Fibre-reinforced-polymer (FRP) composites have been widely used for the flexural strengthening of reinforced concrete (RC) structures. Flexural strengthening methods with FRP include external bonding of FRP composites (EB system) and insertion of FRP strips or bars into grooves cut into the concrete (near-surface-mounted or NSM system). Recently, a prestressed FRP strengthening system has been developed and investigated, whereby the FRP reinforcement is pretensioned prior to attachment to the concrete to maximize the use of the high tensile strength of the FRP reinforcement. Existing studies have shown that the ultimate load carrying capacity and serviceability were greatly improved in FRP flexural strengthened beams. However, the only disadvantage of the FRP strengthening system is the reduction of deformability compared to that of unstrengthened structures due to the limited strain capacity of the FRP reinforcement and premature debonding failure. Structures with low deformability may fail suddenly without warning to evacuate, resulting in catastrophic failure. Therefore, a study on the improvement of deformability is critical for the effective use of FRP strengthening systems. In this study, a partially bonded concept is introduced and applied to various FRP strengthening methods, with the specific objective of increasing deformability in FRP strengthened beams. The FRP reinforcement is usually completely bonded to the concrete tensile surface, while a portion of the FRP length is intentionally unbonded in the partially bonded system in order to improve deformability while sustaining high load carrying capacity. To investigate the general behaviour of the partially bonded system, a new analytical model has been developed because conventional section analysis used for analysis of the fully bonded system is not applicable due to strain incompatibility at the FRP reinforcement level within the unbonded length. The analysis shows that a partially bonded system has a high potential to improve deformability without the loss of strength capacity. An extensive experimental program was conducted to verify the analytical model and to investigate the actual behaviour of the partially bonded beams. A total of seventeen, 3.5m long, RC T-beams were constructed and tested. One of them is an unstrengthened control beam, while the other 16 beams consist of four test groups that were strengthened by different strengthening methods: non-prestressed EB, non-prestressed NSM, 40% prestressed NSM, and 60% prestressed NSM. To allow investigation of the effect of partially unbonding, each group has different unbonded lengths and includes a fully bonded beam. For the non-prestressed EB strengthened beams, the failure mode of all beams was premature FRP debonding failure without regard to the bond condition. The ultimate strength and the ultimate deformability of the partially bonded beams were improved compared to the fully bonded beam. This was because the typical intermediate debonding failure that occurred in the fully bonded beam was avoided due to intentional unbonding in the partially bonded beams. The analytical model predicted the general behaviour of the EB strengthened beams well except at the ultimate response due to the premature debonding failure. A three-dimensional nonlinear finite element (FE) analysis was performed utilizing interfacial elements and contact modeling to investigate the debonding failure of this system. The FE analysis represented the behaviour of the debonding failure and bond stress distributions at FRP-concrete interface of both the fully bonded and partially bonded beams well. For the non-prestressed NSM strengthened beams, the premature debonding failure that occurred in the EB strengthened beams was not observed, and almost the full capacity of FRP was exhibited. Prominent stiffness reduction was observed in terms of load-deflection diagrams at the post-yielding stage with the increase of the unbonded length. This stiffness reduction increased the deformability of the partially bonded beams for a given applied load after steel yielding in comparison to the fully bonded beam. The FRP started to slip at high load levels and the concrete crushed gradually with a gradual loss of the beam’s cross-section, inducing nonlinear behaviour near the ultimate state of the beams. To address this behaviour, an advanced analytical model utilizing idealized section model and slip model is proposed to consider the FRP slip and concrete gradual failure. Prestressed NSM strengthened beams were very effective to improve the cracking load, to decrease the deflection at service load, and to increase the ultimate load compared to non-prestressed NSM strengthened beams. This improvement was greater as the prestressing level increased. The partially bonded prestressed beams showed an improvement in deformability compared to the fully bonded prestressed beams while minimizing the reduction of the ultimate load carrying capacity and serviceability. The partially bonded system was more effective to improve the deformability at higher levels of prestressing force. Based on the model developed, a parametric study was performed varying the main parameters. This showed that the FRP strengthened beam that has an FRP area (Af) less than the balanced FRP area (Af,b) of the beam has a high potential to improve the deformability as the unbonded length increases. The balanced FRP area is increased as the concrete strength and the FRP prestressing force are increased, or as the area of the steel reinforcement decreases. Finally, design recommendations and procedures are proposed for the effective use of the partially bonded system to improve the deformability of FRP strengthened concrete beams.

