Topping concrete refers to the application of a layer of high-strength concrete onto an existing, worn-out concrete surface. The purpose of this process is to create a new, dense and abrasion-resistant surface while also increasing the structural depth and strength of the base concrete.
Fig 1: Section of topping concrete over existing slab.
The procedure for laying topping concrete involves several steps. First, the existing concrete surface must be thoroughly cleaned and prepared to ensure proper adhesion of the new layer. Any loose or damaged areas should be repaired, and the surface should be roughened or scarified to provide a better grip for the new concrete.
Next, a bonding agent is typically applied to the prepared surface to further enhance the adhesion of the new layer. Once this has been done, the topping concrete can be poured and spread evenly over the surface. Troweling or screeding may be required to ensure a smooth and level finish.
There are different types of topping concrete that can be used depending on the specific needs of the project. For example, polymer-modified topping concrete is a popular choice because it provides enhanced durability and resistance to chemical and environmental damage. Self-leveling topping concrete is another option that can be used to create a smooth, level surface over uneven or sloped areas.
Overall, there are several advantages to using topping concrete. In addition to providing a durable and abrasion-resistant surface, it can also improve the overall structural integrity of the base concrete. This can be especially beneficial for older or heavily used concrete surfaces that may be showing signs of wear and tear. Additionally, topping concrete can be a cost-effective alternative to completely replacing the existing concrete surface.
Work Procedure of Laying Topping Concrete
1. Base Concrete Requirement
Before laying topping concrete over an existing concrete surface, a preliminary assessment is necessary. The assessment should determine if the existing concrete is hard and strong enough to support the new layer. Additionally, the base must be free of any cracks, as the topping concrete cannot bridge over them. If there are any cracks present in the existing concrete, they may eventually show up in the topping concrete as well. Therefore, it is crucial to ensure that the base is free of cracks to prevent any future issues.
It is also important to note that weak and fragile concrete bases are not recommended for the laying of topping concrete. This is because a weak base cannot achieve proper adhesion between the old and new concrete layers. As a result, the topping concrete may not properly bond with the existing surface, leading to potential issues down the line. Therefore, it is essential to assess the strength of the existing concrete base before proceeding with the laying of topping concrete.
2. Preparation of Base Concrete
Proper bonding and adhesion of old and new concrete is a critical step in the laying of topping concrete. The surface of the existing concrete on which the topping slab will be placed must be uniform, clean, and free from dust particles or any other contaminants. Any existing topping concrete must be completely removed before proceeding. If the base concrete has been finished smoothly, the surface must be roughened using a mechanical scabbing or scarification machine, exposing the coarse aggregate of the base slab. Loose debris, dirt, and dust should be thoroughly removed using a vacuum cleaner.
Fig 2: Surface preparation for laying topping concrete.
The proper timing of base concrete preparation is crucial to ensure a clean and uncontaminated surface for the topping slab. It is recommended to complete the base preparation at least one day before the placement of the topping slab to prevent any accumulation of dirt or contaminants. This time gap allows for adequate curing of the base concrete and minimizes the risk of contamination, ensuring a high-quality finish for the topping slab. By following this timeline, potential issues that may arise from dirt or debris getting trapped between the base and topping slab can be avoided, resulting in a more durable and aesthetically pleasing final product.
3. Placing of Topping Concrete
Before applying topping concrete, it is essential to assess the condition of the existing slab. This involves pouring a small amount of water on the base slab and observing the rate of absorption. If the water is absorbed within a few minutes, the base slab needs to be kept moist for four hours prior to placing the topping concrete. However, the water must be thoroughly removed just before applying the topping concrete. This precaution is necessary because if the wet concrete is placed on the base slab, it may absorb all the water content from the topping concrete, resulting in a deficiency of water. On the other hand, if no water absorption is observed, the topping concrete can be laid as per the normal process. The thickness of the commonly applied topping slab ranges from 50 to 100 mm, and the steel reinforcement is provided based on the thickness and requirements of the topping slab.
Fig 3: Laying of topping concrete
The surface preparation for laying the topping concrete involves applying a slurry made from cement, sand, and a latex bonding agent. This slurry is poured onto the surface and should be applied while it is still wet. The composition of the concrete mix for the topping will depend on the desired strength requirement, with a minimum recommended grade of M20. The concrete mix will typically consist of aggregates, sand, and cement. Once the topping concrete is poured, it should be spread evenly, compacted, screeded, and bull floated, following the standard procedures used for pouring a concrete floor.
