Most of these problems can be directly attributed to failure of one or more components of the cable corrosion protection systems. As a result of the premature failures, the bridge engineering community is increasing the performance standards for cable systems on cable-stayed bridges. At a minimum, bridge owners are requiring significant increases in durability and corrosion protection for cable stays and designs that permit access for inspection and maintenance. To ensure this improvement is effective, significant development effort has gone into the corrosion protection section of the USA Post-Tensioning Institute's Recommendations for stay cable design, testing and installation. This draft document requires that a minimum of two qualified corrosion protection barriers shall be provided for main tension elements of new structures along the free length as well as at the cable anchorages. Each separate internal or external barrier must be independently tested to verify its integrity against several performance-based criteria. The intention of this specification is to increase the number of effective corrosion barriers and therefore improve reliability of the corrosion protection system on cable-stayed bridges.
The Post-Tensioning Institute's guidelines were first published in 1986 and have been an invaluable tool during the design and construction of cable-stayed bridges ever since. This document is maintained by the PTI committee on cable-stayed bridges which includes international experts in the design, testing, and construction of these structures. But the first three editions have only provided prescriptive definitions of the required corrosion protection materials and systems. Experience has shown that the performance of some such systems has not been problem free and as a result, draft copies of the fourth edition include considerable guidance on the requirements of future corrosion protection systems. The recommendations cover materials for stay cable systems using prestressing steel including wire, strands, and bars. The draft document has been in circulation for more than six months but it has yet to receive the committee's approval.
These proposed requirements are performance-based and require a minimum of two nested corrosion protection barriers. Internal barriers must completely protect the cable for the full free length as well as the anchorage length while the external barrier should completely encase the internal barrier along the free length. The PTI document requires that three specimens of each of the corrosion protection barriers must be independently tested to validate the effectiveness of the barrier in resisting corrosion. Internal barriers such as grease, sheathing, epoxy coating and galvanising on strands must be exposed to a salt fog test for 3000 hours while the cable specimen is under load. After this time the specimen is removed, photographed, and inspected for any signs of corrosion that cannot be removed by wiping with a soft cloth. For comparison and rating purposes a series of photographs is given in the PTI document, but any signs of pitting are immediate grounds for rejection of the internal corrosion protection barrier. Additionally, all non-metallic external barriers such as polyethylene pipe, polyvinyl fluoride tape, and elastomeric cable wrap systems must also be subjected to testing in a weatherometer to simulate exposure to ultra-violet light for the life of the structure. No guidance is currently provided to define the duration of the test or the level of exposure.
Finally, a fully assembled stay cable anchorage, transition zone and one metre of cable free length should be subjected to a 96 hour leak test. To execute this test, the completed assembly is placed in a chamber with a minimum of 3m head of water and dye solution. After 96 hours, the specimen should be dissected; any visible signs of dye on the cable is grounds for rejection. As expected, the full assembly, internal barrier, and external barrier tests must be documented with a comprehensive written report.
Development of DS Brown Company's Cableguard elastomeric cable wrap system was initiated in the early 1990s as a result of the premature failure of corrosion protection systems on cable supported structures. This alternative corrosion protection system was intended for use during the construction or rehabilitation of both cable-stayed and suspension bridges. It has been successfully used on new construction and as a retrofit over other failed corrosion protection systems; it is environmentally friendly, requires no surface preparation, and can be applied directly over strand bundles alone or strand bundles encased in polyethylene pipes.
