Your building’s concrete columns or beams are spalling — the surface is crumbling and rebar is showing. Your contractor quotes you two options: patch it and paint it, or do proper RCC repair.
The patch option is cheaper. Most Mumbai society committees choose it. Then, 18 months later, the patch is cracking again and they are back to square one, having spent twice.
Here is why patches fail, and what proper RCC repair actually requires.
What Is Spalling and Why It Happens
Spalling is the breakdown of the concrete surface layer. The concrete is crumbling away in flakes or chunks, exposing the reinforcement steel (rebar) beneath.
In Mumbai buildings, spalling happens because of three overlapping causes:
**Rebar corrosion.** Water and chlorides (from sea air) reach the steel and trigger oxidation. As steel rusts, it expands — about 3–4 times the original volume. This expansion pressure forces the surrounding concrete to crack and spall away.
**Carbonation of the concrete.** Over 15+ years, atmospheric CO₂ penetrates the concrete surface and reacts with the calcium hydroxide binder, reducing the concrete’s alkalinity and its ability to protect rebar. This process accelerates in the warm, humid Mumbai climate.
**Freeze-thaw cycles and salt crystallisation.** While Mumbai does not freeze, the coastal salt-laden air creates a similar effect. Chloride crystals form inside the concrete pores and expand, fragmenting the surface layer.
Once spalling starts, it accelerates. Exposed rebar corrodes faster because it has direct contact with water and air. The corrosion pushes more concrete away. Within 2–3 years, a small spalled area can double in size.
Why “Patch and Paint” Always Fails
A patch-and-paint repair involves:
1. Hacking away the loose spalling concrete
2. Applying a new thin layer of cement plaster over the exposed rebar
3. Painting it
This approach fails because it does not address the root cause — the corroding rebar.
When you patch over corroding rebar without treating it first, the corrosion continues underneath the new plaster. The rebar expands further. The patch fractures and falls away. You are back where you started.
Additionally, the new plaster patch and the surrounding old concrete have different thermal expansion rates. In Mumbai’s daily heat cycles — cool mornings warming to 35°C+ afternoons — the patch and the substrate expand at different rates, creating stress at the boundary. Hairline cracks develop at the patch edge within weeks.
Within 18 months, the patch is cracking, and the committee is calling contractors again. This time, the cost is higher because the deterioration has progressed further.
A patch is not a repair. It is a temporary cover that costs the full repair cost upfront and guarantees you will pay again.
What Proper RCC Repair Requires
Structural RCC repair means treating the corroded rebar, repairing the concrete loss, and restoring structural strength. The sequence matters:
**Step 1: Remove all deteriorated concrete.** This means hacking away not just the visibly spalled material, but all concrete that is not structurally sound. A structural engineer determines the extent — sometimes this is deeper than it appears.
**Step 2: Clean and treat the exposed rebar.** The steel must be cleaned down to bare metal using wire brushing or light sandblasting. Once clean, a rebar passivation product is applied. This product seals the steel surface and re-establishes the electrochemical barrier that prevents future corrosion.
**Step 3: Apply concrete repair mortar.** Using polymer-modified concrete repair mortar (not ordinary cement plaster), the cavity is filled in layers with proper compaction and curing between layers. Repair mortar has higher bond strength and lower permeability than ordinary plaster.
**Step 4: Apply a protective coating.** Once the repair mortar cures, a waterproof elastomeric coating is applied over the repair and surrounding area. This is the critical step that prevents water re-entry.
**Step 5: Structural monitoring.** For significant repairs on load-bearing elements, deflection or stress monitoring may be specified by the structural engineer to ensure the repaired element performs as intended.
This entire process takes 4–6 weeks for proper curing between steps. It cannot be rushed.
The Cost Difference — And Why It Matters
A patch-and-paint job looks cheaper on the quotation. It is faster and uses cheaper materials.
A proper RCC repair costs more upfront because:
– Skilled labour is required (not general painters)
– Structural assessment is included
– Rebar treatment products are costly but essential
– Polymer-modified repair mortars are more expensive than cement plaster
– Protective coatings add to the cost
– Proper curing time delays the project
But here is the maths that Mumbai committees miss:
If a patch costs Rs X and fails in 18 months, and a proper repair costs 1.5X and lasts 15+ years, the patch actually costs **10 times more per year of service life**.
Patch now = pay again in 1.5 years = pay again in another 1.5 years = pay three times over 4.5 years.
Proper repair now = not paying again for 15 years.
How Society Committees Should Approach RCC Repair
When spalling is identified on a load-bearing structural element (columns, beams, slabs), the managing committee should:
**One — Commission a structural engineer’s assessment.** Not a contractor’s opinion — a structural engineer. The assessment determines the extent of repair needed and whether the structural integrity of the element is compromised.
**Two — Get the structural engineer’s repair specification.** Do not ask contractors to decide what repair method to use. The engineer specifies it. Contractors then quote against that specification.
**Three — Obtain multiple quotations against the engineer’s specification.** Comparing quotes on the same clearly defined scope prevents the “cheaper option” trap. All contractors are using the same materials and method, so the quotes are genuinely comparable.
**Four — Pass a general body resolution.** Structural repairs often exceed committee authority limits. A resolution in a general body meeting ensures member approval and transparency.
**Five — Include monitoring and warranty.** Require the contractor to provide a 5+ year warranty on the repair, and specify that deflection or corrosion monitoring will be done post-repair to confirm the structure is performing as designed.
Watch PS Group’s RCC repair work across Mumbai housing societies on the PS Group YouTube channel. See completed structural repair projects in the PS Group gallery.
For a structural assessment of spalling concrete anywhere in Mumbai or MMR, contact PS Group — PS Civil division. Call +91 97684 16600 or WhatsApp for a site visit. Connect with Sanjay Parmar on LinkedIn for larger project collaborations. For waterproofing that works alongside structural repair, see pscivil.com/waterproofing.
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