Chih-Hung Chiang

Acoustic inspection of bond strength of steel-reinforced mortar after exposure to elevated temperatures

In order to evaluate the bond strength between the reinforcement and concrete after fire damage, a combination of acoustic through-transmission and pull-out tests were used. Previous studies have shown a 25% decrease in the ultrasonic pulse velocity at 90% of the maximum load at room temperature. The specimens were kept in the oven at an elevated temperature for 1, 2, or 3 h. They were then removed and cooled to room temperature. Inspection was conducted using a high-power ultrasonic pulse velocity system while a pull-out load was applied. The correlation between preheated temperature, acoustic wave velocity, and the applied load was analyzed. Initial results show that bond strength and pulse velocity decreased substantially as the temperature or the heating time increased.

ARTIFICIAL NEURAL NETWORKS IN PREDICTION OF CONCRETE STRENGTH REDUCTION DUE TO HIGH TEMPERATURE

This paper demonstrates how artificial neural networks (ANN) can be used to predict the residual strength of heated concrete. The authors report on ongoing research that is trying to account for the effect of exposure time (to high temperatures) on residual strength of concrete. Results reveal that loss of strength becomes significant as exposure times are prolonged at exposure temperatures of 400 deg C. and higher. Analyses of residual pulse velocity and residual strength shows that a certain linear relationship exists for exposure times between 30 and 120 min. Due to the absence of a theoretical relationship, neural network analysis is applied to identify a possible general relationship between residual strength and variables including exposure temperature, exposure time, water-cement ratio (w/c), and residual pulse velocity. The authors note that ANN bears better tolerance than regression analysis to an imprecise data set. The authors conclude that good linear relationships are found when comparing the residual strength predicted by networks with correspondent target values. This type of testing may prove valuable as part of the assessment for fire-damaged concrete prior to the repair of structures.

Experimental study on the acoustic wave velocity in steel-reinforced mortar under external pull-out load

One of the major safety issues concerning the bond stress of mortar and concrete elements under external loads has been investigated. The relationship between bond stress and acoustic wave velocity has been analyzed for different cement/sand ratios. This study pioneers the combination of pull-out tests on standard concrete specimens with acoustic through-transmission measurements, and lays the groundwork for future on-site inspection of steel-reinforced structures. Changes in acoustic wave velocity were observed as the bond stress exceeded a certain level. As the external pull-out load (transferred to induced bond stress) increases, the acoustic wave velocity decreases. This phenomenon becomes very significant as the pull-out load approaches the maximum bond strength of the specimen. For a one-to-one cement/sand ratio, a mere 5% decrease in velocity ratio is critical to structural safety, since a 1.4 safety-factor can assure the bond condition only up to 71% of the maximum load, compared to 70% of the maximum load in more than 60% of the measurements.

Characterization of acoustic wave propagation in a concrete member after fire exposure

The acoustic wave propagation in a concrete member with embedded reinforcing bars was analyzed. Fire exposure was applied to two batches of concrete specimens prior to acoustic wave characterization. The fire duration and maximum temperature were simulated for experimental studies using a custom-built electric oven. A standard ultrasonic pulse velocity testing system for concrete was used to provide the through-transmission wave propagation. Multiple peaks were found in the frequency domain based on the fast Fourier transform of the waveform. This could be due to cracks induced by the incompatibility of thermal deformation of the constituents of concrete. Further study showed bond deterioration between reinforcing bars and concrete would also contribute to the variation in frequency content of the recorded waveform.

Remote monitoring and nondestructive evaluation of wind turbine towers

Wind turbine towers are in need of condition monitoring so as to lower the cost of unexpected maintenance. Wind loading from turbulence and gusts can cause damage in horizontal axis wind turbines even the supporting towers. Monitoring of wind turbines in service using embedded data sensor arrays usually is not targeted at the turbine-tower interaction from the perspective of structural dynamics. In this study the remote monitoring of the tower supporting a horizontal-axis wind turbine was attempted using a microwave interferometer. The dominant frequency of one tower was found to be decreased by more than 20% in 16 months. Numerical modeling using spectral finite elements is in progress and should provide further information regarding frequency shift due to stiffness variation and added mass. Expected outcome will contribute to remote monitoring procedures and nondestructive evaluation techniques for local wind turbine structures during operation.

