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Old 09-17-2012, 04:41 PM   #1
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Concrete/pressure treated interfaces


I've got a half-completed project that includes a 4' retaining wall that is made of 2 ft of concrete stem wall (poured already), with galvanized thread rod embedded in the top to attach an additional 2' of 6x6 PT (not yet added). I'm planning on using a vertical plastic flashing on the earth side of the wall, down the face of the PT and overlapping the concrete by 4-6", to extend the life of the PT portion.

I am wondering about two concrete/PT interfaces on this project:

1) First: I hadn't gotten any GC input that I should put a barrier between the top of the concrete stem wall and the bottom course of PT, but with the concrete in place, I am wondering if I should think about some kind of seal between them. Even if the earth side flashing works, it seems safe to assume that moisture will wick from the front side between them, and rest on the horizontal surface of the top of the concrete stem wall.

2) Second issue: I'd like to pour concrete steps the full 4 ft high, between two short lengths of return walls. (So the bottom steps will be concrete steps between concrete walls - no worries - but the top steps would be concrete steps between PT walls - the point of contact in question.)
a) I could build the PT on the concrete return walls first, and then pour steps between them. In that case I would have the ends of the steps in contact with the PT portions of the walls interfacing on the vertical surface of the wall. (Unless I put a barrier between them with something like whatever comes out of question 1.)

b) Or alternatively, I've thought about pouring the steps first, making them wider by the thickness of the return walls on each side above the stem wall. In this design, I'd run the return wall PT courses along the return walls and onto the top of that extra width of concrete tread, notching them as necessary to fit the profile of the steps. With this approach, I'd also have PT resting on concrete in the horizontal interface, just like the running wall. If there's a recommended layer between top of the concrete stem wall and the PT courses above, I could consider using that.
I can't find a lot about pouring concrete against vertical PT as in 2a, but it seems like a bad idea without some kind of barrier. 2b could be solved by whatever I determine for the main running wall question, which I'm already way past the point of no return on.

I've thought about the theories 6 ways from Sunday, though you may have some I haven't considered; But I'd be especially interested in proven/recommended approach for the stemwall/PT interface, and experience or proven cases that would inform a go/no go on concrete poured next to vertical PT as in 2a.

Thanks for your thoughts!

Allen

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Old 09-17-2012, 05:40 PM   #2
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Concrete/pressure treated interfaces


how is the wood retaining wall being secured to the concrete wall? Inquiring minds want to know .....

will earth be placed against the wood retaining wall to the top of the wall? are you planning on installing weep holes to allow moisture on the back side to pass through?

do you have issues with frozen ground in winter? do you know what your frost depth is if any?

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Old 09-18-2012, 12:37 PM   #3
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Concrete/pressure treated interfaces


Hi GB, I've got 3/4" galvanized thread rod coming up 17" out of the concrete stem wall, located ~8" from the end of each bottom course PT beam, with more thread rod in between those, not more than 3 ft apart. Going to lock down the bottom three courses with a big galv washer and nut on each rod, and bore out recesses on the bottom of the top course beams to accommodate and hide the nut/rod. That top course will be attached with some spikes for better cosmetic appeal than the rod/nut would have, since it doesn't really have any pressure on it.

Earth will be placed to almost the top, and maybe the top. Right now I'm also contemplating a 2x6 pressure treated "cap" on the wall, with the top of the flashing tucked and secured between the cap and the top course. The idea is that the cap, and even the top course, could be "sacrificial" and replaceable after 20 years, while the bottom courses of PT would be protected by the flashing and hopefully last a lot longer before having to be replaced - with all the cringe-inspiring earthwork that implies.

Because of the flashing, I was not planning on weep holes in the PT. Theoretically, nothing would get through to weep. I've got 2.5" drain rock behind the stem wall - wrapped in drain fabric to keep it clear of infiltration - with perf pipe at the bottom that drains to daylight. I plan on keeping 12-18 of highly drainable material above that behind the PT portion, so water that gets close to the wall should hit the flashing and be able to migrate pretty quickly down and out.

Thankfully, we don't have any real frost heave issues up here in the Pacific Northwest. (Our spec for buried water lines is only 18" deep, if that's any indication). I'm curious - what's the relation of that issue to the wall? I should have plenty of footer on the concrete stem wall for our area, btw.

So... any thoughts on the interfaces between the concrete and PT?
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Old 09-18-2012, 12:46 PM   #4
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Concrete/pressure treated interfaces


BTW, forgot to mention: I ran the thread rod design past a structural engineer. He approved a design using 1/2" rod, and I beefed it up more than 100% by using 3/4". (.19" vs. .43" x-section area per rod) because it just wasn't that much more money, and why take chances?
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Old 09-18-2012, 01:34 PM   #5
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Concrete/pressure treated interfaces


I have not seen a retaining wall constructed this wall. If it was approved by a structural engineer then I'm sure you are fine. Just out of curiosity how far do the rods go into the concrete portion of the wall?

In areas I have been we always placed a concrete retaining wall designed to resist the surcharge behind the wall. Where I am currently frost is a major issue (48" frost depth). Ground water that works it way behind the retaining wall adds hydrostatic pressure to the surcharge and should it freeze it would add pressure against the retaining wall. That was the reason for asking about weep holes. They would allow any accumulated ground water to escape. Typically we use a stone french drain (wrapped in filter fabric) just above the low side grade on the surcharge side of the wall with a piece of 2" pvc going through the wall as a drain.

Always curious to see how things are done in other parts of the country.

I would not worry about a barrier between the top of the concrete and the wood retaining wall since you are using pressure preservative wood. Just make sure the wood is rated for ground contact, and not your typical above ground treated wood. When above ground treated wood (what you normally find in the Big Orange Box store) comes into contact with the ground it will not last as long as ground contact.

Again concrete against treated wood is ok so I would think you'll be fine with your steps. Pressure treated wood is what we use to separate concrete foundation from wood floor joists.

Good luck!
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Old 09-18-2012, 03:41 PM   #6
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Concrete/pressure treated interfaces


I haven't seen one like this either - the concrete bottom/PT top design was a combination of aesthetics and integrating with an existing section of 24" concrete wall. The rods go in about 8". I've considered filling the triangle formed in x-section by the top of the concrete, the top of the PT, and a point 3 or so feet back from the top of the PT with something well-draining and super light, like pumice, below the topsoil, to further minimize load since the whole thing is a bit of an experiment on my part, engineer or no.

It sounds like my equivalent of your weep hole is my exit to daylight for the 4" perf pipe (in something similar to a french drain) that I have located in the same position as your 2" pvc; Except I imagine you have those weep holes located every 10-20 ft, and I've just got the perfs picking up water along the 60' length and exiting it at the downhill end. This is a pretty typical setup around here, though weep holes are standard on larger installations, and if I'd thought about it more clearly, probably would have included those for good measure.

Hopefully the whole thing is so ridiculously drained that there won't be any in there to freeze and increase pressure. The very thought is making me lean toward the pumice, to be on the safe side. Plus, the angle of repose of the heavy material would all be leaning on the concrete that way.

As you can imagine, I wouldn't want to have to replace the bottom course of the PT in my lifetime. (The installation of 16' 6x6 beams with holes bored for the rods is going to require a combination of precision, finesse, and brute strength that I won't want to repeat!) So I'm still looking for products that might seal between the concrete and PT, though it's reassuring that you aren't seeing the need for it. If I can't find something that seems perfect, maybe I'll pass on doing anything.

Thanks!

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