Identification and Repair of Corroded Prestressed Tendons
Age and condition of many prestressed highway bridges should be examined for strand corrosion as part of an ongoing inspection program to provide better public safety and to provide a proactive approach to evaluating structural condition, and assessing asset value and its remaining life. Inspecting bridges for corrosion of the strands is important for several reasons. In a new structure, tendons in post tensioned (PT) structures typically are loaded near 70% of their ultimate tensile strength, leaving little room for stress increase due to loss of cross sectional area. Also, PT concrete box girder bridges may develop tension cracks in the deck at negative moment areas (over bents) which allow corrosive materials to enter the ducts and corrode the tendons. Therefore, corroded strands may go undetected and result in catastrophic failure... Further; it is now known that corrosion issues may exist in both reinforced concrete and prestressed concrete bridges.
FHWA funds the extremely expensive biannual inspection of all bridges on the National Bridge Inventory. These structures are comprehensively reviewed by means of physical sounding and/or visual inspection, without any real means to evaluate the condition of the prestressing steel that is internal to the structure. The need for nondestructive and reliable methods for identifying both the magnitude and extent of prestressed tendon corrosion in existing bridges is essential to improving public safety and optimizing the use of the large inspection budget. There is an urgent need to develop reliable and inexpensive techniques to assess the internal prestressing components. The corrosion of prestressing tendons and an inability to assess their condition has led to a number of structural failures in the recent past, illustrated by the examples below:
Varina-Enon Bridge in Virginia (2007) - The Virginia Department of Transportation (VDOT) found evidence of corrosion in one of six steel strands located inside the Varina-Enon Bridge along the southbound approach span. This 17 year old bridge was last inspected in 2005 and at that time VDOT reported that there was no evidence of corrosion. In 2007, after finding the corrosion problem, as a precaution, specially permitted loads over 57 tons (115,000 lbs were restricted from driving over the bridge at I-295 south until repairs were completed. The northbound side of the bridge was not affected by this restriction.
· Pennsylvania, 2005 bridge collapse- In the recent past, catastrophic failures of prestressed precast box beam bridges have occurred in a number of north eastern states in the United States. For example, (December 27, 2005) a 1960 era prestressed concrete box fascia beam over I-70 in Pennsylvania collapsed and fell on the structure below. Inspection of the failed member revealed heavy concrete spalling and corrosion of strands on the bottom flange of the non-composite prestressed concrete box beam. Additional corrosion was revealed on other box beam members and the bridge was subsequently removed from service.
· The Sunshine Skyway Bridge Tendon Corrosion (September 2000)- The Sunshine Skyway Bridge over lower Tampa Bay on Florida’s west coast opened to traffic in 1987. It is part of Interstate 275, which links the major metropolitan areas of Tampa/St. Petersburg and Bradenton/Sarasota. A routine inspection of the interior of the bridge’s high-level approach columns in September 2000 revealed severe tendon corrosion in column 133 northbound. Eleven of the seventeen 12.5-mm (0.5-inch) -diameter strands in the southeast tendon leg had failed in the external region immediately below the column cap The northeast tendon leg exhibited minor surface corrosion and pitting, but no strand failures were observed. Both tendon legs had split polyethylene ducts in the corroded regions. An investigation concluded that there were various levels of corrosion protection designed for the tendons that had failed, and as a result, water and oxygen recharge had occurred at multiple critical locations along the length of the columns. Detailed findings are available in the literature.
· North Carolina, 2000 Pedestrian bridge collapse- On May 20, 2000, a concrete pedestrian walkway spanning US Highway 29 (a divided four-lane highway) collapsed while crowded with NASCAR fans in front of Lowe’s Motor speedway, injuring more than 100 people, some critically. The pedestrian walkway connected the speedway and a nearby parking lot. Inspection of the 30-ft portion of the bridge which collapsed onto the roadway showed that corrosion was the cause of its collapse.
· Berlin, 1980 Congress Hall collapsed- On 21 May 1980, the Berlin Congress Hall roof caved in burying a journalist under the debris. The collapse occurred only 23 years after the building was completed. The engineering report pointed out that the collapse of both the southern outer roof and the peripheral tie of the Berlin Congress Hall was due to inadequate structural planning, unsatisfactorily executed construction of the outer roofs, and corrosion-induced fractures in the tendons bearing the roof arch.
