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Old 10-15-2010, 07:11 PM   #1
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Floating Concrete Outside Pad Questions


I'm installing a 12 foot by 18 foot floating pad that will be left exposed to the weather. I here on good old sandy Cape Cod where the soil is, well, pure large grained sand. I've excavated with a Bobcat, left the bottom sand undisturbed but compacted it anyway with my plate compactor and I'm now installing an 8 in base of 3/4 in stone. I don't think this stone needs to be compacted, but I'm doing it anyway, layer by layer. Once done with the base, I'll be reinforcing the slab with #4 rebar around the perimeter and criss crossed every 18 inches. The slab will be poured 6 in thick using 4000 psi concrete mixed tight and with entrainment. I'm getting a pro to manage the pour and do the finishing work (I do all the preparatory grunt work).

Question 1: Should I install plastic sheeting between stone base and concrete? I'd like to keep concrete as wet as possible during the cure but on the other hand I don't want to prevent water that enters the pad in the future from draining through the stone base (don't know if that will be an issue or not).

Question 2: Should I have an "expansion" joint put into or a cut made into the pad to prevent/manage potential cracking of pad?

Question 3: This pad will be used only in good weather and will not be enclosed. I'd like to just leave it level instead of sloping it. Would that be OK? Will it cause more water to enter the slab (see also Question 1)?

Is there anything else I should be doing to minimize possibility of cracking?
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Old 10-16-2010, 12:13 AM   #2
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Will it be a 'thickened edge' slab? (image attached)
#4 bar is 1/2"?
Sounds like you are on the right path.

One technique is to make the cuts into the slab with a tool and then trowel some slurry into the cut. Trowel the slab flat over the cut so that this becomes the slab's weak point for controlling any cracks.
Dampen the gravel as you compact and just before the pour. Poly under is not required for an outdoor slab.
Keep the slab wet for a few days (1 week) to aid its curing.

No guarantees the slab won't crack, but the rebars should keep it from separating.
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Old 10-16-2010, 07:16 AM   #3
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Originally Posted by diycc2 View Post
I'm installing a 12 foot by 18 foot floating pad that will be left exposed to the weather. I here on good old sandy Cape Cod where the soil is, well, pure large grained sand. I've excavated with a Bobcat, left the bottom sand undisturbed but compacted it anyway with my plate compactor and I'm now installing an 8 in base of 3/4 in stone. I don't think this stone needs to be compacted, but I'm doing it anyway, layer by layer. Once done with the base, I'll be reinforcing the slab with #4 rebar around the perimeter and criss crossed every 18 inches. The slab will be poured 6 in thick using 4000 psi concrete mixed tight and with entrainment. I'm getting a pro to manage the pour and do the finishing work (I do all the preparatory grunt work).

Question 1: Should I install plastic sheeting between stone base and concrete? I'd like to keep concrete as wet as possible during the cure but on the other hand I don't want to prevent water that enters the pad in the future from draining through the stone base (don't know if that will be an issue or not).

Plastic is not required under the slab, and will offer little, if any benefit to the slab, unless of course you plan on building a garage, etc.. on top of it someday.

Question 2: Should I have an "expansion" joint put into or a cut made into the pad to prevent/manage potential cracking of pad?

I would, as 18' is too long in most cases. Just split it once, making 2- 12' x 9' squares.

Question 3: This pad will be used only in good weather and will not be enclosed. I'd like to just leave it level instead of sloping it. Would that be OK? Will it cause more water to enter the slab (see also Question 1)?

Water will have a tendancy to pond on the slab if there is no pitch. Even a minimal 1/8" per foot pitch would keep water moving off of the slab, yet be unnoticeable to the human eye.

Is there anything else I should be doing to minimize possibility of cracking?

What's the use for the slab?

The weather this time of year is condusive to excellent curing of concrete. As a matter of fact, we generally hold back on our curing sealents for a day or two on some job to ensure that the concrete has gained significant strenght before it's subjected to freeze-thaw cycles. It sounds like you have a good plan in place to cure this slab. Good luck.
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Old 10-16-2010, 09:03 AM   #4
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The slab will be used primarily as a working surface (place sawhorses, work table from time to time). I may erect a temporary roof over it in the form of a canopy for short periods of time during non-winter months. But I also may park a vehicle or boat on it occasionally. That's why I'm doing a 6" slab and using all rebar not mesh. Since it will be 6" thick, I was thinking that I would not thicken the edges.

