24' x 35' DIY shed slab Q (durability, etc)

Guys,
I've certainly gathered a lot of information while reading the past posts here. This seems to be the proper place to ask my questions.

We've recently purchased a 24' x 35' metal storage building. We're putting this building on our (relatively) small residential lot, in order to provide more attractive storage, and free up some garage space.

I suspect we'll be living here for no more than 5-7 years, and upon moving we'll:

A) Offer the building with the house
B) Sell / take the building and leave the slab
C) Sell / take building and remove the slab (not likely)

Our current plans are to pour the slab ourselves. I have no concrete experience, however I have 5-6 friends who are willing to help, all with varying degrees of experience. I do have a professional concrete guy who is willing to offer advice / oversee as well, but I don't know if the timing will work to allow him to be there.

We've already done the skid-steer work, and have the forms set, gravel put in place, etc. Here are some links to the current state of affairs:

http://www.speedycycleswest.com/shed/100_8971800pxl.jpg

http://www.speedycycleswest.com/shed/100_8972800pxl.jpg

http://www.speedycycleswest.com/shed/100_8973800pxl.jpg

http://www.speedycycleswest.com/shed/100_8974800pxl.jpg

http://www.speedycycleswest.com/shed/100_8975800pxl.jpg

http://www.speedycycleswest.com/shed/100_8976800pxl.jpg

http://www.speedycycleswest.com/shed/100_8977800pxl.jpg

http://www.speedycycleswest.com/shed/100_8978800pxl.jpg

As for as the forms / dirtwork, I consider the site ready to pour. There are approx 27 tons of gravel in / under the site, and around the perimeter (covering 4" perimeter drain pipe). There -IS- some excavated dirt in the gravel, that was used for some fill. Approx 4-6 skid-steer buckets at most, which isn't a whole lot relative to the pile of gravel. I'm not opposed to tamping with a power tamper if additional compression is necessary.

I still need to sink the floor drain, lay rebar / mesh, vapor barrier, and decide how I want to handle screeding (segment slab into 3-4 parts via 2x4, or drive rebar at 8' intervals for "level indicators", etc). Generally, though, I consider the site ready to pour, dirt-work wise.

You can see we had to build up the left side, and cut down o the right side due to the grade. We have left a small trench -inside- the forms on the high sides, to allow a small "footer" when pouring the slab. This had been planned mostly for aesthetic purposes, and also to keep the fill gravel secured under the slab.

I'm curious about the feasibility of this as a DIY pour. This shed will be light duty, storing motorcycles, assorted tools, and maybe a light duty work truck (F250). Eventually I -may- set up a small mill / lathe in here, but I could pour a pedistal for at that time. To be blunt, utmost performance / perfection isn't necessary. I'd just like a hard, relatively flat / smooth floor. That's about it. As mentioned above, I don't expect a lifetime of use.

The forms are 2x6 forms. I had initially planned on a 5" slab, but it appears that a 6" thickness is the current situation, with additional concrete needed for the trenched footers on the elevated sides. Given the small additional amount of gravel needed, and the premium paid for a less-than-loaded truck, it may make more sense to pour a thicker slab vs adding additional gravel.

The building that will go on the shed is pictured here:

http://www.speedycycleswest.com/shed/Shed1800pxl.jpg

http://www.speedycycleswest.com/shed/Shed2800pxl.jpg

http://www.speedycycleswest.com/shed/Shed3800pxl.jpg

http://www.speedycycleswest.com/shed/Shed4800pxl.jpg

As you can see, it's a fairly light duty, all-metal modular structure. These pictures are taken at the previous owner's lot, on his slab. The building has since been disassembled. It's simply waiting for the completion of a slab / foundation, and reconstruction at our place.

It's mid November in southwest Missouri (where I'm located), and the temps are starting to drop. Current 10 day forecasts are for daytime temps in the mid to low 50's, and night time temps of low 30's to low 40's (all F, not C).

Would should be my concerns here?

Right now, my concerns are ensuring the fill is adequate, such that the slab doesn't drop. Realistically, I'm concerned about a significant drop, as a small (sub 3-4" drop) probably won't affect the building. I think a power tamp should alleviate this situation, if it's even needed.

I'm concerned with the temperatures. I've got the means to cover the concrete after pouring (tarp), but I do not have a concrete blanket. The rental stores I've visited do not offer these. At what temp range is a blanket required?

Lastly, finish. I'm not too concerned with a glass-like finish, but I would like as smooth as possible. I'm considering renting a power trowel to smooth the surface, though I don't know when, exactly, that procedure is done.

