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41 a p p e N D I x e accuracy The ultrasonic echo equipment used in this research study is commercially available. It is the A1220 Monolith developed by Acoustic Control Systems, in cooperation with the Fed- eral Institute for Materials Research and Testing (BAM) in Germany. The equipmentâs accuracy depends mainly on the data acquisition hardware; however, with the A1220, the tun- nel thickness can be estimated within an accuracy of ±3% of the actual thickness. This system and the ultrasonic tomogra- phy testing system (discussed in Appendix D) measured com- parable depths to defects in tunnel linings. precision The A1220 measurements are highly reproducible (i.e., the precision is very good) when no coupling agent is used. Using a scanning system enhances the reproducibility of the mea- surements because the pressing pressure on the transducer and its location can be accurately controlled. Calibration procedures No standard calibration procedures need to be performed before starting to take measurements. testing procedures When using dry-point contact probes, no coupling liquids need to be applied on the surface. However, the surface should be cleaned of dust and sand, and any materials that could prevent the penetration of low-frequency ultrasonic energy should be removed from the surface. The location of the test site and its dimensions should be marked and noted to facilitate the reproduction of measure- ments if necessary and to locate the detected features. For scan- ner testing, the location of the scanner feet and the dimension of the scanner aperture need to be carefully noted. Equally important, the orientation of the probe (i.e., its polarization) with respect to the test area or scanner opening should be recorded. The technical passport of the hardware includes informa- tion about the center frequency of the probe, the delay time, and the voltage level. These constitute all the parameters to be set before starting the measurement process. The choice of parameters depends on the particular application, that is, the test material and the required penetration depth. For testing of concrete tunnel linings of up to 3 ft thick, a center frequency of 55 kHz could be used. The number of test points and grid spacing depend on the required resolution (i.e., the minimum size of the defects being sought) and the time allocated for field investigations. For this project, the team chose a grid spacing of 1 in. in each direction, allowing the scanning operation at about 11 sq ft/h (or 1 sq m/h) for acoustic testing. Investigations revealed that doubling the grid spacing to 2 in. would not compromise the accuracy of the test results. Reconstruc- tion algorithms used for postprocessing the data (e.g., syn- thetic aperture focusing technique) are most effective for grid spacing of 2 in. or less. To achieve the maximum accu- racy, measurements might need to be performed with two polarizations. Cost A handheld unit with one transducer can be purchased for less than $10,000. The cost of a scanning system with the con- trol unit is about $100,000. Limitations The main limitation of conventional ultrasonic techniques is that the sensors have to be in contact with the structure during the measurement process. This leads to several issues Ultrasonic Echo Testing Criteria
42 such as poor repeatability and/or inconsistency of measure- ments, as well as delays from displacing and reinstalling the transducers. Mounting the ultrasonic device on a scanning system accelerates the measurement process and greatly enhances the repeatability and consistency of the results. However, compared with contact-free measurement systems, conventional ultrasonic testing (even with dry-contact trans- ducers like A1220) is relatively slow. Therefore, it is suitable for the assessment of areas deemed problematic during screening. Other limitations of ultrasonic echo testing include the following: ⢠At or near block joints or other structural boundaries, the signals suffer great disturbance due to the reflection of surface waves. This makes the reliable evaluation of mea- surements difficult. ⢠The acoustic waves reflect partially at the interface between the inner shell concrete and roof gap backfill material. If these two materials are well bonded, the reflection is very small or may not be identified. However, a separation often occurs between these two materials. A gap of a few hun- dredths of a millimeter is sometimes enough to completely reflect the sound waves. In such cases, only the thickness of the inner shell is measured (excluding the backfill material). ⢠Even with the phase evaluation, the difference between cer- tain types of defects (e.g., a flaw and an excessively thin cross section of lower acoustic impedance) cannot always be established. ⢠In the case of air-entrained concrete or fiber-reinforced concrete, the range of thickness measurements was report- edly reduced, or carrying out the measurements was more difficult. Data Management The collected data are downloaded from the ultrasonic hard- ware and saved on an external hard disk for safekeeping. Depending on the amount of data acquired, downloading might be necessary in between a measurement cycle; other- wise, an external hard disk can be connected to the instru- ment. Using the A1220 device on a 1-in. by 1-in. grid of size 48 in. by 24 in. (1,225 data points, 1,024 samples per signal, and a sampling frequency of 1 MHz) produces a 16-bit binary file of 2.39 MB. The analysis software delivered with the hard- ware is able to read the binary data format in which the infor- mation is saved. With other analysis software, the data might need to be transformed into a different file format. Data analysis and Interpretation Basic data analysis software is provided by the manufacturer. Other standard data analysis software can be used for further postprocessing of the experimental data. Interpretation depends on the mode of testing (one point [A-scan], linear [B-scan], or surface measurements [C- and D-scans]) and may be enhanced by using advanced analysis and visualization tools. For example, applying the synthetic aper- ture focusing technique to the data improves the signal-to-noise ratio. Phase analysis makes it possible to distinguish among fea- tures and anomalies of different constituents (e.g., steel or air void). Built-in plans or other information about the test area may greatly facilitate the interpretation of the results as well. Data interpretation can be done by experienced trained users and usually demands engineering judgment.
