2024-25_NTCA_Reference_Manual_dig_dwnld

Mortar beds

INTERIOR FLOORS There are ongoing problems with mortar beds, primarily on floors. Problems relate to workmanship, quality control, and a general lack of understanding on requirements for installing successful mortar beds. Areas of concern when installing mortar mixes are: consistent mixture of sand, cement, and water, correct sand to cement ra tio, compaction of the mortar bed, and correct wire placement when required. Two methods of mixing are widely used in the industry. The first, and generally preferred, is use of a powered mortar mixer capable of mixing one bag of cement and various amounts of sand. Pow ered mortar mixers have been used on jobs of 200,000 square feet without any delays to those who were screeding the mortar beds. Rototillers have been used in many parts of the country to mix sand and cement placed on concrete floors near areas where mortar beds are being installed. This method can produce good quality mortar beds, but sand and cement must also be thoroughly turned over with shovels so that materials on the bottom of the pile are properly mixed. With good quality control, mortar beds end up uniformly mixed with a minimum of pure sand spots, which will weaken mortar. Pre-mixed mortars are also readily available and may ease concerns regarding consistent mortar ratios and inconsistent mixing of materials. Rich mixes, using one part Portland cement to three parts sand, typically create excessive shrinkage, which results in uneven shrinkage and curling or warping of the mortar. When bonded to a concrete slab, such rich mixes have a tendency to shrink more on the top side where moisture escapes faster, resulting in curled mortar that delaminates from concrete subfloors. Similar conditions occur when the mortar bed is placed over a cleavage membrane. Damp curing will help, but usually does not fully correct the curling problem. In addition, excessive shrinkage will cause cracks in the mortar. There is misinformation and a lack of understanding on com pressive strengths of mortar mixes and mortar beds. Typically, mortar mixes consisting of one part cement and five parts sand produce compressive strengths of 400 to 600 psi (2,7 to 4,1 MPa). Compressive strength can be increased with additional water, but wetter mortar is not screeded. When compacted in an ASTM cube, compressive strengths can be increased to 1,500 to 1,600 psi (10 to 11 MPa) by more forceful compaction.

Compressive strengths of mortar beds can be increased by com pacting mortar as it is placed but excessive compaction makes mortar difficult to screed. In addition, compaction between layers can cause delamination between the layers. There have been several instances where attempts have been made to reject standard mortar beds because compressive strengths ranged between 450 psi and 700 psi (3.1 and 4.8 MPa) yet many have been successful. With proper compaction in lab oratory testing, various mixtures can range from 1500 – 5000 psi (10 – 34 MPa). Mixtures should be chosen for a project based on the intended service use. VERTICAL MORTAR BEDS Generally, there are fewer problems with vertical mortar mixes and mortar beds. However, the following are avoidable problems. Using fire clay instead of lime can cause delayed shrinkage that may cause mortar beds to delaminate from masonry backing walls, or tiles to delaminate from mortar beds. Rich mortar mixes, such as one part cement to three or four parts sand, frequently cause excessive shrinkage in mortar. When shrinkage is excessive, mortar beds and tiles will crack along joints in metal lath. One part cement to four parts sand mortar beds can crack at horizontal metal lath joints when lath is installed over studs. Under adverse curing conditions, mortar beds may even develop random, spider web cracks. Curing conditions also affect shrinkage of vertical mortar beds consisting of up to 5 or 6 parts sand. Dry, breezy conditions cause excessive shrinkage that can create cracks on horizontal joints between sheets of metal lath. On furring channels and metal studs walls, with mortar applied to only one side, excessive shrinkage will also cause the walls to bow. Mixing mortar too rich can cause curling and shrinkage. Shrinkage results in shrinkage cracks that will crack or delaminate tiles.

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NTCA Reference Manual | 2024 / 2025

Chapter 7 | Thick-Bed Method Installations