Thong M. Pham

New Method of Strengthening Reinforced Concrete Square Columns by Circularizing and Wrapping with Fiber-Reinforced Polymer or Steel Straps

AbstractThis study investigates three methods of strengthening existing reinforced square concrete columns under different loading conditions. Four groups of sixteen reinforced concrete square columns were made from normal-strength concrete. Reinforcement was kept at minimum ratio, simulating columns that need retrofitting. Columns of the first group were reference columns (Group N), while the corners of the second group columns (Group RF) were rounded and wrapped with three layers of carbon-fiber-reinforced polymers (CFRPs). The sides of the columns of the third group (Group CF) were bonded with four pieces of concrete with a segmental circular shape, thus changing the cross section of the column from a square to a circle before each column was wrapped with three layers of CFRP. The columns of the last group (fourth) were modified as the third group to result in a circular cross section, but were confined with steel straps. Results from the study showed that all confinement methods increased the capacity...

Impact Behavior of FRP-Strengthened RC Beams without Stirrups

AbstractThis study investigates the impact behavior of fiber-reinforced polymer (FRP)-strengthened reinforced concrete (RC) beams and FRP contribution to shear strength. Thirteen RC beams (1,500×150×250  mm) were tested against static and impact loads. The beams were strengthened with FRP U-wraps and 45°-angle wraps to investigate the effectiveness of different wrapping schemes. The experimental results show that the debonding strain of FRP under impact loads is slightly smaller than that under static loads. The FRP contribution to shear strength is discussed and verified against a design code. In terms of using the same amount of FRP, 45°-angle FRP wraps provide better performance than the FRP U-wraps in terms of both the load capacity and displacement. Fully wrapping a RC beam with FRP is more efficient than wrapping distributed FRP strips. The impact behavior and failure modes of FRP strengthened RC beams are also discussed.

Optimized FRP wrapping schemes for circular concrete columns under axial compression

This study investigates the behavior and failure modes of fiber-reinforced polymer (FRP) confined concrete wrapped with different FRP schemes, including fully wrapped, partially wrapped, and nonuniformly-wrapped concrete cylinders. By using the same amount of FRP, this study proposes a new wrapping scheme that provides a higher compressive strength and strain for FRP-confined concrete, in comparison with conventional fully wrapping schemes. A total of 33 specimens were cast and tested, with three of these specimens acting as reference specimens and the remaining specimens wrapped with different types of FRP (CFRP and GFRP) by different wrapping schemes. For specimens that belong to the descending branch type, the partially-wrapped specimens had a lower compressive strength but a higher axial strain as compared to the corresponding fully-wrapped specimens. In addition, the nonuniformly-wrapped specimens achieved both a higher compressive strength and axial strain in comparison with the fully-wrapped specimens. Furthermore, the partially-wrapping scheme changes the failure modes of the specimens and the angle of the failure surface. A new equation that can be used to predict the axial strain of concrete cylinders wrapped partially with FRP is proposed.

Predicting Stress and Strain of FRP-Confined Square/Rectangular Columns Using Artificial Neural Networks

AbstractThis study proposes the use of artificial neural networks (ANNs) to calculate the compressive strength and strain of fiber reinforced polymer (FRP)–confined square/rectangular columns. Modeling results have shown that the two proposed ANN models fit the testing data very well. Specifically, the average absolute errors of the two proposed models are less than 5%. The ANNs were trained, validated, and tested on two databases. The first database contains the experimental compressive strength results of 104 FRP confined rectangular concrete columns. The second database consists of the experimental compressive strain of 69 FRP confined square concrete columns. Furthermore, this study proposes a new potential approach to generate a user-friendly equation from a trained ANN model. The proposed equations estimate the compressive strength/strain with small error. As such, the equations could be easily used in engineering design instead of the invisible processes inside the ANN.

Behavior of fiber-reinforced polymer-strengthened reinforced concrete beams under static and impact loads

This study investigates the behavior of fiber-reinforced polymer-strengthened reinforced concrete beams under static and impact loads. The experimental program includes six beams tested in static loads and seven beams tested against impact loads. Longitudinal fiber-reinforced polymer strips and fiber-reinforced polymer U-wraps were used to strengthen these beams. The section of four beams was modified to have a curved soffit in order to reduce the stress concentration of fiber-reinforced polymer U-wraps and provide confinement effect on longitudinal fiber-reinforced polymer strips. The experimental results showed that the proposed modification significantly increased the beam capacities as compared to their rectangular counterparts strengthened with the same amount of fiber-reinforced polymer material. In addition, this article also provides explanations and discussions on the phenomenon of shifting of the flexure failure mode under static loads to the shear–flexure failure mode under impact loads of all the beams tested in the study, as well as the proper interpretations of the measured impact forces in the tests. From the experimental results, it is recommended that the impact force and inertial force at the very early stage of an impact event should be used to design the impact resistance.

