An Innovative Sensing Approach Using Carbon Nanotube-Based Composites for Structural Health Monitoring of Concrete Structures

摘要

Over the time, the integrity and reliability of civil infrastructures are threatened by overloading, fatigue, impact damage, and structural deterioration. Structural health monitoring (SHM) is therefore becoming a viable tool to collect real-time quantitative data from in-service structures concerning structural condition and performance. Being capable of continuously monitoring critical components, SHM systems can instantaneously identify damage, guide necessary repairs, and may ultimately help prevent catastrophic failure. As the core of SHM, the capability, accuracy and reliability of the applied sensing system govern the overall success of the implementation of SHM. To date, conventional sensors such as strain gages, accelerometers, and displacement gages have been widely employed in SHM systems for attaining global or/and local responses of a structure. However, these point-type sensors still suffer from limitations and challenges, which indeed have inspired the development of next-generation sensing methodologies for SHM. Recent advances in nanotechnology offer a variety of self-sensing nanocomposites with integrated nanoscale, noninvasive, electrically percolating networks providing exceptional sensitivity to sense changes in strain as well as the formation and propagation of micro- and macro-damage. By appropriately integrating nanocomposites with distributed sensing schemes, an extensive nerve-like sensing system with enhanced detection capabilities and spatial sensitivity of strain and growing damage can be established for SHM of civil infrastructures.;The research work presented in this dissertation advances the state of the art by introducing an innovative carbon nanotube (CNT)-based nonwoven composite sensor that can be tailored for strain and damage sensing properties and potentially offers a reliable and cost-effective sensing option for SHM. First, a readily scalable two-step process for manufacturing nanocomposites was developed. Specifically, a thin, lightweight and inexpensive nonwoven fabric was selected as the CNT carrier and nanotubes were deposited following a dip-coating procedure. Second, the microstructure, mechanical, and electrical properties of the proposed CNT-based composite sensor were investigated. Its electrical double percolation was observed for the first time and its self-sensing capability, and strain sensitivity was validated and characterized using coupon-level experiments. The sensors were found to be repeatable and respond linearly up to 0.4% strain with achievable elastic strain gage factors of 1.9 and 4.0 in the longitudinal and transverse direction, respectively. Third, the established composite sensors were further integrated with a difference imaging-based electrical impedance tomography (EIT) sensing scheme to offer a true two-dimensional damage sensing methodology, from which damage location, size, and severity can be estimated. This represents a significant extension to the commonly applied direct current (DC)-based point sensing scheme. Next, a systematic characterization of the thermoresistive behavior in these CNT-based nanocomposites and multiscale composites was performed under thermal cycling between 25 to 145 °C. A dynamic dominance for a double-crossover-shaped temperature dependence of their resistances was observed and methodically investigated. Finally, a hybrid composite system was applied on two large-scale reinforced concrete laboratory beams (12 in x 24 in x 16 ft), in which the CNT-modified nonwoven sensing sheet for SHM is integrated with a glass fiber reinforcement to create a combined strengthening and sensing solution. The 14-ft-long nanocomposite sensor was interrogated using a multiplexing approach with multiple electrodes to spatially estimate the damage locations. To date, this is the largest CNT-based composite sensor ever tested.;The findings from this dissertation research have made important scholarly contributions to the fundamental understanding of the sensing networks of the innovative CNT-based nonwoven composites. Important broader impacts have also been made by promoting applications of using CNT-based sensing composites as strain/damage sensors for SHM. The presented methodology has remarkable potential to revolutionize the fields of SHM and structural engineering.