<p>Photo of the prestressed concrete bridge studied herein.</p> Full article ">Figure 2
<p>NDT instruments used in this study: (<b>a</b>,<b>b</b>) photo of the GPR and UEA instruments, respectively; (<b>c</b>,<b>d</b>) illustrations of measurement setups with direct ray wave paths for one measurement position for GPR and UEA, respectively; (<b>e</b>,<b>f</b>) sample recorded GPR and UEA waveforms (or A-scans), respectively. T and R denote the transmitting and receiving GPR transducers, respectively; UEA channels are numbered 1 through 8. <span class="html-italic">s</span> signifies transducer spacings.</p> Full article ">Figure 3
<p>Illustrations of the two main processes: (<b>a</b>) SAFT-based imaging; (<b>b</b>) image fusion.</p> Full article ">Figure 4
<p>Illustration of a reconstruction step used in the imaging process shown in <a href="#ndt-02-00022-f003" class="html-fig">Figure 3</a>a, blue box: (<b>a</b>) discretized 2D reconstruction domain; (<b>b</b>) sample waveform with time-of-flight, <span class="html-italic">T</span>, and corresponding waveform amplitude, <span class="html-italic">A</span>, labeled. Only one transmitter-receiver (T-R) couple is shown for simplicity. <a href="#ndt-02-00022-f004" class="html-fig">Figure 4</a>a was adapted from [<a href="#B29-ndt-02-00022" class="html-bibr">29</a>] with permission from the author.</p> Full article ">Figure 5
<p>Illustration of the image fusion step shown in <a href="#ndt-02-00022-f003" class="html-fig">Figure 3</a>b, green box using a decomposition level of one (for simplicity). DWT = discrete wavelet transform; IDWT = inverse DWT.</p> Full article ">Figure 6
<p>Reconstructed images from measurements taken across the bottom of the selected slab: (<b>a</b>) GPR image; (<b>b</b>) UEA image; (<b>c</b>) fused image.</p> Full article ">Figure 7
<p>Reconstructed images from measurements taken across the top of the selected slab: (<b>a</b>) GPR image; (<b>b</b>) UEA image; (<b>c</b>) fused image.</p> Full article ">Figure 8
<p>Fused images of the selected slab: (<b>a</b>) fused image from bottom measurement [<a href="#ndt-02-00022-f006" class="html-fig">Figure 6</a>c]; (<b>b</b>) fused image from top measurement [<a href="#ndt-02-00022-f007" class="html-fig">Figure 7</a>c]; (<b>c</b>) final fused image.</p> Full article ">Figure 9
<p>Final interpretation: (<b>a</b>) final fused image of the selected slab with features of interest marked by red dotted lines; (<b>b</b>) cross-section of a typical 1960s prestressed concrete voided slab (from <a href="#ndt-02-00022-f0A1" class="html-fig">Figure A1</a>, <a href="#app1-ndt-02-00022" class="html-app">Appendix A</a>).</p> Full article ">Figure A1
<p>Cross-sections of a typical 1960s prestressed concrete voided slab. The drawings were made available by Otak, Inc., Vancouver, WA, USA.</p> Full article ">Figure A2
<p>Laboratory specimens used for calibration purposes. For details see [<a href="#B17-ndt-02-00022" class="html-bibr">17</a>].</p> Full article ">Figure A3
<p>Sample calibration results for Specimen 1: (<b>a</b>) GPR image; (<b>b</b>) UEA image. Waveforms on the right illustrate the polarity of the reflected pulse as a function of the reflector. Note that for the measurements using GPR (shown in <b>a</b>), a steel section was placed on part of the fourth step, which allows for a side-by-side comparison of the reflected pulses shown on the right of <b>a</b>.</p> Full article ">Figure A4
<p>Example of a sigmoid window used to suppress the direct wave as well as near-surface artifacts in a reconstructed image. The vertical dashed line denotes the transition length specified by the user.</p> Full article ">