2024-25_NTCA_Reference_Manual_dig_dwnld

Dry-set mortar fractures within bonding material – mortar on tile and substrate Loss of Bond

Problem

Cause

Cure

Dry-set mortar is present on both tile and substrate. Bond failure occurs near the mid-point of dry-set mortar. Normally, there is no evidence of a powdery cement where the bond is broken.

Vibration or shock before dry-set mortar was cured. Traffic allowed on floor prior to curing of dry-set mortar.

Prevent all trades from disturbing the installation during the curing cycle. Under normal conditions, light foot traffic can be allowed after 48 hours. Heavy traffic must be kept off floor at least 72 hours unless special, fast-setting dry-set mortars have been used. Cold temperatures, 50° F (10° C) or below, will slow the curing process. Never add water to dry-set mortar if it has stiffened beyond the point of usage. Discard old material and mix fresh dry set mortar. Protect from freezing. Maintain substrate and ambient air temperature above freezing. Rapid-setting latex dry-set mortars may offer superior performance. Consult mortar manufacturer. Soak tile. Dampen substrate. Control loss of moisture in high temperature and/or windy conditions. Consider setting method F111 of TCNA Handbook . Adhere strictly to TCNA Handbook method EJ171, and to ANSI Specifications A108.01-2.1.5; Movement Joints. If details on plans or specifications do not call for expansion joints or control joints, the architect should be notified of the ANSI Expansion Joint Requirements. If the architect still does not specify expansion joints, the tile contractor should initiate a letter of release of liability. The tile contractor should not assume responsibility for the design and location of expansion joints. Use mixing drill at or below 300 RPM. Use mixing wand designed for dry set mortars. Follow manufacturer’s directions for mixing times.

Dry-set mortar is weak and powders easily; and may show evidence of shrinkage cracks.

Dry-set mortar retempered with water.

Mortar is crumbly. Failure can occur at any point.

Freeze/thaw.

Bond to substrate and to tile. Center of mortar is weak and powdery.

Rapid water loss to substrate and tile.

Failure will occur at the weakest point. Sometimes entire sections of the tile will crack and/or arch away from the substrate.

Thermal expansion and contraction.

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

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

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NTCA Reference Manual | 2024 / 2025

Chapter 8 | Thin-Bed Method Installations