G. Tadros

Structural health monitoring of innovative bridges in Canada with fiber optic sensors

This paper describes the development and application of fiber optic sensors for monitoring bridge structures. Fiber Bragg gratings (FBGs) have been used to measure static and dynamic loads on bridge decks and columns, including composite repairs for rehabilitation purposes. A new long gage concept that permits overall average strains to be measured has also been developed with gage lengths varying from 1-20 m. These gages can be bonded to the concrete structure or imbedded in the composite repair patch. Six projects undertaken by ISIS Canada to incorporate fiber optic sensing to monitor the structural health of bridges in Canada are described. Data will be presented for several bridges that indicate a measure of system reliability over several years in a hostile environment. The benefits of fiber optic sensors will be highlighted.

Fiber-optic Bragg grating sensors for bridge monitoring

Abstract Fiber-optic Bragg grating strain sensors hold a great deal of potential for structural monitoring because of their exceptional stability and demonstrated potential for long-term monitoring. This sensing technology takes advantage of a spectrally encoded signal which provides inherent immunity from signal intensity fluctuations which plague many other fiber-optic and electronic sensing techniques. This results in measurement stability and lead/interconnect insensitivity which permit longterm and intermittent monitoring with high resolution and accuracy. Fiber-optic grating sensors are intrinsic to the optical fiber, thus capitalizing on its extremely small size and inherent strength and durability. Recent results are provided from a sensor array installed in a road bridge. The strain sensors are attached to both steel and carbon-fiber-reinforced plastic prestressing tendons, which are embedded in the precast girders of the bridge. Measurements of traffic loads and the relaxation behaviour of the tendons are presented. The potential of fiber grating technology is briefly discussed including its application in long-gage strain-sensing and strain-distribution measurements.

FLEXURAL BEHAVIOR OF ONE-WAY CONCRETE SLABS REINFORCED BY FIBER REINFORCED PLASTIC REINFORCEMENTS

Fiber Reinforced Plastic (FRP) reinforcements are currently used for special concrete structures in areas sensitive to magnetic fields and severe environmental conditions that accelerate corrosion of the steel reinforcements, and consequently leads to deterioration of the structure. This paper presents test results of eight one-way concrete slabs reinforced with glass-fiber, carbon-fiber, and conventional steel reinforcements. The slabs were tested under static loading conditions to determine their flexural and shear limit states, including the behavior prior to cracking, cracking, ultimate capacities, and modes of failure. Based on this investigation, design recommendations and guidelines are proposed.

Test Model for the First Canadian Smart Highway Bridge

Carbon fiber reinforced plastic tendons (CFRP) were used for the first time in Canada to pretension six girders of a concrete highway bridge. The bridge was constructed using 13 T-bulb section pretensioned precast concrete girders in each span. Continuity of the two spans was achieved by using post-tensioned steel tendons extending along the entire length of the bridge. This paper reports on an experimental study to examine the behavior of four pretensioned concrete T-beams of the same span-depth ratio as the bridge girders. The beams were tested to examine the various limit state behaviors, ultimate capacities, and failure modes. Two beams were tested in static, and two were tested under cyclic loading. After completion of 2 million cycles, the beams were loaded to failure to evaluate the effect of fatigue loading on the behavior of the beams. Predicted values based on the compatibility and equilibrium approach, and measured values, are compared and discussed. The authors propose a ductility model for beams prestressed by FRP tendons. Design recommendations and construction details of concrete beams prestressed by CFRP tendons are presented.

Fiber optic Bragg grating sensor network installed in a concrete road bridge

The installation of a fiber optic Bragg grating strain sensor network in a new road bridge is described. These sensors are attached to prestressing tendons embedded in prefabricated concrete girders. Three types of prestressing tendons are being monitored: conventional steel strand and two types of carbon fibers reinforced plastic tendons. Sensor durability issues are reviewed and the installation is described. Initial measurements indicate that the sensors are operational and provide some early comparison of tendon performance.

