Concrete Cracks: Causes, Types, and Prevention

Why Concrete Cracks

Contrary to popular belief, some cracking in concrete is normal. Concrete shrinks as it dries and cures, and this shrinkage causes tension that can exceed concrete's tensile strength. Understanding the causes helps you minimize problematic cracking.

Primary Causes

Plastic Shrinkage: Occurs within the first few hours while concrete is still plastic. Caused by rapid surface water evaporation, often in hot, windy, or dry conditions. The surface dries faster than the underlying concrete, creating tension that causes cracks.

Drying Shrinkage: As concrete continues to cure and lose moisture over months, it shrinks. If restrained by friction with the ground, reinforcement, or adjacent structures, this shrinkage creates tension that causes cracking.

Thermal Contraction: Concrete expands when hot and contracts when cold. Temperature changes after initial set create stresses. Large temperature swings during curing or seasonal changes can cause cracking.

Structural Overload: When loads exceed design capacity, concrete cracks. This includes both static loads (weight) and dynamic loads (impact). Foundation settlement can also cause structural cracking.

Chemical Reactions: Alkali-aggregate reaction (AAR) and sulfate attack cause internal expansion and cracking. These are material compatibility issues that develop over years.

Types of Cracks

Hairline Cracks

Very thin cracks (less than 1/16 inch) that often appear within the first few days. Usually caused by surface shrinkage. Typically not structurally significant but can allow moisture penetration.

Plastic Shrinkage Cracks

Appear within hours of placement, often running parallel and somewhat randomly across the surface. Can be several feet long and up to several inches deep. Preventable with proper curing practices.

Settlement Cracks

Occur when the ground beneath concrete settles unevenly. Often diagonal from corners or appear above underground voids. Indicate foundation problems that may require attention.

Structural Cracks

Result from overload, design deficiency, or structural failure. May be horizontal, vertical, or diagonal. These cracks are often wider at one end and may show displacement. Require professional evaluation.

Thermal Cracks

Caused by temperature differentials. In mass concrete, heat from hydration causes expansion while the surface cools. In cold climates, freeze-thaw cycles cause surface cracking.

Crazing

Network of fine cracks in a hexagonal pattern, resembling a spider web. Caused by surface over-troweling or rapid drying. Primarily cosmetic, not structural.

Scaling

Surface flakes off in layers, exposing aggregate. Caused by freeze-thaw cycles, deicing salts, or over-finishing. Indicates surface durability issues.

Prevention Strategies

Proper Mix Design

Use the lowest practical water-cement ratio (0.50 or less)

Include proper air entrainment for freeze-thaw resistance

Use larger aggregate to reduce shrinkage

Consider shrinkage-reducing admixtures

Control Joints

Create planned weakness points where cracking is acceptable:

For slabs, space joints at 2-3 times the thickness in feet

Joints should be 1/4 of slab depth

Create joints within 24 hours of placement

Make joints continuous, not offset

Proper Curing

Begin curing immediately after finishing

Maintain moisture for at least 7 days

Protect from rapid drying in hot/windy conditions

Use curing compounds, wet coverings, or plastic sheeting

Reinforcement

Use wire mesh or rebar to control crack widths

Place reinforcement correctly—mid-depth for slabs

Synthetic fibers help control plastic shrinkage cracking

Proper reinforcement doesn't prevent cracks but keeps them tight

Ground Preparation

Compact soil uniformly to prevent differential settlement

Use proper gravel base for drainage

Remove organic material that can decompose

Address any soft spots before pouring

When to Worry

Most cracks are cosmetic, but some indicate serious problems:

Red Flags:

Cracks wider than 1/4 inch

Vertical displacement (one side higher than the other)

Diagonal cracks in walls, especially near corners

Horizontal cracks in foundation walls

Cracks that continue to grow

Water infiltration through cracks

Cracks accompanied by bowing or leaning

When you notice these signs, consult a structural engineer. Foundation cracks can indicate settlement problems that worsen over time.

Repair Options

Hairline Crack Sealing

Apply concrete sealer or penetrating sealer to hairline cracks. This prevents water intrusion and potential freeze-thaw damage. Simply brush or spray the sealer over affected areas.

Routing and Sealing

For larger cracks (1/8" to 1/4"), route the crack into a V-shape, clean it thoroughly, and fill with flexible sealant. This prevents water infiltration and allows for some movement.

Epoxy Injection

For structural cracks, epoxy injection restores structural integrity. The epoxy bonds the concrete back together. Requires professional application and is relatively expensive.

Crack Stitching

Metal staples or reinforcement are embedded across the crack to prevent further movement. Combined with epoxy injection for structural cracks.

Slab Jacking

If settlement caused the cracking, slab jacking (mud jacking) can raise and level settled concrete. Holes are drilled, grout is pumped underneath, and the slab is raised.

Complete Replacement

Sometimes replacement is more cost-effective than repair, especially for severely damaged concrete. Consider what caused the damage and address underlying issues before replacing.