What Is Spalling Of Concrete?
Spalling of concrete is a phenomenon where few parts of the concrete break itself free from the structure and expose the reinforcement or the insides of the concrete to the atmosphere.
Spalling is caused by certain chemical reactions inside the concrete which lead to the formation of foreign products which are of high volume. These new products due to lack of space inside the concrete, will increase the internal pressure. As a result, cracks are formed to release this pressure outside. These cracks in turn will pave way for the atmosphere to further react with the internal environment of the concrete and potentially increasing the danger of failure. The cracks will further propagate and cause the concrete to fall off. It can be understood that this problem is mainly caused by the formation of new products.
These problems causing new products are formed due to the following conditions:
2. Alkali aggregate reaction,
3. Chemical attack,
4. Sulphate attack,
Spalling may also be caused by exposure to high temperatures mostly in case of fire.
Effects Of Spalling Of Concrete:
1. The concrete debris may injure the people or damage the objects below, thus increasing the risk of people’s life.
2. The reinforcement in the concrete is exposed to the environment thus increasing its vulnerability to corrode. Corrosion will eventually lead up to the failure of steel and may even trigger the collapse of the whole structure.
3. Due to spalling of concrete, the area of the concrete resisting the load is reduced. This reduces the moment, load and shear carrying capacity of the structure.
4. The corrosion of the reinforcement will keep on spreading if not treated properly. This will increase the maintenance costs associated with the repair.
5. The spalling of concrete will spoil the aesthetic appearance of the building, decreasing its value.
Causes Of Concrete Spalling And How Can It Be Prevented?
1. Inadequate Cover:
Inadequate cover will increase the vulnerability of the reinforcement to the environment. Even the slightest crack on the surface will expose the reinforcement. When steel comes in contact with water and air, it corrodes. The products of the corrosion have high volume than that of steel causing high internal pressure and thereby causing spalling.
Solution: Proper structural design, proper supervision or cautious handling of the materials.
2. Freeze thaw Cycles:
In cold weather regions, the seeped water will turn to ice which is higher in volume than that of water.
Solution: This increased volume can be managed by creating artificial air pockets inside the concrete with the help of air entraining agents.
3. Alkali Aggregate Reactions:
When the alkali in the cement reacts with the silica in the aggregates in the presence of water, it will form the Alkali Silica Gel (ASG) which is higher in volume than the conventional products of hydration.
Solution: Alkali aggregate reaction will not take place if any of the three elements – water, alkali or silica are not present. Therefore, the reaction can be incapacitated by reducing the alkali content in the cement or using silica free synthetic aggregates.
4. High Temperature:
Explosive spalling may occur during fire or when concrete is exposed to high temperatures which causes high pore pressure developed by the oversaturation. High strength concrete is more vulnerable to failure under high temperature than that of normal strength concrete due to the increased brittleness.
Solution: Increasing the fire safety of the building will reduce the risk of explosive spalling.
5. Improper Compaction:
Improper compaction may leave the reinforcements exposed to the environment initiating corrosion and thereby spalling.
Solution: Proper supervision and using vibrators to ensure proper compaction will rectify the problem.
6. Improper Water Content:
Low water content will reduce the workability and make it hard for compaction. This may lead to formation of unintended air pockets which may later pave a way for crack formation.
High water cement ratio will reduce the strength of the concrete thus making it more vulnerable to micro cracks and thereby spalling.
Solution: Correct mix design ratio, proper supervision has to be followed to avoid any mishaps during the mixing process.
7. Sulphate Attack:
It is caused by sulphate rich cement or the sulphates present in the atmosphere. The decomposition of the products of the hydration reaction leads to reduction in the strength of the concrete and in doing so causes spalling.
Solution: Using sulphate resisting cement and decreasing the permeability of the concrete.
8. Chloride Attack:
This occurs in the structures exposed to sea water. Chloride attack will directly cause corrosion to the steel reinforcements inducing the spalling of concrete.
Solution: Increasing the concrete cover, coating the reinforcements with anti-rusting paints.
9. Rough Finishes:
A rough surface finished structure tends to accumulate more water on the surface than a smooth surface finished structure. The rough finished structure will increase the seepage of water and may encourage spalling.
Solution: Provide smooth finishes.
10. Polluted Environment:
The pollution in the environment will also cause chemical attack on the concrete. The pollutants with sulphide elements will combine with water and ingress into the structure and cause spalling.
Solution: This can be avoided by applying water repellent paints on the surfaces.
How To Repair Spalling Of Concrete:
Step 1: The loosely attached concrete blocks may be gently removed. Generally, an extra two inches of concrete surrounding the affected area is also removed for added safety.
Step 2: To further increase the safety, sounding devices may be used to find any unevenness in the vicinity of the affected area due to the spalling. Any area of concern can also be treated to avoid problems in the future.
Step 3: The reinforcement may be cleaned with a wire brush to remove any rust in the steel.
Step 4: The exposed area shall be cleaned with compressed air and the reinforcements shall be coated with Anti-rusting polymer or paint.
Step 5: The old concrete surface may be made rough for providing enough teeth for proper bonding or bonding agents may be used to ensure adhesion.
Step 6: The affected area shall be filled with normal concrete or polymer-based concrete. It is important to make sure that the co-efficient of thermal expansion of the old concrete and the new filler material are the same. This will avoid any discrepancies due to uneven heating and cooling.
Step 7: The surface shall be finished smoothly to reduce the water accumulation on the surface.
Step 8: The surface shall be coated with water repellent paints to prevent the seepage of water.
Spalling of concrete is very common in cold weather regions. It can be significantly reduced by proper supervision, introducing air entraining agents, and finishing the surface with a water repellent coat.
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