FRCM Composites for Strengthening Corrosion-damaged
Structures: Experimental and Numerical Investigations
Thèse
Mohammed Elghazy
Doctorat en génie civil
Philosophiae doctor (Ph.D.)
Québec, Canada
© Mohammed Elghazy, 2018
Les matrices cimentaires renforcées de fibres (MCRF) pour
renforcer les structures en béton endommagées par la
corrosion: investigations expérimentales et numériques
Thèse
Mohammed Elghazy
Sous la direction de :
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Résumé
La corrosion des armatures en acier est l'un des mécanismes les plus destructifs pour les structures en béton armé. La corrosion nuit non seulement à l'intégrité structurale et à l’aptitude au service de la structure endommagée, mais peut aussi entraîner des défaillances inattendues ou des ruptures fragiles. Malgré les dispositions rigoureuses de la plupart des codes de pratique pour éviter la corrosion, des signes de dommages dus à la corrosion sont toujours signalés.
Récemment, des systèmes à matrice cimentaire renforcée de fibre (MCRF) ont été proposés comme une technique innovante de renforcement/réparation pour les structures en béton afin de surmonter les inconvénients associés à l'utilisation des systèmes de polymères renforcés de fibres (PRF). Bien que l'utilisation de composites MCRF pour renforcer les éléments en béton non endommagés ait prouvé son efficacité, très peu est connu sur la viabilité de leur utilisation pour renforcer les éléments en béton endommagés à divers niveaux dus à la corrosion. De plus, les comportements de post-réparation et la durabilité à long-terme des éléments corrodés et renforcés par les systèmes MCRF et qui seront probablement exposés aux mêmes conditions environnementales qui prévalaient avant leur réparation, n'ont pas retenu l'attention des chercheurs dans la littérature. De plus, la plupart de nos infrastructures, telles que les ponts et garages de stationnement, sont susceptibles d'être endommagées par la corrosion tout en étant soumises à des charges oscillatoires qui provoquent de la fatigue. À ce jour, aucune information n'est disponible sur l'effet de la combinaison de la charge de fatigue et de la corrosion dans les structures renforcées par les systèmes MCRF.
Dans ce travail, les comportements monotones et de fatigue en flexion des poutres en béton endommagées par la corrosion et renforcées par des systèmes MCRF ont été étudiés en plus de leur performance à long-terme, c'est-à-dire après une exposition à un environnement corrosif après leur renforcement. Le travail comprend des investigations expérimentales et numériques. Les prédictions analytiques et les formulations théoriques actuellement disponibles dans les codes de conception ont été aussi vérifiées par rapport aux résultats expérimentaux. Le programme expérimental consistait à tester trente (30) poutres en béton à grande échelle de 150 × 250 × 2800 mm. Les poutres ont été construites et testées en configuration de charge à quatre points. Un processus accéléré de corrosion a été utilisé pour corroder les armatures d'acier en traction dans le tiers central des poutres. Les paramètres d'essai comprenaient le niveau de corrosion (représenté
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par 10, 20 et 30% de perte de masse dans l'acier de traction), le type de système de renforcement utilisé (Polyparaphénylène benzobisoxazole (PBO-MCRF), MCRF de carbone et PRF), la quantité de composites MCRF (1, 2, 3 et 4 couches), le schéma de renforcement MCRF (couches ancrées aux extrémités par rapport aux couches continues sous forme U) et le régime de chargement (monotone et fatigue).
Les résultats des tests ont montré que l'utilisation de composites MCRF améliorait significativement le comportement en flexion des poutres corrodées. Les composites MCRF ont contrôlé le mode de défaillance des poutres renforcées plutôt que le niveau de corrosion des barres d'acier. Les poutres renforcées par la MCRF ont montré une augmentation de leurs résistances ultimes variant entre 7 et 65% de celles des poutres vierges (poutres ni corrodées ni renforcées) en fonction du type, de la quantité et du schéma de la MCRF utilisée. L'exposition des poutres réparées par la MCRF à d’autres cycles de corrosion a entraîné une réduction de 23% de la perte de masse de l'acier. Le schéma en U était plus efficace que le schéma d'ancrage aux extrémités à retarder le délaminage des couches de MCRF dans les poutres renforcées et testées à court terme. Il a également atténué l'effet des fissures de corrosion longitudinales et, par conséquent, a augmenté l'efficacité du renforcement MCRF. Les essais de fatigue ont montré que la corrosion des barres d'acier diminuait considérablement la résistance à la fatigue des poutres non renforcées. Le renforcement avec des composites MCRF a augmenté la durée de vie en fatigue des poutres endommagées par la corrosion de 38 à 377% de celle des poutres corrodées non-renforcées. Cependant, le renforcement par MCRF n'a pas restauré la durée de vie en fatigue des poutres vierges.
Dans l'étude numérique réalisée dans ce travail, des modèles d'éléments finis (ÉF) tridimensionnels (3D) ont été développés pour simuler le comportement non linéaire des poutres corrodées et renforcées par des composites MCRF et PRF à l'aide du progiciel ATENA-3D. Les résultats de l'analyse numérique étaient en bon accord avec ceux obtenus expérimentalement en termes de modes de défaillance, de déformations, de capacités de charge et de flèches. Les modèles ÉF développés ont été capables de capturer le comportement non-linéaire des poutres testées avec une bonne précision. Une étude paramétrique a ensuite été menée pour étudier l'effet de la résistance à la compression du béton et de l'épaisseur de recouvrement des armatures sur l'efficacité de renforcement des systèmes composites. Il a été observé que la rupture des poutres renforcées
v
par des FRCM était indépendante de la résistance à la compression du béton ou de l'épaisseur de de recouvrement et était régie uniquement par le glissement du tissu dans la matrice.
Sur le plan analytique, les équations de conception de l’ACI-549.4R-13 (ACI 2013) ont été évaluées à l'aide des données expérimentales obtenues à partir des tests. Il a été conclu que les formulations théoriques de l’ACI-549.4R-13 peuvent raisonnablement prédire les résistances ultimes des poutres renforcées ancrées à l'extrémité mais sous-estimer celles des poutres ancrées en continu sous forme U. Un facteur de schéma de 1,1 a ensuite été proposé pour calculer la résistance nominale des poutres renforcées par MCRF sous forme U.
Le résultat de ce travail a été publié (ou soumis pour publication) dans cinq articles de revues et cinq conférences, comme détaillé tout au long de la thèse.
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Abstract
Corrosion of steel reinforcement is one of the most destructive mechanisms for reinforced concrete (RC) structures. Corrosion not only impairs the structural integrity and the serviceability of the damaged structure, but it may also lead to unexpected and brittle failures. Despite the rigorous provisions of most codes of practice to avoid corrosion, evidences of corrosion damage are still being reported.
Recently, fabric-reinforced cementitious matrix (FRCM) systems were proposed as an innovative strengthening/repair technique for RC structures to overcome the drawbacks associated with the use of the well-documented fiber-reinforced polymer (FRP) systems. While the use of FRCM composites to strengthen un-damaged RC members has proven its efficiency, very little is known about the viability of their use to retrofit RC members with various levels of corrosion damage. In addition, the post-repair performance and the long-term durability of the FRCM-strengthened corroded members, which most likely will be exposed to the same environmental conditions that have prevailed prior their repair, have not received attention in the literature. Moreover, most of our infrastructures such as bridges and parking garages are susceptible to corrosion damage while continuously being subjected to oscillatory loads that cause fatigue. To date, no information is available about the effect of combining fatigue loading with corrosion in FRCM-strengthened structures.
