Posts for solar panel mount - Concrete question

[veggie]

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

[Thob]

I've heard of folks around here setting fence posts by digging the hole and pouring in the bag of mix dry - no water added.  The mix absorbs moisture from the soil and hardens, no mixing required!  I'm not sure how long it takes or if that's a good method.

The bags are already mixed (cement/sand/gravel), you really only need to mix them enough to mix in the water, so I think your method is OK.  Depending on how much space you have around the pole, you're also trying make sure there are no voids in the concrete.  I personally like to add some cement to the mix because the bags are pretty poor with cement.  I've also heard that the less water you use, the better - too much water causes the concrete to shrink a lot as it dries, which leads to cracks.

How big is the hole relative to the post?

You're not putting the sonotube all the way to the bottom of the hole, are you?  I can understand using the sonotube near the surface where it forms up the concrete above grade for a nice appearance, but I wouldn't let it extend very far into the hole.

[veggie]

Hi Thob,

Thanks for the reply. Here in Canada we have to set the sono-tubes down to the 5ft. level in order to get below the frost line.
(So that they don't get pushed upward by the re-occurance of ground frost every winter)
The typical procedure here is to bore a 6 ft. hole and pour 1 ft. of gravel into the hole, then set the sono-tube onto the gravel and fill with concrete.
In this case the sono-tube ID will be 6" and the galvanized solar mount pole will be 2" OD.

Veggie

[Hugh Conway]

Good morning Thob
Also in Canada here, but in a warmer part. I used 6" diameter plastic sewer pipe as a form for my posts.
First we poured about 4" of concrete into the bottom of the 2 ft deep holes to give a mushroomed base, with a bit of rebar added.

While the mushrooms were still wet, I placed the pipe forms into the holes. they were already spaced and cross braced with the aluminium that I later used to build the racking. We held it all together with #10 screws while pouring the hand mixed concrete into the pipe forms. This method worked great, as I was able to use the aluminium for racking to correctly line up and space the forms, then tip the whole assembly into  the holes,set them up level and vertical. After the concrete set up, we completely filled and tamped the holes, then removed the aluminium bracing.  I used 3/4" threaded rod in the middle of the forms to make final adjustment of the racking a breeze after the concrete had set up.

The pipe forms were just left in place to make a neat installation, protruding about 2 ft above ground level.
Having used sonotubes before, I found this method far easier and quicker, and the sewer pipe does not get soggy in the rain. The thin walled sewer pipe was relatively inexpensive.

YMMV, but this method worked very well for us.

Cheers,
Hugh

[LowGear]

Wow!  5 feet!  Wow!

I've also heard of the sack in the hole and calling it hard candy in a week but I prefer to premix the premix and have found as I pour the sacks into the wheelbarrow that the three have separated from each other during shipping.  I too throw in a couple of cups of Portland cement per bag as well. 

I am challenged by the sono-tube or cheap sewer pipe all the way or near the bottom of the hole or putting plain gravel in as well.  I'd more likely use a foot or two of concrete at the bottom and then do the upper part with 3/4 minus material.  The minus material really packs in tight and as the fines migrate down the hole it gets a bit tighter with time.

I don't know how isolated you are but I've been known to buy a tool for a couple of hundred dollars on craigslist, use it at my leisure and then resell it for a couple of hundred dollars.  The electric three sack units aren't really that tough to move if you have some child labor to help.

So how many panels?  Stand alone or grid tie?  Details veggie.

Casey

[veggie]

Hi Casey,

I have ten 220W Mono panels and a Magnum 4024 (kw, 24 volt) inverter with a small 550 amp battery bank.
The plan is to take key house circuits off the grid by using a transfer switch that can isolate individual circuits.
This gives the option to run those circuits (fridge, freezer, lights etc) on grid or off-grid with the flip of a switch. (see picture below)
I'm not really interested in grid-tie because I can't be bothered with the bureaucracy. ...and I don't have a grid-tie inverter of that size.
I may not need all the panels for this project, but I do have a solar PV powered greenhouse which would benefit from any left over panels.

