Can I Repair Old Concrete by Pouring New Concrete Straight Over It?
When there is no structural damage to old concrete, it’s perfectly fine to pour a new layer over it. Minor imperfections are not a big deal, but big damage is a sign of big problems. If those problems are not addressed and fixed, the new layer of concrete will not survive for long.
Even if the concrete looks like it’s in good shape, but the work was done a very long time ago (definitely 15 years or more), it may be a good idea to get in an expert to take a look first.
How much damage is too much?
It’s mostly multiple and/or very deep cracks. Pretty much, if the surface looks more like a mosaic or a set of a Marvel movie, you’re in trouble.
This may have happened either because of natural phenomena like earthquakes, poor craftsmanship, or additional construction work. In all of those cases, it is not enough to just fill in the cracks because the damage runs a lot deeper.
What if I there is too much damage?
You will have to break the old concrete and maybe even fix the ground underneath as well. However, the good news is that if the damage is concentrated only in one section, you may need to rebuild that from scratch, and then just pour a thin layer over the other areas to blend it in.
Will the new layer stick to the old concrete?
Yes, as long as the surface is prepared properly. Also, a concrete bonding adhesive will ensure that all layers stick together,
To prepare the surface, start by sweeping and removing all dirt and debris. Then rinse the surface with a pressure washer – you’ll need one that can hit at least 3000 PSI. Hold it about 5 or 6 inches above the surface and clean out every bit of the area.
You can also use a special chemical cleaner that is designed for cleaning concrete. You may also need to use a degreaser if you have any oil spills anywhere.
What is the minimum concrete thickness over existing concrete?
At least 1 to 2 inches. Depending on the type of job, anything less than that may end up cracking or looking uneven.
A layer that small will not usually need wire mesh reinforcement, but for bigger ones, it’s a must. You can find that mesh in any home improvement store, and you will have to lay it down before you add your first layer of concrete.
Keep in mind that thin layers have only a cosmetic purpose and they are far less likely to have the longevity of a project that was done from scratch.
How to mix concrete when applying over the existing one
Always follow the manufacturer’s instructions for correct ratios.
A big slab of concrete is usually applied in two layers – a primer/base coat, and a top/finishing layer. The former is usually twice as thick as the latter. Usually, that means 7 parts concrete to 1 part water for the base and 3 parts concrete to 1 part water for the finish, but it may differ based on the product itself.
What type of concrete to use
Pretty much, anything that is available in stores, as long as you get the right size.
Fine aggregate is good for small repairs and touch-ups (as well as creating household items), while you’ll need the coarse aggregate for creating thick slabs.
Read the back of the package as well. Some brands add adhesive to their product. You can also buy an adhesive separately and add it to plain concrete when you’re mixing everything.
How to pour concrete over the existing sidewalk
First, it’s time to get a lay of the land. The sidewalk will gain at least 2 or 3 inches in height, and you should see how will that work with the rest of the area. Also, take note of any manholes – if there are any on that patch that you are working on, you may need to get the city involved and probably get some permits.
Speaking of permits, check with your local authorities if you need them to start with the project at all. If you are a part of an HOA, you will need to clear the work with them as well. In that case, don’t even buy the materials before you see if they don’t have specific instructions for concrete work in their bylaws.
Then it’s time to take a closer look. Make sure that any cracks or damage are from regular wear and tear, and not from a bigger issue. You want to see only superficial damage. Anything more serious than that will call for the removal of at least the damaged section.
Once you have a green light, it’s time to prep the area first. If there’s any vegetation (especially weeds) peeking through, pull them out completely. Then clean the sidewalk of any dirt and gravel. If the sidewalk is bordering soil and grass, pay a little extra attention to that area.
Speaking of, there is a chance that you may need to do some digging to that side, or that you will do some damage there when you set if the forms for pouring concrete. If you have to do any of that digging, do it before you start with the cleaning and the prep.
Set up the concrete forms. You can use any you prefer if you’re working with straight lines, but plastic ones are highly recommended since they are both versatile and reusable.
Soak the old concrete until it stops absorbing water and it starts running on the sides. Dry slabs don’t bond well, so don’t skip this part.
For the next step, place the mesh reinforcement. Even if you plan on applying a thin layer of concrete, don’t skip this step. A sidewalk is a high traffic area and is very prone to damage. This small additional step (and expense) will lengthen the lifespan of your work.
Next, it’s time for concrete. Larger aggregates are better for the job here, though finer ones will work if you’re trying to keep your layers as thin as possible. In any case, remember to use the same stuff for both layers so that will stick to each other properly.
