Every so often, a construction team or a DIYer will come upon a problem that’s difficult to solve with conventional knowledge. One such problem involves pouring new concrete over an existing concrete structure.
On its face, this appears to be a simple enough procedure. After all, two types of concrete should be able to bond to each other easily…right?
Well, as it turns out, the process requires more foresight than that.
This guide will take you through the steps needed to accomplish this task without leaving your new concrete slab in a weakened state. To that end, this guide should effectively answer if pouring new concrete over existing concrete is even a wise construction method in the first place.
Can You Pour Concrete Over Concrete?
This is a common question among contracting teams as well as DIYers.
Both groups may be surprised to learn that you can, in fact, pouring new concrete over an existing concrete structure. Not only is it possible, but such a procedure is routinely performed in situations where an individual does not want to spend the time and money necessary to fully rip out and replace an existing concrete slab.
Instead, these individuals simply need to take certain steps (outlined below) to prepare their existing damaged slab before pouring on the new wet concrete. However, just because this can be done does not mean that it is a perfect solution to repairing concrete.
Using this method tends to have a shorter lifespan than expected, for example. That means that you’ll likely need to repeat this process before long, especially if your concrete surface in question sustains a lot of wear or pressure on a regular basis.
How to Pour Concrete Over Concrete
In either case, should you decide that this is the best way to get your job done, continue reading for step-by-step instructions.
Step 1: Prepare the Surface
To begin, you’ll need to properly prepare your existing concrete slab’s surface for a fresh application.
This can be done in several ways, including simple brushing with a broom. However, a strong scrub with a power washer is advised in order to remove any concrete sediment and shards. Ground-in dirt, sand, and leaves can also inhibit the adhesion process, so ensure any residual grim is properly lifted from the surface before proceeding.
Step 2: Prepare the Forms
Next, you’ll want to prepare your concrete molds just as you would if you were working from scratch and pouring an entirely new slab.
However, while raising your forms, be sure that their sides are fully flush with the remaining sides of your old slab. Otherwise, there’s a risk for new, wet concrete to seep into that gap and cause the slab to become uneven at that edge.
Often, the process of pouring new concrete over old concrete can cause its overall thickness to increase noticeably. As such, you may find it prudent to add a mesh or rebar structure to your new hybrid concrete slab. If you choose this route, said mesh or rebar should be placed and positioned after your forms are set.
Step 3: Apply Bonding Agent
The secret to successfully welding an old concrete slab to a new batch of concrete lies in the bonding process. However, this process won’t fully take place on its own, regardless of what kind of concrete you decide to use. Instead, you need to plan ahead and apply a specialized bonding agent prior to the application of your desired concrete mixture.
These kinds of bonding agents are commercially available and can often be mixed on-site with only water.
Once you’ve mixed up your bonding agent, you’ll want to apply it across the entirety of your existing concrete surface. This includes every gap, crevasse, and crack.
Once a full layer has been applied, you should pass back over the bonding agent’s surface with a hand trowel to create bonding channels (much as you would when preparing a bonding agent during the tile application process).
Step 4: Mix, Pour and Cure
Once you’ve properly applied your bonding agent, you’re ready to mix up your new concrete and pour it into the mold. Ideally, you should choose a similar concrete mix ratio to your old concrete slab. This will help the new and old slabs work together effectively when mitigating both lateral and horizontal pressure.
As always, be sure to follow your concrete mix’s instructions so that it comes out to proper consistency and cures as expected.
Once your concrete is poured, it’s time to let the whole hybrid structure cure. This includes allowing proper curing time for your bonding agent, which likely requires more curing time than the concrete proper.
Do not attempt to “test” your concrete until well after both drying periods have elapsed.
Mistakes to Avoid, Tips & Tricks
There are a few mistakes that you’ll want to avoid while planning for and carrying out a concrete-on-concrete project.
The first of these is forgetting to use or intentionally forgoing a bonding agent.
No matter what mix ratio you use, new concrete on its own is simply unsuited to bond to an existing concrete structure. Failing to use a bonding agent, therefore, can cause the structure to become unstable and crumble soon thereafter.
Also, while preparing to perform this type of task, some DIYers forget to account for a rise in level resulting from the stacking of two concrete layers. If you’re working on a sidewalk or driveway, this may not be a huge issue (other than introducing a potential tripping hazard). But if you end up raising the level of a concrete patio, for example, you may find yourself in a bind with regards to spacing.
Summary
If you came into this guide wondering if pouring concrete on top of concrete was even possible, your question should be effectively answered.
Not only is this process possible, but it only requires a bit of extra planning (and a quality bonding agent) to be performed properly. You know now those crucial steps, so you should be able to complete a concrete-on-concrete pour in the future when a professional or DIY job requires it.