Spalling refers to the breaking away of the surface layer of concrete, which may extend to the top layers of reinforcing steel. It can result in the formation of spalls, which can be large, with diameters of 150 mm or more, and depths of 25 mm or more. Smaller spalls may also occur, as per ACI 302.1R-4. This phenomenon can affect a wide range of structures, such as framed buildings, multi-storey car parks, bridges, jetties, tanks, and bunds. Spalling leads to a patchy and pitted appearance of the concrete surface, and exposes the aggregate beneath. It is essential to repair spalling as soon as it is detected to ensure the integrity of the concrete surface and prevent compromising the ultimate capacity of the member. Moreover, spalling can negatively impact the aesthetic appearance of the structure.
Causes of Concrete Spalling
Corrosion of embedded reinforcing steel is a common cause of concrete spalling. This occurs because when the steel corrodes, rust is produced, which occupies more volume than the original steel. The resulting pressure causes the concrete to spall off.
Another cause of concrete spalling is fire exposure. When free water in the concrete is exposed to high temperatures, it changes to steam, creating internal expansive pressure in the concrete. This pressure can cause the concrete to spall off.
Freeze and thaw cycling is also a significant cause of concrete spalling. When water in the concrete freezes and expands, it creates internal pressure, which can cause the concrete to spall off when it thaws.
The expansive effects of Alkali Silica Reaction can also lead to concrete spalling. This reaction occurs when certain types of aggregates react with the alkalis in the cement, creating an expansive gel. This gel can cause the concrete to crack and spall off.
Inadequate depth of cover over reinforcement and low quality concrete cover over reinforcing steel can also cause concrete spalling. This is because the reinforcement is not adequately protected from the elements, leading to corrosion and eventual spalling.
Joint spalls are often caused by improperly constructed joints. This can lead to joint spalling because the joints are not able to accommodate the movements and stresses of the concrete.
Finally, bond failure in two-course construction can cause concrete spalling. This can occur due to differences in shrinkage between the topping and base courses, drying of the bonding grout before the topping concrete is placed, and other factors that prevent proper bonding between the two layers of concrete.
Measures to Prevent Concrete Spalling
To prevent spalling in concrete floor slabs, there are several measures that can be taken. One of these measures is early-entry dry-cutting. This technique involves cutting the concrete soon after it has been poured and has begun to set. By doing this, any potential cracking or spalling can be addressed before it becomes a problem.
Another measure that can be taken is to apply a water sealant to the finished surface of the concrete. This will prevent water from entering into the concrete, which can cause damage and spalling over time.
Proper management of the concrete during pouring is also important to prevent spalling. This includes ensuring that there is adequate cover over the reinforcing steel and placing joints at the correct positions to accommodate for any potential movement in the concrete.
Finally, it is important to prevent excess water from being added to the concrete mixture. Excess water can weaken the concrete and increase the likelihood of spalling occurring. By taking these measures, spalling can be prevented and the lifespan of the concrete floor slab can be extended.
Repairing Process of Spalling of Concrete
The repair of concrete spalling can differ depending on various factors such as the severity of the damage, the type of concrete structure, and its location. It is important to take into consideration the recommendations provided by applicable codes such as the ACI Code and BS EN 1504 in order to perform a proper repair.
The severity of the damage to the concrete is a critical factor to consider when deciding on the appropriate repair method. Additionally, the type of concrete structure and its location can have a significant impact on the repair process. It is important to follow the guidelines outlined in the ACI Code and BS EN 1504 to ensure that the repair is carried out effectively.
Following the recommendations of applicable codes is essential in ensuring that the repair of concrete spalling is done correctly. The ACI Code and BS EN 1504 provide important guidelines on the methods and materials to be used in concrete repair. By adhering to these guidelines, the repair process can be completed efficiently and effectively.
1. Clean Spalled Concrete Area
To prepare for the repair, it’s important to clean the area thoroughly to remove any dirt or debris. This is necessary to ensure that there are no bond breaker agents present, which could prevent the repair from properly adhering. If there are heavy stains or a lot of dirt buildup, a regular cleaning may not be enough and a pressure washer may be required to properly clean the surface. By taking the time to clean the area thoroughly, you can help ensure that the repair will be successful and long-lasting.
