Can You Pour Concrete Over Concrete?

We’ve already let the cat out of the bag and stated that you can pour concrete over concrete, so the quick answer is, Yes. However, before you head out to pour a new layer of concrete over your existing layer, you’ll need to check the condition of your old slab. If there are any signs of damage on the old concrete, you will need to address the flaws before pouring a new layer.

Problems like cracks, discoloration, frost heaves, deterioration (crumbling), or a slab that’s settled to different uneven levels cannot be fixed by pouring a new concrete slab over the top. Likewise, if tree roots are causing the old foundation or pad to heave, or lift, a new slab won’t make matter.

Whatever issue causing severe structural damage to the existing layer of concrete can also do the same damage to the newly poured layer of concrete. For it to be possible to pour concrete over concrete, the existing layer of material must be in good condition and properly prepped.

Will New Concrete Stick to Old Concrete?

While you can pour concrete over concrete, you will need to use a bonding agent with the new concrete because the cement inside the concrete doesn’t have its own. Bonding agents help concrete stick to a surface, including old concrete. Without this help, pouring a new layer of concrete will result in two separate layers. The new layer of concrete will sit on top of the old layer but not fuse, causing the material to be weaker than the original layer.

You must apply bonding adhesives (bonding agents) onto the original concrete before pouring the new layer for it to adhere properly. Bonding agents are also necessary when using new concrete to repair gaps and significant cracks in the existing concrete. But for bonding agents to work properly, the existing layer of concrete needs to be free of fine aggregates or dust, and contaminants like grease and oil.

Pouring Concrete Over Concrete: Pros and Cons

There are advantages to pouring concrete over concrete. However, there are also some downsides that we’ll identify. Weigh these factors when debating whether to pour new concrete over your existing layer or to remove the old slab and start from scratch.

Pros

Improved Appearance

One of the main reasons you may consider pouring new concrete over old is purely aesthetics – visually appealing. If your existing concrete looks unsightly due to stains, pitting, or age, adding a new overlay can give your space a fresher look.

Staining or Stamping

Giving your old concrete a new overlay also provides the necessary wet surface to give your concrete a new finish or design using stamping, staining, or etching. Although these design processes can be challenging for the inexperienced, you may want to let the professionals handle an elaborate concrete decoration for the best look.

Cons

Maintenance

Pouring a new layer of concrete over an existing layer can result in a structure that becomes high maintenance. One issue is due to temperature fluctuations, which can cause the new layer of concrete to separate, lift, or crack. The layers can also be susceptible to frost heaves.

When issues become present, you have to promptly address the cause and repair the damage to prevent the problem from spreading and becoming more severe (and expensive). After pouring a new layer of concrete over old concrete, you should use a deep penetrating water sealant to protect the overlay from water damage.

Short lifespan

Concrete can have a lifespan of 30 to 100 years or more, depending on the type of structure, the use, and its thickness. Pouring a new slab of concrete over old concrete can result in a shorter lifespan for your new concrete surface.

Uneven levels

Resurfacing old concrete can also create a trip hazard due to the height change from the old surface height. Besides potential injury risks, overlays cause a height increase of about 2” thickness, which could cause obstructions that block doors, change step or staircase elevations, or interfere with drainage flow.

Joining New Concrete to Old Concrete

When joining new concrete to old concrete, you’ll need to decide on the type or level of bonding you want to use for your overlay. There are three types of bonding levels that you can use when pouring a concrete overlay on existing concrete.

Fully bonded

A fully bonded overlay (BCO – bonded concrete overlay) goes on an existing textured concrete surface and forms a bond, adhering the two layers of concrete together. Fully bonded is the preferred method when working with overlays that are less than 3” thick. You can find multiple commercial bonding agents that you can apply to your existing concrete with a rag or paintbrush before pouring the new concrete overlay.

Another way to ensure that a new concrete overlay fully bonds to the old concrete surface is with reinforcing bars (rebar) or steel mesh. The metal rods or mesh help pin the old concrete with the new concrete; thinner overlays can use mesh fastened to the surface of the old pad, while thicker concrete would need heavier ⅜” or ½” rebar pieces anchored to the old pad.

Partially bonded

Partially bonded overlays mean that the new concrete goes directly on the existing concrete with no bond or barrier in between them.

Unbonded

Unbonded overlays occur when a layer of material goes over the existing concrete base before pouring the new concrete overlay. The material prevents the overlay from bonding with the existing concrete and instead creates two separate layers of concrete.

Adding a separation barrier stops cracks in the existing concrete from spreading to the overlay. Therefore, bond breakers should offer flexibility and an extended lifespan. Common materials are a thin layer of sand, crushed stone, styrofoam, plastic sheets, tar paper (roofing felt), or Typar house wrap.

How to Pour Concrete On Top of Concrete

To pour concrete on top of concrete, follow these steps.

Step 1 – Cleaning

Before you pour your new concrete over your existing concrete surface, you have to prep the area fully. Start by cleaning the old concrete so it is free of debris and dirt. Power-washing is the best way to get all the dirt and gunk off, use liquid detergent or a commercial degreaser to treat heavily soiled surfaces.

Use a stiff bristle broom or brush to loosen any debris or embedded contaminants on the surfaces. For major issues like tree sap or oil stains, chip out the areas with a chisel and hammer or grinder. Cover large chipped out soiled areas with a sealer.

Step 2 – Form a Perimeter

Form a perimeter around the existing slab using 2-by lumber or ¾” plywood staked or blocked to prevent movement. Make a narrow trench around the existing concrete surface so the wooden forms can sit flush with the sides of the old pad. You’ll want the forms sticking 1 to 2-inches above the old surface, or the level of the new pad. Concrete won’t normally stick to wood, but you can coat the insides of the forms with vegetable oil for easier removal once your concrete cures. You’ll also want to mark the edges for control joints if necessary. Joints should line up with the old concrete to not be visible.

