Elevated concrete slab repair

[Diesel Dan]

Who's the concrete guru?
While this isn't a garage, over the years I've seen many builds with basements under shops/garages.
This is a good PSA to PROPERLY repair floors asap if you have one.

What it is:
- an elevated maintenance area about 3 floors up, maybe 8-10" thick.

Have an idea how I'd go about "repairing" this but wondering if anyone has engineering experience in this area.

What I'm thinking:
- knock out any obviously loose previous patches. Drag some chain around or tap with hammer and listen for hollow sound.
- cut perimeter so repair area isn't feathered thin on edges
- use pneumatic industrial needle gun to get down to good concrete
- epoxy pin new rebar adjacent to compromised pieces
- remove rotted rebar
- chemically treat rusty but sound rebar
- treat concrete surface with adhesion promotor
- pour top coat

Of course, imo, this should all be done to engineering specs for rebar size, placement, pinning depth, chemicals and mud mix.
Engineering will also determine what the safe working load will be with equipment and personnel.

 

[strutaeng]

WOW, that looks horrible.

Do you know what happened to the concrete? Almost looks like a chemical attack, and subsequent corrosion from water.

Generally, those are good steps. As to cutting rebar, that really depends on what the engineering says (makes a difference what bars can be cut: flexural bars, shrinkage bars, where the bars are in the span, etc)

Hopefully there's original structural drawings for review. This is not really a cookbook repair like a slab on grade. They may even require shoring/re-shoring. IDK.

 

[Diesel Dan]

WOW, that looks horrible.

Do you know what happened to the concrete? Almost looks like a chemical attack, and subsequent corrosion from water.

Generally, those are good steps. As to cutting rebar, that really depends on what the engineering says (makes a difference what bars can be cut: flexural bars, shrinkage bars, where the bars are in the span, etc)

Hopefully there's original structural drawings for review. This is not really a cookbook repair like a slab on grade. They may even require shoring/re-shoring. IDK.

Decades of water leaks, possibly some water treatment chemicals if I'd guess.
Looks like all repairs in the past consisted of self-leveling epoxy/grout with no real surface prep.

Shoring could be a concern depending on actual slab thickness and how many layers of rebar have been compromised.
The slabs were poured with metal pan underneath that didn't need shoring when installed but that was also before any equipment was placed.

 

[ConCretin]

We've repaired a lot of bridge decks and the process is pretty close to what you describe. This type of work generally occurs after the pavement and membrane are milled off. exposing the concrete deck. They generally spec removing concrete from under the rebar to key in the repair, which requires more than a needle scaler. They also spec coating the repair with something like Armitec 110, which encapsulates the rebar and acts as a bonding agent before applying a repair mortar. I don't recall having to remove and replace much rebar - maybe they catch it before it gets too bad.

In the example you show, it looks ike the concrete cover is inadequate, which makes a successful long term repair pretty difficult unless some additional protection is added.

 

[Firebrick43]

Who's the concrete guru?
While this isn't a garage, over the years I've seen many builds with basements under shops/garages.
This is a good PSA to PROPERLY repair floors asap if you have one.

What it is:
- an elevated maintenance area about 3 floors up, maybe 8-10" thick.

Have an idea how I'd go about "repairing" this but wondering if anyone has engineering experience in this area.

What I'm thinking:
- knock out any obviously loose previous patches. Drag some chain around or tap with hammer and listen for hollow sound.
- cut perimeter so repair area isn't feathered thin on edges
- use pneumatic industrial needle gun to get down to good concrete
- epoxy pin new rebar adjacent to compromised pieces
- remove rotted rebar
- chemically treat rusty but sound rebar
- treat concrete surface with adhesion promotor
- pour top coat

Of course, imo, this should all be done to engineering specs for rebar size, placement, pinning depth, chemicals and mud mix.
Engineering will also determine what the safe working load will be with equipment and personnel.

I have seen a floor similar that had a section for whatever reason had the steel fiber overlay start popping off in two sections.

They sand blasted it, hit edges and problem areas with a long handle needle scaler, vacuumed the piss out of it, sprayed some adhesion promoter(I can’t remember what it was may of been just thin epoxy) and the poured/troweled and epoxy aggregate mix down. It was a sikafloor product.

No one thought it would hold up to the traffic as at least (30) 6000lbs forklifts an hour plus trailer trains they pulled went thru that area and occasionally the 25k and 60k forklifts did 4 or 5 times a day.

The first section they did actually didn’t hold up the first section and they used the promoter the second time (redo) plus kept pouring straight epoxy on the troweled aggregate/epoxy mix as it set up so there were no areas that looked dry.

I held up for years after that with only two small spots (1”) that came up and it so happened that they were only spots after the floor cured that looked dry because they didn’t get enough epoxy I think.

 

[Diesel Dan]

In the example you show, it looks ike the concrete cover is inadequate, which makes a successful long term repair pretty difficult unless some additional protection is added.

There are 1,000s, possibly 10s of thousands of square ft that are in perfect shape after 30+ years. There are other areas quite a distance away that have the top row of rebar at similar depth.

These areas were just exposed to years of water/chemicals from pump condensation, leaks etc. These don't see vehicle traffic other than hand pallet jacks/ small man lifts etc. No fork trucks, industrial power vehicles or such.

 

[Rusted Nut]

I would be concerned with the degradation of the rebar, and lack of cover; especially since this is elevated.

 

[C-S-H]

Maybe this is a candidate for partial-depth hydro-demolition, supplemental bars, and recasting with low-permeability concrete. Let us know how this project turns out.

 

[CFSI]

We've repaired a lot of bridge decks and the process is pretty close to what you describe. This type of work generally occurs after the pavement and membrane are milled off. exposing the concrete deck. They generally spec removing concrete from under the rebar to key in the repair, which requires more than a needle scaler. They also spec coating the repair with something like Armitec 110, which encapsulates the rebar and acts as a bonding agent before applying a repair mortar. I don't recall having to remove and replace much rebar - maybe they catch it before it gets too bad.

In the example you show, it looks ike the concrete cover is inadequate, which makes a successful long term repair pretty difficult unless some additional protection is added.

I agree 100%. We have done the same repairs in parking decks. Saw cut the perimeter, chip 1 1/2 - 2 inches around and under the rebar, sandblast the rebar, apply Sika Armatech 110 epocem to the rebar, apply epoxy bonding agent to entire repair and pour high-strength mortar. Somebody mentioned machinery and condensation on the slab, it does appear to fit that pattern as it is localized.