In this work, the monotonic and fatigue flexural behaviors of corrosion-damaged RC beams strengthened with FRCM systems were investigated in addition to their long-term performance, i.e. after further exposure to corrosive environment following their strengthening. The work includes experimental and numerical investigations. The analytical predictions and theoretical formulations that are currently available in the design codes have been verified against the experimental results. The experimental program consisted of testing thirty (30) large-scale RC beams of 150×250×2800 mm. The beams were constructed and tested under four-point load configuration. An accelerated corrosion process was utilized to corrode the bottom steel reinforcement in the middle third of the test specimens. The test parameters included the level of corrosion damage (represented by 10, 20, and 30% mass loss in the tensile steel), the type of the strengthening system used (Polyparaphenylene benzobisoxazole (PBO-FRCM), C-FRCM, and FRP), the amount of FRCM composites (1, 2, 3, and 4 layers), the FRCM strengthening Scheme
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(end-anchored versus continuously wrapped layers), and the loading regime (monotonic and fatigue).
The test results showed that the use of FRCM composites significantly enhanced the flexural behavior of the corroded beams. FRCM governed the failure mode of the strengthened beams rather than the level of corrosion damage of the steel bars. FRCM-strengthened beams showed an increase in their ultimate strengths that ranged between 7 and 65% of that of the virgin (neither corroded nor strengthened) beam based on the type, amount, and Scheme of the FRCM used. Exposing the repaired beams to post-repair corrosion resulted in 23% reduction in the steel mass loss. The U-wrapped scheme was more efficient than the end-anchoring scheme in delaying the delamination of the FRCM plies in the short-term repaired beams. It also mitigated the effect of the longitudinal corrosion cracks and consequently increased the post-repair strengthening effectiveness of FRCM systems. Fatigue tests showed that corrosion of steel bars dramatically decreased the fatigue life of the unstrengthened-beams. Strengthening with FRCM composites increased the fatigue life of the corrosion-damaged beams by 38 to 377% of that of the corroded-unstrengthened beams. However, FRCM strengthening did not restore the fatigue life of the virgin beams.
In the numerical study carried out in this work, three-dimensional finite element (FE) models were developed to simulate the nonlinear behavior of the corroded beams strengthened with FRCM and FRP composites using the software package ATENA-3D. The results of the numerical analysis were in good agreement with those obtained experimentally in terms of failure modes, strains, load-carrying capacities, and deflections. The developed FE models were able to capture the non-linear behavior of the tested beams with good accuracy. A parametric study was then conducted to investigate the effect of concrete compressive strength and thickness of concrete cover on the strengthening effectiveness of the composite systems. It was observed that failure of FRCM-strengthened beams was independent of the compressive strength of concrete or the thickness of the concrete cover and was governed only by fabric slippage within the matrix.
Analytically, the design equations of ACI-549.4R-13 (ACI 2013) were assessed using the experimental data obtained from the tests. It was concluded that the theoretical formulations of ACI-549.4R-13 can reasonably predict the ultimate strengths of the end-anchored strengthened beams but underestimated those of continuously-anchored beams. A scheme factor of 1.1 was then
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proposed to calculate the nominal strength of beams strengthened with continuously-wrapped shape of FRCM.
The outcome of this work has been published (or submitted for publication) in five journal articles and five conferences, as detailed throughout the thesis.
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Table of Contents
Résumé ………...……….…….. iii
Abstract ………..………..……..………..………… vi
Table of Contents ……….………..……..…...……….………...…..……… ix
List of Figures ……….……….…..………..……..……….………....……xiv
Acknowledgements……….………….…...……..……..………..………....……xx
Chapter 1 ... 1
1.1 Background and Problem Definition... 1
1.2 Thesis Structure ... 3
Chapter 2 ... 6
2.1 General ... 6
2.2 Corrosion of Steel Reinforcement ... 6
2.2.1 Corrosion Mechanism of Steel Bars in Concrete ... 6
2.2.2 Accelerated Corrosion Process ... 9
2.2.3 Effect of Steel Corrosion on Concrete Structures ... 11
2.2.4 Behavior of Corroded-RC Beam under Monotonic Loads ... 12
2.2.5 Behavior of Corroded-RC Beam under Fatigue Loads... 15
2.3 FRCM Composites ... 16
2.3.1 FRCM Acceptance Criteria and Design ... 18
2.3.2 FRCM Tensile Characterization ... 18
2.3.3 Bond Behavior of FRCM Composites ... 20
2.4 FRCM-Strengthened Beams Under Monotonic Load ... 21
2.5 Fatigue and Durability of FRCM-strengthened Beams... 25
2.6 Findings of Literature Review... 26
Chapter 3 ... 27
3.1 Research Significance ... 27
3.2 Research Objectives ... 27
3.3 Methodology ... 28
3.3.1 Experimental Work Program ... 28
3.3.2 Numerical Analysis ... 31
3.3.3 Analytical Investigation ... 32
Chapter 4 ... 33
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4.1 Abstract ... 34
4.2 Introduction and Background ... 34
4.3 Experimental Program... 36
4.3.1 Test Specimen ... 37
4.3.2 Accelerated Corrosion Aging ... 39
4.3.3 Materials ... 40
4.3.4 FRCM Equivalent Axial Stiffness ... 42
4.3.5 FRCM Repair Schemes... 43
4.3.6 Repair Technique ... 44
4.3.7 Test Setup and Instrumentation ... 45
4.4 Test Observations ... 46
4.4.1 Corrosion Cracks and Mass Loss ... 46
4.4.2 Modes of Failure ... 46 4.4.3 Load-deflection Response ... 48 4.4.4 Strength Analysis ... 49 4.4.5 Ductility Performance ... 53 4.4.6 Strain Response ... 54 4.5 Theoretical Predictions ... 58 4.5.1 Design Provision ... 61 4.6 Conclusions ... 61 4.7 Notation ... 63 Chapter 5 ... 65 Résumé ... 66 5.1 Abstract ... 66
5.2 Introduction and Background ... 66
5.3 Experimental Program... 68
5.3.1 Test Specimen and Materials ... 69
5.3.2 Accelerated Corrosion Process ... 71
5.3.3 FRCM Systems ... 72
5.3.4 Strengthening Schemes ... 75
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5.3.6 Instrumentation and Test Setup ... 77
5.4 Test Results ... 78
5.4.1 Corrosion Observations ... 78
5.4.2 Failure Mechanisms ... 80
5.4.3 Strength Response ... 81
5.4.4 Fabric Strains ... 86
5.4.5 Ductility and Energy Absorption ... 87
5.5 Discussion ... 89
5.6 Predicted Strength Results ... 92
5.7 Conclusions ... 95
Chapter 6 ... 97
Résumé ... 98
6.1 Abstract ... 98
6.2 Introduction and Background ... 98
6.3 Experimental Program... 100
6.3.1 Test Specimen and Materials ... 101
6.3.2 Accelerated Corrosion Process ... 103
6.3.3 FRCM Composites... 104
6.3.4 FRCM Schemes ... 106
6.3.5 Repair Methodology ... 108
6.3.6 Test Setup and Instrumentation ... 108
6.4 Test Results and Discussion ... 108
6.4.1 Corrosion Crack Pattern ... 108
6.4.2 Steel Mass Loss... 109
6.4.3 Flexural Cracks Pattern and Failure modes ... 111
6.4.4 Flexural Response ... 114
6.4.5 Load-carrying Capacities ... 116
6.4.6 Ductility and Energy Absorption ... 118
6.4.7 Strain Response ... 120
6.5 Predicted Strengths ... 123
6.6 Conclusions ... 124
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Résumé ... 127
