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Old 08-29-2014, 12:35 PM   #1
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Confused about fence post concretes


I've got some fence posts going in soon and I'm a bit confused about the types of pre-mixed concrete types available.

There's the regular stuff, cement, gravel, sand. Directions are to mix with water, and then pour into the hole.

Then there's the post specific type which I assume is the same mix + some accelerant. The directions for this are to simply dump a bag into the hole, and then add water.

Now from what I understand about concrete mixing in general, the cement requires a very specific water ratio to achieve it's maximum strength, and the strength falls off quite drastically with too little or too much water. The accelerant has very little effect on the overall concrete strength, but does allow it to have a high early strength and set up faster.

So in my mind, by using the post type and just dumping the water in afterwards, I'd likely have some pockets of the concrete with a very low water/cement ratio and some pockets with a really high ratio, both of which reduce the strength of the concrete.

So how can the post type mix properly achieve it's strength without any mixing? And why wouldn't a regular concrete mix be able to do the same? They both claim to have the same 28 day concrete strength so I'm confused about why one has to be so carefully mixed, and the other can just be tossed into a hole with water lol.
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Old 08-29-2014, 12:42 PM   #2
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You can certainly use regular bag mix by dumping it in the hole and adding water. As you note, regardless of mix type, adding water to dry mix is going to result in uneven water/cement ratio, so you will not get full strength of all the concrete. But this is a fence post, not a house foundation, so what difference does it really make if you get 1000 psi, 3000 psi, or 5000 psi? You don't even need concrete around a fence post, many people (myself included) have put them in with just tamped gravel around them, works fine. Unless you are building a high security fence to keep out terrorist invaders, I wouldn't worry about it. If you really want to worry about something, you may want to wrap the underground portion of the wood post in plastic so it does not rot. Enjoy the project.
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Old 08-29-2014, 01:00 PM   #3
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Quote:
Originally Posted by Daniel Holzman View Post
You can certainly use regular bag mix by dumping it in the hole and adding water. As you note, regardless of mix type, adding water to dry mix is going to result in uneven water/cement ratio, so you will not get full strength of all the concrete. But this is a fence post, not a house foundation, so what difference does it really make if you get 1000 psi, 3000 psi, or 5000 psi? You don't even need concrete around a fence post, many people (myself included) have put them in with just tamped gravel around them, works fine. Unless you are building a high security fence to keep out terrorist invaders, I wouldn't worry about it. If you really want to worry about something, you may want to wrap the underground portion of the wood post in plastic so it does not rot. Enjoy the project.
I wouldn't say I'm too worried about it, but I do have an 8ft tall section of fence that I want to make sure stands up, and there is about a +40% cost increase using the post-specific type concrete mix. It's about 400ft of fence in total so the cost + the extra labor to premix the concrete definitely comes into play.

Ideally I'd like to use the cheaper concrete mix for the posts and just mix it in the hole like the post mix, but I just want to make sure there isn't something I'm missing about the post mix, like some space aged additive developed by NASA, that allows it to attain the same strength as regular concrete without any mixing lol.

I'm planning on bracing the posts with 2x4's until the concrete sets up anyways, so the high early strength makes no difference to me.
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Old 08-29-2014, 01:22 PM   #4
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An 8 foot high fence is pretty tall. Typical rule of thumb for fences is to embed the fence about 40% of the height, which allows for resistance to rotation due to sideways forces. For high security fences, you need even more embedment. But the resistance to sideways forces has nothing to do with the compressive strength of the concrete, the lowest concrete is about 2500 psi, which is far above the compressive strength you need for this application. The real issue is how carefully you place the concrete to prevent voids next to the post, which could allow the post to move. Also, if you place the post is soft soil, and there are large horizontal loads, the post can move by compressing the soil. Another good reason for using the 40% embedment rule of thumb.

In your case, an 8 foot high fence would require approximately 3-1/2 feet of embedment, more if this is a high security application.
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Old 09-02-2014, 05:43 PM   #5
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I've installed plenty of fence posts with the cheaper redimix concrete.

I've mixed it first and dumped it in the hole as well as dumping the concrete directly from the bag into the hole dry and added water and tamped it in with a bar to mix it in the hole.

I prefer the dry method and have not had any problems with the posts afterwards.
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Old 09-04-2014, 01:39 PM   #6
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Quote:
Originally Posted by Daniel Holzman View Post
An 8 foot high fence is pretty tall. Typical rule of thumb for fences is to embed the fence about 40% of the height, which allows for resistance to rotation due to sideways forces. For high security fences, you need even more embedment. But the resistance to sideways forces has nothing to do with the compressive strength of the concrete, the lowest concrete is about 2500 psi, which is far above the compressive strength you need for this application. The real issue is how carefully you place the concrete to prevent voids next to the post, which could allow the post to move. Also, if you place the post is soft soil, and there are large horizontal loads, the post can move by compressing the soil. Another good reason for using the 40% embedment rule of thumb.