This transfer switch can individually divert a circuit between grid and inverter (or generator) power.

[vdubnut62]

I use the dry Quikcrete in the hole and tamp exclusively. It'll tamp in surprisingly tight, tighter than dirt and small rocks alone. I did the woodshed this way with 16 foot 4x6 posts and they were held plenty tight, then you have the bonus of the mix setting up. The stuff sets up 'way hard too, I think harder than when mixed with water. I can't give a timescale on how long the process takes because of the variables involved like humidity and ground moisture.
Ron.

[mike90045]

With that large gap from the wall to the pole, I'd put a corkscrew of rebar in there, or wad up some mesh or chicken wire, to help the concrete hold together.

Fenceposts, I set with dry mix and water.  PV mounts, I'd mix and pour.    Maybe you can get a bag of reinforcing FIBER and add to the mix, and not need the rebar.

https://en.wikipedia.org/wiki/Fiber-reinforced_concrete

[Carlb]

When I did my ground array i rented a tracked auger to bore my holes.   I needed to bore to a depth of 6 feet with a diameter of 30" for my 4" schedule 40 steel pipe.  our array has 7 uprights and required about 9 cubic yards of concrete.   Needless to say i wasn't doing any mixing.  The cement truck parked at the street and i used a motorized dump buggy to move the concrete to the holes about 300 feet away.   It only took about 90 minutes to get the job done.   The array is on a adjustable mount that can go from 60 degrees to 0 degrees to  horizontal.  The array consists of 56 200 watt panels for a total of 11.2 Kwh The array is 10 feet tall and 91 feet wide.  It can be adjusted for seasons by one person as it is very well balanced.  I did the install back in late 2009 and it has been performing great with no problems.

[Thob]

I keep forgetting about the ground freezing up north.  Here the maximum frost depth is 6 inches, and I've never seen the ground freeze at all.  But in a few years we're retiring to Wyoming and the frost depth is something like 4 feet there, so I'll need to keep that in mind.  Water lines are 6 feet deep, so I definitely need to add a backhoe to the list of power tools I need to acquire. Good to know about using sonotube or pipe to keep away the frost heaves!

[LowGear]

carlb.  Have you tested tilt to evaluated change in output as a function of degrees from 90 to the sun?

[Carlb]

didn't do a real study but the difference in winter from summer angles is quite substantial when i checked.  if i move the panels from summer to winter position in the winter i gain about 30% and same when i move them from winter to summer in the summer.   I move them 4 times a year and it takes about 10 minutes to change the angle.

[mobile_bob]

fwiw, and having not read all the replies to your post...

and not knowing how many panels you have in mind, how many square feet of panels, and how strong of wind gusts you have in your local...

i personally would want the best foundation i could, especially for anything with more than just minimal sq/ft of panels

back in '83 a shop i worked at, a coworker and myself decided to enter a kite flying contest for the rededication of a ww2 airbase,  there were several classes, the one we were going for was the delta design (big diamond shape)

our kite was 56lbs, 18ft tall and 12ft wide, and we used 3/8 woven nylon rope, and during testing we used a scout terra pickup truck with 5 guys in the back to try to get it up in the air... we also had 50ft of 3/8 tow chain for a tail (which was a horrible idea, but i digress)

the day of the contest, being mid august and as most know in the middle of the doldrums (absolutely no wind) we had to use a 1ton chevy tow truck with a 454cu/in and 4 speed manual trans.... we could get the kit up in low gear at about 14-15 mph, but could not shift to second as the kite would stop the truck dead in its tracks and put the driver up face first into the windshield.  we did calculate the drag force at the time, but i don't recall any details other than it was a huge eye opener for all of us involved.

point being, panels are costly, so if you have many to mount (many sq/ft), and live in an area with brisk winds,  if i were you i would err on the side of more and best concrete i could put down the hole.

remember the wind rules, double the frontal area, you square the forces, double the wind speed you cube the forces.

things can be pretty massive in some cases.

