Flexural Behavior of RC Beams Strengthened with Externally Bonded CFRP Under Cyclic Loading
Flexural Behavior of RC Beams Strengthened with Externally Bonded CFRP Under Cyclic Loading

Author: Mohammad Ahmad Salameh Qaralleh

Publisher:

Published: 2018

Total Pages: 106

ISBN-13:

DOWNLOAD EBOOK

Fifteen percent of the approximately 600,000 bridges in the United States are classified as structurally deficient. In addition, the average age of the reported cases of failure of bridges is less than half of its original service life. The deterioration of the health of bridges due to the environmental and other causes raises a red flag for engineers. Fiber reinforced polymers (FRP) composites gained attention in many fields, as well as, civil engineering field. This is because of the superior properties of FRP composites, such as: high specific strength (strength to density ratio), high specific stiffness (modulus to density ratio), low density, corrosion resistance, long fatigue life, environmental stability, ease of installation, and life-cycle cost effectiveness. Using FRP for strengthening and rehabilitation of reinforced concrete (RC) elements started in the 1980s. The technique of externally bonding FRP sheets to the soffit of RC beams has proven to increase the strength significantly. The behavior of such strengthened RC beams under monotonic loading is well documented and the design guidelines are rather becoming mature. However, the fatigue performance of these beams is lacking and needs more investigation. This work aims at further investigating and provide a better understanding of the fatigue behavior of RC beams strengthened with externally bonded FRP composites. This work will also include presenting the relevant literature to understand the properties of the constituent materials. This work will also focus on developing a procedure to predict the fatigue life of RC beams strengthened with externally bonded FRP. The experimental element of this study will investigate the effect of different factors on the fatigue life such as: variable loading, mean stress, and damage accumulation. The experimental work includes testing eight (8) RC beams with dimensions of: 152.4 mm in width, 152.4 mm in depth, and 1,219 mm in span length. The beams were strengthened by attaching carbon FRP sheets to their soffits. One beam was tested under monotonic loading to serve as a control. A reference beam was tested under constant amplitude fatigue loading. Four beams tested under fatigue loading that contains periodic overloading equals to 10% of the total fatigue life of the beam. Two loading-overloading regimes were used, namely: 9-1, and 900-100. Lastly, 2 beams were tested under constant amplitude loading with different mean stresses. A linear variable displacement transducer (LVDT) and 5 strain gages were used for recording the deflection and strains in the rebars and FRP, respectively. The effect of the stiffness degradation on the classical beam theory was examined using 68 RC beams strengthened with externally bonded FRP reported in the literature. The theoretical calculations of the stress levels in the primary steel are in agreement with the experimental reported values. Thus, the effect of the stiffness degradation of the beam under fatigue loading is negligible. In addition, a new S-N curve was developed based on a combination of the analytical stress ranges and the experimental fatigue life of the literature data points. When the experimental results of this study compared with the reference beam and the literature, the results show that periodic overloading reduces the fatigue life of the strengthened beams. The Palmegren-Miner rule of linear cumulative damage overestimates the fatigue life of beams undergo variable fatigue loading. The location of the steel rupture for the beams tested under fatigue with periodic overloading is different than beams tested under constant amplitude fatigue. This indicates that periodic overloading alters the distribution of the stresses and increases the effect of the shear stress. A model for predicting the fatigue life based on the loading and overloading stress ranges in the steel is presented in this study. Fatigue loading with higher mean stress either with the same or less stress range did not reduce the fatigue life of the beams.

Fatigue Performance of RC Beams Strengthened with Near Surface Mounted CFRP Composites
Fatigue Performance of RC Beams Strengthened with Near Surface Mounted CFRP Composites

Author: Tamer Ghaith Mousa Eljufout

Publisher:

Published: 2019

Total Pages: 184

ISBN-13:

