ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.
Issue link: http://www.aspiremagazinebyengineers.com/i/622975
CONCRETE BRIDGE TECHNOLOGY Impregnation of Post-Tensioning Tendons A solution for post-tensioning steel corrosion by David W. Whitmore and Martin R. Beaudette, Vector Corrosion Technologies Ltd. Post-tensioned concrete is a wellaccepted construction technique that offers appreciable benefits in bridge design, construction, and appearance. Many signature structures across the country have been cost-effectively built with post-tensioned concrete. Posttensioned bridge tendons are normally bonded with cementitious grout after tendon stressing. In addition to bonding the strands in place, the cementitious grout provides an important function in the durability of the structure by protecting the post-tensioning tendons from corrosion. Grout quality for post-tensioning tendons has been an important consideration for many years because strand corrosion was noted in the presence of grout bleed water and bleed water voids. This concern prompted the development of proprietary cementitious grouts with antibleed characteristics.1 Modern grouts have provided improved performance, but problems have been detected on recently constructed structures including grout voids, segregated grout, soft grout, presence of a high level of sulfates, and chloride-contaminated grout. In addition to the noted grout defects, an elevated level of relative humidity (RH) has been suggested as an environmentalcondition factor that affects the risk of corrosion for post-tensioning systems, even without the presence of chlorides.3 High RH influences the effectiveness of cementitious grout, particularly in the presence of voids, by reducing its electrical resistivity, an important material property for corrosion resistance. For example, RH levels in grouted post-tensioning tendons in Virginia have been measured as high as 94.4% as compared to ambient RH of 63.1%.4 According to the Federal Highway Administration, some post-tensioned concrete structures with multiple corrosion protection measures have experienced tendon failure within 6 to 17 years of service.5 Due to the cost of inspection and the consequence of post-tensioning tendon failure, the Texas Transportation Institute states that “. . .bridge owners should do everything economically feasible to prevent the exposure of strands to high relative humidity levels, water, and/or chloride conditions.” Impregnation of post-tensioning tendons, a recent advancement in corrosion mitigation of grouted post-tensioning steel, offers a promising solution to protect both new and existing post-tensioning tendons from the effects of high RH levels, voids, water, sulfates, and chloride-induced corrosion. The system utilizes the naturally occurring interstitial spaces between the wires of seven-wire strands to transport a formulated low-viscosity, dual-acting, hydrocarbon-silicon-polymer resin that displaces moisture, forms a protective barrier on exposed steel surfaces, and impregnates the surrounding grout to form a barrier to moisture and oxygen. Access to the interior of the tendon is made at the grout caps or by an installed port at an intermediate tendon location. After the tendon is air tested and any leaks are