Yu-Fei Wu

Effect of Predamage on the Stress-Strain Relationship of Confined Concrete under Monotonic Loading

Jacketing concrete columns with steel or fiber-reinforced polymer (FRP) has been identified as a highly effective way of restoring and enhancing the strength and ductility of columns damaged by catastrophic events such as earthquakes, blasts, and vehicle impact. However, a stress-strain relationship for damaged and subsequently repaired concrete is not available for design of the repair works. An extensive experimental and analytical investigation was undertaken in this work to study the mechanical properties of repaired concrete columns. Tests were conducted on 102 concrete cylinders involving variations of damage degree, concrete grade, and confinement pressure. The effect of damage on the compressive strength, strain capacity, initial elastic modulus, and strain-hardening modulus of repaired concrete is quantified in terms of a damage index. A stress-strain model is developed for the design of structural repair works. Evaluation of the test data also exhibits that the classic model of confined concrete proposed by a previous study and the classic internal friction theory are qualitatively applicable to failure of concrete materials not only in its original form, but also in the heavily disturbed state. The Hoek-Brown failure criterion is unconservative for heavily damaged concrete.

Plastic Hinge Length of FRP-Confined Square RC Columns

The confinement effect of fiber-reinforced polymer (FRP) jacket on the plastic hinge length of square RC columns is studied through experimental testing and analytical study. Seven half-scale RC square columns with different confinement ratios were tested. The strain of longitudinal reinforcement bars and the extent of yielding were measured by strain gauges mounted inside the reinforcement bars. The variation of the strain field of the external column face was recorded continuously by digital image correlation (DIC) during a test. More rational approaches of data analysis are used for identification of plastic hinge length from measured strain fields. The obtained results show that compared with unconfined RC columns, FRP jacketing increases the plastic hinge length when the confinement level is low, but reduces it when the confinement level is high. The analytical study shows that all existing models of plastic hinge length are inadequate in one way or another because no model includes all important factors. The proposed model can correctly capture the trends but it needs to be further improved for accuracy when more data are available.

Confinement Effectiveness of FRP in Retrofitting Circular Concrete Columns under Simulated Seismic Load

The results of a research program that evaluated the confinement effectiveness of the type and the amount of fiber-reinforced polymer (FRP) used to retrofit circular concrete columns are presented. A total of 17 circular concrete columns were tested under combined lateral cyclic displacement excursions and constant axial load. It is demonstrated that a high axial load level has a detrimental effect and that a large aspect ratio has a positive effect on drift capacity. Compared with the performance of columns that are monotonically loaded until failure, three cycles of every displacement excursion significantly affect drift capacity. The energy dissipation capacity is controlled by FRP jacket confinement stiffness, especially under a high axial load level. The fracture strain of FRP material has no significant impact on the drift capacity of retrofitted circular concrete columns as long as the same confining pressure is provided, which differs from the common opinion that a larger FRP fracture strain is ad...

Fatigue Strengthening of Cracked Steel Beams with Different Configurations and Materials

AbstractThis paper presents an experimental study on the fatigue behavior of cracked steel beams strengthened using different patch systems and high-strength materials. These materials included normal modulus carbon fiber–reinforced polymer (CFRP) laminate, high-strength steel (HSS) plate, and SafStrip (SAF) plate. Adhesive bonding and mechanical anchorage were selected to attach these overlays. A digital image correlation (DIC) system was adopted to detect stress distribution at the vicinity of the crack front. Different failure modes were observed for specimens with different retrofitting schemes. Test results showed that, in comparison with control specimens without strengthening, application of these retrofitting materials significantly retarded crack propagation and extended fatigue life of defected steel beams. The stiffness decay and crack mouth opening displacement (CMOD) were also reduced in repaired cases. Based on the fatigue cycles when the crack propagated to half-height of the steel beam, CF...

Behaviour of Steel Plated RC Columns Subject to Lateral Loading

The main focus of this paper is to describe the behaviour of RC columns that are retrofitted with an alternative technique to “jacketing” or wrapping. This new technique consists of attaching steel plates to the flexural faces of a concrete column using bolts. It is envisaged that this technique would be suitable primarily for columns having rectangular cross-sections and in situations where lateral loading induces predominately a single plane of bending (as opposed to biaxial bending). Effectiveness of this new technique has been demonstrated by experimental testing and numerical simulations. This paper studies the mechanism of the new retrofit scheme, how it works, and the behaviour of columns retrofitted using such a scheme, as well as the important parameters that affect the response of the retrofitted columns. This study forms the basis for the design of the plate retrofitting system.

