Existing bridge as form for slab bridge. 1

I'm a little confused, are you using the existing bridge as form/shoring for the new slab? Aren't voided slabs usually precast thus eliminating the need for formwork?

Why not rip out the existing bridge, drive your piles build a cap and drop in some precast voided slabs (I assume were talking about a couple of short spans).

The Bridges are only about 80' long so we are talking some very short spans. I don't think you would need to make them voided but you could. In any case the precast I have seen still require you to pour a deck over them. Not removing the old bridge is part of the cost savings. Beyond cost saving there should also be a time savings too. I'm not saying this is a great idea I'm just looking for feedback.

Some not all voided slab bridges are precast. Cast-in-place voided slab bridges were very popular in the 1960s and generally provided a low profile bridge when used with grade seperations.

Prestressed, Precast deck beams with voids usually do have a wearing surface placed on top but it is not a requirement.

wolfhnd,

Won't the finished result of your proposal be an engineering monstrosity from the aesthetic point of view? Is minimum cost the only criterion you need to meet?

No one sees the underside of a rural drainage bridge.

Are you are using it as a deck replacement? When you add the new deck, you'll be adding a lot of dead load to the existing bridge.

Are you trying have the deck act compositely with the bridge as a strengthening measure? The cast-in-place deck will only help with live load. the existing bridge will support all dead load (including your additional deck) and its portion of the live load too. This will most likely lower the capacity of your bridge. You might want to consider some more traditional strengthening options (a good source is Xanthakos - Bridge Strengthening and Rehabilitation).

Is existing bridge deteriorated to the point where the slab span supposed to support the entire load with no help from the existing (deteriorated) structure? That's a bad idea. 80' is a long span for a C-I-P slab even without the extra dead load of the existing bridge when it deteriorates to the point where it can no longer support its own weight.