Farhad Ansari

HIGH-STRENGTH CONCRETE SUBJECTED TO TRIAXIAL COMPRESSION

The behavior of high-strength concrete subjected to multiaxial states of stress was studied. An experimental program was undertaken to quantitatively determine the failure surface for high-strength concrete. Results from this study provide the means to predict the failure condition for high-strength concrete under combined stresses. The experimental program was comprised of testing high-strength concretes at three different compressive strength levels. The three strength levels included concretes with compressive strengths of 6 ksi (42 MPa), 10 ksi (69 MPa), and 15 ksi (103 MPa). The triaxial tests were performed on 4-by-8 in (100-by-200 mm) cylindrical specimens. The confining pressures employed in the experiments ranged from 1,200 psi (8.3 MPa) to 12,000 psi (82 MPa). A series of uniaxial tension and compression tests were also performed to develop the necessary data for establishment of the failure criterion. Empirical relationships were developed for prediction of axial strength as a function of confining pressure. In general, the axial strength of high-strength concrete increases with increased confining pressure. However, in comparison with the normal-strength concrete, the effect of confining pressure on the failure strength of high-strength concrete is less pronounced.

Practical Implementation of Optical Fiber Sensors in Civil Structural Health Monitoring

Implementation of successful civil structural health monitoring strategies requires selection and placement of sensors suitable for measurement of key parameters that influence the performance and health of the structural system. Optical fiber sensors have been successfully implemented in aeronautics, mechanical systems, and medical applications. Civil structures pose further challenges in monitoring mainly due to their large dimensions, diversity as well as heterogeneity of materials involved, and hostile construction environment. This article provides a summary of basic principles pertaining to practical health monitoring of civil engineering structures with optical fiber sensors. The issues discussed include basic sensor principles, strain transfer mechanism, sensor packaging, sensor placement in construction environment, and reliability and survivability of the sensors.

STATE-OF-THE-ART IN THE APPLICATIONS OF FIBER-OPTIC SENSORS TO CEMENTITIOUS COMPOSITES

Abstract Optical-fiber sensors are emerging as a superior non-destructive means for evaluating the condition of concrete structures. In contrast to existing non-destructive evaluation techniques, optical fibers are able to detect minute variations in structural conditions through remote measurements. Structures fully integrated with optical fibers will be able to monitor the initiation and progress of various mechanical or environmentally-induced degradations in concrete elements. Recent advances in fiber-optic sensor technology and the possibility of their use in concrete structures have instigated the development of a number of research activities. Owing to inherent interdisciplinary nature of the field of fiber optics, the expertise of the researchers active in the research and development of fiber-optic sensors covers a wide spectrum of disciplines including concrete engineering, as well as opto-electronics and physics. This article is intended for rapid dissemination of the current state-of-the-art in this emerging technology. However, because of the interdisciplinary nature of the subject, a brief discussion on the physical nature of optical fibers is also presented.

Numerical Evaluation of Vibration‐Based Methods for Damage Assessment of Cable‐Stayed Bridges

: This study investigated a number of different damage detection algorithms for structural health monitoring of a typical cable-stayed bridge. The Bayview Bridge, a cable-stayed bridge in Quincy, Illinois, was selected for the study. The focus was in comparing the viability of simplified techniques for practical applications. Accordingly, the numerical analysis involved development of a precise linear elastic finite element model (FEM) to simulate various structural health monitoring test scenarios with accelerometers. The Effective Independence Method was employed to locate the best distribution of the accelerometers along the length of the bridge. The simulated accelerometer data based on the FEM analysis was employed for the evaluation of the four damage identification methods investigated here. These methods included the Enhanced Coordinate Modal Assurance Criterion, Damage Index Method, Mode Shape Curvature Method, and Modal Flexibility Index Method. Some of these methods had been previously applied only to a number of specific bridges. However, the investigation here provides the relative merits and shortcomings of the damage detection methods in long-span cable-stayed bridges.

Fiber optic health monitoring of civil structures using long gage and acoustic sensors

Structural health monitoring with optical fibers provides practical sensing capabilities in many applications including in aeronautics and mechanical structures. A variety of optical fiber sensors have been used including Bragg gratings, intensity or amplitude sensors, and Fabry–Perot ones. Civil structures pose further challenges in monitoring mainly due to their large dimensions, diversity as well as heterogeneity of materials involved, and hostile construction environment. Monitoring of strains, deformations, and deflections provides clues essential for evaluation of design parameters and behavior under service loads. Long gage distributed or multiplexed sensors are excellent candidates for such applications. On the other hand, detection of structural damage and anomalies such as cracking in concrete, splintering of fibers in composites, and fracturing of welds and connections are best accomplished by acoustic sensors. This paper describes principles involved in serial multiplexing of two kinds of optical fibers, namely long gage and acoustic sensors. Both sensor types offer promise in structural health monitoring of large civil structural systems. Representative examples are introduced and described in detail.

