Crack Hairline vs Structural vs Settlement Decision Tree

Why this matters

A homeowner pointing at a crack wants one of three answers: "ignore it," "seal it," or "tear it out." Get the call wrong upward (tear out a working slab) and the customer pays for unnecessary work; get it wrong downward (seal a structural crack) and the slab is back in six months with the customer's lawyer attached. The senior concrete tech needs a decision tree that separates plastic-shrinkage (cosmetic), drying-shrinkage (cosmetic to functional), structural (load or design failure), and settlement (subgrade failure) in field-probability order using only what is visible at the site walk. This tree walks the diagnosis from "how wide is it" to "what is moving underneath."

Symptom presentation - the four call types

You get one of four call descriptions. First, "spider cracks all over the surface" - usually plastic shrinkage on a newly-placed slab. Second, "a long crack across the driveway, getting wider" - drying shrinkage or restraint cracking. Third, "the slab dropped on one side" - settlement, sometimes preceded by a crack. Fourth, "the wall is cracked through" - structural. Each starts at a different question and the answers diverge fast.

Quick checks at the site walk

• Measure the crack width with a feeler gauge or crack-width comparator card at three points along its length. Note the maximum and the mean.
• Measure depth if accessible. ACI 224R defines crack categories partly by depth-to-width ratio. Surface-only cracks under 1/8 inch deep are different problems than full-thickness cracks.
• Note pattern. Random map cracking - plastic or drying shrinkage. Straight line, regularly spaced - control joint failure or restraint crack. Diagonal at corner - load concentration or settlement. Vertical at center of wall - flexural. Stair-step in CMU or block - settlement.
• Note differential elevation. Use a 4-foot level and a straightedge. Two slab panels at the same elevation - shrinkage. Two slab panels with 1/4 inch or more elevation difference - settlement.
• Note moisture and staining. Active water leak or efflorescence at the crack means water is moving through it; that is a sealing/waterproofing decision regardless of the structural call.
• Pull the slab age if known. Cracks under 30 days old - largely plastic and early drying shrinkage. Cracks 30 days to 1 year - drying shrinkage and curl. Cracks over 1 year that opened recently - settlement, restraint, or load change.

Isolation tree - which crack on which axis

Branch 1 - random map pattern, hairline (under 0.005 inch), surface only, no elevation difference. Plastic shrinkage cracking from rapid surface evaporation during placement. Cosmetic. Treat with a penetrating sealer to prevent dirt and moisture intrusion. Do not repair structurally. Document for customer and move on.

Branch 2 - long parallel cracks, 0.005 to 0.030 inch (1/32 inch), spaced 8 to 15 feet, no elevation difference. Drying shrinkage that occurred between control joints, or control joints that were cut too late or too shallow. Cosmetic to functional. Seal with a high-modulus polyurethane or epoxy sealant. Note for the customer that this is the slab finding its own joint pattern and will not progress materially.

Branch 3 - single crack 0.030 to 0.060 inch (1/32 to 1/16 inch), straight line, possibly through-slab, no elevation difference but possibly a slight stairstep where the crack crosses a control joint. Restraint cracking, often from rebar or wire mesh placement issues, or from the slab being cast against a rigid edge (wall, curb, footing) without an isolation joint. Cosmetic if the crack is stable; functional if it leaks. Seal and monitor. Re-inspect at 6 months for width change.

Branch 4 - crack over 0.060 inch (over 1/16 inch), elevation difference of 1/8 inch or more, or progressive widening over 6 months. Structural or settlement. Cannot be resolved by surface treatment. Requires sub-grade diagnosis (settlement) or structural evaluation (load). Recommend escalation to a structural engineer for any wall crack or any slab crack with elevation differential over 1/4 inch.

Branch 5 - diagonal crack at a re-entrant corner (e.g., where a slab notches around a column or post), 0.020 inch or more. Stress concentration cracking. Almost always design-related: missing diagonal corner reinforcement (ACI 318 requires it). Cosmetic if the slab is non-structural; structural concern if the slab carries load. Repair: route, fill with low-viscosity epoxy injection per ACI 224.1R if structural, or seal if cosmetic.

Branch 6 - stairstep crack in a foundation wall or CMU, 0.020 inch or more, with one side dropped relative to the other. Foundation settlement. Cannot be resolved by crack repair alone. Requires sub-grade investigation, likely helical or push-pier underpinning. Escalate to foundation-repair specialty.

Confirming diagnosis - the monitoring window

For cracks in the cosmetic-to-structural ambiguous range (0.020 to 0.060 inch, no clear elevation differential), install crack monitors at three points along the crack and re-measure in 30 days, 90 days, and 1 year. A stable crack at 1 year is by definition not progressive and can be treated as cosmetic. A widening crack of 0.005 inch or more per year is progressive and needs structural evaluation regardless of starting width.

For settlement vs shrinkage when elevation differential is small (under 1/8 inch): set a string line across the slab at two stable benchmarks. Re-measure at 6 months. Any continued differential movement is settlement, not shrinkage (shrinkage is complete within 12 months and does not produce ongoing elevation change).

Remediation - what fix for which crack

Cosmetic plastic-shrinkage: penetrating siloxane or lithium silicate sealer. No filling required. Apply per ASTM C309 cure-and-seal product if within 30 days of placement, or ASTM C1315 penetrating sealer after 30 days.

Sealable drying-shrinkage: rout to 1/4 inch wide by 1/2 inch deep with a crack chaser, vacuum, prime if epoxy, fill with self-leveling polyurethane (e.g., Sika 1a) or two-component epoxy (e.g., Sikadur 35 Hi-Mod LV) per ACI 224.1R-07.

Structural through-crack with no elevation differential: route and epoxy injection per ACI 224.1R, low-viscosity structural epoxy under pressure through surface ports. Restores monolithic action when done correctly.

References

• ACI 224R-01 - Control of Cracking in Concrete Structures
• ACI 224.1R-07 - Causes, Evaluation, and Repair of Cracks in Concrete Structures
• ACI 318-19 - Building Code Requirements for Structural Concrete (Chapter 24 serviceability and crack control)
• ASTM C309 - Standard Specification for Liquid Membrane-Forming Compounds for Curing Concrete
• ASTM C1315 - Standard Specification for Liquid Membrane-Forming Compounds Having Special Properties for Curing and Sealing Concrete
• ICRI Technical Guideline 320.1R - Guide for the Selection of Strengthening Systems for Concrete Structures
• US Bureau of Reclamation Concrete Manual 9th Edition, Chapter VIII Crack Evaluation