Concrete Cracks: Types, Causes & How to Prevent Them — Australian Guide 2026
Most concrete cracks are preventable. The two most common types — plastic shrinkage and drying shrinkage — are both caused by moisture loss. Cut control joints to 1/4 of slab depth (minimum 25 mm) and start curing immediately after finishing.
8 Types of Concrete Cracks
The Boral Australian Concrete Guide (2023 Edition 2.1, Chapter 8) identifies eight distinct crack types that occur in concrete slabs. Each has a specific cause and a corresponding prevention measure. Understanding which type you’re dealing with is the first step to fixing — or avoiding — the problem.
| # | Crack Type | Location / Pattern | Primary Cause | Prevention |
|---|---|---|---|---|
| 1 | Shrinkage cracks | Random pattern across slab | Drying shrinkage without adequate joints | Cut contraction joints along planned lines before concrete sets |
| 2 | Stress concentration at corners | Diagonal from re-entrant corners | Stress concentration at inside corners | Expansion joint at corner, or reinforcing trimmer bars |
| 3 | Settlement cracks | Irregular, often above footings or near edges | Movement of subgrade or footings | Compact and stabilise subgrade; engineer footings correctly |
| 4 | Heaving cracks | Upward displacement through full depth | Poor drainage causing subgrade heave | Correct subgrade drainage; remove expansive clays before placing |
| 5 | Expansion cracks | Where slab meets fixed structure | Thermal expansion with no room to move | Place expansion joints at all fixed structures: walls, columns, drains |
| 6 | Feathered edge cracks | Along thin tapered edges | Narrow feathered sections lack mass to resist shrinkage | Avoid feathered sections; maintain minimum slab depth |
| 7 | Plastic shrinkage cracks | Shallow surface cracks appearing within hours of placing | Rapid evaporation exceeds bleed water rate while concrete is still plastic | Wind breaks, shading, evaporative retarder, and immediate curing |
| 8 | Door / window corner cracks | Diagonal from corners of openings | Stress concentration at wall opening corners | Use reinforcing steel (trimmer bars) at all openings |
Key statistic: The majority of cracks occur within 72 hours after concrete has been placed. This is the window when prevention measures — curing, joint cutting, and wind protection — have the greatest impact. Source: Boral Australian Concrete Guide (2023 Edition 2.1, Chapter 8).
Plastic vs Drying Shrinkage
Shrinkage is the root cause behind the majority of concrete cracks on residential projects. But there are two very different mechanisms, and each requires a different response.
Plastic Shrinkage Cracks
Plastic shrinkage occurs within the first few hours of placing. When wind, low humidity, or high air temperature causes water to evaporate from the concrete surface faster than bleed water rises to replace it, the surface layer begins to shrink while the concrete underneath is still in its plastic (workable) state. The result: shallow parallel cracks, often diagonal to slab edges, that can form in as little as 30–60 minutes after placing.
These cracks are entirely preventable. The key is monitoring the evaporation rate before and during the pour:
Evaporation rate thresholds per Boral Australian Concrete Guide (2023 Edition 2.1, Chapter 5 and Chapter 8). The Boral nomograph combines air temperature, concrete temperature, relative humidity, and wind speed to produce the evaporation rate.
Practical measures for hot or windy days: erect temporary wind breaks (shade cloth or hoarding) before the truck arrives; shade the slab from direct sun; apply a spray-on evaporative retarder immediately after screeding; commence curing compound or plastic sheeting without delay after final finishing.
Drying Shrinkage Cracks
Drying shrinkage occurs over days and weeks as concrete hardens and sheds the water it doesn’t need for cement hydration. Fresh concrete contains significantly more water than the chemical hydration reactions require. As this excess water evaporates, the concrete matrix contracts.
In a completely unrestrained slab, drying shrinkage alone would not produce cracks — the slab would simply get slightly smaller. In practice, however, all real slabs are restrained to some degree: by the friction against the subgrade, by reinforcing steel, by fixed structures at the perimeter, or by their own geometry. Restraint converts shrinkage strain into tensile stress. When that stress exceeds the concrete’s tensile strength, a crack forms.
The amount and rate of cracking from drying shrinkage depends on four factors:
- Rate and total amount of drying — faster and more complete drying means more shrinkage
- Tensile strength of the concrete — higher strength mix resists cracking better
- Modulus of elasticity — stiffer concrete accumulates more stress per unit of shrinkage
- Degree of restraint — subgrade friction, reinforcement, and fixed connections all add restraint
The most effective drying shrinkage control measure is minimising the water content of the mix. Every litre of unnecessary water added on site increases drying shrinkage. Do not add water to the truck on delivery.
