2024-25_NTCA_Reference_Manual_dig_dwnld

Mortar beds

DAMP CURING Normally latex-modified mortar does not require covering to instill damp curing. The surface latex cures quickly, creating a thin film that helps retain moisture in the mortar that aids in damp curing. However, under warm, breezy conditions, sprinkling the mortar with water after it is installed and covering with plastic is rec ommended. When damp sand is substituted for dry sand, damp curing is mandatory for latex-modified mortar beds. Damp curing is also important for standard mortar beds. Damp curing increases compressive strength and produces a denser mortar. Mortar beds as thin as 3/4 in. (19 mm) will cure harder when damp cured. Without damp curing, standard mortar beds, and even latex-modified mortar beds, may not cure properly and become soft and weak under adverse curing conditions. Curing conditions affect the compressive strength of cured mortar. Damp curing produces harder mortar with higher compressive strengths, whether they are standard mixes, or latex-modified.

This decrease in the amount of latex with a rich three to one mix ratio will cause excessive shrinkage in the mortar after it has been installed and screeded. Depending on temperature, humidity, wind/breeze and sunshine, (if the installation is outside or under skylights), the mortar may shrink enough to cause severe cracks around each separate screed area, and curling up to 1/4 in. (6 mm) across an 8 to 10 ft. (2.4 to 3.0 m) wide screed. As discussed previously, repeated freeze/thaw effects can be very damaging to mortar beds as trapped water in these assemblies leads to expansion and stress. Incorporating an ANSI A118.10 waterproofing membrane that also has A118.12 crack isolation properties above the mortar bed will help prevent water from entering and help relieve stress to the adhesive mortar bond interface thus extending the life of the installation. The mortar bed must be properly sloped with strict attention to maintaining flatness to avoid birdbaths where water will rest under tiles. Movement joints must also be bridged with waterproofing per manufacturer’s instructions.

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NTCA Reference Manual | 2024 / 2025

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Chapter 7 | Thick-Bed Method Installations