| Main causes | Rusting reinforcement, thin cover, high water-cement ratio, moisture entering the concrete, fire, freeze-thaw, impact |
|---|---|
| Warning signs | Rust stains, hollow sound on tapping, cracks along bars |
| Repair idea | Find and fix the cause, remove loose concrete, treat the steel, then patch with repair mortar |
| Prevention | Proper cover, low w/c ratio, good compaction and curing, waterproofing |
Spalling looks like a surface problem, but it is often the first sign of corrosion inside the concrete. Left alone, the bar loses section and the member loses strength. Reading why reinforcement bars rust inside concrete will help you understand the cause behind most spalling.
Causes of spalling concrete
| Cause | What happens |
|---|---|
| Rebar corrosion | Rust expands and cracks the cover; the most common cause in India |
| Thin or poor cover | Moisture and air reach the steel sooner; see importance of concrete cover |
| High water-cement ratio / poor compaction | Porous concrete lets water and salts in |
| Leakage and dampness | Constant moisture speeds up corrosion |
| Fire | Heat makes the surface layer burst off |
| Freeze-thaw | Frozen water in pores expands (cold regions) |
| Impact or overload | Local crushing and chipping |
How to repair spalled concrete
1. Find the cause (leak, thin cover, chloride, carbonation) and fix it first, or the spalling will return.
2. Tap the area and mark all hollow concrete.
3. Saw-cut the edges of the patch to a small depth to avoid feathered edges.
4. Remove loose and damaged concrete until you reach sound concrete, and behind exposed bars so the steel can be cleaned fully.
5. Clean rust from the bars by wire brush or grit blasting and apply an anti-corrosion primer. Replace bars that have lost significant section.
6. Apply a bonding coat, then a polymer-modified repair mortar in layers.
7. Cure the repair and finish with a protective coating or waterproofing.
Large areas of spalling, spalling on slab soffits, beams and columns, or exposed corroded main bars should be assessed by a structural engineer before repair. Always check the clause numbers and limits against the latest IS/IRC code and your structural designer before using them on a project.
How to prevent spalling
• Provide the required cover for the exposure condition (see clear cover as per IS 456).
• Keep the water-cement ratio low, compact well and cure properly (see durability of concrete as per IS 456).
• Fix leaks and drainage problems early.
• Check against other defects such as honeycombing and white deposits like efflorescence, which also show moisture movement.
What is spalling of concrete?
What causes concrete spalling?
Is spalling concrete dangerous?
Can spalled concrete be repaired?
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