Can You Pour New Concrete
Over Existing Concrete?

The Honest Truth About This Concrete Patio Make-Over

YES, if done correctly, you can pour new concrete right over existing concrete.

There's some things you have to take into consideration before you make your final decision though.

Is the existing concrete sound? By that I mean it's not settling, heaving, or badly cracked and crumbling.

If it's a solid piece of concrete, maybe some minor cracks and the surface is worn, ugly, or pitted, you can pour new concrete right over it.

I suggest a minimum of 2" thick so you have to be able to accommodate the new slab height whether it's a floor, patio, driveway or walkway.

If the old concrete has settled and cracked badly, or heaved and has some bad cracks, it might be a candidate for a demo and replace.

The old concrete could have some hollow areas under it and continue to settle or heave, breaking the new concrete. It's a little risky in those cases.

How to pour concrete over existing concrete

In the video below, I poured a new stamped concrete patio slab right over the old existing concrete.

The old concrete was over 20 years old. It was originally poured in multiple sections so it had some seams that looked like cracks.

The surface was badly worn and pitted, but the slab was sound and there was ample height to pour a new slab right over the old one.

This old concrete patio was a perfect candidate for pouring new concrete right over the old concrete

Pouring over the old concrete instead of having to demo and remove it saved the customer hundreds of dollars.

Another thing wrong with the old concrete was it sloped back towards the house. So by pouring a new slab over it, I could correct the slope and re-slope the new patio away from the house.

I started this project by pressure washing the old concrete to remove all the mold, dirt, and loose concrete.

I let it dry for 24 hours, then I applied my concrete bonding agent by brushing on a light coating.

My favorite product for this is Weld-Crete. It's so easy to use.

I always bond my new concrete slabs to the existing slabs whenever I over pour and the new concrete is less than three inches thick.

This patio slab was 4 inches thick by the house and 2 inches thick towards the lawn, so I decided to bond it all to the existing concrete.

If the new concrete were going to be 3 inches thick or more, I wouldn't of had to bond them both together.

I formed the perimeter with 2x4's and set them to grade using my self-leveling laser.

I like the Topcon RL-H5B, it so simple to set up and learn how to use. I just set it on the stand, press ON, and is self levels itself and starts the rotary laser.

Then I use my grade stick and the receiver to check my level and set my slopes. Very easy!

After the forms are set, I install the wire mesh and call for concrete.

I'm using a 4000 psi concrete mix with 3/8 inch pea stone because I'm stamping this slab.

This would also be a good mix for a broom finish. I like the 3/8 stone because it stamps a little easier than the 3/4 stone.

The tools I need to pour the new concrete are:

This new concrete patio was 9' x 12' and took 1.5 cubic yards of concrete.

That's more than I would have wanted to mix by hand so I just called my ready-mix supplier.

Once I poured and leveled the concrete, I bull floated it and then let it set until it was ready to stamp.

The minimum thickness I pour new concrete over existing concrete

I don't like to pour new concrete any thinner than 2 inches thick when I pour over existing concrete.

I feel like pouring new concrete that's two inches thick over old existing concrete that's solid and sound is plenty thick enough for most applications.

I'm just going by my experience having done this many times over the years. I'm not an engineer, but I do pour concrete for a living. If the sub-base is solid, there shouldn't be any issues.

If you're driving heavy equipment like dump trucks, bulldozers, excavators, or things like that, you probably want to go thicker. 

How thick is hard to say. That kind of equipment can really punish a concrete floor.

Pouring concrete over an existing concrete driveway or sidewalk

The same principles as above apply to pouring concrete over an existing concrete driveway or sidewalk.

You can if the concrete is sound (solid) with minor cracks, pitting, scaling, surface damage, or just plain ugly.

You just have to consider the added height. How is another 2, 3, or 4 inches in height going to affect door openings, stairs treads, abutting walkways or garage floor heights.

If there's extensive settling, heaving, or cracking, I would want to investigate why it did that in the first place and correct that problem before I poured new concrete over something like that.

Most, if not all of the time, if your concrete has that kind of damage, it's going to be better to demo it, install a good gravel base, and re-pour new concrete.

Will the new concrete stick to the old concrete? 

It will if you use a product called Weld-Crete. Weld-Crete is a bonding agent designed specifically to bond new concrete to old concrete.

My rule of thumb is: If your new concrete is less than 3 inches thick, then you want to make sure it sticks (or bonds) to the old concrete.

If your new concrete is 3 inches thick or more, then you don't want it to stick to the old concrete. You want it to expand and contract on its own to lesson the chances of random cracking.

The way to keep the new and old slabs from sticking to each other is to install a bond breaker like poly, typar, tyvek, roof paper, or and inch of sand in between the two slabs.

If you are bonding the two slabs together using Weld-Crete, the best way to apply it is to use a soft bristle brush and brush it into the old concrete. 

I demonstrate how to do this in the video.

In conclusion:

The simple answer to the question "Can I pour concrete over existing concrete?" is YES.

You just have to understand the limitations and the reasons why it might not be a good idea.

Here's another video showing you how I poured a new garage floor over and old existing garage floor.

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