Pouring a slab around a slab,bad idea?

[fred d]

I have a 20x24 garage

Looking at tearing it down and building a 40x30 building.
Is it a bad idea to use the existing slab as part of the 40x30, or should we demo up the old slab and just pour 1 complete

Could cut the concrete cost by about 40%

 

[J.A.Varela]

Not at all. Use an expansion joint around the existing slab and go at it.

 

[buddyboy]

it's like putting an addition on all 4 sides

 

[K'ledgeBldr]

it's like putting an addition on all 4 sides

That's a really bad idea!

The best scenario is to add-on in one direction (only one control joint). But because of the size the OP is going to want, two control joints will be needed. The lessor amount of control joints lessens the chances of things going wrong.

Can it be done? Sure. But a lot of things have to be done right! Footings, compacted fill, rebar, dowels (a lot), and controlling the drying of the new slab.

 

[Cyberbear]

You are talking about a lot of variables. If funds are tight you do what you can under the circumstances, of course. But if you plan to use the new building for an extended period of time, you may wish to consider how important having a floor without cracks or expansion joints is to you. When I built my 30 x 80 shop a clear smooth floor was of prime importance for my particular usage, especially since I use my slab as a level and smooth building surface for the work I often have done.
If you plan on just using the garage for parking and some storage, perhaps piecing a slab together will work for you. In 1995 I had a monolithic slab poured requiring 16 yards of 6 bag concrete, #3 rebar at 12" o/c and used sealer to retard drying time. After all these years still no cracks. I wish you good luck on your project.

 

[bad_idea]

You called?

 

[fred d]

I really like the idea of a continuous slab. Would make for a nice clean look, and an easy sweep.

My hesitation is that it will be in the front-right corner of the slab. So, not in a inconspicuous area. But probably right in front of the door

 

[bad_idea]

Use the old slab for an apron in front of the shop or a floor for a lean-to behind the shop.

 

[fred d]

Not layer out where either is an option
Old slab will be all in one corner

 

[willymakeit]

Expansion joint and smooth dowels at edges. Smooth dowels will let it move laterally if need but elevation of both slabs remain the same. Tool the expansion joint with backer rod and self leveling caulk.
If the old slab is good, clean and seal it. Use the extra money for something else.

 

[TheEquineFencer]

When the shop I'm in now burned down, money was tight. The Pex in the old concrete was still OK. We were building a steel building back where a post steel was. Everyone wanted to cut in new footers in the old floor. This would have gone through the Pex. A new footer was engineered to go around the outside of the original pad, 2 foot wide on three sides. Worked fine for us. We just built the building a little bigger. So far it's worked for us.

 

[volleyball]

Bottom line is it is fine unless you are **** about it and have the extra cash burning a hole in your pocket. I assume the old slab is fine.

Another thought would be to pour the new concrete higher than the old slab and cover it up. If you clean and etch it properly, you could have a whole new floor.

 

[403is300]

you called?

hahahhhahahahahahah

 

[juiced10]

Just poured mine last weekend. Existing pad was 24x24 and now is 40x29. The main garage is staying the same size. Building new with attic trusses and carports on both sides. I now have footers under new slab to support upper room. I just doweled to existing slab. As was stated I am using money saved to stone the front of new garage. I am doing a full build write up soon.