It is 10x16, going over an existing slab, going to be about 5” thick. Going against a brick wall - best solution is an expansion joint against the wall? Then tar and flashing to keep water out?

Purpose is to dress up and strengthen old slab that has deteriorated.

A friend mentioned an adhesive that can be applied to the old to help the new adhere - educate me on this -

Thanks

This message has been edited. Last edited by: Chris42,

Is the existing concrete pad structurally OK and the purpose of the new pad is just cosmetic?

If that is the case there are companies that can refinish the existing pad and that may be cheaper than pouring a whole new pad on top of the old pad.

Regardless back to your questions. I've always used concrete glue painted on the old when covering with new. Don't let the glue dry before you pour the new. Although probably overkill, we also added concrete glue to the new concrete mix as well.

Definitely add an expansion joint against the brick wall and you'll probably need another expansion joint in the 16' long run as well.

if the slab is deteriorated surface only, refinishing may be all that is needed.

if the concrete is cracked, in-level or breaking into pieces, your over-pour is only a temporary fix. if this is a driveway or parking area heavy traffic would make the slab cracks reflect through the new slab, unless reinforcing is added to strengthen the slab. the cause of the cracking, is a poor subgrade, water intrusion below the slab. this would warrant removal of the existing slab, any wet material and replace with a drainable fill. then re-pour.

expansion joints at the wall is good, but if the other 3 sides are free, it may not be necessary.

there are better sealants than tar. get some polyurethane caulk and seal the joint at the wall. if the expansion joint is below the surface more than a 1/2", use a backer rod to keep the caulk to a minimum thickness. it will adhere better and remain flexible longer.

john

Chris,

Placing a new slab on top of an existing slab as you have described is a NO, NO. What you are doing is creating a bonded overlay. Whatever damage, as in cracking that is exhibited on your existing slab, WILL be transferred through your new slab. If you insist on doing this, you really need some sort of bond breaker or rubblize the existing slab. Typically for paving projects, a few inches of aggregate is used as the bond breaker. The point is, the slabs must be able to move independently.

Honestly, if you want to do it right, demolish the old slab, level up the existing grade with about 3-4" of aggregate, compacting it very well, then place your slab.

quote:

Originally posted by jcsabolt2:
Chris,

Placing a new slab on top of an existing slab as you have described is a NO, NO. What you are doing is creating a bonded overlay. Whatever damage, as in cracking that is exhibited on your existing slab, WILL be transferred through your new slab. If you insist on doing this, you really need some sort of bond breaker or rubblize the existing slab. Typically for paving projects, a few inches of aggregate is used as the bond breaker. The point is, the slabs must be able to move independently.

Honestly, if you want to do it right, demolish the old slab, level up the existing grade with about 3-4" of aggregate, compacting it very well, then place your slab.

Yup. On asphalt, a geotextile fabric layer (petromat) is used to prevent cracks in the base layer from reflecting up. I’d guess it would take a lot more for concrete since it is more rigid.

My folks had a THICK pad poured in to the corner of an L-shaped house. It was pitched into the house and water forever spewed into the basement, through a big crack pushed inward at the L. My thinking is that the slab dropped and pushed the foundation inward.

More weight would have made their issue worse. I jacked out the slab, regraded the earth with a nice clay soil, tamped it to pitch away, and my brother built a deck in the same place. No more water in the basement.

The existing slab is about 5” thick, has rebar in it and is a cover for what was a coal bin. It will never get vehicle traffic, only occasional foot traffic. Two sides are not enclosed by any structure. One 16’ side is along the brick wall and an adjacent side is against 4’ of cast concrete steps.

Sounds like the adhesive and expansion joints on two sides. Polyurethane caulk is possible instead of expansion joint material?

Original slab is supported below with three temporary posts for the pour. Originally, for decades, it has only been supported around the edges.