Infrastructure Technology Institute
Building on its years of experience monitoring cracks in residential structures using wireless sensors, ITI engineers have developed a new sensor to monitor the progression of cracks in bridges. When combined with a commercially available wireless sensor network platform, this sensor gives engineers the ability to deploy an autonomous system that can track the growth of cracks in real-time and alert authorities to small changes in crack size.
In the United States, federal law dictates that bridges must be inspected at least once every two years. This inspection frequency may increase depending on the age, past inspection results, or structural design of the bridge. During these inspections, trained personnel seek out new cracks and document the progress of existing cracks by entering the characteristics of the cracks in a log book, photographing the cracks, and comparing the present position of crack tips with marks made at the position of the crack tips during previous inspections. The data collected using these manual crack monitoring techniques suffer not only from poor repeatability from inspection to inspection but from a lack of information available about the condition of the crack between inspections that may be months or years apart.
The new sensor is applied to a structural member near the tip of an existing crack. The sensor terminals are glued down to the bridge member then metallic traces are drawn onto the surface of the member in such a way that a propagating crack will sever one or more of the traces. The wireless sensor node (opposite page, top) detects the severing of a trace and transmits a message back to its base station, located on or near the bridge, which then sends an alert over the Internet to the proper authorities. The solar-powered wireless sensor network that reads these new sensors is adapted from the agriculture monitoring industry and are water-resistant and solar-powered. With sufficient sunlight, the system can run for five years or more without any maintenance.
Unlike commercially available crack propagation sensors, the sensor developed by ITI does not require the use of heat-cured adhesives and crack measurement is not limited to a small area. ITI’s sensor can be applied using glue and conductive ink, neither of which requires the application of heat. Additionally, the traces can be drawn in any shape or configuration, allowing customized deployment for different shapes of bridge details and expected cracking paths. In contrast, traditional crack propagation sensors require anticipation of the direction of crack growth at the time of installation.
ITI tested these new sensors and their wireless sensor network in the laboratory, and the graph (opposite page, bottom) shows some of the results, in which a crack was made to propagate through a piece of steel. The plot clearly shows severing of each of the four rungs of the sensor as the crack progressed. In the coming months, ITI plans to deploy the new sensing system on an in-service bridge.