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Mortar Beds

EXTERIOR PAVING Mortar Bed Mixtures Gauged with Water

ommended to contact the installation material manufacturer for recommendations to achieve the best assembly and survive these conditions. LATEX-MODIFIED MORTAR Addition of latex to mortar mixes raises compressive and flex ural strength thus increases resistance to freezing and thawing damage. Latex, in varying strengths or concentrations, has been added to different mortar mixes for many years to produce stronger mortar. Again, there is misinformation concerning use of latex in mortar mixes, which states that the addition of latex to mortar mixes decreases the compressive strength. This is true for latex-modified thin-set mortars, which may use greater amounts of latex to make them “flexible,” and for some concrete mixes. However, for mortar beds used with tile work, addition of latex typically increases both compressive and tensile strength. Usually, mortar mixes consisting of one part Portland cement, three parts dry sand and roughly five gallons of 18 to 19 percent solids latex per 90 pound (41 kg) bag of cement, produce mortar with compressive strengths ranging between 2,500 and 5,000 psi (17 and 34 MPa). (Level of compressive strength depends on compaction and curing conditions.) In addition to higher com pressive strength, these mortar beds resist uneven shrinkage, curling and have excellent freeze-thaw resistance. They are typically used on exteriors in freezing climates and over floors requiring extra strength. In addition to mortar mixes, latex is used in concrete road con struction. There is a common phenomenon with this use. With the fresh concrete exposed to sun and a breeze, the surface dries quicker than the interior of the concrete. Under certain condi tions, the surface will crack and curl like drying mud on a lake bed. This condition has been misinterpreted to relate to curling of latex-modified mortar beds. Under certain conditions, latex-modified mortar beds will curl. When latex-mortar beds are installed in windy conditions or air-conditioned areas, they may experience curling at the edges because the air-conditioning removes moisture too rapidly from the surface. Covering with plastic usually eliminates curling under these curing conditions. When damp sand is substituted for dry sand, water in the sand will replace part or most of the latex. Depending on how damp, or wet, the sand is, as much as 2-1/2 to 4-1/2 gallons (9.5 to 17 L) of water can be contained in three hundred pounds of sand. With a normal three to one mix, three hundred pounds of sand is used with each 100 pound (45 kg) bag of cement, along with five gallons of liquid to make a workable mix. With wet sand described above, only 1/2 to 2-1/2 gallons (1.9 to 9.5 L) of latex can be used, instead of the required five gallons.

Mortar beds mixtures gauged with water as defined in ANSI A108.1 have at times experienced failures when used on exterior hori zontal surfaces subject to freezing temperatures. In the 1960s and ’70s, TCNA conducted roof deck experiments with quarry tiles installed over sand and Portland cement mortar beds. In most instances, the mortar beds failed after exposure to freezing winters. Thus, the Limitation in Handbook Method F103 for Roof Deck; “although this is the best known method of installation for a ceramic tile roof deck, it is not reliable in areas where the mortar bed will be subject to freeze-thaw cycles.” Whether used on roof decks, or slab-on-grade construction, when exposed to freezing conditions, standard sand and Portland ce ment mortar beds deteriorate to the point where the mortar beds are only sand. Depending on climate, the time for deterioration ranges from one to several years. After only one freezing winter season, exterior tile installations begin to fail, with a few tiles becoming unbonded. Within two years most tiles may become unbonded. Under extreme con ditions, such as found in the “freeze-thaw belt,” starting near Washington, D.C. and continuing north through New Jersey and New York, and running west to the Rocky Mountains, standard mortar beds turn to sand and tiles become unbonded. These conditions are typically found on exterior mortar beds installed according to TCNA Handbook methods F101 and F103. When the drainage layer is omitted from roof deck installations, total failure frequently occurs during the first winter. Further south, to Atlanta, where the number of freeze-thaw cycles is minimal, failure may consist of delaminated tiles, and early de terioration of the upper and lower portions of the mortar beds, after one to two years. Mortar Bed Mixtures Gauged with Latex Where mortar mixes consisting of one part Portland cement to only three parts sand and liquid latex are used for mixing, instead of water, mortar beds have fared much better under extreme freeze-thaw conditions. On one central New Jersey roof deck, tiles and latex-modified mortar beds were still intact after ten years, even though the drainage layer was omitted. The roof deck had excellent positive drainage to roof drains so water did not puddle on the surface or on the bottom of the mortar beds at the roof membrane. Roughly 1/8 in. (3 mm) of the lower portions of the mortar beds had deteriorated where water drained. The remainder of the mortar bed remained solid with no visible damage. The use of a drainage mat may assist in evacuating water faster to reduce damage to the mortar bed.

Although there are numerous cases of latex-modified mortar beds surviving harsh winter freeze-thaw conditions, it is rec

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NTCA Reference Manual | 2024 / 2025

Chapter 7 | Thick-Bed Method Installations