mixing concrete

[Eric_Phillips]

So what is the consistency the concrete should be for fence posts? I have been mixing them by hand. When I do about a gallon of water to an 80lb bag the concrete is all wetted but still fairly dry consistency. This does pack well when tamped. If I add just a little more water the concrete starts to get some flow characteristics to it. It is a little soupy when tamping in the hole. So if I am going to take the time to mix the concrete instead of just adding it dry to the hole what consistency should it be?

To keep this tractor related I am digging all the holes with a 3PH post hole digger. They are getting straighter.

 

[ray66v]

You want to use the minimum amount of water possible when mixing concrete. This usually means a stiffer mix than most people think. Try to mix it so you can pick up a handful, and it stays in an open hand without running off. It will still work fine for posts, if it a little soupy.

If you get frost where you are, do not run the concrete all the way top the top of the hole. Stopping about 4 inches from the top will dramatically decrease the chances of the frost popping them out of the ground.

If your using wood posts, I suggest only using concrete on the ends. The concrete usually facilitates rotting. You can simply pack stone around the intermediate posts.

 

[MikeD74T]

For setting posts I mix to the consistancy of damp sand. If you can make a ball & it holds it's shape & leaves your hand barely damp that's just right. Tamp it around the posts & it'll hold them upright and cure to high strength. This is essentially the mix used in gunite systems, for free form swimming pools, tunnel or mine shaft lining, etc. Any water added beyond that will loose some ultimate strength but gain ease of use for pouring, etc.
Another tip to reduce frost heaving is make the bottom of the hole slightly bigger than the top & keep the sides as smooth as possible. MikeD74T

 

[Sniggle]

I recently put concrete in about 45 post holes, using 80 80lb bags. I went the other way, as I really did not want to tamp and figured I would sacrifice a little strength for ease of use.

I tried to get my mixture to the consistency of a thick stew or slightly running oatmeal. That way, when I poured it in the hole, it would flow around the pole and fill the voids, aided by some hand vibration of the pole. After cleaning out the hole by hand (tractor did the bulk of the digging) and often using a dig bar to get a few more inches into rock, the last thing I wanted to do was hand tamp concrete. The poles would easily retain their position with this consistency after 20-30 seconds in the holes. (Note that this averion to tamping was a theme for me, and because I used a tensioned fence I did only minor tamping on the line poles, as the corner poles were all in concrete with supporting cross braces).

 

[/pine]

Most of the commercial fencing installation crews I've seen don't even bother pre-mixing/wetting it out...they set the posts with dry mix tamping it in then either soaking it or not even bothering allowing mother nature to finish the job...

 

[Eric_Phillips]

Thanks for the help. It sounds like I am on track. I have mixed it a little soupy when I needed it to flow into areas that were difficult to tamp. I did do the dry method for the vinyl posts since they will only see stress when pushed on not like a tensioned fence. The line posts are going to be pounded so I only need to put concrete in the corner and brace posts.

 

[whitedogone]

Most of the commercial fencing installation crews I've seen don't even bother pre-mixing/wetting it out...they set the posts with dry mix tamping it in then either soaking it or not even bothering allowing mother nature to finish the job...

That's the best way I've found. Concrete don't get any harder than that.

 

[shinnlinger]

I too have allways put some rocks in first to hold the post where I want it and prevent the concrete from making a cup below that post and then just pouring dry, right out of the bag in next and maybe some more rocks on top. If I am feeling it I might run a garden hose over it or run around with a 5 gallon bucket, but throwing it dry sure is easier....

 

[Eric_Phillips]

I too have allways put some rocks in first to hold the post where I want it and prevent the concrete from making a cup below that post

So having an inch or two of concrete under the post is going to be bad?

 

[shinnlinger]

The problem with concrete below the post is it can form a cup that fills with water and rots the post quicker. If you tamp some rocks/gravel in there first it helps the concrete to form a ring which drains better.

 

[Builder]

Most of the commercial fencing installation crews I've seen don't even bother pre-mixing/wetting it out...they set the posts with dry mix tamping it in then either soaking it or not even bothering allowing mother nature to finish the job...

Yep. That's the way the pole barns are built and fence posts are set, they just dump ready mix bags dry, right into the ground. The mix pulls moisture out of the soil and it drys like a rock. I've dug out so many posts done this way, and I can attest that it works.

There's 2 ways of looking at having concrete under load bearing posts. If they're pressure treated ground contact 6x6's, 6x4's, I auger my post holes, pour small 12" wide footings about 10-12" thick at the bottom, smooth the tops, let them dry, then stand the treated posts on the footing, then take a post spade and tamp backfill around the posts to slightly above ground level. This is more important if the building supports a larger load. It needs a wide base under it to keep the post from sinking itself into the ground. Concrete around the sides of the post can seperate and allow the post to sink if it's got enough load on it.

If you just put a post on dirt then put concrete around it, it's not a true concrete footing. Think about it-would you dig a trench and put concrete block on the dirt, then pour conrete next to the wall in the trench? Of course not, you first pour a continuous concrete footing, then put the block walls on top.

However, if we're talking untreated fence posts, it's not critical since it's a non-load bearing post.

The other thing to remember is that if it's a treated post and you cut the bottom, it is more succeptable to rotting. The tips should be put in the ground uncut-factory finish. Re-treat them if they are cut and placed undergound.

 

[rambler]

In my soils and climate, concrete is a bad thing on a wooden fence post. No noe does that a second time! Rots them out real fast.

The post will only rot in the 8-12 inch deep area where the moisture comes & goes. Below grade the wood will come out like new, and above ground it will be like new weathered wood. The rot zone is only a few inches just below the surface.

Some codes want a collar of concrete around pole barn poles to resist pulling out. Some want a pad under the poles to prevent sinking.

But no one likes pouring concrete around a wood post - at least not when they are rebuilding the rotted out fence the second time.

I realize different soils, different climates, things need to be done differently.

--->Paul