Pouring Concrete Over Patio Stones: A Good Idea?

Pouring concrete over patio stones is a complex process that requires careful planning and execution to ensure a successful outcome. While it is possible to pour concrete over large rocks or boulders, as some people suggest, it is not a straightforward task and carries several risks. In this introduction, we will explore the key considerations, challenges, and potential solutions for achieving a durable and aesthetically pleasing result when pouring concrete over patio stones.

Pouring concrete over rocks

Step 1: Clear the Area

Remove any debris or loose rocks from the area where you will be pouring concrete. It is important to ensure that the area is free of any obstacles that may interfere with the concrete pouring process.

Step 2: Prep the Surface

Use a pressure washer to clean the rocks thoroughly. This step is crucial as it ensures that the concrete will properly adhere to the rocks. A clean surface also minimizes the risk of contamination, which can weaken the bond between the concrete and the rocks.

Step 3: Apply a Bonding Agent

Before pouring the concrete, apply a bonding agent to the rocks. This agent helps the concrete adhere better to the rocks, improving the overall strength and durability of the final structure. There are various types of bonding agents available, including epoxy-based products, acrylic additives, and cementitious bonding agents. Choose one that is suitable for your specific project and follow the manufacturer's instructions for proper application.

Step 4: Mix the Concrete

Follow the instructions on the bag to mix the concrete properly. It is important to achieve the correct consistency and ensure that the concrete is thoroughly mixed. You can mix the concrete by hand or use a concrete mixer, depending on the scale of your project.

Step 5: Pour and Level the Concrete

Once the concrete is mixed, carefully pour it over the prepared rocks. Use a trowel or a similar tool to level the concrete and ensure a smooth and even surface. Make sure to fill any gaps or crevices completely, creating a uniform layer of concrete.

Step 6: Cure the Concrete

Allow the concrete to cure for at least 24 hours. Curing is a crucial step in the process, as it allows the concrete to harden and gain strength. Follow the recommended curing techniques, such as maintaining adequate temperature and humidity levels, to ensure optimal results.

Additional Considerations:

• Thickness of Concrete: Ensure that the concrete is poured to a sufficient thickness, typically a minimum of 4 inches. Inconsistent thickness can lead to cracking and structural issues.
• Rebar: Consider using rebar or welded wire for added reinforcement, especially if the concrete slab varies in thickness. This helps prevent cracking and improves the overall strength of the structure.
• Moisture: Rocks may contain moisture, which can cause moisture-related problems such as cracking or weakening of the concrete. Ensure that the rocks are dry before applying the concrete.
• Professional Guidance: Consult with a professional or contractor if you have any doubts or concerns. They can provide expert advice and ensure that the process is carried out correctly.

Preparing the ground

Before pouring concrete over patio stones, it is important to ensure that the ground is stable and level. Here are the steps you can follow to prepare the ground properly:

Assess the Ground Stability:

Start by evaluating the stability of the existing patio stones. Make sure they are firmly in place and do not shift or move when walked on. If the stones are uneven or unstable, it is best to remove them and start with a fresh base.

Clear the Area:

Remove any debris, loose stones, or obstacles from the area where you plan to pour concrete. It is important to have a clean and clear surface to work on.

Compact the Base:

Use a compactor to compact the base layer of soil or gravel underneath the patio stones. This will help create a solid and stable foundation for the concrete. Make sure the base is compacted evenly to avoid any uneven surfaces.

Level the Patio Stones:

Use a level to ensure that the existing patio stones are level with each other. If there are any significant height differences, you may need to adjust the stones or add a layer of sand or gravel underneath to create a more even surface.

Fill Gaps and Cracks:

If there are any gaps or cracks between the patio stones, fill them with a suitable material such as sand, gravel, or a concrete mix. This will help create a smoother surface for the concrete pour and reduce the risk of cracks forming in the finished concrete.

Consider a Moisture Barrier:

If the patio stones are in an area prone to moisture or water intrusion, consider installing a moisture barrier underneath the concrete. This can help prevent the concrete from cracking or deteriorating due to water damage.

Mark the Area:

Clearly mark the area where you plan to pour concrete. Use stakes and string, or spray paint, to outline the perimeter of the concrete slab. This will help you visualise the area and ensure you pour the concrete within the designated boundaries.

Prepare Forms and Reinforcement:

Install wooden forms or frames around the perimeter of the slab to contain the concrete. Ensure the forms are level and secure. Also, consider using reinforcement materials such as rebar or wire mesh to add strength to the concrete. The reinforcement should be placed within the forms before the concrete is poured.

Slope for Drainage:

If necessary, create a slight slope on the ground to ensure proper drainage. This is especially important if the concrete area will be exposed to water, such as in the case of a patio or driveway. The slope should direct water away from nearby buildings or structures.

Final Checks:

Before proceeding with the concrete pour, perform final checks to ensure the ground is ready. Confirm that the patio stones are stable, level, and free of debris. Also, double-check that the forms, reinforcement, and drainage are properly installed and prepared.

Using rebar

Rebar is a steel bar used in concrete construction to reinforce the concrete and prevent cracking. While it is possible to pour concrete without rebar, it is not recommended, as rebar provides structural support and helps control cracking. Without rebar, cracks in the concrete can become quite wide, and the concrete can become uneven, requiring replacement much sooner.

