Condition Assessment of Tendon Anchorages from the I-90/4th Avenue On-Ramp Bridge
FHWA/WSDOT/WSU TRAC Project Transportation Center (TRAC)
Department of Civil & Environmental Engineering
WSDOT Project Manager: Bijan Khaleghi, Concrete Specialist
The east-bound I-90/4th Avenue on-ramp bridge in Seattle, WA was constructed in 1991. In 2003, as part of the Washington State Department of Transportation (WSDOT) Intermodal Improvement Project, a portion of the I-90/4th Avenue on-ramp bridge was demolished and replaced with a re-routed entry ramp for east-bound traffic entering Interstate 90 (I-90). The bridge demolition provided a unique opportunity to assess the condition of post-tensioned tendon anchorages from the prestressed concrete box girders following twelve years of service. The objective of this study was to inspect tendon anchorage specimens from the demolition of the I-90/4th Avenue on-ramp bridge for the presence of corrosion or voids due to poor grout consolidation.
Demolition of the I-90/4th Avenue on-ramp bridge was conducted in two phases during 2003. The first demolition phase occurred during late May and early June, and involved demolition of the highest 2.5 spans of the bridge (from the hinge at Pier 7 to the negative-moment tendon anchorages near Pier 4). Demolition of the remaining 3.5 spans occurred in November 2003, following construction of a new re-routed on-ramp bridge.
Fourteen longitudinal tendon anchorages were visually inspected for evidence of tendon corrosion or voids associated with poor grout consolidation. Thirteen of the anchorages were sawn transversely using a bandsaw to reveal the internal condition of the grout and tendons. No evidence of corrosion or voids was found in thirteen of the fourteen anchorages. A typical tendon anchorage with no corrosion or voids is shown in Fig. 31. It should be noted that many of the tendon anchorages included a large void located at the grout port (as shown in Fig. 32), and small voids (typically £ 0.1 in. thick) were occasionally observed adjacent to the inner surface of the anchorage housing. However, there was no evidence of corrosion at the grout ports or in the small voids where a proper grout mix was used.
Limited corrosion associated with a void in the grout was observed in one tendon anchorage (Tendon Anchorage #8, Fig. 33). The poor grout consolidation and consequent void corrosion appear to have been caused by penetration of concrete into the anchorage at the duct-to-anchorage joint during the construction process. In particular, it is apparent that a standard concrete mix including coarse aggregate (gravel) partially filled the tendon anchorage. This resulted in poor consolidation in the small space between the anchorage housing and adjacent tendons, which led to corrosion of the inner surface of the housing. In order to assess the extent of the void and corrosion, both pieces (from the original transverse cut) of Tendon Anchorage #8 were cut open (longitudinally) with a bandsaw through the largest portion of the void (Fig. 34). The maximum thickness of the void was slightly over 0.5 in., and the void extended approximately 1.5 in. along the anchorage housing length and approximately 2 in. around the inner circumference of the housing.
Based on the results of this study of a 12 year old prestressed concrete box girder bridge in Seattle, WA, it appears that the use of appropriate grout mixes and proper grouting procedures provide adequate grout consolidation within tendon anchorages. Properly consolidated grout appears to adequately protect high strength steel prestressing tendons from corrosion. It is also apparent that improper grout mixes and/or grouting procedures can lead to poorly consolidated grout, voids where moisture can collect, and subsequent corrosion of steel components of the prestressing system. In order to prevent failures associated with corrosion in prestressed concrete bridges, it is critical to maintain proper quality control of grouting procedures during the construction process.