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'Reverse' concrete form?

Posted by uvascanyon (My Page) on
Tue, Sep 6, 11 at 14:09

I realize the title might not make any sense, but I just can't think of how to better describe this in a few words... Here, I have more space though!

What is the best way to make a concrete block that once cured will receive a 4x4 post, as an example?

I'm guessing like many here, I've dug my share of post holes, poured concrete, and dropped-in 4x4's in putting-up fences. I'd like to have the option to later easily slide out the fence posts, for a few reasons I suppose that are not exactly pertinent here. Not sure the best way to make this happen.

If the best way to do this is simply to use a 4x4 post and remove it at some point, exactly when, as doing so too quickly or waiting too long makes for problems...

Thanks for the input.


Follow-Up Postings:

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RE: 'Reverse' concrete form?

There are easier ways to achieve this, if you google "deck footing" and look at the images you will see what I mean.


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RE: 'Reverse' concrete form?

If you google for a fence post sleeve, you'll find what you're looking for.


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RE: 'Reverse' concrete form?

Googling deck footing gets me quite a bit different stuff. However, I did see something that might be what you are saying. If so, I might be getting some of these for some decking actually. Are they the concrete blocks that can receive both the likes of 2x6's, inverted, or a 4x4 post? If I were to use such in a fence application, when removing or replacing the posts, that would entail digging-out the hole and filling it again. I'm looking for something that will leave me with a female 4x4 square, surrounded by concrete, for which posts can slid in and out with relative ease.

Fence post sleeve seems to do it. Actually, one of the first hits on Google is on this forum, from me! That company, Quick Fence Posts, seems to be gone now though. More importantly, their plastic sleeve had me thinking that the concrete's psi is not going to allow the 4x4 to be easily removed, which is what I'm after. I did find these (link) that are made for in-earth applications, but I'd imagine they'd work just fine in concrete also, yes?

Here is a link that might be useful: oz-post


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RE: 'Reverse' concrete form?

The fixture I had in mind for your application was not those concrete 'feet' you mention but a galvanized fixture that is set in a sonotube of concrete with a square to receive your wooden post at the other end. The thing you link to is also known as a Metpost, so this is a separate job to the bridge is it?


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RE: 'Reverse' concrete form?

I think some people make metal sleeves. I hadn't thought about the concrete squeezing the sleeves making them very tight fits but the concrete could also shrink making it a non-issue.

A friend used oz posts just recently. She said it sucks if you have rocky soil as they are tough to get installed and level.


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RE: 'Reverse' concrete form?

My flagpole is made they way you describe. Basically a capped hollow pipe set in the ground with cement and flush with the ground surface. The flagpole of smaller diameter is then set into this hole. Easy to remove and very secure. However, these sections were engineered to fit that way to avoid damaging movement/vibration; is that a concern for your project?


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RE: 'Reverse' concrete form?

Here's an interesting how-to from Instructables. He uses sand around the post, then vacuums the sand out to set a new post.
http://www.instructables.com/id/How-to-fix-and-then-extract-a-fence-post-with-ease/?ALLSTEPS
I doubt you'd be interested in his entire method, but it may give you some ideas.

Here's a blog post on a fence project that involves a removable fence post. He used the Oz post set in concrete, with the wooden post bolted to the Oz post.
http://www.all-about-the-house.com/removable-wood-fence-section-and-gate/

If you want to make your own post sleeve -- something that would have a deeper recess than the Oz sleeve seems to have -- how about cardboard juice cartons? The ones I buy (59 oz.) are about 3 5/8" interior width and 7 1/2" tall (you could add a second one for more height).

One of the qualities of a cardboard sleeve (juice carton or otherwise) is that it can be peeled away. So if you could get the post out once the concrete had put pressure on the sleeve and dried, you could then peel away the cardboard. That would leave you with a hole that would be slightly larger than the post. And that size difference I assume would make it easier to remove the fence posts when you want to change the location of the fence.


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RE: 'Reverse' concrete form?

