Can You Pour Self-leveling Concrete Over Existing Concrete?

Pouring self-leveling concrete over existing concrete can be good for uneven concrete slabs, but it’s not always a foolproof solution.

Self-leveling concrete can be a practical solution for removing uneven or damaged concrete surfaces. However, there’s more to consider before applying it over existing concrete. Proper substrate preparation, including ensuring the base is structurally sound and clean, is critical.

Employing techniques, such as using tapcons to anchor OSB sheets in low spots, can provide a stable base for the self-leveler to be applied more effectively, ensuring a level finish exceeding the typical max depth of 2 inches when required.

At Concrete Chiropractor, we have decades of experience in concrete repair and leveling, and we’ve seen the full spectrum of results with self-leveling concrete.

While it can work well in certain situations, such as raising and re-leveling minor depressions quickly with minimal downtime, we’ve also encountered cases where it failed to hold up over time due to improper preparation or application.

This article will guide you through the circumstances where self-leveling concrete might be the right choice for your project and when it might not be, offering alternatives to help you make the best decision for your concrete surfaces.

How Does Self-Leveling Concrete Work?

Self-leveling concrete is a cementitious mixture designed to flow and level itself when poured over a surface. Unlike traditional concrete, which requires extensive labor to smooth and level, self-leveling concrete spreads out evenly due to its liquid consistency. It acts as a seamless overlay that is particularly ideal for creating flat and smooth surfaces with minimal effort.

One of the primary applications of self-leveling compounds is in the basement, where the basement floor might be uneven or damaged. When mixed with water, self-leveling concrete forms a slurry that can be poured directly onto the existing cement floor.

Due to gravity, the mixture naturally finds its level, filling in low spots and creating a uniform surface. This feature makes it exceptionally suitable as an underlayment for tile, carpet, or other floor coverings.

The mixture typically includes fine aggregates, cement, and special polymers, which enhance its flowability and adhesion to the existing concrete surface. This innovation in building materials ensures that renovations or repairs can be conducted efficiently and effectively.

Additionally, these polymers improve the material’s flexibility, reducing the risk of cracking, which is crucial for maintaining the integrity of basement renovations.

Once poured, the concrete begins to harden relatively quickly, settling into a stable and level state typically within a few hours, depending on the specific product used and the environmental conditions at the time of application.

When Should You Not Use Self-Leveling Concrete?

While self-leveling concrete can be a versatile and convenient option, it’s not always suitable for every situation. Here are some cases where it might not be the best choice:

1. Large Surface Areas

Using self-leveling concrete over large surface areas, such as warehouse floors or expansive outdoor patios, can be challenging. The material must be applied quickly to ensure a consistent, even layer across the entire surface.

This cannot be achieved in large areas like educational facilities or retail spaces, where precision is crucial for a level and uniform finish—especially if the application is not made in one continuous pour.

This can lead to inconsistencies in thickness and finish, which may necessitate additional work, such as grinding to address the “bird baths” issue on concrete surfaces.

In such cases, alternative methods like concrete leveling or grinding may be more appropriate. These methods can offer more control, ensuring a more consistent result over large areas, and they can be instrumental when dealing with concrete sidewalks where isolation joints must seamlessly integrate with other structures.

2. Outdoor Concrete Slabs

Self-leveling concrete is generally designed for indoor use. Outdoor conditions, such as exposure to moisture, temperature fluctuations, and direct sunlight, can adversely affect the material’s curing process and long-term durability.

Alternative solutions may be preferable to address common issues, such as leveling out bird baths on a concrete patio. Moisture can penetrate the surface, leading to potential problems like cracking, peeling, or reduced adhesion over time.

For outdoor spaces requiring maintenance, such as the 13’x20′ outdoor concrete patio, methods like resurfacing or concrete grinding might offer better long-term performance and durability for outdoor applications.

Additionally, when concrete meets other elements, such as a sidewalk or a house, isolation joints with 1/2″ of asphalt-impregnated fiberboard are advisable to prevent further complications and ensure sleek and even transitions.

3. On Settled Concrete

If the existing concrete has settled unevenly due to soil movement or other factors, such as a lack of proper subgrade compaction, self-leveling concrete may not be the best solution. The material can cover the surface and create a level finish, but it won’t address the underlying issue of settlement.

