Wireless and electromechanical approaches for strain sensing and crack detection in fiber reinforced cementitious materials.

摘要

High performance fiber reinforced cementitious composites (HPFRCC) are novel cement-based construction materials with excellent mechanical behaviors. Among those, high ductility and crack resisting are two of the most important features. As these new materials begin to be introduced into practice, there is a need to monitor the health and performance of structures and structural elements made of them. With current sensing technologies either incapable of accurately measuring cracking or too expensive to install, new sensing paradigms are needed. This thesis explores two novel approaches to sensing strain and cracking in HPFRCC structural elements: wireless sensors and the use of HPFRCC materials as their own sensor platform. First, wireless monitoring systems are explored because they are relatively low-cost and easy to install. An order of magnitude cheaper than conventional tethered sensors, wireless sensors can be installed in high density within a single structure. Illustration of a wireless monitoring system using a large number of wireless sensor nodes is provided using the Grove Street Bridge located in Ypsilanti, Michigan. The computational resources of the wireless sensor are leveraged to locally process response data recorded from an HPFRCC element. Damage index methods previously tailored for HPFRCC structural components are embedded into the wireless sensors for automated damage detection. The utility of locally processing response data at the sensor is validated using a cyclically loaded HPFRCC bridge pier.;While wireless sensors are capable of automated data interrogation, they do not fully quantify cracking in HPFRCC elements. To address this limitation, the inherent electromechanical properties of HPFRCC materials are harnessed. Specifically, HPFRCC materials are piezoresistive; in other words, their bulk resistivities change with strain. This work undertakes detail experimental evaluation of the electromechanical properties of one class of strain-hardening HPFRCC: engineered cementitious composites (ECC). First, the piezoresistive properties of ECC are quantified through two- and four-point probe methods. While strain can be accurately measured in the material's elastic regime, microcracking during strain hardening prevents correlations between resistivity and strain to be accurately made. Electrical impedance tomography (EIT) is proposed to map the spatial distribution of ECC bulk conductivity in two-dimensions using repeated electrical measurements taken at the specimen boundary. Hence, EIT conductivity maps can serve as a tool for measuring strain fields in ECC plate elements as well as for imaging cracking in fine detail. This material-level sensing approach holds tremendous promise for future structural health monitoring applications. The EIT sensing approach can also be applied to any semi-conductive material to map conductivity. The universality of the approach is illustrated using a carbon nanotube composite material as a sensing skin, or applique, for structural health monitoring.