Mix the thicker, primer layer per manufacturer’s instructions. The first coat is very similar to the crumb coat in cake baking (here it’s called scratch coat). The layer should not be thicker than 1/8 of an inch and it’s there to help new and old concrete bond with each other. Press hard to spread the mixture and to push it into any possible cracks.
Then, mix the final layer per instructions at the back of the packet. If you need to, add the concrete bonding adhesive as well. Without waiting for the scratch coat to dry completely, pour in your new mixture to the desired level. It’s not a good idea to make multiple pours, so try to get it all even and uniform on the first go.
Before it dries, use a trowel or a rolled to smooth it all over. Don’t forget to work as quickly as possible, especially in summer.
Depending on the product you’re using (or your preferences) you may want to finish the work with a concrete cure. This may mean that for an extra week no one will be allowed to step on that part of the sidewalk, but it will make the concrete stronger and last longer.
How to pour concrete over an old basement floor
When pouring a new layer of the concrete inside (either in a basement, garage, etc), the most important thing you have to pay attention to are the edges that meet the wall. If you’re working from wall to wall, there is no need for forms or any other barriers,
Keep in mind what’s going on with stairs, doors, and other elements – the floor is just about to rise at least an inch or two, and you may need to be able to open a door if there’s one.
When adding only a cosmetic layer, you don’t have to use the wire mesh. Though, it will be advisable to have it since it will both make the final result more durable, as well as it will make it easier to spread the concrete mixture itself.
Speaking of getting everything even. there is actually a trick to it. Use a trowel to even out the mixture close to the walls first and flatten the lip that has formed. Then, instead of rollers, take a wooden plank and smooth the surface by scraping it away from the walls. You’ll end up with excess concrete on the planks – that’s fine, just remove it regularly and continue until everything is even.
Outside of this, the rest of the technique is the same as when applying a new layer of concrete to any other location. Just remember to keep everything well ventilated while you’re working.
How to pour concrete over an old driveway
You are looking at at least 2 inches of extra concrete when layering it over an old driveway. A driveway that can fit one car may be a relatively feasible project for a regular person, but everything bigger than that, you’ll have to call in a contractor.
Start by inspecting the existing surface. Again, look for cracks that may have roots, weeds, and other growth inside, as well as whatever damage that was caused by your vehicle. If any damage is just superficial and cosmetic, you can proceed. Pull all the weeds by the roots. It would not be a bad idea to salt that bit as well (do this at least a day before you start your project).
Clean the driveway. Sweep up the dirt and debris, paying special attention to loose pieces of gravel. Then, give the whole thing a blast with a pressure washer, and finish with a concrete cleaner.
Set concrete forms around the parameter. For a curved driveway, get plastic ones (as a bonus, these guys can be reused). As in other cases, soak the surface with water before you apply the primer layer, and then the final one.
Try not to forget the concrete adhesive since a driveway is bound to experience a lot of wear and tear. Also, it may be a good idea to use a coarse aggregate since you are adding a thicker coat.
How to pour new concrete over old concrete steps
If you are looking to make a major change to those steps (ie make them wider), definitely leave it to the pros. They are most likely going to recommend taking out the old steps and starting from scratch. There is a good reason for this because it is very likely that cracks are going to form, either in corners or in between new and old concrete.
Another reason to call in the pros is if you’re fixing the steps because there is a corner missing. Though it may seem easy to just build a frame and pour in some concrete, it actually takes a bit of experience and expertise to know how to do it so it would not break off again.
Unless there is a real need for touching up the sides of the steps, it’s better to skip that part altogether. Even a thin layer added to the side will be prone to chipping and may even completely break off.
To pour concrete over the top of the steps, start by creating a frame with concrete forms. You will need to get riser forms for this job – they are cut on the bias on one side so there would be no gap between new concrete and the next step.
Create the side frame first, then attach the riser forms in the front. Follow the same steps as if working with any other surface, but make sure that you are working from top to bottom.
If the steps are curved or uneven in size/width, it would be better to leave the job to the pros because there are high chances of you messing up your mold.
How much of this work can you do on your own?
The most difficult part of working with concrete is creating molds and barriers. As long as you can do that, you should be good to go.
Another setback could be the size of your project. The bigger it is, the more concrete you will need. That also means more concrete will be there to mix, so you may need to get your hands on a concrete mixer.
Though most home projects will be fine if you use a plain concrete roller, anything larger may need some heavy machinery as well. Of course, you can rent all of these things, but knowing how to operate them is an absolutely different story.