Durability of GFRP Reinforced Concrete in Field Structures

Synopsis: Recently, ISIS Canada studied the durability of GFRP in concrete in several field structures across Canada. The objective of the study was to provide the engineering community with the results of the performance of GFRP materials that have been exposed to the concrete environment in built structures. Cores of GFRP-reinforced concrete were removed from five field structures. Analytical methods, namely optical microscopy, scanning electron microscopy and energy dispersive x-ray, differential scanning calorimetry and infrared spectroscopy, were used to determine the composition of GFRP after being subjected to the alkaline environment of concrete for five to eight years. Three research teams from four Canadian universities performed microanalyses of the GFRP and surrounding concrete independently. Results indicate that no deterioration of GFRP took place in any of the field structures. No chemical degradation processes occurred within the GFRP due to the alkalinity of the concrete. The overall conclusion of the study is that GFRP is durable in concrete. Also, it was concluded that the CHBDC was conservative in its first edition by not permitting GFRP as primary reinforcement. As a result of the study, the second edition of the CHBDC, currently in the final stages of approval, permits the use of GFRP as primary reinforcement.

Multiplexed Bragg grating laser sensors for civil engineering

A portable, rugged, and compact 4-channel Bragg grating fiber laser sensor demodulation system was developed for interrogating an array of 20-intracore Bragg grating sensors that we embedded within several of the concrete girders used to support the deck of a new two span road in Calgary, Alberta. Another unique feature of this bridge is that two types of Carbon fiber based composite prestressing tendons are being tested in a number of the concrete girders. We have instrumented five of the approximately 60 ft long concrete girders with fiber optic Bragg grating strain and temperature sensors in order to monitor the loads on these girders and their response during the construction of the bridge and subsequently over an extended period of time.

Design Recommendations for Bridge Deck Slabs Reinforced by Fiber Reinforced Polymers

The linear characteristics of fiber reinforced polymers (FRP) up to failure and their relatively low elastic modulus and strain at ultimate has raised concerns with structural engineers regarding their use as reinforcement for flexural members. Based on a nonlinear finite element analysis and testing of a full-scale model at the University of Manitoba, Canada, design guidelines on the use of glass and carbon fiber reinforced polymers (GFRP and CFRP) as reinforcement for bridge deck slabs are proposed. The accuracy of the nonlinear finite element model is demonstrated by comparing the predicted behavior to test results of two models. The influence of the degree of edge restraint, percentage of reinforcement of CFRP and GFRP, type of reinforcement and presence of top reinforcement on the structural behavior and mode of failure of continuous concrete bridge decks is discussed. Based on serviceability and ultimate capacity requirements, reinforcement ratios of CFRP and GFRP for typical bridge deck slabs are recommended.

FRP for Prestressing of Concrete Bridges in Canada

Fiber reinforced polymer (FRP) materials are being used in a variety of configurations as an alternative reinforcement for new and strengthening civil engineering structures and bridges. These materials have high corrosion resistance, high strength and fatigue resistance. This paper describes two bridges that were built in Canada in 1993 and 1997 and are monitored to examine the long term behavior of FRP as prestressing reinforcement in severe environment and under normal traffic conditions. Also presented are summaries of the research work conducted by testing scale models to simulate the behavior of the girder and the deck slab.

Use of Externally Bonded FRP Systems for Rehabilitation of Bridges in Western Canada

Four recent field applications of externally bonded fiber reinforced polymer (FRP) systems for rehabilitation of bridges in western Canada are reviewed in this paper. For the rehabilitation projects described, the ease of handling of the CFRP materials resulted in reduced construction time, when compared with conventional repair techniques. For the three heavily used highway bridges, at least one lane of traffic remained open at all times during the construction. The four projects described in this paper demonstrate a successful transfer of ISIS technology into practice in Western Canada.