Repair of 2" top deck

[Mscoolone]

Our company will be providing a letter report next week on the concrete cores taken for a Pedestrian Bridges. The bridge is constructed of precast double tee beams with a 2” overlay toping with (#4 mesh) reinforcing. The topping is not a structural member except for some composite interaction with the double tees, so the rebar do not give it much strength. The topping essentially acts as a walking surface, levels out the surface, and it protects the structural double tees from the elements and damage from other forces.

We will essentially be saying that the cores were of high strength, chloride content was relatively high but not excessive, and the chloride has not had detrimental effects on the concrete durability but the Double tee 2” topping is in poor condition due to salt attach (spalls or delaminating) on rebars (mesh) in areas.

Total replacement of the 2” top deck is not justified, in our opinion and it would probably do more harm than good since the topping concrete strength is so high, the removal process may crack the flanges of the tees or break the steel connecting plates loose from the joints.

The thickness of the topping cannot be increased due to door elevation, drainage, etc.

One concept of means of repair of the shallow top w/ rebars (mesh) would be to propose that the deck can be scarified and the rebar cleaned, then a durable deck surfacing treatment (latex modified cement mortor) can be placed to minimize the rusting of the rebars (rust inhibitors painted on rebar) and it will seal/minimize the deck from future chloride attack.

Are there other solutions?
All recommendations welcome.

 

[JedClampett]

With all the preconditions you have imposed, I think you're on the right track. You could high pressure water blast the deck after scarification which might remove some more chlorides (or might drive them in deeper) and also remove more material.
Atlas (

http://www.atlastechproducts.com)

makes some sealers like E-Pro Seal and E-Pro Seal SLV that look like they will fill the bill, although I've never used them myself. Pay attention to their warnings, like the one concerning freeze thaw (I think this is mostly for on grade structures) and minimum thicknesses.

 

[Ron]

I agree you're on the right track. You might consider a nitrate admixture to the polymer concrete or a nitrate wash after the water blast to help mitigate chloride/corrosion influence.

 

[RHTPE]

High pressure water blasting could be problematic if the topping is of a higher strength than the tops of the double tees. That aside, it is an excellent process for removal of deteriorated concrete and leaves an ideal surface for new concrete.

You may also want to consider shot-blasting as well. As with water blasting, the amount of concrete removed in one pass is influenced by concrete strength & condition and the machine's settings. It also leaves a great surface for new concrete.

Either method does less damage to the concrete that will remain (in the form of microcracks) than by scarification and does a fair job of cleaning the rebar in the process.

Water blasting can create quite a mess in short order - slurry containment can be costly and must be considered. Shot-blasting (avoiding trade names here) machines can be self-contained and will vacuum up the shot and debris, separate it, and return the shot back for reuse.

Whether the bridge is open to weather (I assume it is) or not, debris containment deserves thorough consideration.

I will defer to others on corrosion mitigation and protection.


Ralph
Structures Consulting
Northeast USA

 

[DRC1]

Trying to scarify the entire deck if much of the topping is hard will be slow and therefore expensive. You may want consider addressing area that have detrioration and then sealing the deck with some of the newer sealers that are out now.

 

[ishvaaag]

By the way, if #4 means some 12 mm diameter mesh, didn't think anyone when making the pedestrian bridge is too much thick for a 2" top cover? At this size the rebar can exert significant thermal forces on the relatively weak concrete, then creating paths for attack and corrossion.