Dynamic survey of wind turbine vibrations

Six wind turbines were blown to the ground by the wind gust during the attack of Typhoon Soudelor in August 2015. Survey using unmanned aerial vehicle, UAV, found the collapsed wind turbines had been broken at the lower section of the supporting towers. The dynamic behavior of wind turbine systems is thus in need of attention. The vibration of rotor blades and supporting towers of two wind turbine systems have been measured remotely using IBIS, a microwave interferometer. However the frequency of the rotor blade can be analyzed only if the microwave measurements are taken as the wind turbine is parked and secured. Time-frequency analyses such as continuous wavelet transform and reassigned spectrograms are applied to the displacement signals obtained. A frequency of 0.44Hz exists in both turbines B and C at various operating conditions. Possible links between dynamic characteristics and structural integrity of wind turbine –tower systems is discussed.

Evaluation of the Dynamic Characteristics of an Extradosed Bridge Using Microwave Interferometer

The microwave interferometer used in the present study is capable of multiple-point displacement measurements. In this paper, a general review of the principle and capacity of such an instrument is given. The vertical displacements of the Ai-Lan bridge corresponding to ambient vibration due to traffic loads are remotely monitored . Several modal shapes of the whole bridge are precisely identified by linking the mode shapes of seven sequential measurements.

Damage Assessment of Reinforced Concrete Beams by Complex Ratios of Transfer Functions

In this paper, a newly developed approach for the determination of characteristics associated with local defects of a beam is briefly introduced. From the numerical studies carried out in this study, the complex ratio between two transfer functions associated with an imaginary intact beam and a damaged beam can provide such information. Based on the numerical studies, the rules in determining degree of damages and locations of defects were first briefly outlined. These idealized rules were then verified by experimental data obtained from dynamic tests of realistic specimens of simply-supported reinforced concrete beams. The preliminary results indicate that the defined ratio of transfer functions can potentially serve as an exaggerated indication for the degree of changes in certain modal responses. Thus, the proposed ratio of transfer functions can be used to assist system identification, while the nature of high sensitivity also restricts its direct application to certain complicated data associated with field tests. A variety of test setup for both receivers and impact sources were studied and the test results appear to agree with designated conditions of the specimens.

Assessment of Post-Fire Residual Strength of Reinforced Concrete Using Ultrasonic Pulse Velocity