7.1 Abstract ... 127
7.2 Introduction and Background ... 127
7.3 Experimental Program... 129
7.3.1 Test Specimen ... 130
7.3.2 Accelerated Corrosion Technique... 132
7.3.3 FRCM Composites... 133
7.3.4 FRCM Strengthening Configuration... 135
7.3.5 Strengthening Procedure ... 137
7.3.6 Test Setup and Instrumentation ... 138
7.4 Test Results and Discussion ... 139
7.4.1 Corrosion Crack Patterns and Actual Steel Mass Loss ... 139
7.4.2 Monotonic Test Results ... 140
7.4.3 Fatigue Test Results ... 143
7.5 Conclusions ... 151
Chapter 8 ... 154
Résumé ... 155
8.1 Abstract ... 155
8.2 Introduction and Background ... 155
8.3 Experimental Investigation ... 158
8.3.1 Test Matrix ... 158
8.3.2 Test Specimen ... 158
8.3.3 Externally-bonded Composite Systems ... 160
8.3.4 Strengthening Procedure and Configuration ... 161
8.3.5 Test Setup and Instrumentation ... 162
8.4 Numerical Simulation ... 162
8.4.1 Constitutive Laws ... 163
8.5 Results and Discussion ... 170
8.5.1 Crack Pattern at Failure ... 170
8.5.2 Load-deflection Response ... 172
8.5.3 Load-carrying Capacity ... 177
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8.5.5 Ductility ... 182
8.6 Parametric Studies ... 183
8.6.1 Effect of Concrete Compressive Strength (fc′)... 183
8.6.2 Effect of Concrete Cover ... 186
8.7 Conclusions ... 188
Chapter 9 ... 191
8.8 Summary ... 191
8.9 Conclusions ... 191
8.9.1 Effect of Corrosion on RC Beams ... 191
8.9.2 Short-term Performance of Corroded Beams Strengthened with FRCM ... 192
8.9.3 Long-term Performance of Corroded Beams Strengthened with FRCM ... 194
8.9.4 Validation of ACI-549.4R-13 Design Equations ... 195
8.9.5 Fatigue Performance of Corroded Beams Strengthened with FRCM ... 196
8.9.6 Numerical simulation ... 197
8.10 Recommendation for Future Work ... 198
8.11 Impact of Current Research... 198
References ... 199
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List of Figures
Figure 2.1: Corrosion process of steel bars inside concrete [5] ... 8
Figure 2.2: A schematic of microcell and macrocell types of corrosion [5] ... 8
Figure 2.3: FRCM composite system ... 17
Figure 2.4: Different fabric configurations [64] ... 17
Figure 2.5: Actual and idealized stress-strain curve of FRCM coupon in tension [67]... 19
Figure 2.6: Failure mechanism of a fiber bundle embedded in cementitious matrix [74] ... 21
Figure 4.1: Typical dimensions and reinforcement details of the test specimen ... 38
Figure 4.2: Specimens connected in series inside the corrosion chamber ... 40
Figure 4.3: Strengthening materials: a) unbalanced PBO fabric, b) unidirectional carbon fabric, and c) unidirectional carbon fabric ... 41
Figure 4.4: Idealized tensile stress-strain curve of FRCM coupon specimen [54] ... 43
Figure 4.5: Repair schemes: (a) Scheme I and (b) Scheme II ... 44
Figure 4.6: Repair procedure: a) removing the deteriorated concrete, b) patch repair, c) roughening the concrete surface with sandblasting, and d) FRCM application ... 45
Figure 4.7: Positions of the electrical strain gauges along the outer fabric ... 45
Figure 4.8: Corrosion cracks pattern for specimen CU ... 46
Figure 4.9: Typical modes of failure: (a) SY-CC in beam CR-1P-I, (b) FD in beam CR-2P-I, (c) FS in beam CR-2P-II, (d) MC-SFM in beam CR-3C-II, and (e) LR in beam CR-1FRP-I ... 48
Figure 4.10: Effect of number of PBO-FRCM plies on the load-deflection curves ... 50
Figure 4.11: Effect of the repair scheme on the load-deflection curves ... 50
Figure 4.12: Effect of FRCM materials on the load-deflection curves... 51
Figure 4.13: Normalized ultimate load versus the equivalent stiffness ... 53
Figure 4.14: Load-strain curves for specimens with repair Scheme I ... 55
Figure 4.15: Load-strain curves for specimens with repair Scheme II ... 56
Figure 4.16: Strain profile in the PBO fabric for specimen CR-4P-I ... 57
Figure 4.17: Strain profile in the PBO fabric for specimen CR-4P-II ... 58
Figure 4.18: Strain profile in the carbon fabric for specimen CR-3C-II ... 58
Figure 4.19: Stress and strain distribution at ultimate stage ... 59
Figure 5.1: Test specimen geometry and reinforcement details. (All dimensions in mm) ... 71
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Figure 5.3: FRCM systems: a) PBO-FRCM (Unbalanced PBO fabric) and b) C-FRCM
(Unidirectional carbon fabric)... 73
Figure 5.4: Stress-strain relationships for FRCM-tensile coupons [97] ... 74
Figure 5.5: Strengthening schemes: a) Scheme I and b) Scheme II ... 76
Figure 5.6: FRCM installation procedure: a) removing the deteriorated concrete, b) patch repairing and sandblasting, and c) installation of PBO-FRCM composite ... 77
Figure 5.7: Profile of steel bars: a) uncorroded bar, b) corroded bar extracted from CSA-4P-I (12.6% mass loss), c) corroded bar extracted from CSB-3C-II (18.6% mass loss), and d) corroded bar extracted from CUC (22.5% mass loss) ... 79
Figure 5.8: Actual and theoretical mass loss versus the duration of corrosion process ... 79
Figure 5.9: Modes of failure: a) FRCM delamination, b) fabric slippage with partial fabric debonding within the matrix, and c) matrix cracking with extensive fabric slippage ... 81
Figure 5.10: Load-deflection relationships for corroded-unstrengthened beams ... 82
Figure 5.11: Load-deflection relationships for corrosion-damaged FRCM-strengthened beams: a) beams of Group A, b) beams of Group B, and c) beams of Group C ... 83
Figure 5.12: Normalized strength versus the FRCM equivalent axial stiffness, Kf ... 85
Figure 5.13: Load versus outer fabric strain for beams strengthened in a) Scheme I and b) Scheme II ... 86
Figure 5.14: Normalized ductility index versus stiffness factor 𝛽𝑓 % ... 89
Figure 5.15: Normalized energy absorption index versus stiffness factor 𝛽𝑓 % ... 89
Figure 5.16: Effect of corrosion damage on the ultimate strength of strengthened beams ... 90
Figure 5.17: Normalized strength versus stiffness factor 𝛽𝑓 % ... 91
Figure 5.18: Predicted versus experimental flexural response for beams strengthened with a) two layers of PBO-FRCM in Scheme I, b) four layers of PBO-FRCM in Scheme II, and c) three layers of C-FRCM in Scheme II ... 94
Figure 6.1: Schematic of the testing procedure of the short- and long-term beams ... 101
Figure 6.2: Typical dimensions and reinforcement details of the test beam (all dimensions in mm) ... 102
Figure 6.3: A schematic of the electrical connection ... 104
Figure 6.4:FRCM systems; a) PBO-FRCM (unbalanced PBO fabric) and (b) C-FRCM (unidirectional carbon fabric) - all dimensions in mm ... 105
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Figure 6.5: Idealized tensile stress-strain curve of FRCM coupon specimen ACI 549.4R-13 [54] ... 106 Figure 6.6: FRCM repair schemes; a) Scheme I and b) Scheme II ... 107 Figure 6.7: Corrosion cracks patterns; a) typical corrosion cracks pattern for short-term
specimens (beam CU); b) beam CRL-4P-I; c) beam CRL-4P-II; and d) beam CRL-3C-II ... 110 Figure 6.8: Failure mode of a) beam CU due to steel yielding and concrete crushing; b) beam CRS-4P-I due to FRCM delamination; c) beam CRL-4P-I due to premature FRCM delamination; d) beam CRL-4P-II due to PBO-fabric debonding from matrix; and e) beam CRL-3C-II due to fabric slippage ... 112 Figure 6.9: Load-deflection relationships of a) short-term beams; b) long-term beams; c) beams repaired with PBO-FRCM (short-term and long-term); and d) beams repaired with C-FRCM (short-term and long-term) ... 116 Figure 6.10: Load-strains relationships for a) beams repaired with PBO-FRCM and b) beams repaired with C-FRCM ... 122 Figure 7.1: Geometry and reinforcement details of the test specimen (all dimensions in mm) . 132 Figure 7.2: Specimens connected in series inside the corrosion chamber ... 133 Figure 7.3: a) Unbalanced PBO fabric and b) Unidirectional carbon fabric. ... 134 Figure 7.4: FRCM strengthening schemes: a) Scheme I and b) Scheme II ... 136 Figure 7.5: FRCM strengthening procedure: a) removing the deteriorated concrete after
