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

What the slab is made of changes both how it cracks and what you are allowed to do to it.

Post-tensioned slab. Steel tendons run through the slab under high tension and hold it in compression. Never core, saw, or drill one without locating the tendons first, and never on a hunch about spacing. Cutting a live tendon releases enormous stored energy, it is a serious injury hazard, and it structurally compromises the slab. Look for the warning stamp at the perimeter or the anchor pockets along the edge, and treat any slab in a region known for expansive soils as suspect until proven otherwise. Locating means scanning, and the repair of a cut tendon is an engineered fix, not a patch.

Conventionally reinforced slab (rebar or welded wire). Cracks are held tight by the steel, so a hairline that stays hairline is normal behavior working as designed. Drilling and coring are routine, but a scan still saves you from cutting a bar in a structural location.

Fiber-reinforced concrete. Fibers control plastic shrinkage cracking during cure, not structural cracking. A fiber mix with no steel still moves at joints and still needs control joints. A wide crack in fiber concrete means the same thing it would mean anywhere else.

Slab with radiant tubing. Cut it and you own a repair inside a floor. Scan, and where the customer has no as-built, warm the system and read the floor with a thermal camera before any penetration.

Monolithic slab-with-footing versus a slab poured inside stem walls. Cracks near the perimeter mean different things. On a monolithic pour, a perimeter crack can involve the footing. On a floating slab inside stem walls, the same crack is often just the slab moving independently, which is what it was designed to do.

Toppings and underlayments. Gypsum-based underlayment and thin self-leveling toppings crack readily and are not structural. Read the crack in the topping as a symptom, then find out whether the substrate below moved.

Decorative work: stamped, colored, exposed aggregate, polished. The concrete behaves normally, but the repair does not. Any crack repair will be visible, color matching is difficult and never exact, and polished floors show every patch under light. Set that expectation before repair, not after.

Lightweight structural concrete (common on elevated decks over metal deck) is softer, drills differently, and has lower pullout values for anchors. Verify anchor ratings for lightweight before hanging equipment.

Old concrete with no reinforcement at all. Common in pre-war basements, garages, and sidewalks. Cracks open wider and shift more because nothing is holding the halves together, and the slabs are often thin and poured directly on soil. Judge these by movement and elevation change, not by crack width alone.

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