Hong C. Rhim

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.

Condition monitoring of deteriorating concrete dams using radar

Remote monitoring of the deterioration process of concrete dams has been studied by using radar to measure laboratory-size concrete specimens. The specimens represent five different physical conditions of deteriorating concrete dams. The investigation includes the manufacturing and the radar measurements of the specimens, and the implementation of signal processing schemes for generating two- and three-dimensional imagery of the specimens. The change in the radar signals that resulted from the specimens is related to the condition change inside the concrete. The results have shown the feasibility of using radar for monitoring the condition change inside concrete dams from a distance.

Radar imaging of concrete specimens for non-destructive testing

Abstract In this paper, the use of radar for non-destructive testing (NDT) of concrete is examined through radar measurements of laboratory size concrete specimens. In the measurements, the emphasis is given to the determination of the specimen thicknesses at three different microwave frequency ranges: 2 ∼ 3.4 GHz (S band), 3.4 ∼ 5.8 GHz (S/C band), and 8 ∼ 12 GHz (X band). The effect of different center frequency and bandwidth on measuring concrete thickness has been investigated. A signal processing algorithm implemented to obtain one-dimensional imagery of the specimens is briefly discussed.

Modeling of electromagnetic wave scattering by concrete specimens

Abstract This paper presents a novel application of finite difference-time domain (FD-TD) modeling to concrete to study the interaction of concrete with electromagnetic waves. The purpose of this work is to visualize the propagation of the electromagnetic fields in a dielectric medium of concrete in an effort to obtain onedimensional images of a concrete target for nondestructive testing purposes. A Gaussian pulse plane wave is directed to laboratory size concrete specimens as an excitation source. Snap shots of computer simulation are shown to display wave propagation and scattering through and by the concrete specimens. Geometry of the targets is varied with different dimensions, and with or without an inclusion.

Radar measurements of concrete for nondestructive evaluation of dams

Remote monitoring of a deterioration process has been studied through radar measurements of laboratory size concrete specimens representing conditions of concrete dams. The investigation includes manufacturing of concrete specimens which model different deterioration stages of a dam, radar measurements, and imaging of the specimens. The specimens have a delamination filled with air or water, which represent different physical conditions of a deteriorating dam. The change of the returned signals from the specimens is related to the condition change inside the concrete. The results show the feasibility of using a radar for monitoring the condition change of concrete dams at a distance.

Radar imaging of reinforced concrete specimens for nondestructive testing

Detection of steel bars embedded inside concrete for reinforcement has been one of the major goals of nondestructive testing methods for concrete. The use of radar in detecting steel bars is advantageous due to the sensitivity of electromagnetic waves to metallic objects and the versatility in using appropriate electromagnetic wave polarization waves to metallic objects and the versatility in using appropriate electromagnetic wave polarization with respect to the orientation of the bars. However, obtaining imagery of steel bars inside a lossy medium such as concrete requires sophisticated problem solving approaches including understanding of the electromagnetic properties of concrete, identification of proper radar measurement parameters as center frequency and bandwidth, and development of image reconstruction algorithms. This paper presents experimental results of radar measurements of laboratory size concrete specimens with or without steel bars. Sample radar imagery processed from raw measurement data is provided with discussion.

Assessment Model for the Safety and Serviceability of Steel Beams Using Terrestrial LiDAR

Structural monitoring is concerned with the safety and serviceability of the users of structures, especially for the case of building structures and infrastructures. When considering the safety of a structure, the maximum stress in a member due to live load, earthquake, wind, or other unexpected loadings must be checked not to exceed the stress specified in a code. Although the steel will not fail at yield, excessively large deflections will deteriorate the serviceability of a structure. Therefore, to guarantee the safety and serviceability of steel beams, the maximum stress and deflection in a steel beam must be monitored. However, no practical method has been reported to monitor both the maximum stress and deflection. In this paper, assessment model for both safety and serviceability of a steel beam is proposed. The model was tested in an experiment by comparing stress level estimated by LiDAR system and stress level directly measured from electrical or fiber optic sensors. The maximum deflection measured from LiDAR system is also compared with the maximum deflection directly measured from LVDTs. In addition to displacement measurement, the proposed system can provide information on deformed shapes of steel beams.

Magnetoelastic Measurements of Steel Stress under Uniaxial Loading

An attempt has been made to measure existing steel stress using magnetoelasticity. A device has been developed and used for the measurement of magnetism in response to the deformation of a steel bar. The proposed technique can be used for the assessment of existing reinforced concrete structures by the measurements of steel stress embedded inside concrete. A traditional technique requires to break the existing steel bar to measure existing strain. However, the proposed technique is developed to measure the stress without damaging the steel bar. A successful application of magnetoelasticity depends on the establishment of relationship between elastic and magnetic response due to loading. To investigate the correlation between the two, steel bars are loaded in tension under uniaxial loading while the magnetic reading is recorded. Based on the test results, equations are suggested to predict stress for steel bars with different diameters.

Shear Strength of the Perfobond Connection of a Steel-Concrete Composite Slim Floor System

The use of steel-concrete composite members has been significantly increased as they have the advantages of the reduction of cross sectional areas, excellent ductility against earthquake loadings and a longer life span than typical steel frame members. In this paper, push-out tests were performed on six specimens to investigate the structural behavior and shear strength of perfobond connection of a steel-concrete composite slim floor system. An equation to theoretically estimate the shear strength of the perfobond connection is proposed, and its accuracy is examined by comparing its predictions with the test results. A finite element analysis model is also developed and used to confirm the effectiveness of the proposed strength equation.

Detection of Debonding in Concrete Members Retrofitted with FRP Using Electromagnetic and Ultrasonic Methods

Microwave and ultrasonic methods have been used for the detection of debonding between concrete and fiber-reinforced plastic (FRP). To determine the capability of the microwave method in detecting thin delamination between two materials of concrete and FRP, concrete specimens are made with the dimensions of 600 mm (length) x 600 mm (width) x 50 mm (depth). Specimens have artificial delamination of 5 and 10 mm thick Styrofoam, which represent debonding in structures. Then, the specimens are partially covered with 1.5 mm thick FRP on the top of 3 mm thick epoxy. A horn antenna with a center frequency of 15 GHz and a frequency bandwidth of 10 GHz is used for the measurements. By transmitting and receiving microwave signals from the horn antenna, differences have been detected for the different types of the targets. Also, measurements using ultrasonic method at 5 kHz have been made on the same series of specimens. This work is directed toward a development of an effective and practical microwave based non-destructive evaluation methodology for the detection and quantification of damages in FRP-covered reinforced concrete members in bridges and buildings.