Over time, the uneven foundation, potentially due to insufficient compaction, may cause the new layer to crack or shift, leading to further problems.

In such cases, it’s essential to address the root cause of the settlement before applying any leveling material. This might involve soil stabilization, mudjacking, or polyjacking to raise and level the settled concrete. Properly resolving compaction-related issues can prevent recurring problems and ensure long-term stability.

4. Vertical Surfaces

Self-leveling concrete is unsuitable for vertical surfaces, such as walls or steep inclines. Its liquid consistency allows it to flow and level on horizontal surfaces, but it won’t adhere or stay in place on vertical areas.

For vertical applications, where a sandwich slab design might be utilized, specialized products like vertical repair mortars or plaster are more appropriate to achieve the desired results.

5. Thick Pours

Self-leveling concrete is typically designed for thin applications, usually between 1/8 inch and 1 inch thick. Pouring thicker layers can lead to several issues, including improper curing, increased shrinkage, and potential cracking. To mitigate these risks, it is recommended to apply a self-leveler in stages.

Suppose a significant thickness is needed to level the surface. In that case, it may be better to explore other methods or use multiple layers of self-leveling concrete, allowing each layer to cure before applying the next.

This approach is endorsed by self-leveling manufacturers for depths exceeding 2 inches and ensures optimal performance of the leveler.

Self-Leveling Concrete Problems You Should Know

Despite its many advantages, self-leveling concrete can present some challenges. Understanding these potential problems can help you make an informed decision and take the necessary precautions to ensure a successful application.

1. Issues with Bonding to Existing Concrete

One of the most critical aspects of self-leveling concrete is ensuring proper adhesion to the existing concrete surface. Initiation of the prep work is paramount as it influences the long-term durability of the flooring project.

If the surface lacks adequate prep requirements, such as not being roughened to an ICR scale of three to five, the new layer may not bond effectively, leading to delamination or peeling over time.

Surface preparation typically involves cleaning the existing concrete to remove dirt, dust, grease, or other contaminants that might interfere with bonding.

Given the importance of prep work, a thorough abrasion or shot-blasting technique may be needed to meet the necessary surface profile for optimum adhesion.

Furthermore, areas exhibiting cracks or damage are a beacon for attention, warranting immediate repair to fulfill prep requirements before the self-leveling compound is applied. Sometimes, a primer may be required to enhance this crucial bond between layers.

2. Cracking and Shrinking

As self-leveling concrete cures, it undergoes a natural process of shrinking. If the material shrinks too much, it can lead to cracking, which fluctuating humidity levels may exacerbate.

Indeed, low humidity can cause the compound to dry too quickly, increasing the likelihood of such defects. This is particularly more likely in environments with extreme temperature changes or if the material is applied too thickly.

To minimize the risk of cracking, it is essential to follow the manufacturer’s recommendations for mixing, application thickness, and curing conditions.

In some cases, adding a reinforcement mesh or using a crack isolation membrane can help reduce the likelihood of cracking and compensate for the effects of humidity and temperature variances on the curing process.

3. Difficulty in Achieving a Uniform Thickness

Achieving a consistent thickness across an entire surface can present difficulties, particularly in large or irregularly shaped areas. If the material is not applied evenly, it can result in trowel marks, or you could end up with areas that are too thin or thick, leading to potential durability issues and a less aesthetic finish.

Using appropriate tools like a gauge rake to spread the material evenly is crucial for maintaining a uniform thickness. In some scenarios, applying self-leveling concrete is an excellent solution to smooth out surfaces and address minor dips without affecting the integrity of the larger structure.

Occasionally, multiple layers may need to be applied to achieve the desired thickness, ensuring all areas lie within the correct depth radius for structural integrity and surface perfection.

4. Troubles with Air Bubbles and Proper Ventilation

Air bubbles can form in self-leveling concrete during mixing or application, leading to imperfections in the finished surface. These bubbles can weaken the material and create an uneven appearance, particularly if they rise to the surface during curing.

Mixing the material thoroughly but gently is essential to minimise air bubbles and avoid excessive agitation. Using a spiked roller during application can also help release trapped air and ensure a smooth finish. Proper ventilation is crucial during curing to prevent moisture buildup and ensure even drying.

