Togay Ozbakkaloglu

Behavior of FRP-Confined Normal- and High-Strength Concrete under Cyclic Axial Compression

An important application of fiber-reinforced polymer (FRP) composites is as a confining material for concrete, both in the seismic retrofit of existing reinforced concrete columns and in the construction of concrete-filled FRP tubes as earthquake-resistant columns in new construction. The reliable design of these structural members against earthquake-induced forces necessitates a clear understanding of the stress-strain behavior of FRP-confined concrete under load cycles. This paper presents the results of an experimental study on the behavior of FRP-confined normal- and high-strength concrete under axial compression. A total of 24 aramid and carbon FRP-confined concrete cylinders with different concrete strengths and FRP jacket thicknesses were tested under monotonic and cyclic loading. Examination of the test results has led to a number of significant conclusions in regards to both the trend and ultimate condition of the axial stress-strain behavior of FRP-confined concrete. These results are presented, and a discussion is provided on the influence of the main test parameters in the observed behaviors. The results are also compared with two existing cyclic axial stress-strain models for FRP-confined concrete.

Lateral Strain-to-Axial Strain Relationship of Confined Concrete

The use of fiber-reinforced polymers (FRP) has become widely accepted engineering practice for strengthening reinforced concrete members. It is well established that lateral confinement of concrete with FRP composites can significantly enhance its strength and ductility. As the confinement pressure generated by FRP on the confined concrete depends on the lateral expansion of concrete, the mechanism of concrete expansion inside the FRP shell is of significant interest. A review of the existing stress-strain models of FRP-confined concrete revealed the need for a model that accurately predicts the dilation characteristic of confined concrete as it provides the essential link between the response of the concrete core and the passive confinement mechanism of the FRP shell. It is also understood that knowledge established from the research area of actively confined concrete can be employed in the development of a model applicable for both FRP-confined and actively confined concretes. Based on a large number of experimental test results of both FRP-confined and actively confined concretes, a generic model is proposed to describe the lateral strain-to-axial strain relationship of confined concrete. The instrumentation arrangements of the tested specimens have allowed for the lateral strain-axial strain relationships of confined concrete to be captured throughout the tests. The trend of the lateral strain-to-axial strain relationship of confined concrete is shown to be a function of the confining pressure, type of confining material and concrete strength. Assessment of models with the experimental databases showed that the predictions of the proposed model are well above existing models and in good agreement with the test results of both FRP-confined and actively confined concretes.

Seismic Behavior of High-Strength Concrete-Filled FRP Tube Columns

This paper reports on an experimental program that investigated the seismic behavior of high-strength concrete (HSC)-filled fiber-reinforced-polymer (FRP) tubes (HSCFFTs), designed to perform as building columns. Five square and one circular concrete-filled FRP tube (CFFT) columns were tested under constant axial compression and reversed-cyclic lateral loading. The main parameters of the experimental study were the axial load level, column cross-sectional shape, concrete strength, amount and type of FRP confinement, and FRP tube corner radius. Examination of the test data resulted in a number of significant conclusions with regard to the influence of the investigated column parameters on the performance of CFFT columns. Of primary importance, the results indicate that square HSCFFT columns are capable of developing very high inelastic deformation capacities under simulated seismic loading. The results also indicate that increasing the FRP tube corner radius up to a certain threshold leads to a significant increase in column lateral drift capacities. By contrast, increasing the corner radius beyond that threshold value provides no additional improvement in the hysteretic behavior of square CFFT columns. The influence of the cross-sectional shape is found to be significant, with the circular CFFT exhibiting a larger lateral drift capacity compared with the companion square CFFTs. A set of comparable columns from previous studies has also been included in the discussion to clarify the influence of each parameter on the column behavior. The results of the experimental program are presented together with a discussion on the influence of the main parameters on the seismic behavior of CFFT columns.

Use of recycled plastics in concrete: A critical review

Plastics have become an essential part of our modern lifestyle, and the global plastic production has increased immensely during the past 50years. This has contributed greatly to the production of plastic-related waste. Reuse of waste and recycled plastic materials in concrete mix as an environmental friendly construction material has drawn attention of researchers in recent times, and a large number of studies reporting the behavior of concrete containing waste and recycled plastic materials have been published. This paper summarizes the current published literature until 2015, discussing the material properties and recycling methods of plastic and the influence of plastic materials on the properties of concrete. To provide a comprehensive review, a total of 84 studies were considered, and they were classified into sub categories based on whether they dealt with concrete containing plastic aggregates or plastic fibers. Furthermore, the morphology of concrete containing plastic materials is described in this paper to explain the influence of plastic aggregates and plastic fibers on the properties of concrete. The properties of concretes containing virgin plastic materials were also reviewed to establish their similarities and differences with concrete containing recycled plastics.

Seismic Behavior of FRP-High-Strength Concrete–Steel Double-Skin Tubular Columns

AbstractThis paper reports on an experimental study on the seismic behavior of fiber-reinforced polymer (FRP)–concrete–steel double-skin tubular (DST) columns. Nine DST and one concrete-filled FRP-tube (CFFT) columns that were made of high-strength concrete were tested under constant axial compression and reversed-cyclic lateral loading. The main parameters of the experimental study were axial load level, amount and type of FRP confinement, concrete strength, sectional shape and thickness of the inner steel tube, and provision (or absence) of a concrete filling inside the steel tube. Of primary importance, the results indicate that DST columns are capable of developing very high inelastic deformation capacities under simulated seismic loading. The results also indicate that the presence of a concrete filling inside the inner steel tube significantly and positively influences the seismic behavior of DST columns. It is found that the performance of the void-filled DST column is superior to that of a compani...

