Concrete is one of the most durable and commonly used building materials for structures like home slabs, driveways, and footpaths. However, small cracks can develop in concrete over time. Minor cracking is quite normal, but how can you determine if concrete cracks warrant repair?
In this article, we’ll look at:
- Typical causes of cracks in concrete slabs and driveways
- Different types and classifications of concrete cracking
- When concrete cracking is considered normal vs a defect
- How weather and soil conditions affect concrete cracks
- Signs that indicate structural issues vs normal shrinkage
- Best ways to prevent excessive concrete cracking
- Repair options for fixing damaged cracked concrete
What Typically Causes Cracking in Concrete?
Concrete is quite strong in compression but weaker in tension and shear strength. Cracks occur when forces exceed the tensile capacity of the concrete. Typical causes of cracking include:
- Shrinkage – As concrete cures, it shrinks slightly causing stress cracks.
- Settlement – Concrete settling over time or compressing from heavy loads.
- Freeze-thaw – Water expanding and contracting during temperature changes.
- Tree roots – Root growth underneath concrete pushes sections upward.
- Poor jointing – Lack of control joints leads to uncontrolled cracking.
- Structural issues – Cracks from foundation or slab failure.
- Poor subgrade – Soft soils or improper base preparation.
- Excess loading – Heavy vehicles exceeding designed capacity.
Concrete cracks are commonly categorised by width and position. Some common types are:
- Hairline cracks – fine cracks less than 1mm wide.
- Shrinkage cracks – small cracks that form as concrete cures.
- Diagonal/map cracking – interconnecting 45-degree cracks.
- Horizontal cracking – parallel cracks along weak spots.
- Vertical cracking – long cracks perpendicular to the pour direction.
- Corner breaks – cracks radiating from corners and joints.
- Random cracking – meandering cracks with no pattern.
What Concrete Cracking is Considered Normal?
Minor shrinkage cracks are expected as concrete cures and ages. These are generally not a structural concern provided they meet the following guidelines:
- Fine hairline cracks less than 1mm wide
- Cracks follow existing control joints and form a rectangular pattern
- No trip hazards or loose pieces near cracks
- No evidence of vertical displacement or sinking
- No ponding water, settlement, or other functional concerns
Factors That Can Worsen Cracking
Certain conditions also make harmful cracks more likely:
- Extreme weather swings – repeated freeze/thaw cycles.
- Expansive or improperly compacted soil – causes shifting.
- Large trees nearby – roots pushing up concrete.
- Poor drainage – allows water pooling and soil saturation.
- Coastal sites – salt erosion exacerbates cracking.
- Modified sites – cut and fill changes increase slab stresses.
Signs That Indicate Structural Concrete Cracks
Cracks that are wide, patterned, and accompanied by other defects may indicate underlying problems requiring repair:
- Cracks over 5mm wide.
- Jagged, spiderweb cracking.
- Cracks running through control joints.
- Cracks not following the pour direction.
- Vertical height differences on sections separated by cracks.
- Loose cracked sections that rock or tilt.
- Cracks with rust staining from exposed rebar.
- Ponding water around wide cracks.
Best Practices to Minimise Cracking
While difficult to prevent entirely, strategic steps can help minimize concrete shrinkage cracks:
- Add control joints to relieve stresses as concrete cures.
- Use mesh reinforcement to distribute stresses uniformly.
- Apply crack control additives to increase tensile strength.
- Ensure subgrade soil is well compacted before pouring.
- Allow proper concrete curing times before loading.
- Seal surface with curing compounds to slow moisture loss.
Options for Repairing Cracked Concrete
For cracked concrete that poses a safety issue or indicates structural failure, common repair options include:
- Patching – Using cement filler for thin cracks.
- Resurfacing – Coating over damaged slabs with new concrete or asphalt.
- Slab jacking – Injecting expanding polyurethane to lift depressed slabs.
- Section replacement – Cutting out and replacing cracked areas.
- Full replacement – Removing and repouring cracked slabs.
Knowing the difference between harmless shrinkage cracks versus more serious structural cracks is important for homeowners. Minor concrete cracks can be monitored, while worrisome cracks require expert inspection and repairs to prevent safety hazards.