On the matter of splitting the pad into two 12x9 pieces, is there a way of doing this so that the surface of the pad doesn't show the split? If I do pitch the pad, the pitch will be longitudinal, along the length of the pad, so any control/contraction joint in the middle will be at a right angle to the pitch. Therefore, the pitch will not help the slot drain assuming I cut in a slot -- maybe that's not an issue assuming some sort of sealant/caulking is used to fill the slot jshaslip has suggested? I guess I'm wondering what my options would be here for the split and what would work best. At this point I'm thinking that a 1 1/2" deep slot cut with a concrete saw a week or so after the pour and filled with appropriate sealant (polysulphide?) would be OK.

Thanks.

Last edited by diycc2; 10-16-2010 at 09:38 AM.
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Old 10-16-2010, 09:41 AM   #5
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The method I suggested does not require sawing or caulking, although that would work as well.

Have the concrete finisher use a 'divider' to make the cut and then fill it in with fines from the troweling.
This will become the weak point in the slab.
If I recall, the divider cut needs to be as close to one-third of the slab thickness as you can get it.

oh, and be sure that the rebar is chaired up at least 2". Mid-slab depth is best.
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Old 10-16-2010, 10:22 AM   #6
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The method I suggested does not require sawing or caulking, although that would work as well.

Have the concrete finisher use a 'divider' to make the cut and then fill it in with fines from the troweling.
This will become the weak point in the slab.
If I recall, the divider cut needs to be as close to one-third of the slab thickness as you can get it.

oh, and be sure that the rebar is chaired up at least 2". Mid-slab depth is best.
Just to make sure that I understand because concrete work is something new to me, the divider is a tool used to essentially create a shallow gap (1 1/2" deep) in the top surface before the concrete has set up? The fines are then used to fill the gap and are troweled smooth with the top so that when complete there is no aggregate linking one side of the gap with the other side and the top surface still appears smooth and complete? I think this is the type of contraction joint they used when they did my garage floor. This pad has cracked along the joint and now there is a fine line crack visible on the surface.

One question regarding this type of contraction "joint" if you don't mind. It's my understanding that what you have recommended and the saw cut that I mentioned both create a weak point as you say and that if the pad cracks it will crack at the contraction joint. Once it cracks then water can enter the crack I suppose. So for a pad exposed to the weather, I'm wondering if a saw cut followed by a sealant might not keep out the water better than a joint "sealed" with fines?

On the rebar, I will tie it all together, bend it around the corners and keep it a couple of inches off the bottom.

Thanks again.
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Old 10-16-2010, 03:54 PM   #7
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Rebar is typically used to increase the tension strength of a slab, although the rebar also can be used to reduce cracking of the concrete. The tension side of a slab is usually on the top, because voids in the supporting gravel often cause settlement of the slab, leading to compression on the bottom and tension on the top. However, if you have frost heave or expansive soils, the tension side could be on the bottom, or even on both sides if the heave is erratic.

In any case, placing the bars at the center does nothing structurally, since the center of a slab is the neutral axis, where you are neither in tension nor compression, so placing bar at the center of the slab is effectively a waste of money.

ACI specifies a minimum of 1-1/2 inches of cover over the bar on the top of a slab, and 2 inches of cover on the bottom. This effectively forces the bars to be placed close to the center on a 6 inch slab, which reduces their structural effectiveness to near zero. In a four inch slab, the bars do in fact have zero structural effectiveness if placed with proper cover.

My recommendation is to skip the bars completely on 6 inch or less thickness slabs such as you contemplate, and minimize cracking by proper selection of the mix, proper placement technique, careful curing technique, use of control joints, and most importantly, proper preparation of the base. Unless you have special loading conditions, the bars will not add much if any performance to your specific project.
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Old 10-16-2010, 07:07 PM   #8
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Rebar is typically used to increase the tension strength of a slab, although the rebar also can be used to reduce cracking of the concrete. The tension side of a slab is usually on the top, because voids in the supporting gravel often cause settlement of the slab, leading to compression on the bottom and tension on the top. However, if you have frost heave or expansive soils, the tension side could be on the bottom, or even on both sides if the heave is erratic.

In any case, placing the bars at the center does nothing structurally, since the center of a slab is the neutral axis, where you are neither in tension nor compression, so placing bar at the center of the slab is effectively a waste of money.

ACI specifies a minimum of 1-1/2 inches of cover over the bar on the top of a slab, and 2 inches of cover on the bottom. This effectively forces the bars to be placed close to the center on a 6 inch slab, which reduces their structural effectiveness to near zero. In a four inch slab, the bars do in fact have zero structural effectiveness if placed with proper cover.