Step 3 – Bonding

If you’re pouring a fully bonded overlay, this is when you add your bonding agent to the existing surface so the two layers bond together. Make sure the layers bond better by creating a soupy cement mixture using a ratio of 1:7 and applying this scratch coat to the old cement, filling all crevices and cracks using a rag or paintbrush. This coat does not need to dry before you pour your concrete overlay.

Bonding with Mesh or Rebar

If you are using mesh or rebar to form a stronger connection between new and old layers of concrete, you’ll need to complete these steps.

Mesh – for 1 to 3-inch thick overlays

Step 1 – Lay the mesh out on top of the old concrete so it covers but is 3” in from all edges. The type of mesh is determined by the thickness of the pour.

Step 2 – Use stainless steel clips and shot-nails or screws to fasten the mesh randomly to the old surface, lifting the mesh slightly off the surface (no more than ½ the thickness of the pour) so that the new concrete can flow around the wire.

Step 3 – Paint a metal primer on the mesh, so there are no problems with rusting.

Rebar – for pours greater 3-inches or thicker

Step 1 – Drill ½” or ⅝ diameter holes ¾ of the depth of the pad, 12” on center, and 6” from edges to avoid chipping.

Step 2 – Flush or vacuum the holes to clean them.

Step 3 – Fill the holes halfway with epoxy injections.

Step 4 – Twist or tap the rebar into each hole, ensuring there’s an even coat of epoxy on the rebar as it sets in the hole.

Step 5 – Depending on the rebar lengths, bend them before or after insertion in the epoxy filled holes. The steel should form a lattice work of reinforcing that is 1-½” below the finished surface of the new concrete.

Step 6 – Paint a metal primer on the rebar, so there are no problems with rusting.

Step 4 – Create Your Mixture and Apply

Once you’ve formed up the new concrete perimeters and have prepped the existing concrete with a bonding agent (or left it unbonded), mix up your cement and pour it into the formed area. A hand trowel or shovel makes it easier to spread the mix and even it out.

You’ll get the best results by using a thicker concrete mixture, achieved by reducing the amount of water added to the dry concrete mix. For thinner concrete use crushed stone or fine sand, for thicker pours use coarse aggregate. Spread the mixture over the entire surface of the existing concrete and any areas where your overlay expands past the old pad.

Use a float or a 2” x 4” to run over the wet concrete to make it level and smooth. Finally, if you’re adding any textures using tools or a broom, staining, or stamping, you’ll need to do it before the concrete fully sets up or you add a curing compound to form a protective layer on the surface. Once the surface is set, cut in the control joints and cover the area with plastic, leaving it to cure (dry) for 3 to 7 days.

Concrete Thickness Overlay on Old Concrete

When pouring concrete over old concrete, 1” to 2 ½” is the minimum thickness for a bonded overlay provided the base is in good condition. For overlay thicknesses up to 5”, bonding the layers provides strength and preventative maintenance.

Overlays less than 1” (25 mm) uses small aggregate (⅜” or less) with high sand content. Thin overlays, though, have more shrinkage with a higher chance of curling, cracking, or debonding. Thus, they are mostly used when restoring worn surfaces but not for adding strength.

Unbonded overlays often have a minimum thickness of 4” to 11” and are suitable for when the base slab has cracks and will serve only as structural base support. Cracks will show through a partially bonded or unbonded overlay; thicker overlays will prevent transfer cracking. Rebar or welded wire fabric reinforcement will also hold the overlay slab and prevent cracking.

Stamping Concrete Over Concrete

You can only stamp concrete when it’s wet, shortly after pouring the new layer. For this reason, you may decide to pour an overlay of new concrete simply to create a stamped pattern. Two methods work for stamping concrete – hand stamps or textured rollers. You can find stamp patterns in designs like pawprints, stones, bricks forming a circle, or basic brick stone textures.

Before you can pour and stamp your new concrete surface, it needs to be thoroughly cleaned, preferably with a power washer. Once the new 1” to 2” concrete overlay goes down and is smoothed out, you have about 15 minutes to apply the stamped design before the concrete hardens too much to impress the design. Due to the short window of opportunity, it works better if you have one or more helpers so you can get the entire surface finished before it dries. It also helps to keep your supplies nearby, so you’re not wasting time having to move around to retrieve items. Finally, when covering large areas, choose a large stamp.

Pouring New Layer Over Existing vs. Resurfacing Old Layer

Before you decide to pour a new layer of concrete over an existing layer, check to see if resurfacing the old layer will work instead. Resurfacing the existing layer of concrete is a fantastic way to save money (and resources) versus pouring a new layer of concrete. However, it will not fix underlying issues like heaving or lifting, cracks or damage from thawing/freezing. For these issues, you’ll likely need concrete replacement.

You can use resurfacing for hiding imperfections, discolorations, outdated finishes, concrete that’s aged and unsightly, or for fixing slight cracks. You have the option of doing a basic resurface or making it decorative, using color, stains, stencils, stamps, or engravings, which is similar to creating an overlay. Similar to pouring a new layer over existing concrete, resurfacing old concrete will require cleaning and prepping the surface.

Conclusion

You can pour a new layer of concrete over an existing concrete surface. You may want to update or enlarge a surface, or to repair minor issues like surface cracks, chips, nicks, or discoloration. Adding a new layer over old can save time, money, and resources. Hopefully, you have a better understanding of pouring concrete over old concrete.