Concrete is known for its durability and strength, but it is not immune to damage. One of the most common and serious types of damage seen in concrete structures is spalling. If you’ve ever noticed patches of concrete flaking off or reinforcement bars becoming visible, that’s a clear sign of spalling. This problem not only affects the appearance of the structure but can also reduce its strength and safety if not repaired in time.
This blog explains everything about spalling — its causes, effects, prevention, and step-by-step repair methods — in simple and easy British English.
What Is Spalling of Concrete?
Spalling (or concrete spalling) refers to the breaking, flaking, or peeling away of concrete from the surface of a structure. It happens when internal pressure builds up inside the concrete due to chemical reactions, corrosion of reinforcement, or environmental effects.
When the pressure becomes too high, small pieces of concrete detach from the surface, exposing the inner layers and sometimes the reinforcement bars. Once exposed, these bars are more likely to corrode, which can lead to further cracking and even structural failure if left untreated.
Main Causes of Spalling in Concrete
Spalling occurs due to a combination of physical, chemical, and environmental factors. Below are the key causes explained in detail:
1. Corrosion of Reinforcement
When moisture and air reach the steel bars inside concrete, corrosion starts. As the steel rusts, it expands in volume — sometimes up to six times more than its original size. This expansion creates internal stress, causing cracks and spalling.
Prevention: Maintain adequate concrete cover and use anti-corrosion coatings on reinforcement bars.
2. Freeze–Thaw Cycles
In cold-weather regions, water trapped inside concrete freezes and expands, creating internal pressure. When it thaws, the pressure is released, and after many such cycles, the surface begins to crack and spall.
Prevention: Use air-entraining admixtures to form tiny air pockets in the mix, giving water space to expand safely.
3. Alkali–Aggregate Reaction (AAR)
This reaction occurs when alkalis in cement react with reactive silica in aggregates in the presence of moisture, forming a gel that swells and causes cracking.
Prevention: Use low-alkali cement and non-reactive aggregates, and control moisture levels.
4. Sulphate and Chloride Attacks
Exposure to sulphates (from soil or water) or chlorides (from sea water or de-icing salts) can chemically attack the concrete. Chlorides especially promote steel corrosion, leading to spalling.
Prevention: Use sulphate-resistant cement and provide protective coatings to reinforcement and concrete surfaces.
5. High Temperature or Fire
During a fire, trapped moisture inside concrete rapidly turns to steam, generating pressure that causes explosive spalling.
Prevention: Use fire-resistant coatings and improve fire safety measures in buildings.
6. Improper Compaction or Curing
If concrete is not compacted properly, voids remain inside, allowing moisture to penetrate easily. Similarly, poor curing causes surface cracks that later lead to spalling.
Prevention: Ensure proper vibration during concreting and adequate curing for at least 7–14 days.
7. Improper Mix or Excess Water
A mix with too much water reduces concrete strength and increases porosity. Low water content, on the other hand, makes compaction difficult. Both lead to microcracks and later spalling.
Prevention: Follow correct water–cement ratio and use proper mix design.
8. Pollution and Rough Finishes
Polluted environments with acidic gases can chemically attack concrete surfaces. Rough finishes tend to trap water, increasing the risk of moisture ingress and spalling.
Prevention: Provide smooth finishes and apply water-repellent or protective paint on exposed surfaces.
Effects of Spalling of Concrete
Spalling might start as a small patch, but it can lead to several major problems if ignored:
- Safety Hazards: Falling concrete debris can injure people or damage property below.
- Exposure of Reinforcement: Steel bars become exposed, accelerating corrosion.
- Loss of Strength: The load-bearing capacity of the structure reduces.
- Increased Maintenance Costs: Repairs become more expensive if delayed.
- Aesthetic Damage: The surface appearance of the building becomes unattractive.
How to Repair Spalling Concrete (Step-by-Step Guide)
Repairing spalled concrete requires careful preparation and execution to ensure long-lasting results.
Step 1: Remove Damaged Concrete
Chip away all loose and weak concrete from the affected area. Usually, about 25–50 mm of the surrounding area is also removed for safety.
Step 2: Clean the Area
Use a wire brush and compressed air to clean dust, debris, and rust from the reinforcement.
Step 3: Treat the Reinforcement
Apply a rust remover or anti-corrosion coating to the exposed steel to prevent further rusting.
Step 4: Prepare the Surface
Roughen the old concrete surface or use a bonding agent to ensure strong adhesion with the new material.
Step 5: Apply Repair Mortar or Concrete
Use polymer-modified repair mortar or high-quality patching concrete to rebuild the surface. Ensure that the thermal expansion properties of the new and old concrete match.
Step 6: Finishing and Curing
Smooth the surface to avoid water accumulation. Then cure the area properly for several days.
Step 7: Apply Protective Coating
Finally, use a waterproof or water-repellent coating to seal the surface against moisture.
Indian Standards Related to Spalling of Concrete
For guidance on preventing and repairing spalling, the following IS Codes are useful:
- IS 456:2000 – Code of Practice for Plain and Reinforced Concrete (for durability, cover, and quality).
- IS 15988:2013 – Guidelines for Repair and Rehabilitation of Concrete Structures.
Difference Between Spalling and Scaling
| Parameter | Spalling | Scaling |
|---|---|---|
| Definition | Breaking or flaking of concrete exposing reinforcement | Peeling away of thin surface layers without exposing steel |
| Main Cause | Corrosion, chemical reactions, internal pressure | Freeze-thaw cycles or poor finishing |
| Appearance | Deep pits or detached concrete chunks | Shallow flakes or thin layers |
| Severity | Structurally significant | Mostly cosmetic |
How to Prevent Spalling in Concrete
- Maintain proper cover to reinforcement.
- Use air-entraining admixtures for freeze-thaw protection.
- Follow correct mix design and water–cement ratio.
- Compact concrete properly using vibrators.
- Use sulphate-resisting cement in aggressive environments.
- Apply anti-corrosion and waterproof coatings.
- Ensure smooth surface finishes and regular maintenance.
Conclusion🎯
Spalling of concrete is a serious problem that indicates underlying structural distress. It starts small but can lead to major safety and durability issues if ignored. However, with proper design, quality materials, good workmanship, and preventive measures, spalling can be largely avoided. Regular inspection and timely repair ensure that your concrete structures stay strong, safe, and durable for decades.
FAQs on Spalling of Concrete
1. What does concrete spalling mean?
It means flaking or breaking off of concrete layers, often exposing steel reinforcement to the air.
2. Is spalling dangerous?
Yes. It can lead to falling debris, corrosion, and even structural failure if ignored.
3. How can you identify spalling?
Look for cracks, rough patches, and exposed steel on concrete surfaces.
4. How can spalling be prevented?
By using proper concrete cover, quality materials, good curing, and protective coatings.
5. How is spalling repaired?
Remove the damaged concrete, clean and coat the reinforcement, then patch with repair mortar and finish with a waterproof coating.
6. Which IS codes deal with concrete spalling?
IS 456:2000 and IS 15988:2013.
7. How long does concrete last if maintained well?
A well-built and maintained concrete structure can last 50 to 100 years.