Cross-sectional unification on the stress-strain model of concrete subjected to high passive confinement by fiber-reinforced polymer

The stress-strain behavior of concrete can be improved by providing a lateral passive confining pressure, such as fiber-reinforced polymer (FRP) wrapping. Many axial stress-strain models have been proposed for FRP-confined concrete columns. However, few models can predict the stress-strain behavior of confined concrete columns with more than two specified cross-sections. A stress-strain model of FRP-confined concrete columns with cross-sectional unification was developed in this paper based on a database from the existing literature that includes circular, square, rectangular and elliptical concrete columns that are highly confined by FRP jackets. Using the database, the existing theoretical models were evaluated. In addition, the ultimate stress and strain models with cross-sectional unification were proposed using two parameters: the cross-sectional aspect ratio and corner radius ratio. The elliptical cross-section can be considered as a rectangular one with a special corner radius for the model calculations. A simple and accurate model of the equivalent corner radius ratio for elliptical columns was proposed. Compared to the other existing models and experimental data, the proposed models show good performance.

Fatigue behaviour of cracked steel beams retrofitted with carbon fibre–reinforced polymer laminates:

In recent years, externally bonded carbon fibre–reinforced polymer has been considered an innovative way to strengthen steel structures attributed to its high strength-to-weight ratio, excellent corrosion resistance and fatigue performance. This article presents an experimental and numerical study on the fatigue behaviour of defected steel beams strengthened with carbon fibre–reinforced polymer laminates, with a special focus on the effect of interfacial debonding. Analytical modelling and numerical simulation confirmed that the interfacial debonding had a pronounced effect on carbon fibre–reinforced polymer strain and stress intensity factor at the crack front. After introducing interfacial debonding from experimental findings into the numerical analysis, the fatigue life and crack propagation versus cycle numbers of the specimens compared well with the test results. Based on the current experimental program, specimens with Sikadur 30 were more prone to debonding failure; therefore, Araldite 420 is suggested for strengthening schemes.

Shear Strength Components in Reinforced Concrete Members

AbstractThe shear strength (V) of reinforced concrete (RC) beams consists of two parts: shear resistance of concrete (Vc) and contribution of the transverse reinforcement (Vs). Previous experimenta...

Prediction of Young’s Modulus of Concrete with Two Types of Elliptical Aggregate

The purpose of this paper is to present a lattice method for predicting the Youngs modulus of concrete with two types of elliptical aggregate. After distributing fine and coarse aggregates within a rectangular unit, the finite element method is adopted to estimate the Youngs modulus of concrete. Compared with previous studies, the method considers the aggregate shape effect and the difference in Youngs modulus between fine aggregate and coarse aggregate. After the validity of the method is verified with two sets of experimental results, the effects of various factors on the Young s modulus of concrete are quantified. It is found that the Youngs modulus of concrete increases with an increase in aggregate aspect ratio, minimum aggregate diameter, or both, but decreases by increasing the interfacial transition zone thickness. It is also found that the maximum aggregate diameter and aggregate gradation have little effect on the Youngs modulus of concrete.

Special issue on applications of engineering composites: Materials, structures, and interfaces

As research and applications of conventional materials are reaching their potential limits in recent years, composite materials and structures find more and more applications in engineering, such as the aerospace, automotive, ship-building, off-shore piping, and infrastructure industries. In fact, research and applications of composites are currently one of the most popular areas in both science and technology, covering fundamental and applied science including physics, chemistry, and material science, as well as applied mechanics, engineering, and processing technologies. Study on composites moves on from conventional macro-scale to microand nano-scale, involving multi-scale and multi-functional issues as well as interdisciplinary approaches. This Special Issue aims to disseminate the most recent research findings and developments in the field. A total of 46 papers on a broad variety of topics were submitted to the Special Issue by the cut-off date. The diversity of topics in the papers is an indication of the broad and vibrant nature of the field. All of the manuscripts were reviewed rigorously and 23 of them were accepted as full papers. The subjects of the accepted papers involved the following: