Installation and performance evaluation of coaxial cable sensors for crack and corrosion detection

Even under service loads, reinforced concrete (RC) structures can develop cracks that result in excessive deflection of the structures and provide passages for moisture to corrode steel reinforcement. It is thus critical to develop a simple, cost-effective tool for real-time crack monitoring and associated corrosion detection that may affect the engineering maintenance of RC structures. The objectives of this study include: (1) to develop a die-cut manufacturing process of coaxial cables with spiral outer conductors, (2) to quantify the sensing properties of a miniaturized topology-based crack sensor, (3) to investigate the effectiveness of various sensor installation procedures in RC applications, and (4) to detect the distribution of corrosion in steel reinforcement. A new manufacturing process was developed to fabricate spirally-wrapped, miniaturized coaxial cables in the order of mm in diameter. To understand their performance and sensitivity, eight miniaturized sensors were fabricated and placed in seven RC concrete members that were tested under three-point loading. Various grout materials were also investigated to compare their effects on sensor sensitivity. Test results indicated that the miniaturized, die-cut coaxial cable sensors are more uniform and more sensitive to cracks than their early versions since the new manufacturing process can refine the topology of their outer conductors. Like embedment, surface attachment of a coaxial cable on a RC member can be effective with appropriate bonding agents such as Sikagrout materials. Preliminary tests by submerging coaxial cables into 3% and 5% NaCl solutions demonstrated that cable sensors can indicate the breaching of small holes on their outer conductor as a result of corrosion, potentially providing a promising technology for distributed corrosion detection --Abstract, page iii

A Novel TDR-Based Coaxial Cable Sensor for Crack/Strain Sensing in Reinforced Concrete Structures

Novel coaxial cable sensors that feature high sensitivity and high spatial resolution are developed for health monitoring of concrete structures using a time-domain reflectometry (TDR). The new sensor was designed based on the topology change of its outer conductor, which was fabricated with tightly wrapped commercial tin-plated steel spiral covered with solder. The cracks that developed within concrete structures will lead to out-of-contact of local steel spirals. This topology change results in a large impedance discontinuity that can be measured with a TDR. A simplified equivalent transmission line model and numerical full-wave simulations using finite-difference time-domain techniques were used to optimize the sensor design. The sensors under test demonstrated high sensitivity and the capability of multiple-crack detection. A plasma-sprayed coating technique was employed to improve sensor uniformity. Engineering implementation issues, e.g., signal loss, signal postprocessing, and sensor design optimization, were also addressed

Electromagnetic modeling of distributed coaxial cable crack sensors in reinforced concrete members

Distributed crack sensors were recently developed with coaxial cables that are composed of inner and outer conductors as well as dielectric layer in between. These sensors were designed based on the change in topology of the cable outer conductor structure under strain effects. Various tests of reinforced concrete (RC) beams and columns indicated that the newly designed sensors are 10~50 times more sensitive than commercial cables to the longitudinal elongation applied on their cable structures. The spatial resolution of the sensors is approximately 50 mm. Limited numerical simulations with the transmission line theory and the finite difference time domain model were performed to understand the general behavior of coaxial cable sensors --Abstract, page iii

Coaxial Cable Sensors and Sensing Instrument for Crack Detection in Bridge Structures -- Phase I: Field Qualification/Validation Planning

The objectives of this study are to pre-test analyze a decommissioned RC bridge that is selected in consultation with New York State Department of Transportation (NYSDOT), and design and plan the field tests of the bridge for the performance qualification and validation of distributed crack sensors and a fast Electrical Time Domain Reflectometry (ETDR) instrument to their full potential. The scope of work includes: (a) Selection of a decommissioned bridge, (b) Pre-test analysis of the select bridge structure to evaluate its progressive damage and determine the locations for sensor deployment, (c) Design and planning of field tests of the select bridge, (d) Field instrumentation with coaxial cable and fiber optical sensors for performance comparison, and (d) Summary of the findings of this study. Once fully validated and demonstrated in field conditions, distributed crack sensors and sensing instruments are expected to play a significant role in routine inspections and bridge ratings and in the rapid assessment of structural conditions for post-event evaluations and responses, improving the safety and security of transportation infrastructure at the height of a crisis. These roles are due primarily to their unique ability of permanently recording the widest crack a RC member experienced during a recent event. Such an attribute ensures the availability of damage data even if a fast ETDR system experiences malfunction during the event, greatly improving the reliability of bridge inspections

