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How to Remove Asphalt From Concrete

For decades, concrete and asphalt have been a staple in residential construction. Driveways, patios, sidewalks, and garages tend to be formed with one of these two materials, and sometimes even both.

It is not uncommon for a home to have an asphalt driveway abutting a concrete pad, like a garage floor or sidewalk. Sometimes in the process of installing these materials, one unintentionally contacts the other and must be dealt with.

Source: progresspaving.com

Today, we will discuss what to do when this occurs and suggest a few methods the professionals use to correct the problem.

Can I Remove Hardened Asphalt From Unsealed Concrete?

In most cases, yes, however, the project can often require significant time and effort. In most situations where the asphalt needs to be removed from concrete, the problem stems from a mistake made during installation.

For example, asphalt is commonly used for driveways because it is flexible, easily repaired, and durable. However, asphalt is not as common as concrete for use with sidewalks and patios, as these features tend to be more decorative. Whereas concrete is often stamped, colored, shaped, and textured, asphalt is not. Therefore, builders tend to use a combination of the two materials for maximum effect.

In many cases, the asphalt is installed after concrete to avoid damaging the surface with heavy machinery during construction. This can result in overspray and drips where the new asphalt joins the existing concrete.

Unless preparations are made in advance to protect the concrete surface, the oil in the asphalt can contact the porous concrete surface and be absorbed. This usually results in having to remove the asphalt with either chemicals or mechanical abrasion.

Typically, the oil in the asphalt will seal the pores in the concrete, preventing sealer or stain from contacting the concrete surface. This can result in splotchy finishing because the stain or paint will struggle to adhere to these areas.

Asphalt is installed at very high temperatures because this liquifies the tar, allowing it to flow more easily. However, this also allows the tar to splash and drip, which is where most staining occurs.

What Methods Can I Use to Remove Asphalt From Concrete?

Pressure Washing

Using a pressure washer is often the first solution to removing asphalt from concrete. This is because most other methods of removing the asphalt involve deep cleaning anyway, so pressure washing is typically the first step. Many times if the asphalt is relatively new, the tar will still be somewhat pliable, making mechanical removal effective.

Pressure washers can generate water pressures capable of actually damaging the concrete, so most pros will only use high-pressure tips as needed. In most situations, the pros will prefer to pressure wash an area several times with a less aggressive spray tip, instead of trying to remove more material with more pressure.

This method also works to completely remove chunks of cured asphalt, however, a stain may remain. If this happens, detergents are usually employed to dissolve the remaining residue and the area is pressure washed again. Not only is this method safer for the concrete, but it is also safer for the user because the use of caustic chemicals can sometimes be avoided. 

Soap and Other Detergents

Interestingly, one of the best solvents for dissolving the oil in asphalt is soap. This method is generally preferred, as soaps and detergents are usually readily available, do a great job of breaking down the oil, and will not harm the concrete. In most situations, repeating this process as needed will return the concrete to its normal appearance. However, if the stain is particularly difficult to loosen, sometimes additional chemicals or solvents will be used as well.

Mineral Spirits and Other Solvents

Mineral spirits, turpentine, and other solvents are also used as an alternative to, or in addition to pressure washing. Asphalt is made from petroleum products, so removing it often requires breaking down the oil contained in the asphalt. This oil gives asphalt its color and is largely responsible for making it sticky.

Therefore, dissolving the oil in asphalt often allows it to release from the concrete, making the removal job easier. This is done by applying the solvent directly to the surface and allowing it to sit from a few minutes to a few hours. This method typically requires less elbow grease, but it does require safety precautions before use.

Since these chemicals are caustic, they can cause serious burns if they come into contact with the user. For this reason, the professionals will often wear eye and skin protection, as well as disposable clothing. These chemicals can also cause lung irritation, so a respirator is also required. 

Concrete Cleaner

Concrete cleaners usually consist of a chemical agent designed specifically for breaking down stains on concrete. These solutions are typically not often used for removing large chunks of asphalt, but rather to remove the residue left behind. Often used in conjunction with pressure washing, these cleaners may contain detergents that dissolve the oil, allowing it to detach from the concrete.

Concrete Etching Solution

A more abrasive way to remove asphalt from concrete involves the use of very strong chemicals to break down not just the stain, but the top layer of concrete as well. This method can be used to not only clean concrete, but also change the texture.

Concrete etching solution usually includes some form of hydrochloric acid, which is extremely caustic. Protective clothing and safety gear (including a respirator) are required when using this type of solution, as it will cause severe burns.

Muriatic Acid

Muriatic acid will dissolve most materials such as metals, plastics (with exceptions), and finishes, which is why it is so effective in removing asphalt. A solution of muriatic acid diluted with water is applied to the surface, allowed to foam up, and rinsed away with water.

However, there may be situations where the ability to neutralize the acid is needed, such as with accidental spills and drips. Readily available products like baking soda and lime are very effective at neutralizing muriatic acid, so it’s a good idea to have some around before using this method.

Unconventional Methods

There are also common products and solvents that are known to be effective in removing asphalt stains from concrete. These common household items are effective and generally safe to use, especially if the stains are light. Here is a partial list and how they work:

  • Cat Litter

If the stain is relatively new and still somewhat liquid, cat litter can be used to absorb the majority of the oil. This often makes removing the stain much easier, because when the stain is still liquid, it can make the stain larger. Removing the oil keeps the stain from flowing and making the job more involved.

  • Oven Cleaner

Oven cleaners are designed to dissolve burnt food waste, and are quite caustic. This method can work to dissolve thicker stains if applied and allowed to work for a few hours, especially in direct sunlight. Care should be taken to wear protective gloves and eyewear when using this method however, because these cleaners are very strong and can damage unprotected skin.

  • Cola

Surprisingly, cola has been used to remove, or at least soften, asphalt stains on concrete. The carbonation of cola tends to soften most stains and will not damage the concrete surface. If the stain is light, several applications of cola and light brushing can lighten an asphalt stain to the point of being unnoticeable.

Patience and Safety Is Key

In most situations, removing asphalt stains from concrete will require a combination of patience, trial and error, and elbow grease. The age, thickness, and size of the stain will often dictate which method is required to do a professional job. Just remember to try the simplest method first and only employ more aggressive methods as needed. This will ensure a good looking job while minimizing any danger to the user, or the concrete.

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