Reinforcing concrete

[GreenMonster]

we're pouring a pad for an outdoor grill. It'll be roughly 4'x7'x1' thick. I'm kind of assuming the rebar should all go in horizontally, but is there a standard as to the grid pattern -- 1' spacing? At a given height of the footer, should rebar start to go in vertically as well?

How about for say a column footer with accompanying wing wall footer?

[CaptainsLS]

Mark, there is not really a standard for that application without determining facts about the above structure. Maximum spacing for rebars is 18" whether it's for flexure or shrinkage. On most applications I would lay 12” on center crossing. Rebar is cheep.

[CaptainsLS]

Also meant to suggest you use #5’s, and I would hammer 6 or so in vertically to tie the horizontal layer too.

[mrusk]

This the footer below the frost line?

[GreenMonster]

This the footer below the frost line?

The 12" pad for the grill, no. It's on 2-3' clean stone.

For a column and wing wall, yes, it would be below frost line.

[mrusk]

The 12" pad for the grill, no. It's on 2-3' clean stone.

For a column and wing wall and any masonry structure, yes, it would be below frost line.

...............................

[GreenMonster]

Yes, thank you for your input Matt. I am aware that technically, vertical masonry structures should be on a footer below the frost line.

I do however, have input from individuals (you could argue experts) in the field that have actual real world experience with this EXACT type of structure, and actual real world installations built in this manner. Soooo, if you have any input as it relates to the original question, I would be glad to hear what you have to say. Otherwise, you are dismissed.

[sancraig]

I'm usually overkill on concrete reinforcement, but I would drive 8 #5 in vertically just to point where their 3" below top of slab. I would also tie 2 horziontal mats of #5 on a 12"x12" spacing tied to the verticals. Bottom mat 3" above subgrade and 3" below top of slab.

I figure if it's something that's not easily replaced later overkill is better from the start.

Oh and yes I would also auger 4 holes blow frost line but thats me.

[CaptainsLS]

Mark, I would put a perimeter footing that penetrates your frost line and pour the pad on top of the aggregate.

Sancraig’s approach is another option. The only spec that I might change is the horizontal rebar placement. On a 12” slab one mat should be sufficient at dead center of the pour, if your bar gets too close to the surface (2-3”) of the concrete you open yourself up to failure.

[GreenMonster]

I'm usually overkill on concrete reinforcement, but I would drive 8 #5 in vertically just to point where their 3" below top of slab. I would also tie 2 horziontal mats of #5 on a 12"x12" spacing tied to the verticals. Bottom mat 3" above subgrade and 3" below top of slab.

I figure if it's something that's not easily replaced later overkill is better from the start.

Oh and yes I would also auger 4 holes blow frost line but thats me.

thanks captain and sancraig. your input on the 2 mats is also what I was wondering about.

I have also talked with my "source" about augering holes and pouring the pad on top of the tubes that are below frost. To tell you the truth, if this was in a customer's yard, I would be hard pressed not to have this below frost in one form or another. However, as it is in my yard, I would like to see how this method holds up.

[danf]

For future arguments sake, is there any reason why a 6x6 mesh couldn't be used for a slab "foundation" like this? Since the slab is going to be for a GRILL and not for any type of actual heavy load (like a house), does it really need to be reinforced with rebar?

My first instinct for reinforcement of any sort of foundation is rebar, BTW. I'm just wondering if the same affect can be achieved with a little less labor.

[sancraig]

It can be but my thought always falls back to whats the effect if the slab fails? Sidewalk,driveway, floor no biggie easily replaced with little damage to other structures.

Outdoor grill or Kitchen slab fails structure above also fails, not so easily fixed. For something this small not much cost difference by the time you buy a roll of mesh or the rebar precut takes an extra half hour to tie the rebar. imho

Im assuming the grill is a permanant structure not a roll around.

[custom patios]

GM, I know you said this is for a grill, but what kind of structure are you building?

[CaptainsLS]

Dan- (again assuming this is a masonry grill) you want that bar in there for the flexural strength and to balance out the compressive loads. This ‘floating slab’ will need structural reinforcement where as the welded mesh only holds the pieces together, it does not provide structural support.

[GreenMonster]

GM, I know you said this is for a grill, but what kind of structure are you building?

cmu structure with thin natural veneer

[custom patios]

The 12" pad for the grill, no. It's on 2-3' clean stone.

