Destructive Verification through Core Samples

Core samples of pipe walls were obtained from two different locations to provide a visual verification of suspected problem areas detected by the ground penetrating radar. In both locations, a 5 m deep, 2.4 m wide, 7.6 m long trench was excavated at predetermined locations with a box shoring used for stabilization. A GPR survey was performed on the exterior surface of the top of the reinforced concrete pipe to determine the exact location to extract the coring sample. Two 75 mm diameter samples were recovered at each of the two locations and preserved for laboratory testing of compressive strength (Edwards and Nowaczyk, 2005). One sample was at a suspected defect location, while the other was in an unaffected section of the concrete pipe wall for comparison. Table 3 presents the results of the compressive strength tests performed to ASTM C39-01 standards.

The cores samples (Figure 9) verified the results found using the non-disruptive in-pipe ground penetrating radar unit transported on a robotic platform. Discoloration of the core samples and diminished compressive strength measures further demonstrate the problems that could occur if hydrogen-sulfide gases were to penetrate the liner and attack the concrete wall.

Table 3. Laboratory testing of concrete core compressive strength (ASTM C39-01)

Core Sample #

Measured Average Length of Recovered Concrete Core (mm)

Compressive Strength (psi)

Notes

1 (a)

102.87

9,770

Core recovered from relatively non-corroded 750mm pipe section

1 (b)

85.09

8,840

Core recovered from suspected corroded 750mm pipe section

2 (a)

106.81

8,020

Core recovered from relatively non-corroded 900mm pipe section

2 (b)

93.17

7,810

Core recovered from suspected corroded 900mm pipe section

Figure 9. Core samples from 750 mm concrete pipe section