Hwa Kian Chai

Characterization of Deep Surface-Opening Cracks in Concrete: Feasibility of Impact-Generated Rayleigh-Waves

This paper studies the feasibility of impact-generated Rayleigh waves (R-waves) for measuring deep surface-opening cracks in concrete structures. The aim is to contribute to a methodology for simple and effective in-place crack depth estimation. Specimens induced with vertical slits of different depths were prepared for measurement. A 2-sensor array was implemented and elastic waves of different central frequencies were generated by mechanical impacts with steel- ball hammers of different ball diameters. R-wave amplitudes were extracted from the waveforms. Attenuation of R-waves due to diffraction and scattering by the slits and the trend of amplitude decaying with slit depth were examined. A reasonable correlation between the amplitude factor and slit depth-to-wavelength ratio was established, indicating a loss of sensitivity in the change of amplitude factor with regard to dominant wavelengths smaller than the slit depth. By comparing results of the P-wave time-offlight (TOF) method, the results by measuring again using the proposed method confirmed the feasibility of R-wave attenuation as an alternative parameter for characterizing surface-opening cracks. In addition, the potential problems associated with the reliability of P-wave TOF method in estimating a crack with limited length were also demonstrated.

Assessment of Reinforced Concrete Surface Breaking Crack Using Rayleigh Wave Measurement

An improved single sided Rayleigh wave (R-wave) measurement was suggested to characterize surface breaking crack in steel reinforced concrete structures. Numerical simulations were performed to clarify the behavior of R-waves interacting with surface breaking crack with different depths and degrees of inclinations. Through analysis of simulation results, correlations between R-wave parameters of interest and crack characteristics (depth and degree of inclination) were obtained, which were then validated by experimental measurement of concrete specimens instigated with vertical and inclined artificial cracks of different depths. Wave parameters including velocity and amplitude attenuation for each case were studied. The correlations allowed us to estimate the depth and inclination of cracks measured experimentally with acceptable discrepancies, particularly for cracks which are relatively shallow and when the crack depth is smaller than the wavelength.

Monitoring Corrosion Process of Reinforced Concrete Structure Using FBG Strain Sensor

A Major factor that affects the durability of a concrete structure is cracks formation induced by expansion of reinforcement corrosion. Therefore, monitoring and evaluating the corrosion level of the structure is essential for its safety. In order to monitor corrosion, an innovative methodology based on Fiber Bragg Grating (FBG) sensing technique was developed and tested in this paper. The method uses the volume of corrosion products to detect the evolution of corrosion. The corrosion process was accelerated by impressed current technique. A correlation between the FBG wavelength shift and corrosion percentage of reinforcement was found.

Torsional Analysis of Multicell Concrete Box Girders Strengthened with CFRP Using a Modified Softened Truss Model

Deterioration and aging of concrete structures are not the only reasons why critical members, such as beams, need to be strengthened. Other possible reasons include upgrading of design standards, compensation for faulty design or construction, changing usage of the structure, and exposure to unpredicted loads, such as impacts or excessive earthquakes. The current study proposes a new analytical method for predicting the torsional capacity and behavior of RC multicell box girders strengthened with carbon-fiber-reinforced polymer (CFRP) sheets. The conventional softened truss model is modified to consider the influence of CFRP strengthening on the torsional response as an external reinforcement of multicell box girders. The proposed method involves solving the concrete torsional problem by combining the equilibrium conditions, compatibility conditions, and constitutive laws of materials, taking into account the confinement of concrete with CFRP sheets. A specific algorithm was developed to predict the torsional behavior of RC multicell box girders with and without CFRP strengthening. Verification of this method was achieved using extensive comparisons between analytically predicted behavior curves and experimentally obtained ones. The experimental work comprised a series of four torsion tests for single- and triple-cell box girders. Good agreement between the results was obtained, confirming the feasibility of the newly proposed method. Based on the outcome of the comparisons, it was revealed that the only currently available, recommended torsion design method was generally conservative, suggesting that further refinement is essential to make it a suitable design method for torsional strengthening using CFRP sheets.

Health Monitoring of Civil Infrastructure and Materials

Despite the generally long life span of concrete structures, they cannot be considered maintenance-free. Several incidents of catastrophic failures remind the engineering world that monitoring of structures is imperative nowadays both for prevention of loss of life and property and also for effective infrastructure management based on a usually finite budget. Due to the variety of structural sizes, shapes, and uses, as well the possible vulnerabilities of the different types of structures, different tools must be used, many times complementary, in order to lead to reliable assessment results. Indeed, the reliability of in situ implementation is a key issue for any health monitoring technique along with other characteristics such as testing speed and cost-effectiveness. Several nondestructive testing (NDT) techniques have already been established, showing their suitability in certain aspects of material and structural characterization and behavior monitoring. As an example, the use of ultrasonic pulse velocity can be mentioned, the correlation of which with the quality of concrete is well documented. Other techniques based on elastic waves such as impact and impulse response, as well as the radar method, have proven their suitability in distinguishing delaminations and inhomogeneities. The assessment based on the above-mentioned methods as well as a number of others, like acoustic emission, radiography, vibration modal analysis, and slightly destructive surface strength methods in conjunction with visual inspection, provides a valuable platform for decision making concerning the maintenance, based on more robust engineering criteria than solely the experience of the engineer. However, it is a common impression that most NDT techniques have not at all reached their full potential especially with regard to in situ implementation. Therefore, the current special issue intends to examine all possible tools for economic and timely infrastructure condition assessment, with emphasis on reliability and connection of the monitoring results with the proper maintenance action that should be taken. The challenges are even higher since new and innovative materials are being increasingly used. These materials include high performance concrete, textile reinforced concrete, and nanomodified and recycled materials, which offer better capabilities for sustainable structures; however their assessment through the same techniques used for conventional concrete should not be taken for granted. In this special issue, the latest advances in different topics of civil structural health monitoring (SHM) are highlighted. More specifically, recent findings in elastic wave methods (acoustic emission, impact-echo, and ultrasonics) are reported. Vibration methodologies which are the most suitable for global bridge SHM are discussed, while the use of radar is reviewed and applied. Advancements in automated robotic inspection and online monitoring of bridge components are also discussed. The issue includes studies on corrosion detection, reinforcing and self-sensing elements for SHM. Algorithms incorporating pattern recognition clustering for the characterization of the location and degree of damage could not but be a strong part of the issue, while monitoring of self-healing and textile-reinforced materials is also discussed. The dual importance of monitoring of old structures is stressed out: their structural safety as well as their cultural heritage significance. The methodologies are successfully tested in several case studies as reported herein. We believe that the present special issue reflects on the recent advances in SHM and NDT for civil structures and materials complemented with insightful findings of various assessment techniques. We wish to thank all the authors for submitting their work in the issue and their patience during the review process. Dimitrios G. Aggelis Ninel Alver Hwa Kian Chai