corrosion, b) patch repair, c) PBO-FRCM application, and d) C-FRCM application ... 137 Figure 7.6: Test setup ... 138 Figure 7.7: Corrosion crack pattern for specimen FCU ... 139 Figure 7.8: Profile of the steel bars: a) Uncorroded bar, b) corroded steel bar extracted from beam MCS-4P-II, and c) fatigue rupture of a corroded steel bar extracted from FCS-3C-II ... 140 Figure 7.9: Failure modes of the monotonically tested beams: a) Steel yielding followed by concrete crushing; b) FRCM delamination; c) Fabric slippage and partial debonding; and d) Matrix cracking followed by fabric slippage ... 141 Figure 7.10: Load-deflection relationships of the monotonically tested beams ... 142 Figure 7.11: Variation of fatigue life of the strengthened beams with the stiffness factor 𝛽𝑓 .... 144
Figure 7.12: Load-deflection Hysteresis for a) beam FCU; b) beam I; c) beam FCS-4P-II; and d) beam FCS-3C-II ... 147
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Figure 7.13: Fatigue cracks at midspan of the strengthened beams (Side view) ... 148
Figure 7.14: Fatigue rupture of steel bars in a) beam FCU and b) beam FCS-4P-I ... 148
Figure 7.15: Effect of fatigue cycles on the stiffness of the tested beams ... 149
Figure 7.16: Effect of fatigue cycles on the concrete and fabric strains ... 150
Figure 8.1: Geometry and details of steel, P-FRCM, and C-FRP reinforcement of the tested beams (all dimensions in mm)... 159
Figure 8.2: a) PBO fabric used in the FRCM composite system and b) Carbon sheets used in the FRP composite system ... 160
Figure 8.3: Strengthening procedure of corroded beams: a) removing the deteriorated concrete, b) patch repairing and sandblasting, c) installation of P-FRCM composite, and d) installation of C-FRP sheets ... 162
Figure 8.4: Constitutive laws of concrete and cementitious matrix: a) compressive hardening law, b) compressive softening law, and c) tensile softening law ... 164
Figure 8.5: C-FRP/concrete interfacial bond stress-slip model according to Lu et al. [117] adopted for various concrete mixes ... 167
Figure 8.6: PBO-fabric /matrix interfacial bond stress-slip model according to D’Ambrisi et al. [120] ... 168
Figure 8.7: a) Meshing of P-FRCM strengthened beams, b) reinforcement layout for beams strengthened with 4 layers of P-FRCM, and c) reinforcement layout for beams strengthened with C-FRP sheet ... 169
Figure 8.8: Numerical and experimental crack patterns at failure for a) beam UUa, b) beam CS-A-1C, and c) beam CS-A-4P ... 172
Figure 8.9: Numerical and experimental load-deflection responses for beams of group A ... 173
Figure 8.10: Numerical and experimental load-deflection responses for beams of group B ... 176
Figure 8.11: Gain and decline in % in the ultimate loads, Pu, with respect to that of the control beam (UU) ... 177
Figure 8.12: Fiber and concrete strain response ... 180
Figure 8.13: Tensile steel strain response for beams CS-A-1C and CS-A-4P... 181
Figure 8.14: Strain profile of the internal and external reinforcement at ultimate ... 181
Figure 8.15: Effect of the concrete compressive strength on the load-deflection response for beams a) CS-A-1C, b) CS-A-2P, and c) CS-A-4P... 185
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Figure 8.16: Effect of the concrete cover on the load-deflection response for beams a) CS-A-1C, b) CS-A-2P, and c) CS-A-4P ... 188
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To the researchers and engineers who appreciate the value of science and
knowledge
To the ones who were
and will always be
in my mind….
in my heart….
.…and in my Soul
xx
Acknowledgements
Every journey has to have an ambitious and a defined end. My journey as a PhD student started in December 2014. My main objective was to acquire my PhD at Laval. During the last three years, I have developed another goal for myself: to explore my own feasibility as a researcher and how to strengthen myself as an individual. The outcomes were greatly dependent on people, financial status, and the surrounding environment.
Therefore, I would like to express my sincere appreciation and gratitude to my thesis supervisor Prof. A. El Refai for his guidance and continuous encouragement during the course of this work. Thanks for being a friend and a firm supervisor. I would like also to express my sincere gratitude to our collaborators Prof. A. Nanni from University of Miami and Prof. U. Ebead from Qatar University, for their encouragement and support during my research.
I would also like to acknowledge and thank all the technicians of the structural laboratory at Laval University, especially, Mr. R. Malo for his precious assistance through the experimental tests. The help provided by Mr. K. Attia, Mr. A. Abbadi, and Mr. N. Allam during the tests is greatly appreciated.
I would like to acknowledge Laval University and Qatar Foundation for the financial support. The donation of composite materials provided by Ruredil, Italy and Simpson Strong-Tie, USA represented by Mr. Brad Erickson is greatly appreciated.
I would like to express my sincere thanks and appreciation to my mother. Without her unconditional love, support, and encouragement, the achievement of this work would not be possible. I cannot end my acknowledgement without expressing my deep appreciation to my sister from the bottom of my heart for her encouragement and endless love
1
1.
Chapter 1
Introduction
1.1 Background and Problem Definition
Repair/strengthening of reinforced concrete (RC) structures is motivated by several factors including aging, change in use, increased loads, code compliance, and environmental damages such as corrosion. Corrosion of steel reinforcement is one of the major durability concerns for concrete structures especially in coastal areas and in cold regions where de-icing salts are heavily used. Pitting corrosion reduces the cross-sectional area of the steel bars and may lead to significant loss of their ductility [1,2]. The expansion of corrosion products causes concrete cracking and impair the composite action between steel and concrete. As a result, the load-carrying capacity and the service life of the corroded member are considerably jeopardized [3–5].
Corrosion of steel reinforcement combined with fatigue stresses significantly reduces the fatigue life of RC structures and may lead to unexpected failures especially in bridges [6,7]. According to the U.S. Federal Highway Administration report published in 2002 [8], approximately 15% of the highway bridges in USA are considered structurally deficient. Their maintenance and repair costs exceed 8.3$ billion dollars annually. In Canada, the total direct costs of corrosion were 23.6$ billion dollars in 2003 [9]. The statistics for Europe, Asian Pacific countries, and Australia are not by any means better than those in North America. Government agencies and industrial firms are looking for more durable and less costly materials and techniques to maintain and repair our infrastructures. Hence, structural engineers are in continuous search for new construction materials and innovative rehabilitation techniques for such deficient structures.
