Pro Tip

Concrete Patching: How to Bond to Existing Concrete for Best Results

Concrete is marvelous stuff but in time it will deteriorate. When it does, you either have to patch it or replace it. Assuming that it is structurally sound, the least expensive alternative is to patch it. However, patching it requires some attention to detail or your patch will not last.

Fact: Fresh wet concrete does not normally bond well to existing dry concrete.

Do you remember elementary school where one of the subjects on which you were graded was “plays well with others”? Concrete would have gotten an F. There is nothing in basic portland cement that will act as a bonding agent. Portland cement concrete works well in mass and provides great compressive strength but not bond.

When patching existing concrete, here’s how to achieve a strong and lasting bond between the new and old concrete.

1. Repair or replace? Determine if the concrete is structurally sound.

If a sidewalk has either heaved or dropped at almost every joint, repairing it will not provide a long-term solution. The slabs are likely still moving. If the slab has so much sand and gravel on the surface that despite sweeping and sweeping and squirting and squirting it just keeps coming back, don’t waste your time on repairs. If it has multiple cracks that run so deep that they appear to run through the slab, a repair would only be temporary. The solution to all of these problems involves a jackhammer and several bags of Sakrete concrete mix. Since this discussion is about best way to bond concrete, we will assume that your slab is good.

2. Prepare the surface.

There are a variety of Sakrete concrete repair products available to fix concrete that has begun to deteriorate. No matter which product you use, the process begins with good surface preparation. All loose sand, gravel, dirt, leaves etc. must be removed. This can typically be done with a garden hose and a good nozzle. Tough areas may require a pressure washer or mechanical abrasion. The two toughest areas to cover are those with oil and tree sap. Both of these will work their way down into the concrete. Simply washing the surface isn’t sufficient. If the stains do not run too deep, you can chip away the concrete using a hammer and chisel. Don’t forget the goggles (not just glasses) as this process will throw concrete all over the place. Also, keep your thumb out of the way. If the spots are too large or too deep for this to be practical, you may need a sealer to cover the stains before patching.

Note: Concrete simply will not bond to all substances. Paint, oil, glue from old flooring tiles are just a few. You must mechanically remove these materials if you want the job to last.

3. Choose a bonding method

There are two basic methods for bonding a portland cement based product to existing concrete: 1) mechanically and 2) chemically.

Mechanical bonding: Let’s discuss the mechanical method first since it is really used in both approaches. The most effective way to ensure a really good bond is with a scratch coat. This is simply a very wet coat made up by mixing the repair product with water. Mix up a small amount of the repair material to a soupy consistency. You don’t need to measure the water—just turn the stuff into slop. Then, using a gloved hand or a rag, smear the material onto the area to be patched. Just think finger painting from kindergarten. The technique is about the same. Apply pressure to ensure that as much as possible is shoved into the nooks and crannies. You only need a thin coat. It is not necessary for this scratch coat to dry. By the time you get the repair material mixed it will be ready. Then mix up additional repair material to the proper consistency and apply over this thin scratch coat.

Chemical bonding: The chemical approach involves mixing up a liquid bonding agent that helps bond new concrete products to old.

Make sure to use concrete products that are compatible with a liquid bonding agent.

Products like Sakrete Sand Mix and Sakrete Fast Set Cement Patcher will work well with a liquid chemical bonding agent such as Sakrete Bonder & Fortifier.

Important: DO NOT USE products like Sakrete Top ‘N Bond and Sakrete Flo-Coat with a liquid bonding agent. These products already contain polymers to aid bonding with portland cement and should never be used with a separate liquid bonding agent.

When using a liquid bonding agent, paint the bonder onto the existing concrete and allow it to dry until it is tacky. This usually takes only a few minutes. Then apply the repair material. The most effective way to ensure that the bonding agent gets into the existing concrete is to apply it directly using a brush or rag. It can be sprayed if you happen to have a sprayer. Although the directions say that you can use it as part of the mix water, direct application works better.

4. Wet the surface (if necessary)

If you are doing a large area and a scratch coat isn’t practical, you will need to spray the surface with water before you apply the repair material. On a warm day, the existing concrete surface will be hot enough to suck the water out of the repair material. In addition, some concretes are quite porous and will rob water from your repair material. If too much water is lost into the old concrete there will not be enough water to hydrate all of the cement particles and a lower strength material will be the result.

5. Check your work

Once the job is complete, you can do a quick check to see if the bond was successful. Wait at least 24 hours and then tap “gently” on the patch using a hammer or some other dull object and listen for a hollow echoing sound. If you just get a dull thud, then the material has bonded well. If you get a hollow sound, the material has not bonded and will crack in time. Which means it is back to the beginning of today’s topic. Here is hoping your concrete work comes across as a dull thud rather than a hollow endeavor.