Control Joints: Spacing & Depth
Shrinkage cracks cannot always be completely prevented — but they can be controlled. A control joint (also called a contraction joint) creates a planned plane of weakness in the slab. When drying shrinkage builds up enough tensile stress, the concrete cracks at the joint rather than randomly across the surface. The result is a clean, neat line instead of an irregular crack.
Three Methods for Cutting Control Joints
Hand-held grooving tool
Used while the concrete is still in its plastic state, before final set. The groove must be at least 1/4 of the slab depth or 25 mm, whichever is larger. This is the most common method for residential slabs. Work quickly — the window closes as the slab stiffens.
Proprietary crack inducers
Plastic or metal inserts placed on the substrate or pushed into the concrete before it sets. They create a weakened plane through the full depth without cutting. Useful in slabs where surface finishing makes grooving impractical.
Concrete saw (early-entry or conventional)
Cut as soon as the concrete is strong enough to resist ravelling at the saw edges — typically 4–12 hours after placing depending on temperature. Depth must be at least 1/4 of slab depth or 25 mm. The standard method for commercial slabs and structural driveways. Do not delay — if the slab cracks before you saw, the joint is useless.
Control Joint Depth: Worked Examples
| Slab thickness | Minimum joint depth | Recommended method |
|---|---|---|
| 75 mm path | 25 mm (rule minimum) | Hand grooving tool |
| 100 mm driveway | 25 mm (rule minimum) | Hand grooving tool or saw cut |
| 125 mm structural slab | 32 mm (1/4 of 125 mm) | Saw cut |
| 150 mm driveway | 38 mm (1/4 of 150 mm) | Saw cut |
Joint Spacing Rule of Thumb
For residential driveways and slabs, control joints are typically spaced at 3–4.5 m in each direction. The general rule: joint spacing in metres should not exceed 25–30 times the slab thickness in millimetres. For a 100 mm slab that gives a maximum spacing of 2.5–3.0 m; most practitioners use 3 m as a practical standard. Closer spacing means smaller panels and more predictable cracking at the joints. Wider spacing increases the risk of random cracks between joints.
Control joints only work if they are cut deep enough and early enough. A shallow groove (less than 1/4 depth) leaves too much material and the slab often cracks elsewhere. A delayed saw cut lets random cracks form before the joint is established. Neither mistake can be corrected after the fact.
Cosmetic vs Structural Cracks
Not all cracks are equal. The most important assessment when you discover a crack is whether it is cosmetic (surface shrinkage, no structural implication) or structural (indicating a problem with load capacity, subgrade stability, or reinforcement). The table below summarises the key distinguishing characteristics.
| Characteristic | Cosmetic crack | Structural crack |
|---|---|---|
| Width | < 0.3 mm (hairline) | > 0.3 mm |
| Depth | Surface layer only | Through full slab depth |
| Pattern | Random map cracking, shallow shrinkage lines | Single run crack, aligned with stress or load path |
| Movement | No differential movement when pressed | Sides move independently; one side higher than the other |
| When it appears | Within 24–72 hours of placing | After loading or over months/years |
| Action required | Monitor; seal if exposed to weather | Engage a structural engineer before loading |
When to Call a Structural Engineer
Engage a structural engineer immediately if a crack meets any of these criteria:
- Width exceeds 0.3 mm at the surface
- Shows differential movement between the two sides (one side is higher)
- Extends through the full slab depth
- Is located near a load-bearing element: column, footing, wall junction, or post
- Is actively growing in length or width over time
AS 3600 and the National Construction Code do not prescribe a universal crack width limit for all slabs — limits vary by exposure classification and structural function. The 0.3 mm threshold cited here is a widely used practical guide for residential slabs in normal exposure conditions. When in doubt, get an engineer’s opinion. The cost of an assessment is trivial compared to the cost of a failed slab.
Prevention Checklist
Use this checklist on every concrete pour. The items are ordered chronologically: subgrade preparation before the pour, placement and joint work during, curing after. Miss any item and the risk of cracking rises significantly.
Source & Standards
Crack type classifications and prevention guidance adapted from the Boral Australian Concrete Guide (2023 Edition 2.1), Chapter 8 (Cracking) and Chapter 5 (Hot & Cold Weather Concreting). Control joint depth requirements per AS 3600 and CCAA guidance. Evaporation rate thresholds per Boral Chapter 5. AS 3600 is the governing standard for concrete structures in Australia; always consult a licensed engineer for structural applications.
Concrete cracking is one of the most common complaints on residential construction projects — and the most preventable. The majority of cracks that builders and homeowners encounter are cosmetic shrinkage cracks that result from skipped curing, delayed joint cutting, or excess water in the mix. Address those three practices and the probability of a crack-free slab rises dramatically.
For the next steps, see our concrete curing time guide for timing and methods, and the site preparation checklist for subgrade and formwork setup before the pour.
Frequently Asked Questions
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