Simpson Company ( a google search will help ) makes all kinds of profession quality post bases for columns, fence posts, arbors .....
You set them in concrete with part of the boot strap set in concrete and the other end is open to receive the post.

different bases for different applications, loads, and size of wood.


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RE: 'Reverse' concrete form?

Wrap the bottom of the post in Mylar before pouring the concrete in the hole, and the post should slide right out. It's similar to how we make the cup holes for synthetic putting greens.


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RE: 'Reverse' concrete form?

Wow, so many replies! I had a response that I thought I had submitted hours ago, but it hung in preview mode. I'll still copy / paste it below...

Mylar, that sounds interesting. I never would have guessed... I have some excess window film, that I believe is Mylar. Wrap just once around, or more? And, it's perfectly fine to let the concrete cure 100% and the post should still slide out?

I did see some Simpson Ties today, most of which have the post sitting above ground though. I did see one where the post would be in 4x4 receptacle below ground, but just a few inches. I'd still prefer to have the post down a ways... Plus, these Simpson Ties are not cheap at about 20 bucks each.

missingtheobvious, really good stuff there! The juice carton idea sounds fascinating. I just ran to our fridge. We have a 64 oz Minute Maid lemonade carton. Outside measurements are about 3.75". Have you tried this before with success? A slightly larger hole is fine as the fence will not budge when all tied together and the weight will keep in down... Winds won't be a problem as it will be an open "country" fence, so not a sail per se...

Isabella, bingo. The flagpole you have there is what I'm after with this reverse form or sleeve idea.

Mylar and / or juice cartons, lol. Good stuff.

A copy / paste from what I had planned to post earlier:

inkognito, thanks for the clarification. That helps a lot!

Yes, separate job. I'm off this week and while the work is not getting done during this break, I'll be having a delivery to the home, so I'm just trying to load up the Lowe's truck with the materials that I'll be using for various projects. I want some of the wood to dry-out for several weeks also...

tanowicki, thanks for the feedback. The old fence here that has fallen apart did not make use of concrete, so as I pull-out the old posts, holes are pretty much there already. That said, I hear ya! The ground in a canyon is probably as hard as it gets, at least from my experiences! Since I have holes already, I think these might do the trick, but I'm going to check-out what inkognito said too.


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RE: 'Reverse' concrete form?

Sorry, uvascanyon, but I have no experience, merely curiosity.

I have to ask -- are there grapes in Uvas Canyon?


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RE: 'Reverse' concrete form?

Yes indeed! Wild here on the property actually. Of course the seedless ones I planted just started cracking open :-( I suppose it has some disease and now it has been two years in a row this has happened... Drat.


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RE: 'Reverse' concrete form?

To hold a post straight up and down in a post hole concrete acts as nothing more than a filler and adds weight to the bottom of the post. It gives it no more strength than the soil around it. Concrete also forms a bowl for water and causes the post to rot faster. So what I do is use gravel or course sand. If ever I need to move the post it pulls out fairly easy and the sand or gravel just falls to the bottom.


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RE: 'Reverse' concrete form?

sounds like you don't have any code compliance , permitting or construction inspections in your area.
In my neck of the woods there are very specific unified building codes on how a fence post must be structurally supported for safety and construction integrity.


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RE: 'Reverse' concrete form?

Thank you deviant. Exactly the point I was trying to make on another of this poster's threads as he talked of suspended bridges and elevated decks. That the safety of the structure - as indicated by compliance with building codes - should be paramount.


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RE: 'Reverse' concrete form?

As I think I had said earlier, the fence that is there now, was there when the house was purchased and has been inspected multiple times, including during surveys when I inquired at that time. It has posts sitting in dirt only. There is also a rather large building on the property with no concrete foundation, just wood into the earth...

The current deck, as I said earlier, maybe in another discussion, has its planks sitting on top of the sill plate of an addition that was done long before I purchased the place. It received a permit and passed inspection, yet as the deck rots, the room addition drops because one of its wall sits on these planks. It torques that I didn't notice this design before I purchased the place; and I'm baffled that it got the blessing to be constructed in that fashion.

