Luận án nghiên cứu thực nghiệm ứng xử dầm bê tông cốt thép chịu uốn bị hư hỏng do ăn mòn được gia cường bằng tấm cfrp

MINISTRY OF EDUCATION AND TRAINING HANOI UNIVERSITY OF CIVIL ENGINEERING TRAN HOAI ANH EXPERIMENTAL STUDY ON THE FLEXURAL BEHAVIOR OF REINFORCED CONCRETE BEAMS DAMAGED DUE TO CORROSION AND STRENGTHENED WITH CFRP SHEETS Major: Civil Engineering Code: 9580201 SUMMARY OF DOCTORAL DISSERTATION Hanoi – 2022 The dissertation has been completed at Hanoi University of Civil Engineering Supervisor 1: Assoc.Prof Nguyen Hoang Giang Supervisor 2: Assoc.Prof Le Trung Thanh Examiner 1: Assoc.Prof Tran Chung Examiner 2: Dr Nguyen Dai Minh Examiner 3: Assoc.Prof Nguyen Ngoc Phuong This doctoral dissertation will be defended at the HUCE-level Board of Examiners at ……… on ………………… This doctoral dissertation can be found at National Library of Vietnam and Library of Hanoi University of Civil Engineering Introduction Context of the study Vietnam is a country located in the tropical region with a hot and humid climate Our country has a coastline of 3260 km with many islands and archipelagos running from the North to the South, with 29/63 provinces and cities adjacent to the sea, including many large and important cities The climatic and environmental conditions of our country can make the process of corrosion of reinforcement on reinforced concrete (RC) structures take place faster than predicted Currently, besides construction with a lifespan of over 30 - 40 years, there are many structures that have been badly corroded and damaged after 20 - 25 years, and even many structures are severely damaged after only 10 - 25 years This fact poses an urgent requirement for studying the behavior of structural members corroded due to chloride ions and repairing methods to strengthen the bearing capacity of the structure Therefore, the topic "Experimental study on the flexural behavior of reinforced concrete beams damaged due to corrosion and strengthened with CFRP sheets" has been proposed in this thesis Purposes of the study - Studying the effect of corrosion degree of longitudinal reinforcement on the flexural behavior of RC beams corroded in the chloride environment; - Experimental study on the flexural strengthening of corroded RC beams using CFRP sheets - Modeling the behavior of RC beams corroded and strengthened with CFRP sheets Object and scope of the study The study was carried out in the laboratory on the samples with dimensions 150x150x150 mm and tested beam specimens with dimensions 150x200x2200 mm The thesis focuses on analyzing the effect of longitudinal reinforcement corrosion in the range of - 15% Corroded beam specimens are strengthened with externally bonded CFRP (Carbon Fiber Reinforced Polymer) sheets Scientific basis Based on the literature review of reinforcement corrosion on the structural members, the thesis has conducted an experimental program on the corroded RC beams as well as flexural strengthening techniques using CFRP sheets Furthermore, nonlinear finite element analyzes are also proposed to simulate the flexural behavior of corroded beams and CFRP-strengthened beams Research methodology - Literature review approach - Experimental approach - Modeling approach Scientific and practical significance of the thesis The thesis contributes to the understanding of the flexural behavior of corroded RC beams in the chloride environment in Vietnam The results of the thesis contribute to predicting the residual bearing capacity of the corroded RC beams and propose a strengthening method using CFRP sheet material New contribution of the thesis - The thesis has provided a data set obtained on a total of 27 tested samples and 14 RC beam samples, with different degrees of reinforcement corrosion, by applying the accelerated corrosion test in actual environmental conditions in Vietnam - Research results determined the effectiveness of flexural strengthening using CFRP sheets for corroded RC beams Then, the thesis proves that the CFRP-strengthened solution is effective for corroded RC beams - The thesis has built nonlinear FE models that allow accurately describing the flexural behavior of control beams, corroded beams, and strengthened beams, especially the failure mechanism due to CFRP debonding Then, the FE model has been developed to investigate the influence of design-oriented parameters on the behavior of