November 3, 2023
Can you pour concrete over existing concrete?
Yes, It is feasible to properly pour concrete over existing concrete, provided that the process is carried out correctly.
Are you looking to give your existing concrete surface a fresh look or fix minor imperfections? Pouring concrete over existing concrete can be a viable option, provided it's done correctly.
This article offers valuable insights into the process, including determining the minimum required thickness for your new layer, understanding bonding restrictions, assessing the condition of your existing slab, and deciding when it's better to repair or replace the current concrete instead.
By following these guidelines, you can ensure the success and longevity of your concrete overlay project.
Assessing your old concrete is crucial to ensure the new layer's structural integrity, durability, and longevity.
Pouring new concrete over old, weak, or damaged concrete slabs can lead to premature failure or further complications in the future. Here are the most important factors to consider when deciding if it's okay to pour concrete over existing concrete:
By carefully assessing these factors, you can determine if pouring new concrete over old is the right choice for your project, or if repair or replacement would be a better option.
The minimum recommended thickness for pouring concrete over existing concrete is generally 2 inches (approximately 50 millimeters).
This thickness is crucial for several reasons:
When pouring concrete over existing concrete, it's common to use a specialized concrete mix designed for thinner pours like a 4000 psi concrete mix with 3/8 peastone.
These mixes often contain smaller aggregate sizes, higher cement content, and additives such as polymers, water-reducers, or fibers to improve bonding and performance.
For newly poured concrete slabs less than 3" thick, I recommend bonding the new concrete slab to the existing concrete slab.
To bond new concrete to old concrete, you can use a concrete bonding agent or a bonding primer. These agents are specifically formulated to enhance the adhesion between the two layers of concrete, ensuring a strong and lasting bond.
Some common bonding agents are:
Before applying a bonding agent, ensure that the existing concrete surface is clean, free of previous coatings, and properly prepared to promote adhesion. This may involve pressure washing, acid etching, or grinding the surface to create a rough texture.
Pouring new concrete over existing concrete involves several essential steps to ensure a successful project with a durable and lasting result.
Here's a step-by-step guide:
When I look at old concrete, I look for settling and heaving first. If the slab has settled, I try to determine why and if I think it's going to continue to settle more.
If the sub-base froze and heaved the concrete slab causing it to crack, why did it do that? Did the slab go back down to its original level after the sub-base unthawed, or did it remain in that position?
If I 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 have cracked concrete and the cracks are small, less than 1/4" wide, and both sides of the crack are flush, then it should be ok to pour new concrete on top of the old concrete.
Yes, in the picture above I suggest pouring a thin layer of new concrete over an old concrete patio.
Remember, concrete has aggregate in it (rocks). The smallest rock size is about 3/8 inches. 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.
The existing layer of 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 new concrete right over the old one.
I poured the new slab 4 inches thick by the house and 1.5 inches (thin) by the lawn. This allowed me to get a 1.5-inch slope on the stamped concrete.
Yes, you can pour concrete over existing painted concrete, if the new concrete is 3 inches thick or more.
When pouring concrete over concrete, if the new concrete slab 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 slab 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 do it if the concrete is sound (solid) with minor cracks, pitting, scaling, surface damage, or just plain ugly.
You just have to factor in the added height. How will another 2, 3, or 4 inches in height 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.
You can pour concrete as thin as 1/8" over old concrete slabs if you use a self-leveling concrete overlay mix. This type of concrete is specially designed for thinner pours of 1/8" - 1" thick.
If you need a level surface and your desired thickness is less than an inch, there's only one way to pour concrete correctly and that's by using a concrete overlay.
Can I pour concrete over existing concrete? Yes, pouring new concrete over existing concrete can be a viable and effective solution to rejuvenate a worn or damaged surface, provided that the process is carried out correctly.
By thoroughly assessing the existing slab, preparing the surface, using appropriate bonding agents, and following the proper steps for pouring, finishing, and curing, a successful and lasting concrete overlay can be achieved.
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