Can You Put New Concrete Over Old Concrete?

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    When redesigning your backyard deck or driveway, you may wonder if you can just pour a new layer of concrete on top of the old layer. You may think you know a bit about concrete and how it works, but there is a lot to consider before making this decision.

    You can put new concrete over old concrete. However, unresolved issues with your old concrete, such as cracks or frost heaves, will carry over to your new concrete if not taken care of. In addition, you must pour it at least 2 inches thick.

    Pouring new concrete, no matter the circumstances can be more of a headache than anticipated if you’re not careful. With that said, you must know what you’re doing before dealing with new concrete joining existing concrete.

    Can You Put New Concrete Over Old Concrete?

    You can put new concrete over old concrete, however, there are many times you will not want to, including:

    • If there is a door or staircase that would be in the way of adding a few inches of cement
    • Tree roots are in the way, causing the existing concrete to lift or move.
    • Substantial cracks in the concrete or it is not level

    In each of these situations, you should remove the old concrete, resolve the existing problem, such as removing the tree roots, and continue with brand new concrete.

    But, here are some instances where you can put new concrete over old concrete:

    • Stepping stones or a pathway to a house
    • Basketball court or driveway

    If you are able to pour new concrete over old concrete, you probably want to know the risks you’d be taking by doing so. Though it can be useful, everything has its pros and cons. Below are a few cons you may want to consider:

    • Maintenance: As you can imagine, layering concrete on top of concrete can eliminate the low maintenance that concrete is often admired for. For example, pouring new concrete on an existing slab brings a chance of frost heaves to the equation. So, knowing how thick a concrete slab should be, is a must for a good result.

    Frost heaves are caused by the combination of freezing temperatures and the soil defrosting below. The fluctuation of the temperatures will cause lifting of the concrete which will cause cracks and separation.

    As soon as you spot damage, make sure to patch them immediately to avoid any spreading. It is also important to seal the new layer and prevent any water damage by using a deep penetrating sealer. If all else fails, bring in a professional to see what the problem is.

    • Shorter Lifespan. Most concrete can last for many years, and in some cases, you may never have any problems with your concrete. Pouring new concrete over an existing slab can, however, cause some issues outlined in this article and limit the lifespan.

    If you were to tear old concrete out and replace it with new, well-poured concrete, you could save yourself a lot of trouble by maximizing the lifespan of your concrete. If you do pour new concrete on the old concrete, you are risking the chance of getting deep potholes. After all, the condition of the existing slab determines the condition of the new concrete.

    • Raising the Level. I mentioned how it is important to be aware of the surroundings before pouring new concrete on top of the existing concrete. You need to be aware of any doors or staircases around that may ruin your chances of adding concrete on top.

    If you were to add a few inches of concrete to a walkway leading to your driveway, you could create a tripping hazard for people that do not realize the gap. The walkway would be several inches taller than the driveway which you surely wouldn’t want. You could solve that though by creating a small ramp.

    • Bonding. Without somehow separating the old concrete from the new concrete, cracks would be guaranteed. If there are any cracks in the old concrete, they would spread into the new concrete in pretty quickly.

    It is important to add a layer of separation between the two slabs for flexibility and longevity. This layer of separation can come in the form of stones, sand, or even plastic. However, plastic isn’t recommended for anyone trying to protect the planet.

    How Thin Can You Pour Concrete Over Existing Concrete?

    The thinnest that is usually used when pouring concrete, in general, is about 2 to 2 ½ inches. That also applies to this pouring new concrete over old concrete.

    Any thinner than 2 inches would be too thin. When considering how thick you want the new concrete to be, it is important to consider the surroundings where the concrete will be. Like I mentioned before, anything like a door or a staircase would warrant getting rid of the old concrete and starting fresh.

    How Do You Attach Concrete to Old Concrete?

    Though it is possible to attach new concrete to old concrete, it is important to do it the right way to keep any cracks or separation from setting in too quickly. If you don’t do everything possible to prevent that, you may be replacing the concrete sooner than you would like.

    Any time one is to attach new concrete to old concrete, it is likely for cracks to form and the sections will begin to separate. One way to help prevent that when attaching them is to use rebar to pin them together.

