In-place Prestressed Concrete Beam Repair

[rpenfi77]

Hello all,

I am working on a small bridge rehab project involving some deterioration on the undersides of some members of a adjacent box girder bridge. It spans over a tidal creek, and removal of the beams is not an option. The undersides of some of the beams show minor spall, however in 2 places, the bottom shear reinforcement is exposed. According to existing bridge plans, the prestressed strands appear to rest directly on the topside of the stirrups. I need to come up with a repair detail. My thoughts are that it would be better to not require exposing a large percentage of the stirrups (as typically required for a concrete repair) rather than risk exposing the prestressed strands. Any thoughts or suggestions??

Thanks alot,
Bob

 

[civilperson]

Manufacture forms to match the shape of the beams, (paper or plywood works best). Chip away all loose concrete and steel brush any visible rust from steel at repair site. Coat with epoxy mix and wait until initial set. Position forms in place with threaded openings and relief tube. Pump epoxy concrete mix into form until visible epoxy at relief tube. After epoxy has set, (45 minutes to an hour with warm temperatures), remove forms and grind imperfections smooth. Trowel on cement based coating to match color or paint to match.

 

[mitchelon]

Verify that you have not lost a lot of PS steel cross section due to corrosion. Spalling could be from the PS steel also. A 0.1% loss on reinforcing steel (maybe an approx loss of 6ksi) is not the same as a 0.1% loss on PS steel (approx loss could be 25ksi).

 

[SlideRuleEra]

As a Bridge Contractor, in 1977, we had a job virtually identical to what you have described. The repair technique specified was virtually identical to what civilperson has recommended. After repairs, the bridge stayed in service for another 20 years (without problems) and was finally replaced because of functional obsolescence - not structural deficiency. Suggest serious consideration of civilperson's advice.

http://www.SlideRuleEra.net

 

[civilperson]

Mitchelon,
A ten per cent loss in cross sectional area would give a decrease of tensile strength in grade 60 rebar of 6 ksi x original area, and a similar ten per cent loss in prestressed strands area would decrease the strength 25-27 ksi x original area. (Probably a typo.)

 

[mitchelon]

Thanks, civilperson. That is exactly what I meant.