For a column and wing wall, yes, it would be below frost line.

if this 12" pad on 2-'3' of clean would work for a cmu grill structure, why wouldnt it work for a column, wing wall, or other structure? why doesnt this footer need to be at/below frost line? I imagine where you are your frost line is propably around 40"+. just trying to unserstand your method of construction, not criticizing.

[GreenMonster]

well, it's not "my" method. I'm going with advice from someone that has done several of these units with this method. I'm willing to try the method out myself, in my backyard for my own information and experience. As I stated earlier, I'm not sure I'd be ballsy enough to try it at a customer's site.

For the sake of arguement, if the clean stone is below the frost line, with a reinforced pad on top, why wouldn't it work in the same manner as a frost free footer? If the open space in the stone is greater than the expansion rate of water, there should be no risk of heaving, correct?

[soopa]

For the sake of arguement, if the clean stone is below the frost line, with a reinforced pad on top, why wouldn't it work in the same manner as a frost free footer? If the open space in the stone is greater than the expansion rate of water, there should be no risk of heaving, correct?

theoretically... but i'm guessing in new hampshire, as here in new york, the frost line is well below 2-3'

probably more like 4-5'

[custom patios]

well, it's not "my" method. I'm going with advice from someone that has done several of these units with this method. I'm willing to try the method out myself, in my backyard for my own information and experience. As I stated earlier, I'm not sure I'd be ballsy enough to try it at a customer's site.

For the sake of arguement, if the clean stone is below the frost line, with a reinforced pad on top, why wouldn't it work in the same manner as a frost free footer? If the open space in the stone is greater than the expansion rate of water, there should be no risk of heaving, correct?

I'm not sure if it's a matter of whether it will work or not. I think it's a matter of how long. I'm very curious? I sure wouldnt try a wall this way.
Hey, guys are dry stacking cmu's these days. they have a special mortar that's applied to the outside. doesnt mean I'm going to follow suit though.
Still, I dont see why this method wont work for some time. especially if you joint the perimeter around adjacent structures

[Mark]

you guys ever seen a garage or shed built on a slab? You typically have a 4-6" slab and the edge of the slab on all 4 sides is deeper. Called a honch (spelling?) This unit is reenforced with rebar and never goes below frost line. Perfectly legitimate practice and 100% up to local codes. Why would this be any different?

[Mark]

Here is a slab we just did an excavation for....this is a 2 car detatched garage for a 50 million dollar property. This was approved by the town and one of the best builders in the lakes region

[CaptainsLS]

Just curious mark, how much cash would you be saving by not going below the frost line? Or are you just looking to experiment?

In order for your kitchen to experience frost heave you will a) need to have a silty soil that will actually freeze b) there will need to be moisture c) the freezing temp’s will need to penetrate the soil.

In theory, you remove all of those aspects with your construction plan…

[GreenMonster]

well, I guess my "source" has shown himself. I didn't want to reveal it, in case Marcus didn't want to feel the wrath here!


you guys ever seen a garage or shed built on a slab? You typically have a 4-6" slab and the edge of the slab on all 4 sides is deeper. Called a honch (spelling?) This unit is reenforced with rebar and never goes below frost line. Perfectly legitimate practice and 100% up to local codes. Why would this be any different?

You know, I was thinking about this, and talking to Dan about it today. At my last house, I had a 24'x40' garage on a floating slab, just as you're describing. There were 3 courses of split face block and then the wood frame on top of that. There was never any cracking of joints or broken units in over 10 years.



Just curious mark, how much cash would you be saving by not going below the frost line? Or are you just looking to experiment?

In order for your kitchen to experience frost heave you will a) need to have a silty soil that will actually freeze b) there will need to be moisture c) the freezing temp’s will need to penetrate the soil.

In theory, you remove all of those aspects with your construction plan…

Chris, I guess I'm somewhat interested in saving the time and the excavation, seeing that we decided after the fact to do the grill. I would say that it's more a matter of Mark has described this method, and I'd like to see it for myself in a real world application. I'm willing to take the risk here, in my backyard, to see if it is truely what it's cracked up to be...... um ,that might be a bad choice of words, but you get my drift.

The more I've talked to Mark about this, and thought about it own my own (sort of in the manner that Mark is drawing parallels to with garage slabs) I tend to think it'll be just fine.