My recommendation is to skip the bars completely on 6 inch or less thickness slabs such as you contemplate, and minimize cracking by proper selection of the mix, proper placement technique, careful curing technique, use of control joints, and most importantly, proper preparation of the base. Unless you have special loading conditions, the bars will not add much if any performance to your specific project.
OK, that all makes sense except I thought that in a slab such as this that a key function of the rebar would be to keep the slab together if it should crack.

Last edited by diycc2; 10-16-2010 at 07:21 PM.
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Old 10-16-2010, 07:41 PM   #9
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If your slab cracks so much that you need rebar to keep it together, you have much more serious issues than the rebar can address. Normal shrinkage cracking of a slab involves hairline cracks that penetrate at most an inch or so into the slab. These can be minimized through proper placement of concrete, inclusion of control joints, proper curing technique, and most importantly proper subsurface preparation. Any cracks large enough to require rebar to keep the slab together can only come from grossly improper subsurface preparation, which you are not likely to have given your careful preparation of the subgrade and attention to detail in the placement of the concrete.
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Old 10-16-2010, 09:59 PM   #10
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If your slab cracks so much that you need rebar to keep it together, you have much more serious issues than the rebar can address. Normal shrinkage cracking of a slab involves hairline cracks that penetrate at most an inch or so into the slab. These can be minimized through proper placement of concrete, inclusion of control joints, proper curing technique, and most importantly proper subsurface preparation. Any cracks large enough to require rebar to keep the slab together can only come from grossly improper subsurface preparation, which you are not likely to have given your careful preparation of the subgrade and attention to detail in the placement of the concrete.

Hmmm . . .foregoing the rebar still makes me feel nervous.

But leaving aside the matter of rebar for a moment, I am trying to do a proper prep job here. I carefully excavated the "hole" leaving an undisturbed base of sand which I compacted with my Stone 5 hp plate compactor (the fact that as a homeowner I own a good plate compactor says something about the importance I place on base prep). It rained overnight, so I compacted the sand one more time while still moist.

I then placed a 2 in. layer of 3/4" "native" stone in place and compacted this layer and repeated this process 3 more times. Although I'm not sure compacting this size stone is of any real value (as opposed to compacting smaller aggregates or mixed sized aggregates designed to interlock) I even wet down the stones before compacting. At this point, it looks like I'll be adding another two inch layer of stones before the concrete is installed so I'll have a stone base about 10 inches deep.

As to the concrete mix, as mentioned above I'll be requesting a 4000 psi, minimum water content mix with air entrainment (not sure of percentage entrainment appropriate yet). The pour and finishing work will be done by an experienced person (17 years experience apparently).

What else can I do to minimize possibility of cracks? When I've poured sonotubes myself, I've always used a 2x2 to jostle the concrete vertically as it was poured into the tube to avoid any voids. Do I do the same thing for the pad? I assume a vibratory-type of tool would be better but that would probably be out of the budget. Should I consider a thicker pad? Say, 8 inches?

Back to the rebar. When I said keeping the slab together, what I really meant was minimizing the width of any cracks that might occur. Does it make any sense to place rebar near the top of the slab in order to minimize the width of any potential cracks?

Last edited by diycc2; 10-16-2010 at 10:04 PM.
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Old 10-17-2010, 07:03 AM   #11
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nothing more you can do to minimize possibility of random cracking EXCEPT place the joint pattern correctly at the proper time & depth altho we'd cut 2 jnts at 90* from each other - i'd rather see 4 1/2' x 6'

that 2x2 in the sonotube pour was for consolidation,,, for vibration, run your compactor just outside of the forms - might work,,, you're already gotten good technical advice on the steel/wire mesh,,, wire mesh will add strength during ' tension ' - after that, it generally holds the cracked pieces together


even filling the grooved contraction jnts w/lo-strength ' cream ' will allow the natural crack to show
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Old 10-17-2010, 08:33 AM   #12
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Hmmm . . .foregoing the rebar still makes me feel nervous.

But leaving aside the matter of rebar for a moment, I am trying to do a proper prep job here. I carefully excavated the "hole" leaving an undisturbed base of sand which I compacted with my Stone 5 hp plate compactor (the fact that as a homeowner I own a good plate compactor says something about the importance I place on base prep). It rained overnight, so I compacted the sand one more time while still moist.

A+

I then placed a 2 in. layer of 3/4" "native" stone in place and compacted this layer and repeated this process 3 more times. Although I'm not sure compacting this size stone is of any real value (as opposed to compacting smaller aggregates or mixed sized aggregates designed to interlock) I even wet down the stones before compacting. At this point, it looks like I'll be adding another two inch layer of stones before the concrete is installed so I'll have a stone base about 10 inches deep.

