In this paper a new low cost, wireless unpowered sensor will be discussed that is designed to monitor the conductivity
of concrete, which may provide information on the ingress of chloride ions during the life of the structure. A method of
extracting temperature information from a previously developed corrosion sensor will also be presented. During a
recent test, both a wireless corrosion sensor and a wireless conductivity sensor were placed in concrete and monitored
throughout the duration of the curing process. Analysis of the data shows it is possible to determine temperature
information based on the corrosion sensor response, allowing wireless in-situ temperature monitoring of the concrete
during the cure. Monitoring curing temperature using the same sensor which would later be used for long-term
corrosion detection would help reduce the cost of such a monitoring system.
The long-term reliability of a prototype, threshold corrosion sensor is demonstrated using data collected during an
eighteen-month accelerated corrosion test. The sensors were embedded in reinforced concrete slabs, subjected to
alternating wet/dry cycles, and interrogated periodically during the test. The frequency signature of the sensor changes
after the steel sensing wire corrodes, providing a convenient and noninvasive technique for determining when a
threshold amount of corrosion has occurred.
The results indicate that the sensor data are reliable, but that some variability of readings should be expected due to the
close tie between the presence of cracks in the concrete and the chloride levels, as the locations of the cracks are not
known at the time that the sensors are embedded in the concrete.
The long-term reliability of a threshold corrosion sensor is demonstrated using data collected during two series of exposure tests. The sensors were embedded in concrete and interrogated in a wireless manner using inductive coupling. The frequency signature of the sensor changes after a steel sensing wire corrodes, providing a convenient and noninvasive technique for determining when a threshold amount of corrosion has occurred in a reinforced concrete structure. In the first series of exposure tests, the sensors were embedded in concrete prisms, which were exposed to a variety of temperature and moisture conditions over a six-month period. In the second series of tests, the sensors were embedded in reinforced concrete slabs. The slabs have been subjected to sustained loads and alternating wet and dry cycles for the past year. Data from both test series indicate that the threshold sensors are functioning as designed.
Prototype sensors have been developed to detect the onset of corrosion in steel reinforced concrete using non-invasive techniques. These sensors are designed to be extremely simple and low cost. The sensors are embedded in the concrete and are powered and interrogated through the use of inductively coupled magnetic fields. A new conductivity sensor is proposed, based on the design of the corrosion sensor. The conductivity sensor design is examined using circuit simulations and initial experimental results. Both sensors could be used together in a corrosion monitoring system.
Eddy current sensing has been successfully used in various applications from testing heat exchange tubes for nuclear power plants to assessing dielectric thickness on printed circuit boards. However, in civil infrastructures cosmetic or cementitious surface material often keeps the probe or reader coil from accessing conductive medium inside the structure, resulting in reduced coupling as the distance increases between the DUT (device under test) and probe. Thus, the direct application of existing eddy current sensing technique is not very useful to detect flaws in civil infrastructures. To address this weak coupling problem, a simple scheme is proposed in which a resonant passive repeater tag is placed between the reader coil and the conducting test target. In this paper, the feasibility of detecting defects like cracks or fractures in conductive medium using a passive resonant tag and remote inductive pick-up as a method of interrogation is shown. Experimental data taken from simple setups to demonstrate the advantage of the proposed scheme are presented.
The prototype sensors provide a low-cost method to detect the onset of corrosion in concrete structures using a noninvasive approach. The embedded sensors are wirelessly powered by inductive coupling and do not require batteries. Unlike traditional techniques for detecting corrosion which require an electrical connection to the embedded reinforcement, the sensors are self-contained and provide information about the environmental conditions within the concrete in the vicinity of the sensor. The sensors were originally envisioned to provide binary information about the onset of corrosion based on the characteristic frequency of the impedance response. However, more complicated signal processing is required to determine the state of the sensor. The viability of the corrosion sensors is being evaluated through a comprehensive series of laboratory tests using small-scale concrete prisms and large-scale reinforced concrete members.
Our society depends heavily on a network of buildings, bridges and roadways. In order to properly maintain this civil infrastructure and avoid damage and costly repairs due to structural failure, it is necessary to monitor the health of these structures. Sensors must frequently be placed in inaccessible locations under harsh conditions and should ideally last the lifetime of the structure the sensors are monitoring. This paper presents the development of a low cost, passive, un-powered wireless analog resistance sensor. The sensor was originally designed for monitoring corrosion in concrete, but there are many other potential applications including remote temperature monitoring, embedded accelerometers, and embedded strain gauges. The passive wireless nature makes the sensor ideally suited for embedding in inaccessible locations under harsh conditions. The sensor consists of a resonant inductor-capacitor circuit containing a resistive transducer. The sensor is interrogated by measuring the impedance through a remote, magnetically coupled reader loop. The width of the resonance is directly related to the resistance of the transducer. The sensor has been simulated under a variety of conditions using a circuit model and compared to actual test sensors built and evaluated in the laboratory.
This paper summarizes the ongoing work at the University of Texas to develop a class of passive, wireless sensors to detect corrosion in reinforced concrete structures. Two prototype sensors have been developed that are designed to be attached to the reinforcing cage before the concrete is placed. The basic and improved sensors are inexpensive to fabricate, sufficiently durable to survive the construction process, can be interrogated through concrete, and provide reliable information about the initiation of corrosion in the adjacent reinforcement. Because the sensors do not contain an internal battery, the expected life of the sensors is expected to be essentially the same as the design life of the structure in which they are embedded.