Kumar IronUpdated
In this guide
Why cracks come before the coat
A coat is thin and cannot stretch much. A crack that opens and closes with temperature or slab movement will split the coat above it. Some cracks are only surface cracks and can be closed easily. Others are a sign that the structure is moving, and no coat will fix that. Sort them first, then seal them.
Hairline vs structural cracks
| Feature | Hairline (surface) crack | Structural crack |
|---|---|---|
| Width | Fine, usually well under 1 mm | Wider, and often over about 2 mm |
| Movement | Stable, not opening over time | Growing or opening across seasons |
| Depth | Surface or shallow, in plaster or screed | Through the slab, beam or column |
| Steel | No rust stain or exposed bar | Rust stain, spalling or exposed steel |
| Shape | Random map or short line | Long, diagonal or near openings and joints |
| Other signs | None beyond the crack | Sagging, bulging, or cracks in several rooms |
For design, the IS 456 surface crack width limit for normal members is 0.3 mm, with stricter limits in aggressive exposure. That is a design limit for new work. It is not a pass or fail test for an existing crack, so ask the engineer.
V-groove and fill for hairline cracks
- Mark the crack line, then cut a V groove along it. One public tender specification for cracks up to 3 mm wide calls for a groove about 5 mm wide.
- Brush or blow out all dust and loose material from the groove.
- Fill with polymer modified mortar or a crack filler, pressing it well into the groove.
- Finish level with the surface and let it cure as the sheet says before coating over it.
- Check the crack again after the monsoon. If it has reopened, the crack is moving and needs a flexible sealant instead.
Polymer modified mortar: what it is and where it fits
Polymer modified mortar is cement mortar with a polymer admixture, often a latex such as SBR or an acrylic. It bonds better to old concrete and is less brittle than plain cement mortar. It fits well for crack filling, coves at the parapet and patching. It is still a rigid material, so it is not the right choice for a joint that moves. Mix it to the product sheet's ratio and do not add extra water.
Sealing expansion joints with PU sealant and backer rod
- Clean the joint fully and check its width and depth.
- Push a closed cell backer rod into the joint to the depth the sealant needs. The backer rod stops the sealant sticking to the bottom and keeps its shape.
- Choose a backer rod slightly larger than the joint. Manufacturers differ on how much, with published ranges from about 20 percent to 50 percent larger, so follow your sealant's sheet.
- Set the sealant depth to about half the joint width, which is the 2 to 1 width to depth ratio many sealant sheets give. Your sealant sheet overrides this rule of thumb.
- Apply primer if the sheet asks for it, then tool the PU sealant so it bonds to both sides.
- Let it cure fully before coating over it, or leave the joint open for the coat if the sheet says so.
| Joint width | Sealant depth at half width |
|---|---|
| 10 mm | About 5 mm |
| 15 mm | About 7.5 mm |
| 20 mm | About 10 mm |
Where to seal
Seal the expansion joints in the slab, the junction between the roof and the parapet wall, the gap between the brick and RCC, and any pipe or cable that passes through the slab. A coat without sealed joints tends to fail at exactly these lines.
When to call an engineer
- A crack wider than about 2 mm, or one that is growing over a few weeks or months. Mark the ends with a pencil and a date to check.
- Rust stains, flaking concrete or any exposed steel bar.
- Sagging or bulging in the slab or a beam.
- Cracks through beams or columns, or diagonal cracks near doors and windows.
- Water coming through in several rooms at once.
- A newly built slab that is already cracking.
A coat hides a leak. It does not repair a structural problem, so do not coat over a crack an engineer has not checked.
Frequently asked questions
- Can I fill cracks with plain cement?
- It is not the best choice. Plain cement mortar is brittle and bonds poorly to old concrete. A polymer modified mortar or a crack filler suited to the job is a better choice.
- How do I know if a crack is still moving?
- Mark both ends of the crack with a pencil and the date, and check after a month or after the monsoon. If the marks have moved apart, the crack is active.
- Why use a backer rod in a joint?
- It sets the sealant to the right depth and stops it bonding to the bottom of the joint, which lets the sealant stretch and stay sound.
- Is a rusted bar in a slab a DIY job?
- No. Exposed or rusted steel means the cover has failed, and the repair needs an engineer's advice. Our guide on rusted TMT bars covers what to check.