Concrete cracks. Every slab, every beam, every column, it's just a question of when and why. The dangerous myth is that all crack repair products work the same. They don't. A polymer-modified mortar might perfectly fix a cosmetic surface crack; put it in a live structural crack and it'll be back in 12 months, wider than before. This guide helps site engineers and contractors pick the right chemistry the first time.
Step 1: Identify the Crack Type
Every crack repair decision starts with a diagnosis. Get this wrong and everything downstream fails.
Hairline Cracks (<0.3 mm), Cosmetic or Early Warning
Hairline cracks in concrete surfaces, typical on column faces, slab soffits, and plaster, are usually shrinkage cracks from the original pour. They are cosmetic as long as they haven't progressed beyond the cover concrete. Test: push a playing card into the crack. If it doesn't enter, it's a true hairline. These can be sealed with a low-viscosity epoxy injection or surface-applied crack sealer, depending on whether waterproofing is a concern.
Wide or Active Cracks (>0.3 mm), Structural Attention Required
Anything wider than 0.3 mm needs closer examination before you choose a repair system. Key questions:
- Is it active or dormant? Stick masonry cement paste across the crack as a tell-tale. Check after 30 days. If the paste cracks, the crack is still moving (active). If the paste is intact, the crack is dormant.
- Is it in a structural member? Cracks in columns, beams, and load-bearing slabs are a structural engineering issue first, a repair chemistry issue second. If in doubt, call a structural engineer before you touch it.
- Which direction does it run? Diagonal cracks in beams and columns are far more concerning than vertical cracks in partition walls.
Wet vs Dry Cracks (This Changes Everything)
A crack with active water ingress, seeping or flowing, eliminates epoxy injection as an option. Epoxy does not bond to wet surfaces. You need a hydrophilic polyurethane foam injection that expands on contact with water to seal the crack, then provides a flexible seal once cured. This is the most common mistake on basement and water tank crack repairs.
Option 1: Polymer-Modified Mortar, When It's Enough
Polymer-modified cementitious repair mortars (like Sika MonoTop 412N, Fosroc Renderoc S, or BS Repail) are the workhorses of concrete repair. They are the right choice for:
- Surface spalling and delamination (removing loose cover concrete and reinstating)
- Honeycombing in columns and beams, fill voids with repair mortar after cutting back to sound concrete
- Dormant cracks wider than 5 mm that have been chased out (V-cut or U-cut) and prepared
- General substrate reinstatement before a waterproofing system is applied
When not to use: Active cracks (still moving), cracks narrower than 2 mm (mortar won't penetrate), and wet/seeping cracks. Don't try to trowel mortar into a 0.5 mm crack, you'll seal the surface, not the crack.
Option 2: Epoxy Injection, The Structural Fix
Low-viscosity epoxy injection is the right system for dormant structural cracks in dry conditions. Products like Sikadur 52 (injection grade, ~100 cP viscosity) can penetrate cracks as narrow as 0.1 mm and cure to a rigid material stronger than the surrounding concrete in compressive strength.
This is what you use when you need to restore structural integrity, bonding the two crack faces back together so the member behaves as a monolith again.
4-Step Injection Process
- Surface preparation: Clean the crack surface with compressed air to remove dust, laitance, and loose material. For wet areas, allow to dry completely before injection.
- Port placement: Drill and set injection ports (packers) at 15-20 cm intervals along the crack, staggered 10-15 cm from the crack line at 45 degrees. Seal the crack surface between ports with epoxy paste (Sikadur 31) and allow to cure. This forces the injected epoxy to travel along the crack rather than escape at the surface.
- Inject from the lowest port upward: Connect the injection pump or gravity feed tube to the lowest port. Inject slowly until product appears at the adjacent port. Move up. Work slowly enough to allow the epoxy to penetrate laterally into the crack depth.
- Remove ports and finish: After full cure (typically 24 hours at 25 degrees C), grind off the ports flush with the surface. If the repair area will be exposed, apply a compatible surface coating.
Critical point: Epoxy injection requires a dry crack. Do not inject if there is any water seepage. The pot life of Sikadur 52 is approximately 30 minutes at 25 degrees C and drops sharply above 35 degrees C. Work early in the morning in summer.
Option 3: Polyurethane Injection, For Wet and Active Cracks
PU foam injection is the answer when epoxy won't work: wet cracks, seeping basement walls, active leaks in water tanks, and bridge expansion joints. Products like Sika Injection 201 CE or Fosroc Nitofill UR are hydrophilic, they react with water and expand to form a closed-cell foam seal.
- Hydrophilic PU: Expands on contact with water (up to 10x volume). Best for active water ingress. The foam remains flexible after curing, so it accommodates minor crack movement.
- Hydrophobic PU: Does not require water to cure. Better for dry or intermittently wet cracks where you need a more durable flexible seal.
The injection process is similar to epoxy, but the product reacts much faster, expansion can begin within seconds of contact with water. Work quickly and have the port sealed before product begins to foam out.
