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Concrete Substrate Issues

Problem

Cause

Cure

Standard flatness deviations

ANSI A108.01 flatness standards: • Large format tile (any one side greater than 23 in. [0.58 m]) require no greater than 1/8 in. in 10 ft. (3 mm in 3 m) and 1/16 in. in 2 ft. (2 mm in 610 mm) • Less than 15 in. (0.38 m) no >1/4 in. in 10 ft. (6 mm in 3 m) and 1/16 in. in 1 ft. (2 mm in 305 mm) ACI 117concrete standards: • Allow 1/4 in. in 10 ft. (6 mm in 3 m); local deviation of 3/8 in. (10 mm) allowed; rough equivalent of overall FF equal to 25-35 Disparity between tile and concrete industries’ maximum allowable flatness tolerances • Laitance/Scaling/Dusting – weak layer of cement 1/16 in.-3/16 in. (1.6 4.8 mm) depth on concrete surface • Crazing – surface hairline cracking due to rapid evaporation of moisture at surface with excess fine aggregate • Improper hard steel trowel surface inhibits adhesion • Segregation of concrete aggregates • Contamination • Concrete admixtures can cause excessive shrinkage cracking or increase density and inhibit adhesion

• Recommend specification/installation of flowable/self-leveling underlayment or bonded/unbonded mortar bed to achieve required flatness for tile • Increase steel reinforcement to cross-sectional area greater than or equal to 1% Concrete slab flatness corrected by: • Grinding/scarification • Mortar bed, patching or self-leveling underlayment

Surface imperfections

• Laitance – adjust mix design; avoid overworking concrete finish and/or freezing of fresh concrete • Crazing – mix design with well graded aggregates; no excess fines; avoid over-finishing; avoid dry shake cement mixtures • Broom trowel finish required for good tile adhesion • Avoid excess water/high slump concrete; avoid finishing if water on surface • Remove dirt residue and clean slab with damp sponge • Test admixtures for compatibility • Crack injection with low viscosity structural epoxy or polyurethane • Remove/replace sections of base and concrete per engineer recommendations Concrete slab must be suitably dry and tested for internal moisture per ASTM F2170 if any moisture sensitive stone tile, underlayments, primers or membranes are part of the tile assembly.

Structural cracking (greater than 1/16 in. (1.6 mm) with vertical edge displacement)

• Poor base preparation/compaction • Excessive loading/deflection • Soil settlement/high ground water • Discontinuity of steel reinforcement

Excessive internal moisture

• Lack of vapor retarder beneath concrete • High (seasonal subgrade water levels) • Inadequate cure/drying time • Excessive water/cement ratio • Water leaks, exposure to rain • Lightweight concrete (aggregate retains moisture)

Chapter 2 | Substrates

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NTCA Reference Manual | 2024 / 2025