Full depth slab replacement for concrete balcony? 1

[smitty9898]

Surprisingly, I could not find this issue on any other threads:

I’m developing a repair spec/drawing for a job involving deteriorated concrete balconies. This job is the typical removal and replacement of the embedded handrail post. The concrete balconies are about 50 years old.

The existing embedded handrail posts are severely corroded, and are cracking the surrounding concrete. There is no other deterioration of the concrete due to carbonation, or corrosion of embedded steel. The slab thickness is approximately 4.5”, the rebar is positioned in an approximate 9”x9” grid. The bar is probably #4 or #5 at about mid-depth of slab.

We are looking at requiring a full depth removal of about a 1’ square surrounding the existing post embedments. My concern is regarding the structural capacity of the newly placed concrete patch. Is there typically adequate bond between existing and old concrete, or is a epoxy bonding agent typically spec’d. Should we “dowel” into the old concrete with epoxy rebar? Or can I just assume that the newly placed concrete will act monolithically with the old concrete? Any help would be great!!!

 

[GeoPaveTraffic]

You will need to treat this like any other cold joint in concrete and given the life safety issues involved (you don't say how high the balconies are) I would think you would want to be very careful indeed. I don't know the structural concrete codes, but I would assume that you will need deformed rebar going both directions epoxed into the existing concrete to develop the required shear and moment capacities. Also, since you are working with a relatively small piece of concrete, you may want to a patch mix with an expoy additive.

 

[JAE]

When you remove the 12" square area of concrete, you might get into a situation where you accidently cut the 9" x 9" grid-rebar. This is your key tie-in to the main slab. Just coring through the slab would be wrong in my mind.

By re-filling the opening with concrete, and leaving the original slab rebar in place, you would at least have one bar crossing the cold joint interface in each direction. This probably would be enough to ensure a good shear-friction capacity (ACI 11.7).

Doweling into 4.5" of concrete is tough to do too, but can be done with care.

Alternatives:

1. Patch back the bad concrete (12x12) but do not rely on it. Instead use steel plating or other shapes to more positively attach to the main body of the balcony slab...possibly plates on top and bottom with through-bolts.

2. Patch back the bad concrete (12x12) and use some other means to provide the lateral stability of the top rail - by possibly creating a rail to span from adjacent walls (if any?) and not rely on a vertical cantilever action to resist the guardrail forces.

 

[Ron]

I'm curious...if only the handrail sockets and embedded sections are bad, why replace so much concrete?

Why not core an oversize hole, then set new handrails in an epoxy grout?

Also, be sure to check what you're doing to the waterproofing on the balcony when you cut into it....assuming there is waterproofing.

 

[smitty9898]

Ron, JAE, Geopave, Thanks for the responses!

JAE -> I would prefer not to have steel plates do to aesthetic reasons. I’m anchoring into an adjacent wall instead of installing posts at some locations. However, most locations require a post to be connected to the concrete deck of the balcony.

Ron -> I thought about core drilling the embedment, but there are numerous locations where the corrosion from the existing embedment caused spalling on the underside of the deck. There is probably about a 6” diameter area around the embedment where the concrete is unsound.

We are installing new waterproofing after the repair of damaged concrete.

Is there typically adequate bond between existing and old concrete, or is a epoxy bonding agent typically spec’d. Should I “dowel” into the old concrete with epoxy rebar? Or can I just assume that the newly placed concrete will act monolithically with the old concrete?

 

[JAE]

I wouldn't depend on pure bond, especially if you core or sawcut the area as the remaining vertical surfaces at the cuts tend to be smooth.

I've also seen cases where a bonding agent was used but the concrete broke free, seemingly indicating that the bonding agent acted more as a de-bonding agent....probably due to poor workmanship.

I would think that any doweling, roughened surface, etc. to develop a mechanical anchorage would be preferable and more dependable than depending on raw bond.

 

[Ron]

smitty...I agree with JAE...smooth surfaces need to be roughened and/or doweled.

One method you can use in demolition is to score the top and bottom surfaces by sawing, then use a demolition hammer to remove the outboard concrete, leaving the rebar intact. Then if you clean the exposed surface of the concrete, apply a good bonding agent, and utilize the existing rebar, you should get by with only minimal doweling.

Be careful with only wall attachment of the rails. You'll have to meet a lateral and vertical load requirement by code.

 

[csd72]

Why are you replacing it with another embedded post, why not use a base plate and chemical anchors into the concrete?

 

[smitty9898]

I am using a base plate and Hilti adhesive anchors.

 

[jike]

Be careful with bonding agents. If you use one, make sure that it is water insensitive. You do NOT want to use a bonding agent that is re-emulsifiable (in other words, reactivates with moisture). The bonding agents that are moist insensitive often have limited working time, and as JAE says, after the allowable working time, is over will often act as "bond breakers".

I would suggest doweling and roughing surfaces.

 

[4wilmar]

I agree with Jike and others above: be careful about bonding agents. If you chip out only the loose or fractured concrete (even if it leaves the holes rough and irregual) the repairs may be small enough to consider an epoxy grout rather than a bonding agent with cementitious material. I think an epoxy material would get a better grip on the existing concrete and hold the area together as it has better tensile strenght than cementitious materials. Clean off any exposed rebar but don't cut it. Fix some formwork to the bottom and fill the holes with free flowing epoxy grout. I like JAE's idea of a base plate to spread the load but if this causes aesthetic problems you might be able to get by without one by embedding a new post in an epoxy material.

Either way, I think I would rig something up to load test a few posts just to be sure. Your building code should have the required loads.