Tiedown spot options?

3000 psi air entrained with so and so aggregate. Just tell the application and tell them to give you the best they have. The point is YOU order and pay for it, not your concrete contractor. Dont accept you ordering it on his account either. He can still call and change (cheapen) the order. Put that you are ordering the concrete in the contract. And stringline and check the depth of the concrete in the MIDDLE!
 
Concrete is a mixture of sand, cement and stone (aggregate). The cement is the glue and the more the better (to a point). Six bag concrete is what driveways are made with and is strong and economical. Of course, the two things that make concrete last are the foundation under the slab and the wire mesh/rebar that has to be in the middle to "absorb" the "stretch." Concrete is only good in compression. Another thing to know is don't let them put chlorine in your mix. Chlorine is used to accelerate the curing and is widely used in the winter. It also is used to make the job go faster, which is bad for concrete.
 
A concrete slab "floats" on the ground beneath it. If the ground under the concrete moves, the concrete moves. Yes Virginia, ground moves. Some ground moves a lot, some a little. What makes it move? Frost heaves, changing moisture content, gradual compacting, expansion due to more water, contraction due to hill drying out, differences in soil (some backfill, some undisturbed), earth movement due to tectonic shifting, hillside slip etc. Expansion joints are put in and you hope it cracks on the joint. Doesnt always. Rebar keeps the crack from separating. Rebar only kicks in AFTER the concrete moves (unless it is prestressed). When the cracks get big enough they should be grouted so water doesnt get it.

Concrete slabs CAN be constructed so they don't "float" on the ground. They are called structural slabs. Structural slabs rest on foundations walls or steel beams which are part of the deep foundation. They are expensive, but are the best there is when done right.
 
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coloradobluesky said:
Also, order you OWN concrete so you actually get the good stuff.
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Or you can simply look at the delivery ticket as soon as the truck rolls onto the site and confirm the mix. :rolleyes:

JOhnH said:
What are you talking about? ;)
(I am sure you know; but I don't).
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5 1/2 bag mix, 6 bag mix, etc. It's an old school way of referring to concrete mix designs, i.e. strengths. It refers to how many bags of cement are included in the mix per cubic yard. 6 bag mix is about a 3,500# mix...maybe as high as 4,000#...depending on everything else in the mix and depending upon whether they substitute fly-ash for some of the cement in the mix.

Also, as I believe someone stated above, exterior concrete should always be air entrained (if you're in an areas that sees hard freezes in the winter). 2% air is what we use around here. That way, there are small bubbles encapsulated in the mixture (forever) and they allow the moisture to expand in the cured concrete when it freezes in the winter without popping the top off of it.

Also, don't use salt to melt snow/ice off of concrete for at least a year...preferably two. This too can cause the top to spall.

As an old-school engineer once said in a concrete design/strength seminar I went to long ago:

Concrete is the only construction material that's dumber than cow chit. At least cow chit eventually dries up and blows away. Concrete must be jack-hammered up and hauled off.
 
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timwinters said:
5 1/2 bag mix, 6 bag mix, etc. It's an old school way of referring to concrete mix designs, i.e. strengths. It refers to how many bags of cement are included in the mix per cubic yard. 6 bag mix is about a 3,500# mix...maybe as high as 4,000#...depending on everything else in the mix and depending upon whether they substitute fly-ash for some of the cement in the mix.
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And none of that matters if too much water is added as is typical...
 
Concrete is a really good idea frequently executed pourly. My industry sticks with modified grout (mostly) and then uses non-destructive, remote sensing to estimate the effectiveness and placement of the grout. The results frequently fail to impress. The one shining example I have is when a job went all wrong and we ended up applying maximum (pipe collapse limit) pressure when placing it. The remote sensing logs were beautiful.
 
cgrab said:
Another thing to know is don't let them put chlorine in your mix. Chlorine is used to accelerate the curing and is widely used in the winter. It also is used to make the job go faster, which is bad for concrete.
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Ummm...no.

Calcium Chloride (not chlorine) is used as an accelerator in colder weather. It is not bad for concrete at all, in fact, you'll wind up with a stronger mixture when it is PROPERLY used in colder weather than when it's not.

