Abstract
Fiber reinforced polymers (FRPs) are used to confine concrete columns and other structural members to enhance the ductility and strength of concrete by providing passive lateral confinement. Extensive theoretical and experimental studies have been conducted on passively confined concrete columns. Even more research has been focused on uniform passive confinement, but research on non-uniform confined columns has been neglected by comparison. This is especially evident from the lack of a reliable triaxial test designed for concrete cubes under non-uniform passive confinement. This type of test is required for developing constitutive material models, and it is also needed to verify the accuracy of currently existing developed models. To date, researchers have used active confined triaxial tests for this purpose. However, these tests are not a consistent measure because the strength and dilation behavior of concrete under active confinement differs under passive confinement. Therefore, designing a test facility and method for conducting triaxial tests of concrete under non-uniform passive confinement is a necessity. Also, extensive non-uniform triaxial tests including versatile concrete grades are needed for developing constitutive models to be used in the numerical analysis of passively confined structural members. Moreover, most of the dilation properties reported so far for square confined columns are by means of strain gauges. Therefore, obtaining the dilation of square columns by a more up-to-date technology is the other necessity in this field.The basic research presented in this thesis begins with an experimental study conducted to examine the stress-strain and dilation behaviors of square concrete columns confined by FRPs. Three different concrete grades were selected from a total number of 84 specimens. The effect of corner radius and number of FRP piles were selected as test parameters. To capture accurate dilation in the corner of columns, the digital image correlation (DIC) system is used. Therefore, three-dimensional dilation behavior was captured at mid-height of square columns. The achieved test results were used to assess the performance and limitations of existing plasticity and damage plasticity models in capturing the strength and dilation of square FRP confined columns. Finite element simulation in this part emphasizes the need for performing true biaxial and triaxial compression tests on concrete cubes. Such tests are essential for calibrating the plasticity model parameters and also for studying the load path dependency of concrete under non-uniform passive confinement. However, no true triaxial test for concrete under non-uniform passive confinement has been developed until now.
Second, a new dilation model was developed as a function of the lateral stiffness ratio only by using FRP confined circular columns. This model was used as the foundation for a new analysis oriented model. The new model allows civil engineering researchers and designers to capture the dilation behavior of the low level FRP confined columns better than extant models. Also, the stress-strain relationship of highly confined columns is easier to capture and the results are clearer than before.
Third, a novel biaxial and triaxial test method was designed and its facility was fabricated. The test results conducted on triaxial concrete cubes were validated by comparing the new results to test results of tests on typical FRP confined columns. Both softening and hardening stress-strain behavior can now be captured accurately. Meanwhile, a method was developed and implemented for decreasing the friction in triaxial compression tests. Also, a method was developed and implemented for calibration of measurement systems, especially for capturing dilation of concrete under non-uniform passive confinement. The invented and implemented test method/facility provides the fundamental and crucial utility for developing and verifying constitutive material models which are proposed for concrete under triaxial state of stress with passive lateral confinements.
Then, by means of the newly fabricated facility, an extensive experimental program was conducted for biaxial and triaxial concrete cube compression tests. More than 100 triaxial concrete cubes were tested under monotonic and/or cyclic compression loads. A variety of concrete grades with different levels of non-uniform passive confinements were designed as test parameters. The results were used to validate the accuracy of existing damage plasticity models. Also, a new effective lateral stiffness ratio has been developed to be used for three-dimensional finite element analysis of confined concrete columns. Moreover, a plastic damage model was developed for concrete under non-uniform passive confinement. The hardening function of the proposed model is a function of concrete grade. The flow rule is only a function of the lateral stiffness ratio. Therefore, the development of such a model is independent of lateral pressure. With the same proposed methodology, it is possible to develop other plasticity based models by relying only on passively confined concrete column tests.
Finally, a modified extended linear Drucker Prager (DP) plasticity model is introduced for finite element modeling of concrete under non-uniform passive confinement. A new form of the hardening and flow rule was developed where the main independent variable is only selected as the lateral stiffness ratio instead of confining pressure to define the hardening law and flow rule. A new approach was also developed for defining hardening, which simplifies the explicit hardening function. While developing this new model, we only used passive confined cube tests. However, this model can be applied to uniform and non-uniform confined concrete.
| Date of Award | 7 Nov 2017 |
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| Original language | English |
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| Supervisor | C W LIM (Supervisor) & Yufei WU (Supervisor) |