Ultrasonic testing of concrete
Gibson, Neil
Ultrasonic pulse velocity (UPV) testing is a technique which has evolved over many years and is well established in the construction industry. Originally developed for testing concrete, the method was soon adapted for use with other materials and has been used successfully on ceramic materials in addition to graphite and timber.
“The instrument detects the first part of the ultrasonic signal to arrive at the receiving transducer. In ‘good’ material, the ultrasonic signal travels in a straight line from the transmitting transducer to the receiving transducer. Testing known good samples easily establishes a reference measurement for ‘good’ material.”
There are a large number of applications in concrete testing; historically, these have been mainly for integrity testing but other applications include the estimation of concrete strength in precast manufacturing and in the field, research into concrete curing and ageing, as well as changes in concrete such as the deterioration due to an aggressive chemical environment or freezing and thawing.
The pulse velocity has been shown to be affected by various fundamental material properties. The elastic modulus, for instance, can be calculated directly from the pulse velocity, given the density and Poisson’s ratio of the material. Other parameters have been shown to affect the pulse velocity, including the strength and moisture content.
Techniques exist, and are well documented(1-3), for calculating material strength from the pulse velocity. This technique is used primarily by precast concrete manufacturers but has also been used successfully in structural integrity investigations(4).
Ultrasonic signal
The instrument detects the first part of the ultrasonic signal to arrive at the receiving transducer. In ‘good’ material, the ultrasonic signal travels in a straight line from the transmitting transducer to the receiving transducer. Testing known good samples easily establishes a reference measurement for ‘good’ material. If a much larger transit time (lower velocity) is measured on the same sample it is a clear indication that the signal has had to travel around, or through, some detect.
Using just the transit time measurement it is possible to detect cracks, voids, delaminations and other defects with relative ease. These tests rely on the fact that the velocity of the ultrasonic wave is much higher in the lest material than it is in air. Any air in the signal path (due to cracks or voids, for example) causes a reduction in the pulse velocity and thus an increase in transit time. This effect produces good results for large cracks as well as micro-cracking of the type induced by thermal shock or freeze/thaw cycling.
Coupling material
One of the minor drawbacks of the method is that it does require a good mechanical ‘bond’ between the test material and the transducers. Failure to achieve this can result in completely erroneous test results. The effect of even a tiny pocket of air between the transducer and the product is the same as that of a crack in the product. To overcome this, some form of coupling material is used (see Figure 1), usually one that has similar acoustic properties to the material under test and that can deform in such a way as to fill the microscopic gaps between the transducer and the product. Most people will have experience of this in the form of the gel used in medical ultrasound applications. On concrete, something relatively ‘thick’, or viscous, is required to cope with the rough surface. A thick grease, or Vaseline (petroleum jelly), is often used.
Even given a good couplant, it is important to understand some of the other possible test variables. Some variation between operators can be experienced due to the differing pressure applied by them to the transducers. This has the effect of varying, very slightly, the thickness of the coupling layer and so reducing the distance travelled by the ultrasonic pulse. This can be easily overcome with a little operator training and the use of the zeroing or calibration facilities built in to the instrument.
Choice of transducer
The choice of transducer is also an important consideration. Transducer frequencies for use with the PUNDIT or PUNDITplus instrument range from 24kHz to IMHz (see Figure 2). Frequencies used for concrete testing are generally in the 24-150kHz range which correspond to wavelengths of around 200mm to 16mm. The wavelength determines the size of the smallest defect that can be detected. The physical size of the transducer may also be a factor in some applications.
One user supplies a range of precast, prestressed concrete lintels to builders’ merchants for the trade and DIY. They have been using the PUNDIT for many years to check the strength of their lintels. If the pre-tensioning load is released too soon, the wire is pulled back through the concrete and the whole batch has to be scrapped.
They have correlated the crushing strength of a series of cubes of different ages with the UPV measured using the PUNDIT. Using the correlation curve, it is then a simple matter of measuring the pulse velocity in manufactured lintels and to read the strength from the chart. By measuring the pulse velocity on the casting bed and using the strength correlation technique, they can check that the concrete has reached sufficient strength to allow them to release the wire tension (see Figure 3).
Concluding remarks
It is a valuable, relatively low-cost and easy-to-use technique. Testing has afforded an improvement to production processes, improved integrity and quality of products and a reduction in scrap and reject rates, thereby saving time and money. There are thousands of users around the world who have proved the value of non-destructive testing using this method.
References:
1. BRITISH STANDARDS INSTITUTION. BS1881 -203:1986. Testing concrete. Recommendations for measurement of velocity of ultrasonic pulses in concrete.
2. AMERICAN CONCRETE INSTITUTE. ACI 228.1 R-03: Inplace methods to estimate concrete strength. Farmington Hills, 2003.
3. BUNGEY, J. Testing of concrete in structures, Blackie, London, 1994.
4. FARR, S. and HERTLEIN, B. Combining destructive and non-destructive evaluation of existing structures. Paper presented at ACI Fall Convention, San Francisco, Oct 2004.
NEIL GIDSON, CNS FARNELL
Copyright The Concrete Society Mar 2005
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