I've contacted a few concrete contractors, and they've quoted labor prices that are roughly 1/2 - 2/3 of what I'm paying for the concrete. Given the (expected) relatively short lifespan of the slab (anticipate 5-7 years), and the lack of "total precision" needed (it will be just a storage building, after all), I'm having a difficult time convincing myself that a contractor is a worthwhile investment.

I should note that I have no concrete experience, but I'm vary mechanically oriented. I've performed high-quality renovation of the house on my own. Moving walls, drywall work, mud work, floor leveling / replacement, tile work, etc. What I don't know, I'm very good at researching and implementing. As such, though I don't know a lot about concrete work, I'm currently researching it, and plan on doing even more prior to the pour.

So...gist of it...how do things look so far? What are your concerns? Any problems you see at this point? Anything you feel I "don't know that I don't know", which could bite me in the butt? Given my expectations, needs, and slab lifetime, is there any reason this pour couldn't be done by myself and my friends?

Sorry for the long post. I'm just trying to provide all the information that I feel may be applicable.

 

Considering the higher ground on several sides (the back?), I would suggest laying one course of 6" or 8" wide block on top of the edge of the slab. After placing the slab and whatever anchor bolts you need, erect the building. This will make clean-up easier and prevent water from washing in after the shed has blocked off the old drainage pattern. - The only question is whether this would affect the doors.

This usually works out well for normal buildings where the doors are purched separately, since it gives you a little more height at little low-tech cost.

If you put the floor drain in the center, form and pour a 2'x2' diamond around it. Then pour the slab and saw cut control joints about 1 1/2" deep from the points of the diamond to the center of the outer walls. This will give you 4 sections about 12' by 17' 6" that should not crack. Cut the day after pouring and caulk with a high quality caulk.

Regarding slab protection. - This depends on your temperature. Concrete can be poured at any temperature. The important thing is not the air temperature, but the temperature of the concrete. You want to keep the concrete above 40 degrees for the first few days. Often, a tarp and straw will do the job since concrete generates some heat as it cures. For a minimal cost, you can go to high early strength concrete that cures faster and generates more heat. If you are in a cold climate, use air entrained concrete for increased frost/salt durabilit. Your concrete supplier can help you on the best concrete mix to use.

Dick

 

Thanks for the input! The front (with garage door) will be facing the house, and a small exterior entry door will be placed on the right side (near the tree).

We placed 4" PVC drain pipe around the perimeter of the slab / form. This pipe is following the grade, moving water from the right rear corner (where the gravel is piled in a corner / over the form) around the building. This is also the highest point of the slab.

Water can flow down the long right side (near the tree), and cross under the gravel ramp at the front. Water can also go across the rear, and down the exposed (will be covered with gravel after the slab has set) pipe, again to the front left corner.

When looking at the first picture:
(http://www.speedycycleswest.com/shed/100_8971800pxl.jpg)

there are PVC "elbows" at the far corner, and the right corner. These connect the rear drain pipe, the right side drain, and the front drain pipe. On the left side of the building (near the fence), there are T junctions. Two sides are connecting the pipes, and the 3rd side has a slotted drainage cap fitted. This should allow water drainage out of the pipe, into the gravel beds (which should not impede flow). The pipes are around the full-perimeter of the slab, and they're 3-hole perforated.

The slab is exactly the size of the building (24x35), which means any blocks and such would have to be off the slab, on the rock backfill or the grade itself. Rather than setting anchor bolts, I had planned on hammer drilling / epoxying bolts, once the concrete had dried / building started going up.

I like that idea. Unfortunately, the drain pipe is not in the center, but rather located ~10 feet back from the garage door opening (front 1/3rd of the building). I had planned on having 1 "long" cut (12' of the 24' width, running the 35' depth), and about 3 "short" cuts (thirding the 35' depth, running across the 24' width).

I'm not sure what qualifies as a "cold climate". I'm in southwest Missouri. Here's a chart of our average temps:

HIGH LOW

January 41.70 21.02

February 46.98 25.02

March 57.34 34.61

April 67.89 44.25

May 75.76 53.61

June 84.25 62.22

July 89.43 66.77

August 88.58 65.15

September 79.83 57.60

October 69.69 46.02

November 56.12 35.27

December 45.36 25.79

I had planned on going the tarp / hay route, for insulation and wind protection.

Thoughts?

 

Your slab protection should be entirely adequate considering your warm climate. - Here we pour at 0 F., but do protect the ground before pouring and the concrete after.

