Fibre Optic Sensors (FOS) are currently being introduced to the structural engineering applications (bridges and other structures) as an attractive alternative to conventional tools such as electrical strain gauges and vibrating wires. Advantages of the FOS from the material point of view include resilience and durability towards environmental parameters. More importantly, FOS can be integrated with the reinforcing element (steel and FRP bars, FRP structural products) during fabrication. This enables continuous monitoring of the strains/ stresses and temperature in the structure with time. The engineers will be able to detect any sign of distress inside the structure as it happens, allowing precise and immediate decision making to solve the problem. The acceptance of the FOS among the construction industry is still hampered by few factors, most importantly cost and long-term durability. The cost of the FOS has been reported to be decreased significantly over the last few years due to larger production and demand rates, and it is expected to reach an affordable rate once their behaviour is fully understood and the research phase is completed. The durability issues are currently the focus of many research projects, including this proposed research activity.
During the first five-year term of the NSERC chair, a wide range of laboratory and field testing on the use of FOS and FRP bars in bridges and structures has conducted. Specially designed laboratory facility at the department of civil engineering has been built and utilized over the past 10 years for this purpose. Two types of FOS will be investigated: Fabry-Perot and Bragg sensors in collaboration with industrial partners (Roctest Ltd. and Avensys Inc.). This research activity includes the following tasks:
In the first technique, the sensor will be hand-laid in a grove made on the surface of the bar and fixed in place using special adhesive. The grove and the sensor in it will be exactly in the direction of the bar. The research will examine the possible adhesives and their durability under different environments and loading schemes. The second technique will focus on more automated-oriented method where the sensor is planted inside the FRP product (bars and FRP bridge decks) during the fabrication. This technique is more efficient for mass production. The research program will investigate this technique in collaboration with the FRP manufacturers (Pultrall Inc. and Arian Sazeh Inc.).
This include elevated and low temperatures, freeze-thaw cycling, wet-dry cycling, alkaline resistance, fatigue (cyclic loading), and creep (sustained load).
Sensitivity to temperature changes: The sensors should have a very stable behaviour against the change in ambient temperature, initial tests made by the manufacturer indicated very linear change in the output wavelength of the sensor when subjected to temperatures between -20 to +60o C. These results will be verified and complemented by series of tests using the sensors (standing alone) or mounted on reinforcing bars/concrete specimens subjected to different temperatures. The bars will be exposed to these temperatures as free bars or while being under stress.
Creep behaviour: The creep test is aimed to assure the reliability of the readings taken by the FOS over extended period of time under loading. This is a typical situation in the field, especially in highway bridges where the own weight of the bridge represents over 60% of the bridge load. This test will be made by subjected steel and FRP bars instrumented with the FOS under study to a constant sustained tensile loading for 5000 hours. The steel bars will be subjected to a sustained stress equal to 0.85 of the yield stress, while the FRP bars will be subjected to 0.4, 0.6, or 0.8 of their ultimate tensile capacity.
Fatigue behaviour: The stability of the FOS output under fatigue loading is of extreme importance for bridge applications, the live load on highway bridges is mainly heavy trucks with high frequency of occurrence. The behaviour of the FOS in fatigue will be evaluated by subjecting the test specimens to cycling loading at two different frequencies; 0.5 and 2.0 Hz. The ratio of minimum to maximum stresses within each cycle with be either 0.1 or 0.9.
Test specimens will be made of steel or FRP bar instrumented with the FOS and placed in two concrete blocks 150x150 mm in cross section and 500 mm long each, and separated by 100 mm along the direction of the bar. The two blocks will be placed in a steel cage and attached to the ends of an MTS loading machine. The loading will continue to achieve 4 Million cycles. The instrumented bars will be subjected to the cycling loading through the concrete cubes attached to the machine ends.
Alkaline resistance: The effect of the pH of the surrounding media on the long-term performance of the FOS will be tested by subjecting steel and FRP rods instrumented with FOS to alkaline solution for extended period and under loading. The rods will be subjected to the same loading levels and durations described in creep behaviour while submerged in alkaline solution.
Environmental cycling: Both freeze-thaw and wet-dry cycling will be done on steel and FRP bars instrumented with FOS. The bars will be subjected to 300 cycling of each type; a full cycle will last for 12 hours divided equally. The bars will not be loaded during the cycling phase, but will be tested under tension after every 100 cycles (up to 0.85 fy for steel bars and 0.8 fu for FRP bars). At the end of the cycling, tension test will be continued till failure.
Laboratory evaluation of the serviceability of the FOS when implemented into reinforced concrete structures will be assessed through a series of tests made on concrete beams reinforced with steel and FRP bars, some of the reinforcing bars will be instrumented with the FOS. The tests will evaluate the performance of the FOS under static, fatigue, and creep effects.
Static tests: Rectangular concrete beams (130 × 180 mm in cross-section and 1800 mm in length) reinforced with steel or FRP bars will be tested in flexure under four-points bending. Electrical strain gages and linear variable displacement transducers (LVDT) will also be used on the beams to compare strain values obtained by the FOS and to measure the deflection of the beam.
Fatigue tests: When FOS are used in highway bridges, they will be subjected to a very large number of loading cycles. To ensure their durability and the consistency of their readings under this condition, reinforced concrete beams instrumented with the sensors will be tested in the laboratory for up to 4 million loading cycles. The beams design, number, and loading scheme will be identical to those described in the previous point. The frequency of the cycles will be set to 2 Hz, enabling the testing of one beam every two weeks. The load will be oscillating between 0.2 and 0.6 of the design service load of the beam, and following a sinusoidal curve. The cycling will be halted at 250,000, 500,000, and 1,000,000 cycles and a static test will be done to reach the design service load of the beam before resuming the cycling loading. At the end of the 4 million cycles, the beam will be loaded to failure under monotonic loading.
Creep tests: The behaviour of the FOS under sustained load while embedded in concrete will be evaluated through beam tests. Eight beams will be fabricated and tested, four at room temperature and another four at low temperature (-18oC), all beams will be identical in size and reinforcement, similar to previous beams described. The duration of the tests will be 300 days; readings will be recorded daily for the first two weeks, then weekly afterwards to monitor the stability of the FOS.
This task is directly connected to the research activity 8 (Field implementation and structural health monitoring). As an integral part of this research, the FOS tested will be implemented into the construction of the new bridges (in collaboration with the MTQ as done before in earlier projects). The FOS, FRP and steel reinforcing bars instrumented with the FOS will be used in part of the bridge deck slab. Conventional electrical strain gages will be installed in very close proximity to the FOS for comparison. Strain measurements will be recorded on continuously during the first three months using remote sensing technique. The stability of the readings and their change with the ambient temperature and humidity will be compared to that found from laboratory tests.