Concrete slab specs and rebar evaluation request

[Wangstang]

I need some input to verify that everything looks good to go for a pole barn slab which will be poured in two days.

Specs are 5000psi concrete minimum 5" thick, 3/8th rebar, with a 3' wide 8" deep section the entire width of the structure in the middle of the barn which could accommodate various bolt down items, such as a lift. Will be poured on top of 3-4" of #67 stone which was leveled on top of compacted/settled crush and run.

I can provide more photos if needed. It's a 30x48 pole barn that will need to be able to withstand farm tractors, a backhoe and a dump truck that will take 10 tons of capacity....i.e. this will need to be sturdier than the average home shop slab but not so over the top that you could land a 747 on the thing.

Please let me know what you think of the layout and specs.

Thanks
Wes

 

[ddawg16]

Where are the blocks to lift that rebar up off the plastic?

Based on my experience....pulling the rebar up as they pour is not a reliable way to do it....the rebar needs to be centered between the top and bottom.

 

[djkeev]

Where are the blocks to lift that rebar up off the plastic?

Based on my experience....pulling the rebar up as they pour is not a reliable way to do it....the rebar needs to be centered between the top and bottom.

Yep, my first thought as well. If the contractor says he will lift it up as he pours...ah....NO! It will only settle back down.....rebar is heavy!!

Get the rebar chairs, do it right! If you don't chair the rebar, you wasted time and money putting it in.
For example...... Here is a plastic type, I prefer the wire chairs.....

http://www.lowes.com/pd_211658-4608...ebar+chair&N=0&langId=-1&storeId=10151&rpp=24

Apron, no rebar? Kinda thin as well. It will easily crack with freeze thaw cycles and ground sturation from rain.

Perimeter form? You can see daylight under the walls, concrete will ooze out easily.

A lot of square feet for a single pour! Hope you are good and fast and don't "lose" it!

Dave

 

[nehog]

I have a bigger problem than that rebar sitting on the plastic...

Look at picture #1 (first image). I see a very small gap at the front of the building and a huge gap at the rear. Either the building is not level, or that floor is not level. Now, a non-level floor is fine, but you want it to slope towards the door, not the rear of the building! The way it is now you are going to have water collect at the back wall--or do you plan to leave that (huge?) gap at the rear open? If so, I'm not sure that is a good move, you may want to consider raising the floor to meet the bottom of the building so there is no gap for vermin and wind to get in.

 

[wssix99]

Get the rebar chairs, do it right! If you don't chair the rebar, you wasted time and money putting it in.

+1 ^^^

If the rebar ends up laying on the plastic its worthless and won't do anything for you, except save you time when you want to rip out the cracked slab and recycle the steel.

You'll want to raise the rebar in to the exact middle of the slab to get the maximum performance for resisting the shrinkage cracking.

 

[Ironcrow]

Yes, plus whatever. Chair that bar. I feel sorry for the contractor who thinks he can pull it up and his customers.

 

[pop pop]

You need to specify a concrete "slump" and test for it. I'd recommend not more than a 3" slump, and test every load delivered! Do the proper ASTM test cylinders so you will know you get 5 ksi concrete in the slab. If your contractor knows you are doing the slump test and making the test cylinders, he'll be a bit more careful about the material delivered to the site. Otherwise you will likely get a bit shorted on cement and added water.

 

[pcmeiners]

Heed Pop Pop's warning, if you do not specify testing, it is unlikely the concrete will be poured to spec. Contractors love to water down crete. Was a foreman for large pours, contractors will sell their mothers to add water to concrete, seriously lowering the PSI strength. Tell the crete supplier the driver will be signing off the load strength.
Since your crete will be pour directly on the plastic, make sure you keep the surface total wet for days, a min of 8 days with misting hoses, better still, misting and blankets.

 

[ConCretin]

You need to specify a concrete "slump" and test for it. I'd recommend not more than a 3" slump, and test every load delivered! Do the proper ASTM test cylinders so you will know you get 5 ksi concrete in the slab. If your contractor knows you are doing the slump test and making the test cylinders, he'll be a bit more careful about the material delivered to the site. Otherwise you will likely get a bit shorted on cement and added water.

With all due respect, no finisher I know is going to want to wrestle a 3" slump and you will not be happy with the finish you get. A better idea would be to spring for a mid range water reducer, which will give the finishers a workable slump and keep the water/cement ratio low.

By the way, 5,000 psi concrete is way overkill for this application.

 

[Wangstang]

So after a bit of confusion between myself and my concrete guy, I think I have cleared a few things up.

1) What’s up with the lack of rebar chairs?....This question was my biggest reason for posting here because I wanted to see what you all thought before I bugged the contractor. The contractor will be moving some of the concrete in by wheelbarrow so he plans to lift and brace the rebar in sections with his crew. If he doesn't do it that way, he can't move the wheelbarrow across the space.

