Posts for solar panel mount - Concrete question

[David Baillie]

In order to support the vertical posts which will hold my solar panel mounts, I intend to set some cardboard
sono-tubes into the ground and fill them with concrete.
I have a question about mixing the concrete.

Will the following procedure work...
-Place my vertical poles into the son-tubes and support the poles in a vertical mode.
-Pour in a dry bag of redy-mix
-Add water and mix with a rod
-Pour in a dry bag of redy-mix
-Add water and mix with a rod
....repeat until the sono-tube is full.

For me this would be much easier than renting and transporting a cement mixer.
Anyone have experience doing this ?

Veggie

I think your basic technique would work.  If you substitute standard ready mix for rapid set you will be good to go.  Some fence installers just pour it in dry and add water.  Personally I use 2 bags of  standard mix at a time, a wheelbarrow, and a hoe it's cheaper and I think the well mixed standard mix is stronger.
My two cents, also in canada, and having hand mixed more concrete in hard to access spots then I care to remember
Best Regards,  David Baillie

[Jens]

As has been mentioned, pre-mix is very low on cement. Fine for non critical stuff but I would not use it to hold up expensive panels. Pre-mix is also relative expensive compared to combining the individual bits yourself. Fine for small quantities but a bad choice if you need any quantity. I would suggest to look at getting in a concrete truck - better concrete, a hell of a lot easier on your back and you might be surprised if you calculate the total cost of getting it delivered vs bags.
The stuff delivered with the concrete truck is a known quality and will give you peace of mind when you got some bad weather coming in!

[Derb]

I have dug out and broken up foundations which have used both bagged concrete and the "proper stuff" mixed before pouring into the hole. Chalk and cheese - the bagged stuff just crumbles off with the 1st smack of a maul but hell, the real stuff takes a good pounding to release from the piles.

[LowGear]

1 cubic yard = 27 cubic feet.  I've heard that those 60 pound bags only have 1/3 of a cubic foot of finished concrete in them.  80 bags of concrete might set you down to think but for small jobs they sure are handy.  The first yard or two that come out the back of a truck are not cheap either.  Really, really handy but not cheap.

Casey

[Ronmar]

Yep, and the truck dosn't give you all day to off-load it either...  A lot of redimix plants won't even talk to you untill you are planning at least a few yard or more for delivery. 

I have seen some of the rental places with 1/2 yard towable mixers(United Rentals used to do this).  They pre-mix it for you, you pick it up, tow it to the site, add the water and pour, then flush out the drum and return it.  Cool little units and way easier to maneuver than a full sized truck to dump less than a full yard of CC...

[Thob]

I've done a few trailer loads from the rental place, and my advice is DON'T, unless you live really close.  The aggregate settles to the bottom and water/cement floats to the top if you travel a few miles.  The result is you can't even open the gate and dump the load without mixing it by hand first, which is a lot of work! There's a reason they haul that stuff in mixers that constantly mix.

And yes, most places around here charge for a full truck load (9 yards?) as the minimum.  There is also a service here that has trucks with dry materials, they mix on-site and will do small batches but I haven't checked out their prices.

[David Baillie]

Well mixed premixed bags of concrete should meet their specs.  How many people actually read the bag though?  Or measure the water.  If you can get a truck in by all means but for a few piers you will pay dearly for small quantities.  Is quality is an issue add a shovel of cement per two bags and increase your strength...
Best regards, David Baillie

[LowGear]

How many people actually read the bag though?

Wow David!  There are actually instructions on the bag?  Again, my reason and purpose for attending this blog is learning.  Grinning aside; I too put some extra portland cement into the mix and do mix up the premix before pouring.  I've also measured the water, cement, sand and gravel when doing repeated mixes in a small mixer.  

I'll be reading that bag on the next project for sure.  Thanks for the tip.

Casey

[Ronmar]

I've done a few trailer loads from the rental place, and my advice is DON'T, unless you live really close.  The aggregate settles to the bottom and water/cement floats to the top if you travel a few miles.  The result is you can't even open the gate and dump the load without mixing it by hand first,

The ones I am talking about WERE a drum mixer on a big trailer.  Gas powered, with a water tank.  Looked like the back half of a cement truck, downsized, with a trailer hitch where the cab should be:).  No handmixing on these badboys. 

I wouldn't want to tow mixed CC in a trailer that wasn't mixing for the very reasons you mentioned.  Over agitation/consolidation is as bad as under consolidation.  One separates the materials, and the other leaves voids...

[Ronmar]

Well mixed premixed bags of concrete should meet their specs.  How many people actually read the bag though?  Or measure the water.  If you can get a truck in by all means but for a few piers you will pay dearly for small quantities.  Is quality is an issue add a shovel of cement per two bags and increase your strength...
Best regards, David Baillie

I agree for the most part.  The bags should be fine and plenty strong if mixed properly.  I have a front step I did with bags of sackrete about a decade ago, mixed with wheelbarrow and hoe, Still solid as a rock, no flaking or chipping with square corners and edges all intact. I did a walkway from driveway to that front step using a stepping stone mold form(1 60# sack per mold) and those are all intact and walked on daily for that same decade...   Don't overwater, mix throughly and don't over consolidate and it is plenty strong for domestic/residential use...

