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First, the acoustic tubes used in road and bridge pile foundations are prone to several issues during installation:
1. The bottom of the pile is a common area where defects may occur. Due to improper construction techniques, the acoustic tube at the bottom can bend inward, reducing the spacing between tubes. This causes the transducers to lose parallel alignment, leading to abnormally high sound velocity readings and reduced amplitude of ultrasonic waves. The resulting signal curves become irregular, making it difficult to interpret the data accurately.
2. Inadequate securing of the acoustic tubes or excessive spacing between fixations can lead to misalignment during concrete pouring. The pressure from the concrete can cause the tubes to tilt or deform, changing the distance between them. This affects the accuracy of the test results, sometimes making it impossible to determine the pile's integrity without resorting to more invasive methods like core drilling.
3. If the steel casing is too long and the welding is not properly sealed, air pockets can form inside the tube. The ultrasonic signals must travel through these air layers, causing significant distortion and affecting the assessment of the pile's structural integrity.
4. It is generally assumed that the transducer is placed at the center of the acoustic tube. However, if the tube has a large diameter, the transducer may swing within the tube, increasing the delay caused by the coupling water layer. This leads to longer sound wave propagation times and can significantly impact the accuracy of the test results.
Second, there are specific operational guidelines for conducting acoustic testing on road and bridge pile foundations:
1. The acoustic tubes should be embedded to the bottom of the cast-in-place pile, with the top extending 300 mm to 500 mm above the pile’s surface. All tubes on the same pile should have the same exposed height to ensure consistency in testing.
2. Sealing is crucial. The bottom of each acoustic tube should be welded with a blind cover or steel plate to prevent slurry leakage. After installation, the top of the tube should be capped or plugged to avoid foreign objects falling in during concrete pouring, which could block the tube.
3. Fixation of the acoustic tubes should be done using spot welding or wire binding, with fixed points no more than 2 meters apart. Special attention should be given to the ends and joints. For areas without reinforcing cages, the tubes can be secured with additional steel bars. To maintain parallel alignment, triangular steel frames can be spot-welded between the tubes.
4. When connecting the acoustic tubes, care must be taken to prevent deformation of the reinforcement cage. The tubes are usually installed in sections along with the cage. The bottom section is fixed to the first part of the cage, while the rest are temporarily attached. When lowering the cage, the upper parts of the tubes are aligned and inserted. Joints can use reverse-thread sleeves or welded sleeves. Reverse-thread joints require soft rubber seals, while welded sleeves should be about 80 mm long and welded securely. The joints must be strong, leak-proof, and smooth to allow the transducer to move freely. If cutting is necessary, a cutting machine should be used, followed by polishing to remove any burrs. Electric welding should be avoided near the tubes.
5. Water filling is an essential step. Each section of the acoustic tube should be filled with clean water as a coupling medium. River water should not be used directly, especially during flood season when the water quality is poor. It should be purified before use to prevent sediment buildup at the bottom of the tube. Before pouring underwater concrete, check the water level in the tube and refill if necessary.
6. Testing should only be conducted after the pile foundation concrete has cured for at least 14 days. Before testing, the pile head should be chiseled to the design elevation. A 20 cm long steel bar, approximately 32 mm in diameter, should be used as a hanging hammer to check if the acoustic tube is clear and to fill it with clean water for proper coupling.