Investigation of Corrosion Protection Systems for Bridge Stay Cables

The use of cable-stayed bridges in medium- and long-span applications in the United States has increased significantly in the past ten years. Of paramount importance with this type of bridge is the protection of the stay cables from corrosive elements. An extensive survey showed a general concern over the adequacy of the corrosion protection of the most widely used current system. In this study the popular two-barrier system (prestressing strand inside polyethylene sheath injected with portland cement grout) was examined experimentally. The effectiveness of the grout in providing secondary corrosion protection in the event of a cut or break in the external sheathing was a major focus of the experimental work. Such breaks have been documented on a US bridge.

Small-scale experimental studies were conducted to develop a low-bleed grout for use in the large scale stay tests. In addition, small-scale accelerated corrosion tests were conducted on grout containing corrosion inhibiting admixtures.

The major portion of the experimental program involved accelerated durability testing of eight large scale stay cable specimens. Each stay specimen was 9.5-m long and was composed of 12 12.7-mm diameter 7-wire prestressing strand. The inclined stay specimens were stressed to bridge dead load levels and then injected with the low-bleed portland cement grout. Additional axial and lateral loads were then applied to most specimens to similate field loading conditions. Selected small areas of the sheathing were removed to simulate breaks in the exterior barrier. The exposed grout in these locations was then subjected to alternate wet and dry cycles with salt solution. Variables included use of temporary corrosion protection, galvanized strand, epoxy-coated strand, and greased and sheathed strand.

One important finding was that when the sheathing was breached for the accelerated corrosion tests, the grout would shrink and crack at that location. As a result of the cracking, corrosion of the underlying unprotected stand occured soon after the application of the salt solution. Another important finding was that significant air pockets formed in the grout in all specimens except one.

Undamaged epoxy-coated, greased and sheathed, and galvanized strand provided significant improvement in corrosion protection along the free length. However, corrosion occured on all of the strands in varying degress at the anchor head.