How to Perform Integrity Testing of Concrete Deep Foundations by Ultrasonic Crosshole Testing According to ASTM D6760-16
How to Perform Integrity Testing of Concrete Deep Foundations by Ultrasonic Crosshole Testing According to ASTM D6760-16
Concrete deep foundations are essential for many structures that require a stable and strong support. However, concrete quality can vary due to factors such as mixing, placement, curing, and environmental conditions. Therefore, it is important to check the homogeneity and integrity of concrete in deep foundations to ensure their performance and safety.
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One of the methods for checking the integrity of concrete deep foundations is ultrasonic crosshole testing, which is standardized by ASTM International as ASTM D6760-16[^1^]. This method uses data from ultrasonic probes lowered into parallel access ducts, or in a single access duct, in the deep foundation element to assess the homogeneity and integrity of concrete between the probes. The data are used to confirm adequate concrete quality or identify zones of poor quality.
In this article, we will explain how to perform ultrasonic crosshole testing according to ASTM D6760-16, including the equipment, procedure, data analysis, and interpretation.
Equipment
The equipment required for ultrasonic crosshole testing consists of:
Ultrasonic probes: These are transducers that can generate and receive ultrasonic pulses. They should have a frequency range of 20 kHz to 100 kHz and a diameter of less than 50 mm. They should also be waterproof and durable.
Access ducts: These are tubes that are installed in the deep foundation element during construction. They should be parallel, vertical, and spaced at equal intervals. They should preferably be made of steel to prevent debonding of the access duct from the concrete resulting in an anomaly. For single hole tests, plastic tubes must be used.
Cables: These are used to connect the ultrasonic probes to the data acquisition system. They should be shielded and have low electrical resistance.
Data acquisition system: This is a device that can control the ultrasonic probes, record the signals, and display the results. It should have a sampling rate of at least 10 MHz and a resolution of at least 12 bits.
Software: This is a program that can process the data, calculate the propagation time and relative energy of the ultrasonic pulses, and generate graphs and reports.
Procedure
The procedure for ultrasonic crosshole testing according to ASTM D6760-16 consists of the following steps:
Prepare the equipment: Check that all the equipment is functioning properly and calibrated according to the manufacturer's instructions. Set up the data acquisition system and software according to the test parameters.
Lower the probes: Lower one probe into each access duct until they reach the bottom of the deep foundation element. Make sure that they are aligned horizontally and vertically.
Perform the test: Start the data acquisition system and software. Trigger one probe to emit an ultrasonic pulse and record the signal received by the other probe. Repeat this process for all possible combinations of probes in different access ducts. Move the probes up by a fixed increment (usually 0.5 m) and repeat until they reach the top of the deep foundation element.
Save the data: Save all the data in a suitable format for further analysis and reporting.
Data Analysis
The data analysis for ultrasonic crosshole testing according to ASTM D6760-16 consists of:
Propagation time: This is the time interval between the emission and reception of an ultrasonic pulse between two probes. It is calculated by identifying the first arrival peak in the signal waveform. It is proportional to the distance between the probes and inversely proportional to the sound velocity in concrete.
Relative energy: This is the ratio of the peak amplitude of an ultrasonic pulse received by one probe to that emitted by another probe. It is calculated by normalizing the peak amplitudes by their respective calibration factors. It is proportional to
the sound attenuation in concrete.
The propagation time and relative energy data are plotted as graphs or contour maps for each pair of probes in different access ducts. The graphs or maps show the variation of these parameters along
the depth of the deep foundation element.
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