Best treated post backfill

[Cody]

Hello everyone,
I am getting ready to set some post for a 16 x 24 cabin Im working on. I have to consider wind uplift (bigtime) frost heave and settling. so my plan was to dig big holes (which I did and it took two weeks) put spikes in the posts and put the post in the holes, then put two or three bags of 5000 psi mix (mixed in a mixer) into the holes. let cure and then backfill the rest with dirt. well then a local says to just pour in the concrete dry then immediately backfill. is that okay? which way is the best?

[ScottA]

IMO dry is better but it might take longer to set. I always dry pour fence posts.

[MountainDon]

Depends.  Define the meaning of best. 

If best is having the strongest concrete, then a wet mix is best.
If best is the least amount of labor expended, then dry is best.

I have installed plenty of posts for playground equipment as well as fences, using the dry method. For simple post holes like that I don't have a problem with that method. However, for the foundation of my house/cabin I would want to use wet mix. That way you know you have the correct amount of water, all the aggregate/sand and cement is mixed and wetted well. Remember concrete cures. The mix has to start with enough water for the chemical process to begin and complete. Too little or too much water results in an inferior concrete.

I'm assuming your holes are dug deeper than the expected frost depth and you're going to use below ground rated PT posts. It also sounds like you don't have any inspections to deal with?

[Ernest T. Bass]

I guess I'll hijack this thread, as I'm thinking making our pier foundation along these lines instead of the shallow pier method I had discussed a little while ago. I have some questions though, since our soil is a bit unique..

We have hard clay under the cabin we will be building, but the top foot or so of silty clay and topsoil holds a lot of moisture. When you first dig a hole down to the frost line, the earth is very hard. However, water quickly drains into the hole from the surface you end up with a foot of water in the bottom. This would probably not be good for pouring concrete, would it? I don't want to compromise the integrity of the footer by pouring dry concrete into tons of muddy water.. The other option is to pre-cast them ourselves, but the 24'' diameter footing pads would be very unmanageable... (We haven't really figured out how to get a 24'' wide hole dug, either...)

My other concern is, normal procedure would be to backfill around the posts with a well-drained gravel or something. I think that this would just make little cisterns around each post, as water would readily flow down from the surface and just get trapped in there, making the clay under the piers very soggy. Could I possibly mix a small amount of redi-mix with the excavated clay and backfill with that? Would there still be any danger of expansive movement or anything?

[mvk]

My understanding is that the correct amount of water is really critical to any concrete and it is usually a lot less then people use if mixing by hand including a mixer. The reason is it's hard to do and also work with. I have some experience with industrial use of concrete where it was tested before and during pours. It was for a ball mill slab 6' thick. I also mixed a lot by hand for fencing and we mixed it wet because it was easier and faster and not really so critical and we were working piece work. When doing piers by hand I mixed it really dry

When I first moved to NH I couldn't believe how lousy they mixed concrete and got away with it. So much for inspector's. They would put they chutes at 1 or  2 places and pour a whole foundation. Just get it really wet. plus no re bar. I was framing then and all most every house had cracked foundations.

Ernest
I think that if you had a form and could get the water out, pump maybe, and put a properly mixed concrete in then you would be OK. What I mean is if you just got the water out till the form was filled even if it came right back in it would be OK. Concrete is supposed to be able to set up under water?

Would like to here from a engineer or technical person on all things concrete though.

Another thing I have poured a lot of pier's and if you are digging by hand and have good soil you can bell out the bottom, kind of make your own big foot and then use a tube and fill in as you go and lift the tube. You can get away with shorter tubes. I used to make my own with felt paper and duck tape. 3 wraps of felt and 3 or 4 wraps of duck tape in 4 places on the 3' length of the tube. Made them as big as 18" inches or so, might go with tape in 5 places if going over that though.  Never had one fail if done right. I have poured many 4' + deep piers this way. You just have to pull it up as you go because if you over fill with out lifting it wont budge and leaves you in a pickle. I never went out of the ground much though, I always like to use posts. Might need a small person to hold by the feet to wangle the bar if you are to deep.

Mike

[ScottA]

From experience I can tell you a post set with mixed concrete will lossen up much easier than one that is poured dry. My guess is the dry poured concrete somehow bonds with the soil around it where the mixed doesn't. I agree getting the correct mix will yeild the strongest concrete but for post setting applications I don't think thats as critical. If your posts are set deep enough and properly braced I really don't see whats to be gained by using concrete at all for a cabin foundation other than for footings.

[Ernest T. Bass]

Yeah, I was planning on only pouring a 4''-6'' thick reinforced footing pad, and coming all the way up with 4x6 PT posts. Actually, I'd rather try out the new TimberSil non-toxic lumber (http://www.timbersilwood.com), but I wonder if that stuff will ever get noticed..

Thanks for the suggestions, Mike. A homemade sonotube would save a lot of money on a big project.. Can anyone confirm that concrete would set OK underwater? It would probably only take about 1/2 hour for the water to come back into the hole after draining it out.

[muldoon]

I agree with ScottA on this as his experience matches mine, (and what I have seen of my fathers and uncles work from before I was born), dry tamped concrete with aggregate seems to hold up better than mixed concrete.  This is especially true if you add a 6" lag bolt or into the post where the concrete surrounds it underground.   From what I have seen the mixed concrete holds very tight to the post, but not as tight to the dirt around it and the whole thing can work loose.  The dry concrete absorbs the moisture from the soil, so maybe this doesn't work well in Arizona, but it works here. 

