Early detection of corrosion of steel reinforcement in a concrete structure can help ensure long-term structural integrity.
Currently, it is very challenging to identify reinforcing steel corrosion until the corrosion has already developed and caused cracking of the surrounding concrete from the rust products.
Once surface cracking of the concrete has occurred, an investigation into the deterioration is time-consuming and destructive to the structure, as cores are taken to investigate the surface of the reinforcing steel and the concentration of the salts in the concrete. This work requires lane closures, causes traffic delays and involves elevated safety risks for maintenance crews and the traveling public. This problem is quite costly, as the annual direct cost in the U.S. of corrosion to structural concrete in bridges and overpasses has been estimated at $8.3 billion.
Sensors the size of a quarter
Although there has been much discussion about the use of sensors to monitor the health of a structure, there are a number of reasons this technology is not widely used. For example, current corrosion sensors for concrete cost more than $100 per sensor, can only measure corrosion potential at one location and require wires to communicate and power the sensor. Because of these challenges, it is rare that advanced sensor technology is used to monitor the health of transportation infrastructure. Instead, owners typically use regular visual inspections to determine the health of their concrete infrastructure. If a sensor were developed that was inexpensive, did not require wires or maintenance and could give the owner the information that it needed to monitor the health of concrete infrastructure, then this would make the development of advanced sensor technology possible.
Researchers at Oklahoma State University (OSU) have developed and patented a new generation of corrosion sensor that is wireless, inexpensive and does not require batteries. This work was sponsored by the Oklahoma Transportation Center (OkTC) and the Oklahoma Center for the Advancement of Science and Technology (OCAST). The research was a collaborative effort between Drs. Nicholas Materer and Allen Apblett from the OSU Chemistry Department and Dr. Tyler Ley of the Civil and Environmental Engineering department at OSU.
The developed sensor can be monitored by using a simple reader that can either be handheld or mounted on a vehicle that drives over the infrastructure and queries the sensors. These sensors are based on passive radio-frequency identification (RFID) tags. The use of RFID sensors is quite widespread to track and count large shipping containers, consumer goods, equipment and medicine in a hospital, and even is used to make automatic payments of toll charges for vehicles. With a passive system, an antenna sends a radio frequency that remotely powers the sensor through induced current; the antenna is then able to receive a transmitted signal from the sensor. Until now, RFID technology has primarily been used as a very rapid way to communicate the location of an object. The research team has modified this technology to do much more.
The developed sensors are about the size of a quarter and use an external wire to sense the presence of corrosive agents at four different locations near the body of the sensor. The current version of the sensor can be read through 1 ft 6 in. of wet concrete. While this sensor could be used in a number of applications, the primary focus has been on embedding the sensor in the surface or cover concrete of a concrete structure.
As the salts penetrate into the concrete, they corrode the external wire that is measured by the sensor and can provide an owner with a warning. When each sensor is read, it responds with a unique identification number and a measurement of the local potential for corrosion. This information may be used to intelligently schedule maintenance such as using a concrete sealer, or to modify the current uses of the structure to prolong its life. The information from the sensors gives the owner the ability to know how their infrastructure is performing and how long it is estimated before major repair is required. The commercial cost of the sensors is estimated to be $20/sensor.
The sensors have been validated with laboratory testing. The sensors were embedded in concrete at a known depth, and the surface of the concrete was exposed to salt solutions. Over time, the salt solution diffused into the concrete, and the response from the sensor was monitored. After a threshold level of corrosion was measured at the location of the sensor, the chloride concentration of the concrete was determined. This was done by taking powder samples of the concrete at different depths and measuring the chlorides in the powder as per ASTM C 1556. Since the sensing wire was embedded at a known depth, the chloride concentration at the height of the wire could be easily determined. This testing showed that the sensors could be used to consistently measure the potential for corrosion at different depths in concrete.
A number of environmental tests were run to investigate the ability of the sensors to show satisfactory long-term performance. To do this, the sensors were exposed to 80 rapid temperature cycles from 73ºF to 175ºF and from 73ºF to -4ºF while embedded in concrete. Failures in the hardware were found and the sensor design was improved. The components used in the sensor are military grade and are designed to have a very long lifespan. In addition concrete specimens with the sensors were cast in different locations. In all of these tests, the strength impact was found to be less than 10%. This is significant, as these sensors were tested in the most critical areas within these members.
Simple mounting techniques have been developed to accurately place the sensor above the reinforcing steel. Plans are being made to use these corrosion sensors in structures in Oklahoma, Texas and Iowa in spring 2012. Presentations about the technology also have been made at national and international conferences. For more information on the sensors, please contact Tyler Ley at [email protected]. R&B