reinforced corrosion beams, such as concrete compressive strength, longitudinal reinforcement ratio, deteriorated bond strength between concrete and reinforcement, and CFRP bonding scheme Content and structure of the thesis Introduction Chapter 1: Litterature review of the corroded reinforced concrete beam structure in the marine environment Chapter 2: Experimental study of flexural behavior of corroded reinforced concrete beams Chapter 3: Experimental study of flexural strengthening of corroded reinforced concrete beams using CFRP sheets Chapter 4: Nonlinear finite element analysis of flexural behavior of corroded RC beams strengthened with CFRP sheets Conclusion: the general conclusions are drawn from the research results of the thesis CHAPTER – LITTERATURE REVIEW OF CORRODED REINFORCED CONCRETE BEAMS IN THE MARINE ENVIRONMENT 1.1 Review of reinforcement corrosion in structural members 1.1.1 Mechanism of reinforcement corrosion The process of reinforcement corrosion is described through metal destruction due to electrochemical reactions, the exchange of ions and electrons at the surface of the metal, and the dissolved solution The formation of a local electrochemical cell on the steel bar between the cathode and the anode in the presence of water and oxygen induces a dissolution reaction on the surface of the metal, as well as the precipitation of iron oxides 1.1.2 Stages of reinforcement corrosion During the initiation phase, the stability of the reinforcing system protected by the concrete cover gradually decreases, while creating favorable conditions for the reinforcement corrosion During the propagation phase, the formation of corrosion products occurs in the form of electrochemical reactions 1.1.3 The main causes of reinforcement corrosion (a) Carbonation of concrete due to the penetration of CO2 in the air into the concrete material (b) Penetration of chloride ions into structures in the marine environment, or structures exposed to inorganic salts 1.2 Review of flexural behavior of corroded RC beams 1.2.1 In the world When the steel reinforcement is corroded, it has a double effect on the mechanical behavior of the structure: (i) reduces the bearing capacity because the area of reinforcement is reduced compared to the original design; (ii) reduces the stiffness of the member by reducing the area of reinforcement and the bond strength between concrete and reinforcement; (iii) the structure is damaged when the applied force and deflection are small As a consequence, the safety and usability of the building structure are affected Structural analysis of the behavior of corroded members depends on many parameters, therefore traditional analysis method used with noncorroded reinforcement has many limitations The effect of those parameters has not yet been fully evaluated, and more experimental studies are needed Therefore, numerical simulation is a useful tool to evaluate and predict the performance of corroded structures 1.2.2 In Vietnam In terms of geographical location, Vietnam is located in the tropics, with a hot and humid monsoon climate Environmental conditions can make the process of reinforcement corrosion on existing structures take place faster than predicted Our country has established a number of relevant standards for concrete and corroded RC structures, such as TCVN 3994:1985, TCVN 9139:2012, TCVN 9343:2012, TCVN 9346:2012, and TCVN 9348:2012 Until now, experimental studies on the mechanical behavior of corroded RC beams due to chloride ions are still limited The previous studies were only carried out on beam samples with relatively small dimensions and low degrees of reinforcement corrosion Moreover, experimental studies on repairing and strengthening corroded RC structures have not been conducted 1.3 Review of repairing and strengthening of corroded RC structures 1.3.1 Methods of repairing and strengthening corroded RC structures (a) Repairing methods - Repair concrete cover - Chemical waterproofing Corrosion inhibitors Surface coating Sprayed concrete Electrochemical treatment (b) Strengthening methods - Concrete/RC strengthening - Steel plate strengthening FRP (Fiber Reinforced Polymer) strengthening External prestressing 1.3.2 Structure of FRP material Commonly, fibers used are carbon fiber, glass fiber, and aramid fiber FRP materials are bonded together by polymer-based substrates such as