2)

I have a bigger problem than that rebar sitting on the plastic...

Look at picture #1 (first image). I see a very small gap at the front of the building and a huge gap at the rear. Either the building is not level, or that floor is not level. Now, a non-level floor is fine, but you want it to slope towards the door, not the rear of the building! The way it is now you are going to have water collect at the back wall--or do you plan to leave that (huge?) gap at the rear open? If so, I'm not sure that is a good move, you may want to consider raising the floor to meet the bottom of the building so there is no gap for vermin and wind to get in.

The floor will be level. It will also have a minimum thickness of 5" all around. There's a blue chalk like going around the base board that indicates the top of the slab, you can't see it in the photos I posted earlier:

3) 5000 PSI concrete...overkill... I'm going heavy on the PSI for a few reasons but the main one is I want to make sure that if I decide to get more advanced with the equipment in the shop, I don't want to have pour a new pad. I'm thinking of a lift that could handle the dump truck we use on the farm, testing our backhoe's hydraulic cylinders/stabilizers on the pad and so on. Also, I'd like this structure to still be standing in three more generations, my house is from the 1800's and I appreciate things lasting FOREVER.

4) The Pour...Additives and Water...I'll keep a close eye on the work. The contractor was upfront and said he'd have retarder added to the mix to make sure he has plenty of time to work the material.

5) Cure time...The contractor advised it would be best to give the slab 28-30 days without any load.

Since your crete will be pour directly on the plastic, make sure you keep the surface total wet for days, a min of 8 days with misting hoses, better still, misting and blankets.

I've heard mixed things on this concept. Some people swear by it, some say it makes a difference but probably isn't worth the effort in 90% of the applications and some say it causes problem. My contractor said he'd be all for it if the concrete were in direct sunlight. The sun tracks across the roof evenly and there's quite a bit of natural shade from the trees. My contractor said if I left things alone he expects everything will be fine but said if I want to mist/dampen the slab to super slow cure, he suggested putting burlap over the surface while misting.

Thanks for all of the input!

Wes

 

[RivennHewn]

What's the plan for keeping the concrete from oozing out under the walls?

 

[ConCretin]

3) 5000 PSI concrete...overkill... I'm going heavy on the PSI for a few reasons but the main one is I want to make sure that if I decide to get more advanced with the equipment in the shop, I don't want to have pour a new pad.

5) Cure time...The contractor advised it would be best to give the slab 28-30 days without any load.

I've heard mixed things on this concept. Some people swear by it, some say it makes a difference but probably isn't worth the effort in 90% of the applications and some say it causes problem. My contractor said he'd be all for it if the concrete were in direct sunlight. The sun tracks across the roof evenly and there's quite a bit of natural shade from the trees. My contractor said if I left things alone he expects everything will be fine but said if I want to mist/dampen the slab to super slow cure, he suggested putting burlap over the surface while misting.

I'm all for overkill. The 5000 mix will have better durability, etc but keep in mind that the sub grade under the concrete does most of the work when it comes to supporting heavy loads.

For a strong surface and minimal shrinkage cracks you need to keep the mix water down and avoid sprinkling water on the surface during finishing. It's a give and take between getting a good finish and ensuring durability but err on the side of less water.

Not much debate when it comes to curing - you should do it. Keep the slab surface continuously moist for at least seven days. It's cheap insurance on a big investment. I'd avoid the burlap - it can stain the concrete.

What are you doing for control joints?

 

[buening]

I'm all for overkill. .....but keep in mind that the sub grade under the concrete does most of the work when it comes to supporting heavy loads.

+1. If you were earlier in this process, I would have doubled your subbase thickness considering the loads you want it designed for. Its a bit late now though considering the rebar is all tied

 

[Falcon67]

I got a 5 sack mix. The min I requested was 3500. The concrete guy said 3500~4000 is their minimum becaue anything less is not competitive with the only other company here and they don't care for call backs or problems. They always hit 4K and my mix should be easy 4500~5000. It wasn't watered down for sure, which actually caused a problem since truck #2 was late to the job site. 28 days is the standard cure time. I waited until the slab started losing it's dark gray color before starting walls. That was about a week. I kept mine wet for 3 days which is all I had to work with it. It was exposed and this was during our killer heat wave last year, so unless the sprinkler was on the slab all the time, the surface temp was over 130F by 11 AM every day. I said to myself "Theres 61,000 lbs of mix, it'll do whatever it'll do and there ain't squat you can do about it" and got on with it.

 

[hdonlybob]

I was a project manager for most of my carrier...and would strongly recommend going with a minimum of 6" tk concrete with the whole floor. That extra inch will not be a major cost and you will not regret it.
I personally would also pay the extra to use Fiber Mesh concrete...
And lastly, compaction is the key to any good floor....make sure it it totally compacted.
Good luck, and it looks like you have a well thought thru plan, and nice shed too..
Cheers.....