Back to the fence... I had asked if those ground stakes that are pounded into the ground are fine to use here, and I was told that they are (no code issues). Putting them in concrete, as opposed to earth, to better ensure that they stay upright is a consideration now. These however only have the post recessed 6 or so inches. I'm looking to drop these posts to more traditional depths. Either way, a notable improvement over what exists now...

Listen, I come here to brainstorm, get ideas from folks with more experience than myself, not to get lectured on what I need to do in addition to the design (what this forum is about). My degrees are in the legal field, so I get it; and more importantly, I appreciate their purpose (building & construction codes). I have at least 20 emails over the years with the permitting department, seeking clarification on matters. When I reach-out to them, I like to have all of my desirable options on the table FIRST, so I'm not having to come back to them later with some other idea. "This" contributes to this end... I also have to check my budget too, and other considerations. Somewhat surprised someone didn't chime-in, repeatedly even, telling me how to spend my time or money as well.

novascapes, thank you... Good point on the concrete forming a bowl of sorts, rotting the wood faster. I'd prefer a heavier base though so it won't lean over time. Recently, we had some pigs come through here, and they seemingly enjoyed pushing around rather large planters. Heck, I've even seen deer standing-up trying to reach our Asian pears, trashing my fencing to keep them out. Maybe rock as you describe would suffice, but I just see the rock compressing into earth side walls of the hole, making the post have more lateral play over time...


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RE: 'Reverse' concrete form?

I do not think any suggestion I make will work in all the different soils through out the land. They are all different. Concrete is simply a filler and it does add weight or ballast to the bottom of the post. Concrete does not necessarily hold the post plumb or vertical. No matter what the fill, it is no better than the soil surrounding it. The depth is much more important in retention. The rock or sand I suggested may not work with your soil not because of holding ability but because of porosity. It allows the water to travel through and possibly weaken the soil around it. In my area we have expansive clays. The soil shrinks when it dries. The gravel or sand works because it continues to fall into the cracks keeping the post tight in the hole. As far as its holding ability, I use this method on cattle corals where 1500 lb. animals are pushing on it. Having said this, Your soil is not mine, and we have no build codes for fence posts.
On to concrete. Concrete shrinks when it dries. Fence posts shrink when they dry. This results in a gap around the post. I would suggest caulking around the post after they dry.
This may seem like overkill to some. My suggestion is only to extend the life of the fence. I am currently working on a house that is supported by 10 X 10 treated wood columns placed in concrete. The columns in concrete all have to be replaced. Everything from the top of the concrete down has rotted away. Some Columns are in dirt alone. They are still in good condition.


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RE: 'Reverse' concrete form?

Another method may work even better. Put gravel in the bottom of the hole. Put the post on the grave. Put the concrete around the post but none below.


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RE: 'Reverse' concrete form?

novascapes, much appreciated.

What you explain makes much sense and I can visualize what you mean. I was thinking about putting some rock at the bottom of the holes to aid in drainage, hopefully not allowing the posts to sit in standing water. What I've done before is to use an "Angelo" bar first though. Once the hole is done, I shoot some water in there and let it sit until it has disappeared. With the hole's bottom now somewhat softer, I hit it hard several times with the pointy end of the bar. This often makes for depressions that go down at least 6 inches further. I'll then add the rock as you describe. I think i'll try that carton idea to see how it works. I'm really curious...

If you don't mind me asking, is there a desirable rock type you'd use, size or type?

Thanks again for sharing your knowledge, including your current job. Really hits home there seeing how the wood rotted in concrete, but not so much those in the earth...


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RE: 'Reverse' concrete form?

novascapes, much appreciated.