What calcium chloride IS hard on is rebar/mesh. it's corrosive and will eventually corrode the reinforcing. This however takes far longer than anyone here will live to see. It shouldn't be used on elevated structural slabs (parking garage floors, bridges, etc.) but for for a simple slab on grade it's fine...recommended actually...unless you want to wait a day or two for the concrete to set up enough to finish. There are non-calcium accelerators available that aren't corrosive but they're quite a bit more money than calcium chloride. For no more yardage than Edddie is pouring though the cost would still be fairly minor ($50 or less).

What you don't want to do is pour concrete if the forecast calls for overnight freezing temps within the first three days or so, or cover it with insulated blankets if freezing weather is forecast. When you order a 4000# mix, that means the concrete SHOULD attain 4000# after curing for 28 days and it will continue to gain strength (albeit slowly) for a long time thereafter...UNLESS...it freezes. Once concrete freezes it's done curing and gaining strength. So, it's best not to pour concrete within 28 days of freezing weather. Another "rule-of-thumb" is that concrete will be at 75% strength after 7 days. A 4000# slab will be at 3000#. The specs on many commercial projects require the contractor to let the slab sit for 7 days before starting walls or steel erection.

https://www.ascconline.org/Portals/...-acceptable-use-calcium-chloride-concrete.pdf

This thread and my runway lighting thread are fine examples of why you don't ask construction questions (or questions about any other specialized trade) on an aviation forum. Everyone (including me!) thinks they're an expert and it's hard for the layman to determine the facts. But, hey, at least Henning is no longer here to share his bottomless pit of misinformation. Find a construction forum (preferably commercial and not residential) to ask these questions at!
 
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Update...

We’re doing it ourselves.

So far, about $300 for a bobcat and operator for two hours to dig down and level. $70 for gravel. $80 or so for lumbar so far.

Ready to build the forms:

24197850538_33fcaefd86_z.jpg


Guessing the concrete for the 16x20 slab will be around $700 for fiber-reinforced concrete delivered.

Thanks for all the advice - I’ll keep you all apprised of the progress.
 
Eddie, even with fibermesh concrete, you'll still want to install rebar (#4 @ 18" ocew) or 6x6-6/6 WWM. Fibermesh doesn't provide the same structural reinforcement that steel does. All it is good for is preventing "micro-cracks." i.e. alligatoring and the like.

Congrats on doing it yourself...it ain't brain science! :cheers:
 
FastEddieB said:
Update...

We’re doing it ourselves.

So far, about $300 for a bobcat and operator for two hours to dig down and level. $70 for gravel. $80 or so for lumbar so far.

Ready to build the forms:

24197850538_33fcaefd86_z.jpg


Guessing the concrete for the 16x20 slab will be around $700 for fiber-reinforced concrete delivered.

Thanks for all the advice - I’ll keep you all apprised of the progress.
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Looks to me that you have a perfectly good hangar just down the hill.
 
Steel or mesh keeps the cracks from separating. Flexibility from plasticisers helps prevent cracks from forming. Most steel and mesh gets stomped on and driven to bottom of slab where it doesnt do any good. Suggest you support it with bricks or standoffs. Some just leave out the steel and go with thicker concrete. YMMV.
 
 
The concrete pourer said what he was pouring could handle 3,500psi (psf?) so that for a normal load up to about 3,000 lbs rebar was unnecessary.

That said, if I chose to go belt and suspenders, what is typical rebar spacing for a 16’x20’ slab? Doesn’t seem like it would add significantly to the cost.
 
Fast Eddie--

For all that you're doing some #4 Rebar, 12" OC, Each way, should be good It will be plenty strong enough

D.
 
