Concrete pad on sloping ground

[michael.white]

Hi all,

I'm installing a generator and I am getting ready to pour my first pad. I've been getting advice from the electrician doing some of the work and the supply store selling me the concrete. Since the generator with fuel will weigh about 1500 lbs and will vibrate, I dug 12" deep footers all around. The pad is 3 1/2' x 5 1/2'. Becuase the pad is on a hillside, the thickness at the top of the hill is 6" (the minimum I was told it should be) and is 14" at the bottom of the hill. The rebar in the footers will be held in place by 6x10 stirrups.

I finished the framing and am about to put in the rebar, but I have a couple of questions:

1) When framing, I may have knocked free some dirt into the footer and created a gap or two between the ground and frame. Is the best way to block these gaps by placing a 2x4 over it? Or is there a better way?

2) How should the rebar be placed? If I follow the slope of the hill, the rebar will be a few inches from the top of the pad at the top of the hill but more than 10" from the top of the pad at the bottom of the hill. Do I make a second layer of rebar?

Thanks!

 

[jel1988]

How big is this generator? Seems like a little overkill. As long as the ground slopes up to your pad at no greater slope than 2 to 1 (run to rise) you would be fine. Second the rebar in the slab could be replaced with 6"*6" welded wire mesh. Place the mesh about 1/3 up from the bottom of the slab. If your still worried get fiber reinforced concrete. Don't forget to put some slope on the slab to create positive drainage, so water doesn't puddle on the slab. Shim the generator as required.

 

[CurlyDave]

The rebar in the footers will be held in place by 6x10 stirrups.

I am not sure of some of your terminology here. What is a 6x10 stirrup? I am not a professional, but have done a lot of concrete. Maybe very advanced amateur.

1) When framing, I may have knocked free some dirt into the footer and created a gap or two between the ground and frame. Is the best way to block these gaps by placing a 2x4 over it? Or is there a better way?

Not quite sure what the frame you mention is, but there should be no wood left embedded in the concrete after the pour. This will almost always crack the concrete around the wood.

If the "frame" here is the form for the concrete either a piece of plywood, or two or 3 thicknesses of 30 lb roofing felt can easily cover minor holes in the forms and prevent concrete from leaking out. Anything on the inside of the form will show up in the concrete.

2) How should the rebar be placed? If I follow the slope of the hill, the rebar will be a few inches from the top of the pad at the top of the hill but more than 10" from the top of the pad at the bottom of the hill. Do I make a second layer of rebar?

Yes. The best way to handle this is with a second layer of rebar.

The ground under the pad should be compacted and covered with gravel which should also be compacted. Do not leave any loose dirt under the pad or the footers. Support the rebar with either "dobies" or plastic rebar chairs to keep it off the ground. A dobie is a small pre-made concrete cube with some tie wire pushed into the top to hold it against the rebar. They come in various sizes, with 3" being the correct on for holding the bar up from dirt.

I have never had a good experience with the 6"x6"x10 ga wire mesh reinforcement. It always ends up on the bottom of the pour, adding no strength. I have demolished dozens on slabs and the wire mesh reinforced ones have all had the mesh on the bottom where it does no good at all.

 

[4310]

I too would go with the wire mesh instead of rebar, and for a pad that small, probably would not even bother with that. I've always used the fiber reinforced concrete. But I order concrete by the truckload so I don't know if it's available if you mix it yourself. It might be,just never checked. Anyway, once it dries you can't see the fibers so don't worry about having a fuzzy looking slab of concrete.

As for the gaps at the bottom you can basically use anything that you want to keep it from creeping out from under the forms. I've used boards, rocks and dirt. It's not going to leak out like water, concrete is pretty thick.

 

[Egon]

Rebar with two inches of concrete cover.

Covering the holes may depend on size and concrete slump. Piling dirt on the outside usually works.

 

[michael.white]

The rebar in the footers will be held in place by 6x10 stirrups.

I am not sure of some of your terminology here. What is a 6x10 stirrup? I am not a professional, but have done a lot of concrete. Maybe very advanced amateur.

