chipping existing concrete for installation of new hold down

[radiocontrolhead]

Question all,

I'm trying to install a new hold down to an existing footing where an epoxy solution just wouldn't work (forces too high).

I'm toying around with the idea of having them chip away at the existing concrete (leaving all existing rebar in place and not to be cut). having them set the anchor rod and then pouring new concrete. embedment depth would be satisfied per design.

Has anyone done this?

The issue i have though is footing supports a post and it all might need to be shored while this takes place.

 

[DCBII]

ACI 546R is a good reference for this kind of repair. When you chip concrete using an impact method you cause micro-cracking or bruising of the surface of the material left behind. Typically you want to use a smaller chipping hammer to reduce bruising, and then waterblast the bruised surface off. The surface preparation to receive the repair material (whether it be concrete, mortar, grout or a proprietary product) is critical, and pull-testing a sample to ensure the contractor is doing it right may be warranted. Often the failure surface will be the bruised concrete rather than the interface between materials.

 

[KootK]

I kind of feel that if it can't be done with an epoxy solution then it probably also can't be done with a chip and embed solution. Sure, maybe you are embedding a steel Frisbee at the bottom of your anchor rod to really knock punching shear out of the park. I think that it's more likely, however, that there's some failure mode in the embedment solution that is simply getting overlooked and making it seem numerically superior to the epoxy solution. If anything, I'd expect the epoxy solution to produce a better result because you're not creating so many new potential failure planes in the concrete doing the anchoring.

Can you describe you situation a bit more? Edge distances, available top rebar in your footing, what it is that you plan to embed?

 

[KootK]

I might be able to get behind coring a hole into the footing an embedding an anchor rod and washer set.

 

[r13]

A sketch will improve understanding of the issue.

 

[radiocontrolhead]

basic sketch attached.

another issue will be an existing steel post that likely has a baseplate that may be used as a hold down mechanism but just do not have existing drawings to justify.

I also have the solution that nobody wants to see and that's to complete shore the framing above, demo existing foundation and pour new.. in which case i would then use the existing post with a specified anchorage solution (embedded anchors that would provide my hold down restraint?

 

[oldestguy]

Regardless f what is to be done for the installation, repairing the chipped concrete surface SHOULD NOT INCLUDE ANY WATER. Reason being that new concrete will bond better if there is an exposed surface that is dry so the cement paste can flow into the "porous" exposed surface. Adding a cement paste to that surface immediately before placing concrete significantly helps. My experience with this sort of bond is a test for bonding always has resulted in a beak elsewhere, not at the joint.

 

[KootK]

More thoughts below. When you run this as an epoxy solution, what is the governing failure mode?

 

[r13]

Where is the uplift force came from, the existing post, the proposed shear wall, or both? If post installed anchor does not work, then your chip away method is no-solution too. Did you consider to bring the footing to level with grade, it may provide a little wriggle room.

 

[radiocontrolhead]

I'll be honest i didn't exactly think this fully but thought maybe a chip away and repour might be a good way to restore cast in place strength.


Retired, the uplift is coming from a new proposed shearwall where there lies an existing steel post at the end of the shear wall. I was going to propose a hold down and post adjacent to it but now I'm considering checking the steel post to see if the baseplate has enough pull out capacity to act as a hold down. I would of course need to add wood nailers to receive the shear wall edge nailing.

 

[Aesur]

What about cutting all the way through, adding additional reinforcing for shear friction and turning it into a bearing/punching shear check rather than breakout?

 

[radiocontrolhead]

Aesur,

This would be one of my go to solutions anyday of the week. The problem i see with this is accessibility where that opening would need to be rather large for drilling equipment to make those holes. Alternatively to this would be to make some horizontal holes completely thorugh the footing to "clamp" it together. I'm anticipating the footing to be a 3 foot square by maybe 18 inches deep. This would make for a lot of drilling.

 

[Aesur]

How much room do you have between the slab and top of footing? Can you add more concrete over the existing footing and tie into the existing with epoxy dowels, setting your anchor in the new concrete?

Maybe there is an FRP solution to reinforce the top of the existing footing after you drill and install the new anchor?

 

[r13]

Wouldn't be the shear wall connected to the floor over entire length? Have you considered the weight-down effect of the floor?

 

[strucbells]

Have you looked at undercut anchors?

I'm not clear on what your load is or what limit state is failing using epoxy, but sometimes they can be a useful (albeit expensive) tool.

 

[Aesur]

I'm anticipating the footing to be a 3 foot square by maybe 18 inches deep. This would make for a lot of drilling.

Are you saying that the footing needed for uplift resistance is 3' square x 18" thick, if so that would give an uplift resistance of 1761 lbs (ASD), epoxy should easily work for that uplift, or are you relying on the building weight for the resistance as well? If you are using the building weight and not the footing or some combination thereof, can you attach the end wall chord to the steel column with welded studs?

 

[radiocontrolhead]

Retired:

Yes I have considered the weight of the building. It will help a bit but not much.

Strucbells:

I'm at about 15 kips for my tension load.

Aesur:

this is existing conditions and i'm estimating based on quick load take down that there is at least a 3 foot square footing under this post. Thickness, i'd guess 18". the size of the pad is not for uplift resistance. The uplift resistance will be resolved separately, through connection of a grade beam. I will need to post more information and i understand my descriptions are vague but i was mainly concerned with teh concept of chipping away and pouring in new concrete if that would effectively restore a cast in place strength.

Some reference on the topic would be helpful as well.

 

[strucbells]

Try a 3/4" dia/M20 undercut anchor. Not the minis intended for hollow core slabs but the full length ones developed for the nuclear industry initially. They're super expensive, but the cost may be worth it if you can avoid futzing around with the existing footing. Looks like it should probably work based on what you've described, but check it out for yourself as I don't have all the details. There's a few different manufacturers with ESRs you can look into.

I forget if we're allowed to name specific products/manufacturers on this site, so I'm being a little vague but a simple google should turn up at least 3 options.

 

[KootK]

...i was mainly concerned with teh concept of chipping away and pouring in new concrete if that would effectively restore a cast in place strength.

I wonder about that often myself. As far as I can tell, the entire business of structural concrete restoration is built on the premise that you can restore concrete to it's original, cast in place strength. I've always found it odd that I never hear that said out loud though. I also have my doubts as to whether or not that assumption would hold up to some targeted laboratory testing, particularly in a situation like yours where the force transfer will be very localized.

 

[Aesur]

I agree with KootK, I am not convinced you can get full bonding strength even when properly cleaning and using bonding agents. I had a conversation with an engineering rep for a major bonding agent supplier a few months back about a similar situation and he claimed you would get full strength with a bonding agent or even just wetting the concrete, however couldn't provide any testing reports, etc. backing up this claim; because of this, we decided to use shear friction and external FRP reinforcing.