Axial Impact Resistance of FRP-Confined Concrete

AbstractThis study investigates the impact resistance of fiber-reinforced polymer (FRP) confined concrete. Concrete cylinders were wrapped with carbon FRP (CFRP) or glass FRP (GFRP) with a varied number of layers and wrapping schemes. The impact tests were conducted by using drop-weight apparatus at different impact velocities. Dynamic behavior of the specimens has been investigated. The experimental results have shown that the failure modes are very different than those from static tests. Identical specimens experienced different damage as the impact velocities changed. The dynamic rupture strain of FRP was found to be substantially lower compared with that under static loads. As a result, the FRP efficiency factors were 0.17 and 0.56 for CFRP and GFRP, respectively. Interestingly, although GFRP has lower tensile strength and elastic modulus, it showed much better performance against impact compared with CFRP in terms of both the strength and ductility. The higher rupture strain of GFRP compared with CFR...

Prediction of the impact force on reinforced concrete beams from a drop weight

It is always a challenge to efficiently and accurately estimate the force on structures from falling objects. This study aims to predict the maximum impact force on reinforced concrete beams subjected to drop-weight impact using artificial neural network. A new empirical model including a comprehensive version and a simplified version is proposed to estimate the maximum impact force. The model was verified against a database collected from the literature including 67 reinforced concrete beams tested under drop-weight impacts. The database covers the concrete strengths ranging from 23 to 47 MPa, the projectile mass from 150 to 500 kg, and the impact velocity up to 9.3 m/s. The prediction of the comprehensive version of the proposed model fits the experimental results very well with an average absolute error of 11.6%. The simplified version of the proposed model is established for easy estimation, with the average error of 23.2% in prediction of the maximum impact force.

Effects of fabrication technique on tensile properties of fiber reinforced polymer

This study investigated the effects of fabrication technique on the tensile properties of fiber reinforced polymer (FRP) flat coupon tests. A total of 20 FRP flat coupons were prepared by two different techniques, which were tested in tension until failure. The first technique of preparing the FRP coupons was based on the recommendation of ASTM D7565/D7565M-10, named the “Cutting Technique,” while the second technique, named the “Folding Technique,” was proposed by this study. Experimental results from this study indicated that preparing FRP coupons using the Cutting Technique resulted in a reduction in the tensile properties as compared to coupons prepared by the proposed Folding Technique. Most notably, the tensile force per unit width obtained by the FRP flat coupons prepared using the Folding Technique was up to 8 % higher than that obtained by coupons prepared using the Cutting Technique. In addition, the effect of the % bending on the tensile properties was also studied. It was found that the % bending about the thickness plane was greater than that of the % bending about the width plane. Furthermore, the tensile properties of the FRP coupons were not sensitive to its % bending.

Failure and impact resistance analysis of plain and fiber-reinforced-polymer confined concrete cylinders under axial impact loads

This study conducts an experimental and numerical investigation on the failure and impact resistance of plain and fiber-reinforced polymer-confined concrete. The impact resistance of concrete cylinders wrapped with different types of fibers including carbon fiber and glass fiber is examined. Drop-weight tests are utilized to conduct the impact tests while the numerical simulation is conducted using LS-DYNA. The experimental and numerical results have proved that fiber-reinforced polymer can be efficiently used to improve the impact resistance of concrete cylinders. In general, fiber-reinforced polymer ruptures at a lower strain than those in static tests and the rupture strain of glass fiber is much higher than that of carbon fiber. The findings in the experimental tests are confirmed by the numerical results. Glass fiber, therefore, exhibits a much better performance than carbon fiber. It is recommended to use glass fiber to enhance the impact resistance of concrete structures strengthened with fiber-reinforced polymer. In addition, the stress evolution of the specimens is analyzed to investigate the failure mechanism.

Axial impact behavior and energy absorption of rubberized concrete with/without fiber-reinforced polymer confinement

This study investigates the axial impact resistance and energy absorption of rubberized concrete with/without fiber-reinforced polymer confinement. The impact tests were carried out using an instrumented drop-weight testing apparatus. The experimental results have shown that rubberized concrete significantly reduced the maximum impact force of up to 50% and extended the impact duration. These characteristics make rubberized concrete a promising material for protective structures and particularly for future sustainable construction of rigid roadside barriers. Glass fiber–reinforced polymer confinement is a very effective method to improve the impact resistance for both conventional concrete and particularly for rubberized concrete. It was found that the rubberized concrete reduced the maximum impact force so that it transferred a lower force to a protected structure as well as a lower rebound force, which is desirable for protection of passengers in an incident of vehicle collision. Interestingly, the rubberized concrete showed a lower energy absorption capacity as compared to conventional concrete, where the exact reason for this is unknown to the authors. Therefore, further research is sought to provide more understanding of the response of rubberized concrete under impact and improve its energy absorption. This study explored experimentally the use of rubberized concrete as a promising sustainable construction material for applications to construction of columns in buildings located in seismic active zones or subjected to terrorist attack, security bollards and rigid road side barriers.