Which Cause to Rule Out First on Slab Cracking: Subgrade vs Mix vs Cure Sequencing Decision Tree

Why this matters

When a slab cracks, three families of cause are in play: the subgrade under it, the mix that went in, and the cure that followed. A scattershot investigation wastes a site visit and often lands on the most convenient culprit rather than the right one. The value of a sequenced approach is that each cause family leaves a different signature, and ruling them out in the correct order means you stop as soon as the evidence converges instead of arguing about water cement ratio when the real problem is a void under the panel. This tree sets the order: subgrade first, mix second, cure third, because subgrade failures are the most expensive to miss and the easiest to confirm or eliminate on site, while cure problems mostly produce surface defects rather than full depth cracks.

Symptom presentation

The crack you are sequencing is a through crack or a wide surface crack, not a finish blemish. Customers report lines that "moved," panels that "dropped," or cracks that "keep getting wider." Per ACI 224.1R, the diagnostic split is between cracks driven by external restraint and support (subgrade) versus cracks driven by the concrete's own volume change (mix and cure).

Quick checks

Arrive with a 4 ft level, a crack comparator, a hammer for sounding, and the batch ticket if you can get it. First three reads:

• Is there vertical step or rocking at the crack? Points subgrade.
• What does the batch ticket say for water added on site and slump? Points mix.
• Was the slab cured, and how soon after finishing? Points cure.

The reason the order is subgrade, then mix, then cure is partly diagnostic economy and partly cost of error. Subgrade failures are the most expensive to discover late (they keep moving and defeat any crack repair), and they are the fastest to confirm or eliminate on site with a hammer and a level, so they go first. Mix problems are next because the batch ticket is a hard document you can read in minutes. Cure and timing problems mostly produce surface and shrinkage cracking rather than full depth structural cracks, so they sit last and are what remains once the higher cost causes are cleared. Resist the urge to grab the most convenient explanation; a clean batch ticket does not mean the mix is innocent until you have also ruled out the slab below it.

Isolation tree

Step 1: rule out subgrade

Sound the panel with a hammer near the crack and at mid panel. A hollow drum tone means a void; a dull thud means full contact. Run the level across the crack: any step over roughly 1/8 in is differential settlement. Check the crack path: settlement cracks tend to run diagonally toward a low corner or follow a trench line (backfilled utility). If you find a void, a step, or a crack tracking a trench, subgrade is your cause. Stop here for those panels. Per ACI 360R, uniform support is the design assumption; lose it and the slab cracks regardless of a perfect mix.

If the panel sounds solid, sits flush, and the crack does not track a trench or low corner, eliminate subgrade and continue.

Step 2: rule out mix

Now look at the concrete itself. Pull the batch ticket. Water cement ratio above about 0.50 for exterior flatwork raises shrinkage and lowers strength; ACI 302.1R targets in the 0.45 to 0.50 range for durable flatwork. Field-added water (tempering) that pushed slump past the design point is a red flag, since each gallon added per cubic yard raises shrinkage. Crack signature for a high shrinkage mix: multiple straight cracks between joints, hairline to moderate, flush, fairly evenly distributed. Wide joint spacing magnifies it. If the ticket shows excess water or high slump and the crack pattern is distributed shrinkage with no subgrade signature, mix is your cause. If the ticket is clean and slump was in spec, eliminate mix and continue.

Step 3: rule out cure (and timing)

What remains points to the cure and the early thermal/moisture history. Plastic shrinkage cracks (short, random, often shallow, forming within hours of placement in wind, sun, or low humidity) signal evaporation outran bleed before any cure was applied; check the day's evaporation rate against the ACI 305R 0.2 lb/sq ft/hr threshold. Lack of curing for the first 7 days raises drying shrinkage and surface cracking. Late sawcutting (joints cut after the slab already cracked) shows as random cracks that ignore the joints. If subgrade and mix are clean and the timeline shows no cure, hot/windy placement, or late joints, cure and timing is your cause.

Confirming diagnosis

• Subgrade: confirm with a deeper probe or core showing the void or soft layer; lift test (loaded panel deflects).
• Mix: confirm with the batch ticket water cement ratio and, if a cylinder was cast, the 28 day break against design strength.
• Cure: confirm against the placement log: weather, evaporation rate, time to cure application, time to sawcut.

Remediation

• Subgrade: stabilize the support (pressure grout/mud jack the void, or remove and recompact) before any crack repair, or the crack returns.
• Mix: structural cracks get rout and seal or epoxy injection if dormant; the lesson is tighter slump and water control next pour.
• Cure: surface and shrinkage cracks get cosmetic fill once dormant; fix the process (wet cure or cure compound within evaporation limits, cut joints early).

Two causes can stack, and the sequence handles that cleanly. A void under the slab (subgrade) combined with a wet, high shrinkage mix produces a crack that is both wider and more displaced than either cause alone; if you stop at the first positive finding you will under repair. Run all three steps even after the first hit when the crack is severe, and weight the remediation toward the highest cost cause found. The lasting payoff of this sequence is process feedback: a subgrade hit tells the crew to fix compaction and lift placement; a mix hit tells them to stop tempering water into the truck; a cure hit tells them to get curing on within the evaporation window and cut joints on time. Each diagnosis closes a loop on the next pour, which is where the real cost savings live.

References

• ACI 224.1R, Causes, Evaluation, and Repair of Cracks in Concrete Structures.
• ACI 302.1R, Guide to Concrete Floor and Slab Construction (w/c, slump, jointing).
• ACI 305R, Guide to Hot Weather Concreting (evaporation rate threshold).
• ACI 360R, Guide to Design of Slabs-on-Ground (subgrade support).