Concrete with low cover, repair options?

[TehMightyEngineer]

Anyone have any good ways to salvage concrete that was cast with low enough cover that we can see the bars poking through in spots? The exterior lifting inserts were too long and pushed the cage toward the inside face of some precast box culverts. It's a DOT job and they're likely not going to accept them no matter what we do (and it's of course their call) but we're going to keep the pieces to resell to a private owner as they've got plenty of reinforcement in them.

Attached is a picture of the worst bar which actually has zero cover. Most other bars in the interior mat have at least 1/2" to 1/4" cover.

My thought was to roughen the concrete surface, install some powder actuated fasteners, and then pour a 1.5" thick thin layer of epoxy cement repair to make up the cover lost. We can seal the edges of the two pours and all the box culvert joints get a conseal gasket as well.

Maine Professional and Structural Engineer
American Concrete Industries
www.americanconcrete.com

 

[TehMightyEngineer]

A farther picture of the piece is attached.

Maine Professional and Structural Engineer
American Concrete Industries
www.americanconcrete.com

 

[JStructsteel]

could you waterproof/wrap them to protect?

 

[KootK]

I initiated this thread a while back that may have some relevance here: Link

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JAE]

Typically we would chip away the concrete back to about 3/4" clear behind the mat of rebar and add a concrete patch -either troweled, formed/pumped, or shotcreted.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[TehMightyEngineer]

Thanks for the replies guys, good info.

jrisebo: That was my initial suggestion but I think my boss wants to go with a more "normal" repair so it looks less out of the ordinary. Plus, it was the top and bottom slab of the culvert and with a lot of people putting natural steam bottoms in these culverts I'm worried that the installation and movement of any gravel/soil/large rocks will damage any coating we put in that isn't highly durable in nature.

KootK: Amazingly useful info as always, I hadn't seen that topic in my google search. I hadn't considered a FRP wrap but I believe it would have the same issues as jrisebo's. While your rebar in the elevator was much less of a corrosion risk this definitely will be exposed pretty heavily to corrosion and needs a highly durable repair. I'll read through it a little more and see what else I can glean from that.

JAE: This was my preferred option as well but hadn't considered chipping away any concrete other than to roughen the surface for a good bond. We're probably going to add about 1.25 to 1.5 inches of a repair agent so I didn't think removing any actual existing material was required. Thoughts?

It's also worth noting that this is 5,000 psi concrete, though will probably have hit 6,000 psi by now. I noticed that I forgot to put that in the original post if it's relevant.

Maine Professional and Structural Engineer
American Concrete Industries

http://www.americanconcrete.com

 

[KootK]

I hadn't considered a FRP wrap but I believe it would have the same issues as jrisebo's.

Definitely not the FRP. I was thinking more along the lines of the Sika 122 corrosion inhibitor business that was discussed (supposedly 2mm is worth 6" concrete cover).

What is the dollar value of one of theses units? Most anything can be repaired but not everything can be repaired and resold profitably. I hate to say it but my gut feel is that these units are scrap unless you've got access to really cheap labour or can justify something pretty close to a "do nothing" solution.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[TehMightyEngineer]

Ah, I see. I've done quite a few industrial concrete repair jobs that utilized the Sika products but never was able to go back to observe how well they performed. Overall I've heard good things about them as well and that was my thought as well.

Well, they're worth less than nothing if we scrap them as the cost of the rebar, concrete, and labor would be a loss then. I guess let's just say that selling these would pay for a good part of my yearly salary. So they're definitely worth the time to work on. We also are approaching our slower time of the year as the New England long winter means not much below grade construction is going to happen between December and April. I'm sure we can find some time for some guys to work on these pieces. It just sucks that we'll probably have a few years before the right kind of job comes along where we can offer these at a low cost.

Maine Professional and Structural Engineer
American Concrete Industries

http://www.americanconcrete.com

 

[KootK]

Yeah, the economics of that might make sense. I'm probably just jaded from my wood truss days. We'd keep a $20K reject truss bundle lying around for two years, bumping into it with forklifts and trucks the whole time, and still end up chopping it up for corner sets. I imagine your products are somewhat less project specific however.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[TehMightyEngineer]

Yeah, normally these would be a problem to sell as box culverts are highly project specific as well. However, the size of these is good for many projects and we had two box culvert jobs to cast at the same time so I designed the rebar to work for both culverts with either low or high soil cover. Thus, these culverts are actually really nicely setup for another job. We also have a pile of reject precast that really should be scrapped, thankfully we still have plenty of room in our yard.

Maine Professional and Structural Engineer
American Concrete Industries

http://www.americanconcrete.com

 

[JAE]

so I didn't think removing any actual existing material was required. Thoughts?

The detail I showed above is a typical concrete repair method used by a lot of concrete repair contractors and suggested, I think, by ACI.

The idea of removing the concrete on the outside, and inside of the rebar is to firstly get rid of infected concrete with chlorides, etc. and secondly, and probably more importantly, to achieve a good lock-in with your patch to the slab by having the patch be wrapped around the slab bars.

In your case you don't have bad concrete but you do need to fine a way to ensure your patch stays in place. I don't trust bonding agents or Portland slurries to do that. (sometimes you get lucky and they work, sometimes not).

If you don't do that, I would have a concern over the long-term bond performance and permanence of your patch.

If you don't remove that much concrete then I would be temped to drill in and epoxy some wire ties of some kind to help hold the patch in place.
We've specified 9 gage stainless steel wire, "L" shaped pins to help with that.



Check out Eng-Tips Forum's Policies here:
faq731-376

 

[TehMightyEngineer]

JAE: That's what I thought. Depending on how long these things sit outside I might be concerned about chlorides and concrete quality. If we get to them right away then I think we can avoid any concern on that regard.

I agree that I don't like relying on bonding agents and cement alone. I was thinking of using powder actuated fasteners staggered around to act as headed dowels. However, I was thinking that this would cause damage to the concrete, especially near the corners where it switches from tension to compression. Drilling and epoxying some wire might be the best way as you indicated. If the shop complains I'll just say you told me to do it.

Maine Professional and Structural Engineer
American Concrete Industries

http://www.americanconcrete.com