Questions about adding onto a concrete slab.

[ben2go]

I have an existing 9.5' by 10' concrete slab.It's very stable with no cracks.My father poured this slab in 1982.It's on a bit of a mound.The ground around it was graded down 28 years ago,after the septic system leech fields were redone.My plan is to put up a 16' wide by 12' deep pole shed.I think it would be best to get the shed poles and roof up before forming up the slab.Where I am getting confused,is connecting the old slab to the new.The ground is very stable, and frost heave is non existent here.I will manually take the grass out,tamp the soil,and lay in a base layer of sand,or possibly crush run with vapor barrier over it.The old slab was done on crush run, with vapor barrier over it,except it was set into the ground in forms.Now the old slab is almost at ground level.The grass and weeds has built up the ground around the slab a little.I think mesh will work well,but I was reading, and one site recommended skipping the mesh and going with fibers instead,1lbs fibers per cubic yard of concrete.The current slab is 4 inches thick with wire mesh in it like weld wire fencing.

The shed will be used for repairing motorcycles and lawn/garden equipment.

How would you go about connecting the new and old slab, so the new slab doesn't  settle lower than the old slab,or vice versa?I don't think the old slab will settle any more.

This is a bad pic of the slab,but this is the front of it.

[rick91351]

What I would do I think would be drill the edge or edges where you are going to pour the new slab.  This would require a impact drill i.e. Hilti drill - or Bosh drill.  You can rent them.....  Or you might have impact drill sitting around some where.  But my experience is use a Hilti or Bosh concrete drill - so much more easy. You might have to buy the bit.  For what you are doing I would use a half inch bit.  I would drill in to the slab edge every couple feet.  Then slide two foot piece of #4 rebar......   

Or I would feel pretty confident if you wanted to just pour the new slab.  Especially if this slab is not going to be heated other than when you are working in there om a project.

[flyingvan]

You could dig a deep perimeter around the existing slab and extend it to surround your new slab, so the deeper footing of the new perimeter would encompass the old and the new

[akwoodchuck]

I would probably just jackhammer it out and pour a new one-piece slab...that one's  done its  time. You're  not looking at much more than a yard of concrete to replace it...

[tommytebco]

It's a shed, for g's sake!!
I was faced with the same situation 10 years ago. I put up my roof (4x4 posts with beams at the front, center and rear. The roof was aluminum interlocking panels. Then I dug out the uncemented area to 6 inches below the top of the slab and poured the cement. There is no tie in with the existing slab.
This crude approach is still standing a decade later with no problems.

(If someone will explain how to add a picture I have a few [img][img] doesn't compute to me.)

[hpinson]

To add an image, upload the image to a photo hosting service like http://imgur.com/ (there are many others that you can use).

Copy the full URL of the image - for example http://i.imgur.com/b19u4QX.jpg   You can usually get this by right clicking on your uploaded image and copying the URL.

Then paste the full URL in between the image tags like this:

[ img ]http://full-path-to/img.jpg[ /img ] (removing the spaces)  Notice the forward slash on the closing img tag.

Which shows the image like this:

[Don_P]

Or another, it looks like a good apron or side shed if possible.

[tommytebco]

Thanks Don.
Picture 1 is during construction. You can see the shed I built over in the left rear.

The second picture is all I have and not very good, but you can see the side door and one 8 foot door.

One downside is the height. The back half roof was a lean-to covering the rusted and leaking shed and firewood storage. It really should have been a foot higher.

I used 8 foot 4x4 for the front and rear posts and 12 foot for the center. Dug to about three feet deep and braced posts and poured a bag of cement mix in the hole (sandy soil here in Florida).

After the roof was up, I dug out and poured the concrete floor and then framed  the walls and applied galvanized steel roofing panels for siding.

[ben2go]

Thanks for the ideas.It will be unheated.I may use my kerosene heater in there during the winter,but I'm not to sure about that.I have one of those tiny Omni 15 heaters.

