NDT&E International 40 (2007) 258–264 Imaging of ungrouted tendon ducts in prestressed concrete by improved SIBIE Ninel Ata, Shinichi Mihara, Masayasu Ohtsu à Graduate School of Science and Technology, Kumamoto University, Kumamoto 860-8555, Japan Received 21 February 2005; received in revised form 11 October 2006; accepted 11 October 2006 Available online 28 November 2006 Abstract The impact-echo method has been extensively applied to nondestructive evaluation of defects in concrete structures. The presence and the location of defects in concrete are estimated by identifying peak frequencies in the frequency spectra. To detect ungrouted tendon ducts, the method is known to be available. However, because post-tensioning prestressed concrete members usually have thin web portions, spectra obtained could include many peak frequencies. As a result, it is often problematic to select appropriate peak frequencies associated with the presence of ungrouted ducts. Stack imaging of spectral amplitudes based on impact-echo (SIBIE) is developed, in order to improve the impact-echo and to visually identify the locations of such reflectors as voids and defects. In the present paper, SIBIE is successfully applied to identify ungrouted metal and plastic sheaths at the hunch portion of a prestressed concrete beam. Two- dimensional dynamic BEM analysis is performed to investigate the relations between peak frequencies and locations of reflectors. At the peak frequencies in the spectra, locations of stress concentration are correlated with the response modes. r 2006 Elsevier Ltd. All rights reserved. Keywords: Impact-echo method; SIBIE; Boundary element method (BEM) 1. Introduction The impact-echo method is well known as a nondes- tructive testing of concrete structures. By interpreting the frequency spectra obtained from the recorded waveforms, the presence and the location of defects are estimated [1,2]. The method has been applied to post-tensioning concrete beams [3–5]. Estimation of ungrouted ducts is, however, limitedly successful, because many peak frequencies are often observed in the frequency spectra due to reflection and diffraction. In order to solve this problem, a new procedure is developed by applying an imaging procedure to the impact-echo data, as stack imaging of spectral amplitudes based on impact-echo (SIBIE) [6]. This procedure is successfully applied to a concrete slab containing a post-tensioning tendon duct. In the present paper, a duct hole located at the hunch portion is investigated. In general, post-tensioning pre- stressed concrete beams are of I-shaped. Thus, to study the effect of this hunch portion on the impact-echo data, a trapezoid-shaped specimen is tested. In addition, SIBIE procedure is improved by using two accelerometers at two detection points. In the case of a plastic sheath, it is reported to be complicated to detect duct holes as plastic ducts have lower acoustic impedance than concrete or grout [1]. Therefore, a concrete specimen containing a plastic sheath is also tested. The impact-echo method is studied numerically, in order to clarify relations between the peak frequencies in the spectra and locations of reflectors. Two-dimensional dynamic Boundary Element Method (BEM) is applied to compare with experimental results. 2. Basic principles of the impact-echo technique In the impact-echo method, elastic waves (typically up to 50 kHz frequency) are generated by a short duration mechanical impact. Applying an impact, detecting elastic waves, and identifying peak frequencies after Fast Fourier Transform (FFT) analysis of detected waves are three basic ARTICLE IN PRESS www.elsevier.com/locate/ndteint 0963-8695/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ndteint.2006.10.008 à Corresponding author. Tel.: +81 96 342 3542; fax: +81 96 342 3507. E-mail address: ohtsu@gpo.kumamoto-u.ac.jp (M. Ohtsu).