Over the past decade, extensive research has been conducted on the strengthening of reinforced concrete (RC) structures using externally bonded fibre reinforced polymer (FRP). More recently, near-surface mounted (NSM) FRP reinforcement has attracted an increasing amount of research as well as practical applications. In the NSM method, grooves are first cut into the concrete cover of an RC element and the FRP reinforcement is bonded inside the groove with an appropriate filler (typically epoxy paste or cement grout). The FRP reinforcement is either prestressed or non-prestressed depending on the required level of strengthening. In all cases, the bond between an NSM bar and the substrate material plays a key role in ensuring the effectiveness of NSM strengthening. The present work investigated experimentally the bond behaviour of non-prestressed and prestressed beams reinforced with near surface mounted carbon fibre reinforced polymer (CFRP) bars under monotonic and fatigue loading. Forty concrete beams were cast and tested in seven groups. The test variables considered in this study were: presence of internal steel reinforcement or not, the type of CFRP rod (spirally wound or sand coated) and the prestressing force (non-prestressed or prestressed). Twenty eight beams were strengthened with non-prestressed CFRP rods; fifteen beams without internal steel reinforcement and thirteen beams with internal steel. Ten beams with internal steel were strengthened with prestressed CFRP rods. The beams were tested in four point bending. In each group, one beam was loaded monotonically. The remaining beams were loaded under different fatigue load levels. The minimum load was kept constant for all beams at 10% of their monotonic capacity and the peak load was varied from one beam to another (denoted as a percentage of the peak load level). Twenty eight beams were strengthened with non-prestressed CFRP rods. Bond failures for the beams with and without internal steel, strengthened with CFRP rods and tested under monotonic or fatigue loads was by debonding between the CFRP rod and the epoxy that started at the loading point and as the load was increased or cycled, the debonding spread towards the support until failure occurred. A comparison of the fatigue life curves for the beams with and without steel, strengthened with CFRP rods revealed that the sand coated rod had better bond characteristics than the spirally wound rod (at the same load range the beam strengthened with sand coated rod had a longer life than the beam strengthened with spirally wound rod). Beams with internal steel, strengthened with CFRP rods and tested under fatigue loading failed in bond at high load levels (short fatigue lives) and by rupture of the steel rebar at low load levels (long fatigue lives). Ten beams with internal steel were strengthened with prestressed CFRP rods. The CFRP rods were prestressed to a force of 62 kN which corresponds to 45% and 40% of the monotonic capacity of the spirally wounded and sand coated rods, respectively. Almost all the beams with internal steel that were strengthened with prestressed CFRP rods failed by slipping between the CFRP rod and the epoxy that started at the support and propagated inwards towards the loading point. The exception to this was the beam strengthened with prestressed sand coated rod and tested under monotonic loading that failed by debonding between the CFRP rod and the epoxy that started at the loading point and propagated towards the support. Comparing the load range (kN) versus life curve for the beams with steel, strengthened with prestressed spirally wound and sand coated rods that failed in bond, shows that the beam strengthened with sand coated rod has longer fatigue lives than beam strengthened with spirally wound rod. A model was used to describe the progress of the debonding crack until excessive slipping occurred. The model predicted the number of cycles until excessive slipping between the CFRP rod and the epoxy occurred and the forces in the CFRP rod at all locations in the shear span at the onset of failure with reasonable accuracy.

Electrical resistance and acoustic emission (AE) measurement are jointly used to monitor the degradation in CFRP laminates subjected to tensile tests. The objective of this thesis is to perform a synergertic analysis between a passive and an active methods to better access how these perform when used for Structural Health Moni- toring (SHM). Laminates with three different stacking sequences: [0]4, [02/902]s and [+45/ − 45]2s are subjected to monotonic and cyclic tensile tests. In each laminate, we carefully investigate which mechanisms of degradation can or cannot be detect- ed by each technique. It is shown that most often, that acoustic emission signals start before any electrical detection is possible. This is is explained based on the redundance of the electrical network that makes it less sensitive to localized damages. Based on in depth study of AE signals clustering, a new classification is proposed to recognize the different damage mechanims based on only two parameters: the RA (rise time/amplitude) and the duration of the signal.