I want to keep this slab.It's strong and solid.It's also the only project I worked on with my father before he passed away, when I was a kid.I don't think I helped to much tho.I got to pull the plastic over the gravel,help hold wood while building the forms,and tear out the forms after the concrete set.

[Redoverfarm]

Unless I am missing something it appears that you will have to extend it in two directions to make it the size of your plans.  I am in agreement with Rick.  I would "pin" it with stubs of rebar.  Not really talking about that many.  Probably 4-5 on that width allowing the pins to extend 6" or so into the new pour area.  More importantly the new area should be excavated to remove the top soil and get to solid ground.  Top soil is too loose to support the weight of the new concrete and will most likely cause the slab to settle.  I would follow up by stone something in the neighborhood of #57.  Then use a whacker to tamp it tight with more attention to the joint area.  Add some rebar grid, wire or livestock panels tied to the stubbed rebar for reinforcement.  Form and pour.

[rick91351]

Unless I am missing something it appears that you will have to extend it in two directions to make it the size of your plans.  I am in agreement with Rick.  I would "pin" it with stubs of rebar.  Not really talking about that many.  Probably 4-5 on that width allowing the pins to extend 6" or so into the new pour area.  More importantly the new area should be excavated to remove the top soil and get to solid ground.  Top soil is too loose to support the weight of the new concrete and will most likely cause the slab to settle.  I would follow up by stone something in the neighborhood of #57.  Then use a whacker to tamp it tight with more attention to the joint area.  Add some rebar grid, wire or livestock panels tied to the stubbed rebar for reinforcement.  Form and pour.

Like John says get a firm foundation to pour on.  Myself I would use what ever the local pit suggests.  Around here (down in the valley) I like or use one something like three quarter road mix, or a product that is referred to as reject sand.  It has too much iron pyrite for mason sand so it was not rescreened and is fairly course and will tamp vary tight.  Both are fairly cheap and have a great compaction value.  If dry take a garden hose and do not over water just kind'a spray the top down.  Number one mistake people down is getting it too wet and then the compactor starts making a mud hole.   Up here in the mountains where we moved off to it is decomposed granite hands down.  Absolutely the best stuff hands down to build roads out of or use for compaction materials around here and is free or very cheep.  I have a borrow site right on the property we have used a crazy amount out of that little pit. If using rebar or concrete wire or the much more likeable panels when you pour reach down and give them a little tug to get them up off the ground or use stools or chairs for the panels to sit on.       

[ben2go]

Unless I am missing something it appears that you will have to extend it in two directions to make it the size of your plans.  I am in agreement with Rick.  I would "pin" it with stubs of rebar.  Not really talking about that many.  Probably 4-5 on that width allowing the pins to extend 6" or so into the new pour area.  More importantly the new area should be excavated to remove the top soil and get to solid ground.  Top soil is too loose to support the weight of the new concrete and will most likely cause the slab to settle.  I would follow up by stone something in the neighborhood of #57.  Then use a whacker to tamp it tight with more attention to the joint area.  Add some rebar grid, wire or livestock panels tied to the stubbed rebar for reinforcement.  Form and pour.

Like John says get a firm foundation to pour on.  Myself I would use what ever the local pit suggests.  Around here (down in the valley) I like or use one something like three quarter road mix, or a product that is referred to as reject sand.  It has too much iron pyrite for mason sand so it was not rescreened and is fairly course and will tamp vary tight.  Both are fairly cheap and have a great compaction value.  If dry take a garden hose and do not over water just kind'a spray the top down.  Number one mistake people down is getting it too wet and then the compactor starts making a mud hole.   Up here in the mountains where we moved off to it is decomposed granite hands down.  Absolutely the best stuff hands down to build roads out of or use for compaction materials around here and is free or very cheep.  I have a borrow site right on the property we have used a crazy amount out of that little pit. If using rebar or concrete wire or the much more likeable panels when you pour reach down and give them a little tug to get them up off the ground or use stools or chairs for the panels to sit on.       

