BY PETER CAULFIELD - A new type of sensor is under development in the civil engineering department of the University of British Columbia that can sense changes in the stress condition and temperature of a concrete structure - such as bridges and other civil infrastructure - by making the concrete electrically conductive.
“Small piezoresistive sensors (which measure changes in electrical resistance) embedded in the concrete and connected by Internet in a network enable us to determine what kind of a load a concrete bridge can carry and how durable it is,” said Nemy Banthia, who is leading the research.
What makes the concrete sensitive to stress are tiny carbon nanotubes inside the sensors.
“Carbon fibres are hybridized with multi-walled carbon nanotubes, which makes the concrete extremely responsive to stress and chemical changes,” Banthia said.
“When installed strategically as tiny sensors in a concrete structure, they can provide engineers with continuous data on the health of the structure they are part of.”
Stress sensitivity of the piezoresistive sensors is 100 times greater than traditional sensors, he said.
“We’re still in the product development stage, but we hope to have a commercial version of the piezoresistive sensor in about a year.”
In addition to sensing, possible future applications of electrically conductive concrete include electrical grounding, lightning arresters and self-heating bridge decks.
“It’s great new technology,” said materials engineer John Zhang, principal of LZhang Consulting and Testing Ltd. in Delta.
“It will be a big improvement on existing sensors, provided a commercial version comes available.”
Banthia’s university research specialty is cement-based and polymer-based fibre reinforced composites. In addition to the piezoresistive sensor, he has developed a high-performance fibre reinforced polymer (FRP) spray that is applied to the surface of concrete to form a compacted composite that is stiff and strong.
The spray has been used on the Safe Bridge on southern Vancouver Island. The bridge, which is located near Duncan and gets a lot of logging truck traffic, was the first bridge of its kind to undergo strengthening using this technique.
“The bridge was built in the 1950s to the standards of earlier building codes, and was in danger of collapse, because of problems in the design,” Banthia said.
He added that after the application of the polymer coating, the channel beam bridge is at least twice as strong and will absorb three times as much energy during an earthquake.
But FRP isn’t as fast or as inexpensive as shotcrete as a way to repair and strengthen concrete, though it is 300 times stronger.
Both the bridge itself and the polymer coating carry fiber-optic sensors which transmit data about the condition of the bridge.
“The signals help to monitor the effectiveness of the strengthening coat, and allows engineers to understand the response of the bridge when heavily loaded trucks drive over it,” Banthia said.
In the event of an earthquake, the sensors will assess the bridge’s dynamic performance.
“And, after the earthquake, the signals will help to determine the residual capacity of the bridge,” he said.
He added that using embedded sensors to monitor the structural condition of bridges can help to prevent failures such as the one that occurred on the Concorde Bridge in suburban Montreal in 2006. The bridge was visually inspected the same week it collapsed.
“Unfortunately, the current tools in our arsenal for bridge inspections and health monitoring are antiquated and unable to predict accurately the true condition of the structures,” Bhantia said. “In particular, they are unable to predict deterioration in steel – due to corrosion and in concrete – due to chemical and physical attack.”
One of the main threats to bridges in Canada is the large amount of de-icing salt which gets dumped on them in the winter, which corrodes the reinforcing steel in the concrete.
“Corrosion produces rust, which takes up about 20 times more volume than steel itself and causes cracking and spalling (flaking) in concrete, and sometimes even collapse,” he said.
Another threat to bridges is global warming.
With atmospheric carbon dioxide (CO2) levels expected to increase from the current level of 600 parts per million to about 1,000 ppm, and atmospheric temperatures expected to rise by as much as 5.5 C, deterioration in concrete structures is expected to accelerate.
“Higher CO2 levels will cause greater carbonation, and corrosion and higher temperatures will lead to greater thermal stresses and shrinkage-induced cracking,” Banthia said.