Can You Put Concrete Over Asphalt?
Installing concrete walkways, sidewalks, and driveways is par for the course for many building owners, but it comes with many questions. One of the most important parts of putting in concrete is what material lies beneath it.
Many builders come across asphalt that is already installed, or are considering installing it themselves, and wonder if it is suitable to pour concrete over.
You can put concrete over asphalt, and it is a great choice as a driveway base for residential homes. Asphalt is only suitable as a driveway base if it is at least between 2” and 4” thick for residences. Homes with driveways intended for many large vehicles, however, the asphalt must be 4” or more to support up to 10” of concrete.
Before you can pour concrete over asphalt, you must first soak the asphalt with water. Typically, builders power-wash the surface to both clean it, smooth out the asphalt, and make the concrete stick to the asphalt more easily. You can also pour new asphalt over old asphalt, however, concrete is a better choice.
Let’s take a look at why you can put concrete over asphalt, and why it’s a great option.
What is Whitetopping?
Whitetopping is a road repair method used by many states and municipalities to save time and money on repairing roads. The process involves pouring concrete over top of asphalt instead of removing the current asphalt. While this is a common technique used by professionals on public roadways, homeowners can also benefit from it.
If you have a worn-out asphalt driveway or walkway in need of replacing and are wondering whether or not you can install concrete over asphalt, the answer is yes. In fact, this whitetopping repair method can actually help reduce labor costs and time that would be spent on the replacement.
However, it should be noted, that this procedure will only work under particular circumstances and when certain conditions have been met. Whitetopping is most suitable for asphalt pavement that has little wear, but, repairs can be made first when necessary. If the asphalt is severely damaged, it should be removed entirely and then replaced with a new concrete pavement.
Is Asphalt Stable For Concrete?
A common concern with putting concrete over asphalt is stability, but asphalt is a stable and supportive base layer. Asphalt is as stable of a base layer for concrete as they come, however, only if the asphalt is compact. The denser and more compact a material is, the more suitable it is as a base layer to support the concrete.
Additionally, it is important to inspect for holes, cracks, and damage before putting concrete over asphalt. It is normal to have cracks in asphalt, but if the cracks measure 1”-3” deep, they need to be filled or patched before applying concrete.
Asphalt repair may be necessary, particularly if you came across an existing asphalt surface that has been driven on heavily. As previously mentioned, if the asphalt is too badly damaged, pouring concrete over top may not be the best course of action.
Advantages of Putting Concrete Over Asphalt
There are advantages to putting concrete over asphalt, particularly if it is a case of installing over existing asphalt, including:
- Asphalt provides a supportive base layer
- Cheap material
- Bonds with concrete well
- Saves money (if it is existing concrete)
First and foremost, the support that asphalt provides as a base layer is a major advantage. If you are installing asphalt for the purpose of putting concrete over it, which is not entirely necessary, it is at least cheap. Asphalt costs $2-$4 per sq. ft., making it cost-effective.
More commonly, however, builders put concrete over existing asphalt. In that case, putting concrete over asphalt can save you money in excavations. Excavations are expensive and can cost between $17 and $20 per sq. ft. but are unnecessary if there is existing asphalt.
On average, 2-car driveways measure 640 sq. ft., but vary in size. If you have existing asphalt, that could save you $10,880 or $12,800 in excavations easily. Instead of excavating and preparing the land for a new driveway, you can simply patch any cracks or holes in the asphalt and pour concrete over it.
How Thick Does Asphalt Need To Be To Put Concrete Over It?
For standard residential driveways, the asphalt needs to be 2”-4” thick if you want to put concrete over it. If there are going to be a lot of heavy vehicles on the surface using the driveway, however, make sure the asphalt is 4”-6” thick so that it can support up to 10” of concrete.
Consult with a professional contractor if you are unsure how thick your asphalt should be if you are planning on putting concrete over it.
Do You Need To Level Asphalt Before Putting Concrete Over It?
It is important for the asphalt to be as level as possible before putting concrete over it. For many builder-owners, that means that they have to level the asphalt using an asphalt repair kit, such as Commercial Grade Blacktop Repair. You can get 50-pound bags of asphalt repair for $10-$20, and it can be used to level the asphalt, and patch cracks.
Shifting and erosion is common for asphalt and other materials, primarily due to exposure to the elements. To ensure that the concrete has the best possible surface to adhere to, leveling existing asphalt is necessary.
How To Level Asphalt Before Putting Concrete Over It
Leveling and patching asphalt is not too difficult, or even time-consuming. Purchase a 50-pound bag of asphalt/blacktop repair. Now, all that you need to do is a gather a few materials before you can get started, including:
- Putty knife
- Broom and pan
- Brush or sponge
- Tack coat
Now, you can level your asphalt to prepare it for concrete.
1. Clean the Asphalt
Sweep any surface debris off of the asphalt. If there is fragile or damaged asphalt, remove it with a sharp object. After you get rid of any weakened asphalt, sweep the surface once again to make sure it is totally cleared off.
Now, take a bucket of warm water and dump it over the asphalt. Some people mix soap or other cleaning solutions with the warm water before dousing the asphalt. Either way, make sure to get all of the asphalt wet without leaving behind soap residue for the best possible adherence.
2. Apply Tack Coat
Tack coat is an asphalt emulsion that acts as a primer for asphalt and makes for a better bond with concrete. A five-gallon tub of asphalt emulsion costs roughly $40-$60, and a little bit goes a long way. Take a flat-surfaced tool and spread the tack coat into the cracks or uneven spots on the asphalt.
Don’t fill the cracks entirely with the tack coat. Using the tack coat liberally makes it so that the asphalt repair mix sticks to the asphalt well.
3. Lay the Asphalt Repair Mix
Now, it is time to apply the asphalt repair mix. For small cracks, apply a small amount of the mix and compress it using a flat, blunt, tool. For less subtle cases of asphalt damage and erosion, apply 1” to 2” of the repair mix and compress it so that it is compact.
Do the same thing for any and all indentations, holes, or cracks in the asphalt. This will make the asphalt level. Allow the newly patched asphalt to have 24 hours of cure time. Once that 24 hours is up, your asphalt is level, thus suitable to have concrete put over it.
Can You Put Concrete Over Old Asphalt?
Depending on the shape that it is in, yes, you can put concrete over old asphalt. However, if the asphalt is so old and untreated that it needs to be redone entirely, you may want to consider doing away with it altogether.
Generally, asphalt is good for 10-20 years. Anywhere within that range, it is worth makes patches or treating the asphalt so that concrete will bond to it well. Concrete is a surface itself, so as long as the asphalt is level, it does not matter so much how old the asphalt is.
As an alternative to pouring concrete over asphalt, you can use an asphalt overlay or resurface. In this case, a new layer of asphalt is installed over the existing asphalt driveway, roadway, parking lot, sidewalk, garage floor, etc.
This option is best suited for when the surface has deteriorated but still maintains its structural integrity. Similar to installing concrete over asphalt, damaged areas must first be repaired and the surface needs to be leveled. In fact, an existing asphalt surface is the ideal base for the installation of new asphalt.
Summing it Up
You can put concrete over asphalt, but make sure that the asphalt is at least 2” to 4” thick. If you plan on storing many heavy vehicles on the driveway, you need 4”-6” asphalt because you need more concrete to support the weight of the vehicles.
As long as you make patches and repairs, you can get your asphalt surface level and ready to serve as a base layer for concrete. Good luck!
Nick Durante is a professional writer with a primary focus on home improvement. When he is not writing about home improvement or taking on projects around the house, he likes to read and create art. He is always looking towards the newest trends in home improvement.
More by Nick Durante