Oral Buyukozturk

PROGRESS ON UNDERSTANDING DEBONDING PROBLEMS IN REINFORCED CONCRETE AND STEEL MEMBERS STRENGTHENED USING FRP COMPOSITES

Abstract Use of fiber reinforced plastic (FRP) composite materials for strengthening and repair of structural members has become an increasingly popular area of research and application in the last decade. However, the method is yet to become a mainstream application due to a number of economical and design related issues. From a structural mechanics point of view, an important concern regarding the effectiveness and safety of this method is the potential of brittle debonding failures. Such failures, unless adequately considered in the design process, may significantly decrease the effectiveness of the strengthening or repair application. In recent years, there has been a concentration of research efforts on characterization and modeling of debonding failures. This paper provides a review of the progress achieved in this area regarding applications to both reinforced concrete and steel members.

Electromagnetic Properties of Concrete at Microwave Frequency Range

Electromagnetic properties of hardened concrete specimens are measured over a microwave frequency range from 0.1 GHz to 20 GHz. The experimentally obtained values provide information about the behavior of concrete as a material in its interaction with electromagnetic waves. In addition to the frequency variation, the effect of different moisture contents on the electromagnetic properties are studied. Properties of mortar specimens and constituents of concrete--that is, coarse aggregates, sand, and cement--are also measured. An open-ended coaxial probe method is used for the measurement of real and imaginary parts of complex permittivity of concrete. The physical significance of the measured data in nondestructive testing, including penetration depth and detectability, is discussed and demonstrated through radar measurement results. The results of this research work will serve as a basis in applying wideband microwave imaging techniques for nondestructive testing of concrete using radar.

Imaging of concrete structures

Abstract Demand for the development of non-destructive testing (NDT) techniques for concrete structures has increased with the growing concern about the deteriorating condition of the Worlds infrastructure. Efficient and accurate imaging techniques are needed for a reliable evaluation of safety and serviceability of concrete structures. Although, presently, imaging is routinely used in various fields, implementation of these technologies in NDT of civil engineering systems, especially of concrete structures, offers many challenges and requires additional development due to the composite nature of the concrete material and the complexities of reinforced or prestressed concrete systems. This paper presents the basic principles of various imaging techniques associated with several NDT methods applicable to concrete structures. The techniques considered are radiography, radioactive computerized tomography, infrared thermography, radar imaging and acoustic imaging. Special considerations regarding the applicability and accuracy of these techniques for the condition assessment of concrete structures are discussed, and examples of imaging applications are given.

Deep Learning-Based Crack Damage Detection Using Convolutional Neural Networks

A number of image processing techniques IPTs have been implemented for detecting civil infrastructure defects to partially replace human-conducted onsite inspections. These IPTs are primarily used to manipulate images to extract defect features, such as cracks in concrete and steel surfaces. However, the extensively varying real-world situations e.g., lighting and shadow changes can lead to challenges to the wide adoption of IPTs. To overcome these challenges, this article proposes a vision-based method using a deep architecture of convolutional neural networks CNNs for detecting concrete cracks without calculating the defect features. As CNNs are capable of learning image features automatically, the proposed method works without the conjugation of IPTs for extracting features. The designed CNN is trained on 40 K images of 256 × 256 pixel resolutions and, consequently, records with about 98% accuracy. The trained CNN is combined with a sliding window technique to scan any image size larger than 256 × 256 pixel resolutions. The robustness and adaptability of the proposed approach are tested on 55 images of 5,888 × 3,584 pixel resolutions taken from a different structure which is not used for training and validation processes under various conditions e.g., strong light spot, shadows, and very thin cracks. Comparative studies are conducted to examine the performance of the proposed CNN using traditional Canny and Sobel edge detection methods. The results show that the proposed method shows quite better performances and can indeed find concrete cracks in realistic situations.

Nonlinear analysis of reinforced concrete structures

Abstract For the prediction of yield and failure of concrete under combined stress, a generalization of the Mohr-Coulomb behavior is made in terms of the principal stress invariants. The generalized yield and failure criteria are developed to account for the two major sources of nonlinearity: the progressive cracking of concrete in tension, and the nonlinear response of concrete under multiaxial compression. Using these criteria, incremental stress-strain relationships are established in suitable form for the nonlinear finite element analysis. For the analysis of reinforced concrete members by finite elements, a method is introduced by which the effect of reinforcement is directly included. With this approach, the stress-strain laws for the constituent materials of reinforced concrete are uncoupled permitting efficient and convenient implementation of a finite element program. The applicability of the method is shown on sample reinforced concrete analysis problems.

