Why Concrete Cracks (Causes and Prevention)
All concrete cracks. This isn't a defect—it's physics. Concrete shrinks as it cures, expands and contracts with temperature changes, and eventually develops cracks. The goal isn't to prevent cracking entirely (impossible), but to control where cracks occur and minimize problematic ones.
Why Concrete Cracks (Causes and Prevention)
All concrete cracks. This isn't a defect—it's physics. Concrete shrinks as it cures, expands and contracts with temperature changes, and eventually develops cracks. The goal isn't to prevent cracking entirely (impossible), but to control where cracks occur and minimize problematic ones.
Understanding why concrete cracks helps you prevent the types that matter: structural cracks that compromise integrity and wide cracks that let water in. Most residential cracking is cosmetic and manageable.
The Main Causes of Concrete Cracking
1. Shrinkage Cracks (Most Common)
What happens: As concrete cures, water evaporates and the material shrinks. If shrinkage is restrained (by forms, subbase friction, or reinforcement), stress builds until the concrete cracks.
When it occurs: Usually within the first few days to weeks after pouring.
What it looks like: Fine, hairline cracks, often in a random pattern. May appear as map cracking (like dried mud).
Prevention:
- Install control joints every 8-10 feet (creates weak points where cracks occur predictably)
- Proper curing—keep concrete moist for 7+ days (see curing guide)
- Avoid rapid drying (wind, hot sun, low humidity)
- Don't add excess water to the mix
2. Settlement Cracks
What happens: The soil beneath concrete compacts unevenly, isn't properly prepared, or washes out, causing sections to sink.
When it occurs: Can happen anytime, often within first few years.
What it looks like: Cracks with vertical displacement (one side higher than the other). Often near edges or at joints.
Prevention:
- Proper subbase preparation (4-6" compacted gravel)
- Adequate compaction of fill soil
- Good drainage to prevent soil erosion
- Avoid building over tree roots or organic material
3. Overload Cracks
What happens: Concrete is stressed beyond its capacity—too heavy a load or concentrated weight.
When it occurs: When load is applied.
What it looks like: Cracks radiating from point of impact or concentrated load. May be accompanied by spalling or crushing.
Prevention:
- Adequate thickness for intended use (5-6" for driveways, not 4")
- Reinforcement (rebar, wire mesh) distributes loads
- Avoid parking heavy equipment on thin residential slabs
- Spread point loads with footings or pads
4. Freeze-Thaw Damage
What happens: Water enters concrete pores, freezes, expands (9% volume increase), and creates internal pressure. Repeated cycles cause progressive damage.
When it occurs: Cold climates, over multiple winters.
What it looks like: Surface scaling, spalling, and cracking—often worse near the surface. Related: concrete spalling.
Prevention:
- Air-entrained concrete (tiny air bubbles accommodate expansion)
- Proper curing before first winter
- Sealing to reduce water absorption
- Avoid deicing salts (accelerates damage)
5. Improper Curing
What happens: Concrete dries too fast, preventing proper hydration. The surface cures faster than the interior, creating differential shrinkage.
When it occurs: First 7-28 days, especially in hot or windy conditions.
What it looks like: Surface crazing (fine network of cracks), weak surface layer, plastic shrinkage cracks (occur within hours of pouring).
Prevention:
- Keep concrete moist for minimum 7 days
- Use curing compound or cover with plastic
- Don't pour in extreme heat or wind without precautions
- Never let surface dry while concrete is still plastic
6. Too Much Water in the Mix
What happens: Excess water makes concrete easier to work but creates more shrinkage as that water evaporates. It also weakens the concrete.
When it occurs: Shrinkage cracks appear during curing; weakness is permanent.
What it looks like: Excessive shrinkage cracking, weaker-than-expected concrete, surface dusting.
