Welded wire mesh controls slab cracking, but a misplaced or low-buried mat opens surface cracks polishing will not hide. Polishing does not repair a moving crack; it densifies concrete, so hairlines and hollows show more clearly under the sealed sheen. This guide covers defect location, crack-width judgment, polishability and acceptance clauses for the PM and owner representative. Figures are working limits for interior commercial slabs; stricter criteria from drawings and mix design go into the specification.

How Mesh Placement Drives the Cracks Polishing Reveals

Mesh works only in the upper third of the slab, restraining top-surface shrinkage. Cover runs 20 to 40 mm; a mat on the subgrade lets the top concrete open random map cracks in the first weeks. Mesh under 10 to 15 mm cover causes spalling, rust bleed and a raised wire pattern once laitance is ground off. Short laps, untied intersections and bent mats leave bands that crack along mesh lines.

Pre-Grind Survey: Map Mesh, Cover Depth and Crack Pattern

Before grinding, survey the slab on a floor plan. Mark every crack with length, width and direction; use a 0.1 mm feeler gauge at the widest point, noting dormant (clean, dry) or active (chalky edges, dust, opening under load). Record slab age, in-situ RH, and hollow-sounding zones by chain sounding. Random hairlines point to shrinkage, diagonal corner cracks to settlement, mesh-line cracks to misplaced reinforcement.

Crack Width Classification and What Polishing Can and Cannot Fix

Polishing can densify a stable hairline so it reads as a faint line; it cannot bridge an active crack, stop movement, or hide a structural fracture once the sealed surface reflects light. The table sets working width classes and the decision for the polish contractor. Width is measured at the widest point after loose material is brushed out.

When a Cracked Slab Should Not Be Polished

Some conditions sit outside the polish scope and must be flagged before quoting. Active cracks over 1.0 mm, diagonal settlement cracks, cracks that widen under load, and through-thickness fractures indicate movement no sealer can restrain. Hollow delaminated zones must be cut out and rebuilt; polishing over a hollow patch amplifies its reflection. Other hold conditions are mesh corrosion, age under 28 days, RH above 75 to 80 percent, or temperature outside 10 to 30 degrees Celsius.

Repair Methods by Width Class, Before the Finish Pass

Repairs are scheduled ahead of the resin finish so each patch hardens fully first. For hairlines up to 0.2 mm, brush out dust, apply densifier at 1:64 or about 15 ml per liter, and mark for a six-month recheck. For 0.2 to 0.5 mm cracks, route a 3 to 5 mm groove, fill flush with epoxy or cementitious compound, grind level once cured. For 0.5 to 1.0 mm cracks, chase a 6 to 10 mm groove, inject epoxy or install a bonded strip; track movement 7 to 14 days. Epoxy cures 24 hours before grinding, 48 hours before traffic; cementitious fills cure 24 to 72 hours.

Polishing Over Repaired Zones and Blend Criteria

Once patches cure, grind the repair flush rather than leaving a ridge. A standard run opens with metal-bond diamonds at 16, 20, 30 and 40 grit to level joints and edges, then resin pads at 100, 200, 400 and 800 grit toward the chosen sheen; skip a grit only when scratches are gone. Check the blend with a 60 degree gloss meter at 2 to 3 m spacing; adjacent areas may differ by up to 5 gloss units, and a difference above 15 units means rework before sign-off. Inspect under 300 to 500 lux oblique light for ridges and swirl marks.

Responsibility Allocation: Slab, Mesh Placement, Finish and Polish Scope

Crack disputes turn on who controlled which stage. Slab design and mesh specification, cover, spacing and lap length, sit with the structural designer. Mesh placement, chairs, tying and laps are the placing contractor's responsibility; curing and finishing belong to the placing crew. The polish contractor surveys the slab, documents pre-existing defects, repairs within agreed width classes, and delivers stated gloss and flatness. Repair of cracks over 1.0 mm or settlement movement needs a separate structural scope and price.

Owner Communication, Acceptance Clauses and Handover Documentation

Most disputes come from unstated expectations: the owner assumes polishing seals every line, while the crew assumes existing cracks are the concrete contractor's problem. Close this gap with a pre-start walkthrough and a written crack log: width, length, location, condition, repair method and accepted status per defect. Agree which dormant widths are accepted as delivered, which cracks must be filled flush, and which bays are excluded pending repair.

1. Survey slab age, RH, soundness and mesh cover before grinding.
2. Measure each crack with a 0.1 mm feeler gauge and classify it into the width table.
3. Hold polishing on active cracks over 1.0 mm, settlement traces and hollow zones.
4. Let epoxy and cementitious repairs cure 24 to 48 hours before the resin finish pass.
5. Verify blend gloss within 5 units; rework any difference over 15 units.
6. Hand over the signed crack log, gloss readings, photos and the annual crack-fill note.

When base conditions, crack classes and repair responsibilities are agreed in writing before the grind, the finish behaves predictably under service traffic and disputes stay limited to touch-ups. The contractor's role is to judge what the slab can carry, lock accepted crack limits into the specification, and deliver measurements the owner representative signs without qualification.