5. Challenges in Maintaining a Level Surface During Curing

Even though self-leveling concrete is designed to level itself, maintaining a perfectly flat surface during curing can still be challenging. Factors such as uneven substrate, improper mixing, or external vibrations can affect the final level.

During application, a laser level or screed can help ensure the surface remains level. Additionally, protecting the area from foot traffic, vibrations, or sudden changes in temperature during curing is crucial to achieving a smooth, level finish.

Alternatives to Self-Leveling Concrete

If self-leveling concrete isn’t the right solution for your project, there are several alternatives to achieve a smooth, level surface.

1. Concrete Leveling

Concrete leveling, also known as mudjacking or slabjacking, involves injecting a mixture of cement, sand, and water beneath the existing concrete to lift and level it.

This method is particularly effective for addressing settled concrete or large surface areas. It provides a long-lasting solution that addresses the root cause of unevenness rather than just covering it up.

2. Concrete Grinding

Concrete grinding involves using specialized machinery to remove high spots or uneven areas from the surface of the concrete. This method is ideal for correcting minor imperfections and achieving a smooth, level finish without adding additional material. It’s often used in conjunction with other methods like resurfacing or patching.

3. Patching Compounds

Patching compounds are specialized materials designed to fill small cracks, holes, or depressions in the concrete surface. They are typically used for minor repairs and can be applied directly to the affected area. While not suitable for large-scale leveling, patching compounds can be an effective solution for addressing localized issues.

4. Resurfacing

Concrete resurfacing involves applying a thin layer of new concrete or a specialized resurfacing material over the existing surface.

This method can cover imperfections, add decorative finishes, or create a new, level surface. Resurfacing is suitable for indoor and outdoor applications and can be combined with other methods like grinding or patching for optimal results.

Is It Advisable to Use Self-Leveling Concrete Over Existing Concrete?

Whether or not self-leveling concrete is advisable for your project depends on several factors, including the condition of the existing concrete, the specific application, and the desired outcome.

While self-leveling concrete, often referred to as self-leveler, can provide a quick and effective solution for many projects, it’s not always the best choice, especially for outdoor areas, large surfaces, or unstable existing concrete.

The self-leveling method is advantageous for interiors when creating a smooth and level surface and can be a practical choice if you aim to cover uneven floors with a depth of less than 2 inches.

Before deciding, it’s essential to evaluate the specific needs of your project and consider alternative methods if self-leveling concrete doesn’t seem to be the most suitable option. Consulting with a professional concrete contractor can also help you make an informed decision and ensure the best results.

Who Can Level Uneven Concrete Slabs Near You?

Finding a skilled professional is essential to achieving the best results when leveling uneven concrete slabs. It’s important to choose a contractor who has extensive experience in concrete leveling, resurfacing, and repair.

They should be well-versed in the various techniques available and capable of offering a comprehensive plan and accurate estimate for your project.

Many reputable concrete contractors provide free consultations. During these, they can evaluate your specific needs and recommend whether self-leveling concrete is the best option or if another method would be more suitable.

Before hiring, check references, read online reviews, and verify the contractor’s credentials and experience. This due diligence will help you select a reliable contractor to deliver high-quality work.

At Concrete Chiropractor, we bring decades of expertise in leveling uneven concrete to homes and businesses throughout New Jersey and eastern Pennsylvania. We are committed to providing professional service and delivering results you can trust.

Frequently Asked Questions

Can self-leveling concrete be used outdoors?

While it’s possible, self-leveling concrete is generally not recommended for outdoor use due to exposure to moisture and temperature changes, which can affect its durability. For outdoor projects, consider alternative methods like concrete resurfacing.

How long does it take for self-leveling concrete to dry?

Self-leveling concrete typically sets within a few hours, but complete curing can take 24 to 48 hours, depending on thickness and conditions. Avoid heavy loads or installing flooring until it’s fully cured.

Do you need to grind concrete before self-leveling?

Grinding the concrete surface is often recommended to ensure proper adhesion of the self-leveling concrete. It removes contaminants and creates a smooth surface, enhancing the bond between the old and new layers.