[Mark]

put a drain pipe in the clean stone if you can and you'll never have an issue. The company I work for has been doing this for 25 years and never a problem. I would not do this for a pillar as it is too small of a foot print and much easier to move, where the kitchens etc are much larger and as pointed out frost is deleted by the stone and lack of water.
You all recall the pizza oven at bedrock....that was on a 12" reenforced slab that weighed 19,000 pounds on top of 3 feet of stone...it won't move or fail

[Mark]

someone here mentioned pouring sona tubes BFL and then the slab on top of them. I would absolutely go to this measure in an unstable environment...on the egde of a slope or newly filled area. They would help anchor the slab and keep it from shifting or slumping to the downhill side.

[CaptainsLS]

Mark- what was the steel layout on the bedrock slab?

[Mark]

we go 12" or smaller on all our work

[CaptainsLS]

Would you mind sharing specifics? You have 8 ½ tons sitting on a 12” slab. I’m just curious if you used a combo mesh/rebar and or fiber, how many feet of aggregate, ect….?

[danf]

you guys ever seen a garage or shed built on a slab? You typically have a 4-6" slab and the edge of the slab on all 4 sides is deeper. Called a honch (spelling?)
Also called a monolithic pour/footer too. Company I worked for in Indiana did an outdoor kitchen that way, it was surrounded on 3 sided by pavers. To the best of my knowledge it has had no ill effects.

[custom patios]

put a drain pipe in the clean stone if you can and you'll never have an issue. The company I work for has been doing this for 25 years and never a problem. I would not do this for a pillar as it is too small of a foot print and much easier to move, where the kitchens etc are much larger and as pointed out frost is deleted by the stone and lack of water.
You all recall the pizza oven at bedrock....that was on a 12" reenforced slab that weighed 19,000 pounds on top of 3 feet of stone...it won't move or fail

these are my thoughts as well. A wall or pillar footer is too suseptible of the movement since it has a small footprint. But a garage or shed pad? if theres any settling at all that would mean big trouble. thanks for posting those pics Mark they are very helpful. I am going to look into my area code requirements and see if its allowable, but I am only aware of codes which require footings below frost.
So without the open aggregate beneath this floating pad, this method is doomed for failure.
What keeps the floating footer from lateral movement?

[custom patios]

Just curious mark, how much cash would you be saving by not going below the frost line? Or are you just looking to experiment?

In order for your kitchen to experience frost heave you will a) need to have a silty soil that will actually freeze b) there will need to be moisture c) the freezing temp’s will need to penetrate the soil.

In theory, you remove all of those aspects with your construction plan…

....settlement or lateral movement become the only issue. and those can be addressed as well. very cool, very clever method!

[GreenMonster]

....settlement or lateral movement become the only issue. and those can be addressed as well. very cool, very clever method!

settlement is easy, geo, compaction, blah blah, what about the lateral movement. How do you ensure that is addressed?

I do agree this is an interesting method. Kudos to someone for thinking outside the box. It's easy to say footer below frost -- I mean that's the accepted fool proof method, but if there is a more efficient and equally safe option, why not try it?

good conversation.

[CaptainsLS]

settlement is easy, geo, compaction, blah blah, what about the lateral movement. How do you ensure that is addressed?


Right, settlement can be addressed just by understanding soils/digging to stable soil. Lateral on the other hand, you would just make the perimeter deeper (12" or so), I think Mark called it "haunch".

[GreenMonster]

Lateral on the other hand, you would just make the perimeter deeper (12" or so), I think Mark called it "haunch".

ok, that's what I was thinking. cool.

alright, now who originally thought this was a cockamamie idea but are thinking about it a little differently now?

[custom patios]

ok, that's what I was thinking. cool.

alright, now who originally thought this was a cockamamie idea but are thinking about it a little differently now?

I for one. Im always skeptical until
I think things through with my
prohardscaper buddies

[GreenMonster]

I'm glad you chimed in Steve, I respect your opinion, and always interested in your thoughts on masonry applications. Chris, too.

Besides, I need to get some warm fuzzies sometimes when I put faith in Marcus.:loco:

[CaptainsLS]

I have seen engineered plans for full masonry walls over compacted aggregate, so I cant say I’m surprised this would work. I am however impressed by some of the more extreme applications that have a floating pad.
What I am most interested in… Is how to determine when this application is not advisable? A pillar is a given, the aspect ratio of the structure is top heavy and will need added support. Marks pizza oven was almost ‘pyramid shaped’ if I recall, and not top heavy. Fireplaces are out, but what about sitting walls? An ‘L’ shaped wall seems like it would have a stable footprint? If the project allows could you expand the size of the pad to accommodate a straight wall???

