Can You Pour Concrete Over Existing Concrete?

Your concrete garage floor, driveway, or patio has a new crack in it every time you look at it. It’s getting to the point where pouring new concrete is a necessity, but can you save money by pouring a new slab over an old one?

You can pour concrete over existing concrete but you need to ensure that it’s done properly. If the proper care isn’t taken, your newly poured concrete will be brittle and deteriorate quickly.

You’ll find the details about pouring concrete over existing concrete below, including a how-to guide.

Let’s get started.

Can you Pour Concrete Over Existing Concrete?

The short answer is YES, you can pour concrete over existing concrete. Whether you should or not is a different question. You will find hundreds of different opinions which say whether you should or should not, and each will have a complicated way of determining what the correct answer is.

However, there are two very easy ways to determine if you cannot pour over concrete.

You cannot pour concrete over existing concrete if:

  • Your existing slab is settling (structural problems)
  • You need the ground level to remain at its current height.

If you witness your existing slab settling, adding the additional weight of a new layer of concrete will only make matters worse. The easiest way to determine if your existing slab of concrete is settling is to look for major cracking.

If you see areas where the floor is cracking and is tilting, then this is a sign that your slab is settling and needs to be removed. Once removed, you will need to repair the sub-base (ground on which the concrete slab sites) to prevent future settling.

Factoring in Height

If you are hoping to pour new concrete over your existing concrete garage floor, you will need to adjust the height of your garage doors. In some cases, this is difficult and costly.

It can also be impossible to pour a new layer if the extra layer of concrete will add a step. Very often people need to have a relatively small gap between their garage floor and their driveway. Pouring a new concrete slab over your existing concrete floor will produce a step of at least 2 inches (5 cm). In many cases, this makes pouring new concrete a bad idea.

Steps to Pour Concrete Over Concrete

1) Clean the existing concrete.

Cleaning the concrete is the most important step if you want to ensure your job withstands the test of time. You need to remove any surface debris (dust), but you must also remove any grease or oil.

Depending on your location, you can find degreasers or similar products at your local hardware store. After everything has been cleaned, use a pressure wash to wash away any debris and chemicals.

2) Set up the Perimeter 

Setting up the perimeter means installing the formwork. When you pour concrete, you need to ensure the formwork acts as a mold and it takes the shape of your final product. This means that you must ensure the height of the wood you use matches the height you want your new concrete slab to be.

In some circumstances, this could involve some trimming of the wood forms. You must also brace the forwork to ensure it remains in place. Remember, once the concrete is poured, it will be pushing against the walls, and the bracing needs to be able to withstand the sidewards pressure from the wet concrete. 

3) Apply bonding agent 

Without a bonding agent, the new concrete will not adhere to the old one. A bonding agent will ensure you have a strong bond between the old and existing concrete slabs. Prior to applying the bonding agent, ensure the existing concrete surface is damp. If your surface is too dry, it will hinder the ability of the bonding agent to bond the two surfaces together.

4) Add meshing and pour the concrete

The next step is to add your wire meshing (reinforcement) and pour your concrete. You may choose to purchase your concrete from a local ready-mix concrete company, and if you do that, you must ensure there is sufficient room for the truck to move around on site.

5) Cure

Curing ensures your concrete hardens properly. In order to achieve a proper cure, you can use a curing compound that is applied using a spray, or you can ensure there is moisture around the surface by installing a wet piece of burlap and keeping it moist.

Curing is crucial because it provides the moisture required for the chemical reaction to occur which hardens the cement powder. If the concrete “dries up” due to a lack of curing, you will end up with a surface that will deteriorate very quickly.

6) Inspect 

Once you have finished pouring your new concrete over your existing surface, you need to inspect it. You want to see if there was good adhesion between the old and new surface. The best way to do this is to take a soft mallet hammer and tap various sections of the concrete.

If you hear a “hollow” sound, it means there is a gap, which is a sign that the old surface did properly adhere to the new surface. If this is the case, you may, unfortunately, have to tear it out and restart the installation, or you risk having to redo the slab earlier than anticipated.

However, if you took your time to properly clean and prepare your existing concrete, then you should not hear a “hollow” sound, and you will know that the two surfaces bonded properly.

Disadvantage of Pouring Concrete Over Concrete

The best surface is always going to be a new and uniform surface. When pouring a new concrete slab over an existing one, you are bonding two surfaces, which will never yield as good a result as a single uniform layer. However, when properly cleaned and with a proper bonding agent, you can still achieve amazing results.

If you are looking at creating something that will last for a very long time, it may be worthwhile to remove the existing concrete, ensure the sub-base is properly compacted, and pour a fresh concrete slab. However, if you are not looking at keeping it for a “lifetime”, then it’s probably adequate to add a properly bonded layer of concrete on top.

Verdict

All in all, pouring a new slab of concrete over an existing slab is fine as long as you do not have any structural damage (settling), and you can allow for a slightly higher floor.

When considering the alternatives, there is the “do-nothing” option, which will lead to your slab deteriorating, or there is the option of tearing the entire slab out. Tearing out a slab is no easy task. It’s laborious, dusty, expensive, and very time consuming. Therefore, if possible, pouring over an existing slab is an excellent option, as long as it is done correctly. 

If you need to do minor garage floor repair, then this article will help you out. Need to level your garage floor? Click here.

Photo Credit: http://theconcreteprotector.com/

About Roy Cohen

I'm Roy, founder of Hack My Garage. I started out like most not knowing how to hammer a nail, but now I know more than most people about home improvement and DIY. I want to spread my wealth of knowledge with the world and hope to make it a better place.