Shotcrete slab repair from below? 2

[davides]

Anyone have any experience with Shotcrete concrete slab repair from below? Our concrete is cinder concrete with corroded or totally failed reinforcement in it. The bay size is approximately 6 feet by 20 feet. There is at least one diagonal brace in each bay.

If the wire is corroded or totally gone, and there is no re-bar in the slab, how would you add structural integrity from below prior to installing Shotcrete?

How do you get the Shotcrete to bond to the old concrete (is it even necessary if you add rebar and/or wire mesh from below first?), do you use a bonding agent?

How much should it cost approximately for Shotcrete (I am in NYC but give me the price where you are and I can factor it up)?

How much to chip off the old spalled concrete, wire brush the steel beams and generally prepare the slab from below?

How much to add reinforcement from below?

Thanks!

 

[JAE]

ACI has some excellent literature on repair of damaged concrete - you'd do well to get this.

As far as the repairs go - a "standard" repair would be to

shotblast and/or hand chip (chipping hammer) away all loose and deteriorated concrete;

inspect the remaining reinforcing and make a decision as to how much needs to be repaired/replaced;

sawcut in and epoxy new rebar in grooves adjacent to and lapping the existing steel (extended to lap the good portions of existing steel);

install wire ties (9 gage or more) into the remaining concrete with drill & epoxy;

install added welded wire fabric if desired;

shotcreting the concrete "from below".

Some other considerations are whether the remaining concrete is loaded with chlorides - if so you can add embedded galvanic anodes or do other activities to prevent continued corrosion of the reinforcing.

Try this site as an example:

http://www.vector-corrosion.com

 

[JAE]

I forgot to add that I just did a parking garage repair with the concrete chipping/blasting, new rebar, and shotcreting - Nebraska - $50 to $60/sf.

 

[Ron]

Davides...have also done shotcrete repair as JAE described. Works well but be sure to get a shotcrete crew who specializes in structural repair...not your local pool builder.

 

[davides]

JAE and Ron.

Thank you for your help on this.

This will all be designed by a structural engineer with experience, I am just trying to understand the process before I speak to the engineer/contractors.

The concrete is Cinder concrete and the only reinforcing is heavy wire (1/4" +/-) about 6" apart with perpendicular wire every foot or so.

When the concrete is chipped off, the wire is typically exposed and the wire is probably 50% to 100% corroded.

When you said;

"sawcut in and epoxy new rebar in grooves adjacent to and lapping the existing steel (extended to lap the good portions of existing steel)";

I assume that you are talking about the steel beams and not the existing reinforcement wire. Essentially the rebar is to run from the flange of one beam to the flange of the next beam?

If I were to 100% replace the reinforcement in any given bay could I just install the rebar below- NOT SAWCUT AND EPOXY- (say 2" below existing slab) and tie it up to the slab with tie wire (drill and epoxy in) to keep it in place prior to shotcrete? I expect a 4" shotcrete installation and this reinforcement would essentially be in the center of the shotcrete.

When the steel beams are cleaned off, should they be painted with anything prior to shotcrete? Should they be wrapped with wire to hold the shotcrete on to prevent future pack rust from blowing off the concrete?

Thanks again.

 

[JAE]

For embedded steel beams, there are two schools of thought - one is that you should clean and paint the steel with a rust prohibitive coating (check out TNEMEC or other specialty coaatings).

The second is that you should clean off the steel and not paint it because if you do, the oxidation process will be now focused right at the ends of the painted areas and even more oxidation will occur there.

The first view is OK if the chloride content in the concrete is low or is concentrated only at the point of rusting and the new paint can extend beyond that.

The second view is good if the chlorides in the concrete are prevalent all over.

We have usually called for the steel beam to be wrapped in WWF and tied to wires that are welded to the steel beam or tied to wires that are embedded into the remaining concrete.

The old practice to encase steel beams in concrete used to be for visual and fire resitant concerns. But over time these steel beams do let go of concrete pieces if there is oxidation of the flanges and/or flex in the beams. I recently had a football sized chunk drop through a Lexus windshield in a garage. The owner was not hurt, elsewhere at the time, but I think he might have cried a bit.

 

[jheidt2543]

This is a little late, but I would also suggest contacting the Master Builders and Sika reps in your area. They both have concrete repair materials and would be willing to come out and look at your structure, maybe even do a test patch. That way you could evaluate a few methods and materials before the project goes out for bid.

Sika has an excellent concrete repair manual that matches their products to the repair problem.

Just a thought!