Slabs and cracking
Why concrete slabs crack: causes, prevention and mesh role
Concrete slabs crack because concrete shrinks as it dries and cools, and whenever that movement is restrained, tension exceeds the material's low tensile strength. Early plastic cracks occur within hours due to surface evaporation; drying shrinkage cracks develop over months. Proper subbase compaction, timely saw cuts, immediate curing and welded wire mesh in the upper third control cracking.
Four distinct types of concrete slab cracking
Understanding why a concrete slab cracks requires diagnosing the timing and visual pattern of the fracture. Slabs crack under four fundamentally different mechanisms:
| Crack Category | Occurrence Timing | Primary Physical Cause | Visual Crack Pattern | Prevention & Mitigation Strategy |
|---|---|---|---|---|
| Plastic shrinkage cracking | First 1 to 6 hours after screeding | Surface water evaporates faster than bleed water rises (>1.0 kg/m²/h) | Parallel, shallow tears 25 to 50 mm deep, spaced 0.3 to 1.0 m apart | Evaporation retarder, wind breaks, synthetic microfibres, immediate fogging |
| Drying shrinkage cracking | Weeks to months after pour | Volume loss as surplus batch water evaporates against subbase restraint | Diagonal or midpoint cracks spanning between joints or slab edges | Control joints at 24–30x thickness; 6×6 W2.9 or 4 ga mesh on chairs |
| Settlement & subbase cracking | Months to years under load | Uneven subgrade settlement, frost heave, or soft organic pockets | Meandering, faulting cracks where one slab side drops lower | 150 mm compacted crushed gravel base; continuous welded wire reinforcement |
| Slab curling & edge lift | 1 to 6 months after cure | Top surface dries and shrinks while bottom stays moist and expanded | Panel edges and corners curl upward, rocking under forklift or vehicle wheels | Low water-cement mix, vapour barrier slip sheets, top-layer steel reinforcement |
1. Plastic shrinkage cracking (The first 6 hours)
Plastic shrinkage cracks form while the concrete is still in a plastic state before setting.
When air temperature is warm, relative humidity is low, and wind speeds exceed 15 km/h (10 mph), the rate of surface evaporation can exceed 1.0 kg/m²/h (0.2 lb/ft²/h). When surface water evaporates faster than internal bleed water can rise to replace it, rapid meniscus depression in surface capillary pores creates intense capillary tensile stresses (Kayondo, Combrinck & Boshoff, 2019).
Because fresh concrete has near-zero tensile strength during this period, the surface tears open in short, parallel cracks. Welded wire mesh does not prevent plastic cracking because the paste has not hardened sufficiently to bond with steel. Prevention relies on:
- Applying sprayable aliphatic evaporation retarders immediately after bull floating.
- Adding synthetic microfibres (0.6 to 0.9 kg/m³) to ready-mix trucks.
- Commencing continuous wet curing or applying membrane curing compound immediately after final troweling.
2. Hardened drying shrinkage cracking (Weeks to months)
Hardened concrete contains more water than required for cement hydration. As surplus water evaporates over the first year, typical concrete shrinks roughly 0.5 mm per linear metre (approximately 1/2 in per 100 ft).
When base friction or foundation connections resist this shortening, tensile stresses accumulate (Weiss, Yang & Shah, 1998). Once tensile stress exceeds the tensile strength of concrete (roughly 2 to 3 MPa · 300 to 450 psi), a crack snaps through the slab.
This is where welded wire mesh performs its primary engineering role. While steel cannot prevent drying shrinkage from occurring, cold-drawn wire positioned in the upper third of the slab bridges the crack plane. As the crack opens, steel engages, holding crack widths below 0.3 mm (0.012 in) to preserve load transfer and prevent water ingress.
3. Slab curling: causes and reinforcement solutions
Curling is the distortion of a slab panel into a curved shape by a difference in moisture or temperature between the top and bottom surfaces (NRMCA CIP 19).
In typical slabs cast on grade:
- The top surface dries out to ambient room humidity, shrinking significantly.
- The bottom surface rests on moist subgrade or polyethylene, remaining damp and expanded.
This differential shrinkage causes the edges and corners of slab panels to curl upward. When heavy vehicle wheel loads pass over an unsupported curled edge, high flexural stress snaps the corner off. Placing welded wire mesh in the upper third of the slab depth provides tensile restraint that counteracts differential upward curling forces.
Summary: the five fundamentals of crack management
- Subgrade: Compact granular base to 98% Standard Proctor density; provide positive sub-drainage.
- Mix design: Specify a low water-cement ratio (0.45 or lower) under CSA A23.1 Class C-2 to minimize shrinkage volume.
- Curing: Keep concrete continuously moist for at least 3 to 7 days, or apply certified curing compound immediately after finishing.
- Joints: Cut contraction joints to 1/4 slab depth spaced at 24 to 30 times slab thickness within 4 to 12 hours.
- Reinforcement: Place 6×6 W2.9/W2.9 (6 ga) or 4 ga welded wire mesh on chairs in the upper third of the slab.
- Weiss, Yang & Shah (1998), Shrinkage cracking of restrained concrete slabs, J. Engineering Mechanics. Link
- Kayondo, Combrinck & Boshoff (2019), Plastic shrinkage cracking in concrete: a review, Construction and Building Materials. Link
- NRMCA, CIP 19: Curling of concrete slabs. Link
- ACI 360R, Guide to Design of Slabs-on-Ground; ACI 302.1R, Guide to Concrete Floor and Slab Construction. American Concrete Institute.
- CSA A23.1, Concrete materials and methods of concrete construction. CSA Group.
Questions
Can you stop concrete from cracking completely?
No. Concrete inevitably shrinks as it dries. The goal of concrete engineering is to force cracks to occur neatly inside cut control joints and use wire mesh to keep those cracks tightly closed.
Why did my new concrete slab crack within hours of pouring?
That is plastic shrinkage cracking, caused by wind or dry heat evaporating surface water faster than bleed water could rise before the slab reached initial set.
What causes concrete slab corners to curl up?
Differential drying shrinkage. The top surface dries and shrinks while the bottom remains moist, pulling the slab edges upward.
Does wire mesh stop concrete slabs from cracking?
No. Mesh holds cracks tightly together once they form so the slab stays level, maintains aggregate interlock, and prevents trip hazards.