For more information on where to buy these products, visit here.

Comments (43)

Rick Thompson says:

Do Pro-Mix products require a bonding agent?

Sakrete says:

No, all Pro-Mix products are heavily polymer-modified and don’t require a bonding agent. Using a bonding agent can actually act as a bond breaker!

Abdyrahman says:

On what time or number of days between old and new concrete to use bonding agent for bonding old and new concrete? I wanna to know the gap between old and new concrete to use bonding aget.
Abdurahman Esmael (Material Engineer)

Sakrete says:

You need to wait at least 28 days before laying anything on top of new concrete.

Robert cortese says:

I’m resurfacing ny cement driveway with Portland cement that hat has a bonding glue added I got a few areas that the mixture not ad herring to the driveway. I’m assuming because I had used an cement paint on the driveway 2 years ago. I had power washed it twice with a pre sealer twice but the paint seems still shows thru. Help

Sakrete says:

Sounds like you’ve got the right diagnosis, Robert. Any old paint still hanging on is going to be a guaranteed bond breaker. You might also have some oil, etc. that’s seeped into the surface, which will also inhibit the bond. Consider some additional mechanical abrasion to make sure you’re down to 100% clean concrete!

Give us a call at 866-SAKRETE if you’d like to dig into this a bit more.

Roy Putnam says:

Thank you for all the info.
What is a good concrete sealer or gray paint for steps and risers?

Sakrete says:

Hey, that’s what we’re here for!

As far as a good sealer goes, give our Concrete & Paver Sealer a try.

Mark says:

Can you mix cement all with mortar mix?

Sakrete says:

Assuming you’re talking about Rapid Set Cement All? We can’t recommend that, unfortunately. Might work just fine, but we’ve never tested it!

Jay says:

This statement is incorrect.
When using a liquid bonding agent, paint the bonder onto the existing concrete and allow it to dry until it is tacky. This usually takes only a few minutes. Then apply the repair material. After the bonder has become tacky, apply a scratch coat as described above and then apply the repair material.

Sakrete says:

Thanks, Jay – we’ll correct!

Thomas Brooks says:

4 days ago, poured a small new slab for stairs landing (6’x6’x5″ deep). Used Sakrete High-Strength Concrete. Upon removing lumber form, discovered several small voids around perimeter. I have both: Sakrete High Strength Ckncrets OR Sakrete Mortor Stucco type S avaliable. Which is best for filling voids? Do I need liquid bonder agent?

Sakrete says:

Thomas, we’d actually recommend going with Top ‘N Bond or some other type of repair material for this application!

John Klem says:

I want to use mortar mix over cured but new concrete because my wife wants it 2 inches higher. Will it be a problem?

Sakrete says:

John, quick word of caution – mortar mix isn’t structural, so we wouldn’t recommend this application. However, if you want to go 2″, any standard concrete will work! Just make sure to do some heavy surface prep AND use bonding agent to give your new pour the best-possible chance of achieving a strong bond.

David Billerm says:

I’m patching a hole in my foundation
I plan to rinse the wall, wipe off loose stuff, then apply the bonding agent, mixe the quick Crete patch.
Should that work?

Sakrete says:

David, you nailed it! That’s the process.

David Biller says:

I need to patch a hole in my foundation and after I wet the opening, apply bonding agent, will that work.

Sakrete says:

David, important note: it’s either bonding agent OR water – don’t use both!

W. Keith Brassel says:

What is the cure time for the brown coat

Sakrete says:

That’s going to vary a bit depending on conditions…but rule of thumb? Let it cure for 48 hours before re-coating.

Jimmy says:

2 questions
1-if a oily spot does not come up completely what type of sealer is recommended
2- after using the flow Crete when should you seal the surface

Sakrete says:

Jimmy, you should always wait the full 28 days before sealing any new concrete. However, we always recommend putting down Concrete Cure ‘N Seal immediately after pouring to help with the curing process. It’s especially valuable for these thin “topping” pours that can be prone to cracking if not treated carefully.

That said, Cure ‘N Seal is the go-to for both steps.

Gary Chase says:

What is the best product to bond concrete to asphalt

Sakrete says:

Gary, don’t have great advice for you there…concrete and asphalt are not typically buddies from a bond strength perspective!

Greg says:

If applied properly, what is the strength (psi) of the bond? I am looking for a high-strength bond, so what product would you recommend and what is the strength of the bond assuming 6000 psi concrete strength on both sides of the bond?

