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

Date of Award


Degree Name

Doctor of Philosophy


Civil and Construction Engineering

First Advisor

Dr. Houssam Toutanji

Second Advisor

Dr. Osama Abudayyeh

Third Advisor

Dr. Yufeng Hu

Fourth Advisor

Dr. Daniel Kujawski


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.

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