Application of fiber-reinforced polymer (FRP) composites as a confining material for concrete, for retrofitting of critical structures as well as for the construction of concrete filled FRP-tubes as earthquake-resistant columns in new construction already have been popular because of the beauty of FRP for providing greater capacity. The reliable design of these structural members for retrofitting and against earthquake-induced forces necessitates a clear understanding of the stress-strain behavior of FRP-confined concrete under load cycles., especially under cyclic axial compression for the seismic retrofit. Although the monotonic axial stress-strain behavior of FRP-confined concrete has been studied extensively over the past two decades, only a few studies have so far investigated the behavior of FRP-confined concrete under cyclic axial compression. Besides there have been only few studies done on FRP confined HSC based on limited experimental data which led to some research gaps and questions. This study is primarily aiming at filling existing research gaps and proposing ideas for further research. The thorough study of existing stress-strain model of FRP confined concrete has led to number of significant conclusions including proposals for the further study in that field. Not unlike that of FRP, the popularity of high-strength concrete (HSC) in the construction industry has steadily increased during the last two decades. HSC structural members are known to exhibit brittle behavior, which jeopardizes their use in seismically active regions. So far there are only few articles for answering the stress-strain relationship of HSC. The behavior of FRP- confined HSC and FRP-confined NSC under monotonic and cyclic axial compression will be reviewed and compared based on other researcher’s articles for the proposal of further study and filling out of the existing research gaps The experimental program done by Togay Ozbakkaloglu and Emre Akin was taken as primary source for this study and the results have been analyzed and compared with the other existing stress-strain model. The experimental program was consisting of a total of 24 FRP-confined concrete cylinders with a concrete core diameter of 152.5 mm and a height of 305 mm which were manufactured and tested under axial compression. The test parameters included the concrete compressive strength (NSC and HSC), type of FRP material (CFRP and AFRP), FRP thickness (2 to 6 layers), and loading pattern (axial monotonic and axial cyclic). AFRP was used as confinement for both NSC and HSC where as CFRP are used for HSC. Stress-Strain Behavior of FRP-Confined Normal- and High-Strength Concrete under Monotonic and Cyclic Axial Compression SHARIFUL ISLAM MS in Civil Engineering, Department of Civil Engineering University of Toledo INTRODUCTION EXPERIMENTAL RESULTS (OZBAKKALOGLU ET. AL) ABSTRACT ACKNOWLEDGEMENT I would like to express my deepest gratitude to my instructor Dr. Azadeh Parvin for her unwavering support and mentorship CONCLUSION • The axial cyclic stress-strain model proposed by Lam and Teng (2009) is highly accurate in predicting both the unloading and reloading paths of FRP- confined NSC. The model closely predicts the shapes of the unloading and reloading curves and accurately estimates the plastic strains for NSC. • When the model is applied to HSC specimens of the experimental program, on the other hand, the model predictions deviate significantly from the experimental results; this is caused largely by the inaccuracies in the calculation of the plastic strains of FRP-confined HSC. Plastic strain equation for HSC was calibrated based on the extremely limited experimental data reported by Rousakis (2001) on CFRP-confined HSC and this might be the reasons behind this inaccuracies. • The predictions of the cyclic stress- strain model proposed by Shao et al. (2006) indicated that this model predicts the reloading paths reasonably accurately, but consistently overestimates the residual plastic strains and does not accurately capture the shape of the unloading paths. Envelop Curve Experimental program by Togay Ozbakkaloglu and Emre Akin shows that the envelope stress-strain curves of cyclically loaded specimens closely follow the stress- strain curves of the corresponding monotonically loaded specimens which was also reported by Lam et al. (2006) and Abbasnia and Ziaadiny (2010). Unloading and Reloading Paths and the Plastic Strain Lam et al. (2006) demonstrated that the relationship between unloading strains ε un;env and plastic strains ε pl was linear for CFRP- confined NSC cylinders for 0.001≤ε un;env ≤0.0035 and ε un;env ≥0.0035. This observation was then supported by that of Abbasnia and Ziaadiny (2010), which was based on an experimental investigation of CFRP-confined NSC square prisms. The experimental results also in agreement with the previousproposal by other researchers. Axial Stress-Strain Behavior • For the monotonic loading, there is an ascending first branch in the stress- strain diagram followed by another ascending or, almost flat second branch for NSC whereas, for HSC, there is a sudden drop (Strength Softening) at the transition point due to the brittle nature of HSC. • Higher compressive strength and ultimate axial strain was observed for higher number of load cycles for NSC in case of cyclic loading which is in harmony with other researchers (Rousakis 2001; Lam et al. 2006). On the other side, HSC didn’t exhibit this behavior which is not in agreement with other researchers. • NSC • HSC Fig 1. Sample Stress-Strain curve with envelop curve for monotonic and cyclic loading. → (1) → (2) The plastic strain of NSC calculated from the experimental results, closely resemblance with the analytical results from Lam and