epoxy, vinylester, or polyester adhesives (a) Specific weight FRP fibers have a specific gravity ranging from 1.7 to 2.6 g/cm3, to 4.5 times smaller than steel material with a specific gravity of 7.85 g/cm3 (b) Mechanical properties (tensile strength, elastic modulus) The stress-strain relationship of FRP material has a linear form from the beginning of tension until failure The failure is sudden and brittle CFRP materials have high tensile strength, in the range of 2400 - 4800 MPa, while the longitudinal strain is quite small at about 1.5% (c) Ability to withstand the impact of environmental conditions Studies have been carried out showing that the mechanical and physical properties of FRP materials are not significantly affected by environmental conditions (d) Fire resistance FRP is a synthetic material, so it is flammable, creating combustion products that are toxic to humans and the environment Therefore, it is necessary to take measures to prevent fire such as coating the surface with flame retardant paint to ensure safety when used 1.3.4 Research on RC structures strengthened with FRP sheets 1.3.4.1 Strengthening design standards - ACI 440.2R-17 fib 14 (2001) ISIS (2008) TR55 (2000) JSCE CES41 (2001) 1.3.4.2 Calculation procedure for strengthening RC structures in flexion Normally, the calculation process for strengthening RC structures with FRP sheets consists of 11 steps 1.3.4.3 Failure modes of RC beams strengthened with FRP sheets Vùng tập trung ứng suất Vị trí đứt FRP Hướng phát triển vết nứt (a) Dạng – Đứt FRP (d) Dạng – Bong tách lớp bê tông bảo vệ Vùng bê tông nén vỡ (b) Dạng – Phá hoại uốn (e) Dạng – Bong tách đầu Vết nứt nghiêng (c) Dạng – Phá hoại cắt (f) Dạng – Bong tách Fig 1.19 Typical failure modes of FRP-strengthened RC beams 1.3.4.4 Prediction models of FRP debonding strength - Tangential stress models - Concrete tooth models - Shear resistance models 1.3.4.5 Researches in Viet Nam It is a fact that Vietnam does not have design standards, as well as construction and acceptance standards for structures strengthened with FRP materials The previous studies have only been carried out on test samples with non-corroded reinforcement So far, experimental studies on strengthening corroded RC structures under chloride ions attacks are greatly limited 1.4 Conclusion of Chapter Chapter focuses on presenting three main contents, including (i) a review on the reinforcement corrosion in the structural members; (ii) a review of the flexural behavior of corroded RC beams; (iii) a review of repairing and strengthening of corroded RC structures In Vietnam, experimental studies to quantify the effect of reinforcement corrosion on the performance of RC structures are still limited, as well as a lack of scientific basis for calculating design to strengthen the loadcarrying capacity of the structure From that fact, " Experimental study on the flexural behavior of reinforced concrete beams damaged due to corrosion and strengthened with CFRP sheets " is a topic of scientific, practical, and urgent significance for the construction industry, especially for scientists and project management agencies CHAPTER – EXPERIMENTAL STUDY OF FLEXURAL BEHAVIOR OF CORRODED REINFORCED CONCRETE BEAMS 2.1 Setting up the accelerated corrosion test 2.1.1 Purpose Accelerated corrosion tests are electrochemically performed on testing samples with the aim of creating the desired corrosion degree in a much shorter time period than is practical, expressed in hours/day 2.1.2 Principle Electrochemical corrosion is the corrosion of metals due to the action of the electrolyte solution and the creation of an electric current (a) Electrolyte solution The electrolyte solution used in the electrochemical corrosion test must describe the salinity of seawater Therefore, this solution is made by diluting at the concentration of 35 g of NaCl into liter of water (b) Requirement for active current The smaller the applied voltage and current, the closer the corrosion state of the reinforcement is to reality, but the test time will be prolonged 2.1.3 Testing diagram The testing diagram, as illustrated in Fig 2.2, can be applied to two cases: (i) Beam specimens are immersed almost completely in salt water; (ii) Beam specimens are only partially submerged in salt water In the framework of this thesis, the tested sample and the beam specimens are immersed in the