High temperature may cause the degradation of concrete strength and subsequent loss of steel-concrete bond. Based on the results of a previous study on the correlation of pulse velocity and residual strength, this research aims to examine further the feasibility of nondestructive testing toward in-situ assessment of concrete structures after fire exposure or under constant exposure to high temperature. Ultrasonic pulse velocities of cylindrical concrete specimens are measured both before and after simulated fire exposure. Residual compressive strengths are obtained from compressive tests. The effects of exposure time, up to two hours, and maximum exposure temperature, up to 800 degree Celsius, are both analyzed. It is confirmed that residual compressive strength of standard 12 by 24 centimeter concrete specimens decreases as exposure time is increased. Regression analyses show strong linear relationship exists between pulse velocity and residual strength over the exposure time ranging from 30 to 120 minutes and the maximum exposure temperature ranging from 500 to 700 degree Celsius. The captured waveforms are also processed using continuous wavelet transform that provides simultaneously frequency and time information. Signal features can thus be identified. The findings can be used as a basis for assessing the post-fire residual concrete strength. Introduction The post-fire assessment of residual material properties of concrete structures has been studied extensively. Maximum exposure temperature, exposure time, and heating and cooling rates are among the most important factors to be considered in such assessment. The effect of temperature on compressive strength of concrete has been thoroughly explored and data well established for various concrete mixtures in the literatures [1, 2]. The effect of exposure time, on the other hand, was barely noticed. Early work by Mohamedbhai [3] discussed residual strength of concrete due to transient temperature variation. He concluded that exposure time beyond one hour has a significant effect on residual strength. However, the effect diminished as the level of exposure temperature increased. Almost all the loss of compressive strength occurred within two hours of exposure to the maximum temperature. Rates of heating and cooling had no effect on the residual strength of concrete heated to 600 o C and beyond. Their effects on concrete heated to lower temperatures were more pronounced. No further analysis was presented for the underlying observation. The residual strength is apparently related to the transient temperature variation. Appropriate model is to be developed for analyzing such a nonlinear relation. Condition assessments of structure and material deterioration often include the application of nondestructive testing methods such as the stress wave techniques. For example, areas of critical damage were examined by the ultrasonic pulse velocity method as part of the evaluation prior to repair of fire damaged concrete structures [4-6]. The pulse velocity can be applied quickly and indicate the area of concrete of inferior quality. It can also be used as an estimate of compressive strength following the procedures suggested by ACI [7]. Some uncertainties may still be involved and caution should be taken. Nevertheless, the relatively fast implementation of the pulse velocity method makes it quite attractive to the assessment of fire-acted sites, as the extreme environmental conditions may limit even prevent more complex experimental investigations. Key Engineering Materials Online: 2004-08-15 ISSN: 1662-9795, Vols. 270-273, pp 1506-1512 doi:10.4028/www.scientific.net/KEM.270-273.1506

Proper source-receiver distance to obtain surface wave group velocity profile for flaw detection inside a concrete plate-like structure

A technique leads to rapid flaw detection for concrete plate-like structure is realized by obtaining the group velocity dispersion profile of the fundamental antisymmetric mode of the plate (A0 mode). The depth of a delaminating crack, honeycomb or depth of weak surface layer on top of the sound concrete can all be evaluated by the change of velocity in the dispersion profile of A0 mode at the wavelength about twice of the depth. The testing method involves obtaining the A0 group slowness spectrogram produced by single test with one receiver placed away from the source of impact. The image of the spectrogram is obtained by Short-Time Fourier Transfer (STFT) and enhanced by reassigned method. The choice of window length in STFT and the ratio between impactor-receiver distance and plate thickness, d/T, is essential as the dominant surface wave response may simply a non-dispersive Rayleigh wave or following the A0 or S0 (fundamental symmetric mode) modal dispersion curve. In this study, the axisymmetric finite element model of a plate subject to transient load was constructed. The nodal vertical velocity waveforms for various distances were analyzed using various STFT window lengths. The results show, for certain d/T ratio, S0 mode would be dominant when longer window is used. The best window lengths for a d/T ratio as well as the corresponding largest wavelength which follows the A0 theoretical dispersion curve or Rayleigh wave were summarized. The information allows people to determine the proper impactor-receiver distance and analyzing window to successfully detect the depth of flaws inside a plate.A technique leads to rapid flaw detection for concrete plate-like structure is realized by obtaining the group velocity dispersion profile of the fundamental antisymmetric mode of the plate (A0 mode). The depth of a delaminating crack, honeycomb or depth of weak surface layer on top of the sound concrete can all be evaluated by the change of velocity in the dispersion profile of A0 mode at the wavelength about twice of the depth. The testing method involves obtaining the A0 group slowness spectrogram produced by single test with one receiver placed away from the source of impact. The image of the spectrogram is obtained by Short-Time Fourier Transfer (STFT) and enhanced by reassigned method. The choice of window length in STFT and the ratio between impactor-receiver distance and plate thickness, d/T, is essential as the dominant surface wave response may simply a non-dispersive Rayleigh wave or following the A0 or S0 (fundamental symmetric mode) modal dispersion curve. In this study, the axisymmetric fini...