Yiching Lin

Transient response of thick circular and square bars subjected to transverse elastic impact

The objective of the studies presented in this paper was to understand the transient response of thick circular and square bars subjected to transverse elastic point impact. It is shown that the transient response is composed of a number of resonant frequencies caused by cross‐sectional modes of vibration. The individual cross‐sectional modes and their corresponding natural frequencies were determined using plane strain and three‐dimensional finite‐element models of circular and square cross sections. The first few modes for both circular and square cross sections are shown. Subsequently, three‐dimensional finite‐element models of circular and square bars were used to determine the transient response caused by transverse point impact. To verify the results obtained from the numerical models, experimental studies were performed on a 0.4‐m‐diam circular bar and a 0.3‐m square bar. These specimens were representative of full‐size concrete columns—typical barlike structural elements. The results of these expe...

USE OF STRESS WAVES FOR DETERMINING THE DEPTH OF SURFACE-OPENING CRACKS IN CONCRETE STRUCTURES

The objective of the studies described in this paper was to determine the depth of surface-opening cracks in concrete structures via stress waves so that a safety evaluation of such cracked structures could be conducted. Laboratory samples containing vertical and inclined surface-opening cracks with known depths were used in the studies. Two displacement transducers were placed on opposite sides of a surface-opening crack to monitor disturbances caused by the arrivals of stress waves generated by elastic impact. The recorded displacement waveforms were analyzed primarily in the time domain. Frequency analysis was conducted to complement the signal analysis. It was learned that as impact is applied on one side of a surface-opening crack, the P-wave arrival diffracted from the bottom edge of the crack causes an initial disturbance and is easy to identify in the waveform recorded at a transducer found on the opposite side of the crack. Also, the time of impact initiation can be obtained by identifying the R-wave arrival in the waveform recorded at a transducer on the impact side. As a result, the P-wave travel time from the impact point through the crack tip to the receiver on the opposite side of the crack can be obtained to determine the crack depth. In addition to the time-domain analysis, frequency analysis can be used to ascertain whether the surface-opening crack under testing is approximately perpendicular to the surface.

IMPACT-ECHO RESPONSE OF CONCRETE SHAFTS

Numerical and experimental studies of the transient response of concrete shafts subjected to elastic impact were carried out using the finite element method and the impact-echo testing technique. Two- and three-dimensional finite element studies of concrete shafts were performed for: (a) solid shafts; (b) shafts containing cracks, voids, layers of low-quality concrete, and changes in cross section, such as bulges and necks; and (c) shafts in soil. These studies were carried out to gain an improved understanding of the impact response of concrete shafts containing flaws, problems for which there are currently no analytical solutions. It was also the intent of the numerical studies to determine whether information in addition to that obtained with existing nondestructive testing techniques could be obtained during impact testing of concrete shafts. Laboratory studies of solid concrete shafts were carried out to verify the finite element models. Subsequently, experimental studies were carried out on concrete shafts embedded in soil. These shafts contained flaws at known locations. The results of these studies show that additional information about the integrity of drilled shafts and piles can be obtained using impact techniques. The numerical studies also show that the finite element method is a powerful tool for studying the impact response of concrete foundation elements.

Transient response of thick rectangular bars subjected to transverse elastic impact

The objective of the research presented in this paper was to study the transient response of thick rectangular bars subjected to transverse elastic point impact. It is shown that the transient response is composed of a number of resonant frequencies caused by cross‐sectional modes of vibration. The individual modes and their corresponding natural frequencies were determined using plane‐strain, finite‐element models of rectangular cross sections. The effect of the cross‐section aspect (depth/width) ratio on the natural frequencies is shown. Three‐dimensional, finite‐element models of rectangular bars were analyzed to determine the transient response caused by transverse point impact. To verify the results obtained from the numerical studies, experimental studies were performed on rectangular bars with aspect ratios of 0.60, 0.75, 1.33, and 1.67. The bars used in the experimental study were representative of full‐scale concrete beams and columns—typical barlike structural elements. Excellent agreement was o...

