In my neighborhood, some of the older homes went through a phase where they installed asphalt driveways. Some of them still look good, but others are starting to buckle or break apart. Now people are opting for new concrete driveways by digging up the asphalt. But I wanted to know if you can pour concrete over the asphalt, instead.
You can pour concrete over asphalt, provided the asphalt base is stable and free from defects formed due to frost heave or another shifting. If the asphalt surface is still structurally sound or has remained intact for years, it is acceptable to use as a base for a concrete garage floor or driveway.
There are many different scenarios where pouring concrete over asphalt may or may not be a good idea. For instance, it may be fine to pour concrete over asphalt when redoing a garage floor but not for a driveway.
In this article, we’ll go over all the instances for when you should – and shouldn’t – consider pouring asphalt over concrete. As well, we’ll go over the mechanics of concrete and why it can work with asphalt if certain measures are taken.
Will Concrete Bond to Asphalt?
No, an asphalt surface provides a weak bond to concrete. Asphalt is bitumen-based, which means its primary ingredient is oil. As an oily product, asphalt will not bond well to any other substance unless heated artificially.
Just because concrete doesn’t bond well to asphalt doesn’t mean you cannot pour concrete over the top of it. Many roads – especially highways – will pour concrete over asphalt roads and they work just as well or better than asphalt roads.
A common construction method used on roads today is to pour concrete curbs over asphalt roads. Through the use of mechanically sawn-out channels in the asphalt, the bottom of the concrete becomes “locked” into the asphalt.
Other mechanical fastening methods, such as using rebar, can also be used to fasten concrete to asphalt. The two mixtures will not naturally adhere to one another, but there are many relatively simple methods to affix the two building materials together.
Can You Pour Concrete Over Asphalt Driveway?
Yes, you can pour concrete over an asphalt driveway. While asphalt provides a poor bond with concrete, it does serve as an excellent base as it is solid, stable, and very strong. Provided the asphalt is in decent condition, there is no reason you can’t pour concrete over the top.
There are instances when you cannot pour concrete over your asphalt driveway. If your driveway has large chunks of asphalt missing or is severely heaving, then it is recommended you not pour concrete over it.
The substandard base will crack your new concrete driveway as parts of your new concrete pad will rise and fall where the asphalt beneath was damaged or missing.
However, if your asphalt was in decent to excellent condition, then you can pour a new concrete driveway right over the top. Concrete doesn’t bond well to asphalt, meaning water could get between the two layers and harm the new concrete with cracking or moisture damage.
To mitigate water infiltration between the two layers, consider drilling holes in the asphalt in a grid pattern before pouring the concrete. Any water then trapped between the two layers will find a way out through the drilled holes.
Pouring Concrete Over Asphalt in a Garage
You can also pour concrete over asphalt in your garage, provided your garage walls can accept the new concrete. If your existing asphalt garage pad is relatively intact, you can pour concrete over it as long as the garage walls have a concrete footing or blocks that are high enough to accept the new depth of the concrete.
If your garage framing sits on the grade of the existing asphalt, then you’ll have to remove the asphalt. You cannot pour concrete on the asphalt as it would cover the bottom of your framing, which is not acceptable under any circumstance.
Since the concrete pad should be at least 4” thick, you can quickly gauge in your garage if you can pour over the top or not. Much like a driveway, if the asphalt is too cracked and undulating, then you’ll need to remove it first.
Finally, while the space between the asphalt and concrete in a garage won’t likely fill with water, it doesn’t hurt to drill some holes through the asphalt if water does find its way between the layers. Otherwise, the foundation of your garage could be compromised.
How to Put Concrete Over Asphalt
When installing concrete over asphalt, you’ll need to secure it to the asphalt surface to prevent any sort of movement. Remember that a bitumen-based asphalt doesn’t bond well to concrete, so constant movement over the concrete or a significant impact could shift the concrete slightly over time.
Here are some tips to consider when installing concrete over asphalt:
- Fasten vertical rebar to asphalt to support concrete.
- Drill and check the asphalt depth. If too thin, remove it.
- Punch extra holes through asphalt for drainage.
- Make keyways in asphalt for extra concrete adhesion.
When pouring over asphalt, the most important thing you can do is to mechanically fasten the rebar to the asphalt. Drill into the asphalt several inches and install vertical #5 rebar every 18” on center in a grid pattern. Ensure that it will be at least an inch and a half below the top surface of your concrete pad.
