Type N - One part portland cement, one part hydrated lime or lime putty, and six parts sand; or, one part type II masonry cement and three parts sand. Type N mortar is suitable for general use in above-grade exposed masonry where high compressive or lateral strength is not required.

Type O - One part portland cement, two parts hydrated lime or lime putty, and nine parts sand; or, one part type I or type II masonry cement and three parts sand. Type O mortar is recommended for load-bearing, solid-unit walls when the compressive stresses do not exceed 100 pounds per square inch (psi) and the masonry is not subject to freezing and thawing in the presence of a lot of moisture.

MIXING MORTAR

The manner in which mortar is mixed has a lot to do with the quality of the final product. In addition to machine and hand mixing, you need to know the requirements for introducing various additives, including water, to the mix in order to achieve optimum results.

Machine Mixing

Machine mixing refers to mixing large quantities of mortar in a drum-type mixer. Place all dry ingredients in the mixer first and mix them for 1 minute before adding the water. When adding water, you should always add it slowly. Minimum mixing time is 3 minutes. The mortar should be mixed until a completely uniform mixture is obtained.

Hand Mixing

Hand mixing involves mixing small amounts of mortar by hand in a mortar box or wheelbarrow. Take care to mix all ingredients thoroughly to obtain a uniform mixture. As in machine mixing, mix all dry materials together first before adding water. Keep a steel drum of water close at hand to use as the water supply. You should also keep all your masonry tools free of hardened mortar mix and dirt by immersing them in water when not in use.

Requirements

You occasionally need to mix lime putty with mortar. When machine mixing, use a pail to measure the lime putty. Place the putty on top of the sand. When hand mixing, add the sand to the lime putty. Wet pails before filling them with mortar and clean them immediately after emptying.

Mixing water for mortar must meet the same quality requirements as mixing water for concrete. Do not use water containing large amounts of dissolved salts. Salts weaken the mortars.

You can restore the workability of any mortar that stiffens on the mortar board due to evaporation by remixing it thoroughly. Add water as necessary, but discard any mortar stiffened by initial setting. Because it is difficult to determine the cause of stiffening, a practical guide is to use mortar within 2 1/2 hours after the original mixing. Discard any mortar you do not use within this time.

Do not use an antifreeze admixture to lower the freezing pint of mortars during winter construction. The quantity necessary to lower the freezing point to any appreciable degree is so large it will seriously impair the strength and other desirable properties of the mortar.

Do not add more than 2-percent calcium chloride (an accelerator) by weight of cement to mortar to accelerate its hardening rate and increase its early strength. Do not add more than 1-percent calcium chloride to masonry cements. Make a trial mix to find the percentage of calcium chloride that gives the desired hardening rate. Calcium chloride should not be used for steel-reinforced masonry. You can also obtain high early strength in mortars with high-early-strength portland cement.

MODULAR PLANNING

Concrete masonry walls should be laid out to make maximum use of full- and half-length units. This minimizes cutting and fitting of units on the job. Length and height of walls, width and height of openings, and wall areas between doors, windows, and corners should be planned to use full-size and half-size units, which are usually available (figure 8-6). This procedure assumes that window and door frames are of modular dimensions which fit modular full- and half-size units. Then, all horizontal dimensions should be in multiples of nominal full-length masonry units.

Both horizontal and vertical dimensions should be designed to be in multiples of 8 inches. Table 8-2 lists nominal length of concrete masonry walls by stretchers. Table 8-3 lists nominal height of concrete masonry walls by courses. When 8-by-4-by-16 units are used, the horizontal dimensions should be planned in multiples of 8 inches (half-length units) and the vertical dimensions in multiples of 4 inches. If the thickness of the wall is greater or less than the length of a half unit, a special-length unit is required at each