Corrosion Behavior of ASTM A416 Steel in Simulated Pore Solution

Cable used in post-tensioned bridges usually consists of multiple strands encased within a plastic sleeve. The annulus or space between the steel tendon and the plastic sleeve is filled with grout, which is intended to provide corrosion protection for the steel.  Often the grout does not provide adequate protection due to a number of factors, including insufficient grout-cover, i.e., air voids; insufficient grout quality and/or strength; faulty grouting procedure; or inadequate waterproofing sealant.  Because post-tensioned concrete members rely on tensile strength of prestressed steel to resist loading, the loss of even a few wires or strands could have catastrophic results.  And yet, because these members are encased in grout, a method to detect corrosion or corrosion risk does not yet exist. 

Since post-tensioning technology is still relatively new, these problems were not evident until the 1980’s.  The first instance occurred in 1980 when the southern outer roof of the Berlin Congress Hall collapsed 23 years after it was constructed.  Soon thereafter, two bridges were found with similar serious corrosion issues – the Taf Fawr Bridge on A470 in Wales, England and the Angel Road Bridge on the A406 North Circular in London, England.¹ The Taf Fawr Bridge was demolished in 1986, and the Angel Road Bridge was significantly retrofitted in 1982.  In 1985, the single-span segmental post-tensioned Ynys-y-Gwas Bridge in Wales collapsed as a result of corrosion of longitudinal tendons at its segmented joints.  This structure was only 32 years old, and there had been no previous indication of distress prior to collapse.  In 1992, the British Department of Transportation conducted a study on these corrosion issues and concluded that there was no method that could guarantee complete corrosion prevention.  Later that year, post-tensioned bridges were effectively banned in the United Kingdom.²

The United Kingdom was not the only country with post-tensioned bridge issues.  The post-tensioned Melle Bridge, which was built in Belgium in 1956, collapsed in 1992.  In this instance, the bridge had been inspected, load tested, re-waterproofed, declared adequate, and just restored to service two years prior to its collapse.  More recently, the Saint Stefano Bridge in Italy (1999) and the Lowe’s Motor Speedway footbridge in North Carolina (2000) collapsed due to similar corrosion-related failures.³

Locally, in Florida, corrosion in post-tensioned bridges is a major concern as well.  The first reported observation of post-tension corrosion was at the 18-year old Niles Channel Bridge in the Keys (1999).⁴  Similar issues were reported at the 7-year old Mid Bay Bridge in the Western Panhandle (2001)⁵ and the 15-year old Sunshine Skyway Bridge in Tampa (2002).⁶

As part of an effort to develop an impedance-based sensor for detecting corrosion of pre-stressed tendons, experiments were conducted using rotating and stationary disk electrodes fashioned out of the tendon steel and immersed in a simulated alkaline pore solution. A model for the impedance response was developed that accounted for a porous electrode behavior that gradually disappeared over a period of 36 hours. In some cases, a small corrosion rate could be discerned from the impedance response, but, in general, the metal was stable under these conditions.

References

1. Proverbio, E. and Bonaccorsi, L. M. (2002).  Failure of prestressing steel induced by crevice corrosion in prestressed concrete structures.  Proceedings 9^th International Conference on the durability of building materials and components, Brisbane, Australia, March 17-21. 
2. Lewis, A. (1996).  A moratorium lifted.  Concrete, November/December, 25-27. 
3. Goins, D. (2000).  Motor speedway bridge collapse caused by corrosion.  Materials Performance, 36 (7) 18-19. 
4. Sagüés, A. A., Kranc, S. C., and Hoehne, R. H (2000).  Initial development of methods for assessing condition of post-tensioned tendons of segmental bridges.  Final Report Florida Department of Transportation, Tallahassee, FL, May. 
5. Hartt, W. H. (2002).  Corrosion evaluation of post-tensioned tendons on the mid bay bridge in Destin, FL.  Final Report, Florida Department of Transportation, Tallahassee, FL, April 15. 
6. Sagüés, A., Powers, R. G., and Wang, H (2008).  Mechanism of corrosion of steel strands in post tensioned grouted assemblies.  NACE International, paper no. 03312.