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Extending a pad - how to?

I currently have a 10x8 pad which my old shed sat on. I'm looking to replace the shed with a 12x8 and want to extend the pad by two feet. What size rebar should I use, how far do I need to drill into the existing pad and how far apart should they be spaced? Also what kind of concrete or cement should I use and admixes? 4 or 6 inch deep? it will be an A-frame wood shed. Hairline cracking where they meet arent a concern since it will be covered but shifting would be. Thanks

A. Spruce
Re: Extending a pad - how to?

1/2" rebar, or even bolts would suffice to keep the slaps from shifting. Stay back 6-8" from the edges and place 2 feet, or so, apart.

Re: Extending a pad - how to?

#4 rebar will be fine. I'd drill as deep as the bit allows in the center of the existing slab about 4"-6" in. If you want, that can run around the perimeter of the new slab (better). Doing similarly 6" more toward center (best). Across the middle, drill the existing slab every 12" or so. Now the fun- blow the hokes clean and for all but perimeter rebar epoxy in rebar stubs cut twice the depth of the holes. Bend and test-fit your perimeter rebar before epoxying those in. You can buy epoxy in caulking-sized tubes that mix the two components in the nozzle as you use it. Do one stub to get the right amount, you want some squish-out with the rebar fully in to know it's going to be solid. See the epoxy label for curing time needed. Given good soil a 4" slab is OK for utility purposes. Better is a thicker perimeter, say 6"-8" about a foot wide. Best is 6"+ thickness throughout.

At the most, I'd run one full perimeter bar since with this narrow of a pour the second bar usually run won't add much to strength. Also with a pour this small and the summer heat I'd cover the finished pour with plastic and leave it 3 days, more is better. Make sure the plastic laps the existing slab a couple feet and weight all edges to seal it to the ground (dirt does well for this). Give it a week minimum from pouring to building and treat the concrete gently, especially the corners. Two weeks is better.

The real key to success here is what's under the slab. I'd compact the soil with a 4X4 post till it firms up and resists further compaction, then I'd lay poly sheeting under the pour. The joint won't be waterproof, but running a bead of silicone caulking across the existing slab above the rebar and letting it cure will help. Be sure to 'juke' the new pour well with a shovel there to avoid air pockets but don't hit the silicone bead. Tap the edge forms with your hammer for the same reason. Lots of medium taps, not a few big ones. Float and finish as usual.

You might get by without the rebar or the stubs but you might not. They're more insurance that nothing ever goes wrong as they are strength with this small of a slab. If you can't do rebar just do the rest with a 6" pour and chances are you'll be fine.


Re: Extending a pad - how to?

So the two edge rebars should be bent into an "L" shape and tied together?

What is the purpose of the plastic poly under the pour?

The plastic sheet d****d over the pour, is it touching the new concrete? How soon after finish should this sheet be laid on? Should I be wetting it everyday?

A shed is going over the pad so the joint doesn't need to be water proof. But should I try to make it waterproof so in the winter it wont split the crack more?

Will I be using concrete or cement also what kind of additives will I need?

Thanks for the responses!

A. Spruce
Re: Extending a pad - how to?

Personally, I think MC's rebar suggestions are a little overkill for this project, will they result in a stronger connection and slab, absolutely, but is it necessary for a shed, probably not.

The plastic under the slab is to help prevent moisture intrusion.

The plastic over the pour is to slow dehydration of the cement, resulting in a stronger cure. It will be fine to put the plastic in direct contact with the new pour, once it has "dried to the touch", meaning that it's not a soupy mess anymore. Basically, you've troweled it to the extent you want and have put a brush finish over it (non-skid ). It will be soft to the touch, but won't leave cement residue on your finger when you do so. Covering will only be necessary if it is hot where you are and the slab is in direct sun. As for wetting the pour, again, depends on the temperature, wetting (and the plastic ) are done to slow the drying process, which prevents spalling and cracking of the surface.

As long as you are using redi-mix (U-cart/truck delivery ) or bags of structural cement mix (not post mix ), you don't need to add anything to the concrete or the joint. Adding latex to the mix aids in the finishing, but since this is a woodshed, you don't need do any more than screed it off, knock down the big rocks with a trowel, then put a brush finish over it for traction.

Re: Extending a pad - how to?

So this is what I got done so far today. I was going to bend the bar but I had enough on my plate today... lol Now I see the poly only comes in huge rolls.... Can I just put two 4mil garbage bags down? The original pad is only 4 inches thick and some parts of the edge are only 3 1/2. I drilled, cleaned holes, epoxied and hammered in the bars. Cracked one hole in the middle....:rolleyes: I dug to 4 inches and compacted down with water and it came out to 6 inches thick. The dirt I took out was full of roots and didnt pack well so I'm just going to pour the 6, I think it will be easier mixing a few more bags than getting that dirt packed. Will be putting some anchors down to keep the frame in place before the pour. Thanks for all your help guys.

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