In most cases the answer is YES! If the existing concrete is structurally sound, is not heaving or settling, then you can pour concrete over concrete.
My rule of thumb is this:
My name is Mike Day, I own Day's Concrete Floors, Inc. I've owned my own concrete business for 39 years. I specialize in all aspects of concrete flat work.
In the picture above, we're pouring a new concrete floor for a garage over the existing concrete floor.
The old concrete floor was very worn on the surface. You could see the aggregate in many places. Some areas had deteriorated up to a half inch deep.
This old concrete floor also had a gradual slope towards the back wall, right where you see us in the picture, so water would puddle back there whenever it rained or when water dripped off the cars.
When the customer called me to ask about what a solution to this problem would be, I recommended pouring a new concrete floor right over the old one.
The only issue was the garage door openings. He would have to move the garage doors up the thickness of the new floor for them to close properly.
The good thing was, he was installing new doors. So this problem was solved easily. If he wasn't installing new doors, the solution would have been to un-bolt each track (on the sides of the doors) and move them up so the door would close tight on the top.
In the video below, you can watch us pour this new concrete floor right over the existing concrete floor.
In most cases, I don't like to pour any new concrete less than 2 inches thick over an old, existing concrete floor, slab, patio, or driveway.
Two inches thick (or more) gives you adequate strength for a wear surface, especially if you're driving over it.
Although it is possible to pour 1.5 inches thick with a pea stone mix, I don't really recommend pouring that thin. If that's all the height you can afford to lose, it will work but you may develop more shrinkage cracking than normal because of it being so thin.
A solution for that is to saw cut some extra expansion joints to help minimize any shrinkage cracks if you're concerned about that.
And remember, if you're new concrete pour is less than 3 inches thick, you will want to BOND the new concrete to the old concrete using a bonding agent.
The bonding agent I use is called WELD-CRETE.
This stuff is very easy to use and is designed to bond new concrete to old.
You just paint it on, using a roller or a scrub brush, an hour or up to 10 days before your new pour.
When you pour your new concrete over this bonding agent, it activates it and bonds the new concrete to the old concrete.
If you don't use Weld-Crete, there's no guarantee the new concrete will stick to the old concrete.
If you want to check out the Weld-Crete data sheet and application instructions CLICK HERE.
There are cases when pouring new concrete over old just isn't a viable solution.
If you're concrete has large cracks and the slab may still be moving due to freeze and thaw conditions or settling, then a tear out and replace is a better solution.
If tree roots are causing your concrete to crack, then tear out the old concrete, resolve the tree roots problem, and pour new concrete.
If door thresholds or stair risers are an issue, pouring new concrete may cause some other problems you would have to resolve.
If you're existing concrete has a lot of cracks, but is structurally sound, isn't moving, heaving, or settling, then it's ok to pour new concrete over it.
I recommend installing a bond breaker between the old slab and the new one so the cracks won't transfer up through the new concrete.
Some of the things we have used as bond breakers are:
Using a bond breaker will keep the two slabs of concrete separate and the cracks will not mirror up through the new slab.
You'll still need to saw cut or groove expansion joints in the new slab to help control shrinkage cracks, just like you would any new concrete pour.
The thinner the new over-pour, the closer the expansion joints should be.
How close should they be?
This is just a general rule of thumb to give you an idea of how close your crack control joints should be to minimize any new cracks from developing.
After you've determined you can pour new concrete over your old concrete, here's how you prepare your old concrete.
If you watch my video above, you'll see how we installed forms for the garage door openings to stop the new concrete.
YES, you should use some kind of reinforcement in your new concrete pour.
For all my over-pours, I use fiber mesh reinforcement in the concrete. This type of reinforcement is tiny polypropylene fibers that get mixed into the concrete when the concrete is loaded onto the concrete truck.
Also, like in the video, I use wire mesh if the new pour is thick enough to allow for it.
Sometimes I'll use rebar instead of wire mesh if the new pour is thick enough and a lot of weight is being driven over the new pour.
But the bottom line is YES, use some type of reinforcement in your new concrete over-pour to help control any cracks that may develop and keep them from spreading.
In most cases it's OK to pour new concrete over existing concrete. Use my above recommendations and work experiences as a guideline to determine if your concrete is a good candidate or not.
I've personally poured hundreds of jobs where we poured over existing concrete that was worn out, cracked, pitted or sloped the wrong way.
If you have a question about your project, watch the video on YouTube and leave me a question in the comments section. I'll try to help you any way I can.
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That's us Pumping a slab!