The Truth About Concrete Cracks

Here’s the reality: all concrete cracks. The goal isn’t to prevent cracking entirely—it’s to control where and how it cracks. Understanding this changes your approach from impossible prevention to effective management.

Types of Concrete Cracks

1. Plastic Shrinkage Cracks

When: Within hours of pouring

Appearance: Short, irregular cracks, often parallel

Cause: Surface dries faster than interior due to:

Hot weather
Wind
Low humidity
Direct sun

Prevention:

Dampen subgrade before pouring
Use windbreaks
Apply evaporation retarder
Start curing immediately

2. Drying Shrinkage Cracks

When: Days to weeks after pouring

Appearance: Random pattern or at control joint locations

Cause: Natural concrete shrinkage as water evaporates (concrete shrinks about 1/16” per 10 feet)

Prevention:

Proper control joints (see below)
Good curing practices
Correct water-cement ratio
Quality aggregates

3. Settlement Cracks

When: Weeks to months after pouring

Appearance: Uneven, following void patterns below slab

Cause:

Poor subgrade compaction
Soil erosion under slab
Organic material decomposition

Prevention:

Proper excavation and compaction
Remove all organic material
Adequate gravel base
Compact in lifts

4. Structural Cracks

When: Any time, often worsening

Appearance: Wide cracks, often with displacement

Cause:

Overloading
Poor reinforcement
Design errors
Foundation movement

Prevention:

Proper engineering
Adequate reinforcement
Correct slab thickness
Professional design for heavy loads

5. Crazing (Map Cracking)

When: Days to weeks after pouring

Appearance: Fine spider-web pattern on surface

Cause:

Over-finishing
Too much water in mix
Rapid surface drying
Adding water during finishing

Prevention:

Don’t overwork the surface
Never add water during finishing
Time finishing properly
Proper curing

Control Joints: Your Primary Defense

What Are Control Joints?

Pre-planned weak points in the slab that encourage concrete to crack in a straight, hidden line rather than randomly.

Control Joint Rules

Spacing:

Maximum: 24-36 × slab thickness in feet
Example: 4” slab = 8-10 feet max spacing
General rule: Keep panels square (length ≤ 1.5× width)

Depth:

Minimum 1/4 of slab thickness
4” slab = 1” deep joint
6” slab = 1.5” deep joint

Joint Cutting Methods

Tooled joints (during pour):

Cut while concrete is still workable
Use jointing tool after floating
Creates cleanest appearance
Must be deep enough

Saw-cut joints (after curing):

Cut within 4-12 hours of pour
Use early-entry saw if possible
Traditional saw: 12-24 hours
Creates precise lines

Joint Layout Best Practices

Plan joints before pouring
Mark locations on forms
Avoid re-entrant corners (L-shapes crack at inside corner)
Run joints through to edges
Avoid triangular panels
Keep panels as square as possible

The Mix: Getting It Right

Water-Cement Ratio

Too much water causes:

Increased shrinkage
Lower strength
Surface dusting
Craze cracking

Ideal ratio: 0.45-0.50 for typical residential

Never add water to pre-mixed concrete to make it more workable. Use plasticizer additives instead.

Aggregate Quality

Problems from poor aggregate:

Dirty aggregate traps air
Wrong gradation causes segregation
Reactive aggregate causes expansion

Use clean, well-graded aggregate from reputable suppliers.

Admixtures That Help

Water reducers: Improve workability without added water
Shrinkage reducers: Decrease drying shrinkage
Fibers: Control plastic shrinkage cracking
Air entrainment: Required for freeze-thaw resistance

Proper Curing Prevents Cracks

Why Curing Matters

Concrete that dries too fast:

Shrinks more
Develops less strength
Cracks more readily

Minimum curing: 7 days for residential flatwork

Curing Methods

Method	Effectiveness	Effort
Water curing	Excellent	High
Wet burlap	Excellent	Moderate
Plastic sheeting	Good	Low
Curing compound	Good	Very low

Temperature and Weather Effects

Hot Weather Pouring (above 85°F)

Risks:

Rapid moisture loss
Plastic shrinkage cracking
Accelerated set time

Solutions:

Pour early morning
Cool materials (ice in water)
Use retarding admixtures
Erect sunshades
Fog spray if needed
Begin curing immediately

Cold Weather Pouring (below 50°F)

Risks:

Slow strength gain
Freeze damage
Thermal cracking

Solutions:

Use accelerators
Heat materials
Insulated blankets after pour
Extend curing period
Never pour on frozen ground

Rain During Pour

Before concrete sets: Protect with plastic

After initial set: Light rain usually OK

Heavy rain on fresh concrete: Can damage surface, cause scaling

Reinforcement for Crack Control

Wire Mesh

Holds cracks together (doesn’t prevent them)
Position in upper third of slab
Use chairs to maintain position
6×6 welded wire mesh common choice

Rebar

Better for driveways and heavy loads
#3 or #4 at 18” on center typical
Position in middle to upper third
Maintain 2-3” concrete cover

Fiber Reinforcement

Reduces plastic shrinkage cracking
Distributed throughout concrete
Doesn’t replace wire mesh or rebar
Use as supplement, not replacement

Subgrade Preparation

Proper Base = Fewer Cracks

Steps:

Remove all topsoil and organics
Compact subgrade soil
Add 4-6” of gravel base
Compact gravel in lifts
Moisten before pouring

Vapor Barriers

Place 6-mil poly under interior slabs
Cover with sand to protect from punctures
Helps prevent moisture-related issues
May increase surface drying (consider curing)

Common Mistakes That Cause Cracks

Mistake	Type of Crack	Prevention
Too much water in mix	Drying shrinkage	Use correct water ratio
No control joints	Random cracking	Plan and cut joints
Poor subgrade prep	Settlement	Proper compaction
Finishing too soon	Crazing	Wait for bleed water
No curing	All types	Cure minimum 7 days
Adding water during pour	Weak zones, cracks	Never add water
Re-entrant corners	Corner cracks	Add joint at corner

Repairing Existing Cracks

Hairline Cracks (less than 1/8”)

Method: Apply concrete crack filler or caulk

Clean crack with brush
Apply filler with caulk gun
Smooth with putty knife
Let cure per product instructions

Medium Cracks (1/8” to 1/2”)

Method: Epoxy or polyurethane injection

Clean and dry crack
Apply injection ports
Inject repair material
Cap ports after filling

Large Cracks (over 1/2”)

Method: Routing and sealing

Widen crack with grinder
Clean thoroughly
Fill with flexible sealant
Or consider slab replacement

When to Call a Professional

Cracks with vertical displacement
Cracks that continue growing
Structural concrete cracking
Foundation cracks
Cracks with water infiltration

Crack Prevention Checklist

Before pour:

During pour:

After pour:

Curing begins immediately
Saw cuts made in time (if using)
Protected from weather extremes
Traffic restricted appropriate time

Calculate Your Concrete Needs

Planning a crack-free concrete project? Use our free Concrete Calculator to estimate materials and costs.

Pro Tip: The three most important factors in crack prevention are: proper control joints, correct water-cement ratio, and adequate curing. Get these three right, and you’ll avoid 90% of concrete crack problems. When in doubt, add more control joints—they’re invisible when cracks form in them, which is exactly what you want.