In the last decades, externally-bonded strengthening technologies based on organic matrices, referred to as fiber-reinforced polymers (FRP), have proven success in restoring the serviceability and strength of RC structures [10–12]. However, several problems associated with the use of FRP have been documented. FRP materials are flammable and prone to deterioration and the loss of their mechanical and bond properties at high temperatures [13]. Their epoxy-based agents lose their stiffness and strength and change from a stiff material to a viscous material with poor properties when exposed to elevated temperatures, which affect their bond strength [14,15].
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Structures located in hot climates or those at risk of fire can easily be vulnerable when exposed to elevated temperatures. The toxicity nature of epoxy and its poor thermal compatibility to the concrete substrate add another dimension to the drawbacks of FRP systems [16].
In order to overcome the drawbacks associated with the use of FRP composites, the need to replace the organic binder (epoxy) by an inorganic binder has been raised. The use of fabric-reinforced cementitious matrix (FRCM) systems has been introduced as an alternative promising strengthening technique to FRPs. The FRCM system is a composite material consisting of fabric meshes made of long dry-woven intermittent in two orthogonal directions and embedded in cement-based matrix that serves as a binder. The embedded grid is shielded between the mortar layers thus minimizing its fire vulnerability. In addition, the compatibility between the mortar used and the concrete substrate is inherited since both materials have cement as a common “base”. The cement-based mortars used in FRCM also act as barriers against chloride ions penetration thus protecting the main reinforcing bars from corrosion attack. Its lightweight, high tensile strength, and ease of application makes the system very appealing to engineers. With their innovative features, FRCM systems can ensure the endurance of the rehabilitation process and consequently the sustainability of the strengthened structure.
Recently, significant efforts have been made to introduce FRCM composites in the construction industry and the use of FRCM composites to strengthen RC structures was initiated in several field applications. A significant amount of research has been devoted to study the flexural and shear behaviors of undamaged RC members strengthened with FRCM composites. Nonetheless, the feasibility of using FRCM composites to strengthen corrosion-damaged RC structures has received little attention. The challenge in using FRCM to repair/strengthen corrosion-damaged concrete members rises from the fact that the technique uses cementitious mortars rather than epoxies, which necessitates proper surface preparation to ensure adequate mechanical and chemical adhesion to the concrete substrate. Factors such as the absorption properties of the substrate, its degree of carbonation, its moisture condition, and its cleanness, in addition to the existence of micro-cracks and/or contaminants on the repaired surface may affect the desired performance. Unfortunately, surfaces of corroded concrete structures are characterized by their random texture, unpredictable crack distribution, and concrete fragmentation. Volume expansion resulting from corrosion of steel bars in addition to the existence of corrosion products cause the weakness of the
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surrounding concrete and the loss of its integrity. In fact, these conditions might consist a serious obstacle to the use of FRCM in strengthening corroded structures. To the authors’ knowledge, very few of the previous studies in which FRCM technique was adopted has addressed this problem, not to mention the long-term performance of the strengthened members being exposed to the same conditions that have caused their deterioration. Therefore, the work presented herein aimed at tackling these problems and filling the gap in our knowledge on the use of FRCM composites in strengthening corrosion-damaged RC elements.
1.2 Thesis Structure
The purpose of this work is to investigate the short- and long-term structural performance of corrosion-damaged RC beams strengthened with FRCM composites. This thesis is organized in nine chapters as follows:
• Chapter (1) provides background on the subject as well as the definition of the research problem.
• Chapter (2) provides a comprehensive literature review on topics that are related to the work of this study. This includes a review on the corrosion mechanism in RC structures, a review on the flexural behavior of corrosion-damaged RC members under both monotonic and fatigue loads, a full historical review on previous research that was carried out to determine the mechanical properties of FRCM composites, and an inclusive review on the flexural behavior of RC members strengthened with FRCM composites.
• Chapter (3) highlights the research significance along with the research objectives of this work. A detailed description of the utilized methodology to achieve these objectives is also included. • Chapter (4) presents the first journal paper submitted to the American Society of Civil
Engineers (ASCE) Journal of Composites for Construction. The paper is titled
“Corrosion-Damaged Reinforced Concrete Beams Repaired with Fabric-Reinforced Cementitious Matrix (FRCM)”. The paper reports on the feasibility of using FRCM to strengthen/repair RC beams
with moderate level of corrosion damage. The test results of eleven large-scale RC beams were presented including those of nine beams that were subjected to accelerated corrosion process for 70 days to obtain an average mass loss of 12.6% in the tensile steel reinforcing bars and
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those of two beams that were tested as controls. The effect of various levels of corrosion damage on the flexural response of the FRCM-strengthened beams was investigated throughout Chapter 5. In this paper, the test parameters included the number of fabric plies (1, 2, 3, and 4), the FRCM repair scheme (end-anchored and continuous U-wrapped strips), and FRCM materials (carbon and PBO). The analysis and discussion of the test results were presented in terms of mode of failure, load-deflection response, load carrying capacity, ductility performance, and strain responses. In addition, the experimental results were compared to the current design guidelines of ACI-549.4R-13.
• Chapter (5) presents the second paper that was published in the Journal of Composite Structures. The paper is titled “Effect of Corrosion Damage on the Flexural Performance of RC
Beams Strengthened with FRCM Composites.” The paper reports on the effect of the level of
corrosion damage on the flexural response of RC beams strengthened with FRCM composites. Three theoretical tensile steel mass losses were considered in this study, namely 10, 20, and 30 %, which represented moderate, severe, and very severe degrees of corrosion damage. The test parameters also included the fabric type (PBO and carbon), the number of FRCM layers (two, three, and four), and the strengthening scheme (end-anchored and continuously wrapped). The test results were analyzed and discussed to highlight the effect of various test variables on the failure mechanism, the flexural performance, the fiber, steel, and concrete strain responses, the ductility, and the flexural strengths of the tested beams. In addition, the experimental results were compared to the current design guidelines of ACI-549.4R-13.
• Chapter (6) presents the third journal paper that was submitted to the Journal of Construction and Building Materials and was titled “Post-repair Flexural Performance of Corroded Beams
Rehabilitated with Fabric-Reinforced Cementitious Matrix (FRCM) under Corrosive Environment”. The test results of nine RC beams were reported in this paper including those of
one beam that were neither corroded nor repaired, another beam that was corroded and not repaired, and six beams that were corroded and then repaired in two phases. Beams of Phase I (short-term) were subjected to an accelerated corrosion process for 210 days before being strengthened with FRCM whereas beams of Phase II (long-term) were initially subjected to accelerated corrosion for 70 days, then repaired with FRCM and exposed to further corrosion
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for 140 days. The test parameters also included the type of FRCM materials (PBO and Carbon) and the FRCM repair scheme (end-anchored and continuous wrapping).
• Chapter (7) presents the fourth journal paper that was submitted to the ASCE’s Journal of Composites for Construction. The paper is titled “Fatigue and Monotonic Behavior of
Corrosion-damaged Reinforced Concrete Beams Strengthened with FRCM Composites.” This
paper aimed at investigating the potential of using FRCM composites to restore the fatigue life of RC beams that were severely damaged due to corrosion (20% tensile steel mass loss). The paper provides better understanding on the flexural behavior of FRCM-strengthened beams damaged due to corrosion. The test results of twelve beams were reported. The test parameters included the fabric material (PBO and Carbon), the number of FRCM plies, the strengthening configuration, and the type of loading (monotonic and fatigue). The results were discussed and presented in terms of fatigue life, fatigue behavior, modes of failure, strain response, and stiffness degradation.
