Post Installed Anchors to CMU

[bookowski]

I posted this in the fastening group but didn't get any useful tips, maybe someone here has some experience with this:

I need to attach plates to an existing cmu wall, my loads are in the 400#-600# range per anchor in tension and about 150# shear. These plates will support cantilever tubes so I want to have a good factor of safety in there. I have some questions about how people typically handle the following:

- Most likely the anchors will hit a mix of solid and hollow cells. If I go with Hilti they recommend a different anchor for each condition - do you spec both and make the contractor switch back and forth? Powers recommends the same epoxy for both hollow and solid cells, this seems less likely to contractor mix up.

- Most anchors are shown as not applicable at or near the mortar joint, I can't really control the location of these anchors. Do you typically deal with this by adding additional anchors assuming that some will be ineffective?

thanks

 

[ash060]

Grout the cells that need to have anchors. Tell the contractor to verify that the cells are grouted before installing the anchors.

 

[frv]

That's a good suggestion.

Ordinarily you'd try to avoid grouting existing cells, as it is a pain to do; but in application critical situations, you may be justified in requiring it and not having to worry about ungrouted cells.

 

[bookowski]

How do you grout just one cell, break open the face of the block above and grout down? How do you prevent the grout from falling all the way down? I worry about doing more damage than good - have you done this before?

thanks

 

[ash060]

You grout the whole cell. Most masonry anchors require a 20" or greater edge distance, so grouting the whole vertical cell is necessary.

Sawcut the face shell and grout. Have done it many times before.

 

[rscassar]

I would not feel comfortable specifying a chemical anchor into ungrouted cmu, especially a cantilever where the anchor will always be subject to tension. I would ask the builder to locate the grouted cells prior to anchoring or to grout the cells.

 

[bookowski]

ash060 - Can you explain this a bit more, I like the idea but what you are saying is that I would grout the whole cell vertically? This is actually the top story of a multi story wall so I'd want to stop it, at least at the floor level. I could have them open the face at the floor and block that up and then grout from there up. These anchors are at almost every cell so I'd basically be grouting the wall solid... Maybe I only need to grout several courses above and below.

kikflip - The anchors into hollow cells use the screen tube insert so they 'mushroom' on the backside and become almost like a mechanical anchor. I've used them before and never had an issue. Although since this is a cantilever I agree that it's a bit more risky.

 

[frv]

There would be a bond beam at the floor level.

 

[bookowski]

frv - Good point. Still, isn't grouting the wall solid a lot of work. Do you guys really not trust anchors to hollow cells that much? I've used them with no problems in the past but maybe in this case it's worth grouting.

 

[RacingAZ]

You don't really need to grout full height unless you need to for high axial loads. For lightly loaded connections that you can calc out to work with ungrouted cells, you can just grout a certain number of blocks to satisfy whatever edge requirement for the anchors.

You break out the face shell then push down some pieces of empty grout bags to block out the cell cavity below. That's how I see the masonry guys do it.

I would normally thru bolt a cantilever connection and still grout 2-3 blocks above and below the connection.

 

[a2mfk]

frv and Racing both explained techniques I have used dozens of times. Don't ever hesitate to grout in-place CMU, its necessary sometimes and done all of the time. If you loads get really high you can have them sawcut a slot and add a vertical rebar, cover the slow and solid grout.

I would never anchor anything structural to hollow block, maybe a flat screen TV using tap cons or something. Take a hammer to a hollow face shell once and you will have a good feel for it.

I am sure you are doing something architectural, but anyway to add a small angled tube brace for truss action to reduce this to smaller axial forces?

 

[bookowski]

Thanks for the tips. I'm going to have them grout the cells. I actually don't know the composition of the wall yet so a good majority of them may already be solid.

a2fmk - I can't add kickers due to arch and I don't have access to the backside so no through bolting.

This is actually a stair, the treads are cantilevering off of the existing block wall. It's a residential stair and is only 2'-6". I've designed it for 350# live load at the tip (3') or the equivalent of 700# at midspan. I have a continuous deep stringer bolted flat to the wall and I'm assuming that only one anchor resists the tension while in reality I have one per cell so even if one isn't up to strength I will be engaging several through the stringer. Since this is a stair and a cantilever I want to have a lot of extra capacity in there which is why I was so concerned with the hollow cells. I'll have them grout and possibly even add rebar. Thanks.

 

[hokie66]

This is all about the anchors, and I agree with providing anchors only in solidly grouted sections of the wall. The other thing you need to check is the bending capacity of the wall itself.

 

[concretemasonry]

Just make sure you use a proper masonry grout per ASTM standards (ASTM 476?) to insure complete filling and bond to the masonry units. It may seem too wet due to the 8" slump, but this makes sure you have complete filling and enough moisture to be absorbed by the units and create the bond and continuity. You may want to refill the void as the excess moisture is quickly absorbed by the older, in place dry masonry units. - It is not like concrete and it works better since it is intended for that purpose.

This will allow the loads to be distributed to the entire masonry wall.

Dick



Engineer and international traveler interested in construction techniques, problems and proper design.

 

[bookowski]

hokie66 - Checked the wall and I think it's fine, but again I don't exactly know what's in there for grout/reinf yet. I think that will be ok though, it's a small moment and if need be i'll add reinf when I have them grout the cells.

concretemasonry - thanks, good tip.

So my plan is to grout any hollow cells that we find. Do people agree with using chemical anchors to grouted cells? Any reason to use mechanical instead? How long after grouting is it safe to start installing anchors?

thanks again.

 

[a2mfk]

Mechanical anchors, specifically expansion anchors in CMU, have critical edge distance requirements. I have seen exp. anchors spall CMU when installed/specified incorrectly. I prefer epoxy in CMU for this reason, make sure it is an epoxy rated/tested for "sustained tension"...

You are bolting a channel stringer at every cell to the wall? If the spacing were further out, you may need to consider the torsion you are putting into the channel, but then again it is braced to the wall and sounds like you are only "spanning" it 8". Otherwise I may suggest using an HSS for that member.

 

[rscassar]

I would use epoxy over expansion anchors for this application. I would specify you wait 28 days before installing the chemical anchors. Similar to propping a CMU lintel.

 

[hokie66]

Where you have to grout, you can cast in bolts at those locations. Would require good dimensional control, but would avoid having to wait on the grout to harden sufficiently for post-installed anchor installation.

 

[anchorengineer]

Post installed anchors is all I do.
Just use the Hilti HY-20. It can be used in hollow or grout filled cmu. Just assume the loading for the hollow cmu. It will be conservative. Use the 3/8" HIT-A anchor with a short screen tube. I've tested these anchors and (assuming the CMU is new/good) you will easily get the loads you're looking for. If you're going into old or "cinder" blocks it's a different story. Always insist the installer is trained or know's what they are doing or else you might as well put mud in the holes.

Small mechanical sleeve (not wedge) anchors might work if the CMU is good.

 

[bookowski]

anchorengineer - Hilti does not show the HY-20 as a preferred anchor in their adhesive anchor selection chart for solid blocks. They are pretty sparse in their info in general.

This is a building from the mid 80's so I don't know how great the cmu will be. I'm going to have a pull test done on a few anchors to get a feel for what I'm dealing with.

Hokie66 - You are saying to embed a bolt and leave the threaded portion out? I like it... but I doubt they'll get them in the right spot.