Pouring a new slab around and over VERY OLD slab.

Old 01-18-05, 02:31 PM
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Pouring a new slab around and over VERY OLD slab.

Hey Folks,

This is my first post. I'm building a shed/workshop in my backyard.

I have a 12'x18' 4" slab that was poured nearly 60 years ago. It has brick piers set every 4 feet around the perimeter. I am planning to pour around and over this slab to make a new 16'x24' slab with 12"x16" footers. There was a shed on it and it had NOTHING anchoring it to the slab! The old shed has since rotted and fallen down.

How deep should I pour over the existing slab?

What prep should be done to the existing slab for the new slab to adhere to it?

I live in the Deep South, so cold weather is not a problem.

Also, (If this makes sense) one footer of the new slab is going to be up against the old slab. In other words, on one side, the new form is going to be exactly 12" from the old slab.

I have plans if anyone would like to look. I can fax or email.

Old 01-18-05, 03:08 PM
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The two slabs will need to be tied together to try to keep the combined slabs stable. They will tend to move independently of each other.

All in all, it would really be better to remove the old slab and make a fresh monolithic pour. 1. You have a 60 year old slab of unknown construction: what the composition of the concrete is, what kind of base is underneath it, no vapor barrier. 2. Piecing together is not as satisfactory as a monolithic pour. 3. If problems develop later, any correction will be costly.

I imagine that the cost of building a building on this slab far exceeds the cost of the slab. I am having a 24 x 28 slab poured for $2900 this week for my new shop. If the combined slab shifts after construction, your building could suffer structurally.

Most of the time a slab would be 4 inches, so you would pour the same 4 inches over the old slab as you would install on a new base. You would have to pour more than 4 inches for the rest to come to the same level, since the base will likely be lower than that of the old slab. So this is already more concrete than an new slab.

I don't see an upside to pouring over the old slab.
Old 01-21-05, 08:09 PM
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Smile New over old....

With all due respects to "chfite"....the last thing you want to do is tie the old slab and the new slab together since, as he said, the slabs will move (read expand/contract) independantly. I have found that a thin layer of sand on top of the old slab prior to pouring is recommended. Since this slab is only going to be used for storage/shop and not vehicle parking, 1.5" of concrete over the old slab should suffice with the remainder of the slab at 3.5" thick(4"....as in lumber....2"x4" on edge) and 12'' x 16" footers.

"chfite" is right in that it would be "better to remove the old slab and make a fresh monolithic pour", but in your application you can save some money and labor by pouring over the old slab, which I can appreciate after over 35 years in the concrete trade.
Old 01-22-05, 12:29 PM
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concrete slab

ive just had to do the exact same thing. but being a bricklayer i dug a strip footing around the old concrete base , took brickwork up100mm above old base, painted the old base with a strong adhesive for a key. poured new concrete to top of brick. job done
Old 02-25-05, 07:43 AM
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Thanks so much for the info guys!

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