• Chapter (8) presents the fifth paper that was submitted to the Journal of Engineering Structures and titled “Finite Element Modeling and Experimental Results of Corroded Concrete Beams
Strengthened with Externally-bonded Composites”. This paper presents the numerical
simulation of corroded RC beams strengthened with carbon-FRP (CFRP) and PBO-FRCM composites under flexural loading using ATENA software package. The CFRP composite was modeled as discrete reinforcement bonded directly to the concrete substrate without binder while the PBO-FRCM was modeled using a more detailed approach that involved modeling the fabric and the matrix layers. Interfacial bond stress-slip models were adopted at the CFRP/concrete and the PBO-fabric/matrix interfaces to simulate the failure mechanism observed during the test. The carrying capacities, deflection responses, and load-strains responses were evaluated and compared with the experimental results to validate the accuracy of the model. The validated models were used in a parametric study to investigate the effect of varying the concrete compressive strengths on the flexural behavior of the FRCM-strengthened beams after corrosion.
• Chapter (9) includes a summary of this study and the overall conclusions based on the experimental, analytical, and numerical results obtained. Recommendations for future work are also suggested in this chapter.
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2.
Chapter 2
Literature Review
2.1 General
The corrosion of the steel reinforcement in concrete structures is a major challenge facing structural engineers while maintaining ageing infrastructures. Corrosion usually causes engineering and economic problems, which has led to a considerable amount of research work devoted to study the performance of corroded-RC flexural elements under both monotonic and fatigue loads. In recent years, advanced composite materials in the form of externally-bonded fabric-reinforced cementitious matrix (FRCM) have been introduced as innovative strengthening techniques for RC structures. Several publications have been published in recent years to document the effectiveness of FRCM composites to strengthen undamaged concrete structures. Nevertheless, their application to strengthen/repair corrosion-damaged concrete elements is still lacking.
This chapter provides a comprehensive review on the corrosion of steel reinforcement in concrete structures as well as the use of externally-bonded FRCM composites to strengthen RC flexural elements. A review of the available literature concerning the research work on corroded FRCM-strengthened flexural elements is presented along with the factors that affect their structural behavior.
2.2 Corrosion of Steel Reinforcement
2.2.1 Corrosion Mechanism of Steel Bars in Concrete
The high alkaline environment of concrete normally protects the steel reinforcement against corrosion by creating a passive film of iron oxides at the steel/concrete interface [5]. In addition, the concrete cover works as a barrier to protect the steel bars against harsh environmental conditions [5]. Exposure to sea salt spray in marine environment or de-icing salts in cold regions are the most common causes of corrosion in concrete worldwide. Steel corrosion in concrete can be triggered by chloride attacks and concrete carbonation. Chloride ions diffuse through the concrete pores and cracks until they reach the surface of the steel bars. Chlorides work as catalysts to initiate the corrosion reactions in the weak spots on the steel surface. In case of concrete
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carbonation, the carbon dioxide gas (CO2) in the atmosphere dissolves in water (H2O) to form the carbonic acid (H2CO3). The acid then penetrates into the concrete and drops its pH level to less than 8.5. Consequently, the passive layer previously formed on the bar surface decades and corrosion reactions are then initiated [5,17].
Corrosion of steel in concrete is an electrochemical process. It involves the transfer of charge (electrons) from one location to another [17]. The corrosion reaction is similar to a galvanic cell that consists of an anode and a cathode as shown in Figure 2.1. At the anode, the iron (steel bar) dissolves or oxides and the ferrous electrons are released as illustrated in Equation (2.1). The released electrons react with water and oxygen at any other location of the steel bar that acts as a cathode to form dissolved hydroxide ions (Equation (2.2)).
Fe Fe+2 + 2e− (Anode reaction) Eq. (2.1)
2e−+H 2O +
1
2O2 2OH
− (Cathode reaction) Eq. (2.2)
The ferrous ions (Fe+2) at the anode react with the hydroxide ions (OH−) that are released from the cathode to form ferrous hydroxide [Fe (OH)2] as per Equation (2.3). In the presence of
sufficient amount of water and oxygen, the ferrous hydroxide reacts to produce ferric hydroxide [Fe (OH)3] as shown in Equation (2.4). Finally, the ferric hydroxide [Fe(OH)3] decays to hydrated ferric oxide [Fe2O3. H2O] or rust [5]. This reaction is shown in Equation (2.5). The volume of the corrosion products is much greater than the volume of steel (volume of rust is about 6 times of that of iron). Therefore, longitudinal cracks parallel to the reinforcing bars generally appear in the concrete cover as indication of corrosion. The cracks allow to more quantities of moisture and oxygen to reach the steel bars, which boosts the rate of corrosion reactions.
Fe+2+2OH− Fe (OH)
2 Eq. (2.3)
4Fe (OH)2+ O2 + 2H2O 4Fe (OH)3 Eq. (2.4) 2Fe (OH)3 Fe2O3. H2O + 2H2O Eq. (2.5)
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Figure 2.1: Corrosion process of steel bars inside concrete [5]
Corrosion of steel in concrete can occur as micro-cells (uniform iron removal) or as macro-cells (local iron removal). Figures 2.2a and 2.2b show schematics of micro-cell and macro-cell types of corrosion, respectively. At the beginning, corrosion pits start to take place before they expand and propagate and uniform corrosion can be seen along the steel bars. The pits are usually initiated by the chloride attacks at the weak spots on the bar’s surface, which creates an electrochemical potential difference between the anodes and cathodes. In micro-cells, both the cathodes and anodes are very close as shown in Figure 2.2a. Macro-cell reactions can occur over large distances on the surface of the steel bar or even between two different bars. In macro-cells, the typical pits are separated by large passive area of steel that acts as small concentered anodes (Eq. (2.1)) that are surrounded by large cathodes as shown in Figure 2.2b (Eq. (2.2)).
Figure 2.2: A schematic of microcell and macrocell types of corrosion [5]
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2.2.2 Accelerated Corrosion Process
A significant amount of experimental work has been carried out to investigate the influence of corrosion of steel reinforcement on the structural performance and durability of concrete structures [3,18,19]. Most researchers used accelerated corrosion techniques to achieve the desired level of corrosion damage within reasonable amount of time. Table 2.1 summarizes some of previous accelerated corrosion tests that have been conducted.
Accelerated corrosion can be used to simulate natural corrosion without remarkable change in the structural response that would be encountered due to natural corrosion [20–22]. The corrosion process is accelerated by electrical polarization of the steel reinforcement. Chloride salt is usually used to activate the corrosion process. Some researchers added chloride salts to the concrete mix [10,20,23] while others partially submerged the test specimens in a salted solution [18,22,24,25]. During accelerated corrosion, the steel bar is connected to the positive terminal of a power supply to enforce the bar to act as anode and consequently Fe2+ ions start to dissipate from the surface of the bar. Embedded stainless steel bars or tubes or external plates made of copper or stainless steel are then connected to the negative terminal of the power supply to act as cathode. Both methods are presented in Table 2.1.
The level of corrosion-damage of steel bars can be quantified as the percentage of the mass of the steel bars lost to rust. The steel mass loss due to corrosion can be measured experimentally or predicted theoretically. Measuring the mass of lost metal, after the completion of the tests, is the most widely adopted measure of corrosion levels in laboratory tests. The actual mass loss can be determined according to ASTM G1-90 [26] standard, which provides mechanical, chemical, and electrolytic techniques to remove corrosion from the steel bars.
On the other side, the mass loss of steel bars can be predicted theoretically using Faraday’s law [18,22,24,27]. This law relates the mass loss to the corrosion current and to the time of exposure to corrosion as follows:
m =
I t an F
Eq. (2.6)
where m = mass loss (in grams); I = intensity of the impressed current (in Ampere); t = the time of corrosion (in seconds); a = the atomic mass of iron (55.847gm); n = the number of electrons
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transferred during the corrosion reaction (n = 2 in case of iron); and F = Faraday’s constant, which equals to 96 500 C/equivalent.