Once you’ve installed your vertical rebar, you can install horizontal rebar to connect the grid using #4 rebar. Use rebar ties to bond the intersections together. This will ensure that your concrete pad stays fixed to the asphalt beneath it for the life of the material.
While 4” of concrete over the asphalt is the minimum you should pour for a driveway or garage, it is recommended that you increase that amount by an inch. While this will add to your costs – up to 20% – it will allow you to hide the ends of your rebar more and provide a more solid base atop the asphalt.
As mentioned above, another important practice is ensuring you have drainage beneath your concrete. Gravel, typically a substrate for concrete, usually acts as the drainage for a concrete pad. Since asphalt is not porous, you must mechanically drill holes through the asphalt for drainage. They should be placed in areas where the grade is lowest.
Finally, keyways – or channels – can be scored into the asphalt with an angle grinder or circular saw outfitted with a diamond blade. Keyways can hold the concrete in place, as the concrete will not move out of the channels. Many municipalities use this technique when forming curbs for roads.
When Can’t You Pour Concrete Over Asphalt?
You cannot pour concrete over asphalt if the asphalt is too thin, coming out in chunks, or heaving too much. If the asphalt has degraded to the extent that it is failing, you must assume that anything poured over the top of it will eventually fail similarly.
Here are specific instances when you should not pour concrete over asphalt:
- Heaving or buckling asphalt.
- Asphalt that is cracked, with large chunks removed.
- Improper substrate beneath the asphalt such as clay.
- The asphalt itself is too thin.
Like any concrete project, you need a proper base to ensure your concrete won’t shift or crack prematurely. Understanding what’s under the surface of your asphalt is critical. If your asphalt is buckling significantly, then it is likely a sign that the subsurface of the asphalt was not prepped properly. In that case, you must remove the asphalt and start from scratch.
Similarly, if you have an asphalt surface that is fairly level but with many cracks and sections missing, then there are underlying problems. Pouring over a surface like this would result in shifting, as the asphalt itself is already shifting – the cracks – and heaving would also occur due to the missing chunks of asphalt beneath the new concrete.
If the asphalt is in poor shape, it is likely that either the asphalt itself is too thin – less than 5” – or the surface beneath it is not appropriate. A base of gravel – at least 8” thick – should be beneath your asphalt. If you drill down and find clay or topsoil, then you must dig up your asphalt and start again.
The asphalt should also be a minimum of 5” thick. If you drill down only to find that the asphalt is one or two inches thick, then you should not pour new concrete over the top. The exception to that rule is if the thin asphalt has a proper sub-base of gravel, and the asphalt itself is in good condition.
Can I Fill Asphalt Cracks With Concrete?
Yes, you can fill asphalt cracks with concrete. However, it is not the recommended solution for repairing cracks in asphalt. The main reason is the nature of the two materials – asphalt is more flexible than concrete. Movement from the asphalt will crack the concrete, causing it to fail before an asphalt patch.
Over time, a concrete patch on an asphalt driveway will either pop out due to forces exerted on it by the asphalt or act oppositely and crack your asphalt all around the concrete. AReas that experience significant freezing and thawing will speed up this process even further.
So, while you can patch asphalt with concrete, I recommend patching asphalt with asphalt and concrete with concrete.
What Else Can You Put Over Asphalt?
You can install pavers, bricks, or other surface materials over asphalt, but the same considerations apply to those materials as they would to concrete. You must first consider the condition of the asphalt itself. Is it thick enough? Is the sub-base appropriate? If so, the asphalt may be an adequate base for other materials.
Then again, if the asphalt is heaving, too thin, or has no adequate sub-base, then you should not install pavers or any other material over it. If you do, your new material will, over time, experience those same issues.
Mixing and matching building materials is always a tricky business. While asphalt and concrete both seem extremely durable and strong, they don’t always mix well.
Common sense can help when considering whether you want to pour concrete over your asphalt. Ask yourself, “does my asphalt seem like it’s in decent condition?” If you aren’t sure if the answer is a definitive “no”, then you need to rip up your asphalt and start fresh.
If you can look at your asphalt and objectively find that it is in good shape, then you can go about seeing how thick it is and what is underneath. If the asphalt’s thickness and sub-base are good, you can proceed with pouring concrete on your asphalt.