Topping concrete is a technique used to enhance the durability and strength of an existing concrete surface. It involves placing a layer of high-strength concrete on top of an older, worn-out surface. This layer provides a dense and abrasion-resistant surface, which can significantly increase the structural depth and strength of the underlying concrete.
The process of topping concrete involves preparing the existing concrete surface by cleaning it thoroughly and ensuring that it is free from any contaminants that could hinder the bonding of the new concrete layer. The new layer is then placed on top of the old surface and spread evenly, usually to a thickness of several inches. The surface is then smoothed and finished to achieve the desired appearance and texture.
Topping concrete is often used in commercial and industrial settings where the surface is subject to heavy traffic, abrasion, and wear. It can be applied to a variety of surfaces, including floors, walls, and roofs. The use of topping concrete can help extend the life of an existing concrete surface, reduce maintenance costs, and improve the overall appearance and functionality of the space.
Fig 1: Section of topping concrete over existing slab.
The focus of this article is to provide an understanding of the process involved in laying topping concrete, along with its various types and benefits. The article aims to explain the procedure of laying topping concrete in a comprehensive and informative manner.
In addition to discussing the work procedure of laying topping concrete, the article also highlights the different types of topping concrete that are available. The information provided in the article is intended to help readers choose the right type of topping concrete for their specific needs.
Furthermore, the article emphasizes the advantages of using topping concrete for construction projects. These benefits include increased durability, improved aesthetics, and better resistance to wear and tear.
Overall, this article serves as a guide for individuals looking to learn about the work procedure of laying topping concrete, as well as its various types and benefits. The information provided can help readers make informed decisions when it comes to using topping concrete in their construction projects.
Work Procedure of Laying Topping Concrete
1. Base Concrete Requirement
Prior to the application of topping concrete onto an existing concrete surface, it is necessary to conduct a preliminary assessment. One crucial aspect of this assessment is ensuring that the existing concrete is both hard and strong. Additionally, it is important to ensure that the base is devoid of any cracks, as topping concrete does not effectively bridge over these cracks. Over time, these cracks may manifest in the topping concrete, which can lead to structural issues.
Moreover, weak and fragile concrete bases are not ideal for this purpose as they do not establish proper adhesion between the old and new concrete. This can result in a weak bond between the two layers, leading to the formation of air pockets and other imperfections. As a result, a thorough examination of the existing concrete surface is essential before proceeding with the laying of topping concrete.
2. Preparation of Base Concrete
When laying topping concrete, it is crucial to ensure that the old and new concrete adhere properly to one another. To achieve this, the surface on which the topping slab is to be placed must be uniform, clean, and free from any contaminants such as dust particles. Any existing topping concrete must be removed before the new concrete is laid.
If the base concrete is smooth, it must be made rough to expose the course aggregate of the base slab. This can be done by using a mechanical scabbing or scarification machine. The loose debris, dirt, and dust must be removed thoroughly using a vacuum cleaner.
By taking these steps, the surface will be ready to receive the new topping concrete. This preparation ensures that the topping slab will bond correctly with the base slab, creating a strong and durable structure.
Fig 2: Surface preparation for laying topping concrete.
The preparation of the base concrete is a crucial aspect that requires careful consideration. It is essential to ensure that the base is prepared a day before the placement of the topping slab. This measure is necessary to prevent any potential contamination or accumulation of dirt on the base. By allowing a day for the base to be prepared before placing the topping slab, one can effectively mitigate any risks associated with contamination or dirt buildup. Therefore, it is imperative to give due attention to the timing of base preparation in order to achieve optimal results in the construction process.
3. Placing of Topping Concrete
The first step before placing the topping concrete is to examine the existing slab to determine its nature. This involves pouring a small amount of water onto the base slab and observing the absorption rate. If the water is absorbed quickly, the slab must be kept moist for four hours prior to the placement of the topping concrete. Before placing the topping concrete, all the water on the slab must be removed. This is because when the wet concrete is placed on the base, the base absorbs all the water content from the concrete, leaving the topping concrete without enough water. On the other hand, if there is no absorption of water, the topping concrete can be laid normally. The typical thickness of a topping slab ranges from 50-100 mm. Based on the desired thickness and requirements of the topping slab, steel reinforcement is provided.
Fig 3: Laying of topping concrete
A common method for creating a durable surface is to first apply a slurry made of cement, sand, and a latex bonding agent. This is spread over the surface and then left to partially dry. Once the slurry has set, the concrete is placed on top.
The specific mixture used for the concrete will depend on the desired strength of the topping. Generally, the mix will contain aggregates, sand, and cement, with a minimum grade of M20.
When laying the topping concrete, it’s important to ensure that it is spread evenly and compacted thoroughly. This can be done using screeds and bull floats, similar to the process for creating a concrete floor. By following these steps, a strong and durable surface can be created that will last for many years.
