When engineers examine a road or bridge, many wish they could just see inside the thick concrete from the surface. Like a doctor can use an ultrasound device to see inside a patient from the skin’s surface, engineers now can use a new ultrasonic testing method, ultrasonic shear-wave tomography (UST), to examine deep inside thick concrete for internal defects from the concrete’s surface. The test is portable, wireless and shows immediate test results without damage to the road or bridge.
This UST technology was developed by Russian and German researchers and is now available as a new testing method in the U.S. The Skokie, Ill.-based CTLGroup’s nondestructive testing and evaluation department is using UST as a new testing method to look for internal defects such as voids and honeycomb inside bridge piers, girders, dams, structures and concrete roads. Currently, there is only one such UST test unit being used in the U.S.
The low-frequency ultrasonic equipment is a multifunctional phased array system applicable for nondestructive concrete testing using low-frequency ultrasonic wave (20 to 100 kHz) and tomographic methods of signal processing.
This system represents one of the most advanced techniques currently available in diagnosing defects in concrete, especially for large concrete structures, such as a bridge pier. This equipment is used to image the internal condition of concrete structures with access to only one side of the test element via a pulse-echo technique. The presence of internal defects such as cracks and voids and their approximate depths and extents can be located. This method also has been used successfully to locate voids in tendon ducts embedded in stem walls of a box girder bridge.
UST (branded in the U.S. under the trade name MIRA) is one of over 20 testing methods that CTLGroup engineers are using today to perform field examinations of concrete structures. Since concrete is not a homogeneous material, the signals from test equipment are rapidly attenuated. The penetration of ultrasonic waves is typically restricted to a shallow depth for traditional pulse-echo devices because of attenuation. UST, the newest addition to the nondestructive evaluation (NDE) toolbox, is the first ultrasonic device that uses shear waves instead of compression waves, combined with an array of sensors, to greatly enhance the penetration capability.
One significant advantage of UST testing over other testing methods is the near-3-D imaging function, which allows easier visualization of the internal conditions. In addition, the speed of field testing with MIRA in most cases surpasses other NDE techniques. For example, on a recently completed project, use of UST drastically reduced the time it took to complete the void detection in tendon ducts compared with what it might take if another test method such as impact echo was used.
A taste of medicine
When it comes to progress in testing methods of concrete, NDE of structures is a relatively new way to determine the extent of damage or defective construction at minimal cost. Recent advances in testing techniques, equipment and on-site computer software have brought reliability and industry-wide acceptance to this discipline.
Construction elements ranging from deep foundations to cladding on tall buildings, bridges and pavements can be evaluated with little or no disturbance either to the structures or to their occupants. The use of NDE techniques in civil engineering is analogous to the use of indirect sounding methods in medical examinations. The obvious advantages of a noninvasive approach to patient care has led to the development of very sophisticated sounding techniques, and a number of these have been adapted for the NDE of structures.
The most complete field investigation of structural problems usually is achieved through a judicious mix of visual inspection, NDE and minimally intrusive material sampling. Properly managed, this approach typically costs less than a more traditional investigative approach and results in a clearer definition of the problem.
Typically in a concrete bridge evaluation, the challenge that bridge engineers have to face is finding the hidden problems early enough without any disruption to the structure in use, so that rectification actions can take place before failure occurs. In some other cases, the actual condition of a bridge structure needs to be checked before a scheduled rebuild or rehabilitation program is implemented.
Some common problems related to reinforced concrete bridges include:
- Deck:
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- Overlay debonding;
- Cracking;
- Delamination;
- Spalling; and
- Defects associated with the post-tensioned deck, such as a distressed tendon or a void in a grouted duct.
- Superstructure (girders, stringers, etc.):
-
- Honeycomb;
- Cracking; and
- Defects associated with a post-tensioned element, such as a distressed tendon or a void in a grouted duct.
- Substructures (piers, abutments, footings, piles, etc.):
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- Honeycomb;
- Spalling;
- Scaling;
- Unknown depth of foundation; and
- Defects associated with post-tensioned piers, such as a distressed tendon or a void in a grouted duct.
Fortunately, a lot of hidden defects can be diagnosed with a number of currently available nondestructive testing techniques, depending on the structural element and the nature of the problem. Implementation of the latest technologies into an evaluation program is a key to success.
The select many
Based on CTLGroup’s engineering consulting experience, the following representative civil NDE techniques have been used effectively to evaluate and inspect bridges:
Ground Penetrating Radar
GPR has become a very well-known and versatile diagnosing tool in civil engineering in the past 10 years. Radar application in bridge evaluation includes location of reinforcing steel and tendons, detection of large-area delamination or debonding in the deck, etc.
Impact Echo
As a classic stress-wave testing technique, IE applications in bridge evaluation include determination of depth of delamination in concrete and detection of voids in a post-tensioned duct.
Impulse Response
Similar to the IE test, the impulse response test is based on stress-wave reflection to detect anomalies in concrete. However, IR testing uses a compressive stress impact generated by a 1-kg sledgehammer, with magnitude approximately 100 times that of the IE test. This greater point stress input causes the plate to respond to the IR hammer impact in a bending mode over a lower frequency range (0-1 kHz for plate structures) as opposed to the reflective mode of the IE test. Typical applications of the IR test include the detection of delamination, honeycomb, cavity or weak support of a structure.
Infrared Thermography
The infrared thermography camera remotely senses the emission of thermal radiation from an object and produces a visual image from this thermal signal. Surface or near-surface defects such as delamination will show up as a hot spot during the heating process. Typical application of this method in bridge evaluation is the detection of the bridge-deck delamination or overlay debonding.
Videoscope/Borescope Inspection
The modern videoscope possesses high resolution with the capability of real-time video and audio recording functions. Their common applications in bridge evaluation include bearing and tendon inspection.
Ultrasonic Pulse Velocity
UPV is a function of both the concrete dynamic elastic modulus and its density. The velocity for typical modern concrete is about 4,000 m/s. Typically, the lower the velocity, the worse the concrete quality is. Applications of UPV in bridge evaluation include detection of honeycomb in bridge piers and beams or stringers.
