Durability of a Precast Prestressed Concrete Bridge Structure at a Salt Mine After 17 Years of Service


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Title: Durability of a Precast Prestressed Concrete Bridge Structure at a Salt Mine After 17 Years of Service

Author(s): Randall W. Poston and Morris Schupack

Publication: Special Publication

Volume: 122


Appears on pages(s): 363-384

Keywords: bridges (structures); chlorides; concrete durability; conveyors; corrosion; cover; cracking (fracturing); evaluation; precast concrete; prestressed concrete; prestressing steels; reinforcing steels; rock salt; serviceability; sodium chloride; Structural R

Date: 6/1/1990

An investigation was conducted to assess the structural integrity of a 17-year-old precast prestressed concrete conveyor bridge used to transport sodium chloride rock salt from a storage building to an outside stockpile area. The stockpile, depending on storage requirements, quite often buried most of the structure and/or subjected it to sodium chloride dust. The investigation revealed that the structure had performed remarkably well, considering the small concrete cover used to protect the reinforcing elements and the inadequate consideration of structural cracking induced by unanticipated loading from stockpiled salt. The concrete strength of the single tee members was estimated to be 7000 psi (48 MPa), with cover to the stirrups varying from virtually 0 to 1 1/2 in. (0 to 38 mm) and cover to the prestressing strands varying from 3/4 to 2 in. (19 to 51 mm). It was observed that aggressive prestressing strand corrosion causing pitting and some brittle wire failures occurred locally at flexural crack locations in single tee column members with little corrosion activity immediately adjacent to the cracks, even after 17 years of aggressive chloride exposure. This observation seems to conflict with the prevailing theory of the role of cracking on corrosion--that cracks perpendicular to steel reinforcement should result in limited early localized corrosion but, with time, chloride ions penetrate even uncracked concrete and initiate widespread corrosion.