The simple answer is YES, you can pour new concrete right over existing concrete.
But, there's some things you have to take into consideration before you make a final decision.
Is the existing concrete structurally sound?
How you determine that, could be the most important part to answering that question.
When I look at old concrete, I look for settling and heaving first. If the slab settled, I try to determine why and if I think it's going to settle more.
If it heaved and cracked, why did it do that? Did the slab go back down to its original level after it heaved or did it remain in that position?
If you can't come to a conclusive answer to those questions, then it's probably a better idea to remove the old concrete and figure out why it settled or heaved, then fix that problem so it doesn't do it again.
If you just pour new concrete over it and don't fix the problem, it's highly likely the same thing will happen to the new concrete.
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 pouring a minimum of 2" over the old concrete, so you have to be able to accommodate the new slab height whether it's a floor, patio, driveway or walkway.
You can pour as thin as 1.5 inches over the old concrete.
Remember, concrete has aggregate in it (rocks). The smallest rock size is about 3/8 inch. If you pour any thinner than 1.5 inches you don't have enough cement paste over and around the aggregate to get strong concrete.
It'll end up cracking and breaking off if you pour it too thin.
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 structurally sound and there was ample height to pour a new slab right over the old one.
I had to slope the new concrete away from the house because the old concrete sloped towards the house.
I ended up pouring the new concrete 4 inches thick up by the house and 1.5 inches (thin) by the lawn. This allowed me to get a 1.5 inch slope on the new concrete away from the house.
Pouring over the old concrete instead of having to demo and remove it, ended up saving the customer hundreds of dollars.
Step 1. I started this project by pressure washing the old concrete to remove all the mold, dirt, and loose concrete.
Step 2. I let it dry for 24 hours, then I applied a concrete bonding agent by brushing on a light coating. My favorite bonding agent for this is Weld-Crete. It basically glues the two concrete slabs together.
This patio slab was 4 inches thick by the house and 1.5 - 2 inches thick towards the lawn, so I decided to bond it all to the existing concrete.
If all the new concrete was 3 inches thick or more, I wouldn't have to bond them both together.
Step 3. 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 it 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!
Step 4. 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.
Step 5. I poured and leveled the concrete, I bull floated it and then let it set until it was ready to stamp.
Yes you can, if the new concrete is 3 inches thick or more.
When pouring concrete over concrete, if the new concrete is less than 3 inches thick it's best to bond the new concrete to the old concrete.
If the concrete is painted and you're pouring less than 3 inches of new concrete over it, you have to remove the paint (pressure wash it or grind it off) then apply a bonding agent to "glue" the new concrete to the old.
If the new concrete is 3 inches thick or more, then you want them to move (expand & contract) independently from each other. This is when we use a "bond breaker" between the old and new slabs.
You can leave the paint on the concrete, lay some plastic down, and pour the new concrete over both the paint and the plastic.
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.
Newly poured concrete will stick to old concrete if you use a bonding agent called Weld-Crete.
Weld-Crete is a "glue like" 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 plastic, 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.
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 an old garage floor.