steel reinforcement. A flexural test of a concrete beam strengthened with CNSM technique was performed and the results were then compared to those for a concrete beam strengthened by the conventional NSM technique. The embedment length of the CFRP strips was varied in order to increase the effect of the anchoring depth of the NSM and CNSM CFRP strips in the beam specimens. From the results of the test, the beam with the CNSM CFRP strip showed typical structural behavior similar to that of the beam with the NSM CFRP strip. Moreover, there was no apparent structural degradation resulting from the stirrup partial-cutting. Consequently, the CNSM strengthening technique can be suitably utilized for extensively damaged concrete structures where it is difficult to apply the conventional NSM technique.
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제안 방법
In their study, it was reported that the composite action between the embedded CFRP strip and the concrete was partially lost by flexural cracks at midspan and that the ultimate load and corresponding deflection could be significantly reduced by the flexural cracks, particularly in the beams with an embedment length shorter than 68% of the net span length. In addition, a model was presented for the calculation of the development length of the NSM CFRP strips as shown in the following equation (2) and was evaluated by a parametric study. The maximum critical shear stress is also given by equation (3).
A more feasible method should therefore be considered to enhance the integrity of the rehabilitation system with NSM. In this study, an alternative method is attempted, whereby CFRP strips are inserted at a deeper location. However, this alternative method requires partial cutting of the existing stirrups with an enclosed shape, which interrupts the installation of CFRP strips.
In this study, the stirrup-Cutting Near Surface Mounted (CNSM) CFRP strip technique, where the CFRP strip is installed at a deeper level than the NSM technique, was evaluated through flexural tests and the results were compared with the conventional NSM technique. The number and the embedment length of the CFRP strip were considered as a test variables in order to clearly observe the effect of the partial cutting of stirrups on the failure mode and load carrying capacity between both strengthening techniques.
In addition, more convenient loading work was accomplished with the configuration. The beams were instrumented with a linear variable differential transducer (LVDT) at midspan and at 675mm from each support in order to measure the deflection. At midspan, a strain gauge was glued to the top fiber of the concrete in the compression zone using an epoxy resin after carefully sandblasting the concrete.
The minimum embedment length of the CFRP strips of the test beam was set at 864mm, which corresponds to 32% of the net span length, in order to prevent the IC debonding failure. The embedment length of the CFRP strip in beams was varied in order to investigate the change in the failure mode and moment capacity of a beam strengthened with NSM and CNSM CFRP strips. The considered embedment lengths were 32%, 48%, 70%, 80%, and 90% of the net span length of the beam.
Nevertheless, the predicted moment capacity concurred with the experimental results. Therefore, the ACI formulation and the relationship of the effective strain of the NSM CFRP strip can be utilized for the preliminary analysis and for the experiment of flexural member strengthened with CNSM technique.
대상 데이터
CFRP strips produced by a Korean manufacturer were used. The width and thickness of each strip was 25mm and 1.
Table 1 shows the test specimen. Fourteen beam specimens were cast for this experimental program, including two non-strengthened beams, which were used as control specimens. Stirrups with a closed shape were distributed at 100mm spacing along the beam axis over the entire length.
Normal weight ready-mixed concrete was used for the specimens. The average compressive strength of concrete was 30 MPa as evaluated by tests on three cylindrical 100 mm high specimens.
The considered embedment lengths were 32%, 48%, 70%, 80%, and 90% of the net span length of the beam. The names of specimens comprise the name of the strengthening technique, that of NSM or CNSM, and the number of the CFRP strip, that of 1 or 2, and the embedment length of the CFRP strip, that of 32, 48, 70, 80, or 96, as listed in Table 1.
성능/효과
As shown in Figure 6, the embedded CFRP strips were split longitudinally and were buckled locally due to a large deformation. Based on these results, it can be assumed that in both techniques the embedded CFRP strips could have been partly damaged or buckled before the failure of the beam.
In addition, when the CNSM CFPR strips have a sufficient length of not less than 70% of the net span length to achieve full composite action with the concrete, the load carrying capacity can be improved by around 40%. Furthermore, based on the experimental observations, an identical flexural behavior to those beams that were strengthened with the conventional NSM technique was observed, which confirms that this CNSM strengthening technique can be effectively used for structurally deficient concrete structures that have a limited or severely damaged concrete cover.
When the embedment length of the CFRP strip was below the 50% of the net span of the beam, the beam collapsed at a very early stage. The load and deflection at the failure of beams CNSM2-32 and 48 were only around 83% and 61% of those of beams CNSM2-70, 80 and 96. These significant reductions in failure load and in the amount of deflection seemed to be caused by an inability to develop the tensile capacity of the CFRP strips due to the insufficient embedment length of the CFRP reinforcement.
참고문헌 (18)
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