Guidelines for Sampling, Assessing, and Restoring Defective Grout in Pre-stressed Concrete Bridge Post-Tensioning Ducts.

摘要

A significant proportion of the U.S. bridge inventory is based on bonded post-tensioned (PT) concrete construction. An important aspect of maintaining corrosion protection of these PT systems is assuring that tendon ducts are properly grouted with an acceptable material. Grout is a cementitious material typically used to provide corrosion protection to the strands used in PT concrete bridges. However, inspections have revealed fractured strands and, in some cases, failed tendons as a consequence of corrosion, even with the newer prepackaged, preapproved thixotropic grouts. Studies to-date have attributed this corrosion to physical or chemical grout deficiencies (or both), the former consisting of air voids, free water, and unhardened, segregated, or separated grout and the latter of chloride concentration in excess of what is specified by the American Association of State Highway and Transportation Officials and other specifications. Based on collected information and data analysis, State transportation departments can evaluate if grout deficiencies are present in the tendons of their PT bridges and determine the significance of any deficiencies. Durability concerns associated with PT tendons were raised as early as 1999 when tendon failures were seen in some PT bridges as a result of strand corrosion due to the collection of bleed water in grout voids at tendon profile locations like anchorages and crest areas. While the development of prepackaged thixotropic grout was thought to provide a solution to the bleed water problem, corrosion-caused tendon failures on relatively new PT bridges have occurred, and forensic studies have revealed separation and segregation of grout materials as well as the presence of soft material, free water, and high chloride and sulfate content.(13) Consequently, it has become important to examine the overall quality of materials and construction for some in-place grouts in existing PT bridges. The purpose of this study is to provide State transportation departments with guidance regarding tendon inspection, grout sampling, data analysis, and interpretation.