electrolyte solution, so that all the test samples have the same saturation state Trường hợp dầm thí nghiệm ngập nước Trường hợp dầm thí nghiệm trạng thái khơ ướt Dầm thí nghiệm Dung dịch NaCl 3.5% Thanh đồng Fig 2.2 Diagram of the accelerated corrosion test on the beam sample 2.1.4 Testing operation The test time is predicted based on the mass loss of metal due to corrosion according to Faraday's law in Eq (2.1), where I (A) is the amperage in the metal, t (second) is the corrosion time, M = 56 is the atomic mass of iron, n = is the number of electrons exchanged, F = 96500 is Faraday's constant I t  M m  (2.1) n F Based on the mass loss of metal due to corrosion ∆m (g), the degree of corrosion is determined by the coefficient c (%) according to Eq (2.2), where mo (g) is the initial mass before corrosion, m (g) is the remaining mass after corrosion m - m Dm c= o = (%) (2.2) mo mo 2.2 Accelerated corrosion test on tested specimens 2.2.1 Materials (a) Concrete In the laboratory, tested specimens were made of concretes with different compressive strengths corresponding to B30, B40, and B50 strength grades, respectively Table 2.1 Concrete mixes used Concrete Cement (kg) Sand (kg) Gravel (kg) Water (liter) B30 B40 B50 477 480 480 596 740 760 1250 1080 965 185 160 147 (b) Reinforcement Fly ash (kg) 60 85 Superplasticizer (liter) 5 W/C 0,39 0,33 0,31 The reinforcements used are steel rebars with a nominal diameter of d12 mm and the CB300-V grade according to TCVN 1651-2:2008 2.2.2 Tested specimens Tested specimens with dimensions of 150x150x150 mm are made of materials specified in section 2.2.1 The bonding length between steel reinforcement and concrete is 60 mm, and the thickness of the concrete cover is 69 mm 2.2.3 Accelerated corrosion test For the test specimens of concrete B30, the total time of the corrosion test was 312 hours For concrete B40, the test times for the three sets of specimens are 195, 290, and 366 hours, respectively For concrete B50, the test times for the three sets of specimens are 239, 334, and 413 hours, respectively Experimental results Bê tông B30 70 60 Mức độ ăn mòn, c(%) Mức độ ăn mòn, c(%) 80 50 40 30 20 10 Bê tông 40 Bê tơng B50 Mức độ ăn mịn, c(%) 2.2.4 2 1 0 M1 M2 M3 M4 M5 M6 Mẫu thí nghiệm M7 M8 M1 M9 M2 M3 M4 M5 M6 Mẫu thí nghiệm M7 M8 M9 M1 M2 M3 M4 M5 Mẫu thí nghiệm M6 M7 M8 (a) (b) (c) Fig 2.8 Corrosion degree of reinforcement in tested specimens: (a) Concrete B30; (b) Concrete B40; (c) Concrete B50 2.2.5 Determination of the correction factor for Faraday's law The relationship between the correction factor K and the concrete compressive strength is represented by a linear function, as shown in Fig 2.11 0.6 Hệ số hiệu chỉnh K 0.5 0.4 0.3 y = -0.0178x + 1.2033 R² = 0.9792 0.2 0.1 30 35 40 45 50 55 60 65 Cường độ chịu nén (MPa) Fig 2.11 Relationship between correction factor and concrete compressive strength 2.3 Accelerated corrosion test on RC beam specimens 2.3.1 Materials (a) Concrete 11 Fig 2.19 Load-deflection curves of corroded beams D5-C and D6-C Beam D5-C has the maximum load Pph=32.4 kN and the corresponding deflection at middle span fph=24.88 mm Beam D6-C has the maximum load Pph=31.9 kN and the corresponding deflection at middle span fph=26.69 mm (d) For specimen set IV (cm = 13,69% - 14,84%) Fig 2.20 Load-deflection curves of corroded beams D7-C and D8-C Beam D7-C has the maximum load Pph=33.1 kN and the corresponding deflection at middle span fph=29.80 mm Beam D8-C has the maximum load Pph=31.1 kN and the corresponding deflection at middle span fph=22.83 mm 3.2.4 Effect of longitudinal reinforcement corrosion on the flexural behavior of RC beams Fig 2.22 Comparison of load-deflection curves between control beams and corroded beams 12 2.5 Crack pattern of concrete on RC beams Fig 2.37 Crack pattern due to loading on control beam D1-NC Fig 2.38 Crack pattern due to loading on control beam D2-NC Fig 2.39 Crack pattern due to loading on corroded beam D3-C Fig 2.40 Crack pattern due to loading on corroded beam D4-C Fig 2.41 Crack pattern due to loading on corroded beam D5-C 13 Fig 2.42 Crack pattern due to loading on corroded beam D6-C Fig 2.43 Crack pattern due to loading on corroded beam D7-C Fig 2.44 Crack pattern due to loading on corroded beam D8-C 2.6 Conclusion of Chapter - When the corrosion degree of longitudinal reinforcement ranges between - 6%, there is little effect