Strain Transfer of Fiber Bragg Grating Sensor Externally Bonded to FRP Strip for Structural Monitoring after Reinforcement

Abstract

To date, a method of attaching a FRP (fiber-reinforced polymer) to concrete members with epoxy has been widely applied to increase the strength of the member. However, there are cases in which the adhesion of the epoxy deteriorates over time and the reinforcing effect of the FRP is gradually lost. Therefore, monitoring whether or not the reinforcing effect is properly maintained is needed in order to prevent a decrease in the structural performance of the member improved by FRP reinforcement. In this regard, this study examines FRP with OF (optical fiber) sensors to monitor the reinforcing effect of FRP in concrete structural members. In particular, this paper seeks to determine an appropriate adhesion length when FBG (fiber Bragg grating) based OF sensors are externally bonded to FRP strips with epoxy resin. To this end, a tensile test was carried out to evaluate the sensing performance according to the adhesion length. In addition, an analytical approach was performed and the result were compared with test result. The results of the experimental and analytical studies showed that the strain generated in the FRP is sufficiently transferred to the OF if the total adhesion length of it is 40 mm or more in consideration of the error in the epoxy thickness.

Short-Term Creep Effect on Strain Transfer from Fiber-Reinforced Polymer Strips to Fiber Bragg Grating-Optical Fiber Sensors

“…Regarding the OF sensors bonded with epoxy resin or epoxy resin combined with CN adhesive, the EM showed better efficiency compared to the EBM in the case of the OF sensors having a bond length of 80 mm. According to the theoretical approaches of Ansari et al and Seo et al [ 22 , 23 , 29 ], the shear transfer coefficient from the FRP strips to the embedded OF sensors can be increased due to shear transfer from the FRP strips on both sides compared to the OF sensors bonded externally on one side. However, when the bond length was 150 mm, the TSP of the OF sensor in both the EM and EBM showed more than 97%, and no difference was observed between them.…”

“…The TSP can be considered the sensing capability of the OF sensor, which shows the accuracy level for the strain transfer capability from the FRP strip to the OF sensor. From the studies of Seo et al [ 22 , 23 ], the strain can be transferred sufficiently from the FRP strip to the OF sensor with a bond length of 40 mm if not exposed to sustained loading conditions. When evaluating the sensing capacity of OF sensors bonded to an FRP strip with a bond length not less than 40 mm, the test result indicated a deviation range of 5%.…”

“…The same type of epoxy resin (SK-CPA10, SK Chemicals Co., Gyeonggi-do, Republic of Korea) used in previous studies by Seo et al [ 22 , 23 ] was applied as an adhesive. The epoxy resin was mixed with a hardener in a 2:1 ratio in accordance with the manufacturer’s guidelines.…”

“…In order to reduce the fabrication period and simplify the fabrication procedure, Seo et al [ 22 , 23 ] performed studies to suggest a minimum bond length between OF sensors and FRP materials through the tensile test of FBG-FRP composite using an analytical approach. However, in these studies, the creep effect on the epoxy resin used as an adhesive between the FBG sensor and the FRP material, which may affect strain transfer, was not considered.…”

Numerical and experimental analysis of the reliability of strain measured by surface‐mounted fiber‐optic sensors based on Bragg gratings

“…An approbation of the results of measuring strains in carbon fiber is given. In Seo et al, 29 an optical fiber glued to a fiberglass tape, which is mounted to the surface of reinforced concrete for additional external reinforcement, is considered. FBG‐based sensor provides control of the connection of the tape with reinforced concrete.…”