I’m getting ready to dig some holes for my deck. I’ve got to go 36” deep with a diameter of 12”. I plan to use Quikrete (50lb) bags. After checking their web site it looks like I’ll need 6 bags per hole.

Here’s the interesting part. This supposed expert told a friend of mine that you can mix dirt with the cement mix reducing the Quikrete you need by half. He also claims this will make it more structurally stable as well. I’ve never heard of such a thing. Is this guy on dope? Or what?

 

I think the use of dirt in concrete is a bad idea. I've seen it a few times after the fact, and it crumbles like a chocolate chip cookie. I never see the pros mix dirt in their concrete. If they could do just as good a job and save half the money, they'd do it. Concrete is cheap...a lot cheaper than having to remove it and re-pour when the rain erodes the dirt-laden stuff you put in the first time.

 

My suggestion: buy him rolling paper or a bong next Christmas and ignore his dirt recommendation. I paid under $3 an 80lb bag at Lowes last weekend.

 

kmarks,

It looks like the concensus is the advice you got on dirt in the mix is bad.
If you think about how concrete is made it would be obvious.

Concrete is made up of aggregate that is bound together with powdered cement which is just a glorified glue.
If you introduce dirt into the mix it would get between the cement and the gravel, weakening the bond between the two.
This is why the gravel must pass a certain standard of cleanliness for use in concrete in critical locations.

If you wanted to stretch your mix you could add clean screened gravel.

 

BTW: modified stone is a bad choice to use for this purpose.

 

Whether he is on drugs or not is irrelevant, because mixing dirt in cement is a method used by professionals.

Soil cement has been used to construct highways since 1935. (over 100,000 miles of roads)

Cement & Concrete Basics: Soil-Cement | Portland Cement Association (PCA)

"Soil-cement is also used as slope protection, ditch lining, and foundation stabilization."

"Soil-cement is used in every state in the United States as well as in all the Canadian provinces. "

I am currently using this method to upgrade a dirt floor in a barn.
I plan to use various mixture ratios of dirt per bag for different sections of the floor.
Only time will tell me which mixture works best, and it is a cheap experiment for anyone to give a try.

As for a deck footer I would use the Quikrete concrete without any additives and pour about one to one and a half bags in per hole.
I would pour them in and later set the post in and backfill with tamped earth like a fence post.
The concrete draws moisture from the ground so you only want it as a footer for the post, not poured around it.
Your requirement for that hole may be for the post to be that deep, and not the entire hole to be used as a concrete footer. (way too much)
Your footer for a 12" round hole should be about 8" thick or so depending on your state and local codes. (one to one and a half bags per hole should do)

 

User2, welcome to the forums if your post was not meant to advertise or spam. The moderators look carefully at first posts. Especially when you are responding to a 6 1/2 year old thread. Hopefully the OP got their holes filled in that time.

 

Stabilized soil is a completely different application than setting fenceposts. It may not be spam, but it is totally wrong in answer to the OP.

 

yes i know this thread is old and this is my first post but i do feel i need to jump in here. i have to agree with all that user2 said.

i came here (and elsewhere) looking for info on whether it is ok to mix cement with the soil dug from the post-hole instead of concrete mix. after all, that is what a fencer did for my front fence many years ago, plus i may do a fence myself soon so need to know what is acceptable, good, and best practice. sure, the OP was about decking but the theory is the same for fences.

there is a lot of differing information out there so it is often not easy to work out what is the best. from what i have read many of the professional "old timers" out there, plus many of the more experienced younger ones, say that concreting posts in the ground will reduce the life of that post and lead to faster rotting of the timber - a solid casing like concrete traps moisture inside. posts back-filled with compacted earth on its own has been a technique used for a very long time, and many of those fences/posts are still standing today, many many decades later. just ask yourself this question: how many recently made fences are still standing today? a bit of cement mixed in with the soil will help to "harden" the soil faster than waiting for mother nature to do it, and provide a bit more strength while still allowing water to wick away from the wood. most important is the end grain - it should be placed on top of eg a pointy rock inside the hole and then back-filled since that allows any water to drain lower than the end grain in the softer soil that has been dug up. placing the post on the hard soil at the bottom of the hole, or worse still on a bed of concrete, is a big no-no.

i have always used concrete in the past, but after my research i will never do so again. and think about these too: 1) your hardware store will not tell you that rammed earth is better because they will lose sales of concrete mix, and 2) many tradespersons will not tell you that either because ramming the earth is physically harder work than mixing concrete or throwing in a few bags of quick set and turning on the hose.

 

Hello and welcome....

Thanks for the additional reply... Since this thread is rather old I will close it. Feel free to start a new thread with any issues..

Thanks..