2024-25_NTCA_Reference_Manual_dig_dwnld

Concrete Substrate Issues

Problem

Cause

Cure

Concrete slab-on grade curling

Curling of concrete slab occurs from top to bottom as a result of differential shrinkage (shrinkage of slab where ex posed to air; slow or delayed shrinkage at bottom where wet)

Differential curling can be minimized by: • Location of vapor retarder directly beneath slab • Damp curing of slab* • Proper drainage beneath slab • Increased steel reinforcement • Avoid exposure to air movement and protect from hot, dry conditions Shrinkage can be minimized by: • Proper placement/design of control joints • Damp curing of slab* • Increased steel reinforcement • Cool temperatures, high humidity and protect from wind Concrete slab flatness improved by: • Quality control testing of slab per ASTM E1155 after placement** • Check for changes prior to tile installation • Damp curing of slab • Increase steel reinforcement Concrete slab flatness corrected by: • Grinding/scarification • Mortar bed, patching or flowable/self leveling underlayment • For post-tension slabs, check with contractor before grinding.

Random shrinkage cracking (less than1/16 in. [1.6 mm] width)

• Concrete mix design (water, cement) • Aggregate size, grading, type • Admixtures • Restraint of slab (subgrade drag) • Omission, improper placement or in sufficient steel reinforcement • Hot, dry, air movement • Curling of slab on-grade (especially at control joints) • Dead load (weight) deflection of ele vated concrete slab • Improper finishing of concrete surface • Segregation of aggregates

Excessive flatness deviations

* Covering of concrete is the recommended method of curing in preparation of tile installation; curing compounds may be used in lieu of covering, but must be removed by scarification or brushing/washing of concrete as recommended by curing product manufacturer. Water misting or ponding not recommended due to water infiltration through joints and increased potential for slab curling or increase in slab internal moisture that could affect the adhesion of underlayments, primers, or membranes. ** ASTM E1155 allows/does not address excessive concrete flatness deviations that could occur greater than 72 hours after concrete placement, at perimeters, across expansion, control or cold joints or after form removal of elevated concrete.

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NTCA Reference Manual | 2024 / 2025

Chapter 2 | Substrates