Types of Cracks in Buildings and Their Remedies

Almost every building develops some cracks over its life. Most are harmless — a sign that concrete and masonry are simply settling, shrinking or breathing with temperature. A few, however, point to a real structural problem. The skill lies in telling the two apart, and this guide walks through the crack types you're most likely to see on an Indian site, what causes each one, and how it is usually repaired.

First — is it structural or non-structural?

• Width: hairline to about 1.5 mm is usually cosmetic; wider than 3 mm, or still growing, needs a look.
• Direction: vertical, random map cracking is usually cosmetic; diagonal or stepped cracks, especially at the corners of openings, are a structural warning sign.
• Location: on the plaster surface only is usually cosmetic; running through the wall or member, visible on both faces, is structural.
• Behaviour over time: stable after a season is usually cosmetic; widening, or paired with sagging or sticking doors/windows, is structural.

As a rule of thumb from IS 456, a surface crack under about 0.3 mm is not considered harmful in ordinary exposure conditions. Anything wider — especially if it keeps growing — deserves inspection before it's simply plastered over.

1. Plastic shrinkage cracks

Fine, short, irregular cracks appearing on slab or plaster surfaces within hours of casting, often in a map-like pattern. They happen when surface water evaporates faster than bleed water can replace it — usually on a hot, dry or windy day — so the surface shrinks while the concrete below is still plastic.

Remedy: shade fresh concrete, use wind breaks, and start curing immediately after finishing. Existing cracks are cleaned, kept moist, and filled with a cement-sand slurry or polymer-modified repair mortar — these are almost always cosmetic.

2. Drying shrinkage cracks

Fine, closely spaced cracks appearing days to weeks after casting on plaster, screeds and slabs, as the concrete loses moisture and shrinks. If this shrinkage is restrained by reinforcement or an adjoining rigid element, the weaker section cracks.

Remedy: proper curing (7 days minimum for OPC, 10 days for PPC/PSC per IS 456), correct water-cement ratio, and shrinkage/expansion joints in long runs of slab or wall. Existing cracks are raked out and filled with a shrinkage-compensated mortar.

3. Thermal cracks

Long, often vertical cracks in exposed walls, parapets and roof slabs, caused by daily and seasonal temperature swings. Where this movement is restrained — a long unbroken wall, an uninsulated RCC roof — stress builds up at the weakest point.

Remedy: provide expansion joints (IS 456 suggests considering one for structures longer than about 45 m), insulate or waterproof roof slabs, and seal existing cracks with a flexible sealant rather than a rigid patch.

4. Settlement cracks

Diagonal, stepped cracks in masonry — often following the mortar joints — or diagonal cracks radiating from the corners of door and window openings, sometimes with doors and windows starting to stick. The cause is uneven foundation settlement: variable soil, poorly compacted fill, nearby excavation, or an undersized foundation.

Remedy: this is a structural symptom and needs an engineer's assessment before repair — monitoring the crack, checking the soil, and reviewing the foundation design. Fixes range from underpinning or grouting to jacketing of affected members, followed by epoxy injection or stitching of the crack itself.

5. Structural (load-related) cracks

Vertical cracks at a beam's mid-span soffit suggest flexural distress; diagonal cracks near supports suggest shear distress; vertical cracks with spalling in a column are a warning sign of overloading or corrosion. Causes include overloading beyond design capacity, design or construction errors, or loss of section from corrosion.

Remedy: always get a structural assessment first. Typical repairs include epoxy injection to restore monolithic behaviour, and jacketing (RCC, steel plate or FRP wrap) to restore or enhance capacity.

6. Corrosion-induced cracks

Cracks running parallel to an embedded bar, often with rust staining, sometimes with the cover concrete visibly bulging or spalling off. Corroding steel expands to several times its original volume, and the cover concrete simply can't accommodate it. Common triggers: inadequate cover, high water-cement ratio, chloride ingress, or carbonation of old concrete.

Remedy: break out the affected cover concrete, clean or replace the corroded bar, apply a rust-inhibiting primer, and reinstate with repair mortar or micro-concrete. Long-term prevention is a durability issue — correct nominal cover and concrete grade for the exposure condition (IS 456, Table 16).

7. Chemical attack cracks

Sulphate attack shows as expansion and pattern cracking, often with a whitish deposit, typically in foundations in sulphate-bearing soil. Alkali-silica reaction shows as random map cracking, sometimes with a gel exuding from the cracks. Both are internal-expansion problems that surface cracking alone won't fix.

Remedy: for new work, use SRC/PSC and improve drainage for sulphate exposure, and non-reactive aggregate with low-alkali or blended cement to avoid ASR. Existing affected concrete generally needs a specialist assessment, since the deterioration is ongoing.

Quick decision guide

• Hairline, random, on plaster only, not growing → shrinkage or thermal; seal and move on.
• Diagonal, at corners of openings, doors/windows sticking, still widening → likely settlement; call a structural engineer before repairing.
• Rust stains, bulging cover, crack along a bar line → corrosion; needs to be opened up and treated, not just filled.
• Any sign of deflection, sagging or spalling in a beam/column under load → structural; get it assessed immediately.