Rectangular Stress Block for High-Strength Concrete

The use of high-strength concrete (HSC), with strengths reaching 130 MPa, has increased in recent years due to its superior performance. Structures are designed and built using HSC, especially in columns of multistory structures. The rectangular stress block specified in the ACI 318-02 Building Code for design of reinforced concrete elements was developed, however, on the basis of normal-strength concrete column tests. The applicability of the ACI rectangular stress block to higher-strength concretes becomes questionable, especially for members under high levels of axial compression. A new rectangular stress block is introduced in this paper for a wide range of concrete strengths between 20 and 130 MPa. The proposed stress block is verified against available test data. Column strengths computed using the stress block show good agreement with those recorded experimentally.

Axial Compressive Behavior of FRP-Concrete-Steel Double-Skin Tubular Columns Made of Normal- and High-Strength Concrete

This paper presents the results of an experimental study that was undertaken to investigate the effects of key parameters on the compressive behavior of fiber-reinforced polymer (FRP)-concrete-steel double-skin tubular columns (DSTCs). A total of 24 normal-strength and high-strength concrete-filled DSTCs were manufactured and tested under axial compression. The key parameters examined included the concrete strength; thickness of FRP tube; diameter, strength, and thickness of inner steel tube; and presence (absence) of concrete filling inside it. The results indicate that both normal- and high-strength concretes in a DSTC system is confined effectively by FRP and steel tubes, resulting in a highly ductile compressive behavior. The results also indicate that increasing the inner steel tube diameter leads to an increase in the ultimate axial stress and strain of concrete in DSTCs. It is observed that the concrete filling of the inner steel tubes results in a slight decrease in the ultimate axial strain and a slight increase in ultimate stress of DSTCs. No clear influence of the strength of inner steel tube is observed on the ultimate condition of concrete in DSTCs. It is found that, for a given nominal confinement ratio, an increase in the concrete strength results in a decrease in the ultimate axial strain of DSTCs.

Concrete-Filled FRP Tubes: Manufacture and Testing of New Forms Designed for Improved Performance

This paper reports on the development and testing of three new concrete-filled fiber-reinforced polymer (FRP) tube (CFFT) systems. These CFFT systems were designed to enhance the effectiveness of square and rectangular FRP tubes in confining concrete. In the design of the rectangular CFFTs two different enhancement techniques were considered; namely, corner strengthening and provision of an internal FRP panel. The technique used in the development of the square CFFT system involved the incorporation of four internal concrete-filled FRP cylinders as an integral part of the CFFT. The performance of these systems was investigated experimentally through axial compression tests of 10 unique CFFTs. The results of the experimental study indicate that the new CFFT systems presented in this paper offer significantly improved performance relative to conventional CFFTs with similar material and geometric properties. Examination of the test results have led to a number of significant conclusions with respect to the confinement effectiveness of each new CFFT system. These results are presented and a discussion is provided on the parameters that influenced the compressive behavior of these CFFT systems.

Unified Stress-Strain Model for FRP and Actively Confined Normal-Strength and High-Strength Concrete

AbstractAccurate modeling of the complete stress-strain relationship of confined and unconfined concrete is of vital importance in predicting the overall flexural behavior of reinforced concrete structures. The analysis-oriented models, which utilize the dilation characteristics of confined concretes for stress-strain relationship prediction, are well recognized for their versatility in such modeling applications. These models assume that at a given lateral strain, the axial compressive stress and strain of fiber-reinforced polymer (FRP)–confined concrete are the same as those of the same concrete when it is actively confined under a confining pressure equal to that supplied by the FRP jacket. However, this assumption has recently been demonstrated experimentally to be inaccurate for high-strength concrete (HSC). It was shown that at a given axial strain, lateral strains of actively confined and FRP-confined concretes of the same concrete strength correspond when they are subjected to the same lateral con...

High-performance fiber-reinforced concrete: a review

In recent years, an emerging technology termed, “High-Performance Fiber-Reinforced Concrete (HPFRC)” has become popular in the construction industry. The materials used in HPFRC depend on the desired characteristics and the availability of suitable local economic alternative materials. Concrete is a common building material, generally weak in tension, often ridden with cracks due to plastic and drying shrinkage. The introduction of short discrete fibers into the concrete can be used to counteract and prevent the propagation of cracks. Despite an increase in interest to use HPFRC in concrete structures, some doubts still remain regarding the effect of fibers on the properties of concrete. This paper presents the most comprehensive review to date on the mechanical, physical, and durability-related features of concrete. Specifically, this literature review aims to provide a comprehensive review of the mechanism of crack formation and propagation, compressive strength, modulus of elasticity, stress–strain behavior, tensile strength (TS), flexural strength, drying shrinkage, creep, electrical resistance, and chloride migration resistance of HPFRC. In general, the addition of fibers in high-performance concrete has been proven to improve the mechanical properties of concrete, particularly the TS, flexural strength, and ductility performance. Furthermore, incorporation of fibers in concrete results in reductions in the shrinkage and creep deformations of concrete. However, it has been shown that fibers may also have negative effects on some properties of concrete, such as the workability, which get reduced with the addition of steel fibers. The addition of fibers, particularly steel fibers, due to their conductivity leads to a significant reduction in the electrical resistivity of the concrete, and it also results in some reduction in the chloride penetration resistance of the concrete.