This paper summarizes ongoing work on applying passive magneto-inductive (MI) waveguides as wireless sensor arrays
to monitor corrosion in infrastructure systems. The passive uniformly-spaced sensor array provides a low-cost and quick
method to detect the onset of corrosion in concrete structures using a noninvasive approach. The embedded sensors
communicate with neighboring sensors through inductive coupling. The corrosion information is interpreted based on
both frequency and time domain characteristics. Bandpass characteristics in the frequency domain and received reflected
time domain waves are investigated to locate the defects along the wireless sensor array. Using the relationship between
the relative positions of defects and MI waveguide performances, a new combined technique to determine location of
defects has been developed and proven to provide both improved sensitivity and defect location capability.
Highway bridges are vital links in the transportation network in the United States. Identifying possible safety problems
in the approximately 600,000 bridges across the country is generally accomplished through labor-intensive, visual
inspections. Ongoing research sponsored by NIST seeks to improve inspection practices by providing real-time,
continuous monitoring technology for steel bridges. A wireless sensor network with a service life of ten years that is
powered by an integrated energy harvester is targeted.
In order to achieve the target ten-year life for the monitoring system, novel approaches to energy harvesting for use in
recharging batteries are investigated. Three main sources of energy are evaluated: (a) vibrational energy, (b) solar
energy, and (c) wind energy. Assessing the energy produced from vehicular-induced vibrations and converted through
electromagnetic induction is the focus of this paper.
The goal of the study is to process acceleration data and analyze the vibrational response of steel bridges to moving truck
loads. Through spectral analysis and harvester modeling, the feasibility of vibration-based energy harvesting for longterm
monitoring can be assessed. The effects of bridge conditions, ambient temperature, truck traffic patterns, and
harvester position on the power content of the vibrations are investigated.
With sensor nodes continually recharged, the proposed real-time monitoring system will operate off the power grid, thus
reducing life cycle costs and enhancing inspection practices for state DOTs. This paper will present the results of
estimating the vibration energy of a steel bridge in Texas.
Real-time monitoring of fracture critical steel bridges can potentially enhance inspection practices by tracking the
behavior of the bridge. Significant advances have occurred in recent years on the development of robust hardware for
field monitoring applications. These systems can monitor, process, and store data from a variety of sensors (e.g. strain
gages, crack propagation gages etc.) to track changes in the behavior of the bridge. Thus, for a long-term monitoring
system to be successful, the reliability of gages that are to be monitored for several years is very important. This paper
focuses on the results of a research study focused on developing a wireless monitoring system with a useful life of more
than 10 years. An important aspect of the study is to identify strain gages and installation procedures that result in long
lives as well as characterizing the effect of temperature fluctuations and other environmental factors on the sensor drift
and noise. In long-term monitoring applications, slight sensor drift and noise can build up over time to produce
misleading results. Thus, a wide variety of gages that can be used to monitor bridges have been tested for over a year
through environmental tests. The environmental tests were developed to determine the durability of the gages and their
protective coatings (e.g. zinc-based spray, wax and silicon, etc.) against humidity, sun exposure and other environmental
effects that are expected in long-term bridge monitoring applications. Moreover, fatigue tests were performed to
determine the fatigue category of the weldable gages and to reveal any debonding issues of the bondable gages. This
paper focuses on the results of laboratory tests on gage durability that were conducted as part of a research project
sponsored by the National Institute of Standards and Technology (NIST).
This paper describes the ongoing research efforts to develop a novel class of low-cost, unpowered, wireless sensors for
detecting corrosion of reinforcement in concrete structures. The sensors are powered through magnetic coupling
between an external reader coil and an embedded sensor. Measured AC impedance is used to interpret the state of the
embedded sensor. The sensors are envisioned to be placed during construction and interrogated as part of routine
The sensor prototype incorporates a sacrificial corroding element that is placed entirely outside the sensor components
and interacts with the resonant circuit by inductive coupling and shielding of the magnetic fields. As the resistance of
the sacrificial element increases due to corrosion, the measured frequency response changes gradually indicating
corrosion initiation within concrete. In this paper the potential for detecting multiple levels of corrosion damage is
A reconfigurable and portable wireless reader has been designed for embedded passive Electronic Structural
Surveillance (ESS) sensors, used to monitor corrosion in infrastructure systems. The passive ESS sensors have been
developed and proven effective in monitoring localized defects in their environment. They are interrogated by
inductively coupled magnetic field of a reader coil. The input impedance of the reader coil is monitored to determine
whether a corrosion threshold has been reached. We have previously used an impedance analyzer to obtain the
impedance data. Such systems have good sensitivity and moderate speed but are bulky and heavy. The new reader
approach presented in this paper is designed addressing the need of portability, sensitivity and read range. The reader
electronics is implemented on a reconfigurable National Instruments (NI) modular transceiver platform, capable of
software defined radio. The design employs a reflectometer, which is implemented using a 3-port directional coupler and
a single coil as both the driver and reader, along with the transceiver. The NI transceiver is used to generate a swept
frequency input signal and analyze reflected signal from the reader, which is related to the input impedance of our ESS
sensor. The configuration of the reader coil is optimized for reader range and sensitivity. We have acquired analog data
using this design, showing that the real-time reader system facilitates especially fast detection and long read ranges for
Real-time, monitoring systems can enhance the bridge inspection process by providing data for estimating the health of
the bridge and potentially notifying bridge owners of problems between inspection visits. A low-power, wireless, strain
data acquisition device has recently been developed to acquire dynamic strain data. Strain gages can be used to monitor
the number and size of stress cycles in fatigue-sensitive members. From the cycle count, Palmgren-Miner's rule can be
used to determine an effective stress range. The remaining fatigue life can then be calculated and compared to existing
conditions and the age of the bridge. Because damage is expected to escalate over time, more frequent inspections may
be needed when a bridge approaches its fatigue life. The strain node can be programmed in LabVIEW WSN to detect
critical events or perform a rainflow analysis. To aid in system interaction, a software interface will be designed to
allow for automated processing and transmission of data to a cloud server, thereby allowing engineers and bridge owners
to access the data from anywhere so as to make informed decisions when prioritizing inspections. This paper will
present the development of the strain node and the software interface.
A passive, wireless sensor has been developed at the University of Texas at Austin to monitor the insitu conductivity of
concrete within civil infrastructure systems. Electrical conductivity is one possible indicator of corrosion of embedded
reinforcement and thereby provides information on structural performance. The sensors would be attached to the
reinforcement cages before placement of the concrete and interrogated as part of a routine inspection over the service
life. A new sensor design, a non-contact conductivity sensor, is being developed to minimize the likelihood of damage to
the sensor during placement of the concrete; a metal element is positioned above the sensor body but is not connected to
the resonant circuit within the sensor. In order to verify the response of the non-contact conductivity sensors, they were
submerged in liquids of increasing conductivity. Analysis of the measured data demonstrated that the noncontact
conductivity sensors successfully detected conductivity variations in liquids.
A passive, wireless and inexpensive sensor has been developed to monitor the conductivity of concrete and thereby
provide information on the progress of chloride-induced corrosion of the embedded reinforcement in concrete structures.
Sensors are designed to be attached to the reinforcement cages before placement of the concrete in new construction or
in portions of rehabilitated structures. Sensors will then be interrogated intermittently over the service life during routine
inspections. The results of two experimental investigations are discussed in this paper. In the first, conductivity sensors
were submerged in liquids of increasing conductivity. In the second, conductivity sensors were embedded in concrete
cylinders and interrogated over a 25-week period during initial set and curing of the concrete. Analysis of the measured
data shows that the passive conductivity sensors were successful in detecting a variety of conductivity levels in the
A passive sensor platform has been developed at the University of Texas at Austin to monitor corrosion of embedded
reinforcement in concrete structures. The sensors are powered and interrogated in a wireless manner. Initial sensor
designs used a sacrificial corroding steel wire to indicate the risk of corrosion within concrete. The wire was
physically connected to the sensor circuitry and passed through the circuit protection layer. Consequently, it allowed
contaminants to reach the circuit electric components causing corrosion and limiting the service life of the sensor. A
novel sensor configuration that relies on wireless inductive coupling between a resonant circuit and the transducer
element is presented. The non-contact design eliminates the breach concern and enhances the durability of the senor.
Preliminary test results of the new design will be discussed in this paper.
This paper provides a summary of ongoing research sponsored by the National Institute of Standards and Technology
(NIST) that seeks to improve inspection practices for steel bridges by providing the technology and methodology for
real-time monitoring. In order to reduce the time and cost of installing a monitoring system, the research team elected to
use wireless communications within the sensor network. The investigation considered both IEEE 802.11 and IEEE
802.15.4 communications protocols and identified the latter as more practical for bridge monitoring applications. Studies
were conducted to investigate possible improvements in the network performance using high-gain antennas. Results
from experiments conducted outside and on bridges with different antennas are presented in this paper. Although some
benefits were observed using high-gain antennas, the inconsistent performance and higher cost relative to the current
stock, omni-directional antennas does not justify their use.
This paper summarizes ongoing work to develop low-cost, wireless, resonant sensor nets that can be used to monitor
corrosion in infrastructure systems. A magnetically coupled sensor array is analyzed using a circuit model. The array
acts as a magneto-inductive waveguide and the impedance discontinuities caused by corrosion (or other defects) lead to
reflection. The relationship between the relative position of defects and pass band ripples is investigated, providing a
technique to determine the location of targets. A configuration for increased sensitivity and a method for defect
localization are presented.
A class of low-cost, wireless sensor has been developed at the University of Texas at Austin to monitor the performance
of reinforced and prestressed concrete members in civil infrastructure systems. The sensors are designed to be
interrogated in a wireless manner as part of a routine inspection. The sensors do not require batteries or connections to
external power supplies. As such, the sensors are intended to be maintenance free over the service life of the
Research efforts to date have focused on detecting the onset of corrosion. It is envisioned that the sensors would be
attached to the reinforcement cages before placement of the concrete. The results of long-term exposure tests will be
used in this presentation to demonstrate the potential and reliability of the resonant sensors.
Results from our efforts to improve the performance of low-cost, unpowered, wireless, resistance based Electronic
Structural Surveillance tags (ESS) will be presented. The ESS tags use an unpowered embedded sensor read by an external reader using an inductively coupled impedance measurement. Read range of coupled tags is largely dependent on the strength of inductive coupling which is influenced by the relative shape and size of the coils. Reader coil geometries can be optimized to increase read range. Additionally, an enhanced circuit model, for data extraction, is developed and tested with the corrosion sensor. The model provides increased information about the sensor and its surroundings. Better coil design and circuit model based data extraction methods can improve the reliability in reading the sensor. Recommendations for design and analysis resulting from this study can be extended to optimize other electronic structural surveillance tag sensors.
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.
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.
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.
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.
This paper summarizes the ongoing work at the University of Texas to develop wireless sensors to monitor the condition of civil engineering structures. The state sensors will be attached to or embedded in the structures during construction and will be interrogated during routine inspections of the structure. Each sensor comprises two components: a switch to detect the damage within the structure and a resonant circuit to transmit the information to the inspector. Prototype sensors have been developed to detect the presence of cracks in welded steel construction and corrosion in reinforced concrete construction.