Concrete Beam Repair

[suzpari]

I am trying to find the best way to repair severely deteriorated concrete beams supporting a concrete deck in a marine environment. The beams are 20" deep by 18" wide and about 19' long. In some cases, the bottom steel is exposed and badly corroded and some of the stirrups are also exposed. There is a significant amount of concrete spall.

I know that I can replace badly corroded tensile steel, but how can I deal with the stirrups which are only partially exposed? Also, if the loose/spalling concrete is removed and new concrete poured to restore the beam to its original dimensions, will the repaired beam act as one monolithic unit for bending as well as shear? Would some bonding compound or dowel bars be sufficient to get this action? I am thinking it won't but any advice would be appreciated. How about the connection at the end of the beams to their supports? What should I check for?

I've been searching the internet for similar construction projects but no luck. I do not have a lot of experience with concrete repair so any help would be greatly appreciated!

Thanks!

 

[JAE]

If the beams are simple span the bottom sections of concrete do not do much of anything for flexural capacity. The concrete is assumed to be cracked and ignored in your calculations.

Removing and replacing this concrete cover is usually done by chipping away at the lower sections and removing the concrete at least back to a clear distance over the bottom bars - usually 3/4" minimum. You can then possibly insert new longitudinal bars and more fully inspect the stirrups.

Damaged stirrups are difficult to repair but can be done. Typically the bottom sides of the stirrups are damaged which is the "anchorage" portion of the stirrup. Lapping new stirrups may not be possible so adding depth to the beams for this lap may be done.

This book is a great reference for this kind of work: Link to ACI Concrete Repair Manual

The concrete that is put back can be either gunite (shotcrete) or normal concrete - or a modified polymer patching compound such as Durapatch VOH by L&M - Durapatch VOH

Go here for a description of Mod. Polymers - Mod Polymers

 

[BAretired]

A cross section would help. Are you talking about T beams where the slab and beam are monolithic? Are the beams simple spans or continuous?

How about providing a photo of the beam corrosion?

BA

 

[hokie66]

This type repair in a marine environment is very difficult at best, and is often unsuccessful in the long term. There are only a few specialist contractors who are meticulous enough to give acceptable results. Repairing stirrups involves overhead welding to the undamaged part of the existing. The repairs, if attempted, should be done in conjunction with installation of cathodic protection.

 

[suzpari]

Thanks All. I have attached a couple of photos showing the damage to some beams as suggested by BAretired. I hope the links work. What do you think? Most of the beams are continuous and are monolithic with the concrete slab, which in some areas is also damaged.

Hokie66, I have already suggested cathodic protection to the client but they are not interested and do not think it necessary.

JAE, thanks for the links. I will check them out.

 

[ztengguy]

Do you have headroom where you could add a new layer of rebar, and tie it all together with some skin friction u bars drilled up into it (which also would offer some shear resisitance

 

[suzpari]

@ztengguy, I have limited headroom....maybe just a couple of inches. The idea is to restore the beams to their original dimensions though.

 

[BAretired]

The first thing to do is check the drawings, assuming you can find them. Is the design tight or is there a bit of fat in it?

The next thing to do is wire brush the reinforcement to check the depth of rust penetration. The photo suggests that it is not very deep.

If the reinforcement is deemed adequate for the job, I would suggest chipping of all soft and loose concrete, then waterproofing the steel using galvicon paint.

The underside of beams needs to be inspected on a regular basis to ensure that no new corrosion is commencing. If the client wants a more attractive finish, I would suggest something which could be easily removed for inspection purposes, then replaced. I would not attempt to add concrete cover as it will simply trap moisture and allow corrosion to continue unseen.

As hokie has said, this type of repair is often not successful, but I believe the above measures will provide a temporary solution. At some time in the future, replacement will likely be required.

BA

 

[Teguci]

Step one should be to understand why there is a problem. My concern for your situation is that the concrete is probably contaminated by high levels of chlorides (common in marine environments). Without addressing this problem, the "good" steel you leave in place will soon be rusting and spalling the concrete you left in place just above your new repairs (I would not want to have that conversation with the client).

Cathodic protection is one possible solution to that problem, but there are other answers.

 

[hokie66]

BA,
I see the degree of corrosion as almost complete. Zoom in a bit on his photo and I believe you will agree.

 

[BAretired]

Yes hokie, I believe you are correct. In that case, my earlier suggestion would not be valid, but I still do not like the idea of adding rebar and concrete to the bottom of the beam. Maybe a galvanized steel channel or truss on each side bolted through the beam?

BA

 

[hokie66]

That might be a possibility...anything they do is going to be temporary, and I think the OP has his hands full, especially with a client that rejects out of hand the idea of cathodic protection for a marine structure.

 

[suzpari]

The bars (in some beams) are definitely in a very bad way (deep rust penetration). The same for some slab areas. As I mentioned before, the client does not want to use any cathodic protection even though I explained that there would be accelerated corrosion of rebar due to the halo effect. They want to use epoxy-coated rebars and think that would be sufficient. @Teguci, is that one of the "other answers" you're referring to?

My thinking is that it would be best to reconstruct but I wanted to know what might be the best repair method because that's what the client wants. I am reluctant to just add rebar and concrete to the bottom of the beam because I do not believe that the beam will act as one unit once the 'repair' is complete.

Thanks to all for your advice and discussion.

 

[hokie66]

I think what you believe is correct. You can't fix this by just sticking some bars in and patching the bottom.

As for epoxy coated reinforcement, without meticulously preventing the epoxy from being damaged during construction, you would be better to use plain bars.

 

[Teguci]

Other solutions:
Cut out bad concrete and replace. I expect the bad concrete is a full 2 inches of depth all around (you won't have much remaining). You can determine the depth of contamination by drlling in increments and collecting the dust to send out for testing. Cores can be tested as well.

Fully replace (enough said).

Those are the only others I could think of.

The main problem is the concrete has already soaked up enough salt to eat all of your rebar. Its just a matter of time. If you leave the existing concrete in place, you will get further rusting of the existing rebar followed by spalling.

Repair or replacement concrete should be a "tight" mix (GGBFS or silica fume F additive and low water/cement ratio). There are further additives that can be mixed in to buy some additional time for the new concrete.

As for the repair concrete acting together with the existing, that is not difficult to determine. Check VQ/I against shear friction and provide some steel dowels