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 CORROSIO
Trang 1MINISTRY 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
Trang 2The 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
Trang 3Introduction
1 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
2 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
3 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 5 - 15% Corroded beam specimens are strengthened with externally bonded CFRP (Carbon Fiber Reinforced Polymer) sheets
Trang 4also proposed to simulate the flexural behavior of corroded beams and CFRP-strengthened beams
5 Research methodology
- Literature review approach
- Experimental approach
- Modeling approach
6 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
7 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
8 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
Trang 5CHAPTER 1 – 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 non-corroded 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
Trang 61.2.2 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
- 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, 3 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
Trang 7CFRP 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
1.3.4 Research on RC structures strengthened with FRP sheets
1.3.4.1 Strengthening design standards
1.3.4.3 Failure modes of RC beams strengthened with FRP sheets
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
Trang 8It 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 1
Chapter 1 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 carrying capacity of the structure From that fact, " Experimental study
load-on the flexural behavior of reinforced cload-oncrete 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 2 – 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 1 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
Trang 92.1.3 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
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 =
2 is the number of electrons exchanged, F = 96500 is Faraday's constant
I t M m
Table 2.1 Concrete mixes used Concrete Cement
(kg)
Sand (kg)
Gravel (kg)
Water (liter)
Fly ash (kg)
plasticizer (liter)
Trang 10The 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 2.2.4 Experimental results
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
Fig 2.11 Relationship between correction factor and concrete compressive
0 0.1 0.2 0.3 0.4 0.5 0.6
Trang 11The beam specimens are made of concrete with compressive strength grade B30, with the composition as shown in Table 2.1 The results of the compression test are presented in Table 2.7
Table 2.7 Compressive strength of concrete B30 at 28 days Specimen P (kN) R n (MPa) m (MPa) s (MPa) cv (%)
2.3.2 Experimental beam specimen
The beam specimens are made of reinforced concrete with dimensions
of 150x200x2200 mm In this chapter, eight beam specimens were fabricated and maintained under the same environmental conditions
Fig 2.12 Dimensions and layout of the beam specimen
2.3.3 Accelerated corrosion test
Set I: two control beam specimens, denoted D1-NC and D2-NC;
Set II: two corroded beam specimens, denoted D3-C and D4-C, have corrosion degrees of longitudinal reinforcement of 5 - 6%;
Set III: two corroded beam specimens, denoted D5-C and D6-C, have corrosion degrees of longitudinal reinforcement of 9 – 10%;
Set IV: two corroded beam specimens, denoted D7-C and D8-C, have corrosion degrees of longitudinal reinforcement of 13 - 15%
2.4 Experimental program of flexural behavior of corroded RC beams
2.4.1 Testing purpose
Four-point bending test was performed on each beam specimen to determine the characteristics of flexural behavior, such as load-deflection curve, cracking load, load-carrying capacity (maximum
2200
1 2Ø12
1 2Ø12
2 Ø6a150
2 Ø6a150
1 2Ø12
Trang 12load), deflections at first crack and failure, crack pattern, and failure mode of beam
2.4.2 Experimental diagram
Fig 2.15 Diagram of 4-point bending test 2.4.3 Relationship between load and deflection
(a) For specimen set I (control beam)
Fig 2.17 Load-deflection curves of control beams D1-NC and D2-NC
Beam D1-NC has the maximum load P ph=35.0 kN and the corresponding
deflection at middle span f ph=24.74 mm Beam D2-NC has the maximum load
P ph =38.5 kN and the corresponding deflection at middle span f ph=24.82 mm
Fig 2.18 Load-deflection curves of corroded beams D3-C and D4-C
Beam D3-C has the maximum load P ph=36.2 kN and the corresponding
deflection at middle span f ph=28,25 mm Beam D4-C has the maximum load
P ph =35.7 kN and the corresponding deflection at middle span f ph=28.24 mm
Trang 13Fig 2.19 Load-deflection curves of corroded beams D5-C and D6-C
Beam D5-C has the maximum load P ph=32.4 kN and the corresponding
deflection at middle span f ph=24.88 mm Beam D6-C has the maximum load
P ph =31.9 kN and the corresponding deflection at middle span f ph=26.69 mm
Fig 2.20 Load-deflection curves of corroded beams D7-C and D8-C
Beam D7-C has the maximum load P ph=33.1 kN and the corresponding
deflection at middle span f ph=29.80 mm Beam D8-C has the maximum load
P ph =31.1 kN and the corresponding deflection at middle span f ph=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