For practical application, the current density, i, instead of the current intensity, I, as follows:
m =
(S×𝑖 ) t an F
Eq. (2.7)
where S is the surface area of the corroded steel and i is the impressed current density.
Table 2.1: Summary of some previous accelerated corrosion tests
Study Specimen type Current density (μA/cm2) Cathode type Corrosion environment Almusallam et al. (1996) [25] Bond pull-out 10400 External stainless-steel plate Constant immersion in 3% NaCl solution Alonso et al. (1998) [28] Bond pull-out 3, 10, or 100 External stainless-steel plate
Concrete cast with 3% NaCl by weight of cement
Mangat and Elgarf
(1999) [24] Beams 1000, 2000, 3000, or 4000 External copper plate Constant immersion 3.5% NaCl solution Masoud et al. (2001) [10] Beams 150 Internal stainless-steel bar
Concrete cast with 3% NaCl by weight of cement
El-Maaddawy and
Soudki (2003) [20] Concrete prisms
100, 200, 350, or 500
Internal stainless-steel bar
Concrete cast with 5% NaCl by weight of cement Malumbela et al. (2010) [21] Beams 189 External stainless-steel bars 4 days immersion in 5% NaCl solution followed by 2 days drying Mancini et al.
(2014) [23] Concrete prisms 200
External stainless-steel plate
Concrete cast with 5% NaCl by weight of cement Ou and Nguyen (2016) [22] Beams 600 External copper plate Constant immersion 5% NaCl solution
In natural corrosion, the measured corrosion current density is in the range of 1 to 100 μA/cm2 [28–31] while the impressed current density used in accelerated corrosion ranges between 3 and 10400 μA/cm2 [25,30]. The density of the impressed current should be kept constant during the
accelerated corrosion to ensure uniform corrosion mechanism [20]. Much research has been carried out to investigate the optimal current density that should be used in accelerated corrosion techniques to simulate natural corrosion within reasonable amount of time [24,27,28]. It has been reported that the use of a current density higher than 250 μA/cm2 in accelerated corrosion increases
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the corrosion cracks width [32], decreases the bond strength [18,27], and may affect the mechanical properties of the steel bars [19,22] in comparison to those measured during natural corrosion at similar mass losses. These observations were attributed to the change of the composition of the corrosion products due to the use of high current densities [27]. El-Maaddawy and Soudki (2003) [20] concluded that varying the current density level between 100 and 200 μA/cm2 had no impact on the concrete strain response. However, increasing the current density
above 200 μA/cm2 caused a significant increase in the concrete strain response and the widths of
corrosion cracks due to higher concentrations of corrosion products around the steel bars. In addition, it was concluded that the use of current densities less than 200 μA/cm2 would be small enough to obtain corrosion-damage similar to that reported in the field.
2.2.3 Effect of Steel Corrosion on Concrete Structures
Corrosion of steel reinforcement in concrete structures damages the reinforcement itself, the surrounding concrete, and ruins the composite action between the steel bars and concrete. Corrosion significantly affects the tensile behavior of the steel bars. The external surfaces of the corroded bars significantly change due to the formation of irregular pits. Corrosion alters the shape of the bar section, which varies randomly along the corroded length. It also reduces the cross-sectional area of the bar and consequently its load-carrying capacity. The non-uniform distribution of corrosion pits cause stress concentration, which reduces the residual strength and ductility of the corroded bars compared to that of the uncorroded ones having the same cross section [33,34]. The level of corrosion-damage (i.e. the mass loss of the bars) and the distribution of pits along the length of the corroded bars significantly influence their stress-strain response in both tension and compression [33]. For the same corrosion mass loss, corrosion has a more significant effect on the tensile behavior of plain bars than that of deformed bars and has a more pronounced impact on the smaller bars than on the larger bars [2].
In an interesting study by Ou et al [22], the tensile behavior of naturally-corroded and artificially-corroded steel bars were compared. The current density used was 600 μA/cm2. The test results indicated that the tensile strength and ductility of both naturally- and artificially-corroded bars were decreased by increasing the level of corrosion damage. The yield and ultimate tensile strengths of artificially-corroded bars were similar to those of naturally-corroded bars. However, the ultimate strains of the naturally-corroded bars were smaller than those of the
artificially-12
corroded ones. These observations indicated that corrosion pits were more uniformly distributed in artificially-corroded bars than in naturally-corroded ones.
Corrosion damage has a more pronounced impact on the fatigue behavior of the corroded steel bars rather than their tensile behavior [35,36]. Corrosion of steel bars significantly decrease their fatigue life under tensile and compressive cyclic loads [33]. In addition, corroded bars fail in a very brittle manner by sudden fatigue rupture. The fatigue life of corroded bars decreases dramatically with the increase of corrosion level and the fatigue stress range [19].
On the other side, the expansion of the corrosion products results in expansive stresses in the concrete surrounding the corroded bars. Once these stresses exceed the concrete tensile strength, cracking and spalling of concrete cover occur. Moreover, corrosion products work as an isolation layer at the steel/concrete interface, which deteriorates bond and loosens the composite action between steel and concrete [37]. When deformed bars corrode, the height of their lugs decreases, and rust fills the inner cracks of the deteriorated concrete. Therefore, their bond performance become similar to that of plain bars, where just friction is the dominant parameter [38,39]. In advanced stages of corrosion, the cracked concrete can peel off, and bond between the two materials can be totally lost. Many researchers have investigated the effect of corrosion on the bond characteristics at concrete/steel interface. Most studies have found that bond increases slightly as the steel bar corrodes and decreases rapidly as the corrosion cracks take place [40]. It is important to note that the reduction in bond are dependent of the thickness of the concrete cover, the bar diameter, and the presence of stirrups (confinement) [41,42].
2.2.4 Behavior of Corroded-RC Beam under Monotonic Loads
Much research work has been devoted to study the structural behavior of corroded RC beams under monotonic loads. In the following, a summary of some of the previous work is presented.
In an early study by Almusallam et al. (1996) [43], the structural behavior of RC elements with different levels of corrosion damage was investigated. The test specimen was 63.5×305×711 mm and reinforced by 5 steel bars of 6 mm diameter. The whole length of the steel reinforcement was subjected to an accelerated corrosion process. A sharp reduction in the ultimate strength was observed as the degree of corrosion increased. For instance, 10 and 28% mass loss resulted in a reduction in the ultimate strengths of the specimens by 20 and 65%, respectively. The corroded
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specimens failed by bond-shear failure due to the loss of bond between steel and concrete. Thus, the ductility of the corroded specimens was significantly reduced.
Vidal et al. (2007) [44] conducted a long-term experimental program to study the structural behavior of 36 beams of 150×280×3000 mm. The beams were reinforced with two steel bars of 12 or 16 mm diameter. The beams were stored in a chloride environment under service loads for 14 and 17 years to corrode naturally. The test results showed that the steel mass loss was 15 and 30% after 14 and 17 years respectively. In addition, the flexural cracks during the corrosion exposure had no significant impact on the rate of corrosion. The inspection of the corroded bars indicated that neither the distribution of the corrosion pits nor their intensity was uniform along the steel bars despite the fact that all specimens were subjected to uniform environmental conditions. The flexural strength and stiffness of the corroded beams were significantly influenced by the level of corrosion damage.
Torres-Acosta et al. (2007) [45] investigated the behavior of twelve simply-supported RC beams (100×150×500 mm) with several levels of corrosion damage. The whole length of the steel bars was subjected to an accelerating corrosion process. Longitudinal cracks parallel to the steel bars were observed at the end of the corrosion process. Their width increased as the corrosion level increased. All of the corroded beams showed a more brittle response as the degree of corrosion damage became higher. The residual flexural strengths also decreased at higher corrosion levels and were significantly influenced by the depths of the pits on the surface of steel bars.
