Why this matters
A crack in concrete is never just a crack. It is a piece of evidence — about the water-cement ratio, the curing discipline, the joint layout, the restraint from neighbouring structure, or the load the element was never designed to carry.
For Indian site conditions, IS 456:2000 Clause 35.1 sets the surface crack-width limits for reinforced concrete at 0.1 mm to 0.3 mm, depending on exposure. Anything wider than that, and durability starts to suffer.
This guide walks through the seven types of cracks you will actually meet on an Indian site, what causes each one, and what to do about it — before and after.
The 7 types
Plastic Shrinkage Cracks
Appear: 30 min – 6 hr after placing · Depth: shallow (surface)
Form on the surface when water evaporates faster than it can bleed to the top. Look like a spider's web or branching map. Common on hot, windy days — exactly the conditions covered in Hot Weather Concreting.
- High evaporation rate (ACI 305: > 1.0 kg/m²/hr is critical)
- Delaying curing
- Wind + low humidity
- Excess bleed water
- Start curing immediately after finishing
- Use windbreaks / sun-shades in summer
- Cool the aggregates / mix water
- Fog-spray the surface
Drying Shrinkage Cracks
Appear: weeks to months · Depth: through-section
Concrete shrinks as it dries — typically 400 to 700 microstrain. When shrinkage is restrained (by adjacent structure, reinforcement, or subgrade friction), tensile stress builds and cracks form. Most common in slabs, walls and long thin elements.
- High water content / high w/c ratio
- Excess cement content
- Small aggregate (high paste volume)
- Restraint from adjoining members
- Keep w/c as low as workability allows
- Use larger max-aggregate size
- Proper joint layout (contraction / expansion / construction)
- 7+ days moist curing (IS 456 Table 5)
Thermal Cracks
Appear: 1–14 days · Pattern: through-section
Heat of hydration raises the core temperature 30–50 °C above ambient in mass pours. When the surface cools faster than the core, the surface goes into tension and cracks. Especially severe in thick foundations, pile caps, dams, and large piers.
- High cement content (OPC, no SCMs)
- Large section thickness
- Sudden cooling (form removal, cold rain)
- High ambient temperature during pour
- Use PPC / PSC / fly ash / GGBS
- Pre-cool aggregates or use chilled water
- Insulate forms — don't strip suddenly
- Limit pour lifts to manageable depth
Settlement Cracks
Appear: 30 min – 24 hr · Pattern: over reinforcement
Form when fresh concrete settles around obstacles (rebar, embedments) and the paste continues to bleed water, leaving a void that opens as a crack over the obstruction. Common in deep beams, columns and heavily-reinforced slabs.
- Excess bleed (high w/c, deep sections)
- Obstructions to uniform settlement
- Insufficient vibration / revibration
- Revibrate after initial set (within ~2 hr)
- Lower w/c, reduce bleeding
- Use proper cover and bar spacing
Structural Cracks
Appear: any time · Pattern: diagonal in beams, shear walls
These are the ones you take seriously. They form when applied loads exceed what the section — and its reinforcement — was designed to carry. Diagonal cracks in beams (typically 45°), vertical cracks in columns, and cracks radiating from beam-column joints are the classic signs. Always investigate, never patch and move on.
- Overloading beyond design
- Inadequate reinforcement / detailing
- Corrosion-induced section loss
- Settlement of foundations
- Stop work, document, mark crack ends
- Get a structural engineer to investigate
- Non-destructive test (rebound hammer, UPV)
- Strengthen or replace as advised
Chemical Reaction Cracks
Appear: months to years · Pattern: map / random
Result from internal or external chemical attack. Alkali-Silica Reaction (ASR) is the most common cause of map-pattern cracking in Indian conditions when reactive aggregates are used without SCMs. Sulphate attack and carbonation progressively widen cracks and spall cover.
- Alkali-Silica Reaction (ASR)
- Sulphate attack from soil / groundwater
- Delayed ettringite formation (heat curing)
- Carbonation-induced corrosion
- Test aggregates for ASR (IS 2386 Part VII)
- Use sulphate-resisting cement where needed
- Low w/c, adequate cover, good curing
- Limit heat curing temperature
Crazing
Appear: hours to days · Pattern: fine random hex pattern
Very fine surface cracks — usually less than 0.1 mm deep — that form a chicken-wire or hexagonal pattern on the surface of hardened concrete. Mostly cosmetic, but they reduce surface durability and worsen with freeze-thaw cycles or aggressive exposure.
- Over-floating or finishing while bleed water is present
- Rapid surface drying
- Rich, wet mixes with high fines
- Don't finish while bleed water is on the surface
- Start curing as soon as finishing is done
- Use air entrainment for exposed slabs
Identification at a glance
When you see a crack on site, the first three questions are: when did it appear?, what pattern does it make?, and how deep is it?
| Crack Type | When | Pattern | Depth | Severity |
|---|---|---|---|---|
| Plastic shrinkage | 30 min – 6 hr | Branching / map | Surface | Medium |
| Drying shrinkage | Weeks – months | Random / through-joints | Through | Medium |
| Thermal | 1 – 14 days | Through-section | Through | Medium–High |
| Settlement | 30 min – 24 hr | Over rebar | Surface | Low–Medium |
| Structural | Any time | Diagonal / at joints | Through | High |
| Chemical | Months – years | Map / random | Progressive | High |
| Crazing | Hours – days | Hex / random fine | < 0.1 mm | Low |
The prevention checklist
Mix Design
- Lowest workable w/c
- SCM replacement (fly ash / GGBS)
- Max-aggregate size as large as practical
- Aggregate grading near middle of IS 383 zone
Placement & Finishing
- Cool the mix in hot weather
- Place within initial-set time
- Revibrate deep sections
- Never finish bleed water back in
Joints & Curing
- Joint layout before pour — saw-cut early
- Start curing the moment finishing ends
- Min 7 days moist curing (IS 456 Table 5)
- 10–14 days for severe exposure / blended cement
Repair — by width
| Width | Method | Material |
|---|---|---|
| < 0.3 mm | Surface sealing | Epoxy / PU / acrylic sealants |
| 0.3 – 3 mm | Pressure injection | Low-viscosity epoxy or PU resin |
| > 3 mm (dormant) | Routing & sealing | Backer rod + elastomeric sealant |
| > 3 mm (active) | Flexible sealing | Silicone / polysulphide, allow movement |
| Multiple / wide | Structural investigation | Strengthening as advised by engineer |
Related articles
Hot Weather Concreting — Complete Guide for Indian Conditions
IS 456 Clause 8.2.2 thresholds, ACI 305 evaporation limits, and the curing rules above 35°C.
Cube Testing Procedure as per IS 516
Sampling, casting, curing and testing of 150 mm cubes with the IS 456 acceptance criteria.
Amit Haridas
Founder & Proprietor, ConcreteInfo. 25+ years of experience in construction QA/QC and concrete technology — mix design, RMC plant operations, on-site quality control and code-referenced training. NRMCA Certified Trainer (USA) and ISO Lead Auditor.