If you use rebar support it with the plastic standoffs. If you don't support it somehow, might as well not have it. Concrete finishers dont like rebar, they trip over it when they wade in it. I think 2' x 2' grid is usually enough and gives the finishers a square to stand in so they dont HAVE to walk on the rebar. If the slab is small enough so they wont be wading in it then thats not an issue. Cut expansion joints so you arent more than 3' from a joint anywhere. Put the rebar specs and requirements in the written contract. Order your own concrete in your account. Inspect and approve or disapprove BEFORE pouring. If you get into a dispute YOU dont order the concrete which puts YOU in control. As in "Well, if you dont do it the way its specified, Im not ordering the concrete". If you wait to approve until the truck arrives or AFTER pouring you are ****ed.

Mistakes in concrete are extremely expensive and heartbreaking. Dont go cheap. Having said all that a simple slab by itself out in the boonies is not exactly critical if its not perfect, I guess.
 
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I was just at Home Depot, and they have 2” plastic rebar supports for about $25 for 40 of them.

Going to work on framing now, and watching the weather to schedule the pour - they need 2 or 3 days notice.
 
FastEddieB said:
The concrete pourer said what he was pouring could handle 3,500psi (psf?) so that for a normal load up to about 3,000 lbs rebar was unnecessary.

That said, if I chose to go belt and suspenders, what is typical rebar spacing for a 16’x20’ slab? Doesn’t seem like it would add significantly to the cost.
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Rebar is for tension, not compression. Like @coloradobluesky said, 18 to 24" ocew is adequate.

To clarify sawcut spacing & depth: The cut should be a minimum of 20% the depth of the concrete, preferably 25% and should be spaced no more than twice the concrete thickness in feet. i.e. cut a 6" slab no more than 12' ocew and a 4" slab no more than 8' ocew. In your case since it's a 16 x 20 slab, regardless of the thickness, cut it once in the long direction and twice in the short direction, i.e. 8' x 6.666' "squares". You want to keep it as close to square as possible because concrete likes to crack down the middle of a long rectangle.

I don't agree with ordering your own concrete. You can simply check the delivery ticket to make sure it's the mix you agreed on. A local contractor will likely get a discount on the concrete that you won't get if you order it yourself.

It ain't rocket surgery.

Oh, and one other thing, if the weather is warm and/or breezy you'll want to cut the slab the same day. If it's cool and calm you can get by cutting it the next morning.
 
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FastEddieB said:
The concrete pourer said what he was pouring could handle 3,500psi (psf?) so that for a normal load up to about 3,000 lbs rebar was unnecessary.
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The load of the aircraft (or just about any wheeled vehicle) isn’t a problem. The pressure exerted on the concrete cannot exceed the tire inflation pressure until a tire goes flat. The suggestions for rebar or mesh are to help with cracks. If ya don’t mind cracks spreading or lifting in a parking pad then don’t bother. Some folks are bothered by cracks, I’m not one of those. I have a crack and it has never bothered me.
 
Long shot, but...

Is there anyone in the Knoxville area or nearby that has done rebar and could maybe help me out with that part of the slab?

Today I'm going to finalize the leveling (2" drop over 20' front-to-back) and sink the tiedown rings. There are several suppliers of rebar in the Knoxville area once I know what to order.

Thanks in advance!
 
Here ya go Eddie...

Rebar is typically sold in 20' sticks. You need 18...#4. If you have a really good cut-off saw you can cut it to length yourself. If not (grade 60 rebar is pretty tough stuff) then have the place you buy it from cut it to length. They should do this for a nominal charge.

Have them cut 8 sticks to 19'
Have them cut 10 sticks to 15'

You can keep the 5' cut-offs and toss one or two into the concrete, diagonally, at each corner for extra reinforcing if you want, but not necessary. Or, better yet, install a diagonal box around each tie down ring. Make the box 1' square and let each leg stick out 2' past the box.

You need 80 of the 2" stand-offs (chairs) from home depot (or the rebar supplier will likely have them also, maybe at a better price). Put one under each intersection and you're done!

Layout attached.
 

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One other thing, there are rebar chairs that have just one clip...and clip to only one stick of bar. With these you still have to tie the rebar together with wire at the intersections.

The home depot chairs have two clips, are installed at the intersections and keep the bar from moving. No rebar tying is required...thus this is the type you want unless you want to tie a bunch of bar which is a backbreaking PITA for us old farts.

