This article proposes a new technique that advances long-gauge carbon fiber line sensor technology, with and without post-tensioning of the sensor, to measure changes in strain levels in structural areas. Carbon fiber line sensors were fabricated to produce a slim high-strength sensor with a diameter of less than 1.4 mm using a carbon fiber tow with a width of 6 mm. A theoretical analysis of these sensors as well as several series of experiments was conducted to investigate the effect of fiber arrangement on the error compensation of the carbon fiber line sensors. The results revealed that using two sets of carbon fiber line sensors, one as an active sensor and the other to compensate the errors of the first, is an effective method when both sensors have a convergent fiber arrangement and change in resistance. A post-tensioning method was implemented to enhance the overall behavior of the sensor. The results showed that the post-tensioning method yields significant improvement in the linearity and cyclic ability up to 6000 microstrains and reduces the fluctuation errors in the change in resistance from ±0.031% to ±0.007%. Finally, the possibility of repairing damaged carbon fiber line sensors is also discussed.

Owing to fabrication defects in carbon fiber (CF) tows, the unevenness of fiber roves, such as local bends, misalignments, and skewness, results in irregular distribution of the electrical resistance in the transverse direction along the gauge length of a sensor, which affects its performance. In this study, a pre-tension approach was developed according to the creep mechanism of composites to straighten the CFs. In addition, the resin relaxation was controlled by tensioning the fibers during and after hardening of the epoxy resin using a double-tension method to enhance the electrical sensing properties of long gauge carbon fiber line (CFL) strain sensors. Different levels of sustained tensile stresses were studied to obtain the optimal tensile stress level both during and after hardening to be applied in the double-tension method. The results of static and dynamic tests showed that the double-tension technique could significantly straighten the fibers, and stabilize the transverse connections of CFL sensors in the case of tensioning the fibers during and after hardening under a sustained stress of 60% of the ultimate tensile stress of the CFs. The proposed double-tension method was utilized to improve the response of the CFL sensors with short gauge lengths.

Currently, corrosion of steel reinforcements is a major topic for several researchers because of the early deterioration and shortening of the service life of structures, particularly those prone to salt attacks. This study proposes a new technique for monitoring corrosion of steel reinforcements in flexural reinforced concrete structures using distributed embedded long-gauge packaged carbon fiber line sensors with self-compensation under service loads. Three different approaches including continuous-strain ratio, distributed-strain ratio, and section fiber model were proposed to evaluate corrosion levels using strain measurements. Different groups of distributed packaged carbon fiber line sensors were installed on the concrete surface and steel reinforcements of the reinforced concrete beam to verify the proposed approaches experimentally using the accelerated corrosion technique. The sensors installed on the concrete surface affected by cracks can only localize the corrosion locations but are unable to determine the actual values of corrosion levels. The degree of corrosion calculated using each approach was compared with the weight loss of an experimental corrosion model. The results showed that the continuous-strain ratio approach will be suitable to localize and evaluate the corrosion degrees if corrosion occurs before the formation of cracks, while the distributed-strain ratio is more effective in the presence of cracks. In highly damaged conditions, the section fiber model will be the most accurate method to evaluate and localize corrosion in steel reinforcements.