2. Remove Loose Concrete
To conduct a successful concrete repair, it is necessary to remove any loose or broken concrete from the area. This can be accomplished using a hammer and chisel. In some cases, shot blasting or grinding may also be required for proper surface preparation.
It is recommended to remove approximately 3.81cm deep of concrete to ensure a successful repair. Before beginning the repair process, it is advised to strongly tap the surrounding area with a hammer to check for possible deterioration. A hollow sound indicates deteriorated concrete and should be eliminated to prevent the need for future repairs.
To facilitate the patching process and contain the concrete within the specified area, it is recommended to cut a rectangular part of the concrete. This will result in better repair results.
Once the repair process is complete, it is essential to eliminate dust and debris using a vacuum. This will help to ensure a clean and safe working environment. By following these steps, a successful concrete repair can be achieved.
3. Clean and Coating Corroded Steel Bars
To clean the rebar and remove any corrosion or dirt, it is recommended to use a wire brush as shown in Figure 2. This will help to prepare the surface for the application of a protective coating. After cleaning, it is important to apply a coating to prevent the rebar from rusting. This can be seen in Figure 3, which illustrates the application of a protective coating. By following these steps, the rebar can be maintained and protected from corrosion, which can prolong its lifespan and ensure its structural integrity.
4. Apply Repair Material Mixture
There are different products to use when repairing concrete depending on the size and depth of the damage. For damages with a spalling depth of less than 1/3 of the concrete thickness, a surface repair can usually suffice. However, for damages greater than 1/3 of the concrete depth, a full depth restoration may be required, which may entail installing steel bars.
To prepare for patching, the affected surface should be roughened, and a suitable adhesion agent should be applied to improve bonding. Once the surface is ready, the patching can commence using a suitable admixture. The best recommended patching material is Portland-cement-based or epoxy, and it is important to select a patching material with thermal expansion close to that of existing concrete.
During patching, it is crucial to consider the ambient temperature since it can affect the patching material. The ambient temperature should be above 4.5°C. For driveways, walkways, and other horizontal surfaces, a cementitious overlay may be used for repair. If the affected area is adjacent to a joint, it is important to ensure that it is restored properly, allowing for some allowance for expansion of the member.
5. Finishing
A margin trowel should be used to scrape off any excess product and apply finishing touches to the surface in order to level it. Figure 4 provides a visual representation of this process. It is important to ensure that the surface is level before moving on to any further steps. By using a margin trowel, the excess product can be easily removed and any imperfections can be smoothed out for a more even finish. This step is crucial for achieving a high-quality final product, as it ensures that the surface is both level and smooth.
6. Curing
To ensure that a material has the necessary strength, it is important to properly cure it. This involves a process of treating the material to improve its properties and performance, such as hardness, toughness, durability, and resistance to wear and tear.
The curing process may vary depending on the type of material being used, but generally involves subjecting it to heat, pressure, or a chemical reaction. The material is often shaped into the desired form before curing, such as in the case of concrete, which is poured into molds and left to set.
Once the curing process is complete, the material should have achieved the desired strength and other properties. However, it is important to follow proper curing procedures, as failure to do so can result in weaker and less durable materials that may be prone to failure or degradation over time.
Overall, proper curing is essential for ensuring that materials meet the required strength and performance standards, and it is important to carefully follow the appropriate curing procedures for each specific material.
7. Apply Pain or Coat on Repaired Surface
Once the material has fully set, it is important to apply paint or a waterproofing membrane to prevent spalling from occurring again. This additional layer of protection helps to seal the surface and prevent moisture penetration, which can lead to spalling or the deterioration of the material over time. By applying paint or a waterproofing membrane, you can extend the lifespan of the material and reduce the risk of future spalling issues. It is crucial to follow proper application techniques and use high-quality products to ensure effective protection against spalling and other types of damage. Taking this preventive measure after the material has set completely can help maintain the integrity and durability of the surface, ensuring its long-term performance.
Fig. 5: Apply paint, coat, or waterproof membrane to prevent recurrence of spalling