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NUMERICAL AND ANALYTICAL MODELING OF CONCRETE CONFINED WITH FRP WRAPS.
Abstract
This thesis is intended at studying and comparing empirical models that have been proposed for the modeling of the stress-strain response of a FRP confined concrete subjected to axial load. An attempt has been made to model the experimental set up for the compression test of a concrete cylinder confined with FRP sheet in AbaqusCAE. The results so obtained have been compared and analyzed against the experimental test results & the results obtained from a chosen mathematical model (Modified Lam & Teng). An attempt was made to create a new material model in Opensees that follows the chosen mathematical model. However, this was not achieved due to the reasons that will be explained in the later sections.
Reinforced concrete confined with steel is typically designed by considering the Manders model (Mander et al., 1988), which assumes a constant confining pressure. This is true with the case of steel as it is a ductile material and one assumes the steel to be yielded. However with the case of FRP jackets, this is not true. FRP is a linear elastic and brittle material and does not yield, which makes the Manders model inaccurate for its analysis. Many models have been proposed which take into account the increasing confining pressure due to the FRP wrap. A comparative study of the constitutive models proposed for FRP confined reinforced concrete has been done in this study.
Finally after a series of numerical interpretations of different specimens and their comparison with the experimental data, the utility and accuracy of the new modified Lam & Teng’s model was validated. The validation process included comparison and analytical data obtained via finite element simulation in Abaqus, empirical model results and the experimental data.
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Contents
2.2 Mechanism for Concrete Confinement by Transverse Reinforcement 4
2.3 Modeling of Concrete in Compression 6
2.3.1 Modified Hognestad Model: 6
2.4 Stress-Strain Response of FRP-Confined Concrete 9
2.5.1 Samaan and Mirmiran Model (1998) 16
2.5.1 Mander’s Model (1984) 18
2.5.3 Lam & Teng Model (2003) 24
2.5.4 Modified Lam & Teng (Liu et al., 2013) 26
2.5.5 Drucker-Prager Plasticity Model 30
Chapter 3 Experimental Database & Preliminary results 35
Chapter 4 AbaqusCAE Finite Element Modelling 45
4.3 Fiber Reinforced Polymeric Jacket 48
4.4.2. Boundary Conditions & Analysis Step 49
Chapter 5 Comparison Study and Analysis of Results 52
5.2 Performance of the Drucker-Prager Model 54
5.3 Performance of the Modified Lam & Teng model 57
5.3 Field Retrofitting cases 66
Appendix A : Numerical and Analytical modeling results (Tabular) 76
Appendix B : Numerical and Analytical modeling results (Graphical) 79
Appendix C : Graphical comparison with Confinement ratios 106
Appendix D-1 : Bridge Retrofitted Data 107
Appendix D-2 : CalTrans retrofitting guidelines 109
Chapter 1 Introduction
1.1 Introduction
Today, many reinforced concrete structures are in a bad condition. According to the ASCE report card 2013 for America’s infrastructure, one in nine of the bridges in the United States is structurally deficient. (2013 Report card for America’s Infrastructure,ASCE) The report also mentions that the average age of the bridges in the country is 42 years. Most of them need some rehabilitation and repair work to either restore them to their full capacity or to increase their design capacity in order to meet their growing demand.
Causes of deterioration can range from corrosive environmental conditions, damage due to natural cause such as earthquakes & tornadoes or by human factors such as traffic accidents, use of substandard quality of construction material, faulty construction practices or increase in the load demand for the structure.
Indication of a deteriorated reinforced concrete column is the spalling action of the concrete cover leading to exposure of the steel reinforcement in the column which leads to corrosion of the steel, eventually leading to reduced performance of that structure element. With respect to deteriorated reinforced concrete columns, one could conclude that the causes stated above result in deterioration because of lack of lateral confinement. The longitudinal reinforcement in the reinforced concrete columns provide very little lateral confinement effect, which is not adequate for most loading conditions.
As a structural designer one always tries to design the reinforced concrete structures in a manner so that they exhibit ductile behavior. Lateral confinement in a reinforced concrete column provides the column with the required ductility. Under seismic loading, this additional confinement could ensure adequate strength for the column and increase its deformation capacity which improves its performance in an event like an earthquake. (Park et al., 1982; Mander et al., 1988; Shams & Saadeghvaziri, 1997)
Many confinement techniques have been developed over the years; designing the columns with steel hoops (stirrups) or by providing steel jacketing techniques. The steel jacketing technique has been proved quite useful in the field of retrofitting the columns. However, corrosion of the steel can be of concern. It also increases the self-weight of the structure to a great extent which is always a tradeoff. In situations where the concrete cover is very loose and weak one cannot use the steel jacketing techniques as it might damage the column even more due to the bolting of the jackets.
During recent decades, many researchers have been trying to replace the conventional steel jacketing technique by usage of fiber reinforced polymer (FRP) wraps. FRP wraps used as confinement can increase the ultimate compressive strength and the ultimate strain of the concrete. (Samaan et al.,1998; Toutanji, 1999). A lot of research has been carried out on developing a retrofitting technique with these FRP wraps. The main advantages these FRP wraps possess over the steel jackets are very high strength to weight ratio & high resistivity to corrosion.
1.2 Scope of the study
The objective is achieved and restricted within the following scope of study:
- Literature review to identify and choose the most relevant models for modeling of concrete confined with fiber reinforced polymers in compression.
- Modelling and finite element analysis of the confined concrete compression test in AbaqusCAE.
- Developing the stress-strain curve from several proposed empirical model (Modified Lam
& Teng).
- Survey of experimental data on confined concrete with FRP in order to generate an experimental database.
- Comparison of the analytical results in order to define the strengths and limitations of the empirical model chosen to study.
- Validation of the model chosen and a study on its relevance for use in typical bridge columns retrofitted with FRP jackets.
Proposing & validating changes to the empirical model for more accurate results.
NUMERICAL AND ANALYTICAL MODELING OF CONCRETE CONFINED WITH FRP WRAPS.