What you want:

shopping (1).jpg

What you don't want:

shopping.jpg
 
Thanks, Tim! That helps a lot.

I just went by the Home Depot in Lenoir City, TN. They stock the 20’ lengths.

I’ve cut a lot of rebar with an angle grinder fitted with a cutoff disk. A bit tedious but not too hard.

May pick up the rebar next week and get it placed. I’ll keep you up to date.
 
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Picked up the rebar and standoffs Monday.

Got the forms squared and leveled with my son-in-law Sunday. 2" drop back-to-front over the 20'. Tiedowns sunk with Quickcrete. 4" PVC to act as "surrounds".

We had a torrential downpour Monday night :

38211066506_e3a6f1b85b_z.jpg


I do have another load of gravel in the back of the Ridgeline and need to fill in a lot of low spots. Next step is cutting and placing the rebar, probably in a few days.

With rebar, should I still specify fiber in the concrete, or can I safely save there?
 
@FastEddieB

If you don't over-trowel it (and DON'T) then you shouldn't really need fibermesh.

But...

Fibermesh only costs about $5/yd in this area and you're only pouring 4 to 6 yards. If it's only $5 in your area then $20 to $30 is very cheap insurance.

It certainly won't hurt.
 
Cheap insurance for the win.

Now, about “scoring” the pad. I see on YouTube they’re called control joints.

Am I right in thinking a two scores, one lengthwise and one widthwise would be called for?

The options seem to be...

1) Saw grooves after the concrete has set.

2) Use a special trowel to make two shallow grooves.

3) Something done with felt strips.

Any suggestions?
 
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Don't think the ground has the heaving problem where Eddie's at, but do check as I'm not sure.
 
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Grade beams support the slab edges. The one you drive over might outta have a rebar in the grade beam, too. Think of a piece of plywood sitting on the ground. Compare it to a piece of plywood nailed to a 2x4 perimeter frame underneath. Which is stronger/more rigid?
 
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You don't need a thickened edge! It's a slab not a building pad. It doesn't matter if it heaves a bit when the ground freezes. The rebar will hold it together and it's not like you're going to be driving a loaded dump truck up on to it applying pressure on the edges. That slab will laugh at the loading your plane or a standard vehicles imposes upon it.

Re: "scoring"...

See post #61
 
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8245BD14-E963-42C0-8952-44E67EBA97AA.jpeg Hmm. My Texas house has a fire pit with a concrete patio around it to hold lawn chairs. It has grade beams around the perimeter. I guess those pros don't know nuthin!
 
Stewartb said:
View attachment 57655 Hmm. My Texas house has a fire pit with a concrete patio around it to hold lawn chairs. It has grade beams around the perimeter. I guess those pros don't know nuthin!
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Most of Texas has expansive soils, totally different construction methods needed there. The hills of Tennessee/N Georgia...not so much...

It is indeed another insurance policy but really unnecessary for a lightly loaded slab.
 
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Progress update...

Over the last several days I filled in and leveled the gravel, got the rebar, cut it to size, and placed it with the supports.

In (back breaking and tedious) progress as caught by Karen:

38403382686_fe5e4b12f3_z.jpg


(As a data point, neither of the tiedown slabs on the other side of the runway utilized rebar and to date neither has cracked.)

Pretty much ready for the pour:

38403384586_b484589d3e_z.jpg


I'm thinking 4 yards will be enough, per this online calculator:

37745134884_e7204b7518_z.jpg


Looks close, but the 16x20 was the outside dimensions of the forms, and 2x4's are apparently only 3.5" wide. I don't want to come up short, since there's a $100 fee for the truck to fetch more.

Anyway, watching the weather forecasts for next week and ready to schedule the pour. Still open to anyone wanting to drop in and help/supervise!

As an aside, costs so far:

38460305381_736ea981a4.jpg


Even with the concrete and tool rental/purchase, I should still be under 50% the original estimate of $3,800 all in from the contractor. Assuming my labor has no value!
 
When I order concrete I plan on a little extra and have a plan for pouring the extra on my site. maybe a ramp up to the slab...
 
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