Some people just like the 8" high concrete block course when it comes to cleaning up and set the building on top of it.

Dick

 

Your site looks OK except for a couple of things. First, you should have more braces or "kickers". They should be about 4 feet apart and all along the "high sides" of the slab (where you've got more than one board stacked). The weight of a foot of concrete will push the forms out and your edges will be as wavy as a pregnant snake. Some of your braces are at too steep an angle and will not work. They should be at 45 degrees or less.
Second, the drain pipe in the slab should be completely covered by gravel, so that the top portion of the pipe is at or below the bottom of the concrete slab. If the pipe sticks up even a little into the slab, the concrete will crack right above it. If you don't want to move it now, just make sure you saw a control joint in the concrete right over the length of the pipe.
Third, you should rent a plate compactor to tamp down the fill prior to pouring.
As to the pour: Rent a power trowel with COMBINATION BLADES. These will help float and finish the slab better than either float blades or finish blades alone.
Set the rebar stakes for elevation as you mentioned. You can set a lot of them to make it easy when pouring.
I like your joint placement ideas. The spacing is about right.
After the slab is finished, only cover/straw it if it's going to be below freezing within a few days of pouring. Concrete will continue to cure at low temps, but much more slowly than at higher ones. If you're in doubt, straw and tarp for a week or two.
It would greatly benefit you to have your concrete finisher buddy (with his finishing tools) be there when you pour. I could go on some more, but this is getting too long already. Good luck!

Pecos

 

********Your site looks OK except for a couple of things. First, you should have more braces or "kickers". They should be about 4 feet apart and all along the "high sides" of the slab (where you've got more than one board stacked). The weight of a foot of concrete will push the forms out and your edges will be as wavy as a pregnant snake. Some of your braces are at too steep an angle and will not work. They should be at 45 degrees or less.********

Pecos, thanks for the tips! I just went and measured the braces. Many are less than 4', but there are 2 that are 5' and 6' apart. I'll add another brace in these two "holes".

Also, the braces are not free-standing, but nailed into the forms. Will this cure the "too steep" situation, or should I lower them all to a less steep angle? The slab will only be ~6" at the top, and about 10-12" x 8" or so against the inside of these "tall" forms (where we've dug out for a small "footer" for gravel retention).

I should note that I'll be backfilling gravel against this "high" edge, over the drain pipe when the slab is finished. As such, neither the pipe, form marks, or slab edge will be visible. I'd like the slab to be as nice as possible, but it's not critical as it'll all be "hidden" anyway.

********Second, the drain pipe in the slab should be completely covered by gravel, so that the top portion of the pipe is at or below the bottom of the concrete slab. If the pipe sticks up even a little into the slab, the concrete will crack right above it. If you don't want to move it now, just make sure you saw a control joint in the concrete right over the length of the pipe.********

Correct. I may not have made it clear in my original post, but this drain pipe has not been set. It's in the proper location, but it's not been set in the gravel at all. All of the pipe (save the elbow) will be at, or below, the base level of the pad.

********Third, you should rent a plate compactor to tamp down the fill prior to pouring.********

I'll definitely do that.

********As to the pour: Rent a power trowel with COMBINATION BLADES. These will help float and finish the slab better than either float blades or finish blades alone.********

I stopped by the rental place I'll be using, and priced equipment. The power trowel I'll be renting does indeed come with combination blades. Thanks for the tip, though!

********Set the rebar stakes for elevation as you mentioned. You can set a lot of them to make it easy when pouring.********

Would it make sense to tack-weld rebar (or mesh) to the elevation stakes? This would ensure the rebar is properly placed -inside- the slab, as opposed to getting mashed down while tamping, screeding, etc. ??

********I like your joint placement ideas. The spacing is about right.********

Excellent. It's good to have a pro give a "thumbs up" to an idea!

********After the slab is finished, only cover/straw it if it's going to be below freezing within a few days of pouring. Concrete will continue to cure at low temps, but much more slowly than at higher ones. If you're in doubt, straw and tarp for a week or two.********

Daytime temps should be above freezing, but nighttime lows may be at, or below, freezing. For instance, there's a 26 degree night time low projected in the next 10 days. Will a single night time low below freezing be sufficient for straw / tarping?

Lastly...as I check on this more, watch more video / footage, etc, I'm starting to get a bit spooked. Not by the work, or what's involved, but by the size of my slab. And more appropriately, time.

