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Masonry Systems Data Mortar And Grout Mixes And Specifications
1) Mortars
2) Grout
3) Guide Specifications: Section 04100 Mortars And Grout
1. Mortars
Centuries ago, combinations of sand and lime were used as mortar. These combinations took months and even years to harden as the lime slowly combined with carbon dioxide from the air to form calcium carbonate. Because it took so long for these mortars to harden and gain strength, it was necessary to use very thin joints. In many instances the joints were so thin that adjacent masonry units would bear on each other in direct contact. This type of construction required an excessive amount of labor to carefully fit and place each masonry unit. However, sand-lime mortars were adequate for the then massive construction and slow-paced construction procedures.
The development of mortars that hardened and gained strength rapidly made it possible to place masonry units rapidly. Also, thicker joints provided cushions for dimensional variations in the masonry units. The stronger mortars were first obtained by "sweetening" the lime with a small amount of portland cement. Later the amount of portland cement in the ratio was increased until the process involved 'sweetening" the cement with a small amount of lime.
Mortar for masonry is designed not only to join masonry units into an integral structure with predictable performance properties, but also to: (1) effect tight seals between units against the entry of air and moisture; (2) bond with steel joint reinforcement, metal ties, and anchor bolts so that they perform integrally with the masonry; (3) provide an architectural quality to exposed masonry structures through color contrasts or shadow lines from various joint-tooling procedures; and (4) compensate for size variations in the units by providing a bed to accommodate tolerances of units.
Masonry mortar is composed of one or more cementitious materials, clean, well-graded masonry sand, and sufficient water to produce a plastic, workable mixture. Modern specifications call for proportions by volume ranging from one part of cementitious material to 2-1/4 to 3-1/2 parts of damp, loose mortar sand. The choice of cementitious material -- masonry cement, a portland cement combination -- is largely a matter of economics and convenience. Any of these combinations will produce mortar with acceptable properties as long as the specifications are met.
1.1. Desirable Properties. Good mortar is necessary for good workmanship and proper structural performance of masonry construction. Since mortar must bond masonry units into strong, durable, weathertight walls, it must have the properties described below.
1.1.1. Workability. Probably the most important quality of plastic masonry mortar is workability, because of its influence on other important mortar properties in both the plastic and hardened states. Workability is difficult to define because it is a combination of a number of interrelated properties. The properties considered as having the greatest influence on workability are: consistency (flowability), water retentivity, setting time, weight, adhesion, and penetrability.
An experienced mason judges the workability of mortar by the way it adheres to or slides from his trowel. Mortar of good workability should spread easily on the masonry unit, cling to vertical surfaces, extrude readily from joints without dropping or smearing, and permit easy positioning of the unit without subsequent shifting due to its weight or the weight of successive courses. Mortar consistency should change with weather to help in laying the units. A good workable mix should be softer in summer than in winter to compensate for water loss.
1.1.2. Water Retentivity. Mortar having this property resists rapid loss of mixing water (prevents loss of plasticity) to the air on a dry day or to an absorptive masonry unit. Rapid loss of water causes the mortar to stiffen quickly, making it practically impossible to obtain good bond and weather-tight joints.
Water retentivity is an important property and is related to workability. A mortar that has good water retentivity remains soft and plastic long enough for the masonry units to be carefully aligned, leveled, plumbed, and adjusted to proper line without danger of breaking the intimate contact or bond between the mortar and the units. When low-absorption units such as split block are in contact with a mortar having too much water retentivity, they may float. Consequently, the water retentivity of a mortar should be within tolerable limits.
Entrained air, extremely fine aggregate or cementitious materials, or water add workability or plasticity to the mortar and increase its water retentivity.
Mortar of proper workability is soft, but with good body; it spreads readily and extrudes without smearing or dropping away.
1.1.3. Consistent Rate of Hardening. The rate of hardening of mortar due to hydration (chemical reaction) is the speed at which it develops resistance to an applied load. Too rapid hardening may interfere with the use of the mortar by the mason. Very slow hardening may impede the progress of the work because the mortar will flow from the completed masonry. During winter construction, slow hardening may also subject mortar to early damage from frost action. A well-defined, consistent rate of hardening assists the mason in laying the masonry units and in tooling the joints to the same degree of hardness. Uniform color of masonry joints reflects proper hardening and consistent tooling times.
Hardening is sometimes confused with a stiffening caused by rapid loss of water, as when low-water-retention mortar is used with highly absorptive units. Also, during very hot, dry weather mortar may tend to stiffen more rapidly than usual. In this case, the mason may find it advisable to lay shorter mortar beds and fewer units in advance of tooling.
Tensile bond strength is perhaps the most important property of hardened mortar. Mortar must develop sufficient bond to withstand the tensile forces brought about by structural, earth, and wind loads, expansion of clay brick, shrinkage of concrete masonry units or mortar, and temperature changes.
Many variables effect bond, including: (1) mortar ingredients, such as type and amount of cementitious materials, water retained, and air content; (2) characteristics of the masonry units, such as surface texture, suction, and moisture content; (3) workmanship, such as pressure applied to the mortar bed during placing; and (4) curing conditions, such as temperature, relative humidity, and wind. The effects of these variables on bond will be briefly discussed.
All other factors being equal, mortar bond strength is related to mortar composition, especially the cement content. The bond strength of the mortar increases as the content of cement increases.
Bond is low on smooth, molded surfaces, such as glass or die skin surfaces of clay brick or tile. On the other hand, good bond is achieved on concrete block or on wire-cut or textured surfaces of clay brick. The suction rates of concrete masonry units are low enough that they never require wetting prior to laying of mortar. Some clay brick units have such high suction rates that poor bond will result unless the brick are wetted; surfaces of the wetted brick should be dry.
The NW Masonry Guide Table of Contents
Masonry Institute of Washington
Washington State Conference of Mason Contractors