Structural Damage Detection Using Modal Strain Energy and Hybrid Multiobjective Optimization

Modal strain energy (MSE) is a sensitive physical property that can be utilized as a damage index in structural health monitoring. Inverse problem solving-based approaches using single-objective optimization algorithms are also a promising damage identification method. However, the research into the integration of these methods is currently limited; only partial success in the detection of structural damage with high errors has been reported. The majority of previous research was focused on detecting damage in simply supported beams or plain structures. In this study, a novel damage detection approach using hybrid multiobjective optimization algorithms based on MSE is proposed to detect damages in various three-dimensional (3-D) steel structures. Minor damages have little effect on the difference of the modal properties of the structure, and thus such damages with multiple locations in a structure are difficult to detect using traditional damage detection methods based on modal properties. Various minor damage scenarios are created for the 3-D structures to investigate the newly proposed multiobjective approach. The proposed hybrid multiobjective genetic algorithm detects the exact locations and extents of the induced minor damages in the structure. Even though it uses incomplete mode shapes, which do not have any measured information at the damaged element, the proposed approach detects damage well. The robustness of the proposed method is investigated by adding 5% Gaussian random white noise as a noise effect to mode shapes, which are used in the calculation ofMSE.

Crack propagation in concrete composites influenced by interface fracture parameters

Abstract The mechanical behavior of concrete composites is influenced by the characteristics of mortar aggregate interfaces. Initiation and propagation of cracks at the interface or penetration of cracks into the aggregate can greatly influence the global behavior of the material. In the interfacial regions of concrete composites the crack path criterion will involve relative magnitudes of the fracture toughnesses between the interface and the constituent materials. This study investigates fracture of two-phase composites in terms of parameters that influence the cracking scenarios in the interfacial regions and affect the fracture behavior of the concrete. These parameters include elastic moduli mismatch between the mortar and the aggregate and the ratios of the interface fracture toughness to the fracture toughness of the aggregate and the mortar. Numerical and physical model tests were performed to study the influence of these variables on the global load-deformation behavior of composite beams. Physical beam models consisting of circular aggregate inclusions in mortar matrices were tested in three-point bending. An analysis capability is developed for cracking in the composite incorporating transgranular or interfacial fracture scenarios using finite element simulations performed with the material fracture properties. The simulation is of a cohesive force type that allows parametric variation of fracture parameters to study influences on load-deformation performance of the composite. Results of the simulation are used to quantify the effect of different interfacial properties on the fracture and load-deformation behavior of the specimens. The results of both the experimental and analytical model studies show that ductility improves when cracks propagate through mortar-aggregate interfaces and also improves with rougher aggregate surfaces. This study advances the understanding of the role of interfaces in the global behavior of the cementitious composites and furthers the development of high-performance cementitious materials.

BEHAVIOR OF FIBER REINFORCED HIGH-STRENGTH CONCRETE UNDER DIRECT SHEAR

Experimental and modeling studies were performed on two types of fibers, polypropylene and steel fibers, in conjunction with or without conventional stirrups. In general, fibers proved to be more effective in high-strength concrete than in normal strength concrete, increasing both ultimate load and overall ductility. For specimens with steel fibers, significant increases in ultimate load and ductility were observed. With polypropylene fibers, a lower increase in ultimate load was obtained when compared to the increase due to steel fibers. Ductility of the polypropylene fiber specimens was greater than that of the steel fiber reinforced specimens. In tests with combinations of fibers with stirrups, slight increases in ultimate load with major improvements in ductility were noted in comparison to the values for plain concrete specimens with conventional stirrups.

Nondestructive Testing of Materials and Structures

Condition assessment and characterization of materials and structures by means of nondestructive testing (NDT) methods is a priority need around the world to meet the challenges associated with the durability, maintenance, rehabilitation, retrofitting, renewal and health monitoring of new and existing infrastructures including historic monuments. Numerous NDT methods that make use of certain components of the electromagnetic and acoustic spectrum are currently in use to this effect with various levels of success and there is an intensive worldwide research effort aimed at improving the existing methods and developing new ones. The knowledge and information compiled in this book captures the current state of the art in NDT methods and their application to civil and other engineering materials and structures. Critical reviews and advanced interdisciplinary discussions by world-renowned researchers point to the capabilities and limitations of the currently used NDT methods and shed light on current and future research directions to overcome the challenges in their development and practical use. In this respect, the contents of this book will equally benefit practicing engineers and researchers who take part in characterization, assessment and health monitoring of materials and structures.

Constitutive modeling of concrete in finite element analysis

Abstract Approaches generally used in defining constitutive relations for concrete are reviewed. A computer program developed for the three-dimensional finite element analysis of complex reinforced, prestressed, and refractory concrete systems is described. The constitutive models based on isotropic elastic, orthotropic elastic, and plasticity formulations, which are implemented in that program, are discussed in detail. The program incorporates nonlinear material properties, cracking in concrete, shear transfer in cracked reinforced concrete sections, and time dependent effects such as creep, shrinkage, and transient temperature distributions. A wide range of structural problems are analyzed to demonstrate the applicability of the computer program. Comparisons between predictions with different constitutive models, and between predictions and test results are made.