DOWNLOAD EBOOK

Bridges have a fundamental role in improving the effectiveness of highways and providing an expedient and express traffic system. Over time, Reinforced Concrete (RC) bridges degrade due to gradually increased traffic loads and environmental deteriorations. Subsequently, service loads might cause higher stresses in concrete and steel reinforcement than stresses considered in the design stage. This affects the structural performance of RC beams and leads to sudden fatigue failure. As such, there is an essential need for rehabilitation to avoid hazards and tragedies. Carbon Fiber Reinforced Polymer (CFRP) composites are becoming widely used to strengthen RC bridges. Near Surface Mounted (NSM) technique has proven its advantages over other applied strengthening techniques. This study investigates several factors associated with the fatigue performance of RC beams strengthened with NSM CFRP reinforcement, such as: the effect of strengthening techniques on the fatigue limit of RC beams; the influence of loading history on the fatigue behavior of rehabilitated RC beams; the development of deflections, stiffness degradation, and energy dissipation of strengthened RC beams under different loading patterns; and the viability of an accelerated fatigue approach for developing a fatigue stress-life predication model of RC beams. Analytical, experimental, and numerical analyses were performed to achieve the study’s objectives. Empirical fatigue stress-life prediction models for non-strengthened and strengthened RC beams were developed based on least-squares regressions of eighty experimental data points obtained from the literature. The proposed models have a satisfactory precision for design purposes. The experimental program in this study includes eleven cast-in-place RC beams with dimensions of 152.4 × 152.4 × 1,521 mm. Specimens were tested under four-point bending configuration to simulate long-span RC beams. Monotonic tests were performed to determine the flexural static capacity, ductility index, and cracking patterns of non-strengthened and strengthened RC beams. The effect of CFRP strengthening techniques on the fatigue limit of RC beams was examined using Locati method. A non-strengthened RC beam, a RC beam strengthened with NSM CFRP reinforcement, and a RC beam strengthened with Externally Bonded (EB) CFRP sheet were tested under a step-like constant amplitude cyclic loading to failure. Accordingly, CFRP strengthening techniques increase the fatigue limit of RC beams and improve their fatigue responses. Strengthened RC beams showed less stiffness degradation and energy dissipation when compared to non-strengthened RC beams. NSM CFRP technique demonstrated a better flexural static strength and fatigue life than EB CFRP technique. Six RC beams were tested to investigate the influence of loading history on the fatigue behavior of rehabilitated RC beams. Two of the six RC beams were strengthened with NSM CFRP reinforcement and tested as a reference under constant amplitude cyclic load. To simulate the condition of service traffic loading, the other four RC beams were cyclically pre-loaded then rehabilitated with NSM CFRP reinforcement. These specimens were tested under the same loading conditions of the reference specimens. The rehabilitated pre-fatigued RC beams had stiffness degradation and failure modes similar to the reference specimens. The pre-fatigue induced an under-stressing effect that extends the fatigue life of the rehabilitated RC beams. Moreover, the post-fatigue monotonic behavior of the rehabilitated RC beams showed an increase in the elastic modulus and a decrease in ductility within the flexural static capacity. Finally, numerical analyses were performed to predict the fatigue responses of RC beams strengthened with NSM CFRP reinforcement, and to check the viability of an accelerated fatigue approach for developing a fatigue stress-life predication model. The accelerated fatigue loading has a higher rate of damage accumulation than the standard testing approach. The developed model fits the upper 95% prediction band of RC beams tested under constant amplitude cyclic loading.

Fatigue Damage Modeling of Cfrp Strengthened Reinforced Concrete Beams
Fatigue Damage Modeling of Cfrp Strengthened Reinforced Concrete Beams

Author: Asad-Ur-Rehman Khan

Publisher: LAP Lambert Academic Publishing

Published: 2010-08

Total Pages: 212

ISBN-13: 9783838371368

DOWNLOAD EBOOK

Improvement in the short-term behavior of deficient reinforced concrete (RC) beams is very well known through extensive testing of reinforced concrete beams strengthened by externally bonded carbon fiber reinforced polymer (CFRP) strips. However, long-term performance of such beams needs to be assessed before the method can gain full acceptance. Modeling of the overall system of CFRP strengthened RC beams requires modeling of the individual components that ultimately leads to failure i.e., concrete, CFRP and concrete-CFRP interface. Component elasto-damage constitutive models are developed for concrete crushing, CFRP plate rupture and concrete-CFRP interface failure, calibrated by data from both suitably designed experiments and from experiments reported in literature, and are utilized to investigate the low cycle fatigue behavior of reinforced concrete beams strengthened by CFRP strips. The predictive ability of the models is tested by comparison to data from two different beam strengthening schemes i.e., with end anchorage and without end anchorage.

Flexural Performance of Reinforced Concrete Beams Externally Strengthened with Carbon and Basalt FRP Sheets
Flexural Performance of Reinforced Concrete Beams Externally Strengthened with Carbon and Basalt FRP Sheets

Author: Sahar Samir Choobbor

Publisher:

Published: 2015

Total Pages: 104

ISBN-13:

DOWNLOAD EBOOK

"Different strengthening systems have been widely used for many years to retrofit and repair deficient structural members. Reinforced concrete (RC) slabs and beams are commonly strengthened in flexure by externally bonding Carbon Fiber Reinforced Polymer (CFRP) sheets to the bottom side of the member. The CFRP sheets used in strengthening applications have high strength; however, they are brittle materials with low ductility. Basalt Fiber Reinforced Polymer (BFRP) sheets on the other hand have relatively lower strength compared to CFRP, however they have higher ductility. As a result, there is growing interest among researchers and practitioners in combining different types of FRP sheets to produce an enhanced strengthening system in terms of strength and ductility. This study investigates the flexural behavior of RC beams externally strengthened with CFRP sheets, BFRP sheets, and their hybrid combination (CFRP-BFRP). This hybrid system is designed to enhance the properties of composites, where it combines the high strength of CFRP and high ductility of BFRP sheets, respectively. To investigate the behavior of the different strengthening systems, an experimental program was conducted on ten RC beams that were tested under four-point bending. The load versus mid-span deflection data were recorded and used to compare the performance of the strengthened specimens. The test results indicated that all strengthened specimens yielded higher flexural capacity and lower ductility values compared to the unstrengthened control beam. The increase in the flexural capacity of the strengthened beams ranged from 23% to 68% of the control beam. Moreover, the beams strengthened with BFRP and hybrid CFRP-BFRP sheets achieved higher ductility compared with the beams strengthened with CFRP sheets. Thus, it was concluded that the use of a hybrid combination of CFRP-BFRP sheets could achieve the desired increase in the flexural capacity of RC beams with an improved ductility compared to that with CFRP sheets only. Finite element (FE) models were also developed and were able to capture the behavior of the tested beams with a good level of accuracy. The predicted flexural capacity along with the associated mid-span deflection differed by 1% to 10% from the experimental values."--Abstract.

