A pile test is used to assess the integrity of concrete piles and to locate any hidden defects. It is conducted by motion and sound sensing, using a light hammer to hit the pile to generate sound waves, and also as a sound measurement device, such as a geophone. Data from sensors are relayed to a computer that displays the results in a graph. The reflected acoustic data, called a reflectogram, allow for measurements of the length of the pile as well as the location of any breaks in the concrete. Pile testing can be accomplished by testing sonic echo and frequency response, while sonic logging is another way to test pile integrity.
Pile testing has been standardized by the American Society for Testing and Materials (ASTM). Organizations that conduct tests for pile integrity use standard ASTM – D 5882-00, “Standard Test Method for Low Strain Integrity Testing of Piles.” The standard details proper placement of motion sensors and where to hit the pile in relation to the sensors. Following international guidelines is one step in conducting a pile test, and further testing is needed to assess any abnormalities, such as changes in impedance due to poor-quality concrete or erosions of the pile diameter.
The length of the concrete pile is tested, but a pile test determines how long the concrete is based on an average wave velocity. Sound wave speed is determined by the age and grade type of the concrete. Variations from one pile to another can lead to wide errors in accuracy.
Using sound is the only way to test foundation integrity. It is not logical physically or economically to dig up an entire structure for a pile test, so the piles that are present serve as structural supports. One method of sound analysis is sonic echo testing, which is used along with data from a frequency response test. Data can be gathered from depths of about 20 to 30 times the diameter of the concrete pile, while soil quality also plays a role in the depth the pile can be measured to.
Sonic logging enables a pile test to work at any depth or be used with any size or shape pile. Tubes previously cast into the concrete are required for the test and are filled with water to conduct acoustic measurement. For any method used, if the foundation integrity is questioned, then the pile is removed and subject to load testing.