The deterioration of prestressed concrete structures due to corrosion is a costly problem. This problem is accelerated in cold weather climates where de-icing salts are used. These salts accelerate the corrosion of the steel tendons greatly reducing the service life of the structures and leading to constant costly repairs. Recent research has shown composite materials such as Fibre Reinforced Polymers (FRP) to be suitable alternatives to steel, providing similar strength without being susceptible to electrochemical corrosion. Carbon FRP in particular has great promise for prestressed applications, showing resistance to corrosion in environments that might be encountered in concrete and experiencing less relaxation than steel. This thesis outlines the testing and implementation of a post-tensioned system that uses CFRP tendons to replace corroded, unbonded post-tensioned steel tendons. This system was then implemented in a parking garage in downtown Toronto. To the author’s knowledge, this is the first example of an unbonded, post-tensioned tendon replacement using FRP tendons. The system used split wedge anchors designed specifically for CFRP tendons at the University of Waterloo. The dead end was anchored by directly bonding the tendon to the concrete slab. Overall, the system was shown to work and provide a durable solution for unbonded post-tensioning systems. The CFRP tendon was successfully inserted in the opening left by the removal of the corroded tendon and stressed. It was found that the current anchorage configuration experienced large load losses of up to 60 % during the transfer. Changing the orientation of the anchor was found to reduce the load lost to a range of 1 % to 9 %. / Thesis (Master, Civil Engineering) -- Queen's University, 2009-04-09 15:30:59.865

This thesis investigates strengthening slender steel columns with carbon fibre reinforced polymer (CFRP) plates of various moduli. Three different types of CFRP were used in the study: Ultra-high modulus (430GPa), High modulus (212GPa) and Normal modulus (168GPa). In this study, specimens were grouped according to measured initial out-of-straightness values. The first section examines the effect of adding CFRP plates to the column flanges when buckling about the weak axis. Twelve columns, with a slenderness ratio of 197, were tested, of which nine were strengthened with CFRP. The main parameters tested were the level of initial out-of-straightness (length (L)/8387 to L/1020), CFRP modulus (168 to 430 GPa), CFRP reinforcement ratio (13% to 34%) and the length of CFRP plate (33% to 95% of L). The gain in axial strength due to CFRP retrofitting ranged from 11% to 29%, depending on the various parameters. The gain generally increased as CFRP modulus, initial out-of-straightness, or CFRP reinforcement ratio increased. Global buckling consistently governed the maximum load. In the case of the 430 GPa CFRP, buckling was followed by CFRP crushing in compression, then rupture in tension. The second section of the thesis examines the effect of CFRP plates added to the flanges and tested for buckling in the strong axis. Eight columns, with a slenderness ratio of 83, were tested of which five were strengthened with CFRP. The main parameters examined were the level of initial out-of-straightness (L/28889 to L/1635), CFRP modulus (168 to 430 GPa), CFRP reinforcement ratio (13% to 34%) and the axis of bending. The gain in axial strength due to CFRP retrofitting ranged from 0% to 25%, depending on the various parameters. The gain generally increased as initial out-of-straightness, or CFRP reinforcement ratio increased. The higher modulus did not perform as expected, showing no gain in strength, because the compressive strains were too large and the CFRP crushed before the specimen experienced any gain. Specimens compared with the weak axis, strengthened with normal modulus CFRP, had similar percentage gains in strength. / Thesis (Master, Civil Engineering) -- Queen's University, 2014-06-27 15:19:03.397

Tests results of three flanged and two rectangular cross-section concrete beams reinforced with carbon fibre reinforced polymer (CFRP) bars are reported. In addition, a companion concrete flanged beam reinforced with steel bars is tested for comparison purposes. The amount of CFRP reinforcement used and flange thickness were the main parameters investigated in the test specimens. One CFRP reinforced concrete rectangular beam exhibited concrete crushing failure mode, whereas the other four CFRP reinforced concrete beams failed owing to tensile rupture of CFRP bars. The ACI 440 design guide for FRP reinforced concrete members underestimated the moment capacity of beams failed owing to CFRP tensile rupture and reasonably predicted deflections of the beams tested. A simplified theoretical analysis for estimating the moment capacity of concrete flanged beams reinforced with FRP bars was developed. The experimental moment capacity of the CFRP reinforced concrete beams tested compared favourably with that predicted by the theoretical analysis developed.

