Mapping Voids, Debonding, Delaminations, Moisture, and Other Defects Behind or Within Tunnel Linings (2013)

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

312 a p p e N D I x t These specifications are based on the ground-penetrating radar (GPR) reflection from a large metal plate. A typical metal plate reflection (MPR) is shown in the upper part of Figure T.1. The amplitude of reflection (i.e., volts) is measured from the maximum positive peak to the preceding negative. No filtering, averaging, or signal clean up, such as sky wave removal (and reflection subtraction), is allowed. Performance specifications are as follows: 1. Noise-to-Signal Ratio Test. The antenna will be positioned at its recommended operating height (between 12 in. and 18 in.) above a minimum square foot (4 ft by 4 ft) metal plate. The radar unit shall be turned on and allowed to oper- ate for a 15-min warm-up period. After warm up, the unit shall be operated at maximum pulse rate, and fifty (50) radar waveform pulses shall be recorded. The recorded waveforms shall then be evaluated for noise-to-signal ratio. The noise- to-signal ratio is described by the following equation: Noise Level Signal Level 0.05 5% mp( ) ( ) ( )≤A A n The signal level, Amp, is defined as the average metal plate reflection amplitude as measured from the peak to the preceding minimum. The noise level, An, is defined as the worst-case maximum amplitude occurring between 1 and 10 ns after the surface echo. The noise level is mea- sured from any positive peak to either the preceding or trailing negative, whichever is greater. The noise-to-signal ratio shall be less than or equal to 0.05 (5%). 2. Signal Stability Test. The same test configuration shall be used as described in the noise-to-signal ratio test. Fifty (50) traces shall be recorded at the minimum data rate of 25 traces per second. The signal stability shall be evaluated using the following equation: 0.01 1% max min AVG ( )− ≤A A A where Amax is defined as the maximum amplitude for all 50 traces, Amin is defined as the minimum amplitude for all 50 traces, and AAVG is defined as the average trace amplitude of all 50 traces. The signal stability test results for the GPR shall be less than or equal to 1%. 3. Long-Term Signal Stability. The same test configuration shall be used as described in the noise-to-signal ratio test. The radar shall be switched on with no warm up and allowed to operate for 2 hours continuously. At a minimum, a single waveform shall be captured every 2 min, 60 in total. The amplitude of reflection shall be calculated and plotted against time. To check for signal drift, the time at which the metal plate reflection occurs shall be captured and plotted against time. For the system to be performing adequately, the amplitude should remain constant after a short warm- up period, and the system should have little or no drift. The stability criterion is as follows: 0.03 3% any 20 20 ( )− ≤A A A where A20 is the amplitude measured at 20 min, and Aany is any amplitude measured after 20 min. The drift criterion is as follows: 0.05 5% any 20 20 ( )− ≤t t t where t20 is the time when the peak metal plate reflection occurs at 20 min, and tany is the time when the MPR occurs in any trace after 20 min. 4. Variations in Time Calibration Factor. The same test con- figuration shall be used as described in the noise-to-signal ratio test. Fifty (50) traces shall be collected, and the height of the antenna shall be measured. The test shall be repeated at two other heights. Typically, heights of approximately 12 in., 16 in., and 20 in. are used. The time delay from the Radar Specifications for Air-Coupled Ground-Penetrating Radar Antennae

313 Figure T.1. Examples of acceptable and unacceptable metal plate reflections.

314 end reflection at the tip of the antenna to the metal plate reflection shall be is measured for each trace, and their mean is time ti (where the subscript represents height position at i). The difference between t2 and t1 represents the time to travel a fixed distance in air. For bistatic antennae, the travel distance must be calculated on the basis of the system geom- etry. The factor C1 is calculated by dividing the distance by the time difference (e.g., in. per nanosecond). The factor C2 represents the same between heights 2 and 3. The variation in time calibration factor is as shown below: Mean of and 0.02 2% 1 2 1 2 ( )− ≤C C C C The variation in time calibration factor shall be less than or equal to 2%. 5. End Reflection Test. The same test configuration and results from the noise-to-signal ratio test shall be used. The ampli- tude of the end reflection directly preceding the metal plate reflection shall be measured. This is a measure of the ade- quacy of system tuning. The size of the end reflection shall be 0.15 15% mp ( )<A A E where AE is the mean of the amplitude of end reflection defined as any peak occurring from 1 ns to 5 ns before the metal plate reflection, and Amp is the mean of the ampli- tude of reflection from the metal plate. The end reflection in the metal plate test shall be less than 15% of the amplitude of the metal plate reflection. 6. Symmetry of Metal Plate Reflection. The same test con- figuration as used in the noise-to-signal ratio test shall be used. Two different criteria have been established for sym- metry, as described below: 6.1. The first criterion is that the time from the maximum negative peak following the surface reflection to the zero crossing point shall be measured. This time (tf) is shown in Figure T.1. The required specification is 0.7t nsf ≤ An example of metal plate reflections that pass and fail this specification are shown in Figure T.1. 6.2. The second criterion is based on the symmetry of the legs of the metal plate reflection. The amplitude is measured from the positive peak to both the preced- ing and trailing negative. The specification is 0.95 95%min max ( )>A A where Amin and Amax are the minimum and maximum metal plate reflections measured using the preceding or trailing negatives. The ratio should be at least 95%. 7. Concrete Penetration Test. The antenna shall be placed at its recommended operating height above a 6-in.-thick concrete block. The concrete block shall be nonreinforced, have a minimum age of 28 days, and have a minimum 3,000 psi compressive strength. The block shall be 3 ft (36 in.) by 3 ft (36 in.) or greater to ensure that all the GPR energy enters the concrete. The concrete block shall be placed on top of a metal plate. Two hundred (200) traces shall be recorded. The reflection amplitude from the top and bottom of the concrete block shall be measured. The concrete penetration test is defined by the following equation: 0.25 25% bottom top ( )≥A A where Atop is the mean of the measured return amplitude from the top of the concrete slab, and Abottom is the mean of the measured return amplitude from the metal plate. The concrete penetration test results for the GPR should be greater than or equal to 25%.