White-light interferometric fiber-optic distributed strain-sensing system

Abstract A novel technique of using different lengths of optical fiber as strain sensors in a white-light interferometer to measure the strain distribution is presented. The technique for connecting the fiber sensors and the method of calculating the intensity of the transmitted and reflected light are given. The measuring principle and a four-sensor distributed fiber-optic strain-measuring system are demonstrated. Calibration experiments results show that the relationship of the strain between the embedded fiber and the matrix is linear and the sensitivity of the fiber sensor is a function of the sensing length of the embedded fiber.

Theoretical and Experimental Investigations into Crack Detection with BOTDR-Distributed Fiber Optic Sensors

AbstractDevelopment of a model for the analysis of strain transfer mechanism in Brillouin-based sensors with strain singularities is provided in this study. The main objective of the research pertained to the development of a method for accurate detection of cracks and their locations in sensing with Brillouin-based fiber optic distributed sensors. The work involved formulation of a shear lag–based model considering the elastic as well as elastoplastic stages of the fiber optic coating strains. Feasibility of the proposed approach is evaluated through an experimental program. The experimental program involved use of a Brillouin optical time domain reflectometer (BOTDR) for distributed measurement of strain and detection of simulated cracks in a 15-m-long beam. The results indicate that the discontinuities in the strain distribution based on the theoretical analysis provide the means to accurately pinpoint the location of simulated cracks. On the other hand, the distortion effect of the BOTDR system due to...

Vibration-Based Method and Sensor for Monitoring of Bridge Scour

Scour is the major cause for many bridge failures and damage to piers and abutments. Scour is not easily discernible because it is hidden under the channel flow. Over the years, a number of sensors have been developed for detection of scour depth. Development, testing, and field implementation of a new and simple type of scour sensor is described in this paper. The scour depth detection concept is based on measuring the fundamental frequency of vibration of a rod embedded in the riverbed. The sensor uses a single fiber-optic Bragg grating (FBG) sensor for transduction of the vibration frequency. The inverse relationship between the fundamental frequency and the length of the sensor rod is used for detection of the scour depth. A computational approach is developed based on the Winkler spring reaction soil model for automated calibration of the scour sensor during installation in the riverbed. The scope of the research included development of the theoretical basis for the sensor, establishment of the computational methodology for detection of the riverbed foundation properties, proof-of-concept laboratory tests, small-scale field verification tests, and installation and remote monitoring of scour in a multispan scour critical bridge in Illinois. The results include laboratory test data from the measurements in soil, simulated scour tests in a hydraulic flume, and real-time data from remote monitoring of scour at the bridge site.

Embedded fiber optic sensor for characterization of interface strains in FRP composite

Fiber reinforced polymer composites (FRP) have found widespread usage in repair and strengthening of concrete structures. FRP composites exhibit high strength to weight ratio, corrosion resistance, and convenient to use in repair applications. Several methods have been devised for repair of concrete beams with FRP fabrics, including wrapping of the cracked members, or adhesion of the fabric to the tension face of the members. A common cause of failure in such members is associated with debonding of the FRP substrate from the concrete in an abrupt manner. The mechanism of debonding is investigated through embedment of a distributed optical fiber sensor at the interface between the cracked concrete and the FRP fabric during repair of reinforced concrete beams under load. The fiber optic system consists of segmented long gauge length sensors along the length of an embedded optical fiber. This arrangement allows for complete interrogation of the interface deformations and debonding phenomenon. Experimental results pertaining to the load testing of FRP-repaired reinforced concrete beams are reported.

HIGH-STRENGTH CONCRETE IN TRIAXIAL COMPRESSION BY DIFFERENT SIZES OF SPECIMENS

This paper introduces some experimental data for very high-strength concrete in triaxial compression with very high confining pressures and comparison with available data for different sizes of cylindrical specimens. The experimental data pertain to failure strength and stress-strain curves of high-strength concrete (HSC) in biaxial compression with very high confining pressures by 3-by-6 in. cylinders. Comparisons with the available publications were made. The comparison shows a slight size effect on the failure strength of HSC in triaxial compression.