Finite element studies of the impact-echo response of plates containing thin layers and voids

Quantitative nondestructive evaluation based on the use of transient stress waves generated by point impact is hindered by the fact that the governing partial differential equations admit closed-form solutions for only the most trivial cases, an infinite half-space and an infinite plate. In previous studies carried out by the authors, the finite element method has been shown to provide useful numerical solutions for a variety of cases involving bounded solids containing flaws. Numerical results have been verified with carefully controlled experiments. Currently, the method is being used to establish the basis for a new nondestructive evaluation technique for civil engineering structures. This technique is called impact-echo, and it is based on the use of low frequency, transient stress waves generated by elastic point impact. In this paper, the impact-echo response of plates containing thin layers is studied using finite element models. The purpose of these studies was to determine the applicability of using the method for detecting voids in layered civil engineering structures. Results of the numerical studies show that it is feasible to use the impact-echo method for this application.

Investigation of Pulse Velocity-Strength Relationship of Hardened Concrete

The purpose of this paper is to investigate the relationship between the ultrasonic pulse velocity (UPV) and the compressive strength of concrete. The specimens used in the studies were made of concrete with a paste content of 36% and the constituents of the, specimens varied in different water-cement ratios (w/c) and coarse aggregate contents by weight. The UPV measurement and compressive strength tests were carried out at the concrete age of 1, 3, 7, 14, and 28 days. The experimental results show that the relationship between UPV and the compressive strength of concrete is significantly influenced by age and coarse aggregate content. The UPV and the compressive strength of concrete grow with age, but the growth rate varies with mixture proportion. To simplify the analysis task, this study chose hardened concrete (at an age of 28 days) as the subject for analysis. It is found that with the same content of coarse aggregate, a clear relationship curve can be drawn to describe the UPV and compressive strength of hardened concrete. This paper proposes the UPV and strength relationship curves for concrete having different contents of coarse aggregate. These curves were verified to be suitable for prediction of hardened concrete strength with a measured UPV value.

Method for determining internal P-wave speed and thickness of concrete plates

This paper discusses the development of the time-of-flight reflection method, a new method for determining the internal P-wave speed of a plate-like concrete structure. The authors performed numerical and experimental studies in a two-step test procedure that combined the time-of-flight reflection method and the impact-echo technique. The studies examined P-wave behavior fundamentals and identified its arrival via the waveform recorded by a receiver located on the impact surface. Results showed that the arrival of the P-wave reflected from the plate bottom surface caused a noticeable upward disturbance in the waveform that is easily identified, due to the receiver and the impact point being 2.43 times the plate thickness.

EVALUATING BOND QUALITY AT STEEL/CONCRETE INTERFACES USING THE NORMALIZED IMPACT-ECHO SPECTRUM

The objective of this paper is to develop a nondestructive test technique based on stress wave propagation for evaluating the bond quality of concrete structures strengthened by externally bonded steel plates. In this paper, a newly developed technique based on the normalized impact-echo spectrum was used to quantitatively detect the size of air voids at the interfacial epoxy layer between steel plate and concrete. Experimental results show that in the normalized impact-echo spectra, the amplitude of the peak at the frequency corresponding to the wave reflections at the steel/epoxy interface increases with an increase in the air void size. The bond strength decreases with an increase in the peak amplitude due to the increasing size of the flaw.

Impact-Echo Response of Plates Containing Thin Layers and Voids

An ongoing research program initiated in 1983 by Carino and Sansalone has been aimed at developing the theoretical basis and practical applications for a new nondestructive technique for detecting flaws in reinforced concrete structures. The technique, known as impact-echo, is well documented [1–6] and only a brief overview of the principle of the method, signal processing techniques, and instrumentation is presented. This paper highlights the results of a recent investigation into the feasibility of using the impact-echo technique to detect voids in plates containing thin layers of materials having different acoustic impedances.

Nondestructive evaluation of concrete quality and integrity in composite columns

This paper summarizes the results obtained from the studies aimed at determining the feasibility of using the impact-echo technique for nondestructive evaluation of concrete quality and integrity in composite columns. Both numerical and experimental studies are carried out to gain a thorough understanding of the transient impact response of composite columns with/without flaws. Results obtained from these studies show that the impact response of a solid composite column is composed of a number of cross-sectional modes of vibration generated by multiple wave reflections between close proximity of boundaries. The frequency of the fundamental mode can be used to determine the P-wave speed in concrete. The presence of flaws disrupts the spectral pattern associated with the solid case. A shift of the fundamental frequency to a lower value is the key to identifying the presence of a flaw. In addition, the spectrum will also show a high amplitude peak at the frequency corresponding to the depth of the flaw.