A+

As to the concrete mix, as mentioned above I'll be requesting a 4000 psi, minimum water content mix with air entrainment (not sure of percentage entrainment appropriate yet). The pour and finishing work will be done by an experienced person (17 years experience apparently).

A+

What else can I do to minimize possibility of cracks? When I've poured sonotubes myself, I've always used a 2x2 to jostle the concrete vertically as it was poured into the tube to avoid any voids. Do I do the same thing for the pad? I assume a vibratory-type of tool would be better but that would probably be out of the budget. Should I consider a thicker pad? Say, 8 inches?

No to 8", you're already far thicker than necessary. To give you an idea, I have about 100 yds. of concrete around our shop. It always go's in over a good, hard gravel base, 5" of 4000 psi concrete, #3 rebar on ~ 3' centers, and I spray a coat of curing sealent as soon as possible. Not one shrinkage crack in anything, although there are two hairline cracks:

- The first is from the quad axle garbage truck emptying our dumpster this spring. The truck ran over the corner of the slab, as he always does, but the gravel was pure mush, as the frost ws coming out of the ground and the driveway was completely saturated. My fault really, as I should have just pushed the dumpster to the end of the slab, but forgot.

- The other crack is from frost heave on the other side of the building, where the concrete heaves and hinges every winter. In spring, the slab returns to it's original place, with a very minimal crack.

Due to steel re-enforcement, these cracks are only aesthetic, and by no means become a nusaince.

Back to the rebar. When I said keeping the slab together, what I really meant was minimizing the width of any cracks that might occur. Does it make any sense to place rebar near the top of the slab in order to minimize the width of any potential cracks?

I think you're plan for rebar is a little overkill, but it's your's and do what you feel is best for your situation. If you decide against using any, odds are you'll kick yourself down the road, and there is NO way to add rebar at a later time.

One last thing, I'd trust the opine of your concrete finsher, who's obviously very familiar with your conditions & climate, rather than take the opinion of someone a country away reading old blanket ACI specs.

Good luck.
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Old 10-17-2010, 12:48 PM   #13
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I think you're plan for rebar is a little overkill, but it's your's and do what you feel is best for your situation. If you decide against using any, odds are you'll kick yourself down the road, and there is NO way to add rebar at a later time.

One last thing, I'd trust the opine of your concrete finsher, who's obviously very familiar with your conditions & climate, rather than take the opinion of someone a country away reading old blanket ACI specs.

Good luck.
Many thanks. I think I can get the rebar locally at a decent price (I'm able to trailer it in 20 ft lengths). In any event, relative to the amount of my labor and cost of other materials going into this thing I think the rebar cost and placement, again my labor, will be relatively minor and as you say you can't add it later. My strong inclination is to err on the side of caution and use the rebar. I see no "engineering" downside to using the rebar, and it seems to me that it should minimize the size of any potential cracking if not the propensity for cracking.

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Old 10-17-2010, 01:26 PM   #14
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The only downside to using rebar is that it rusts and expands if it gets wet. In a thin slab like a 4" thick slab, per ACI code the rebar must be placed at the center (minimum 2" cover from soil). If the rebar is placed closer than about 2 inches to any surface, it has a bad tendency to rust in place due to moisture penetration through the concrete, made worse if you use salt to melt snow over the concrete. Rusty rebar will create cracks in the concrete, since the rebar expands substantially when it rusts.

So if you can properly place the rebar with adequate cover, it does no harm, only costs money and time. If you fail to place the bar with adequate cover, there is the potential for rusting and cracking down the road.
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Old 10-17-2010, 01:34 PM   #15
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The only downside to using rebar is that it rusts and expands if it gets wet. In a thin slab like a 4" thick slab, per ACI code the rebar must be placed at the center (minimum 2" cover from soil). If the rebar is placed closer than about 2 inches to any surface, it has a bad tendency to rust in place due to moisture penetration through the concrete, made worse if you use salt to melt snow over the concrete. Rusty rebar will create cracks in the concrete, since the rebar expands substantially when it rusts.

So if you can properly place the rebar with adequate cover, it does no harm, only costs money and time. If you fail to place the bar with adequate cover, there is the potential for rusting and cracking down the road.
Assuming I place it correctly (and why not) will it likely minimize the width of any cracks that may occur? Or prevent potential vertical shear if a crack goes through the pad?

I do recall reading about rusting rebar, now that you mention it. Maybe I should look into galvanized re-bar? Probably twice the price . . .

Last edited by diycc2; 10-17-2010 at 01:52 PM.
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