Almost every home has some part of it that is composed of or supported by concrete. As concrete ages, it begins to wear, eventually needing replacing or refurbishment.
Many homeowners have two questions about the topic: Can you pour concrete over concrete? Is it a good idea to pour concrete over concrete?”
The answer to the first question is “yes.” The second question is a bit harder to answer and includes stipulations.
The following is a guide to pouring concrete over concrete when you can do it, when it is advisable, and when it is not something you should do.
Common Reasons To Pour Concrete Over Concrete
There are a few reasons it makes sense to lay concrete over existing concrete. In each case, the existing concrete is damaged, but not so much it poses a structural or safety issue just by existing.
Leveling Uneven or Damaged Concrete Surfaces
Leveling surfaces is the most common instance where a contractor or homeowner will pour new concrete over an existing concrete structure. As long as the existing concrete does not have significant damage, you can use concrete to address the following issues:
- Leveling out an uneven surface
- Filling divots or minor cracks
- Replacing chipped concrete
- Providing a smooth surface
Adding Thickness to an Existing Slab
Concrete can also serve as a support substance for an existing concrete slab. By adding concrete to the existing slab, you are adding thickness and strength. The tradeoff is that you are also adding weight.
Depending on what the structure is, that can have major consequences. Adding too much weight, even to a slab, can cause the concrete underneath to buckle, crumble and break apart. If that happens, your new concrete, if it remains in place, will only be a thin shell.
Resurfacing or Restoration
If the damage to the existing concrete is not one of the following exceptions, you can use concrete to resurface an existing concrete structure to restore or resurface it:
- Large cracks, holes, or divots
- Adding weight that compromises load capacity
- Exceptionally thin existing concrete
If the structure you want to resurface or restore meets this criterion, you run the risk of a cave-in.
Restoring decorative concrete is probably the safest time to add concrete to a structure. Slapping a fresh coat of concrete on a structure can completely alter its appearance and render it freshly detailed. The key to this type of project is not adding so much new concrete, the old structure crumbles.
You will often have to pour concrete over an existing slab to bring it up to the level of a new slab. You can also use concrete to bridge a seam if the seam is large.
Concrete How To Guides: Polishing Concrete or Sanding Concrete
Technical considerations for pouring concrete over existing concrete come down to X factors:
- Load strength of the existing concrete
- Weight of the new concrete
- Age of the older concrete
- Whether the existing concrete has fully cured
Once you have accounted for that, you can pour your new concrete.
Not adhering to the figures above or pouring concrete over concrete that is not fully cured can lead to a collapse of the existing concrete.
Complicating that is that you should pour at least two inches of concrete over anything but the structure you are just touching up. The two inches add significant weight to the structure underneath, ensuring it will cure hard without crumbling.
The type of concrete you use depends on the type of project you are tackling. Collecting input from professionals on the type to use is a good idea. You do not want to research it yourself, guess, and be wrong.
Further Reading: How Much Does a Yard of Concrete Weigh?
Benefits of Pouring Concrete Over Existing Structure
Adding concrete to an existing structure has at least three primary benefits.
It is much less expensive to add a layer of concrete to an existing layer than to “reinvent the wheel” by ripping out the old structure and redoing it. You save money on materials, and if you have it professionally done, a lot of money on labor. If you are DIY’ing it, your financial labor costs are less, but your actual labor costs will be much more.
Slapping on two inches of concrete and letting it dry will be much quicker than ripping out the old concrete, prepping the area for the new concrete, laying the new concrete, and then waiting for it to dry. Additionally, adding a layer of concrete avoids creating or discovering new problems, which is always the case when you rip out the old to put in the new structure.
If you have a pro do it, you still save time. The time to complete the project depends on the type of project you have, but as a rule, you should add at least two full days to any project you rip out, prep and re-lay versus just going over the old structure.
Ripping out old concrete structures and smoothing over the area you want to add new concrete is labor-intensive, difficult, and thankless work. You will also have to rent equipment to break the concrete apart, even if it is rotten and crumbling. Then there is disposing of the old concrete, which is no small task if you do it yourself.
Drawbacks of Pouring Concrete Over Existing Concrete
The largest risk of pouring a layer of concrete over an existing concrete structure is that the old structure crumbles and collapses. The chances of a collapse make it imperative that you carefully plan your project to ensure you know what you are dealing with at all times.
Another risk is not adding enough new concrete and a combination of thinness and moisture, creating a weak spot in the new concrete. That can lead to cracking and holes breaking through. The risk of a collapse is exacerbated by an external influence wearing the old concrete down. If you do not treat the problem, you risk covering it up with new concrete.
Finally, there is the prep work that goes into working with concrete. While not nearly as intensive as ripping old concrete out, new concrete requires a lot of prep, or you could have to replace the old concrete and the stuff you added.
Preparing the Existing Slab for a New Pour
To prep your existing concrete, you must first clean it thoroughly. After that, you must outline a perimeter, add forms, and then add your rebar or mesh on top of the old concrete. Once that is done, you must prime your existing layer of concrete, mix your new concrete and apply it. When the new concrete is applied, you must smooth it over and add more if needed.
Types of Concrete Mixes Suitable for Pouring Over Existing Concrete
The first rule for picking concrete is to consult with a professional. You will have choices that include, but are not limited to:
- Using lighter concrete than the old
- Concrete mixture composition
- Whether to use self-leveling concrete
- Whether to use quick-drying concrete
Importance of Consulting with a Professional
You need to discuss your project with someone that knows what they are doing. It is too complex, with too much of a downside to using the old “try it and see what happens” method. Also, if you cut corners, you will have negative consequences, including having to rip out all of it and start over.
You can pour new concrete over existing concrete if the old concrete is stable enough. Doing so will save you time, money, and labor. There are drawbacks and limitations to laying new concrete over old. If you have a project where this might work, you should consult a professional.