Site Information Required For the Tests
- Location of the site;
- Pile types including size, material and reinforcement;
- The layout of piles;
- Details of pile installation (including construction & driving sequence and rest periods );
- Number of piles to be tested;
- Subsurface profile/driving details of the piles (more if variations are noted);
- Depth of water table and soil investigation report, if any;
- The density of concrete;
- Strength of concrete;
- Abnormal conditions were noted while driving/boring or concreting piles. The normal daily report produced by the piling site should contain this information. In addition, any other information concerning planning and conducting the tests including relevant past experiences covering similar test(s) in the area; and
- Details of test piles(s), if any.
Different Types of Tests Integrity Test
Various methods are available for checking the integrity of concrete piles after installation. In the most widely used method, impulses or vibrations are applied to the pile and Measurements are made of timings and attenuation of reflected signals.
The commonly used sonic methods, vibration methods, sonic logging techniques, etc, have been tried within the last 15-20 years in different parts of the world. However, the methods based on the One Dimensional Stress Wave approach known as Sonic Integrity Testing, Low Strain Integrity testing or Sonic Echo Testing have been used successfully in various parts of the world. The method is simple and quick enabling dozens of piles to be examined in a single working day without much interference in site activities.
The work carried out on sonic integrity testing of piles in the country has shown its efficiency.
Low Strain Integrity Testing OR Sonic Integrity OR Sonic Echo Test
This is a system of assessing the integrity of piles by the use of low-stress waves imparted to the pile shaft and is also known as Sonic Integrity or Sonic Echo Test.
A small metal/hard rubber hammer is used to produce a light tap on top of the pile. The shock travelling down the length of the pile is reflected back from the toe of the pile and recorded through a suitable transducer /accelerometer (also held on top of the pile close to the point of impact ) in a computer disk or diskette for subsequent analysis.
The primary shock wave which travels down the length of the shaft is reflected from the toe by the change in density between the concrete and sub-strata. However, if the pile has any imperfections or discontinuities within its length these will set up secondary reflections which will be added to the return signal.
By a careful analysis of the captured signal and knowledge of the conditions of the ground, age of concrete, etc, a picture of the locations of such problems can be built up. The reflected stress wave can be monitored using either processing technique, the observed signals are amplified and converted into the digital display as velocity versus length or frequency versus mobility records, providing information on the structural integrity of piles.
The stress wave velocity and approximate pile lengths are provided as input for the integrity testing. The stress wave velocity is dependent on Young’s Modulus and the Mass density of pile concrete. This value generally lies between 3000-4000 metres per second depending on the grade of concrete used (M15-M25).
Normally more than one recording of signals is done until repeatability of signals is achieved. If necessary, averaging of signals is also done to achieve more informative signals. In a suspected pile the test should be repeated at more than one location on top of the pile.
In the case of large-diameter piles, the tests shall be conducted at 5-6 places to cover the entire section of the pile.
The tests shall be conducted on piles whose length is correctly recorded or on test piles where available, to determine the value of stress wave velocity and characteristic or reference signal for comparing the signals for testing subsequent piles.
The method of testing involves high skill and the use of computerized equipment. Therefore, the tests should be performed and interpreted by trained and experienced personnel.
Frequently Ask Questions
What is the curing time in cast-in-situ concrete piles?
The cast-in-situ piles should not be tested normally before 14 days of casting. It should be properly cured in the curing period.
What will be the maximum size of the pile for the Integrity Test?
The present experience of Non-Destructive Testing of piles is up to a diameter of 1500 mm.
What will happen when the same soil density is available below the pile?
Soil stiffness or founding on a rock of similar density as the pile will attenuate the signals such that there will be little or no toe reflection.
At which part of the pile, the test will be conducted?
An Ultrasonic Pulse Velocity Test is carried out at the head of the pile in order to arrive at the speed of sound propagation through the cast concrete of the pile.
Why does Penetration Test conduct during Integrity Test?
This test is used to assess the strength of the concrete at the pile head to ensure that the pile is sufficiently cured and ready for NDT.
Why does Density Testing of Cast Cubes during Integrity Test?
In order to determine the density of the concrete used each cube should be measured for density by weighing in air and water.
What one can measure with this test?
The test measures :
1). pile length or depth to anomalies
2). pile head stiffness
3). pile shaft mobility, which is dependent on pile section and concrete properties
How to show pile integrity test results?
It shows abrupt changes in response, due to
1). changes in the cross-sectional area – such as a reduction in diameter or
2). material – such as a void in concrete