on the flexural capacity and deflection at the failure of the beam - When the corrosion degree of longitudinal reinforcement ranges between - 10%, the load-carrying capacity of the beam is reduced by about 12.8%, corresponding to the residual capacity of 87.2% compared to the control beam - When the corrosion degree of longitudinal reinforcement ranges between 13 - 15%, the load-carrying capacity of the beam can be reduced by 19.2% compared to the control beam, which has the potential for brittle failure due to the longitudinal steel bar rupture caused by pitting corrosion - At low load levels, the deflection at the middle span increases with increasing corrosion degree of longitudinal reinforcement However, with corrosion degrees of - 15%, there is little effect on the deflection at failure, but deflection values tend to be dispersed among beams with high corrosion degrees 14 CHAPTER – EXPERIMENTAL STUDY OF FLEXURAL STRENGTHENING OF REINFORCED CONCRETE BEAMS USING CFRP SHEETS 3.1 Experimental program of corroded RC beams strengthening 3.1.1 Materials (a) Concrete The concrete used is designed with a B30 strength grade The test result shows that the mean compressive strength is 40.9 MPa, with a standard deviation of 2.4 MPa and a coefficient of variation of 1.6% (b) Reinforcement To fabricate the beam specimens, the reinforcement used has the same origin and the same production batch as the reinforcement used to fabricate the corroded beams in Chapter (c) Strengthening material In this study, unidirectional CFRP sheets, product code UT70-20G of Toray, Japan were used to strengthen the flexural capacity of corroded RC beams Table 3.1 Mechanical properties of CFRP sheets Tensile strength (MPa) 3400 Modulus of elasticity (GPa) 245 Specific weight (g/cm2) 200 Proportion (g/cm3) Thickness (mm) 1,8 0,111 3.1.2 Strengthened beam specimens    Specimen set V: two beams, D9-CFRP and D10-CFRP, strengthened with a single CFRP sheet having b = 150 mm and L = 2000 mm Specimen set VI: two beams, D11-CFRP and D12-CFRP, strengthened with a single CFRP sheet having b = 150 mm and L = 1400 mm Specimen set VII: two beams, D11-CFRP and D12-CFRP, strengthened with a single CFRP sheet having b = 150 mm and L = 1000 mm 3.1.3 Flexural-strengthening process of corroded RC beams using CFRP sheets Step 1: Prepare the surface for repairing corrosion-induced cracks Step 2: Make drill holes and install the cylinders Step 3: Seal all surfaces of cracks with adhesive Step 4: Pump two-component glue to fill the internal cracks Step 5: Bond the CFRP sheet on the surface of the beam 3.2 Experimental program of flexural behavior of CFRPstrengthened corroded RC beams 3.2.1 Testing purpose Four-point bending test was performed on each beam specimen for the purpose of determining the mechanical behavior of the corroded beam 15 strengthened with CFRP sheets Then, the effectiveness of the CFRPstrengthening method can be determined 3.2.2 Testing diagram The four-point bending test is carried out according to the same scheme as for the control and corroded beams in Chapter 3.2.3 Relationship between load and deflection (a) For specimen set V: beams D9-CFRP and D10-CFRP Beam D9-CFRP has the maximum load Pph=59.9 kN, the corresponding deflection fph=24.23 mm, and ultimate deflection fu=33.73 mm Beam D10CFRP has the maximum load Pph=59.1 kN, the corresponding deflection fph=23.19 mm, and ultimate deflection fu=37.79 mm 60 Tải trọng P (kN) 50 40 30 D9-CFRP 20 D10-CFRP 10 0 10 20 Độ võng f (mm) 30 40 Fig 3.16 Load-deflection curves of beams D9-CFRP and D10-CFRP (b) For specimen set VI: beams D11-CFRP and D12-CFRP Beam D11-CFRP has the maximum load Pph=47.3 kN, the corresponding deflection fph=15.82 mm, and ultimate deflection fu=22.46 mm Beam D12CFRP has the maximum load Pph=45.2 kN, the corresponding deflection fph=9.59 mm, and ultimate deflection fu=22.57 mm 60 Tải trọng P (kN) 50 40 30 D11-CFRP 20 D12-CFRP 10 0 10 15 Độ võng f (mm) 20 25 Fig 3.17 Load-deflection curves of beams D11-CFRP and D12-CFRP (c) For specimen set VII: beams D13-CFRP and D14-CFRP Beam D13-CFRP has the maximum load Pph=41.5 kN, the corresponding deflection fph=8.14 mm, and ultimate deflection fu=16.34 mm Beam D14CFRP has the maximum load Pph=39.3 kN, the corresponding deflection fph=6.79 mm, and ultimate deflection fu=15.50 mm 16 50 Tải trọng P (kN) 40 30 D13-CFRP 20 D14-CFRP 10 0 10 