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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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Chunks of external plaster falling off a Mumbai building are not a cosmetic problem — they are a public safety hazard, and the society committee is legally responsible for addressing them.

If your building’s facade is showing cracks, hollow sections, or plaster that has already fallen, the window to fix it before monsoon is closing fast.

Why External Plaster Falls Off Mumbai Buildings

Three forces work together on every Mumbai building facade, and they are all at their worst just before and during monsoon:

Moisture-driven bond failure. Mumbai’s monsoon drives water horizontally into surface cracks at force. Once water gets behind the plaster, it sits between the plaster layer and the concrete substrate. When the surface heats up in sunlight after a rain shower, that trapped moisture expands. The bond fails and the plaster delaminates — first invisibly, then visibly, then in falling chunks.

Carbonation of the concrete substrate. Over time, CO₂ from the atmosphere reacts with the calcium hydroxide in concrete, reducing its alkalinity and weakening the bond between substrate and plaster. Buildings over 15 years old across Mumbai — which covers most of the housing society stock in Andheri, Borivali, Kandivali, Chembur, and Dadar — show measurable carbonation in the outer concrete face.

Salt crystallisation from sea air. Mumbai’s position on the Arabian Sea coast means chloride-laden air deposits salt into the pore structure of external walls across the entire city. When moisture evaporates, salt crystals form inside the plaster and expand, exerting pressure from within. This is why buildings near the coast in Worli, Mahim, Bandra, and Juhu typically show faster facade deterioration than inland suburbs.

The Tap Test: Do This Today

Before calling any contractor, walk the external face of the building and tap the plaster systematically with your knuckles or a small tool. A solid sound means the plaster is bonded. A hollow sound — like tapping an empty box — means the plaster has delaminated and is held in place only by surface tension.

Hollow plaster will not survive the monsoon. It will fall — in wet, heavy chunks — onto walkways, compound areas, vehicles, and people.

Any section that sounds hollow must be removed before June. This is not a repair option — it is a safety obligation.

The Correct Repair Sequence

Patching fallen plaster with fresh cement and painting over it fails within one season. The new plaster expands and contracts at a different rate from the old surrounding plaster and cracks at every patch boundary. This is the most common and expensive mistake Mumbai societies make on facade repairs.

The correct sequence:

1. Remove all delaminated plaster — everything that sounds hollow, plus a minimum 150mm border into sound plaster around every hollow area. The repair patch must be large enough that its edges are bonded to solid substrate on all sides.

2. Treat the exposed substrate. If rebar is visible and rusting, apply a passivation coat to stop further corrosion before replastering. If the substrate is sound concrete, apply a bonding agent to improve adhesion of the new plaster.

3. Apply plaster in two coats — scratch coat first, cured for minimum 7 days, then finish coat. Single-coat thick plaster cracks during curing. Both coats should use a polymer-modified plaster mix for improved bond strength and crack resistance.

4. Apply a waterproof elastomeric finish coat over the repaired and surrounding area before painting. An elastomeric coating bridges hairline cracks and provides the first line of defence against the next monsoon.

5. Seal all window frame junctions while scaffolding access is available. The joint between window frame and surrounding plaster is one of the most common sources of seepage into flats in Mumbai — and it is almost always missed during facade repairs because it looks minor.

What the Managing Committee Is Legally Required to Do

The external facade is common property. Under the Maharashtra Co-operative Societies Act, the managing committee has a duty to maintain common property in a condition that does not pose a hazard to residents or the public.

Plaster that has already fallen — or that is demonstrably about to fall — creates direct liability for the committee if an injury occurs and the committee was aware of the condition and failed to act.

The correct steps once hollow or fallen plaster is identified:

One — cordon off any area below the affected sections immediately to prevent injury.
Two — get a written assessment documenting the extent and location of the damage.
Three — pass a committee resolution authorising the repair work, or call a special general body meeting if the cost exceeds the committee’s authorisation limit under the society’s bye-laws.
Four — obtain at least two written quotations with clearly defined scopes before appointing a contractor.

If BMC has already issued a structural notice about the building, the timeline above becomes compressed. The notice will specify a response deadline — typically 30 days — and non-compliance leads to further action including potential demolition orders for severely deteriorated structures.

What You Can Realistically Do Before June

A full facade replaster programme takes 3–4 weeks minimum under dry conditions. That window has largely closed for May 2026.

What is achievable before monsoon: remove all hollow plaster sections, treat exposed substrate and rebar, apply a temporary waterproof coating to the exposed areas. This removes the immediate safety hazard and protects the structure through the monsoon. The full replaster is then done properly in October–November when conditions are suitable.

Half the job done correctly is more valuable than the full job done wrong in a hurry before the rains.

See PS Group’s facade and civil repair work on the PS Group gallery and the PS Group YouTube channel. Follow PS Group on Facebook for pre-monsoon maintenance reminders.

For a pre-monsoon facade assessment anywhere in Mumbai or MMR, call +91 97684 16600 or WhatsApp PS Group. Full civil repair details at pscivil.com/civil. Where waterproofing of the external wall is needed alongside plaster repair, see pscivil.com/waterproofing. For unexplained seepage source identification, see pscivil.com/leakdetect.

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RCC Repair vs Patch and Paint: Why Mumbai Building Committees Keep Paying Twice