Reading Concrete + Slab Cracks Reference

Why this matters

Customers often have concrete cracks - driveways, sidewalks, foundations, slab floors. Some are normal; some signal problems. The tech who can distinguish helps customers prioritize OR avoid unnecessary worry. This is the field card.

Types of cracks

Hairline / shrinkage cracks:

• 1/16" or less wide
• Normal in concrete (shrinks as cures)
• Cosmetic
• Most concrete has some

Plastic shrinkage cracks:

• Form during initial curing
• Pattern of small cracks
• Surface only
• Cosmetic

Structural cracks:

• Wider (1/8"+ typical)
• Indicate movement / settlement
• Concerning
• Investigation needed

Diagonal cracks:

• Slope downward
• Settlement indicator
• Concerning if widening over time

Vertical cracks:

• Less concerning typically (especially in foundation walls)
• Shrinkage OR minor settlement

Horizontal cracks:

• In foundation walls
• Indicate lateral pressure (soil pushing in)
• Serious concern

Stair-step cracks (in masonry):

• Follow mortar joints
• Differential settlement
• Concerning

Map cracks:

• Network like cracked porcelain
• Often surface issue
• Sometimes ASR (alkali-silica reaction)

Where cracks form

Slab floors:

• Center of large areas (shrinkage)
• Corners
• Where new pour meets old
• At control joints (intentional)

Driveways:

• At expansion joints
• Where vehicle loads concentrate
• Near edges
• Cold-weather (freeze-thaw)

Sidewalks:

• At joints
• Tree root displacement
• Erosion underneath

Foundation walls:

• Vertical at corners (shrinkage)
• Horizontal (lateral pressure)
• Diagonal at openings (window, door)

Pool decks:

• Movement around pool
• Thermal cycling
• Settlement

Why cracks form

Shrinkage:

• Concrete shrinks as water evaporates during curing
• Hairline cracks normal
• Most concrete has these

Settlement:

• Soil below settles unevenly
• Concrete cracks following stress
• Foundation movement cause

Thermal expansion / contraction:

• Hot summer days expand
• Cold winter contract
• Cracks at joints OR points of restraint

Freeze-thaw:

• Water in concrete freezes (expands)
• Surface flaking
• Crack propagation

Heavy loads:

• Vehicle on inadequate slab
• Concentrated point loads
• Cracks underneath

Tree roots:

• Lift OR displace concrete
• Cracks following root path
• Common at driveways near mature trees

ASR (alkali-silica reaction):

• Chemical reaction in concrete
• Slow process
• Map cracking pattern
• Rare in residential

Corroded rebar:

• Steel inside concrete rusts
• Expands; cracks concrete from inside
• Saltwater areas, coastal
• Old concrete

Severity assessment

Hairline (less than 1/16"):

• Normal
• Cosmetic only
• Optional: seal with crack filler
• Continue monitoring

Small (1/16" to 1/8"):

• Often shrinkage
• May widen over time
• Seal to prevent water entry
• Monitor

Medium (1/8" to 1/4"):

• Concerning for movement
• Investigate cause
• Repair appropriately
• Continue monitoring

Large (1/4"+):

• Significant movement
• Structural concern (foundation especially)
• Engineer evaluation
• Don't ignore

Visible separation OR displacement:

• Sides offset (not just gap)
• Active failure
• Address immediately

Cracks in foundation walls

Specific concern:

Vertical:

• Common
• Often shrinkage
• Less serious
• Seal + monitor

Horizontal:

• Soil pressure pushing wall inward
• Serious concern
• Engineer evaluation
• Sometimes need reinforcement (carbon fiber strap, steel I-beams)

Diagonal at corners / openings:

• Settlement
• Investigate cause
• Address foundation OR drainage

Cracks with offset:

• Walls shifted relative to each other
• Significant settlement
• Engineer evaluation

Cracks in floor slabs

Cracks not creating problems:

• Hairline
• Slow widening
• Stable
• Filled / sealed if cosmetic concern

Cracks creating problems:

• Water entry from below
• Insect intrusion
• Trip hazard
• Visible movement

Tracking changes

Customer asks "is it getting worse?":

Crack monitor:

• Glass slide across crack (breaks if movement)
• Crack monitor card (specific tool)
• Marked endpoints (measure over time)
• Photo with measurement

Documentation:

• Date + measurement
• Photo with coin / ruler
• Recheck periodically

Customer's record of progression helps engineer evaluate.

Cross-trade observations

Plumber:

• Slab leak suspicion
• Cracks near water lines
• Pool deck crack near pool plumbing

Pest control:

• Crack as termite entry point
• Ant trail through crack
• Mouse entry

HVAC:

• Slab crack under outdoor unit (settling)
• Garage floor crack affecting equipment placement

Roofer:

• Foundation movement reflected in roof issues
• Sagging beams

When to recommend repair

Cosmetic crack filler:

• Hairline cracks
• Customer wants aesthetic improvement
• Caulk OR epoxy filler

Structural repair:

• Wider cracks
• Movement evident
• Engineer-recommended approach

Foundation repair:

• Significant settlement
• Wall bowing
• Specialty contractor (helical piers, etc.)

Repair methods

Epoxy injection:

• Inject low-viscosity epoxy into crack
• Bonds the crack
• For structural cracks (sometimes)

Polyurethane injection:

• Flexible filler
• For non-structural cracks
• Water-stopping properties

Carbon fiber strap:

• Reinforces bowing walls
• Prevents further movement
• Doesn't fix existing crack but stabilizes

Wall plates / steel beams:

• For severely bowed walls
• More aggressive than carbon fiber

Mudjacking / polyjacking:

• Lift settled slabs
• Cement OR polyurethane foam
• Driveways, sidewalks

Pier reinforcement:

• Helical piers OR push piers
• Support foundation
• Significant cost
• Engineer-designed

Replace:

• Sometimes easier than repair
• Particularly badly damaged slabs
• Old + multiple issues

Drainage as common cause

Many concrete issues from drainage:

• Water at foundation = saturated soil = movement
• Water in cracks = freeze-thaw damage
• Erosion under slab = settling

Address drainage first; repair concrete second.

Customer education

Cosmetic cracks:

• Normal
• Don't worry
• Seal if concerned about appearance

Concerning cracks:

• Get engineer evaluation
• Track progression
• Address underlying cause

Documentation

For observations:

• Photo with measurement
• Date
• Location
• Suspected cause
• Recommendation

If customer asks later about progression: documentation matters.

When to refer to engineer

Refer when:

• Cracks 1/4"+ in foundation
• Horizontal cracks
• Multiple cracks expanding
• Customer planning major work on home
• Floors sloping
• Customer's concern significant
• Cosmetic vs structural unclear

Engineer evaluation cost reasonable; saves overspending on unnecessary repair OR underspending on real problem.

Specific concrete types

References

• ACI (American Concrete Institute) standards
• IRC Chapter 4 (Foundations)
• ASCE foundation guidelines
• Manuall internal: Foundation Problem Signs, Concrete Crack Diagnosis, Drainage + Grading Around Foundation