Abstract

PT construction provides efficient bridge with advantages in construction, economy, and serviceability. In bonded post-tensioned systems, the introduction of stresses to the reinforced concrete element is made through tensioned high-strength steel strand that are encapsulated in a cementitious grout. The hardened grout provides corrosion protection by the presence of the alkaline pore water to passivate the steel strand and by creating a barrier from external contaminants. However, there have been documented cases of strand corrosion relating to deficiencies of the grout materials. Segregation of thixotropic grouts allowed for the development of chemically deficient grouts (characterized as having high sulfate concentrations) in vertically-deviated tendons in part attributed to an upward moisture and ion mobility. The Schupak pressure bleed test and vertically-cast grout specimens was utilized to assess the performance of grout materials subjected to adverse construction practices including prehydration and addition of excess mix water. Results indicated that there is limited availability of sulfate ions upon initial grout casting but would still be able to accumulate via transport of the free water. Greater pressures and vertical deviations allowed for greater moisture transport and sulfate accumulation.

Introduction

Bridge construction utilizing post-tensioned (PT) tendon systems have become increasingly popular.1-2 PT construction allows engineers greater options for the design of highway bridges and provides efficient structures with advantages in construction, economy, and serviceability. In bonded post-tensioned systems, the introduction of stresses to the reinforced concrete element is made through tensioned high-strength steel strand that are anchored and encapsulated in a cementitious grout within a tendon.3-4 The hardened grout allows development of the stress along the length of the tendon and also provides corrosion protection by the presence of the alkaline pore water to passivate the steel strand and by creating a barrier from external contaminants. However, there have been documented cases of strand corrosion relating to deficiencies of the grout materials.5-12 In the 1990's, the corrosion was attributed to the development and accumulation of bleed water that allowed for volume changes grout void formation. In the early 2010's segregation of zero-bleed thixotropic grouts allowed for the development of chemically deficient grouts characterized as having high sulfate and alkali ion accumulation.13-20 The formation of the deficient grout was in part attributed to the susceptibility of the material to segregate in presence of excess water. Material subjected to aging and inadequate storage are less robust in the presence of the excess mix water.21-24 In a case study bridge in Florida, the steel corrosion typically occurred at higher elevations;25-30 although, there were instances of segregated grout a low points as well. The presence of elevated sulfate concentrations (that can allow for corrosion initiation)31-34 and alkali ions in the segregated grouts in presence of the large vertical deviation was in part attributed to an upward moisture and ion mobility.35-36

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