Fatigue Flexural Behaviour of Reinforced Concrete Beams with Non-prestressed and Prestressed Basalt Fiber Reinforced Polymer Bars
Fatigue Flexural Behaviour of Reinforced Concrete Beams with Non-prestressed and Prestressed Basalt Fiber Reinforced Polymer Bars

Author: Taha Younes

Publisher:

Published: 2015

Total Pages: 64

ISBN-13:

DOWNLOAD EBOOK

Basalt fibers have recently been introduced as a promising alternative to the existing fiber reinforced polymer (FRP) family. The mechanical properties of basalt FRP (BFRP) bars are, generally, better than those of glass FRP (GFRP) bars. However, they are still lower than those of carbon FRP (CFRP) bars. Also BFRP bars have now been developed that have a higher modulus of elasticity than typical GRFP bars. Only a limited amount of research is available on BFRP bars in structural concrete applications and there is no information on the performance of prestressed basalt bars in reinforced concrete elements subjected to fatigue loading. Most studies that are available deal only with the flexural behaviour of concrete beams reinforced with non- prestressed and prestressed GFRP and CFRP bars under monotonic and fatigue loading. This thesis presents an experimental study of the flexural behaviour of concrete beams reinforced with non-prestressed and prestressed basalt bars under monotonic and fatigue loading and compares these beam fatigue results with the fatigue behaviour of similar machined basalt rebars tested under fatigue loading in air. Sixteen beams with dimensions of (2400x 300x150mm) and thirteen BFRP bare rebars were tested. The parameters that varied were the level of prestress of the bars (0%, 20% and 40% of their static tension capacity) and the fatigue load ranges. The experimental findings showed a difference in the long life fatigue strength between the beams prestressed to 40% 20% and 0% of the bar strength with the beams with the bars prestressed to 40% of the bar strength showing a higher fatigue strength than of those prestressed to 0% and 20%. For 40% and 20 % prestressed beams, there is no benefit in fatigue performance above 20% and 13% of the ultimate capacity of the beams a level at which calculations showed that the remaining prestress did not close cracks at the minimum load in the fatigue load cycle. When compared on the basis of load range versus cycles to failure, the data for the three beam types fell onto a single curve at load levels where the remaining prestress after fatigue creep relaxation no longer closed the crack at the minimum load.

The Fatigue Performance Assessment of Corrosion Damaged RC Beams, Patch Repaired and Externally Strengthened Using CFRP
The Fatigue Performance Assessment of Corrosion Damaged RC Beams, Patch Repaired and Externally Strengthened Using CFRP

Author: Steven Ivan Gregan

Publisher:

Published: 2012

Total Pages: 0

ISBN-13:

DOWNLOAD EBOOK

The focus of the dissertation was to provide an in depth investigation towards the fatigue performance of Carbon Fiber Reinforced Polymers (CFRP) which were externally bonded onto concrete beams as a repair and strengthening technique for internally corrosion damaged RC beams. It was identified that more research concerning the fatigue performance of externally bonded CFRP laminates used as a composite material for originally damaged concrete structures was required. Therefore, there was a need to study the failure mechanisms between the externally bonded CFRP, corrosion damaged internal steel, and patch repaired section and the original substrate concrete with respect to the long term performance, whilst treating the system (CFRP, substrate concrete, patch repair and bonding agents) as a composite material. The methodology of the dissertation included the introduction of accelerated corrosion techniques to degrade the internal steel reinforcement. The damaged RC beams were repaired by removing the damaged concrete, treating the corroded internal steel reinforcement, replacing the damaged concre te section removed with a rapid-hardening high strength patch repair mortar, and finally externally bonding CFRP laminates along the patch repaired section and entire tensile face to restore the bending capacity lost due to the reduction of internal steel and subsequent patch repair. Two of the six RC beams which were patch repaired and CFRP strengthened, were subjected to a monotonic load in order to establish the ultimate static load at failure for the RC beams. The ultimate static load at failure was then used to derive the maximum imposed cyclic fatigue loading that was applied. The remaining four RC beams were then subjected to constant sinusoidal cyclic loads at varying amplitudes, the range of amplitude dependent on the corresponding static load at failure for the identical RC beam.