Spider Cracks vs Structural Cracks Decision Tree

Why this matters

The homeowner's first call is "my driveway is cracking." The contractor's job in the next 10 minutes is to distinguish a cosmetic surface pattern that needs a wash and a stain from a structural movement that needs slabjacking, steel, or a tear-out. Wrong call left costs a callback; wrong call right loses the job to a competitor selling the cheaper repair. ACI 224.1R and ICRI 320.2R-2009 frame the technical distinctions; this article translates them into a field decision tree a tech can run with a flashlight, a crack gauge, and a level.

The four crack categories

Field cracking on residential and light-commercial slabs sorts into four categories that drive different remediation paths.

1. Plastic shrinkage cracks (spider cracks, map cracks, crazing). Formed during the first 6 hours of placement when surface evaporation exceeds bleed-water rise. Cosmetic, surface-only, usually under 1/16 inch wide, and never extend through the slab.

2. Drying shrinkage cracks. Formed in the first weeks to months after placement as the slab loses moisture and contracts. Typically follow control-joint patterns that should have been cut but weren't, and may be 1/32 to 1/8 inch wide. Through-cracks but not movement-active.

3. Structural cracks from settlement, heave, or load. Movement-active. Width varies; offset between crack faces is the marker. May continue to grow over time. Repair requires addressing the cause (slabjacking for settlement, root barriers for heave, structural reinforcement for load).

4. Curling-edge cracks. Slab edges curl upward as the top dries faster than the bottom; loaded edges fracture. Often diagonal corner cracks 12-36 inches from the slab edge. Distinct cause but treated similarly to drying shrinkage if not progressive.

Decision 1: Surface pattern

Step back and look at the overall pattern.

• Hexagonal or polygonal network covering broad areas, individual crack lengths under 24 inches: plastic shrinkage / crazing. Surface-only. Confirm with the flashlight at oblique angle - crazing catches light differently from through-cracks.
• Straight or gently curved cracks running across the slab, often connecting joint stops to slab edges or appearing where a control joint should have been: drying shrinkage. Typically through-slab.
• Single crack with consistent width and direction following a line of load or following a settlement pattern (parallel to a foundation edge, radiating from a point load): structural. Investigate further.
• Cracks at corners of slab edges, diagonal, with curling-up edge: curling-edge crack.

Decision 2: Width measurement

Pull a crack gauge or feeler gauge.

• Under 1/64 inch (0.4 mm): cosmetic. No structural concern. Sealer or stain hides it.
• 1/64 to 1/16 inch (0.4 to 1.6 mm): minor. Through-slab is possible; structural concern is low for non-load-bearing flatwork. Polyurethane crack-filler is sufficient for moisture/aesthetics.
• 1/16 to 1/8 inch (1.6 to 3.2 mm): meaningful. Investigate cause before sealing. May be drying shrinkage that should have had control joints; may be early structural movement.
• Above 1/8 inch (3.2 mm): structural until proven otherwise. ICRI 320.2R-2009 flags this width as the threshold above which engineering evaluation is the conservative call.

Width alone does not classify the crack; combine with the other measurements.

Decision 3: Vertical offset between crack faces

Run a straightedge across the crack perpendicular to the crack direction. Measure any step between the two faces.

• Zero offset: no settlement or heave currently active. Could still be a past movement that has stabilized; combine with growth-history evidence.
• Offset under 1/16 inch: minor differential movement. Could be plastic-state subgrade settlement that completed; could be early-stage active movement.
• Offset 1/16 to 1/4 inch: structural movement, current or past. Investigate cause. For slab-on-grade in expansive soil, this is the diagnostic signature of heave or settlement.
• Offset above 1/4 inch: structural failure. Engineering evaluation required. Do not over-coat or seal without addressing.

Decision 4: Through-slab vs surface only

Flashlight at oblique angle across the crack. If the crack catches light differently along its length (lighter in some segments, darker in others), it's variable depth. If it appears as a uniform dark line, it's likely through-slab.

For a more definitive test: water on the surface adjacent to the crack on a dry slab. Water that disappears into the crack is through-slab. Water that pools at the crack is surface-only.

For high-stakes diagnosis, core sampling per ASTM C42 confirms depth. For residential troubleshooting, the water test is sufficient.

Decision 5: Growth history

Ask the homeowner: when did the crack first appear, has it grown, has new cracking appeared along the same line.

• Crack present since first month after pour, no growth since: drying shrinkage or completed plastic-state movement. Not structural.
• Crack appeared 6+ months after pour: drying shrinkage in progress or early structural. Re-measure in 30 days to determine if growing.
• Crack visibly grown in the last year: active movement. Investigate cause.

Crack monitors (ICRI 320.4R) installed across the crack and read at 30 and 90 day intervals quantify growth definitively. For diagnostic certainty on a borderline case, a $15 plastic crack monitor and a 90-day visit answers the question.

Decision 6: Cause

For cracks that pass the structural threshold (width above 1/8 inch with measurable offset and through-slab confirmation), the cause has to be identified before remediation:

• Settlement: subgrade has compacted unevenly. Slabjacking (mudjacking or polyurethane lifting per ASTM standards) can re-level if the settlement is complete. If settlement is ongoing, slabjack after the cause is addressed.
• Heave from expansive soil: clay subgrade has swelled with moisture. Root cause is soil moisture variation; drainage corrections and root barriers address the cause. Slab repair only after.
• Heave from tree roots: tree roots have lifted the slab. Root cause is the tree; root pruning or removal is the upstream fix. Slab repair after.
• Frost heave: in cold climates, frost penetration below the slab. Insulation under the slab or replacement at appropriate frost depth per IRC R403.1.4 is the long-term fix.
• Load: slab exceeded design load. Re-engineer the slab or limit load.

Quick decision tree summary

References

• ACI 224.1R-07, Causes, Evaluation, and Repair of Cracks in Concrete Structures.
• ICRI Guideline 320.2R-2009, Guide for Selecting Application Methods for the Repair of Concrete Surfaces.
• ICRI Guideline 320.4R, Guide for the Field Evaluation of Cracks in Concrete Surfaces.
• ASTM C42/C42M, Standard Test Method for Obtaining and Testing Drilled Cores and Sawed Beams of Concrete.
• IRC R403.1.4, Frost Protection, 2021 International Residential Code.