Bonding to Existing Concrete

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.

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. So that you don’t waste too much time or money, we should probably discuss what “structurally sound” means. If your 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 your 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 you have 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 bags of one of the Sakrete concretes.

Since this discussion is on the best way to bond concrete, we will assume that your slab is good.

There are a variety of Sakrete concrete repair products available to fix concrete that has begun to deteriorate. However, without good surface preparation, none of them are going to perform satisfactorily. 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.

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

Let’s discuss the mechanical approach 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 nocks 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.

The chemical approach involved mixing up a liquid bonding agent that helps bond new concrete products to old. Products like Sakrete Top'n Bond and Sakrete Flo-Coat already contain polymers that greatly improve the bond of portland cement and should NEVER be used with a liquid bonding agent. I know in America bigger is better but it’s just not so with these products. Other products like Sakrete Sand Mix and Sakrete Fast Set Cement Patcher benefit from the use of a liquid chemical bonding agent such as Sakrete Bonder & Fortifier. 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. Just as in the process described above, after the bonder has become tacky apply a scratch coat and 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.

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. 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.

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 (not like some of my party guests) rather than a hollow endeavor.



  • Lee-Technical Service

    Roy, the Top-N-Bond would be the better product to fix your areas that puddle and it can be colored. The problem that you will have is actually matching the color that you have already. The part will take some trial and error with small samples and measuring each of them meticulously. This will not be an easy task and almost impossible in most cases.
  • Roy

    I have a newly poured, colored patio slab with a ~4'x4' section that puddles and slightly slopes toward the house. Would Top-N-Bond be the right product to slope this portion of the slab away from the house and eliminate the puddling? Can this product be colored? I understand I'll need to wait 28 days - is any other surface prep or bonding agent needed? Thank you!
  • Lee-Technical Service

    Shane, It really depends on what you are trying to achieve and what exact products that you will be using. IF you are using concrete mix, you are correct, you may not get a bond without a bonding agent. Also keep in mind that concrete need to be poured a minimum of 2", so you need to make sure that the added height will work. There are a lot of variables to consider before starting this project. Can it be done, with the right products, yes. You will need do some measuring and planning. Let us know if we can help, call 800-334-0784.
  • Lee-Technical Service

    Mike, give the area 28 days, from the day you poured it, to cure. Then you can take the Top-N-Bond Concrete Patcher and fix the low spot. Top-N-Bond can be placed from 1/2" down to a featheredge. If you need to do multiple layers, then give it an hour to harden and then place the next layer. Be sure that all layers are placed before 24 hours. If you wait longer than 24 hours then you will have to wait 28 days to finish. You may also want to consider doing a thin skin coat over the entire newly poured area so that the color will be uniform. If you are not worried about the color then you would not need to skim coat it.
  • Shane

    My wife wants to use quikrete on our concrete side walk I told her I didn't think it would Bond but she's wanting to do the do it yourself cobblestone look. Do you have any suggestions
  • Mike

    I just poured a new threshold for a garage door. Now that it is dry and the forms were pulled, one part settled a bit more and I have a dip. Can I pour a patch over this new, but dry, concrete to bring it level?
  • Lee-Technical Service

    SkyMaster3, yes, you will still have to wait 28 days before sealing. Unless the sealer manufacturer states otherwise. Breathable sealers can be placed sooner but you would have to consult the manufacturer for that info.
  • SkyMaster3

    This may be along the same vein as Adam's question, but I have a large concrete slab that has a few holes and hair-line fractures. I am using a sand mix and scratch coat to patch the inperfections. Do I need to wait the 28 days to seal and paint if its just several small patch holes? Thanks!
  • Lee-Technical Service

    Gerald, the Bonder and Fortifier will turn clear when it dries. This means the material is ready for you to begin your project.
  • Gerald

    How do you know for sure that bonder/fortified has adhered properly...before putting mud down?...ty in advance....
Load More View All

Leave a Comment:

Back to Blog