Additional cement pour over existing concrete.

[machmeter62]

I need to add more depth to a 25 year old concrete slab, but the slab will join over another slab, which restricts me to a depth of 2-1/2" or slightly >. I pressure washed the existing slab, and the question is: how to add additional strength to the pour other than by adding additional cement ("six-sack mix"), or some other method?... I think it is too shallow for any rebar or wire? Also any thoughts on these fiber-glass additives, etc. to boost the strength and reduce cracking.

The entire job will be about 1/4 yd., and the pour is on a slab that has no cracks or settlement. The pour will have some occasional vehicle weight... after it cures. Thanks in advance.

 

[buck12]

I don't have any real advice but I had a contractor do something similar at my last house. The house was 40 years old and I had a 25' deep carport added onto to make a 32' deep garage. The contractor formed up 4.5" over the existing slap and used fiber reinforced concrete. I owned that house for another 8 years and never had any trouble with the garage floor. Take this for what it is worth.

 

[MarkV]

I would talk to a concrete supplier and see what they advise. Pours that thin don't have a good track record normally.

MarkV

 

[wildwillie]

If you are only going on top of a old pad it will be fine so long as there is no cracks in old pad. If you are extending the pad with a 2.5'' over lap it will crack here, steel will only keep it from shifting I'd use welded wire if this is the case. Fiber glass in the mix will help but its hard to get a good finish and you end up with some fiber standing up . No bare feet foa a while.

 

[Stimw]

IIRC they sell a bonding agent that you can put on the old CC so the new can stick. I wouldn't go less than 2 1/2" though.

 

[dodge man]

I work for an engineering company, and for some city streets, we will design a system called "white topping". You actually pour the concrete over existing asphalt at about the depth you are talking about. The trick is to make alot more saw cuts than normal. I would think if you have an existing slap that is in good shape, it will work out fine, and you wouldn't have to go through the trouble of all the extra saw cuts. I might try to line the saw cuts up with any existing saw cuts. As someone else suggested, any place you widen the slap, it will crack and may sperate in a vertical manner leaving a bump between slabs. You can use dowel bars to keep this from happening. The fibers in it work really well, and we spec it on city streets where we white top. It works good for a broom finish, but not for a smooth finish, because it ends up pretty rough.

 

[/pine]

IIRC they sell a bonding agent that you can put on the old CC so the new can stick. I wouldn't go less than 2 1/2" though.

Agree, you can get concrete bonding agent from Home Depot...if you use ready-mix they may have it too...

 

[yooperdave]

Use a bonding agent. You may ALSO want to roughen up the top of the old concrete.

If the 2 slabs bond, the final product will work in a composite manner.

Good Luck,

Yooper Dave

 

[herd]

I have poured a lot of concrete and really don't like the fiber, I have been used welded wire for the past few years with a lot of luck. If you're only pouring 2.5'', you may want to consider pea gravel for your aggregrate. Unless you pour strainght lines, you will have cracks anywhere you change directions. Saw cuts work good but I prefer the keyways, just make sure you backpack them good.

You might also want to go to either a 4000# or even 5000# pour, it's not that more expensive Redi-mix.

 

[MikeD74T]

Concrete is routinely poured at this thickness over precast planks. Wet the old concrete & scrub in a paste ( use a push broom/brush )of portland cement/water as a bonding agent. If you use reinforcing wire get sheets vs rolls as it lays down better. Use a peastone mix. Mark or cut a relief joint where the thickness changes. MikeD74T