Short Answer:

Spalling of concrete means the breaking, flaking, or peeling off of the surface layer of concrete. It is usually caused by the internal pressure developed when steel reinforcement corrodes and expands, pushing the surrounding concrete outward.

Spalling can also occur due to freeze-thaw cycles, poor workmanship, or chemical attacks. It leads to visible damage, exposes steel reinforcement, and reduces the strength and durability of the structure. Preventing spalling involves using good quality concrete, proper cover to reinforcement, and regular maintenance.

Detailed Explanation

Spalling of Concrete 

Spalling of concrete is a surface damage where parts of the concrete layer break away, peel off, or chip out, typically exposing the underlying steel reinforcement. It is a common deterioration issue in RCC (Reinforced Cement Concrete) structures and significantly affects the appearance, strength, and durability of the structure. It starts as minor cracks or bulging and gradually worsens if left untreated.

The most common cause of spalling is the corrosion of embedded steel reinforcement. When reinforcement corrodes, it produces rust, which occupies more volume than the original steel. This expansion creates internal tensile stress inside the concrete. As a result, the concrete cracks and eventually breaks away from the surface, a process known as spalling.

How Spalling Occurs

1. Corrosion of Reinforcement

• The most frequent cause of spalling is the rusting of steel bars inside concrete.
• In presence of moisture and oxygen, especially when carbon dioxide or chlorides enter the concrete, the steel starts corroding.
• Rust expands and creates pressure around the reinforcement.
• This pressure cracks the concrete cover, causing the outer layers to break off.

2. Freeze-Thaw Cycles

• In cold regions, water inside concrete pores freezes and expands.
• Repeated freezing and thawing cycles cause cracks and loosening of concrete surfaces.
• Over time, these actions dislodge pieces of concrete.

3. Alkali-Aggregate Reaction (AAR)

• Sometimes, a chemical reaction between alkalis in cement and certain types of aggregates occurs.
• This forms an expansive gel that absorbs water and swells, resulting in cracking and surface disintegration.

4. Fire or Heat Exposure

• High temperatures can cause thermal expansion and break the bond between concrete and steel.
• The outer layers become weak and fall off, leading to spalling.

5. Poor Quality of Concrete or Workmanship

• Improper compaction, inadequate curing, or low-grade materials increase concrete porosity.
• Porous concrete allows faster moisture and gas entry, increasing spalling risk.

6. Structural Overloading or Impact

• Heavy loads or external impacts can cause surface cracking.
• If moisture enters through these cracks, it leads to corrosion and spalling.

Effects of Spalling

• Exposes reinforcement, accelerating corrosion.
• Reduces cross-sectional area of concrete, weakening the element.
• Affects load-bearing capacity and durability.
• Leads to safety hazards, especially in overhead structures.
• Degrades aesthetics and requires expensive repairs.

Conclusion

Spalling of concrete is a damaging condition where concrete flakes or breaks off due to internal stress, mainly caused by steel corrosion, freeze-thaw, chemical attack, or poor workmanship. It weakens the structure, exposes reinforcement, and reduces service life. To prevent spalling, good quality concrete, adequate cover, and regular maintenance are essential. Timely identification and repair help maintain the integrity and durability of RCC structures.