When deciding whether or not to use rebar in a concrete patio, there are several factors to consider:

• Size of the patio: Larger patios are more susceptible to cracking and movement due to increased surface area and potential for uneven settling or ground movement. Rebar becomes more important as the patio size increases to provide additional support and prevent cracking.
• Intended use: If the patio will be subjected to heavy loads, such as furniture, planters, or vehicles, rebar reinforcement may be necessary to distribute the weight evenly and prevent cracking or settling.
• Climate and environmental conditions: In regions with extreme temperature fluctuations or freeze-thaw cycles, rebar can help mitigate the effects of expansion and contraction, reducing the risk of cracking. Additionally, areas prone to seismic activity or soil movement may benefit from the added stability provided by rebar.
• Soil composition and stability: If the soil beneath the patio is unstable, prone to shifting, or has poor drainage, rebar can help distribute the load more evenly and prevent cracking or settling caused by soil movement.
• Thickness of the concrete patio: Thicker patios generally require less rebar reinforcement, while thinner patios may require more substantial rebar reinforcement to provide necessary structural support. Patios less than 4 inches thick typically require rebar to prevent cracking and ensure long-term durability.

When using rebar in a concrete patio, it is important to place it in the bottom third of the pour, rather than vertically centered, to provide tensile strength and help control cracking. Additionally, pulling the rebar up during pouring so that it is not resting on the bottom of the slab can help prevent it from sinking to the bottom.

While rebar offers undeniable benefits in terms of structural integrity, prevention of cracking, and durability, there are alternative reinforcement methods such as wire mesh, fiber reinforcement, and polymer additives that can also be effective, depending on the project requirements and budget constraints. Consulting with a professional contractor can help determine the best reinforcement method for a specific patio project.

Adjusting the concrete mix

Water Ratio

It is easy to make concrete too wet by adding excess water. This can reduce the strength of the concrete and increase the likelihood of cracks. The consistency of the mix should be similar to stiff cake batter. Adding more water to the mix is also not recommended when pouring concrete over rocks or boulders, as it can reduce the strength of the concrete and lead to potential issues with durability.

Mixing Your Own Concrete

If you are mixing your own concrete, use a ratio of one part Portland cement, two parts sand, and two parts crushed stone. This will ensure you achieve the correct consistency and strength for your patio.

Reinforcement

Reinforcing your concrete will help minimise cracking. You can use either reinforcing steel mesh or 1/2-inch rebar on a 12-inch by 16-inch grid. Place the reinforcement on the bottom of the form and then lift it up during the pour so it sits two inches off the bottom. This will provide better support for the concrete.

Concrete Fibres

Concrete reinforcing fibres will make your patio more durable and crack-resistant. You can add these fibres to site-mixed concrete or ask for them to be included in ready-mixed concrete. Using these fibres in addition to reinforcing rods will ensure the best results and help prevent cracks.

Consistency

When pouring and finishing the concrete, use a straight piece of wood drawn across the top edges of the form to smooth and level the surface. This process is called "screeding". It is important to leave the concrete alone until the surface water dries but the concrete itself is still soft. At this point, you can continue smoothing the surface with a trowel.

Surface Texture

If you desire a rough, high-friction surface on your patio, use a broom to texture the concrete before it hardens. Keep the surface damp with a garden hose or wet sheets and blankets to achieve the desired effect.

Filling gaps between boulders

Step 1: Assess the Situation

Before deciding on a filling material, it is important to understand the cause of the gaps. Are the gaps due to improper construction, weathering, erosion, or damage? Identifying the root cause will help determine the best course of action.

Step 2: Choose the Right Filling Material

The choice of filling material depends on the size of the gap and the desired level of flexibility. For smaller gaps (less than 1/4 inch), flexible caulk, epoxy, concrete patching compound, or sealants (silicone, polyurethane, or butyl rubber) can be used. For larger gaps, a combination of backer rod and non-sag caulk is recommended. Other options include expansion joint filler, concrete repair mortar, and joint covers (metal or plastic).

Step 3: Prepare the Area

Before filling the gaps, it is important to clean and prepare the area. Power washing the spaces between the boulders will help remove any loose debris and create a better surface for the filling material to adhere to. Ensure the area is thoroughly dried before proceeding to the next step.

Step 4: Fill the Gaps

For larger gaps, start by inserting a backer rod into the gap. This will create a uniform base and prevent the filling material from sinking or cracking. Then, apply the chosen filling material, such as non-sag caulk, concrete repair mortar, or a similar product. Smooth and shape the material as needed.

Step 5: Allow for Curing

Once the gaps are filled, allow the filling material to cure and harden. The curing time will depend on the product used, so be sure to follow the manufacturer's instructions.

Step 6: Monitor and Maintain

After the filling material has cured, monitor the area periodically to ensure that the gaps remain filled and no new issues arise. If any problems are noticed, address them promptly to prevent further damage or erosion.

As always, if you are unsure or uncomfortable with any part of the process, it is best to consult a professional for guidance or assistance.

Frequently asked questions