Slab Cracks Three Days After Pour - Whose Problem?
A 120 mm thick suspended concrete slab on a 6 m by 8 m grid is poured in summer with a 25 MPa, 100 mm slump mix placed by pump. Ambient temperature peaks at 34°C and there is a moderate wind. The slab is finished with a power trowel and covered with plastic sheeting the following morning. At 72 hours you see a network of fine random cracks across the surface, plus several longer cracks running roughly parallel to the short span. The client's inspector says the mix is at fault and wants the slab rejected. The contractor says it is normal shrinkage and wants to tile over it.
1. Classify the two crack patterns you can see and explain the most probable mechanism for each, given the pour conditions.
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Fine random surface cracks (crazing / plastic and drying shrinkage):
- The combination of 34°C, wind and a 100 mm slump mix means the surface loses moisture faster than bleed water can replace it. The surface layer shrinks before the underlying concrete, and because it is restrained, it cracks in a fine, random, shallow network - crazing or plastic shrinkage cracking.
- Covering with plastic the following morning is too late: the critical period is the first few hours after finishing, when evaporation is highest. The plastic may also have trapped bleed water and created a weak, high water/cement surface layer that is more prone to crazing.
- Power trowelling a surface that is still bleeding, or trowelling too early, closes the surface and traps water, worsening the weak surface layer and the crazing.
Longer cracks roughly parallel to the short span:
- These are consistent with restrained drying/thermal shrinkage of the slab as a whole. The slab is restrained by the beams/edges, and it wants to shorten; the restraint induces tension. Cracks form perpendicular to the restraint, i.e. roughly parallel to the short span, because that is the direction of least restraint.
- The 6 m by 8 m grid and 120 mm thickness mean the slab is relatively lightly reinforced and has a large surface-to-volume ratio, so it shrinks and loses heat quickly. The 72-hour timing fits early-age thermal and drying shrinkage.
Important distinction: neither pattern is automatically a structural failure, but neither is 'normal and just tile over it'. The random cracks are a durability and finish problem; the longer cracks need to be checked for width, depth and whether they are through-section. The inspector's 'reject the slab' is also premature - the mix is one factor, but placement, finishing and curing in those conditions are at least as significant.
2. The contractor proposes tiling directly over the cracks. Assess this and set out what you would do before any finish is applied, including how you would decide whether the cracks are structural.
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Direct tiling is not acceptable. Tiling over live or unclassified cracks:
- transfers the movement to the tile adhesive and tiles, causing debonding, cracked tiles and lippage;
- hides a possible structural or durability problem;
- does nothing about water ingress to the reinforcement through the cracks.
Before any finish:
- Measure the cracks. Record width (a crack gauge or comparator), length, pattern and whether they are full depth. A common rule of thumb is that cracks wider than about 0.3 mm in an exposed slab warrant assessment; any crack that is widening or that shows through the soffit is more serious.
- Monitor. Install tell-tales or measure over 1-2 weeks to see whether the cracks are stable or still moving. Cracks that are still growing are not ready to be sealed.
- Check for structural significance. Look for: cracks through the full depth (visible on the soffit), cracks at supports or over reinforcement, deflection, or cracks wider than the design allowance. If any are present, refer to the engineer for a structural assessment - the slab may still be adequate but needs a documented verdict.
- Confirm cover and reinforcement. If cracks are wide or there is any doubt, a cover meter check and possibly a local breakout confirm the bars are where they should be and not corroding.
- Repair appropriately. For stable, fine, non-structural cracks, route and seal or use an approved crack-injection/impregnation system, then apply a suitable membrane or crack-isolation system under the tiles. For structural cracks, the engineer specifies the repair (e.g. epoxy injection) before finishes.
- Fix the cause. Review the mix, finishing and curing for the next pour - earlier curing, evaporation retarder, windbreaks, and a finishing sequence that does not trowel while bleeding.
Decision rule: stable, fine, non-structural cracks → seal and use a crack-isolation membrane under tiles. Full-depth, moving, or wide cracks → engineer's assessment and specified repair first. Tiling directly over unclassified cracks is a defect waiting to happen.