4. Finishing
The finishing required for topping concrete is influenced by various factors, such as the location, intended use, and surface requirements. If the topping concrete is placed on an existing slab, a rough finish is typically applied to facilitate the installation of flooring material. However, if the concrete is being placed on top of a floor concrete surface, a smooth finish can be achieved.
It’s essential to consider the intended use of the topping concrete to determine the appropriate finishing technique. For instance, a rough finish may be suitable for outdoor areas or places where slip resistance is a priority. In contrast, a smooth finish may be more appropriate for indoor areas that require an aesthetically pleasing surface.
Ultimately, the finishing technique chosen will depend on various factors, such as the location, usage, and desired appearance of the surface. By considering these factors, the appropriate finishing technique can be selected to ensure that the topping concrete meets the necessary requirements.
5. Curing
In order to achieve maximum surface strength, maximum resistance to surface abrasion, and low impermeability of concrete, it is essential to ensure effective curing. This process also helps to minimize the development of drying shrinkage cracks. Furthermore, effective curing can reduce the effect of differential shrinkage, which in turn reduces curling, by delaying the impact of differential drying until the concrete is better equipped to resist it.
It is crucial to note that curing must be done for a minimum of 7 days to obtain the desired results. Failing to properly cure the concrete can result in reduced strength, increased porosity, and the formation of cracks, which can compromise the structural integrity of the concrete. Therefore, it is important to prioritize effective curing to ensure the longevity and durability of any concrete structure.
Types of Topping Concrete
Topping concrete is differentiated into 2 types based upon the type of bonding that is formed between old and new concrete.
1. Fully Bonded Topping
When placing a fully bonded concrete overlay on a base slab, it is recommended that the minimum thickness be between 1 to 2 inches (25 to 50 mm). This is assuming that the base slab is in good condition, with no significant cracks and the concrete is of good quality. In such cases, welded wire fabric reinforcement is usually not necessary. However, it is worth noting that if the existing base slab has cracks, they can be expected to appear through the new overlay.
For conventional concrete overlays that are less than 1 inch (25 mm) thick, the use of small aggregate with a maximum size of 3/8 inches (10 mm) and a high sand content is necessary. This is because thinner overlays have a higher water demand and are more prone to shrinkage, which increases the likelihood of cracking, de-bonding, and curling. It is also worth mentioning that most thin-bonded overlays are used to restore the wearing surface rather than to add strength to the slab. This is typically sufficient for light-duty floors that require restoration of serviceability.
Although the formulas for placing overlays were originally developed for highway and airport pavements, they can be adjusted and used for floors as well.
2. Partially Bonded or Unbonded Topping
Unbonded construction with a minimum thickness of 4 inches (100mm) is recommended in situations where the base slab has cracks or where it is difficult to achieve good bond between the new and existing surfaces. Both light-duty and heavy-duty floors can benefit from this type of construction because the existing slab provides a good base for the new floor surface. However, unbonded overlays are typically thicker than bonded overlays, which can create challenges for other service requirements such as doorway clearances.
Even if cracks in the base slab are repaired, they may eventually reflect through a partially bonded overlay. To address this issue, properly designed welded-wire fabric reinforcement can be used to hold the overlay slab together at any cracks that may form. However, placement of the fabric in a thin, partially bonded or unbonded overlay requires special precautions to ensure that it is positioned at the proper level below the surface.
To reduce reflective cracking, thicker overlays should be used.
Advantages of Topping Concrete
Increasing the thickness of a flooring system can offer various benefits. One of the significant advantages is that it provides a clean surface to hide any flaws that may be evident in the original concrete surface. Furthermore, a thicker flooring system can be leveled and flattened, providing a polished and uniform appearance or acting as a solid base for alternative flooring products.
Another advantage of thicker flooring systems is that they can strengthen weak and fragile concrete surfaces. This can be particularly important in industrial or commercial settings where heavy equipment is frequently used. A thicker flooring system can also reduce wear and tear on equipment by providing a smoother drive surface, ultimately leading to reduced repair costs for the finished construction.