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I hired a reputable local company to install 6 foot vinyl fence around my yard. Salesman was telling me how they would be using cement to hold the post in place. To my surprise I see the workers use dry cement mix with no water or sand or anything in the holes for poles. When asked about it, they saying that "water from the ground and rain" will activate the cement and its better to do it this way than pouring wet cement. Is this correct? I even called the company and they said that this is how they are doing it for everyone.

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    How much rain do you normally get where you live? In a wet place, I could very easily see how just dry cement mix could easily pick up enough water to achieve a workable W/C -- too much water in concrete is a very bad thing, because it makes for a weak concrete mix – ThreePhaseEel Jun 5 at 3:29
  • People install fence posts with no concrete and the fences in general are fine. Concrete with no water is better than filling the space with gravel. This is a fence so it isn't like you need a 6000 psi mix. Does seem odd that they'd use a concrete mix and not bother with water. What is the expected lifetime of a vinyl fence, are the posts made of vinyl/metal or wood? – Fresh Codemonger Jun 5 at 5:52
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    To set your mind at ease, I built a fence around my backyard nearly 25 years ago. I put a bit of gravel at the bottom of each hole, set the 4x4 PT posts on the gravel then dumped about a bag of dry concrete mix into each one. We've replaced the wood fencing twice, but the posts are all original and as straight as when we set them. We do live in the Midwest US and get a fair amount of rain - there are very few irrigation systems on the farms around us - crops grow based on rainfall, and they grow quite well. – FreeMan Jun 5 at 13:04
  • @ThreePhaseEel in in NY area so not too much rain but we do get it every week. – Agoor Jun 5 at 13:39
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    I'd worry more about the depth of the whole, that's what make the post stable. – Quora Feans Jun 7 at 21:52
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I put some posts into concrete - made a dry mix and tamped it down.

Next day it was just about done - the mix absorbed moisture from the surrounding soil and was fine.

If you need a really smooth top surface then adding a wetter screed is a possibility.

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Unless you are in a extremely dry area this is fine.
The concrete mix will absorb moisture from air and surrounding soil and will slowly set. Usually a couple of days is all it takes.
Some rain or a bucket of water will speed up the process, but isn't really needed.

In fact, poured concrete is more in danger off being over-saturated with water, which dilutes the mix too much or flushes part of the mix away.

If you expect the posts to get a lot of lateral pressure while curing (like the fence catching a lot of wind) you may want to speed up the curing a little bit to prevent the posts being pushed out of vertical before the concrete is set enough. But a temporary brace at an angle serves just as well.

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    Even in extremely dry areas, there is a surprising amount of moisture in the soil. It may take an extra 48 hours or so, but it will cure. I would also note that a heavy aggregate mixture can take a moderate amount of lateral pressure while still dry. – RyanJMcGowan Jun 6 at 22:07
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This is a common technique for setting posts in the ground.

Here is a video from Quickcrete that explains how to do it:

Quickcrete Dry Mix Post Setting

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    No, this isn't the same thing - with quickcrete you pour the dry mix in first, and then water the hole so it sets. But this question appears to be about just pouring that dry mix in and adding no water, which is a lot more unusual (at least I've never seen that before.) – berry120 Jun 5 at 14:39
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    It's not exactly the same but the idea is the same. Different concrete products have different recommendations. We don't know what exact product the installer was using. But if you try putting Quickcrete in a post hole without any additional water, you will see that it hardens just the same. – jwh20 Jun 5 at 16:13
  • @berry120 the quick setting premix ive used on many occasions all have you fill the hole with water and then pour the mix in - doing it the other way washes the fine particles away and doesnt leave anything like a good solution. – Moo Jun 6 at 1:02
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This is perfectly fine unless you live in a desert where it never rains.

I've seen Florida Public Works workers build a concrete catch basin for a large pipe that passed under a road, and they used full, sealed bags of concrete mix and simply stack the bags in a gradual stair-step arrangement. When they were done and everything was neatly stacked, they punched a hole in each bag with a tool -- that's all -- and they left. It rains every day for half of the year in this part of Florida, and the catch basin was soon a solid mass. A year later, the paper and plastic that the bags were made of was noticeably breaking down, and two years later all the paper and plastic was gone and the catch basin looked like it was made of concrete pillows. It's been at least 20 years at this writing, and the catch basin still looks fine.

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    I've seen the same thing on Crete 22 years ago, albeit unintentional. There was an abandoned building site that looked like the owner had run out of money. (Not a rare occurrence in Greece, even back then.) There was a strange, almost "sculpture-like" concrete construction to the side of the house, and it took me a while to figure out what it was: it was the place where they had stacked the bags of concrete mix to be used on the site. Apparently, when the site was closed, they just left them there, and the concrete cured in the bags on its own, and at some point, the bags were completely gone. – Jörg W Mittag Jun 7 at 10:57
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Concrete hardens through a process called hydration. As you'd guess by the name, water is required. It's common practice to place dry post mix with a pool of water standing on top. I haven't seen anybody just pour in the dry mix and walk away, but I guess if the climate is sufficiently wet and/or humid, then maybe.. In either case, eventually that concrete will get enough water to cure to some degree.

The concrete resulting from dry-placed methods ends up weaker than a wet mix with the right amount of water. It's easy to tell the difference when breaking the concrete off a post with a sledge hammer -- the stuff that was done dry often falls apart with a single blow. As long as the soil is stable (no expansion/shrinking) it's probably alright to have weak concrete surrounding the post.

In my opinion, the only reason it might be "better" to do concrete this way is that it saves the installer the trouble of mixing concrete, placing it before it sets too much, cleaning tools, and disposing of excess/waste concrete. In other words, it's "better" because it's faster, cheaper, easier for the contractor.

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  • It's also "better" because it works sufficiently well for many situations. It's certainly not the way to go for an 8' privacy fence. For a 6' vinyl (i.e. light) fence, it's probably just fine in the humid summer of NY, not so much in arid Arizona... I find your comment about removing the concrete interesting and encouraging - I've got a post or two that are splitting and I've considered replacing them, but didn't want to deal with the concrete. Sounds like it won't be that bad! – FreeMan Jun 5 at 18:01
  • I found that a heavy steel digging bar readily breaks up the concrete in a fence post hole. Weak points are where the corners of the post are or were. – Jim Stewart Jun 5 at 18:07
  • that's a good thing if you need to replace the post. – Jasen Jun 6 at 4:26

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