Seeking concrete guidance on....er...concrete anchors...
August 3, 2020 12:11 PM   Subscribe

I need to anchor an aluminum post with a 4" base and eight mounting holes to a concrete porch. Easy, right? Just use commercially available concrete anchors like TapCon or RedHeads, right? Problem is that the slab is only 2" thick.

I am not able to tell what's under the concrete - at the borders of the porch it is standard red bricks, but I expect those are the perimeter and that there are likely large voids under most of the concrete slab, which means most fasteners I've seen are too long for this application.

The aluminum post manufacturer says they need 4" of material for the wedge anchors they supply. My slab is only 2" thick, so I'm thinking I would have to drill a pilot hole in the concrete up to 1.5/1.75" deep. Do they make something that will work for this?
posted by Thistledown to Home & Garden (11 answers total) 1 user marked this as a favorite
 
Could you use a beefy toggle bolt? You'd have to drill the hole bigger than normal, but you could put the toggle through and then fill with epoxy. The toggle would expand under the slab (if you're right about it being only 2" thick).
posted by spacewrench at 12:18 PM on August 3, 2020


Can you give us more info on the post, like what it is for or connected to? Your options will be very different if you're talking about a railing as opposed to a leg supporting a second floor to the porch.
posted by NotMyselfRightNow at 12:21 PM on August 3, 2020


Response by poster: Thanks - @notmyselfrightnow - The post will be supporting a handrail, so only 36" high - nothing major load-supporting.

Can't use a toggle bolt for this, I think.
posted by Thistledown at 12:42 PM on August 3, 2020


On a slab that thin I wouldn't use any type of expansion bolt. Instead I would use epoxy and stainless or galvanized anchor bolts.

Here's a video. The tricky part is keeping the bolts aligned while the epoxy sets up. You might want to make a small wooden or cardboard jig.
posted by JackFlash at 1:00 PM on August 3, 2020


Best answer: I think 1/4" Tapcons should have enough holding power.

These have a min installation depth of 1"

I would be careful drilling too many holes close together in thin concrete as you can easily break it all into rubble. Start with 3 or 4 and see if you need more. Worse comes to worst you may just need to break up a small bit of the slab, dig it out a bit, and pour a new deeper footing just in that area. This is pretty common practice when setting heavy machinery in areas with insufficient slab depth, just on a smaller scale.
posted by dudemanlives at 1:02 PM on August 3, 2020 [2 favorites]


Best answer: ITW makes a Trubolt wedge anchor that goes with 1.5-2" embedment lengths (their online catalog is a PDF, which is a PITA but it does have capacity tables for a given embedment depth, which will let you determine if you need to pour a new footing or not).

Also, personally, I'd go for setting a new footing as dudemanlives suggests. Note however that I universally tend to over-engineer as a matter of personal practice.
posted by aramaic at 1:09 PM on August 3, 2020


Best answer: Oof, a handrail does not have much weight but a 36” lever is going to do a great job multiplying sideways force to rip those fasteners right out.

I assume handrails are going to be fallen into by a full grown adult person carrying a 50 pound box.

Maybe drill multiple holes to explore and if you only have 2” of concrete, I’d make 4” x 4” hole in the slab and the dig deeper into the dirt with a shop vac and trowel enough to pour a proper footing. Bonus there is you can embed bolts into the concrete as it sets.

On preview: what the others said.
posted by sol at 1:11 PM on August 3, 2020


I think you need some kind of toggle bolt - this blind toggle bolt from Anchor Fixings is a design I haven't seen before (also check out their boxbolt) and is more rugged than the usual things that look like giant drywall anchors which look like this - second item from left in this image.

If this was my job (and depending on the application) and given the slab thinness I can imagine alternating blind toggle bolts and boxbolts to take the strss off the slab and minimise movement in the aluminium post.
posted by unearthed at 1:36 PM on August 3, 2020


Best answer: I assume your aluminum post might be hollow with an internal diameter less than 2".

I would be tempted to drill a hole in the slab matching that i. d., install a steel pipe of that size in the hole with ~18" sticking out, and slide the post onto the pipe even if I had to drill away material from the bottom of the post to get access to the hollow core of the post.

If the slab is on or above dirt you could just pound the pipe down into the dirt; if it's above a relatively limited hollow space you could pour concrete into the hole to set the pipe into; but if it's right on top of brick this idea probably wouldn't work. So you would need to drill a 1/8" pilot hole to test the space with something like a coat hanger probe, for example.

You would likely also want a shim such as a wire wrap around the pipe to make the fit between post and pipe as tight as possible, or you could install set screws in threaded holes if the wall of the aluminum post is thick enough.
posted by jamjam at 1:43 PM on August 3, 2020


Response by poster: I think 1/4" Tapcons should have enough holding power.

These have a min installation depth of 1"

I would be careful drilling too many holes close together in thin concrete as you can easily break it all into rubble. Start with 3 or 4 and see if you need more. Worse comes to worst you may just need to break up a small bit of the slab, dig it out a bit, and pour a new deeper footing just in that area. This is pretty common practice when setting heavy machinery in areas with insufficient slab depth, just on a smaller scale.
posted by dudemanlives at 4:02 PM on August 3 [mark


The slab is on an elevated tier of bricks backed by cinderblocks - sorry, I should have mentioned that - so digging out and pouring a new footer isn't going to be feasible. I wish it was - that would be my preference.

My dad has a machine shop and he has suggested that he can make a base plate of considerably broader dimensions to space the holes out going into the concrete, and then just mount the post to the baseplate.

I also really, really love @jamjam 's idea on the pipe. That's terrific.
posted by Thistledown at 2:22 PM on August 3, 2020


Since the slab was evidently not poured in place and might seem a little on the thin side, is there any chance it's reinforced with mesh or rebar?

You might want to test it with a strong magnet before proceeding.
posted by jamjam at 4:02 PM on August 3, 2020


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