The impact-echo method is applied to evaluate the grout quality in tendon ducts of posttensioned structures nondestructively in the present study. As the main evaluation index, the dominant frequency (DFQ) is obtained from the frequency-domain spectrum, which is transformed from the original time-domain signal. The influences of several factors on the DFQ are investigated in detail both experimentally and numerically. These factors include size effect, slab thickness, tendon duct depth, affected range, DFQ shift, and grout strength. The experiments are conducted in the construction field on posttensioned box girders under various grouting conditions. The DFQs of each test point in a test area are integrated to draw a contour image based on their coordinates. Through the analysis of the contour image, grout quality in the tendon ducts can be evaluated accordingly. Grout defects are detected in some box girders and verified by chiseling these suspected ducts. The numerical studies, which can provide...

Experimental and Numerical Studies on Nondestructive Evaluation of Grout Quality in Tendon Ducts Using Impact-Echo Method

Numerical and experimental study on multi-directional SAFT to detect defects inside plain or reinforced concrete

In the case of a two-layer system, the impact-echo test has classically assumed that two frequency peaks are expected in the spectrum. In this paper, the spectrum of the impact-echo test is...

Guided-Wave Approach for Spectral Peaks Characterization of Impact-Echo Tests in Layered Systems

/pdf/damage-detection-in-metamorphic-stone-blocksutilizing-impact-n8cjt0zsol.pdf

Damage Detection in Metamorphic Stone BlocksUtilizing Impact-echo Testing and Modal Analysis

The suitability of the synthetic aperture imaging of defects using a matched filter approach on finite aperture transducers was investigated. The first part of the study involved the use a finite-difference time-domain (FDTD) algorithm to simulate the phased array ultrasonic wave propagation in an aluminum block and its interaction with side-drilled hole-like defects. B-scans were generated using the FDTD method for three active aperture transducer configurations of the phased array (a) single element and (b) 16-element linear scan mode, and (c) 16-element steering mode. A matched filter algorithm (MFA) was developed using the delay laws and the spatial impulse response of a finite size rectangular phased array transducer. The conventional synthetic aperture focusing technique (SAFT) algorithm and the MFA were independently applied on the FDTD signals simulated with the probe operating at a center frequency of 5 MHz and the processed B-scans were compared. The second part of the study investigated the capability of the MFA approach to improve the SNR. Gaussian white noise was added to the FDTD generated defect signals. The noisy B-scans were then processed using the SAFT and the MFA and the improvements in the SNR were estimated. The third part of the study investigated the application of the MFA to image and size surface-crack-like defects in pipe specimens obtained using a 45° steered beam from a phased array probe. These studies confirm that MFA is an alternative to SAFT with little additional computational burden. It can also be applied blindly, like SAFT, to effect synthetic focusing with distinct advantages in treating finite transducer effects, and in handling steered beam inspections. Finally, limitations of the MFA in dealing with larger-sized transducers are discussed.

Application of a matched filter approach for finite aperture transducers for the synthetic aperture imaging of defects

This paper simulates an impact-echo array technique for the two-dimensional imaging of defects in thick concrete structures. A finite difference time domain (FDTD) Model was employed for simulating the impact-echo method. The absorbing boundary conditions were employed in the FDTD for efficient calculation. The signals predicted using the FDTD models were then used for 2D image reconstruction of the cross-sectional region of the structure. The reconstruction algorithm uses time shifting (migration) of the signals based on point-source assumption for the impact sources. The images are represented in a typical “B-scan” format. Several types of defects were simulated, and parametric studies on the different parameters that influence the image quality during imaging of defects in concrete structures were examined.

A simulation study on the impact echo array technique for concrete structure NDT

This paper simulates an impact‐echo array technique for the two‐dimensional imaging of defects in thick structures such as concrete. A Finite Difference Time Difference (FDTD) Model was employed for the forward model of the impact‐echo method. The absorbing boundary conditions were employed for the efficient modeling of the technique. The signals predicted using the FDTD models from a linear array of impact‐echo transmitter‐receiver system in a multiplexed configuration was used for 2‐D image reconstruction of the cross‐sectional region of the structure. The reconstruction algorithm uses time shifting (migration) of the signals based on point‐source assumption for the impact sources. The images are represented in a typical “B‐scan” representation. Several types of defects were simulated and parametric studies on the different parameters that influence the image quality during imaging of defects in concrete structures were examined.

Impact Echo Array Technique for Concrete Structure NDT: Simulation Studies

The 2006 ultrasonic benchmark problem involves pulse‐echo angle beam scanning of a notch located on an inclined planar back surface. The response from a side‐drilled hole is to be used as a reference. The models are to simulate (a) the peak‐to‐peak B‐scan P‐ and SV‐ responses of the slots normalized by the appropriate SDH response and (b) the maximum peak‐to‐peak corner response of the slots (either mode‐converted or not). At CNDE, several simulation tools are being developed to assess/predict UT response for various geometries. The Finite‐Difference‐Time‐Difference (FDTD) scheme is one such simulation tool that has been under development in 1D, 2D and 3D. The FDTD is an explicit time domain tool that can simulate pulse propagation characteristics in acoustic/elastic media. The computational domain is limited by implementing Perfectly Matched Layers (PMLs) at the domain boundaries. We present the results of calculations based on 2D FDTD to determine the response of rectangular shaped surface‐breaking defe...

C. Sridharan

Papers

A simulation study on the impact echo array technique for concrete structure NDT

Impact Echo Array Technique for Concrete Structure NDT: Simulation Studies

The 2006 Ultrasonic Benchmark Problem — FDTD Simulations in 2D