Most of the recent structural failures on the list did not exhibit signs of distress prior to collapse. Internal voids were identified in the Varina-Enon Bridge through routine inspections and these voids were filled with good quality grout. It was assumed that completely filling the voids would stop further corrosion from occurring. However, corrosion increased even after the voids were completely filled with grout. It appears that corrosion and potential failure of post tensioned structures is more problematic than previously thought.
A typical investigation of the corrosion of prestressing tendons maybe performed by visual inspection augmented with the use of a bore scope, use of impact echo testing, an evaluation of corrosion potentials, and an assessment of chloride contamination. It is now known that this approach has not always identified ongoing corrosion and has actually provided a false sense of security while tendons continue to corrode.
It has also been verified that tendons can corrode even when completely covered by grout. Within voids and despite low pH grouts, corrosion has occurred irrespective of low chloride concentration. At times, the rate of strand corrosion may increase when voids are filled with fresh grout. The cause of this phenomenon is not fully understood and requires study. Thus, there is a need for a specific, targeted investigation of non-destructive techniques for the evaluation of prestressing tendons so that the overall structural condition may be determined. Structures often exhibit initial signs of distress (wire breaks, structural cracks, spalls, delaminations, efflorescence, misalignments, or other similar issues) that may be indicative of tendon corrosion within the internal structure. The correlation between the presence of initial distress signs and tendon corrosion should be investigated to determine when extensive evaluation of internal tendons is warranted. Ability for engineers to adequately evaluation the condition of internal structural members will lead to a better understanding of the time and the options available for rehabilitation.
This research project is focused on five focus points. These are:
1. Expand the National Bridge Inspection program to incorporate inspection guidelines for both pre-tensioned and post-tensioned bridges. The criteria should include dates and inspection methods with updated requirements for inspection of tendons directly embedded in concrete or running through ducts.
2. Conduct a thorough literature search to determine state-of-the-art methods and means for inspecting and evaluating the condition of prestressed bridges in service with emphasis on early detection of corrosion. The means and methods should be ranked by reliability, ease of use, and cost. Promising techniques that are being tested in the laboratory should be evaluated, but the emphasis should remain on methods that are currently reliable and easily applied in the field. The researcher should evaluate both typical inspection methods, as well as in-depth evaluation systems for discovering or mapping corrosion. Sources should include, but not be limited to state agencies, local agencies, international municipalities, and research available from universities and consulting resources.
3. Recommend inspection methods appropriate for prestressed/pretensioned concrete structures that have been repaired. Determine whether corrosion is likely to occur at the repair location, and if so, recommend repair procedures to decrease the likelihood of re-occurrence.
4. Include photographic and video examples of the inspection means and methods, as well as concrete removal and repair that demonstrate effectiveness of the means and methods. (Note: demonstrate the effectiveness of which means and methods?...the inspection or the removal/repair?)
5. Provide guidance on how necessary inspection procedures should be considered during the bridge design and construction. This step alone may make the difference in field application of some of the better available corrosion detection methods. It is anticipated that these efforts will become part of the bridge designer’s process of ensuring their structure meets the current requirements for 75 year bridge life spans and to allow future extension to 100 years or beyond, as needed.
Corrosion, pre-tensioned, post-tensioned, prestressed, bridge, cable stayed bridges, ducts, grout, voids, chemical additives, concrete, pedestrian, segmental construction, reinforced, NDE methods, bridge inspection, NBIS, FHWA.
It is now clear that checking for tendon corrosion is essential to insuring that public safety is met and the condition of the structure can be realized for purposes of evaluating maintenance and repair strategies. It is also clear that there are no national standards as to how to evaluate if tendons are corroded and there is no national standard as to how often this type of inspection should be conducted. Prestressed bridges which have been in service for 30 to 40 years need to be evaluated for corrosion problems.
This should be a 5 year research project with a budget of $1,000,000.
When the DOTs, FHWA, AASHTO, local municipal agencies, and private design consultants use the tools proposed in this research, the public reaps the benefit. The deliverables from this work should include:
a) An addendum to the existing NBIS manual. This addendum should provide a standard for all government agencies to follow; and
b) A manual of inspection methods for evaluating if corrosion is present and if so, the extent of the damage.
The results of this work would immediately be used by transportation owning agencies.
This research will give bridge designers, bridge inspectors, and transportation system owners the tools they currently lack but need to ensure that the traveling public is safe when crossing the nation’s bridges. If methods, techniques and equipment recommendations are provided to the states, they will be better informed of structural condition and will be better positioned to make informed decisions for necessary bridge physical improvements.
As NDE methods become more reliable for field use, the confidence as to the health of our bridges can be better realized. These are direct benefits to our bridge departments and decision makers.