2024-25_NTCA_Reference_Manual_dig_dwnld

Loss of Bond (Dry-set mortar adheres to tile but not to substrate)

Problem

Cause

Cure

Dry-set mortar separates from the sub strate

Oil or wax based sweeping compounds used

Mechanically remove to clean concrete

Dry-set mortar releases (clean shear) from the surface of the waterproofing.

Waterproof materials not compatible with the dry-set mortar.

Dry-set mortar application over water proofing requires strict adherence to manufacturer's instructions. Have the manufacturer confirm the compatibility of the materials. In hot weather conditions, mix dry-set mortar with cool water. Dampen sub strate prior to application of dry-set mor tar. Apply dry-set mortar with flat side of trowel and notch just before tile is ap plied. Take special precautions to retain the moisture in setting materials applied over substrates heated by the sun, radi ant heat, etc. Use liquid applied water proof sealer. Burnished concrete must be scarified to open pores of the concrete to achieve sufficient bond of the dry-set mortar. Dry-set mortars that are not designed as large and heavy mortars are limited to 1/4” in. (6.4 mm) thickness between the back of tile and the substrate. Maintain substrate and ambient air temperature above freezing per the manufacturer recommendations. Fast setting latex dry-set mortars may offer superior performance. Consult mortar manufacturer. Use mixing drill at or below 300 RPM. Use mixing wand designed for dry-set mortars. Follow manufacturer’s direc tions for mixing water or additive and mixing times

Flash set or rapid evaporation of water causes dry-set to be weak at substrate interface, resulting in release from sub strate.

Substrate too hot, causing flash set of mortar.

Dry-set mortar separates from the sub strate with very little residue of dry-set mortar remaining on the concrete.

Concrete substrate burnished during the finishing process providing little porosity at the surface for a mechanical bond. Dry-set mortar applied in excess of thickness limitations inducing exces sive shrinkage during curing.

Dry-set mortar separates from the sub strate; can exhibit cracking within the dry-set. May leave dry-set residue on substrate. Bond failure may occur at any interface and within matrix of dry-set mortar. Mortar is weak and crumbles.

Dry-set mortar freezes before curing.

Bond can fail at either interface or with in the matrix of the dry-set. Dry-set is weak and has signs of air entrainment.

Mixing with high speed drill, over mix ing or using improper mixing wand will entrain air and substantially weaken the dry-set mortar.

NTCA Reference Manual | 2024 / 2025

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Chapter 8 | Thin-Bed Method Installations