Concrete Tensile Stress Limit in AASHTO LRFD 1

[concredent]

1. AASHTO LRFD Bridge Specifications, 2nd Ed. SI Unit(Interim 2003)
(1) C5.9.4.2.2
Severe corrosive conditions include exposure to deicing salt, water, or airborne sea salt
and airborne chemicals in heavy industrial areas.
(2) Table 5.9.4.2.2-1 Tensile Stress Limits in Prestressed Concrete
at Service Limit State After Losses, Fully Prestressed Components
= Bridge Type : Other than Segmentally Constructed Bridges
= For components with bonded prestressing tendons or reinforcement
that are subjected to not worse than moderate corrosion conditions
===> stress limit 0.50 * sqrt(fc’) (MPa)
= For components with bonded prestressing tendons or reinforcement
that are subjected to severe corrosive conditions
===> stress limit 0.25 * sqrt(fc’) (MPa)

2. Question
I am participating in the design of Prestressed Concrete Box Girder.
We usually use deicing salt in this area in winter.
In the case of top slab, which criteria(above 1.(2)) we should follow?
Is there any way to use 0.50 * sqrt(fc’)?
If we install water proofing on top slab, can we apply 0.50 * sqrt(fc’)?

I am looking forward to your reply.
Thanks in advance for all information.

 

[TpaRAF]

The commentary specifically mentions de-icing salts are considered an extremely corrosive environment. The lower stress limit should be used.

Are the box girders closed or "tub" shaped? If the top slab is cast independently, the p/s tendons may not be directly exposed to the corrosive environ (e.g. over seawater).

Deck coatings would wear down or deteriorate in time. It may initially offer some protection, but you're designing for a 70yr+ lifespan. It would be better to use an admixture (e.g. microsilica) for corrosion resistance.

Does your local DOT allow higher stresses at the beam ends? In Florida, the DOT allows the outer 15% of the span length twice the stress of the middle 70%, for tension at release.

Good luck!