Over the past decade, extensive research has been conducted on the strengthening of reinforced concrete (RC) structures using externally bonded fibre reinforced polymer (FRP). More recently, near-surface mounted (NSM) FRP reinforcement has attracted an increasing amount of research as well as practical applications. In the NSM method, grooves are first cut into the concrete cover of an RC element and the FRP reinforcement is bonded inside the groove with an appropriate filler (typically epoxy paste or cement grout). The FRP reinforcement is either prestressed or non-prestressed depending on the required level of strengthening. In all cases, the bond between an NSM bar and the substrate material plays a key role in ensuring the effectiveness of NSM strengthening. The present work investigated experimentally the bond behaviour of non-prestressed and prestressed beams reinforced with near surface mounted carbon fibre reinforced polymer (CFRP) bars under monotonic and fatigue loading. Forty concrete beams were cast and tested in seven groups. The test variables considered in this study were: presence of internal steel reinforcement or not, the type of CFRP rod (spirally wound or sand coated) and the prestressing force (non-prestressed or prestressed). Twenty eight beams were strengthened with non-prestressed CFRP rods; fifteen beams without internal steel reinforcement and thirteen beams with internal steel. Ten beams with internal steel were strengthened with prestressed CFRP rods. The beams were tested in four point bending. In each group, one beam was loaded monotonically. The remaining beams were loaded under different fatigue load levels. The minimum load was kept constant for all beams at 10% of their monotonic capacity and the peak load was varied from one beam to another (denoted as a percentage of the peak load level). Twenty eight beams were strengthened with non-prestressed CFRP rods. Bond failures for the beams with and without internal steel, strengthened with CFRP rods and tested under monotonic or fatigue loads was by debonding between the CFRP rod and the epoxy that started at the loading point and as the load was increased or cycled, the debonding spread towards the support until failure occurred. A comparison of the fatigue life curves for the beams with and without steel, strengthened with CFRP rods revealed that the sand coated rod had better bond characteristics than the spirally wound rod (at the same load range the beam strengthened with sand coated rod had a longer life than the beam strengthened with spirally wound rod). Beams with internal steel, strengthened with CFRP rods and tested under fatigue loading failed in bond at high load levels (short fatigue lives) and by rupture of the steel rebar at low load levels (long fatigue lives). Ten beams with internal steel were strengthened with prestressed CFRP rods. The CFRP rods were prestressed to a force of 62 kN which corresponds to 45% and 40% of the monotonic capacity of the spirally wounded and sand coated rods, respectively. Almost all the beams with internal steel that were strengthened with prestressed CFRP rods failed by slipping between the CFRP rod and the epoxy that started at the support and propagated inwards towards the loading point. The exception to this was the beam strengthened with prestressed sand coated rod and tested under monotonic loading that failed by debonding between the CFRP rod and the epoxy that started at the loading point and propagated towards the support. Comparing the load range (kN) versus life curve for the beams with steel, strengthened with prestressed spirally wound and sand coated rods that failed in bond, shows that the beam strengthened with sand coated rod has longer fatigue lives than beam strengthened with spirally wound rod. A model was used to describe the progress of the debonding crack until excessive slipping occurred. The model predicted the number of cycles until excessive slipping between the CFRP rod and the epoxy occurred and the forces in the CFRP rod at all locations in the shear span at the onset of failure with reasonable accuracy.

The research work focuses on implementing intelligent measurement and diagnostic techniques for the non-destructive evaluation (NDE) of aircraft carbon composites. The outcome of this research work developed reliable and faster techniques to aid in the rapid assessment of defects in anisotropic carbon composites by applying ultrasonic and infrared thermography NDE methods. To fulfill the requirement of the intelligent non-destructive evaluation methods, this research is divided into four sub-researches: fuzzy logic based delamination detection, super-resolution image reconstruction for ultrasonic C-scan, ultrasonic 3D reconstruction, and polynomial fitting techniques for infrared thermography inspection. These researches focus on the improvement and optimization of current ultrasonic testing and infrared thermography inspection. They are independent but interrelated component, and they all serve the same goal which is to interpret data correctly and provide detailed information about the region of interests (ROI) for intelligent non-destructive measurement and evaluation. Details of these researches are presented in Chapter 2, 3, 4, and 5 respectively. For the ultrasonic testing, a fuzzy inference classifier will be used to generate the rule base and knowledge base for different kinds of defects in composites. It will automatically manage large amounts of signal data sets and extract the important information. Data features and NDE expert knowledge are seamlessly combined to provide the best possible diagnosis of the potential defects and problems. As a result, the outcome of this research work will help ensure the integrity and reliability of carbon composites. The C-scan image resolution of ultrasonic testing system was improved by applying super-resolution algorithms to overcome the inherent resolution limitations of the existing ultrasonic system. It greatly improves the image quality and allows for more detailed inspection of the ROI with high resolution, making defect evaluation easier and more accurate. The ultrasonic 3D reconstruction technique will be able to provide NDE inspectors with more detailed information on defect depth, volume, and 3D structure, as well as help them make quick, accurate, and reliable decisions. For the IR inspection, the thermography methods based on the thermal contrast are strongly affected by non-uniform heating which due to the heat source alignment and specimen thickness variation. The proposed polynomial curve fitting and surface fitting techniques were applied to eliminate the non-uniform heating effect by subtracting the estimated non-uniform heating pattern from the corrupted IR images. Mainly, aircraft composite material: carbon fiber reinforced polymer (CFRP) panels will be considered for this research work. Based on the preliminary study, delamination defects due to impact damage and foreign object inclusions artificially embedded in CFRP panels were successfully detected by immersion ultrasonic testing (UT) and IRT inspection. Therefore, the next step will be in improving the detection algorithm and developing an intelligent quality inspection technique for NDE testing. Powered with multiple image processing techniques and mathematical algorithms, the research result will provide high resolution images and detailed information about defect areas. In addition, it will also capable of identifying the type, shape, size, and the distribution of defect.