[GreenMonster]

hmmm, I'm not sure how Mark is doing some of the walls they're doing. He was explaining how some walls are "slushed in". Where a stone wall is built on a compacted agg base, and mortar is used on the interior and appears dry-stacked. but a wall that is pointed, agg base and a reinforced pad....?????

[custom patios]

i think i would be willing to try this on a project
too bad i got nothin to try it on.lol
i was very skeptical of celtek, but tried it and
liked it. thanks for the props Mark. i
value very much what you say as well.

[custom patios]

What I am most interested in… Is how to determine when this application is not advisable?





excellent question. i dont think any wall
would be a good candidate, but i learn new atuff
each day

[mrusk]

What about codes? Here in nj we have building inspectors, codes and all that good stuff. Regardless if that worked, we could never implment it here. Also no floating slabs for garages here.

[danf]

What about codes?
Did you see this?

Perfectly legitimate practice and 100% up to local codes.
There's building inspectors and codes here too. Same in Indiana.

Are you sure your codes prohibit this type of construction? If so, sucks for you. :D

[GreenMonster]

Come on Matt, try to keep up here. I know you can't spell, but chit, you can read can't you?

Yes, up here in Hickville we have building inspectors and code enforcement. Sometimes they do double duty as the dog catcher, or lever man on the trash compactor at the dump but I assure you they exist. Every once in a while, they actually show up for an inspection.

Matter of fact, everyone "thinks" you can get away with stuff in this town, but our CEO is a true hard ass. We went around a few times during our construction last summer. On the subject of foundations, CEO inspects the footer forms before he allows a pour. then he inspects the footer, then inspects the forms for the walls, then inspects the pour. A bit excessive if you ask me.

As far as slabs go, whether they are allowed or not is at the town level. In my town, they were ok 12 or so years ago when I did that first garage I mentioned, but they are not allowed anymore. Next town over (where I believe Mark's garage slab is) they are ok.

[Terraventure]

CEO's vary widely around here. Anytime I have talked with other contractors about codes it really comes downto what the building inspector understandes and will allow. The engineering behind floating slabs has been around for mor than half a century. Frank Llyod Wright first came up with a clean stone foundation. As long as the stone will drain you are good to go. As Mark said adding drain in the bottom of the stone to drain water may not be needed but would be extra insurance.


Just my two cents

[GreenMonster]

CEO's vary widely around here. Anytime I have talked with other contractors about codes it really comes downto what the building inspector understandes and will allow.

very true. A better way to put it might be the inspector's interpretation of the rules. The first page of the code book has that statement that essentially says all decisions come down to the local level. That's the little statement that gives these guys essentially all the power to make you do whatever they want, with no recourse on your end... regardless of how silly it may be.

[ZX12R]

'you guys ever seen a garage or shed built on a slab? You typically have a 4-6" slab and the edge of the slab on all 4 sides is deeper. Called a honch (spelling?) This unit is reenforced with rebar and never goes below frost line. Perfectly legitimate practice and 100% up to local codes. Why would this be any different?"

As someone else stated,we call it a monolithic slab.I poured my first one last week in my yard as I am putting a 12X24 shed on it. It's overkill for a shed but I went 12" deep on the perimeter(3500 PSI concrete where most use 3000) and 6 " in the center.I compacted the soil,layed geotextile,applied clean stone,a vapor barrier,3 seperate runs of rebar were used along entire outside wall and wire mesh was also used.

By the way monolithic slabs are commonly used for houses without basements in southern jersey where the soil is sand.I know as my neighbor is a builder.

[custom patios]

I watched my dad build his shed on a floating slab. I know it wasnt 12" thick. I must have been around 12. Im 37 now. The shed hasnt moved. I know he did not put in 2'-3' of clean either. 6" maybe uncompacted. I cant remember but it was not substantial Im sure. The shed is on a slope as well. He built a stone wall around the back so the area was level. I dont know why I didnt think of this earlier. Of course lots of people build sheds on a slab and not a footer. I would do that as well. Im not sure why I was thinking this can not be applied to garages and other structures. Maybe weight and size? Codes?
Im talking out loud to myself right now: If I used a floating slab for a garage I would defineatley do a monolithic pour. I think It would be at least
6" in the middle and the perimeter going to at least 12". the monolithic pour is what shall anchor the structure from lateral movement. I would rebar the slab to china.
Do you guys think that I could have used a floating slab in this application?