On Leslie Street, in the northern part of Toronto, a forest of concrete columns supports a multi-lane bridge, part of the city’s main thoroughfare of Highway 401. It doesn’t look much different from any other highway bridge in the city, but the columns supporting it possess two unique features — both firsts in the province.

• Two of the columns have dormant intelligence, instrumented with the first experimental long gauge fiber optic sensors ever manufactured or field tested in Canada.

• Four columns are wrapped to a height of about seven feet with advanced composite materials (ACMs) — extremely strong and lightweight synthetic material such as carbon-fiber or glass-fiber reinforced composite materials.

The columns are an experimental part of a new trend in civil engineering called smart structures, incorporating sensors in some of the most advanced building materials ever used in bridge construction. The critical deterioration of transportation infrastructure across the continent, including highways and bridges, has driven the search for new methods of concrete rehabilitation and repair. These advanced composite materials are the leading edge of civil engineering technology, at a fraction of the weight and orders of magnitude stronger than conventional construction materials.

Ontario’s Ministry of Transportation scheduled the conventional concrete column repair on the Leslie Street bridge for 1996 and decided to experiment with ACM wraps instrumented with fiber optic strain sensors, and with reference cells to measure the progress of corrosion. A readout is shown in the photo above.

The optical sensors were designed, manufactured, and installed by research engineers at the Fibre Optic Sensing laboratory of the University of Toronto Institute for Aerospace Studies. The UTIAS FOS laboratory, headed by Dr. R.C. Tennyson, is a research member of Intelligent Sensing for Innovative Structures (ISIS) Canada, a federal Network Centre of Excellence.

"Since the [ACM] wraps are impervious to air, water, and chloride, they may protect steel reinforcements," said Frank Pianca, of MTO’s Research and Development Branch, "but we need to see if moisture and oxygen enter from the top or bottom of the column. We’re monitoring to see how effective it is and we’ll need to follow the corrosion performance of the column for about three to five years."

Advanced experimentation with ACMs includes the installation of fiber optic sensors under the wraps that allow highway engineers to keep track of what is happening to the concrete columns underneath. The sensors precisely measure expansion or contraction of the concrete, and these measurements may indicate the effect of corrosion inside the column.

The fiber optic sensors are the same kind of hair-thin fiber optic cables used for telecommunications, but wrapped around or embedded in concrete structures. A short section of fiber optic cable is stripped of its protective coating and treated with high-intensity, ultra-violet beams to create a sensing region. This sensing region is, in fact, the sensor, which can deliver measurements of concrete stresses and strains for computer analysis.

Most fiber optic sensors for civil engineering are approximately 1- to 1.5-in. long and provide measurements of temperature and defined points of strain.

The experimental sensors on the Leslie Street bridge, however, are 10-ft. long — one-of-a-kind, custom-made sensors with length equal to the circumference of the columns. When the steel reinforcement inside the column is contaminated by salt and moisture, the steel begins to rust, expand, and crack the surrounding concrete columns.

Highway engineers are testing ACM wraps to see if they can reduce and/or delay the corrosion inside the columns, and, consequently, bridge repair and road closures. The long gauge sensors installed on the Leslie Street bridge precisely measure the total column expansion, providing valuable data on the state of the column underneath the repair wraps, while the reference cells are used to measure corrosion potential.

"What we are looking for is displacement of any kind," said UTIAS FOS laboratory research engineer Paul Mulvihill. "If the columns have widened, that could be an indication of corrosion. If there is no displacement — if the columns haven’t expanded — that’s an indication that the ACM wraps are doing their job."

The sensor readings were calibrated at the time of installation in 1996. In March, 1998, the second set of readings provided the first hard evidence of the viability of these experimental long gauge sensors, and the clear potential of ACM wraps. When the readings from the bridge were analyzed shortly after they were taken, all five sensors initially installed were found to be fully operational, and the two wrapped columns instrumented with the sensors changed in circumference only a minuscule amount — the first column expanded by an average of 5871 microns (about 5.9 millimeters), and the second column expanded by an average of 6259 microns (about 6.3 millimeters). While readings will continue to be taken, statistically, these measurements indicate no significant column change.