I will be excavating 6 inches down by hand.Pinning with 3/8 rebar on 12 inch centers?Yes,I'll be pouring in two directions,rear and to the left.In the front,I plan for a concrete ramp.I have seen sand,pea gravel,and 3/4 gravel used under slabs.Crush run used to be norm,but things around here are starting to catch up with the times.I'll ask around with some of the concrete finishers.How do you feel about hand tamping?Weld wire fence,similar to live stock panels here,was used in the original slab.I was thinking of doing the same thing.My father used hand size stones to space the weld wire re-enforcement.I think I'll go with the stand offs I have seen at my local home improvement stores.I think these are the same as stools or chairs for setting rebar.They are open and allow the concrete to flow around, and through, them.I think I am getting a solid plan together.I'll be putting up the poles,framing,and roof before pouring the slab.I'll be doing as much as possible by hand and by myself.

[rick91351]

 

[Redoverfarm]

I will be excavating 6 inches down by hand.Pinning with 3/8 rebar on 12 inch centers?Yes,I'll be pouring in two directions,rear and to the left.In the front,I plan for a concrete ramp.I have seen sand,pea gravel,and 3/4 gravel used under slabs.Crush run used to be norm,but things around here are starting to catch up with the times.I'll ask around with some of the concrete finishers.How do you feel about hand tamping?Weld wire fence,similar to live stock panels here,was used in the original slab.I was thinking of doing the same thing.My father used hand size stones to space the weld wire re-enforcement.I think I'll go with the stand offs I have seen at my local home improvement stores.I think these are the same as stools or chairs for setting rebar.They are open and allow the concrete to flow around, and through, them.I think I am getting a solid plan together.I'll be putting up the poles,framing,and roof before pouring the slab.I'll be doing as much as possible by hand and by myself.

You might want to go ahead and excavate for the slab before erecting the building.  Makes it a lot easier maneuvering without obstacles in the way.  Just a thought. 

[JRR]

Because you are building on "good earth", I doubt if the new concrete, after curing, will shift far away from the original slab to any noticeable degree.  I don't think it is necessary to tie the new to the old with re-bar or anything else.  If you want to accent the original slab, you might consider wrapping it in expansion-joint material before pouring the new.  Just a thought.  (http://www.homedepot.com/p/Reflectix-4-in-x-50-ft-Expansion-Joint-for-Concrete-EXP04050/203151902).

[ben2go]

You might want to go ahead and excavate for the slab before erecting the building.  Makes it a lot easier maneuvering without obstacles in the way.  Just a thought. 

Oh yes, of course.

[ben2go]

Because you are building on "good earth", I doubt if the new concrete, after curing, will shift far away from the original slab to any noticeable degree.  I don't think it is necessary to tie the new to the old with re-bar or anything else.  If you want to accent the original slab, you might consider wrapping it in expansion-joint material before pouring the new.  Just a thought.  (http://www.homedepot.com/p/Reflectix-4-in-x-50-ft-Expansion-Joint-for-Concrete-EXP04050/203151902).

 

I would feel better if the two were pinned together.

[Redoverfarm]

You might want to go ahead and excavate for the slab before erecting the building.  Makes it a lot easier maneuvering without obstacles in the way.  Just a thought. 

The Pins are mainly to tie the two slabs together and will help prevent the new from settling as well.  There will not be a large amount of weight so if it is pinned every 12" and tied into the new pour it should help.  With the added re-enforcement if the ground would settle the slap should stay in place.

[ben2go]

The Pins are mainly to tie the two slabs together and will help prevent the new from settling as well.  There will not be a large amount of weight so if it is pinned every 12" and tied into the new pour it should help.  With the added re-enforcement if the ground would settle the slap should stay in place.

[glenn kangiser]

I poured part of my garage 13 years ago or so and added to it the next year with no problems.  I did leave 6 6 10 10 remesh out of the old one as I recall.  No problems.

[Dave Sparks]

Hey Glen !  I did get your voicemail.  Been swamped and will be into May.  Thanks and I did what you mentioned at my Dad's old house and the cement is still is good!