Gu et al. (2010) [46] investigated the flexural behavior of naturally- and artificially-corroded RC beams. The naturally-corroded beams with steel mass loss up to 15% failed due to flexural while increasing the mass loss to 20% changed their mode of failure to rupture of steel bars. On the other hand, the beams subjected to an accelerated corrosion process up to 25% mass loss failed due to flexural whereas those subjected to 40% mass loss failed due to rupture of steel bars. This observation was attributed to the non-uniform characteristics of the corrosion damage and the significant change in the mechanical properties of naturally-corroded steel bars. The load-carrying capacities of the corroded beams were reduced due to the reduction in cross-sectional area of the steel bars and the degradation in their mechanical properties due to corrosion while the decline in the beams’ stiffness was mainly attributed to the loss of bond at the steel/concrete interface.
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Malumbela et al. (2010) [47] conducted an experimental study to investigate the influence of varying the steel mass loss on the ultimate flexural capacity of corroded RC beams. The test specimen had dimensions of 153×254×3000 mm and were reinforced with three deformed steel bars of 12 mm diameter. The middle 700 mm of the steel bars were subjected to an accelerated corrosion process. The results showed that the maximum mass loss was obtained in the middle of the corroded zone with less mass loss reported at the two edges of the corrosion zone. The use of a sustained load during the corrosion stage had no significant effect on the rate of corrosion. The ultimate flexural capacity of the tested beams decreased linearly with the measured steel mass loss. The ultimate capacity of the beams decreased at a rate of 0.70% per each 1% steel mass loss.
Xia et al. (2012) [48] conducted an experimental study on the effect of corrosion of steel reinforcement on the stiffness and flexural strength of RC beams. The beams were tested under four-point load configuration. The tensile reinforcement was corroded within the maximum moment zone only to provide sufficient anchored length to avoid debonding failure. Nine levels of corrosion damage (4 to 12% steel mass loss) were achieved using an accelerated corrosion process. At the end of the corrosion process, longitudinal cracks parallel to the steel bars were observed. The average and maximum crack widths were in the range of 0.32 to 2.38 mm, and 0.68 to 4.5 mm, respectively. The average and the maximum crack widths increased as the degree of corrosion increased. The test results showed that the corroded and uncorroded beams had the same stiffness until 60% of the ultimate load after which the stiffness of the corroded beams significantly decreased. The residual flexural strength of the corroded specimens decreased as the level of corrosion increased. The corrosion level changed the failure mode from under-reinforced ductile failure to brittle collapse due to the rupture of the steel bars at the highest degree of corrosion damage (12% tensile steel mass loss).
Dang and François (2014) [49] studied the structural behavior of RC beams (280×150x3000) mm naturally-corroded under harsh environmental conditions over several years. The test observations revealed that the non-uniform distribution of the corrosion intensity due to the random formation of pits on the surface of the corroded bars not only affected the flexural strength of the beams but also changed their ductile failure (steel yielding followed by concrete crushing) to brittle rupture of the steel bars. The reduction in ultimate capacities corresponding to a given steel mass loss was lower than the reduction in ultimate deflections. Therefore, it was concluded that the service life
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of the corroded structure may be limited by the reduction of ductility rather than the reduction of the load-bearing capacity. This observation was related to the significant change in the mechanical properties of the corroded steel bars.
2.2.5 Behavior of Corroded-RC Beam under Fatigue Loads
The fatigue life of concrete structures is usually governed by the endurance of the steel reinforcing bars under repetitive loads and is rarely controlled by concrete [50–52]. Corrosion damage significantly decreases the fatigue endurance of the steel bars, which significantly reduces the fatigue life of RC beams. Many researchers investigated the effect of corrosion on the fatigue performance of RC beams. A summary of some of these investigations is presented as follow:
Yi et al. (2010) [53] carried out a laboratory study to investigate the performance of RC beams with corroded reinforcement under fatigue loading. The test specimens (150×300×3600 mm) were reinforced with two deformed steel bars of 20 mm diameter each. The beams were subjected to accelerated corrosion process to achieve eight degrees of corrosion damage in the range of 3.25 to 11.6 % steel mass loss. All of the beams were subjected to the same level of fatigue load oscillating between 11 and 52% of the load-carrying capacity of the control (virgin) beam. The test results revealed that corrosion of the steel reinforcement significantly reduced the fatigue life of the beams. The fatigue life decreased as the level of corrosion damage increased. For example, the non-corroded beams safely sustained 2 million fatigue cycles while beams with 3.25 and 11.6% steel mass loss failed after 626,000 and 89,000 cycles, respectively. In addition, all of the corroded beams failed due to the sudden rupture of one of the steel bars. The rupture occurred at locations of corrosion pits where maximum mass losses were encountered.
The performance of corroded RC beams under repeated loads were investigated by Sun et al. (2015) [6]. The test results showed that corrosion damage of the steel reinforcement had a significant adverse effect on the fatigue life of the beams. All of the corroded beams failed by brittle fatigue fracture of one of the steel bars. The fatigue performance of the corroded RC beams was described by two stages. At the first stage, flexural cracks occurred accompanied by rapid increase in the mid-span deflection of the beams and reduction in their flexural stiffness. The second stage represented the largest part of the fatigue life of the beams. In this stage, deflections
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Recently, Zhang et al. (2017) [7] investigated the effect of corrosion on the fatigue behavior of seven RC beams (120×200×1500 mm). Six out of the seven beams were subjected to an accelerated corrosion process to achieve different levels of corrosion damage in their tensile steel reinforcement (2.8 to 15.4 % steel mass loss). The corroded beams were subjected to fatigue with lower and upper limits of 10 and 60% of the ultimate flexural capacity of the virgin beam, respectively. The test results showed that the fatigue life of the corroded beams decreased rapidly with the increase of the level of corrosion damage. Strains in both concrete and steel increased with the increase of the fatigue life due to the accumulated fatigue damage. The midspan deflections of the beams increased rapidly at approximately 5% of the fatigue life followed by a stable stage until sudden failure of the beams occurred. The beams failed by sudden fatigue rupture of the steel bars. The microscopic scan of the fracture surface of the bars showed that fracture consisted of three zones namely, the crack initiation region, the crack growth region, and the abrupt rupture zone.
2.3 FRCM Composites
Fabric-reinforced cementitious matrix (FRCM) system is a composite material consisting of fabric meshes made of dry fibers embedded in cement-based matrix (serving as binder) as shown in Figure 2.3. When adhered to concrete or masonry, the FRCM system acts as an externally-bonded reinforcement. FRCM is considered as the natural evolution of ferrocement and FRP composites. FRCM have recently emerged as a viable technology for repairing and strengthening RC and masonry structures [54]. FRCM has been mentioned in the literature review with many acronyms such as textile-reinforced mortar (TRM) [55–57], textile-reinforced concrete (TRC) [58], and mineral-based composites (MBC) [59].
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Figure 2.3: FRCM composite system
The fabric is a manufactured planar textile structure made of fibers, yarns, or both, which is assembled by various means such as weaving, knitting, tufting, felting, braiding, or bonding of webs to give the structure sufficient strength and other properties required for its intended use [54]. Figure 2.4 illustrates different fabric configurations. The fabric can be made of high performance fiber such as carbon, Polyparaphenylene benzobisoxazole (PBO), alkali-resistant glass (AR-glass), and basalt. Many researchers reported the significant influence of fabric material, geometry, and configuration on the mechanical properties of the FRCM composite [54,60–63]. The penetration of the cementitious matrix to the fabric is a fundamental factor for bond between the fabric and the matrix to develop and consequently for the mechanical properties of FRCM composites. Therefore, the fabrics consist of meshes with total coverage area less than 2/3 of the total area of the fabric to provide sufficient matrix penetration [54,64].