Teng model but plastic strain of HSC was greatly underestimated by Lam and Teng model. This is because of Eq. (2) by Lam and Teng model that predicts a reduction of plastic strain with increasing unconfined compressive strength. The trend line for HSC, based on the experimental data, does not change significantly with the unconfined compressive strength. Fig 2. Comparison of experimentally recorded plastic strains with predictions of Lam and Teng’s model: (a) AFRP-confined NSC; (b) AFRP-confined HSC In addition to the studies cited previously, the model proposed by Shao et al. (2006) provides a set of equations for the calculation of the plastic strains (Eq. 3 & Eq. 4). The plastic strains also overestimated by Shao’s model and this shortcoming is caused by the overestimation of E secu which is based on the σ un;env f ∕ o.co ratio. Fig 3. Comparison of experimentally recorded plastic strains with predictions of Shao’s model: (a)AFRP-confined NSC; (b) AFRP-confined HSC. Effect of Unconfined Compressive Strength By comparing samples of same f’ lu /f’ co , indicate that the ultimate strength is lower for HSC than NSC under cyclic loading. k ε values of HSC are consistently lower than that of NSC which suggests that it is strength dependent which was also reported by other researchers (Wang,2009;Li,2011). Effect of FRP Type The comparison of the average k ε values given in Table 1 for the AFRP- and CFRP-confined HSC specimens suggests that the type of FRP does not have significant influence on k ε . This observation, however, is not in agreement with the findings of on going research by Li (2011) in Ozbakkaloglu’s research group at the University of Adelaide. Li (2011) observed higher k ε values have for AFRP-confined concrete compared to CFRP-confined concrete. Effect of Loading Pattern Strain-reduction factor, k ε value does not depend on the load cycles according to Ozbakkaloglu et al. so is the hoops strain є h,rupt but Lam et al. 2006 and Demir et al. 2010 proposed an observed increase in є h,rupt PROPOSAL FOR FURTHER STUDY Among over 15 design-oriented stress- strain models studied in this project, none is able to correctly predict the stress- strain behavior of FRP-confined HSC. There is a need for a stress-strain model that can accurately predict the behavior of FRP- confined HSC, and more work is required to better understand and model the behavior of FRP-confined HSC. If this gaps can be filled, FRP-confined HSC would be promising in the prolific field of retrofitting. → (3) → (4) COMPARISON AND FINDING EXISTING RESEARCH GAPS PROJECT- SPRING_2016 CIVE 6460 Table 1. Test Results of FRP-Confined Concrete Cylinders (Ozbakkaloglu et al.2012)
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Application of fiber-reinforced polymer (FRP) composites as a confining material for concrete, for retrofitting of critical structures as well as for the construction of concrete filled FRP-tubes as earthquake-resistant columns in new construction already have been popular because of the beauty of FRP for providing greater capacity. The reliable design of these structural members for retrofitting and against earthquake-induced forces necessitates a clear understanding of the stress-strain behavior of FRP-confined concrete under load cycles., especially under cyclic axial compression for the seismic retrofit. Although the monotonic axial stress-strain behavior of FRP-confined concrete has been studied extensively over the past two decades, only a few studies have so far investigated the behavior of FRP-confined concrete under cyclic axial compression. Besides there have been only few studies done on FRP confined HSC based on limited experimental data which led to some research gaps and questions. This study is primarily aiming at filling existing research gaps and proposing ideas for further research. The thorough study of existing stress-strain model of FRP confined concrete has led to number of significant conclusions including proposals for the further study in that field.
Not unlike that of FRP, the popularity of high-strength concrete (HSC) in the construction industry has steadily increased during the last two decades. HSC structural members are known to exhibit brittle behavior, which jeopardizes their use in seismically active regions. So far there are only few articles for answering the stress-strain relationship of HSC. The behavior of FRP-confined HSC and FRP-confined NSC under monotonic and cyclic axial compression will be reviewed and compared based on other researcher’s articles for the proposal of further study and filling out of the existing research gaps The experimental program done by Togay Ozbakkaloglu and Emre Akin was taken as primary source for this study and the results have been analyzed and compared with the other existing stress-strain model. The experimental program was consisting of a total of 24 FRP-confined concrete cylinders with a concrete core diameter of 152.5 mm and a height of 305 mm which were manufactured and tested under axial compression. The test parameters included the concrete compressive strength (NSC and HSC), type of FRP material (CFRP and AFRP), FRP thickness (2 to 6 layers), and loading pattern (axial monotonic and axial cyclic). AFRP was used as confinement for both NSC and HSC where as CFRP are used for HSC.
Stress-Strain Behavior of FRP-Confined Normal- and High-Strength Concrete under Monotonic and Cyclic Axial Compression
SHARIFUL ISLAM MS in Civil Engineering, Department of Civil Engineering
University of Toledo
INTRODUCTION
EXPERIMENTAL RESULTS (OZBAKKALOGLU ET. AL)
ABSTRACT
ACKNOWLEDGEMENT I would like to express my deepest gratitude to my instructor Dr. Azadeh Parvin for her unwavering support and mentorship throughout this project.
