Concrete cracks — it's not a question of if, but when and how. The crack repair strategy must match the crack type: structural vs. non-structural, active vs. dormant, wide vs. hairline. Using the wrong repair method can create a floor defect worse than the original crack.
Structural vs. Non-Structural Cracks
Structural cracks extend through the full depth of the slab and may be accompanied by vertical displacement (one side higher than the other) or horizontal movement. These cracks may indicate foundation settlement, expansive clay soil movement, or inadequate slab design. Structural cracks require geotechnical evaluation before coating — no amount of surface crack repair makes a structurally moving slab appropriate for coating. Non-structural cracks (shrinkage cracks, control joint failures, surface cracks) are cosmetic and do not affect the slab's structural integrity. These are appropriate for repair before coating.
Active vs. Dormant Cracks
An active crack continues to move — opening and closing with thermal cycling or ongoing settlement. A dormant crack has reached equilibrium and shows no further movement. The repair material must match the crack status. Rigid repair materials (epoxy injection) work well in dormant cracks — they restore structural continuity across the crack. In active cracks, rigid repairs simply crack again along the repair boundary. Active cracks require flexible repair materials — semi-rigid polyurea or polyurethane sealants that can accommodate movement without failing. The field test is simple: mark the crack ends with pencil marks and check after a week in changing temperatures; movement indicates an active crack.
Structural epoxy injection uses low-pressure (5–20 psi) ports bonded to the slab surface over the crack at regular intervals. Liquid, low-viscosity epoxy (unfilled, 200–500 cP) is introduced at the lowest port and allowed to travel by capillary action through the crack until it appears at the next higher port — indicating the crack volume between those ports is filled. The process continues port by port until the crack is filled and sealed. The cured epoxy restores the concrete to near its original tensile strength across the crack plane — in some cases stronger than the original concrete.
Polyurea Filler for Surface Preparation
For coating applications, cracks wider than hairline are typically routed (ground wider with a diamond router bit to create a uniform channel) and filled with a rapid-set, semi-rigid polyurea filler. Polyurea fillers cure in minutes, can be ground flush with the slab surface within 15–30 minutes of application, and provide a slightly flexible repair that resists crack re-reflection better than rigid epoxy mortars. After fill and grind, the repair is essentially invisible in the finished coated floor. The decision between polyurea surface fill and epoxy injection depends on crack width, depth, and whether structural consolidation or surface sealing is the goal.
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