Prevention:
- Follow mix specifications—don't add water "to make it easier"
- Use plasticizers if better workability is needed
- Order correct slump from ready-mix supplier
- Resist the temptation to spray water on setting concrete to extend working time
7. Missing or Inadequate Control Joints
What happens: Without control joints, concrete chooses where to crack—usually the most inconvenient, visible locations.
When it occurs: During initial shrinkage or later temperature cycles.
What it looks like: Random cracking across the slab instead of at joint locations.
Prevention:
- Cut control joints at proper intervals (see table below)
- Joint depth should be 1/4 of slab thickness
- Cut within 6-18 hours of pouring (timing varies by conditions)
| Slab Thickness | Maximum Joint Spacing |
|---|---|
| 4 inches | 8-10 feet |
| 5 inches | 10-12 feet |
| 6 inches | 12-15 feet |
Rule of thumb: Joint spacing (in feet) = 2-3 × slab thickness (in inches)
Types of Cracks: Cosmetic vs. Concerning
Cosmetic Cracks (Usually Not Serious)
- Hairline cracks (<1/16"): Normal shrinkage, typically stable
- Crazing: Fine surface network, doesn't affect structure
- Cracks at control joints: Working as intended
- Stable cracks: Haven't changed in years
Concerning Cracks (May Need Attention)
- Growing cracks: Getting longer or wider over time
- Wide cracks (>1/4"): Allow water penetration, may indicate structural issues
- Displaced cracks: One side higher than the other (settlement)
- Multiple parallel cracks: Possible structural problem
- Cracks with rust staining: Corroding reinforcement
When to Worry
| Crack Type | Concern Level | Action |
|---|---|---|
| Hairline, stable | Low | Monitor, seal optional |
| 1/8"-1/4", stable | Moderate | Fill and seal |
| >1/4" or growing | High | Professional assessment |
| With displacement | High | Professional assessment |
| Foundation cracks | High | Structural engineer |
Prevention Summary
The most effective crack prevention strategies:
| Strategy | Prevents | Difficulty |
|---|---|---|
| Control joints | Random cracking | Easy |
| Proper curing | Shrinkage, weakness | Easy |
| Correct water ratio | Shrinkage, weakness | Moderate |
| Good subbase prep | Settlement | Moderate |
| Adequate thickness | Overload damage | Planning |
| Reinforcement | Crack spreading | Moderate |
| Air entrainment | Freeze-thaw | Specify in order |
| Sealing | Freeze-thaw, water damage | Easy |
Frequently Asked Questions
Is it normal for new concrete to crack?
Yes—some cracking is inevitable and normal. Hairline cracks from shrinkage typically appear within the first month. Control joints are designed to create intentional crack locations. Concerning cracks are those wider than 1/4", growing, or showing displacement.
How do I prevent concrete from cracking?
The most effective prevention: (1) Install control joints every 8-10 feet, (2) cure properly by keeping concrete moist for 7+ days, (3) don't add excess water to the mix, and (4) prepare a proper compacted gravel base. These four steps prevent most problematic cracking.
When should I worry about concrete cracks?
Worry about cracks that are wider than 1/4 inch, actively growing, have one side higher than the other (displacement), or appear in foundation walls. These may indicate structural issues. Stable hairline cracks are cosmetic and normal.
Do control joints prevent cracking?
Control joints don't prevent cracking—they control where cracks occur. Concrete will crack due to shrinkage regardless. Joints create intentional weak points where cracks form predictably and neatly, rather than randomly across your slab.
Key Takeaways
- All concrete cracks—the goal is controlling where and how
- Control joints are the #1 prevention tool (every 8-10 feet)
- Proper curing prevents most shrinkage cracks (keep moist 7+ days)
- Excess water in the mix increases shrinkage and weakness
- Settlement cracks indicate subbase problems—address the cause
- Hairline cracks are cosmetic; wide or growing cracks need attention
For repairing existing cracks, see how to repair concrete cracks. For complete project guidance, visit our concrete guide hub or use the concrete calculator for your next project.