CONCLUSION• The axial cyclic stress-strain model proposed by Lam and
Teng (2009) is highly accurate in predicting both the unloading and reloading paths of FRP-confined NSC. The model closely predicts the shapes of the unloading and reloading curves and accurately estimates the plastic strains for NSC.
• When the model is applied to HSC specimens of the experimental program, on the other hand, the model predictions deviate significantly from the experimental results; this is caused largely by the inaccuracies in the calculation of the plastic strains of FRP-confined HSC. Plastic strain equation for HSC was calibrated based on the extremely limited experimental data reported by Rousakis (2001) on CFRP-confined HSC and this might be the reasons behind this inaccuracies.
• The predictions of the cyclic stress-strain model proposed by Shao et al. (2006) indicated that this model predicts the reloading paths reasonably accurately, but consistently overestimates the residual plastic strains and does not accurately capture the shape of the unloading paths.
Envelop Curve Experimental program by Togay Ozbakkaloglu and Emre Akin shows that the envelope stress-strain curves of cyclically loaded specimens closely follow the stress-strain curves of the corresponding monotonically loaded specimens which was also reported by Lam et al. (2006) and Abbasnia and Ziaadiny (2010).
Unloading and Reloading Paths and the Plastic Strain Lam et al. (2006) demonstrated that the relationship between unloading strains εun;env and plastic strains εpl was linear for CFRP-confined NSC cylinders for 0.001≤εun;env≤0.0035 and εun;env
≥0.0035. This observation was then supported by that of Abbasnia and Ziaadiny (2010), which was based on an experimental investigation of CFRP-confined NSC square prisms. The experimental results also in agreement with the previousproposal by other researchers.
Eq. (1) and (2) were calibrated by Lam and Teng for the prediction of plastic strain for NSC and HSC.
Axial Stress-Strain Behavior• For the monotonic loading, there is an ascending first branch
in the stress-strain diagram followed by another ascending or, almost flat second branch for NSC whereas, for HSC, there is a sudden drop (Strength Softening) at the transition point due to the brittle nature of HSC.
• Higher compressive strength and ultimate axial strain was observed for higher number of load cycles for NSC in case of cyclic loading which is in harmony with other researchers (Rousakis 2001; Lam et al. 2006). On the other side, HSC didn’t exhibit this behavior which is not in agreement with other researchers.
• NSC • HSCFig 1. Sample Stress-Strain curve with envelop curve for monotonic and cyclic loading.
→ (1)
→ (2)
The plastic strain of NSC calculated from the experimental results, closely resemblance with the analytical results from Lam and Teng model but plastic strain of HSC was greatly underestimated by Lam and Teng model. This is because of Eq. (2) by Lam and Teng model that predicts a reduction of plastic strain with increasing unconfined compressive strength. The trend line for HSC, based on the experimental data, does not change significantly with the unconfined compressive strength.
Fig 2. Comparison of experimentally recorded plastic strains with predictions of Lam and Teng’s model: (a) AFRP-confined NSC; (b) AFRP-confined HSC In addition to the studies cited previously, the model proposed by Shao et al. (2006) provides a set of equations for the calculation of the plastic strains (Eq. 3 & Eq. 4).
The plastic strains also overestimated by Shao’s model and this shortcoming is caused by the overestimation of Esecu which is based on the σun;env∕fo.co ratio.
Fig 3. Comparison of experimentally recorded plastic strains with predictions of Shao’s model: (a)AFRP-confined NSC; (b) AFRP-confined HSC.
Effect of Unconfined Compressive Strength By comparing samples of same f’lu/f’co, indicate that the ultimate strength is lower for HSC than NSC under cyclic loading. kε values of HSC are consistently lower than that of NSC which suggests that it is strength dependent which was also reported by other researchers (Wang,2009;Li,2011).Effect of FRP Type The comparison of the average kε values given in Table 1 for the AFRP- and CFRP-confined HSC specimens suggests that the type of FRP does not have significant influence on kε. This observation, however, is not in agreement with the findings of on going research by Li (2011) in Ozbakkaloglu’s research group at the University of Adelaide. Li (2011) observed higher kε values have for AFRP-confined concrete compared to CFRP-confined concrete.
Effect of Loading Pattern Strain-reduction factor, kε value does not depend on the load cycles according to Ozbakkaloglu et al. so is the hoops strain єh,rupt but Lam et al. 2006 and Demir et al. 2010 proposed an observed increase in єh,rupt with increase in loading/unloading cycles.
PROPOSAL FOR FURTHER STUDY Among over 15 design-oriented stress-strain models studied in this project, none is able to correctly predict the stress-strain behavior of FRP-confined HSC. There is a need for a stress-strain model that can accurately predict the behavior of FRP-confined HSC, and more work is required to better understand and model the behavior of FRP-confined HSC. If this gaps can be filled, FRP-confined HSC would be promising in the prolific field of retrofitting.
→ (3)
→ (4)
COMPARISON AND FINDING EXISTING RESEARCH GAPS
PROJECT- SPRING_2016
CIVE 6460
Table 1. Test Results of FRP-Confined Concrete Cylinders (Ozbakkaloglu et al.2012)