Độ võng f (mm) 15 20 Fig 3.18 Load-deflection curves of beams D13-CFRP and D14-CFRP 3.3 Analysis of experimental results 3.3.1 Load-carrying capacity 60 Tải trọng P (kN) 50 40 30 20 10 D9-CFRP D10-CFRP D11-CFRP D12-CFRP D13-CFRP D14-CFRP 0 10 20 Độ võng f (mm) 30 40 Fig 3.19 Comparison of load-deflection curves between strengthened corroded beams 3.3.2 Deflection When decreasing the CFRP length from 2000 to 1000 mm, the deflection at maximum load decreased from 23.7 to 7.5 mm, corresponding to a reduction of 68.4% Meanwhile, the ultimate deflection also decreased from 35.8 to 15.9 mm, corresponding to a decrease of 55.6% 37.8 40 35 33.7 Độ võng (mm) 30 25 20 15 10 24.2 23.2 22.6 22.5 16.3 15.8 9.6 8.1 15.5 6.8 D9-CFRP D10-CFRP D11-CFRP D12-CFRP D13-CFRP D14-CFRP Dầm TN Fig 3.20 Comparison of deflection at maximum load and ultimate deflection between strengthened beams 17 3.4 Conclusion of Chapter - The CFRP strengthening method can be applied to improve the load- - - - carrying capacity of corroded RC beams with - 10% corrosion degrees since the residual bearing capacity is 75% greater than that of the control beam and the concrete cover has not been detached, although occurred a large number of corrosion-induced cracks By bonding the CFRP sheet at the bottom of the beam, the flexural capacity of strengthened beams increases from 1.10 to 1.62 times that of the control beam, and from 1.26 to 1.85 times that of the corroded beam For corroded beams, it is recommended to bond CFRP sheets on the entire beam span to achieve maximum performance on flexural strengthening The failure mechanism of the beam strengthened with the CFRP length equivalent to beam span is the intermediate crack debonding and the flexural failure that is characterized by the crushing of concrete in the compression zone Meanwhile, when the CFRP length is reduced, the failure can change from flexural mode to flexural-shear mode, characterized by detachment of the concrete cover at the bottom of the beam, and web-shear cracks, at the same time, the concrete in the compression zone is broken The initial stiffness of the strengthened beam is equivalent to that of the control beam However, the deflection at maximum load and the ultimate deflection of the beam were significantly reduced (55.6 – 68.4%) when reducing the CFRP length from to 0.5 beam span CHAPTER – NONLINEAR FINITE ELEMENT ANALYSIS OF FLEXURAL BEHAVIOR OF CORRODED RC BEAMS STRENGTHENED WITH CFRP SHEETS 4.1 Introduction Based on the experimental results of the load-displacement curve, crack pattern, and failure mechanism, the data are used to verify the numerical model and develop a series of numerical models Then, a parametric study was carried out to investigate the influence of designoriented parameters on the flexural behavior of CFRP-strengthened corroded RC beams 4.2 Summary of the experimental program 4.2.1 Materials and beam specimens Six beam specimens were selected to expand the study of the behavior of strengthened corroded RC beams using the finite element (FE) method Set I consists of two control beams D1-NC and D2-NC Set III consists of two corroded beams D5-C and D6-C Set V consists of two corroded beams strengthened with CFRP sheet, D9-CFRP and D10-CFRP 18 4.2.2 Experimental result The experimental results of six beam specimens are summarized in Fig 4.2 Fig 4.2 Load-displacement curves obtained from -4-point bending test 4.3 Nonlinear finite element model 4.3.1 Definition of finite element Fig 4.3 FE model of corroded RC beam strengthened with CFRP 4.3.2 Constitutive model of materials 4.3.2.1 Concrete modeling (a) Un-damged concrete (b) Damaged concrete Fig 4.6 Constitutive model of concrete 4.3.2.2 Steel-concrete interface modeling Fig 4.7 Constitutive model of steel-concrete bond 19 4.3.2.3 Support and loading points modeling In the three-dimensional model, steel material with the properties of an isotropic elastic material was used, with a modulus of elasticity of 210 GPa and a Poisson coefficient of 0.3 4.3.2.4 CFRP and concrete interface modeling (a) Stress – strain relationship (b) CFRP – concrete bond Fig 4.8 Constitutive model of CFRP-concrete bond 4.3.2.5 Reinforcement modeling (a) Non-corroded reinforcement (b) Corroded reinforcement Fig 4.9 Constitutive model of steel reinforcement 4.3.3 Validation of FE models 4.3.3.1 Sensitivity analysis of mesh The model with a