Engineering composites and especially long fibre carbon composites have been in high demand not only in aerospace and automotive applications, but also in high end everyday applications. In aerospace, carbon composites are used predominantly for secondary structures attached by joints or fasteners to various alloys or even different composites, and are exposed to service loads and repetitive impacting. Impact fatigue (IF) is not studied adequately for long cycles and relevant literature is investigating mainly drop weight tests and high speed projectile experiments. The main aim of this research was to investigate the behaviour long fibre CFRP'S exposed to repeated low-velocity, low energy impacts, and to observe the damage effects of this regime on the structural integrity of these materials. Two types of specimen configurations using CFRPS's were used and exposed to loading conditions relevant to the Izod impact fatigue test (IIFT), and the tensile impact fatigue test (TIFT), in order to determine the fatigue behaviour of the specimens for each of these load conditions. For the IIFT, the fatigue life was investigated using IM7/8552 unidirectional specimens and T700/LTM45 cross-ply specimens were utilised for the TIFT. The specimen thicknesses were altered in both cases and parametric studies were carried out, where it was seen that IF results in high level of scatter and the apparent decrease in life was seen at relatively modest levels of maximum force after relatively few cycles. In the case of the IIFT, a durability limit was not apparent which increases the complications when designing against IF. In the case of the TIFT the stiffness deterioration was reflected as an increase of the loading time, in the force vs time graph, over the total fatigue life span. Fatigue crack growth was investigated using fractography and X-ray micro-CT at the micro and macro level. It was seen, that IF had the potential to initiate cracks and to cause their propagation at low levels of loading. For the IIFT, a single crack was growing substantially in the fibre direction and across the sample width causing matrix cracking and probably breaking of some fibres, which acted as impact wave guides since matrix cracks were propagating initially along the length of the fibres. In the case of the TIFT multiple damage modes were presented (matrix cracks, axial splits and delaminations). Their sequence and progression was successfully v captured and contrasted against the number of impacts. Axial splits governed the damage scenario, with delaminations extending between them and the free edges. For the TIFT, IF was studied using the force-life (F-Nf) and energy-life (E-Nf) curves. The tests undertaken showed that when halving the thickness of the laminates the fatigue life presented a 10-fold decrease as well as higher scatter. Finite element modelling was undertaken to validate the experimental data of the TIFT test. Successful simulation of a single impact was carried out using a fully transient 3-D model of the actual experiment configuration which involved geometric non-linearities in addition to the multiple contact conditions. The analysis was undertaken using the Abaqus 6.11 explicit solver. Since the numerical single impact results (force vs time response) was in agreement with the experimental results, the crack modes, experimentally observed, were also incorporated in the model utilising the use of the cohesive zone elements (CZE).

AbstractThis thesis describes research on the use of eddy current techniques for nondestructivetesting of carbon fibre reinforced plastic (CFRP). The research hasinvolved bulk conductivity testing, fibre direction characterization and 3D FEMmodeling of the CFPR and eddy current probes geometry. In the conductivity testing,how the sample thickness, fibre volume content and fibre conductivity affects thesignal from the eddy current has been evaluated. Eddy current testing shows gooddirectionality as CFRP is an anisotropic material, thus is very suitable to characterizethe fibre orientation. Direction sensitive probes have been developed and tested toreveal information about the fibre direction and layer. Computer FEM software hasbeen used to analyze the magnetic field inside the sample and probes. Specific probegeometries have been designed depending on the electrical properties of thecomposites and testing requirement. The experiment, simulation and analysis resultsshow very good agreement. However, when the measuring frequency increases, noisesand parasitic capacitance inevitably become significant and have a negative influenceon the results. Improvements and further research are proposed which are believed tomake eddy-current techniques a more feasible and efficient measurement method, willcontribute to the development and maintenance of light weight CFRP composites.