Sakrete says:

Greg, all of our repair mixes are rated for 5000 psi, which is SIGNIFICANTLY stronger than almost all concrete that you’d typically have in the wild.

If you want something stronger, Pro-Mix All-Purpose Cement Mix is rated for 9000 psi – definite overkill!

Bart LaBram says:

I need to fill a low area on my drive. I want to feather from 2” down to 1/16 a small section about 2’ wide by 6’. Which secrete material and bonding agent do you recommend

Sakrete says:

Bart, the BEST option would be Pro-Mix All-Purpose Cement Mix. Extremely rapid-setting, no bonding agent required and feathers down beautifully. Great, great mix.

We have also seen some luck with using Fast Setting Cement Patcher, polymer modified with Bonder & Fortifier. While the 1/16″ feather isn’t technically in spec, we have seen applications where the bonding agent adds enough strength to allow you to get down pretty darn thin.

That said, Pro-Mix all the way if you can get it in your area!

Russ Capellari says:

1 of the slabs of my driveway has cracked and 1 side is about a 1/4-3/8 in. Difference in height. I have cleaned out the crack and caulked but would like to blend one into the other to try to get to an original look. The concrete has stoped heaving. What product do you recommend for this repair? I would like to feather the edges to try and get a look better than the caulked crack. Would I have to dig out the caulk or will your product, that I hope you carry to make this repair, will adhere to the concrete and caulk? I’ve tried to grind one edge of the crack to blend in with the other with my 4 inch grinder but I’m not getting any where. I’m sure I could rent a commercial grinder but I would rather patch if possible. Thanks for any info

Sakrete says:

Russ, sounds like Top ‘N Bond would be the PERFECT mix to build a nice, smooth transition between those heaved areas.

Peggy Miller says:

we have a 50 year old concrete slab that we have in-trusted a self employed company to ‘dye’ it for us. I stripped the top layer and failed to place the moisture barrier on top before re-creating the top colored surface. Now we have bubbled-powdered-caked areas through-out our floor which was to be our rec.hall for our RV Park close to $7,000. in loss. how Can I remedy this situation? do I use your Bonding method to try to stop moisture seeping through the surface.

Sakrete says:

Peggy, sorry to hear about that! Hate to hear about bad jobs like this one.

Sounds like a bit of a complex remedy, so it’d be best to give our team a call at 866-SAKRETE to discuss in detail! If you have any photos handy, those are always helpful too.

John Teague says:

What is the best method to patch and match 20 year old joints between slate in a walkway? The joints go deep and some are cracked all are dark with age. I can use hammer and chisel to clean up where broken. Thank you.

Sakrete says:

John, that depends on what the original joints are made out of. Assuming it’s probably some sort of a mortar, but they could also be a Sand Mix Concrete or even just straight Portland cement!

Send us some photos on social, at or give us a call at 866-SAKRETE and we’ll dig into this one together.

Ronnie Standridge says:

Does this bonding mixture react with galvanized steel metal studs?

Sakrete says:

Ronnie, what type of application are you looking at? Bonder & Fortifier is definitely designed to be non-reactive, but we’d like to understand your project a bit more before making a definitive statement. Give us a call at 866-SAKRETE and let’s talk specifics!

Dennis Rasco says:

Repairing holes in cement block. Plan to use darkened Pro Mix Concrete (red striped bad).

1). Do I need to add a binding agent to the mix, or moisten the concrete with a bonding agent, or is water moistening sufficient?
2) Do I need to add anchor screws studs like tapcon, to aide in the repair?

Thank you,

D. R

Sakrete says:

Dennis, great choice of mix!

No bonding agent required for Red Stripe – just water moisten (SSD) and you’re good to go

Tapcons might be helpful if you’ve got a BIG hole, but typically not necessary.

If you’ve got any photos, shoot ’em our way and we can give you some better advice.

Kyle S says:

Hi, question about the bonding coat?!

For best practice, do you require the bonding coat on the existing “concrete” to be dry or a bit wet before applying the bonder on top of it? or is it better to be dry and make it tacky before applying the new concrete on top of it.

Also can this product be added to the cement mix also and coat the existing concrete for more strength?

Looking for best application of the bonding material.


Sakrete says:

Good questions!

You do want to allow the bonding coat to dry A BIT before application. Depending on the product you’re using, you have 12 – 24 hours to top coat before the bonder must be reapplied. If you let it dry to “tacky,” you’re perfect.

You can also add bonding agents directly to the mix, absolutely.

Check out the TDS Reports before starting your project – they’ll have everything you need to know. If you have any other questions, give our team a call at 866-SAKRETE!

Joseph says:

Thank you for the advice
I will shop for sakrete bonder

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