mesh size of 20 mm showed an acceptable time of calculation with high accuracy, therefore it was selected for FE model validation and parametric study 4.3.3.2 Validation of FE models of control and corroded beams (a) (b) Fig 4.12 Comparison of load-displacement curves from experiment and FE model: (a) Control beams; (b) Corroded beams 20 The predicted load-carrying capacity of both sets of specimens through the FE model indicates that the difference between the two approaches is not significant, with the largest difference of about 12% (a) (b) Fig 4.13 Stress in steel reinforcement at the beam failure: (a) FEM-D1-NC and (b) FEM-D5-C (a) (b) Fig 4.14 Comparison of crack pattern due to loading obtained from experiment and FE model: (a) Beam D1-NC; (b) Beam D5-C 4.3.3.3 Validation of CFRP-strengthened corroded beam model Fig 4.15 shows that the load-displacement curves of the modeled beams are almost similar to the experimental beams The failure mechanism of beams D9-CFRP and D10-CFRP is divided into three stages and compared with the modeled beams, as shown in Fig 4.16 Fig 4.15 Comparison of load-displacement curves of CFRP-strengthened corroded beams from experiment and FE model 21 (a) Giai đoạn Sự liên kết vết nứt uốn vết nứt dọc cốt thép ăn mòn (b) Giai đoạn (c) Giai đoạn Fig 4.16 Crack pattern of CFRP-strengthened corroded beams obtained from FE model at failure stages 4.4.1 Concrete compressive strength, longitudinal reinforcement ratio, and bond strength Figs 4.19 and 4.20 show the results obtained from the parametric study in the FE model (a) (b) Fig 4.19 Load-displacement curves when changing (a) Concrete compressive strength; (b) Ratio of longitudinal reinforcement Fig 4.20 Load-displacement curves of CFRP-strengthened corroded beams when changing the bond strength 4.4.2 CFRP bonding scheme 22 The obtained results from investigations of CFRP bonding schemes are shown in Figs 4.22 – 4.24 (a) (b) Fig 4.22 Load-displacement curves when changing (a) CFRP bonding scheme; (b) CFRP length (a) (b) Fig 4.23 CFRP bonding scheme and crack pattern at the beam failure: (a) Sided bonding; (b) U-wrap anchorages (a) (b) Fig 4.24 Crack patterns and shear stress distribution in the CFRP sheet at the failure: (a) L = 1400 mm; (b) L = 1000 mm 4.5 Conclusion of Chapter - For the same configuration of the beam, the flexural behavior of CFRPstrengthened corroded beams is strongly influenced by the compressive 23 - - - - strength of concrete rather than the ratio of longitudinal reinforcement, because the beam specimen is fractured when the concrete in the compression zone is broken Strengthened corroded beams can be more ductile when higher strength concrete is used, compared with increasing longitudinal reinforcement content On the other hand, the residual load after CFRP debonding is larger with increasing reinforcement content When reducing 50% of corroded reinforcement-concrete bond strength, the strengthened beam still works similarly to the beam with a reduced bond strength of 13.5% However, when the bond strength is severely reduced (90%), the beam becomes more brittle, and the failure occurs due to the development of flexural cracks For corroded beams, flexural strengthening is only effective when the CFRP length is large enough If the CFRP length ranges from 50-70% of the beam span, the failure can occur suddenly due to excessive propagation and opening of shear cracks, which commonly appeared in shear failure U-wrap anchorages contribute significantly to limiting the CFRP debonding at the plate-ends, thus allowing an increase of nearly 14% in the load-carrying capacity of strengthened beams Furthermore, using sided bonding technique of CFRP sheets on the opposing sides of the beam, the development of vertical cracks in the tension zone is limited, while the beam maintains the flexural failure due to yielding of the longitudinal reinforcement and crushing of the concrete in the compression zone CONCLUSION CONCLUSION  The thesis has provided a data set obtained on a total of 27 tested samples and 14 beam specimens, with different degrees of corrosion, by applying the accelerated corrosion test in actual environmental conditions in Vietnam  The load-carrying capacity of the corroded beams with corrosion degrees of - 6% does not change significantly compared to the control beam When the corrosion degree of longitudinal