Integration testing of concrete piles is a crucial step in ensuring the proper functioning of pile foundations. This testing is conducted before the construction of pile caps to detect any potential issues with the piles.
Defective pile shaft necking, intrusion of foreign matter, and improper toe formation due to contamination of concrete at the base with soil particles are some of the reasons that lead to the failure of cast-in-situ piles. Other factors include discontinuity of concrete, leaching of concrete due to high water current, poor construction methods, and poor quality control of concreting.
Precast concrete piles are also susceptible to failure due to the development of cracks during handling operations. Therefore, it is essential to conduct an integration test of piles to ensure proper placement, strength, and durability of the piles.
The integration test of piles, conducted before the completion of pile caps, is critical in detecting any potential issues that could lead to pile failure. Sonic Echo Testing, as outlined in IS 14893:2001, has been successfully used to check the integrity of piles after installation.
Integrity Testing of Concrete Piles
Pile integrity testing is a quick and straightforward method that allows for the testing of multiple piles in a single working day. This technique provides information about the continuity and defects of piles, such as cracks, necking, soil incursions, changes in cross-section, and approximate pile lengths, unless the pile is exceptionally long or has high skin friction. It also provides an indication of the soundness of the concrete. However, it is crucial to have experienced individuals interpret the results specific to piling.
During the test, a small metal or hard rubber hammer is used to tap the top of the pile lightly. The resulting shock travels down the pile’s length and is reflected back from the toe of the pile. A suitable transducer or accelerometer, held close to the point of impact at the top of the pile, records the signal in a computer disk for subsequent analysis. The primary shock wave that travels down the pile shaft is reflected from the toe by the change in density between the concrete and the sub-strata. However, if the pile has any defects or discontinuities within its length, these will set up secondary reflections that will be added to the return signal.
By carefully analyzing the captured signal and considering the conditions of the ground, age of the concrete, and other factors, it is possible to build up a picture of the location of any problems within the pile. Usually, more than one recording of signals is done until repeatability of signals is achieved. For large diameter piles, the tests are conducted at 5-6 locations to cover the entire section of the pile.
General Requirements of Piles Integrity Tests
Before conducting a low strain integrity test on a pile, it is important to ensure that the pile has been trimmed to the appropriate level and that any laitance has been removed. The area surrounding the pile should also be free of standing water and kept dry during testing. In the case of cast in situ piles, it is generally recommended to wait at least 14 days after casting before testing.
If test piles are available on site, they can be used to determine the pulse velocity and characteristic or reference signal of the pile concrete. This can also be done for piles with accurate depth records. Additionally, there are complementary tests that can be used to assess the soundness of the concrete at the test level and determine the stress wave velocity of the pile concrete based on its density and in-situ condition.
One such test is the ultrasonic pulse velocity test, which is conducted at the head of the pile to determine the speed of sound propagation through the concrete. Another test is the penetration test, which assesses the strength of the concrete at the pile head to ensure that it is sufficiently cured and ready for non-destructive testing (NDT). Lastly, the density of the concrete can be determined by measuring each cast cube’s weight in both air and water.
Limitations of Piles Integrity Tests
The current state of Non-Destructive Testing (NDT) for piles is limited to a maximum diameter of 1500 mm. This method is applicable to both cast in situ concrete bored and driven piles. However, it cannot be performed after the pile cap has been provisioned or cast. Additionally, it is not suitable for testing piles that are surrounded by water, as this can lead to inaccurate results.
It is worth noting that NDT of piles does not provide information regarding the verticality or displacement of the piles. Furthermore, it is not capable of detecting local loss of cover, small intrusions, or other conditions that may exist at the base of the pile. As a result, this method is limited in its ability to provide a comprehensive evaluation of the pile’s condition.