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...!

 

[michael.white]

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...!

I had to look that one up: Concrete slump test - Wikipedia, the free encyclopedia

So do you mean a drier mix or wetter mix?

 

[JimRB]

Wet will flow more than dry. If you get it too wet then you end up with weaker concrete. If you watch commercial pours they use vibrators to get the air out of pours. You do a Google search of concrete vibrators and youtube to see some videos. Wacker Neuson Plug and Play External Vibration - YouTube

 

[/pine]

I had to look that one up: Concrete slump test - Wikipedia, the free encyclopedia

So do you mean a drier mix or wetter mix?

Sorry I may have misstated the terms (high/low slump)...

If your mix is too wet gravity will keep pulling it down the hill... a stiffer mix is harder to work but will stay in place...

Also a stiffer mix will tend to leave cavities and honeycombing...so it is important to be judicious until all the rebar is encapsulated...if all your re-enforcing bars are tied together banging (no so hard to displace them) them with a hammer etc. will help...a little honeycombing along the form boards should not be an issue...

 

[GPintheMitten]

Vibrators? Slump? Sounds like a lot of overkill to me.

The generator weighs 1500 pounds not 15,000.

 

[/pine]

Vibrators? Slump? Sounds like a lot of overkill to me.

The generator weighs 1500 pounds not 15,000.

If you're not familiar the word "slump" is a way of describing the viscosity of a concrete mix...it does not necessarily mean getting a test cone and and ASTM manual...!

As for "vibrating"...I suggested using a hammer...!

 

[Western]

Keep your slab damp for a few days or a week, the slower it cures, the stronger it will be. A 6 slump is pretty thin, used allot to fill block and foam walls. 4 is common and likely what your supplier uses standard. Just echoing what Pine said. Small pour, so good size to learn and fairly easy.

I would put the re-bar at 14-16" oc both ways.

 

[CurlyDave]

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?

The dobies are not very expensive, and the tie wire in them is very helpful. The one time I used rocks, when the fresh concrete was pumped into the forms, I watched the concrete push the rocks out from under the rebar. Fortunately, it was only a 4" slab, and we had a bountiful supply of rocks. I could pick up the rebar and place a fresh rock under it at many places. The original rocks are still under the concrete, just in the wrong places.

The gravel will help keep water drained under the slab.

 

[EddieWalker]

Wire is the worse thing in the world you can use for concrete pads because it is impossible to walk on it when spreading the mud and keep it in the middle of the mud. Like Dave said, it always ends up at the bottom of the pad accomplishing nothing. I've seen this 100 percent of the time in every slap that I have opened up or demolished. It's a nice theory, but totally impossible to accomplish. The little game of saying they pull up the concrete as the work on it is a compete lie. It doesn't work and the only times you will see a crew doing that is when they first start working the mud for show, once the mud gets to flowing, there isn't any time for that little game, and it gets pushed to the bottom.

You didn't say what sized rebar you are using, but since this is such a small pad, number 3, which is also called 3/8's rebar is more then big enough. I cut lengths of it about four to six inches longer then the depth of your footings and hammer them into the dirt straight up and down every two feet all the way around so that the top of the rebar is about an inch higher then you want your rebar going across the pad. Then I tie one stick of rebar about four inches or so off the bottom of the footing onto those vertical pieces of rebar. Then I do it again with another stick at the level you want the rebar going across your pad. Then I lay out my grid with sticks every two feet from footing to footing and I put chairs under it to keep it off the ground. You don't have to buy the chairs for something this small. A rock works well, or you can even use short lengths of rebar hammered into the ground like you do in the footings. Just make sure not to have it higher then a couple inches below the height of your forms.

Are you mixing the concrete yourself with sacks like Quikrete from Lowes or Home Depot? I found that the 60 pound sacks are easier to handle and pound for pound are .2 cents cheaper then the 80 pound sacks. This isn't good quality concrete, but it's fine for what you are doing. If you want to make it stronger, you can buy a sack of Portland cement and add some of that to each sack you mix. Or you can buy the higher quality stuff, but I don't think that's really needed for something like this.

Mix it up as dry as you can to get the most strength out of it. It should not flow, you should have to force it where you want it once you dump it into the form. Jabbing it with a stich and tapping on the forms will help get rid of air pockets along the edge of the forms, but you will not get all of them and it really wont make a big difference strength wise as long as you get most of them and do it all the way around the forms.

Make sure you have your stakes on the outside of the form. I like to have them below my forms so I can screed over the forms without hitting the stakes. It's not that big a deal if you have the stakes higher, it just means you have to work around it. I use screws on my stakes and forms in case I need to adjust them before the pour. It is also easier to take them apart in my opinion. 2 inch sheetrock screws are great for this.

If you use too much water, it will form on the top of your pad and you will have to wait longer until you can work it smooth. There is a point where it's too wet, then it gets just right to work it. If it's drying too fast for you, add water from a bottle. If it's too wet and you have standing water, you have to wait. This might take an hour or more. If you don't wait and work it at the right time, it will be a mess. Finishing it off is an art and a big part of what you pay for when hiring a crew. Sticking with it and getting it right takes a lot of effort and elbow grease, but there isn't any other way to get it done.

Post pics of your forms before putting the rebar in and we will comment on how it looks. Then do the same after you install your rebar. Pictures are worth a million words!!!!

Eddie

 

[s219]

Are you mixing the concrete yourself with sacks like Quikrete from Lowes or Home Depot? I found that the 60 pound sacks are easier to handle and pound for pound are .2 cents cheaper then the 80 pound sacks.

I found the same thing here. Was busting my butt carrying 80# sacks down a steep hill when installing stairs to a pier this fall (it was so steep I had to rope down). Switched to the smaller sacks which made life easier, and turns out they saved me money too.

 

[michael.white]

Thanks for the advice!

I'll be mixing the concrete with a mixer I picked up for my TC-30, so I should only have to deal with the weight twice - unloading it from the trailer and dumping it into the mixer. However, I'll definitely ask on the price of the 60 lb bags.

I had a bit of time to work on it this weekend and snapped a couple of pictures.



The ground is pretty hard and dry here with a lot of caliche, and driving in 2x4s is pretty much out of the question; on the plus side, the ground doesn't crack. The T-posts seem to hold well, although I'll have to cut them off at the top in order to get the mixer in there.



I finished the first level of rebar and am adding a partial second level no higher than 3" below the top of the form. I'll probably add a bit of gravel to the bottom of the footer, as I made it a few inches deeper than I needed to.