reinforcement ranges from - 10%, the flexural capacity of corroded beams is reduced by about 12.8%, corresponding to 87.2% of the control beam Meanwhile, for beams with corrosion degrees of 13-15%, the flexural capacity can be reduced by 19.2%, corresponding to 80.8% of the control beam, and there is a potential for brittle failure due to pitting corrosion  The method of flexural strengthening using CFRP sheets has been 24   implemented on beam specimens with corrosion degrees of 10% As a result, the load-carrying capacity of strengthened beams increases from 1.10 to 1.62 times that of the control beam, and from 1.26 to 1.85 times that of the corroded beam This study recommended that the CFRP sheet is bonded on the entire beam span to achieve maximum performance and limit the change of failure mode The failure mechanism of CFRP-strengthened corroded beams is the intermediate crack debonding In the corroded beam, the steel reinforcement-concrete bond is reduced, and the cracks due to corrosion appear along the length of the steel bar When the cracks due to loading cross the corrosion-induced cracks in the shear span, the tensile stress in concrete is transferred to the CFRP sheet, resulting in the initiation of the debonding process The failure mode of the beam can change, which depends on the bonding technique and the CFRP length The nonlinear FE model built in DIANA FEA allows accurately describing the flexural behavior of the experimental beams, especially the failure mechanism due to debonding on CFRPstrengthened corroded beams Then, the parametric study determines the effect of concrete compressive strength, longitudinal reinforcement ratio, deteriorated bond between concrete and corroded reinforcement, and strengthening scheme on the behavior of corroded and strengthened beams OUTLOOK ON FURTHER RESEARCH  Further research for RC beams with larger dimensions and higher corrosion degrees of reinforcement At the same time, the effect of shear reinforcement corrosion on the behavior of corroded RC beams is needed to investigate  Research can be expanded for strengthening both flexural and shear strength of corroded RC beams LIST OF PUBLICATIONS RELATED TO THE THESIS 1] Tran Hoai Anh, Nguyen Thanh Quang, Nguyen Ngoc Tan, Nguyen Hoang Giang, Tran Anh Dung (2018), “An experimental study to identify the influence of steel corrosion on concrete – steel bond”, Proceedings of 7th International Conference on Protection of Structures against Hazards, Hanoi, Vietnam, 511-518 ISBN: 978-981-11-7777-4 [2] Trần Hoài Anh, Nguyễn Ngọc Tân, Nguyễn Hoàng Giang (2019), “Nghiên cứu thực nghiệm khả chịu lực dầm bê tông cốt thép bị ăn mịn mơi trường xâm thực clorua”, Tạp chí Xây dựng Việt Nam, 09 2019, 81-86 ISSN: 2734-9888 [3] Trần Hoài Anh, Nguyễn Ngọc Tân, Nguyễn Hoàng Giang (2019), “Một số đặc điểm vết nứt dầm bê tơng cốt thép bị ăn mịn mơi trường xâm thực clorua”, Tạp chí Xây dựng Việt Nam, 10 2019, 101-107 ISSN: 2734-9888 [4] Trần Hoài Anh, Nguyễn Ngọc Tân, Nguyễn Hoàng Giang (2021), “Nghiên cứu thực nghiệm hiệu gia cường kháng uốn dầm bê tơng cốt thép bị ăn mịn sợi composite CFRP”, Tạp chí Khoa học Cơng nghệ Xây dựng, 15 (1V), 1-16 ISSN: 1859-2996 https://doi.org/10.31814/stce.nuce202115(1V)-01 [5] Tran Hoai Anh, Nguyen Ngoc Tan, Nguyen Trung Kien, Nguyen Hoang Giang (2021), “Finite element analysis of the flexural behavior of corroded RC beams strengthened by CFRP sheets”, International Journal of GEOMATE, 21 (88), 42-47 ISSN: 21862982 https://doi.org/10.21660/2021.88.gxi255 [6] Nguyen Trung Kien, Tran Hoai Anh, Nguyen Ngoc Tan, Nguyen Hoang Giang, Phuong Tran (2022), “Nonlinear finite element analysis of corroded RC beams strengthened by CFRP sheets”, Composite Structures (under review) ISSN: 0263-8223 ... Giang (2021), ? ?Nghiên cứu thực nghiệm hiệu gia cường kháng uốn dầm bê tơng cốt thép bị ăn mịn sợi composite CFRP? ??, Tạp chí Khoa học Cơng nghệ Xây dựng, 15 (1V), 1-16 ISSN: 1859-2996 https://doi.org/10.31814/stce.nuce202115(1V)-01... [2] Trần Hoài Anh, Nguyễn Ngọc Tân, Nguyễn Hoàng Giang (2019), ? ?Nghiên cứu thực nghiệm khả chịu lực dầm bê tơng cốt thép bị ăn mịn mơi trường xâm thực clorua”, Tạp chí Xây dựng Việt Nam, 09 2019,... Experimental results Bê tông B30 70 60 Mức độ ăn mòn, c(%) Mức độ ăn mòn, c(%) 80 50 40 30 20 10 Bê tông 40 Bê tơng B50 Mức độ ăn mịn, c(%) 2.2.4 2 1 0 M1 M2 M3 M4 M5 M6 Mẫu thí nghiệm M7 M8 M1 M9