bottom line, just dumping the dry mix down the hole, while it works for fence posts will result in very low quality mix in my opinion.  better off mixing by hand in a one time use framed up out of wood with a plywood bottom basin, using a hoe and shovel in my opinion.   even if not perfectly mixed it will be far better than dry dumping and adding water i would think.

again fwiw, which ain't much these days



bob g

[Ronmar]

Are these poles attached in a framework, or are they free-standing(each pole supports a standalone array)?  Got the 6" tube 5' long with 2" pole.  How many of these do you need to do?  If my math serves me a 6" tube 72" long with a 2" pipe will take just over 1CU/FT of ConCrete(1800 CU/IN). An 80# plus a 60# bag = 1.05 CU/FT of CC.  5' of tube would probably be OK with 2 60# sacks.  Got 10 of these to do, that's 20 sacks to mix?...       

If part of an interconnected frame, the frame can provide the rigidity, so the legs merely need to be restrained by the weight of the CC around the legs in the holes.  With a plate attached to the bottom of the pole you could probably fill the tubes with tamped crushed rock which would provide the necessary weight to restrain the legs...  Havn't used CC for fence posts for years, crushed rock drains better so the posts last longer.

If freestanding posts, the issue becomes withstanding lateral force.  For this you need that base to be bonded throughout.  Since the powdered CC resists the transmission of water, and proper wetting is necessary to fully fire the CC chemical reaction, just putting the dry mix in the hole and wetting will make for an inconsistent mix.  The sonotubes will also restrict the dry mix from absorbing moisture from the earth.  If I recall correctly a 60# sack of quickrete takes about 2 quarts of water to activate properly.  But mixing throughly with that little water by hand is laborious.  Using more water is a problem as the more fluid mix will separate as it falls into the hole, with the aggregate settling to the bottom.  The cement glued to the agregate spread evenly thru the mix is what gives it it's strength. With a proper mix, the sonotube helps the mix retain the moisture and cure properly. 

Depending on how many of these posts you have to do you can mix it by hand in a tub or wheelbarrow, just resist the urge to add more water...  Way easier to throw 2 sacks into a mixer, start it up and add 4 measured quarts of water. This way it gets mixed properly and evenly with the proper ammount of water.  As soon as you see it fully wetted, tip it down the hole, repeate as necessary...  With a small mixer you could probably do 10 posts in under 2 hours as it goes very fast...   

CC only really needs re-enforcement(rebar/mesh) when placed in tension.  Not any real tension loads associated with a pole/post in CC, so no re-enforcement necessary... that is why crushed rock typically works well for posts.   

[LowGear]

Last month we had a bit of a blow. So I notice the tent roof over the tractor and farm truck tried to run away but was still there as were the 1" conduit pipes set in about 5 gallon poured pads (6).  This is a kite that measures out to about 20' by 30'.  "Wow!" mumbles me as I fire up the tractor for some mission or another.  Two women and I got it replaced in about two hours.  Ok, the two women got it replaced in about two hours but I provided important supervision.  The weather service reported 40 mile an hour gusts.

As I putted by MEL, Menehune Energy Laboratory and not Melvin, I noticed the metal roof had curled up on one side and was caught against the ridge cap.  "Holy Wow" or something like that.  Five minutes later I'm up there with a jar of screws and screw driver.  "Holy Shit" I had forgot to put most, yes - MOST, of screws in the leading steel roof panels on BOTH sides.  The South side has 30 solar panels!  Yeah the unit has paid for its self but now is the sweet part of solar panels.  The decade plus of nearly complimentary power generation.  40 - 60 screws later and I'm sleeping better.

So how does this tie into this thread?  How much concrete do you need and what is it's mission?  I escaped catastrophic consequences on the tent building because the sides of the tent material ripped loose and allowed much of the force to escape.  There are no walls so there wasn't any entrapment going on either.  OK, this is the way I usually know it's time to replace the roof.  I got three years out of this one while I normally get only two.  The even better news is that I'm ready to go for the wet season that starts next month if the plants are lucky.

The second part is there's more to building the greatest brick out house in the county if you don't secure the roof because you're too pooped to think straight.  Relying on luck is not always the best plan but be thankful when she stops by and gives you a hand

Casey