Sami Rizkalla

Near-Surface-Mounted Fiber-Reinforced Polymer Reinforcements for Flexural Strengthening of Concrete Structures

Use of fiber-reinforced polymer (FRP) materials to strengthen bridges has been adopted extensively in the last decade. FRP has been used in different configurations and techniques to use the material effectively and to ensure long service life of the selected system. One of these innovative strengthening techniques is near-surface mounting (NSM) that consists of placing FRP reinforcing bars or strips into grooves precut into the concrete cover in the tension region of the strengthened concrete member. This method is relatively simple and considerably enhances the bond of the mounted FRP reinforcements, thereby using the material more effectively. This paper presents test results of reinforced concrete (RC) T-beams strengthened in flexure with different strengthening systems using FRP reinforcing bars and strips as NSM reinforcement and externally bonded FRP strips. The FRP reinforcements used in this investigation include carbon FRP (CFRP) reinforcing bars and strips and glass FRP (GFRP) thermoplastic strips. Behavior and effectiveness of the materials used for the various strengthening systems are compared. The structural performance and modes of failure of the tested beams are presented and discussed. Test results indicated that using NSM FRP reinforcing bars and strips is practical, significantly improves the stiffness, and increases the flexural capacity of RC beams. The limitations of using NSM FRP reinforcing bars and strips are controlled by serviceability requirements in terms of overall deflections and crack widths rather than delamination, observed by many researchers, of externally bonded FRP reinforcement. Strengthening of RC beams using NSM FRP strips provided higher strength capacity than externally bonded FRP strips using the same material with the same axial stiffness.

CONFINEMENT MODEL FOR AXIALLY LOADED CONCRETE CONFINED BY CIRCULAR FIBER-REINFORCED POLYMER TUBES

This paper introduces an analytical model to predict the behavior of axially loaded circular concrete columns confined by fiber-reinforced polymer (FRP) tubes. The model, an extension of an earlier confinement model for concrete confined by steel reinforcement, is based on equilibrium, compatibility conditions, and the biaxial strength failure criteria of FRP tubes. It can be used to predict the behavior of prefabricated FRP tubes totally or partially filled with concrete, as well as concrete wrapped with FRP sheets. The model can account for the case of axially loaded concrete core only as well as the composite section of the concrete core and FRP tube, and is verified by experimental results reported by the authors and other researchers. A parametric study is presented to examine the effect of stiffness of the FRP tube, the effect of loading the FRP tube axially, and the effect of presence of inner hole inside the concrete core. The study shows that increasing the central hole size reduces the confinement effect, increasing the stiffness of the tube improves the confinement, and axial loading of the FRP tube significantly reduces the confinement.

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.

BEHAVIOR OF AXIALLY LOADED CONCRETE-FILLED CIRCULAR FIBER-REINFORCED POLYMER TUBES

The aim of this paper is to describe the structural behavior of concrete-filled glass fiber-reinforced polymer (GFRP) tubes subjected to axial loads. The research included completely- and partially-filled tubes with a central hole as well as a tube-in-tube system with concrete filling between the 2 tubes. The GFRP tubes were designed to provide strength in both axial and transverse directions and were axially loaded with the concrete core. The study showed that the strength and ductility of concrete are improved due to confinement using GFRP tubes. The highest confinement level was achieved for completely-filled tubes. Using a central hole lowers the confinement effect; however, using an inner tube can enhance the confinement for this type of member. Results show that loading of GFRP tubes reduces the confinement effectiveness. The effects of laminate structure, hole size, interface condition between the tube and the concrete core, stiffnes of the tube, and failure modes are discussed.

Bond Mechanism of Near-Surface-MountedFiber-Reinforced Polymer Bars for FlexuralStrengthening of Concrete Structures

This paper presents both experimental and analytical investigations undertaken to evaluate bond characteristics of near-sutfacemounted (NSM) carbon fiber-reinforced polymer (CFRP) bars. A total of eight concrete beams. strengthened with NSM CFRP bars, were tested under monotonic static loading. Different embedment lengths are studied to determine the development length of the fiber-reinforced polymer (FRP) reinforcement. The performance of two different adhesives used to bond the bars to the surrounding concrete is examined. A general methodology to evaluate the development length of NSM FRP bars of different configurations and types of fibers is presented. A quantitative criterion governing debonding failure is established. The proposed bond model assumes linear elastic behavior jar the concrete, adhesive, and the NSM FRP bars, following the same philosophy oj the ACI provisions for bond analysis and design. The proposed analytical model is validated by comparing the predicted values with test results as well as to nonlinear finite element modelling. The influence of key parameters, including the thickness of the adhesive cover, groove width, groove spacing, and inte171ai steel reinforcement configuration, are discussed.

Experimental and Analytical Modeling of Concrete-Filled Fiber-Reinforced Polymer Tubes Subjected to Combined Bending and Axial Loads

This paper presents test results of an experimental program, and proposes an analytical model to describe the behavior of concrete-filled, fiber-reinforced polymer (FRP) tubes subjected to combined axial compression loads and bending moments. The experimental program included 10 specimens subjected to eccentric axial loads, 2 specimens tested under concentric axial loads, and 2 specimens tested in bending. Glass FRP tubes with 2 different laminate structures were considered, and axial load/bending moment interaction curves are given. An analytical model is presented that accounts for variable confinement of concrete as a result of the gradual change of the biaxial state of stresses developed in the tube as the eccentricity changes. A parametric study was conducted to evaluate effects of diameter-to-thickness ratio and laminate structure of the tube, including different fiber proportions in the axial and hoop directions. The study evaluated the confinement as affected by the eccentricity of the applied axial load as well as the influence of the FRP laminate structure. Findings indicate that the interaction curves are significantly affected by both the laminate structure and diameter-to-thickness ratios of the tubes.

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.

Bond Behavior of CFRP Strengthened Steel Structures

Recent research has focused on rehabilitation and strengthening of steel structures and bridges using fiber reinforced polymer (FRP) materials. The bond behavior of FRP materials to steel structures is quite different from that of concrete structures. Preliminary test results showed the occurrence of very high bond stresses for most strengthening applications due to the amount of strengthening required for steel structures and bridges. In this paper, surface preparation methods and means of preventing galvanic corrosion are discussed. The results of an experimental program for selection of suitable adhesives through determination of the development length is discussed as well as preliminary testing showing the importance of proper detailing of the ends of the FRP strips. The shear stress distribution determined in the experimental program is compared to analytical models using a stress-based approach. The remainder of the paper focuses on the current methods for determining bond stresses and their use for the design of FRP strengthening system for steel structures.

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.