You've got a lot more experience than me if you've done at least one - I've never poured a pad before. The stirrups are U-shaped rebar with curled ends. These hang down into the footer to support the rebar in the footer.

1) When framing, I may have knocked free some dirt into the footer and created a gap or two between the ground and frame. Is the best way to block these gaps by placing a 2x4 over it? Or is there a better way?

Not quite sure what the frame you mention is, but there should be no wood left embedded in the concrete after the pour. This will almost always crack the concrete around the wood.

If the "frame" here is the form for the concrete either a piece of plywood, or two or 3 thicknesses of 30 lb roofing felt can easily cover minor holes in the forms and prevent concrete from leaking out. Anything on the inside of the form will show up in the concrete.

Yes, I meant the form - sorry. The holes are below ground level, so I'll just screw some pieces of wood. Thanks.

2) How should the rebar be placed? If I follow the slope of the hill, the rebar will be a few inches from the top of the pad at the top of the hill but more than 10" from the top of the pad at the bottom of the hill. Do I make a second layer of rebar?

Yes. The best way to handle this is with a second layer of rebar.

The ground under the pad should be compacted and covered with gravel which should also be compacted. Do not leave any loose dirt under the pad or the footers. Support the rebar with either "dobies" or plastic rebar chairs to keep it off the ground. A dobie is a small pre-made concrete cube with some tie wire pushed into the top to hold it against the rebar. They come in various sizes, with 3" being the correct on for holding the bar up from dirt.

I have never had a good experience with the 6"x6"x10 ga wire mesh reinforcement. It always ends up on the bottom of the pour, adding no strength. I have demolished dozens on slabs and the wire mesh reinforced ones have all had the mesh on the bottom where it does no good at all.

OK, thanks for the confirmation of a second layer of rebar.

The ground is already compacted as it's undisturbed dirt. And since the dirt was undisturbed, my understanding is that gravel is not needed.

I was just planning on using some rocks to hold it off the ground instead of dobies. Bad idea? Or just easier to use the dobies?

 

[michael.white]

How big is this generator? Seems like a little overkill. As long as the ground slopes up to your pad at no greater slope than 2 to 1 (run to rise) you would be fine. Second the rebar in the slab could be replaced with 6"*6" welded wire mesh. Place the mesh about 1/3 up from the bottom of the slab. If your still worried get fiber reinforced concrete. Don't forget to put some slope on the slab to create positive drainage, so water doesn't puddle on the slab. Shim the generator as required.

It's this one here from Central Maine Diesel: Isuzu 21 kW Diesel Generator with Sound Enclosure

The generator is about 3' x 5'. 21 kW and about 1500 lbs when full of fuel. And you're right - probably overkill.

I'll probably stick with rebar - it's already sitting the driveway.

Got the slope in the form - slight off centered bubble on the level, sloping down hill.

 

[michael.white]

Rebar with two inches of concrete cover.

Covering the holes may depend on size and concrete slump. Piling dirt on the outside usually works.

Another level of rebar then - thanks.

I'll probably jam some rocks in the holes then, and stuff some felt around them per CurlyDave. Lots of rocks available from another project. Too many projects....

 

[michael.white]

I too would go with the wire mesh instead of rebar, and for a pad that small, probably would not even bother with that. I've always used the fiber reinforced concrete. But I order concrete by the truckload so I don't know if it's available if you mix it yourself. It might be,just never checked. Anyway, once it dries you can't see the fibers so don't worry about having a fuzzy looking slab of concrete.

As for the gaps at the bottom you can basically use anything that you want to keep it from creeping out from under the forms. I've used boards, rocks and dirt. It's not going to leak out like water, concrete is pretty thick.

I've already got the rebar sitting in the driveway, so I'm going to stick with rebar.

Glad to get a lot of confirmation about stuffing material around the holes - thanks!

 

[/pine]

To pour on a slope you will need a high slump mixture...so as long as any gaps are below finish grade don't worry about them...with a high slump the gaps will plug themselves...you won't be wasting a bunch of concrete etc...!