In your case, an 8 foot high fence would require approximately 3-1/2 feet of embedment, more if this is a high security application.
Thanks for the help. The 8ft section isn't a high security situation. I just want a little extra height as it backs onto a major road and want to block as much noise as possible.

Just another theoretical question, does spacing the posts closer together reduce the embedment depth at all? I know the soil is quite rocky, so I'm not sure the actual depth the skidsteer will be able to drill to. (I'm shooting for 4 feet deep)

I've already planned on 5ft spacing for the posts on the 8ft section of wall.
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Old 09-23-2014, 12:38 PM   #7
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I just thought I'd post a picture of the back section of fence we finished on the weekend.

http://imgur.com/szacGZv

I know, the fence board tops aren't even yet.

Bonus pic of the freak snowstorm we got the week before lol. So many broken tree limbs.

http://imgur.com/e3jWAGr

Now onto the side fence.
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Old 09-23-2014, 01:03 PM   #8
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Placing the post close together does not determine the embedment.You need to place the posts below the frost line so they do not heave with freezing soil.Since you did not give your location,it's hard to tell.
I have been installing fences for many years and mix my own concrete from sand ,pearock and Portland.
If I have a small job I buy the premixed bags.Always mix it before adding to the hole.No scientific studies or anything like that.It's been working for 40 years that way and I trust it more than dumping water on top.
Don't know where your located d189379 but some codes will not allow a 8' fence and some need to be permitted.Just something you may want to look into before you go to the trouble.
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Old 09-23-2014, 01:17 PM   #9
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Years ago, I helped my dad build a 50'x100' pole shed out on the farm. Rafters are 20' off the ground.

At the time, he was told to dig the holes, dump half-bag of portland cement in each one, leave them over night, then come the next day and put the poles in, tamping them in with dirt. So that's what we did. 30 years later, the shed is as straight and solid as when it was built.

In recent years, I've built quite a few cedar privacy fences. I now dump some crushed limestone in the hole, put in the post, and tamp it in using the crushed limestone. Absolutely rock solid results, and I've been told that the crushed stone allows moisture to get away from the posts - as opposed to concreted, which holds it in.

From what I understand of your question, if I was doing your project, I'd use crushed limestone.
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Old 09-23-2014, 01:50 PM   #10
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DrHicks:I still see them do it daily like that here in Illinois.I don't always agree with the idea of the rock being better for drainage.If you have heavy clay soil the water will pool and sit in the rock pocket.One reason I always ask for location before trying to help.It makes a big difference.
My grandpa used to sharpen his knifes on the concrete floor of the garage and did a good job of it.I do it different these days because hopefully over the years we have learned a few things.
Not saying it's wrong or won't work but wood and other materials are different now and we have hopefully learned a few things too.
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Old 09-23-2014, 02:16 PM   #11
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Quote:
Originally Posted by mako1 View Post
Placing the post close together does not determine the embedment.You need to place the posts below the frost line so they do not heave with freezing soil.Since you did not give your location,it's hard to tell.
I have been installing fences for many years and mix my own concrete from sand ,pearock and Portland.
If I have a small job I buy the premixed bags.Always mix it before adding to the hole.No scientific studies or anything like that.It's been working for 40 years that way and I trust it more than dumping water on top.
Don't know where your located d189379 but some codes will not allow a 8' fence and some need to be permitted.Just something you may want to look into before you go to the trouble.
I'm in Calgary so the frost line is 7ft down. There is no way the posts are going that deep.
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Old 09-23-2014, 02:29 PM   #12
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Quote:
Originally Posted by mako1 View Post
DrHicks:I still see them do it daily like that here in Illinois.I don't always agree with the idea of the rock being better for drainage.If you have heavy clay soil the water will pool and sit in the rock pocket.One reason I always ask for location before trying to help.It makes a big difference.
My grandpa used to sharpen his knifes on the concrete floor of the garage and did a good job of it.I do it different these days because hopefully over the years we have learned a few things.
Not saying it's wrong or won't work but wood and other materials are different now and we have hopefully learned a few things too.
FWIW, dad's farm is in NW Iowa - home of extremely thick black soil. Also though, the poles were actually "repurposed" telephone poles - so they were probably about 1 part wood & 1 part creosote. In this case, moisture doesn't seem to have been an issue.
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Old 09-27-2014, 11:50 AM   #13
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Concrete mix for the posts and just mix it in the hole like the post mix.If you have heavy clay soil the water will pool and sit in the rock pocket.One reason I always ask for location before trying to help.It makes a big difference.
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