Crack Detectability and Durability of Coaxial Cable Sensors in Reinforced Concrete Bridge Applications

The working mechanism and the measurement principle of topology-based crack sensors made of coaxial cables are briefly reviewed. The sensitivity, spatial resolution, and ruggedness of two coaxial cable sensors, respectively made of rubber and Teflon dielectric materials, were compared and validated with laboratory testing of a 4/5-scale, T-shaped, reinforced concrete beam-column specimen. Two Teflon sensors were installed on one of the solid decks of a three-span continuous highway bridge to investigate their durability and measurement repeatability. Laboratory tests indicated that both types of sensors have high sensitivity, but the Teflon sensor has a higher spatial resolution and a negligible spillover effect of any significant cracks. At a 90-degree bend, however, the Teflon sensor is more susceptible than the rubber sensor to the rubbing action of the outer conductor of a coaxial cable against its dielectric layer. No cracks were observed during the field load tests of the instrumented bridge. Both sensors indicated high durability in realworld application but a certain variation of waveforms was measured over a period of 5 years because of the use of different instruments. Future research is directed to develop an online calibration of crack sensors with a small portion of built-in standard cable at the end of the cable sensor

Novel Distributed Cable Sensors for Detection of Cracks in RC Structures

In this paper, the development of a fundamentally new, topology-based cable sensor design concept is summarized for crack detection in reinforced concrete (RC) structures. The sensitivity, spatial resolution, and signal loss of sensors are investigated both numerically and experimentally. Two sensors were fabricated and validated with small- and large-scale laboratory tests under different loads. Both were proven sensitive to crack of various sizes from visually undetectable to excessive, giving the location and severity of damage simultaneously. One sensor has been installed on a three-span bridge for its long-term monitoring. It is capable of recording damage that has occurred during a recent event

Dual Mode Sensing with Low-Profile Piezoelectric Thin Wafer Sensors for Steel Bridge Crack Detection and Diagnosis

Monitoring of fatigue cracking in steel bridges is of high interest to many bridge owners and agencies. Due to the variety of deterioration sources and locations of bridge defects, there is currently no single method that can detect and address the potential sources globally. In this paper, we presented a dual mode sensing methodology integrating acoustic emission and ultrasonic wave inspection based on the use of low-profile piezoelectric wafer active sensors (PWAS). After introducing the research background and piezoelectric sensing principles, PWAS crack detection in passive acoustic emission mode is first presented. Their acoustic emission detection capability has been validated through both static and compact tension fatigue tests. With the use of coaxial cable wiring, PWAS AE signal quality has been improved. The active ultrasonic inspection is conducted by the damage index and wave imaging approach. The results in the paper show that such an integration of passive acoustic emission detection with active ultrasonic sensing is a technological leap forward from the current practice of periodic and subjective visual inspection and bridge management based primarily on history of past performance

Experimental validation of an integrated FRP and visco-elastic hardening, damping, and wave-modulating system for blast resistance enhancement of RC columns

In today\u27s society, terrorist attacks and accidental explosions pose a major threat to critical infrastructure. Vulnerable to blast loading, structures must be rehabilitated to ensure structural stability and protect human life. The goal of this study is to develop and validate a sandwich composite technology for column retrofitting. The new technology consists of an inner fiber reinforced polymer (FRP) sheet, an outer FRP sheet, and a visco-elastic (VE) layer sandwiched between the two FRP sheets. The inner FRP sheet is wrapped around an existing column for confinement, while the outer FRP sheet is for anchoring of the VE layer into the column supports. The compact, inexpensive, and easy to construct system has been shown effective under seismic loads. In this study, the blast performance of the engineering system is investigated with two main objectives: to field validate the effectiveness of the system for hardening, damping, and wave-modulating (HDM) of a reinforced concrete (RC) column under blast loads, and to validate the performance of coaxial cable crack sensors for dynamic measurements under blast loads --Abstract, page iii