Partial Depth Bridge Deck Replacement

[damorim]

Hello all,

Looking at rehabbing an old bridge deck by means of a partial depth deck replacement on the entire bridge. Existing deck and overlay would be hydrodemolished off to free the existing top mat of black steel. The top mat would be removed and replaced with new stainless steel reinforcement as per the Client's wishes. Deck would be recast to original thickness and asphalt overlay added. Doing some research,I haven't been able to find much regarding complete replacement of the top mat of reinforcement... most things I find involve limited areas of removal in which the top mat is maintained in the rehab.

Anyone have any experience conducting something similar? Comments/concerns with procedure?? My concern right now is the significant depth of removal results in a pretty thin deck that needs to support the hydrodemo equipment. Haven't crunched any numbers yet but not feeling too comfortable about that. Another thing I've thought about is that the existing bottom deck layer and the new top deck layer would really have no mechanical connection between the two. Composite action would rely on the interface shear bond. With such a large plan area, maybe this is a non-issue. Curious to hear thoughts on this rehab approach.

Thanks

 

[JGard1985]

stainless steel reinforcement as per the Client's wishes. Nice!

With no experience in this approach....
Seems like you are opening Pandora's box in terms of repair and retrofit. Even if you have good moment positive moment capacity from reduced concrete section (very reduced!) How will you deal with areas of negative bending moment? I would expect cracking over beams and supports. It seems like the client will be chasing repairs in the remaining deck section as well




Jeff
Pipe Stress Analysis Engineer

http://www.xceed-eng.com

 

[SlideRuleEra]

...the significant depth of removal results in a pretty thin deck...
Haven't crunched any numbers yet...

Crunch the numbers. I'm not talking about a sophisticated computer model. Just take a preliminary simplified approach. Consider the remaining span depth (after top mat removal), the self weight of that span, the length of the span, and a reasonable construction live load and see what "should" happen with those assumptions.

As a former bridge Contractor, I've rehabilitated a fair number of bridges... but none in that (IMHO, risky) way.
Can the span be supported with falsework during the rehab?

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[damorim]

I guess both my concerns are pretty easy checks. The weight of the specified hydro demo and construction live load equipment can be compared against the thin remnant of the existing deck after hydrodemo. Also, the concrete bond been the two layers can be checked to see if it can handle the shear forces, especially the ones caused by longitudinal negative moments above the piers in the final condition. Any other concerns I'm not seeing?

 

[SlideRuleEra]

damorim - Have you performed the simple calcs yet? You are skipping the most important point - What happens when the upper rebar mat (including concrete) are removed?

Strongly suggest that you put an assumed construction load of 50 PSF on the slab (with upper mat / concrete removed) and find out what will happen. If the span collapses... then the detail repair design is irrelevant.

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[BridgeEI]

If the beams were able to support the original construction equipment when the deck was initially poured, why would removing the top couple of inches make any difference? You're beams are now composite and have a much higher load capacity compared to the initial construction. I'm not seeing an issue right off hand especially since this is something we routinely do. Maybe if it's a slab only and not supported by beams/girders.

 

[bridgebuster]

A bonded overlay isn't anything new; replacing the entire top mat seems novel. Usually, only the damaged/deteriorated bars are repaired. This type of work is typically done in stages in order to have access for equipment. Depending on your work zone width you could use couplers to splice the bars or if you have room use a traditional lap splice.

The key thing with a bonded overlay is surface preparation. Depending on the material, finishing and curing is also important. In NY we use mircosilica concrete - works very well - but it's not very forgiving.

 

[damorim]

BridgeEI - I have zero concern with the girder capacity (just for some more info, this bridge was constructed in 1952 and is a steel girder bridge with CIP concrete deck - non-composite construction). It is more of a question regarding the integrity of the roughly 4" deck that will be left after removal of the top layer and top mat. I would restrict equipment axle GVW and travel to always be atop the full-depth portion of the deck as it is hydrodemolishing (which is usually how the equipment moves anyways) and I would also require equipment to travel centred on the girders. However, there is still concern regarding the flexural capacity (transverse) and punching shear of the thin portion. A colleague is working through the exercise right now. I agree with you that partial depth deck replacements are extremely common, I'm just having trouble finding one in which the top mat was entirely replaced. The fact I can't find any such examples is what's making me second guess some things.

Bridgebuster - we would be using a high-performance concrete overlay. We've locally completed entire bridge decks all at once but the top layer is usually maintained... concerns regarding structural integrity are almost non-existant. In order to avoid the many many splices, etc., doing this in stages in not an option (we are already doing the bridge in two stages to be able to maintain traffic). There will be a closure pour required down the middle of the deck.

 

[Canuck65]

It's been a while, but I have worked on a fair number of parking structure projects where we stripped off the top half of the slab and on several of them we opted to replace the entire top mat of steel. The hydrodemo equipment typically consisted of the main pump unit that was in a tractor trailer. This was parked quite a ways away, off the suspended slab. The much lighter unit that did the actual hydro demo (about 5 or 6 ft width of removal per pass) operated on the slab and is typically supported on the 'thick' untouched portion of the slab and leaves behind the demo'd section. For a typical two way r/c slab with a design live load capacity of 50 psf, the hydrodemo equipment operated with minimal shoring below. However, the entire area below was closed to any entry in case the hydrodemo equipment broke through the slab where the soffit was deteriorated. Being a bridge deck instead of a two way slab, the initial design load is higher, however, as the slab system is different, calcs are warranted for the various loading conditions with the top half removed, unless it can be shored from below.

As far as a new steel mat is concerned, typically the replacement mat is based upon a structural design as a new slab, with consideration of what is being left behind ... all the pieces of the puzzle need to fit together. The new should meet current code.

Bonding new to old should not be difficult as long as the proper procedures are followed. No mechanical connection required on any of my projects. You should be able to develop a bond strength of over 1.5 MPa, generally in excess of the tensile strength of the 1950's parent concrete. Keep in mind that the hydrodemo equipment produces an excellent surface to bond to as long as it is kept clean and the replacement concrete is placed in a reasonably short time frame after demo. Some form of bonding agent is necessary, a good quality one can help your bond strengths. Have bond testing done at least several days after the pour.

 

[damorim]

Thanks Canuck65, that is some good info. I have reached out to a couple hydrodemolition experts to go over the plans with them and get their feedback. They'll be able to give me all of the equipment loads required as well for the analysis. The structure is fairly low over a small creek. We were looking to do a number of repairs and repainting to the girders and we were planning on using an underbridge platform.... But come to think of it, maybe a scaffolding system supported off of the ground would be more appropriate. In this case, the deck can also be shored from below if necessary.

Indeed, the new top mat will be designed with several concerns in mind including the fact that we are incorporating new (large) deck overhangs and higher crash level traffic barriers as well as the fact that the deck is currently overstressed and under-reinforced for negative moment over the piers. Design of the new top mat will be relatively straight forward.

I'll look into your recommendation for a bonding agent and will see what we have done on other partial depth repairs. As you mention, I haven't ever seen a mechanical connection provided between the layers so the bond must be sufficient for the two concrete layers to act as a single unit.

There has been some history with repairs/rehabs and engineering work on this project so I just want to cross all my t's and dot my i's on this one.

Thanks