Ultrasonic Crosshole And Crossmedium Tomography For The Detection Of Defects In Structural Concrete

The use of ultrasonic crossmedium and crosshole tomography is described as a means for nondestructive<br>evaluation (NDE) of flaws or imperfections in structural concrete. Tomography is an inversion procedure that can<br>provide for ultrasonic images of a concrete specimen from the observation of transmitted compressional or shear first<br>arrival energy. Two applications for ultrasonic tomographic imaging are presented here. Crossmedium tomographic<br>images are obtained from concrete walls or columns with two-sided access. Crosshole tomographic images are acquired<br>from drilled shafts or slurry walls containing steel or PVC access tubes set in place during the concrete placement.<br>In order to study the application of crossmedium tomography for imaging flaws in concrete, four concrete<br>walls containing simulated flaws or defects were cast-in-place in the laboratory as part of a 1993 National Science<br>Foundation (NSF) SBIR study. A new specialized rolling ultrasonic scanner source was developed for this testing and<br>was used for rapid scanning of structural concrete with two-sided access. Using the new scanner, a typical 116 cm (45.5<br>in) survey line for 1764 different source-receiver combinations was acquired in 1% hours. A comparable tomographic<br>test line, performed on a single source-receiver measurement basis, takes approximately 40-60 hours. Therefore, the<br>scanner source now makes locating and imaging flaws in critical concrete structures practical. Tomographic analysis<br>was performed using two series expansion algorithms from geotomography: Simultaneous Iterative Reconstruction<br>Technique (SIRT) and a least square matrix inversion approach using an optimized Conjugate Gradient (CG) approach.<br>The results of this study indicated that travel time tomography was most successful in locating simulated voids in<br>concrete. The crossmedium tomography was also used to locate and image simulated honeycombs, simulated open<br>cracks, microcracks, and weak concrete.