4. Finishing
The finish required for topping concrete can vary depending on a number of factors, including the location, intended use, and desired surface characteristics. For example, if the topping concrete is being placed on an existing slab, a rough finish may be necessary in order to provide a suitable surface for laying flooring material. On the other hand, if the concrete is being placed on top of floor concrete, a smooth finish may be more appropriate. Ultimately, the type of finish that is chosen will depend on the specific needs of the project and the goals that the finished product is intended to achieve. By carefully considering these factors and choosing the right finishing technique, it is possible to create a durable, functional, and aesthetically pleasing surface that will meet the needs of the end user.
Fig 4: Finishing of topping concrete.
5. Curing
To achieve optimal surface strength, abrasion resistance, and impermeability, concrete must undergo proper curing. This process also helps to prevent or minimize the formation of drying shrinkage cracks. In addition, effective curing delays the onset of differential shrinkage, which can lead to curling. By delaying this differential drying until the concrete is more capable of withstanding its effects, the overall structural integrity of the concrete is improved.
It is crucial to note that curing must be performed for a minimum of seven days to ensure that these desired outcomes are achieved. Neglecting to provide adequate curing time can result in weaker, less durable concrete with a greater propensity for cracking and other forms of damage. Therefore, the importance of effective and thorough curing practices cannot be overstated when it comes to ensuring the long-term integrity and performance of concrete structures.
Types of Topping Concrete
There are two types of topping concrete that can be used when working with concrete surfaces. These types are differentiated based on the type of bonding that is formed between the old and new concrete.
The bonding between the old and new concrete is an essential aspect of any concrete repair or renovation project. It ensures that the new concrete layer adheres properly to the existing surface, creating a strong and durable bond that can withstand the test of time.
The first type of topping concrete is known as a bonded topping. This type of topping is applied directly to the existing concrete surface after it has been properly cleaned and prepared. The bonding agent used in this process helps to create a strong bond between the old and new concrete, ensuring that the topping will adhere properly and last for a long time.
The second type of topping concrete is known as an unbonded topping. In this process, a layer of insulation is placed between the existing concrete surface and the new topping. This prevents the new concrete from bonding directly to the old surface, allowing it to move independently. This is particularly useful in situations where the existing concrete surface is prone to cracking or movement.
In summary, the type of topping concrete used in a project depends on the specific needs of the job and the condition of the existing concrete surface. Whether a bonded or unbonded topping is used, the goal is always to create a strong and durable surface that can withstand the stresses of use and time.
1. Fully Bonded Topping
When applying a concrete overlay on a base slab that is in good condition, it is recommended to have a thickness of 1 to 2 inches (25 to 50 mm) and welded wire fabric reinforcement is not typically necessary. If the existing base slab has cracks, these may appear in the bonded overlay as well. To place a thinner overlay of less than 1 inch (25 mm) thickness, small aggregate with a maximum size of 3/8 inch (10 mm) or less and high sand content must be used. However, this increases the likelihood of shrinkage, cracking, de-bonding, and curling due to the high-water demand. Thin-bonded overlays are primarily used to restore the wearing surface of light-duty floors, rather than to add strength to the slab. The formulas developed for highway and airport pavements can be adapted for use with floors.
2. Partially Bonded or Unbonded Topping
Unbonded construction is a recommended method for constructing floors with a minimum thickness of 4 inches (100 mm) if there are cracks in the base slab or if achieving good bond between the new and existing slabs is not possible. Both light and heavy-duty floors can benefit from this method as the existing slab provides a good base for the new floor surface. However, unbonded overlays are usually thicker than bonded overlays, and the higher final floor elevation may cause interference with other service requirements, such as doorway clearances.
Even if the cracks in the base slab are repaired, they may eventually reappear through a partially bonded overlay. Therefore, properly designed welded-wire fabric reinforcement is necessary to hold the overlay slab together at any cracks that may form. However, placing the fabric in a thin, partially bonded, or unbonded overlay requires special precautions to ensure that it is positioned at the correct level below the surface.
Thicker overlays can help reduce reflective cracking. However, the placement of thicker overlays may cause interference with other service requirements. Therefore, it is crucial to carefully consider the thickness of the overlay and its impact on other aspects of the construction project.
Advantages of Topping Concrete
There are several benefits of increasing the thickness of a flooring system. One of them is the ability to create a clean and flawless surface, even if the original concrete had flaws. By leveling and flattening the concrete, it can be polished or used as a base for alternative flooring products.
Another advantage of increasing the thickness of the floor is that it strengthens weak and fragile concrete. This can help reduce wear and tear on equipment by providing a smoother surface for equipment to operate on. Additionally, it can lead to reduced repair costs for the finished construction since the floor will be less likely to crack or suffer damage from heavy usage.