    Rebar is short for reinforcing bar, meaning the bars are used to support and strengthen the bond between the two slabs. The size of the rebars really depends on the size of the concrete. Heavy concrete will, of course, use heavier rebar, but smaller concrete such as sidewalk or driveway will only require about half an inch of rebar.

    Here are the steps to make sure you bond new concrete with old concrete correctly:

    1. Drill ⅝ diameter holes to be 6 inches deep into the old concrete. Be sure to drill the holes 12 inches on center. Place the holes horizontally if the joint is wider than it is tall, but place then vertically if it is taller.
    2. Be sure the holes are at least 6 inches from any edges to prevent any chipping.
    3. Flush water into the holes.
    4. Place injections of epoxy into the backs of the holes while making sure that it is enough to fill the holes halfway.
    5. Twist 12-inch rebars into the holes making sure there is an even coat of epoxy around the rebar surround every inch that is in the hole.
    6. Patch the holes with duct tape by sticking it through the end of the rebar and bringing it down to the hole to prevent any epoxy leakage while curing.
    7. Use metal primer to tape the exposed pieces of rebar to prevent rusting.
    8. It is now time to pour the new concrete for it to flow and surround the rebar pins.

    Can You Stamp Concrete Over Existing Concrete?

    When stamping concrete, it still has to be a fresh pour of concrete so it is wet. Therefore, you have to pour a new layer over existing concrete to stamp concrete, but make sure the existing concrete is in good condition.

    Like mentioned before, the new layer of concrete is only as good as the existing layer below so make sure to patch any cracks below and separate the two layers.

    When stamping concrete, there are two different methods to consider. You can use textured rollers or hand stamps. It may be handy if you buy multiple handstamps when dealing with a large surface to avoid it drying before finishing.

    There are many different stamp patterns to consider like basic brickwork stone texture. There are stamps that are formed to look like stones or bricks placed in a circle. You can even have a little bit of fun with pawprint stamps.

    Make sure the existing concrete is as clean as possible. You should consider power washing to avoid any debris getting in the way. Once you pour the new concrete, you must be quick with the stamps as they need to be set within 15 minutes of pouring the concrete. To avoid any stress, pace all tools beside you to speed up the process.

    You may need to ask a few helpers to assist you to help with the process of laying down the new concrete and stamping it. The concrete should be about 2 inches thick and be quick to make sure the new concrete is leveled and smooth before using the stamps. That is when your helpers will be of assistance to you.

    For more help on apply stamps to concrete, click HERE to watch a YouTube video.

    Can Concrete Be Resurfaced?

    If you do not know what resurfacing concrete is, you wouldn’t be the only one. It is actually a way to revamp or repolish your concrete to look new again. It actually isn’t that uncommon, and it is a viable option.

    Many people will tell you that you should just replace the concrete rather than resurfacing, but replacing might not be necessary. You can cover your concrete patio in different ways. Resurfacing will help save you some money and conserve your resources.

    Resourcing old concrete might be perfect if you are looking to:

    • Cover unpleasant concrete
    • Conceal imperfections
    • Fix cracks
    • Deal with discoloration
    • Update outdated finishes

    Before talking about the options and how to prepare the concrete for resurfacing, keep in mind that resurfacing will not solve any underlying issues your concrete may have. If it is starting to heave or lift, resurfacing is not an option and you may need to replace it. Furthermore, it will not help with any severe cracks or freeze/thaw damage.

    There are many decorative overlay options when resurfacing. You can stamp concrete which was mentioned earlier, but you can also color it, stain it, engrave it, or you can even use stencils.

    Just remember that preparing the concrete is just as crucial as anything else. In fact, it is probably the most critical part. You must have a clean base for the overlay to cling to or you may have a case of overlay failure.

    Can You Put New Concrete Over Old Concrete? Now You Know

    • Putting new concrete over old concrete is definitely an option to consider, but before deciding, you must consider all options and what will go into the process. You should check to see if adding a couple of inches of concrete will disrupt a door shutting or a bottom step to a staircase etc. And if you are interested in building concrete patios, check out that article to find out all you need to know about building one!
    Justin Wylie

    Justin Wylie, owner of All Pro Decks & Patios is a Deckorators® Certified Pro Elite located in the San Antonio Hill Country area. He specializes in building decks, patios, patio covers, pergolas, concrete slabs, fireplaces and more while providing quality and service for his customers.

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