What you explain makes much sense and I can visualize what you mean. I was thinking about putting some rock at the bottom of the holes to aid in drainage, hopefully not allowing the posts to sit in standing water. What I've done before is to use an "Angelo" bar first though. Once the hole is done, I shoot some water in there and let it sit until it has disappeared. With the hole's bottom now somewhat softer, I hit it hard several times with the pointy end of the bar. This often makes for depressions that go down at least 6 inches further. I'll then add the rock as you describe. I think i'll try that carton idea to see how it works. I'm really curious...

If you don't mind me asking, is there a desirable rock type you'd use, size or type?

Thanks again for sharing your knowledge, including your current job. Really hits home there seeing how the wood rotted in concrete, but not so much those in the earth...


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RE: 'Reverse' concrete form?

3/4 to 1 1/2" washed is what I used.


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RE: 'Reverse' concrete form?

  • Posted by bahia SF Bay Area (My Page) on
    Fri, Sep 9, 11 at 21:06

Avoiding any earth to wood contact by way of inset metal post anchors set into concrete footings is far more permanent than sinking wood into the concrete. If strength against being pushed out of plumb is a real issue, then use heavier duty post anchors designed to resist loads. Yes it is more expensive this way, but it will last without rotting away. It sounds like there's been quite a bit of construction around your place that didn't have much logic behind its construction. If you end up doing what you propose, a recessed hole within a concrete footing will still tend to hold water and stay moist, leading to faster rate of potential rotting. Personally, when I design new fences these days, I prefer to use 1 1/2 " diameter steel poles and wrap them with 2 x 4's before framing the rest of the fence. No earth to wood contact and the fence won't fail from base of posts rotting.


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RE: 'Reverse' concrete form?

novascapes, thanks for the follow-up.

bahia, I heard that before, as someone hear had said the same thing, but framing the post with pressure-treated 1x1's.

I like this idea, especially with more meaty 2x4's, as you said. I suppose you mean the steel poles as used in cyclone fencing. I guess the 2x4 framing is done with screws, and piece of wood to close the top opening too, and this just sits on top of concrete anchoring the pole. Getting true 2x4's is very important, I would think because warped ones will make the boxing process more challenging.

Thank you bahia.


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RE: 'Reverse' concrete form?

I like the idea of the steel posts. As long as they are galvanized and are of sufficient strength to withstand wind loads they should be great. If using the normal cyclone posts I would think it would at least be the corner post size not the line post.
In my area we use a lot of oilfield drill pipe and it holds up for a long time but it might not be so available in your area.


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RE: 'Reverse' concrete form?

  • Posted by mjsee Zone 7b NC (My Page) on
    Mon, Sep 12, 11 at 9:10

Listen, I come here to brainstorm, get ideas from folks with more experience than myself, not to get lectured on what I need to do in addition to the design (what this forum is about)

MINOR RANT FOLLOWS-(you have been warned.)

In general, people are trying to help. It's the internet, take what you can use and ignore what you don't can't. Those who are worried about code compliance/safety are just trying to be helpful.If it doesn't apply to where you are,or you don't care...all well and good. But try not to get irritated by people who are concerned for your (or your guests) safety.


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RE: 'Reverse' concrete form?

Thank you novascapes.

mjsee, no problem. I understand. I've had other recent discussions where this topic surfaced as well, and not exactly in a caring tone, but a mocking one. Just so you understand that my above expression was in response to more than what's seen here...

Thanks much, everyone who took time to post.


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RE: 'Reverse' concrete form?

  • Posted by mjsee Zone 7b NC (My Page) on
    Mon, Sep 12, 11 at 16:24

Fair enough!


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RE: 'Reverse' concrete form?

My fovorite expresion;
I don't always know what I think I know.

Forums are all about different ideas. They help us do things right the first time. They can help some of us overcome ignorant ways of doing things with more intelligent ideas/or facts.
If we are willing to give them some thought it may prevent us from proving our stupidity.


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RE: 'Reverse' concrete form?

Very much agreed on that novascapes. I'm on 5 forums off and on, and truly enjoy what they offer. I actually created a forum and run it too. They are truly amazing.


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