Decision Matrix
| Crack Condition | Width | Recommended System | Product Examples |
|---|---|---|---|
| Dormant, dry, cosmetic | <0.3 mm | Low-viscosity epoxy injection or surface sealer | Sikadur 52, Fosroc Concresive 1080 |
| Dormant, dry, structural | 0.3-5 mm | Epoxy injection | Sikadur 52 injection grade |
| Dormant, dry, wide void | >5 mm | Polymer mortar (chased out first) | Sika MonoTop 412N, BS Repail, Fosroc Renderoc S |
| Active, dry | Any | Flexible PU injection, then monitor | Sika Injection 201 CE |
| Active, wet/seeping | Any | Hydrophilic PU injection | Sika Injection 201 CE, Fosroc Nitofill UR |
| Spalling / delamination | Surface | Break out to sound concrete + repair mortar | Sika MonoTop 612, BS Repail Coat |
Tools You Actually Need
Don't let a supplier upsell you on a 50,000 rupee injection rig for a small job. Here's the practical toolkit:
- Drill + 6-10 mm bits for drilling port holes at 45 degrees
- Mechanical packers (ports) with zerk fittings, reusable, 150-300 rupees each
- Manual hand pump or gravity-feed tubes (sufficient for hairline cracks), 1,500-5,000 rupees
- Low-pressure injection pump (0-3 bar, hand-operated) for most site crack repair, 5,000-15,000 rupees. Not the 1 lakh hydraulic rig, that's for specialised grouting.
- Angle grinder for surface prep on spalled areas
- Mixing paddle + drill for mortar
- Compressed air (compressor or can) for crack cleaning
Cost Comparison per Linear Metre of Crack
| System | Approx. Material Cost / Linear Metre | Notes |
|---|---|---|
| Surface sealing (crack filler) | 150-400 rupees | Cosmetic only |
| Polymer mortar repair | 500-1,500 rupees | Requires chase-out labour |
| Epoxy injection (Sikadur 52) | 1,000-3,000 rupees | Varies with crack width and depth |
| PU injection (wet/active) | 1,500-4,000 rupees | Higher product cost than epoxy |
Material costs only. Labour for drilling, port setting, and injection is typically 200-400 rupees per linear metre for a skilled applicator.
When to Call a Structural Engineer First
Do not guess on these:
- Any crack in a column, beam, or load-bearing wall wider than 0.3 mm
- Diagonal shear cracks in beams (45 degrees to horizontal)
- Cracks with rust staining, the rebar is corroding and the cover concrete will eventually fall
- Cracks accompanied by deflection visible to the naked eye
- New cracks that are growing, mark the ends with a pencil and date them. If they grow more than 10 mm in a month, stop and call an engineer
- Cracks in post-tensioned slabs, specialist territory, full stop
Product selection doesn't matter if the structure is compromised. Get the engineering assessment done first, then procure materials.
What to Order for a Typical 100 sq ft Wall Repair
A rough guide for a typical 100 sq ft plastered wall with moderate hairline cracking (dormant, dry):
- Low-viscosity epoxy (Sikadur 52): 1-2 litres for the injection + 0.5 kg epoxy paste for port sealing
- Mechanical injection ports: 8-12 nos.
- Polymer repair mortar: 5-10 kg for any spalled areas
- Surface waterproofing coat after repair: per area requirements
For precise material estimates, use our Construction Material Calculator or WhatsApp a photo of the crack to Divya at +91 92568 16832 for a product recommendation within minutes.
Technotrade Associates, authorised distributor for Sika, Fosroc, and Buildsmart concrete repair products across India. Technical guidance available by WhatsApp, 7 days a week.
Frequently Asked Questions
How do you permanently repair cracks in concrete floors?
Start by diagnosing whether the crack is active (still moving) or dormant. For dormant cracks under 0.3 mm, low-viscosity epoxy injection penetrates deep into the crack and cures to a bond stronger than the surrounding concrete. For cracks wider than 5 mm, chase out the crack in a V-cut, prime with bonding agent, and fill with polymer-modified repair mortar such as Sika MonoTop 412N or BS Repail. Active cracks need flexible polyurethane injection that accommodates continued movement without re-cracking.
Is epoxy injection better than polyurethane for crack repair?
It depends on the crack condition, not the product cost. Epoxy injection (Sikadur 52) cures rigid and restores structural load transfer across dormant, dry cracks from 0.1 mm to 5 mm wide. Polyurethane injection (Sika Injection 201 CE) cures flexible and works on wet or active cracks where epoxy would fail to bond. Rule of thumb: if the crack is dry and dormant, choose epoxy for structural strength. If the crack is wet, seeping, or still moving, choose polyurethane for a flexible, watertight seal.
Can you repair concrete cracks during monsoon when the crack is wet?
Yes, but only with hydrophilic polyurethane injection, never with epoxy. Hydrophilic PU foam (such as Sika Injection 201 CE or Fosroc Nitofill UR) actually needs water to react; it expands up to 10 times its volume on contact with moisture and forms a closed-cell flexible seal inside the crack. Epoxy requires a completely dry substrate to bond, so monsoon crack repairs in basements, retaining walls, and water tanks should always use PU injection.
How much does concrete crack repair cost per metre in India?
Material costs vary by repair method. Surface crack fillers run 150 to 400 rupees per linear metre. Polymer mortar repair (after chasing out the crack) costs 500 to 1,500 rupees per metre. Epoxy injection for structural cracks costs 1,000 to 3,000 rupees per metre depending on crack width and depth. Polyurethane injection for wet or active cracks costs 1,500 to 4,000 rupees per metre. Labour for drilling, port setting, and injection typically adds 200 to 400 rupees per metre.
When should I call a structural engineer before repairing a crack?
Always consult a structural engineer before repair if the crack is in a column, beam, or load-bearing wall and wider than 0.3 mm; if diagonal shear cracks appear in beams at roughly 45 degrees; if cracks show rust staining (indicating rebar corrosion beneath); if visible deflection accompanies the crack; or if the crack is growing, mark the crack ends with pencil, date them, and if extension exceeds 10 mm in a month, stop all repair work and get an engineering assessment. Cracks in post-tensioned slabs are specialist territory and should never be injected without an engineer's clearance.
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