|Related Entries: Ultrasonic Testing (UT): Civil Engineering,Other Methods: Schmidt hammer,|
The increasing load on highway bridges due to increasing heavy goods vehicle traffic, aging and problems with the durability of structures may lead to obstructions of traffic with ensuing severe economic damages. Effective and reliable condition assessment tools are an important part of the ongoing efforts to evaluate and maintain bridge structures. In most of countries in the world, bridges and other concrete structures are regularly inspected, normally at least once every two years. However, the inspections are mainly carried out visually. Therefore, damages are only identified when the deterioration becomes visible. In Germany the interval for a simple test must be carried out every 3 years and a "deep" inspection every 6 years accordingly to DIN 1076.
Many bridges in the world collapsed every year and this not only in poor or underdeveloped nations. Many people lost their lives in recent disasters in August 2007 in Minneapolis and in September 2006 in Canada. A recent U.S. Department of Transportation survey found that 27% of the nation's bridges are structurally deficient. The U.S. has about 600,000 bridges, with 17,000 considered "scour critical". About 1,500 collapsed between 1966 and 2005, according to Jean-Louis Briaud. In the past most bridges have failed for a handful of known reasons: weak foundations, corrosion, metal fatigue, ship collision or design error.
Nondestructive testing can be an effective tool in the inspection and condition assessment of bridge structures. It can provide knowledge that may not be possible to deduce from visual observation alone. The integration of both visual and nondestructive inspection methods is key to complete bridge condition assessment and management. Some simple nondestructive techniques, such as hammer sounding, rebound hammer testing, dye penetration, and magnetic particle testing, can be easily integrated into visual inspections. The results of these integrated inspections will improve bridge data files, and will yield more technically based recommendations for further inspection and maintenance, and more accurate estimations of remaining service life. Once a full representation of the overall bridge condition is determined, appropriate and economical decisions regarding the possible rehabilitation or replacement of bridge members or the entire structure can be made.
In the case of concrete bridge decks the decks consist of a concrete slab covered by an asphalt coating. The concrete slabs generally have a thickness of approximately 25 cm and contain two mats of steel rebar reinforcements. The most serious form of deterioration in concrete bridge decks is the corrosion of steel rebars caused by the excessive use of chloride deicing salts during winter for the maintenance of the structures. As the reinforcement steel corrodes, it expands and creates a cracks or surface fracture plane in the concrete at, or just above, the level of the reinforcement. The fracture plane, or delamination, may be localized or may extend over a substantial area.
Recent advances in NDE techniques have improved the functional characteristics of many NDE methods and have led to systems that are more reliable. Increased use of NDE methods will depend on several factors including the ability of the systems to accurately detect deteriorated conditions, the ease of use and field portability of the systems, and the total cost of completing the NDE based inspections.
Since bridges are build in almost hundreds of different kind of types and using so many different materials of supporting components, so that it is not possible to use just one NDT method for all tasks. For instants, microwave or ground penetrating radar (GPR) may be used for reinforced concrete decks but is not suitable for weld testing of steel members. Also there are many tasks which need further research to make NDT methods suitable.
A lot of application reports of NDT methods for bridge testing are frequently published. Several methods are available, are in research or are used for further inspection after regularly inspections indicates their needs. They include:
Older bridges need more than just a regular inspection. As bridge get older, it needs more tests. What is missing, experts say, is short-term cash and a long-term commitment by governments to fund more research into new technologies and innovations.
GPR data collection
Deterioration Map of GPR Data of Bridge Deck
Testing with Impact-Echo (left) and GPR (right)
Crevice corrosion in layers of steel can bend plates, or strike between concrete and steel. Surface corrosion may be visible in routine bridge inspections, but non-destructive ultrasonic testing can yield information on the extent of unseen damage(WisDOT
Rolf Diederichs created: 2007-08-06 Login
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