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 reinforced concrete under Indian site conditions, IS 456:2000 Table 35.1 read with Clause 35.3.2 sets surface crack-width limits at 0.1 mm to 0.3 mm, depending on exposure class. Durability risk rises when measured widths exceed the applicable limit; the table values are design limits, not warning thresholds for routine service.
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)
- Moist curing per IS 456 Cl 13.5 — 7 d OPC, 10 d blended; extend to ~14 d where mineral admixtures or hot/dry conditions apply
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 (reinforcement, 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. Treat as serious; investigate before patching or further loading.
- 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, shallow surface cracks forming a chicken-wire or hexagonal pattern on the surface of hardened concrete. Individual crack depth is typically well under 0.1 mm; widths are small but variable. Mostly cosmetic in mild exposure, though they reduce surface durability and worsen under freeze–thaw or aggressive exposure. Crazing is distinct from the map / random pattern associated with ASR or DEF (see Type 6) — the latter is a chemical-swell reaction, not a surface moisture effect, and warrants petrographic assessment.
- 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 reinforcement | 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
- Do not work bleed water back into the surface
Joints & Curing
- Plan joint layout before the pour — saw-cut depth 1/4 to 1/3 of slab thickness (ACI 360 / ACPA)
- Time saw cuts: early-entry saws within a few hours of finishing; conventional saws as soon as the concrete holds the cut, and before random cracking starts
- Start curing the moment finishing ends
- Moist curing per IS 456 Cl 13.5 — 7 d OPC, 10 d blended; extend to ~14 d where mineral admixtures or hot/dry conditions apply
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 |
Frequently Asked Questions
Plastic shrinkage cracks form on the surface when water evaporates faster than it can bleed to the top — common on hot, windy days. They look like a spider's web or branching map. Prevention: start curing immediately after finishing, use windbreaks or sun-shades in summer, cool the aggregates or mix water, and fog-spray the surface.
IS 456:2000 Table 35.1 read with Clause 35.3.2 sets surface crack-width design limits from 0.1 mm (severe exposure) to 0.3 mm (mild exposure). In service inspections, widths approaching or exceeding 1 mm are commonly used as a practical trigger for closer examination — a conservative project threshold, not a code limit. Cracks wider than the applicable code limit compromise durability and should be assessed and treated as the project specification requires.
Crazing is very fine, shallow surface cracking in a chicken-wire or hexagonal pattern, with individual crack depth typically well under 0.1 mm — mostly cosmetic in mild exposure, though it reduces surface durability and worsens under freeze–thaw or aggressive exposure. It is distinct from the map / random pattern associated with ASR or delayed ettringite formation (DEF): the latter is a chemical-swell reaction, not a surface moisture effect, and warrants petrographic assessment.
Structural cracks form when applied loads exceed what the section and its reinforcement were designed to carry. Diagonal cracks in beams (typically 45°), vertical cracks in columns, and cracks radiating from beam-column joints are the classic signs — treat these as serious and investigate before patching or further loading. Stop work, document and mark the crack ends, get a structural engineer to investigate, use non-destructive testing (rebound hammer, UPV), and strengthen or replace as advised.
Moist curing per IS 456 Clause 13.5 — 7 days for OPC, 10 days for blended cement — extended to about 14 days where mineral admixtures or hot/dry conditions apply. Starting curing the moment finishing ends, alongside keeping w/c as low as workability allows and proper joint layout, is the key defence against drying shrinkage cracking.
Repair is matched to width and activity: cracks under 0.3 mm get surface sealing with epoxy, PU or acrylic sealants; 0.3–3 mm cracks are treated with pressure injection of low-viscosity epoxy or PU resin; dormant cracks over 3 mm are routed and sealed with a backer rod and elastomeric sealant; active cracks over 3 mm need flexible sealing (silicone or polysulphide) that allows continued movement; and multiple or wide cracks call for structural investigation and strengthening as the engineer advises.
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. Prevention includes using PPC / PSC / fly ash / GGBS, pre-cooling aggregates or using chilled water, insulating forms instead of stripping suddenly, and limiting pour lifts to a manageable depth.
Related articles
Hot-Weather Concreting: Planning, Temperature Control and Curing
IS 456 temperature limits, ACI 305 evaporation guidance, and the curing rules for hot conditions.
Cube Testing Procedure (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.