Understanding the Role of Materials in the Durability of Prestressed Concrete in Marine Environments

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Paul, Alvaro
Kurtis, Kimberly E.
Kahn, Lawrence F.
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The durability of prestressed concrete structures in marine environments is of increasing concern. Traditionally, emphasis of research on this topic has been placed on the chloride diffusivity and permeability of the cement paste in order to increase the service life of prestressed concrete structures, while the characteristics and properties of the aggregate and steel reinforcement has received limited attention. This thesis encompasses two relatively unexplored topics regarding prestressed concrete durability in coastal regions. Recently, stainless steel alloys have been proposed to replace conventional prestressing reinforcement for concrete bridge piles in coastal regions in order to provide a 100+ year service life. Investigation of the performance of precast, prestressed concrete piles using duplex high-strength stainless steel (HSSS) 2205 prestressing strands and austenitic stainless steel (SS) 304 transverse, spiral reinforcement shows that these piles can be built following conventional construction procedures and driven to refusal without visible damage. Flexural and shear capacities of the piles are greater than predicted by AASHTO LRFD and ACI 318 specifications, while experimental prestress losses and development and transfer lengths are lower than values predicted by AASHTO LRFD. These results, in addition to the superior corrosion resistance providing a 100+ year service life, demonstrate that duplex HSSS 2205 can be used for prestressing strands in combination with austenitic SS 304 for the transverse confinement and shear reinforcement for prestressed concrete piles, using the same design requirements and construction procedures used for conventional prestressing strand and wire reinforcement. Additionally, this research evaluates the suitability of a sulfide- and sulfate-bearing aggregate obtained from a previously unexploited deposit in a coastal lowland region in Georgia. These sands produce low-pH condition in contact with moisture. Their impact on the early-age behavior, mechanical properties, and durability of cement-based materials is examined on mortar and concrete specimens. Results show a high variability of the composition of the sands and their performance when used in concrete and mortar, delays of the early-age hydration kinetics and setting time of mortar mixtures, earlier onset of reinforcement corrosion in structural concrete, and varied mechanical properties when acidic sands mined from the same source are used in concrete. The presence of sulfate and alkali feldspar minerals in the acidic sands was also shown to increase the potential of mortar mixtures to develop delayed ettringite formation (DEF) when exposed to high-temperature curing, such as during production in precast facilities. This thesis demonstrates that prestressed concrete bridge piles, reinforced with the proposed stainless steel alloys, can be effectively implemented in coastal regions but that aggregates from lowland coastal deposits may negatively impact the performance of cement-based materials. Overall, these findings demonstrate that the selection of reinforcement materials and aggregate should be examined as rigorously as the design elements (e.g., cementitious materials composition, concrete mixture proportions) more routinely assessed for marine structures.
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