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What’s Under the Hood (Concrete)?
Using ultrasonic echo tomography is a very useful structural diagnostic technique for diagnosing surface and internal concrete defects.
Cracks, spalls, and surface defects such as voids, honeycombing, exposed rebar, etc., are typical symptoms of distress associated with reinforced concrete structures. These symptoms are typically identified as a precursor to a potentially larger issue hiding under the concrete surface. To an owner, this can mean unanticipated costs, potential impacts to building operations, and even safety concerns. The process of finding what has caused the defects will include an evaluation of distress symptoms such as cracks, voids, and surface defects. This effort requires a combination of visual assessment, and field and lab testing to develop a prognosis of the issue. One very useful and powerful tool used for assessment is ultrasonic echo tomography.
Defects in new or existing construction come from improper construction practices, improper design details, accidents, or sometimes natural disasters. These defects can impact the integrity of the structure and lead to sustainability and usability issues.
What is Ultrasonic Echo Tomography?
Ultrasonic echo tomography is a non-destructive test method used for evaluating the condition of hardened concrete by measuring the time of ultrasonic shear-wave transmitting in the hardened concrete. A shear-wave which is transmitted in an isotropic medium will be partly reflected when it reaches another medium with a different acoustic impedance. The amount of energy reflected depends on the significance of difference in the acoustic impedance of the two media. The effectiveness of ultrasonic echo testing in identifying defects inside concrete has been proven in many field applications.
Diagnosis of A Reinforced Concrete Aeration Tank
This project involved a newly constructed cylindrical aeration tank that was designed to hold wastewater. The tank had an inner diameter of 63 feet, and a height of 18 feet. Our client observed concrete construction defects, including map pattern cracking, cold joints, and honeycombing on the exterior face of the reinforced 1-foot thick concrete wall of the tank after the forms were removed. These defects raised serious concerns related to the water-tightness of the concrete structure and its structural integrity.
To identify the probable causes of the observed defects, and determine if subsurface defects existed that were not visible on the surface, Terracon performed Ground Penetrating Radar (GPR) test and ultrasonic echo tomography test on the wall (on both interior and exterior faces) at several locations. The results of GPR test indicated inconsistent and less than specified concrete cover thickness near the exterior face of the wall which had contributed to the map pattern cracking mimicking the pattern of the reinforcing steel cage. Based on the 3-D models obtained from ultrasonic echo tomography test, the defects appeared to be limited to the vicinity of the exterior surface of wall, with no additional subsurface concrete flaws. This was verified by through-wall coring performed on the concrete wall.
The findings allowed the client to determine that the new tank structure did not require demolition. They could be confident that the extent of observed surface defects were limited in nature. The structural engineer of record proposed a surface sealing repair coating to verify the water tightness of the structure. Significant demolition cost and schedule impacts were avoided.
Ultrasonic echo tomography is just one of the non-destructive techniques that can be utilized in evaluating concrete structures. With each unique project, a combination of a reasonable structural/materials evaluation and use of appropriate advanced non-destructive testing technology can help save time and money for all stakeholders.
Zhengqi (Jason) Li, Ph.D., E.I.T., LEED AP, Jason is a staff engineer in our Houston office. He utilizes his doctorate in engineering to specialize in concrete diagnostics - non-destructive testing, and research on concrete materials and structures.
Jigar Desai, Ph.D., P.E., is office manager as part of Terracon’s Facilities Services team in our Houston office. He serves as a technical leader in Terracon’s Engineering Diagnostics services and consults on materials/structural diagnostics projects with a focus on concrete structures.