More cement won't necessarilly make it stronger. Only if it was lacking in cement content in the first place.  Most of those bags are rated to produce a 2500-4000 PSI mixture, and most of that probably depends on the ammount of water that is used and how throughly it is mixed... Cement by itself is not very strong.  It is the aggregate that makes CC strong with just enough cement and sand to bond it together.  Adding cement will only reduce the ammount of aggregate per CU/FT.  Just as too much water will make the mix fluid enough that the sand and gravel will be able to separate more easilly during the pour and consolidation with the force of gravity before the cement chemically fuses...

[mobile_bob]

i just did a 1 yd pour of 5k psi mix, for a base to install an 8k dollar washing machine that spins out to 400g's

we checked into bags of premix and found that it would take around 200 bucks worth of bags and a lot of work and even then i was unsure of the quality of the finished product with cold joints and such, let alone what psi the stuff would be good for if all was done perfectly, which it never is.

i called a concrete plant 20miles away and they would deliver 4 yds or more at the regular price of 105 bucks/yd of 5kpsi mix
when i asked for 1.5 yd (had a small side project to do at the same time) they told me that they would deliver a 5 gallon bucket of mix for 40 bucks... hmmm 40 bucks!  why not?

it is common thinking at least for me, that small quantities are horribly expensive, well i guess that is not the case?

i guess "horribly expensive" is a relative term?

all i know is i will never mix concrete again!  at least for anything over a couple bags of premix.

bob g

[LowGear]

https://www.google.com/search?q=log+pole+building+round&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwiY8Zb3lPjSAhWKjFQKHfEUAa4QsAQIHw&biw=1098&bih=527&dpr=1.75#imgrc=--oQ09jCLw0d9M:

Give it a minute.  Interesting illustrations.

[glort]

I Don't understand all the concern for the strength of the concrete.

It's not going to be like a floor or anything that holds significant compression loading and that's what concrete strength is all about. This is more going to be about surface area and the ability of the surrounding soil  to hold the concrete in place. Anything that set up properly is going to do this job, it's very low demand and the soil and the post is going to be the weak link rather than the cement.
I'd be doing as suggested, Digging the hole bell shaped at the bottom to get more purchase on the soil rather than worry about the strength of the cement.

Looking at it the other way, if you were trying to push these over or pull them out, I'd guarantee either the 2" posts or the soil is going to give way before any properly prepared cement from bags or other wise gives way.
The weak point is going to be where the post meets the cement because that's the point of most force/ leverage on the pole.  No bagged cement is going to break away from that!
Those posts are not that strong as I have well seen helping out a mate with a road sign business.  Never seen the cement break away from the posts but I have seen plenty ripped out of the ground with the cement still attached and just as many posts bent like pretzels..... Usually with the concrete still attached perfectly to the pole but with soil movement.  If the ground is wet, you can push the posts over with surprising ease even with 100Kg of concrete attached. For signs they always use bagged mix unless it's one of those huge things hanging over the road where the structure is 8" box section and you need a mixer full for all the anchor points anyway.

It's the leverage on the posts and soil that is the problem, not the strength of the concrete.

You'd be better off with larger posts and deeper holes with crappy cement than you would with the best cement out there and lesser poles in not so deep quality cement.
Probably better off still with no cement at all and just pile driving the posts.

[LowGear]

Aloha glort,

I pretty much agree with you but I found the thread interesting and informative.  The different approaches are always fun.  Sometimes we're confronted with building code or the challenges brought out by isolation not to forget that old adversary: being under capitalized. 

Casey

[vdubnut62]

I Don't understand all the concern for the strength of the concrete.

It's not going to be like a floor or anything that holds significant compression loading and that's what concrete strength is all about. This is more going to be about surface area and the ability of the surrounding soil  to hold the concrete in place. Anything that set up properly is going to do this job, it's very low demand and the soil and the post is going to be the weak link rather than the cement.
I'd be doing as suggested, Digging the hole bell shaped at the bottom to get more purchase on the soil rather than worry about the strength of the cement.

Looking at it the other way, if you were trying to push these over or pull them out, I'd guarantee either the 2" posts or the soil is going to give way before any properly prepared cement from bags or other wise gives way.
The weak point is going to be where the post meets the cement because that's the point of most force/ leverage on the pole.  No bagged cement is going to break away from that!
Those posts are not that strong as I have well seen helping out a mate with a road sign business.  Never seen the cement break away from the posts but I have seen plenty ripped out of the ground with the cement still attached and just as many posts bent like pretzels..... Usually with the concrete still attached perfectly to the pole but with soil movement.  If the ground is wet, you can push the posts over with surprising ease even with 100Kg of concrete attached. For signs they always use bagged mix unless it's one of those huge things hanging over the road where the structure is 8" box section and you need a mixer full for all the anchor points anyway.

It's the leverage on the posts and soil that is the problem, not the strength of the concrete.

You'd be better off with larger posts and deeper holes with crappy cement than you would with the best cement out there and lesser poles in not so deep quality cement.
Probably better off still with no cement at all and just pile driving the posts.

+1  Ron