24'x35' x 6" is 15.56 yards, not taking into account overage or the "channel" footings. I'm assuming I'll be approaching 17-18 yards (I'll obviously measure more accurately prior to ordering) for the total project. An instructional video I watched stated not to pour more than 7 yards of 6" at a time, or it'll get ahead of you. That's slightly less than 1/2 of my project. Granted, they were talking pouring prior to mucking and screeding, but still...for a first timer, it's probably better not to break that limit at all.

Aside from tool costs / time / PITA factor, would there be a problem orchestrating 2 (or more) pours for this? Assuming a minimum load of 6 yards (I can confirm tomorrow), I could divide this pour neatly into 1/3rds. 24x12 would be nearly 6 yards (5.3 or so + trench), and would allow us to screed using just a single 2x4x12. Basically, 3 24x12(ish) pours would seem to incur the same concrete costs, provide much more time to "work" the concrete (for a given area), and be easier to work as well.

Would there be any downsides to doing this in multiple pours? I realize there may be tool rental expense, and / or getting help on-site 3 times, but a smaller pour like this would also reduce the amount of help required. Thoughts?

Also, thanks very much for the input and information I've received. It's very helpful, and very appreciated.

 

- I wouldn't attach the wire to the grade stakes. You should tap the stakes below grade with a hammer when you're done with them so you don't encounter them while finishing the slab.
- Nailing the braces to the forms should aid in them not sliding up. I still like to place them 45 degrees or less though.
- With night time temps in the 20s, you should cover the concrete.
- Since the slab has a thickened edge for a footing, I would not pour in 3 different pours. However, I must admit that I had some doubts about a DIY with no concrete experience pulling off a slab of this size with good results. Hence, the suggestion that your experienced friend be there.
- I am curious about what you call "tamping" before screeding. Are you talking about using a jitterbug? Concrete doesn't need to be tamped. Just pour, screed, bullfloat, and commence with the machine when the concrete is ready. Knowing when to get on it with a machine and how to use it correctly are other reasons to have your friend there.
- There is a lot more to concrete work than meets the eye. Having the right tools and knowing how and when to use them is only part of it. Not pouring too wet a mix (creating weak concrete) or too dry (difficult workability) is also very important. I know this is a DIY forum, but I truly believe you'd be better off to hire out the finishing to a contractor, especially with a slab of this size. You may find someone to do it "on the side" for cash. That should make it a bit cheaper. However, I also know the desire to save money and get the satsfaction from doing it yourself. Good luck with the slab whichever way you go.

Pecos

 

Thanks for the information. The more and more I look at this, the more I wonder if the DIY approach isn't a false economy. At least as far as tackling a project of this size as my -first- concrete project. At ~18 yards @ ~$84 / yard, that's $1500 I'm playing with.

While I'm pretty confident we could handle it with enough guys...man...being wrong would be a pretty hard sting.

So, along that line, what would be a "going" rate for a finisher? The site is already graded, already formed, already backfilled, and it'll already have steel / vapor barrier and be compacted before the crew gets here. It'll strictly be a "pour the truck, finish the concrete, and cover" proposition. I'm curious what would be a reasonable retail rate, and what would be a good "side job" rate. The #'s I've been given so far have been on a "per square foot" basis. I'd like to see what numbers are suggested, before I say what I've been told locally.

Also, should I expect a crew to have their own tools (including power trowel), or should I expect to have to rent those even after hiring a crew?

Again, thanks for all the help and information.

 

It really depends on where you are, and what the market is like there. Around here (Indy), finishers make between $15-$22 per hour (non-union). However, they may work for less (for cash money). After all, Christmas is coming up, as well as winter which is a slow time for many of us.
If you're getting guys "on the side", they probably have hand tools but I would be surprised if they own a finishing machine. Since you've got the site all prepped, you could probably find someone to do it. I personally would love it if someone offered me a ready-to-pour job on the side.
I would call around. Call some local ready-mix concrete plants and ask about small crews or individuals that they deal with. By the way, it would be much more attractive to them if you do the strawing/covering, and sawcutting of the control joints. Tell them you'll help so they don't have to provide their own laborer. All these things should get you a competent crew and a better job. Good luck.

Pecos

 

Concrete is easy (if you know what you are doing), but the consequences of failure are very expensive. The cost of breaking out and hauling away a failed slab , plus the cost of DIY, are less than the cost of letting someone else asume the risk and doing it right the first time.

If you insist on doing it yourself, practice on a sidewalk or a pad for a small building, like a pumphouse first.