What I would add to this conversation is that from looking at alot of old posts, (mostly fence posts and pole barns) the ones with problems all have some common failure points and thats where you should pay attention.  The following two things will go along way to adding the life of any posts.  (and they dont cost any significant extra to do it). 

1) Dont put the post directly into the hole, instead add about 3-4 inches of pea gravel before placing the post or the concrete.  This allows water that travels down the post to drain away from the wood end.  Without doing this, water will soak into the wood and rot, this rot will travel up the post and literally destroy it out from the inside out. 

2) Where the post enters the ground, water will commonly stand on the top of the concrete top and wick into the wood.  Take a few minutes to taper the top of the concrete forcing water away from the wood.  Ideally, bring the concrete above surface level a few inches and taper it off above grade. 

Again, this is just my experience. 

Can anyone confirm that concrete would set OK underwater?

Also from experience doing concrete piers for a fishing pond dock, concrete does just fine in water so long as you do it in a single pour. 

[MountainDon]

Clay:  I have no experience with clay (thank goodness) so can't comment on it.

Concrete: Concrete will set up under water. Concrete gets it's strength from a chemical process that requires the presence of moisture. I know that for a fact as back home I knew an old civil engineer whose specialty was concrete. He had an old house and replaced the basement floor. Once the concrete was set he gently flooded it and left it submerged for 28 days, the time it takes to reach maximum strength.

I would think that your footing would be okay if the concrete was placed in a dry hole to begin with. Can't say for sure, since I'm not an engineer, but I believe it would be okay. Maybe pumping it dry and lining the hole with a layer of polyethylene would be advised. 

As for the thickness of the footings I believe a poured footing should be 8 inches thick with rebar, either # or X configuration.

And yes as mvk added, water content is critical to maximum strength. It has to be just right. And yes, most people mixing their concrete own make it too sloppy.

And I agree with ScottA that there's little reason to add concrete around a post foundation. The footing under the post is what spreads the load out. Rammed earth backfill is all that's usually required. That even works for fence posts... dry fill concreting a fence post is simply easy to do, not necessary, IMO.

[Ernest T. Bass]

If the water content is so critical, wouldn't pouring concrete dry (in wet soil) make a perfect ratio unlikely? Also, how long does it take to "set" before it can be submerged?

I just don't see gravel draining around here with this hard clay... Maybe if there was a hard-packed layer of dirt near the surface around the post to keep water from seeping down, the clay below would stay pretty dry?

As far as rot goes.. Would it be advisable (in my soil) to put a couple inches of pea gravel under the post (between the post and footing), to keep moisture from getting trapped between the two? Wind upheaval shouldn't be a serious concern, so I don't have to anchor the post to the concrete. I could just tack some blocks to the sides of the posts to grab the dirt a little...

Also, would the concrete-stabilized clay work as a backfill around the posts? It would be cheaper than gravel, and seems like it would help keep surface water from seeping down.

[bobtheengineer]

Concrete will actually set harder and more completely when its wet (thats why they do controlled cures of concrete for testing 7 or 28 days under water).  As long as you get the water out, and pour the concrete, the water won't displace the concrete (concrete more than 2x as heavy as water).  Unless you have flowing water that will erode the concrete as it sets up, you'll be fine. 

Another note, a drilled pier, or cassion, derives its strength from 2 places.  One is the tip of the pier in bearing, and the other is the skin friction between the pier and the soil.  Pier's are designed differently than spread footings.  A pier with the same bearing area, as a spread footing, will have significantly higher bearing capacity.  I don't know why you would want to put the sonotube inside the hole at all, it would actually make a weaker foundation in bearing.  Uplift is another condition, as long as the weight of the scructure above, is heavy enough to counter act the uplift forces, you don't have anything to worry about.  With drilled piers, its actually better to have the pier loaded closer to its capacity to eliminate the possibility of uplift.  You get into a problem when you have too many piers, that aren't loaded anywhere near their bearing capacity.

Not to bash the bigfoot type foundation on here, that some people use, but I don't know why you would want to do all that excavating, and put in a spread type footing.  The pier has such a higher capacity, for the same amount of concrete, and for significantly less work.  As soon as you excavate and backfill around the bigfoot, you loose all the potential strength that the skin friction provides.

Just a rant from an engineer, who has used drilled pier foundations a time or two.

Good Luck!

[Ernest T. Bass]

Doesn't that same skin friction pop the pier out of the ground in a cold climate with expansive clay soils?

Anyway, I'm not planning on pouring a whole concrete pier; just a footer for a PT post. We'd never be able to get all that concrete to our building site..

[mvk]

Well once again I start yapping with out reading or at least understanding I missed that you were packing the post's in the ground.

I don't favor that I would not put post in the ground unless I was sure they could drain.

Bobthe engineer

Your post just popped up I had to leave and when I came back it was there. Thanks. I though that there was some forces working on the sides of piers, all you have to do is try to pull them. I dig most of my holes by hand or I did so if it was clean I wouldn't use a tube but mostly the holes get bigger to move out rocks or in soft dirt there is cave in and the tubes save concrete. There still must be some hold between ground and tube if you tamp it good no? The other thing was inspectors wanted tubes.

Keep on ranting I like to hear the best way to do something always good to know. Don't have to do it but it's a option. How much wind would it take to roll a house over anyway?

Mike