SECTION II. MORTAR
7-6. Good mortar is necessary for good workmanship and good masonry service because it must bond the masonry units into a strong well-knit structure.
7-7. The mortar that bonds concrete blocks,bricks, or clay tiles together will be the weakest part of the masonry unless you mix and apply it properly. When masonry leaks are encountered, they are usually through the mortar joints. The strength of masonry and its resistance to rain penetration depends largely on the strength of the bond between the masonry unit and the mortar. Various factors affect bond strength including the type and quantity of mortar, its workability or plasticity, its water retentivity, the surface texture of the mortar bed, and the quality of workmanship in laying the units. You can correct irregular brick dimensions and shape with a good mortar joint.
7-8. Mortar must be flexible enough to work with a trowel. You can obtain good plasticity or workability by:
Mortar properties depend largely upon the type of sand the mortar contains. Clean, sharp sand produces excellent mortar; too much sand causes mortar to segregate, drop off the trowel, and weather poorly.
7-9. Mortar property resists rapid water loss to highly absorbent masonry units. Mortar must have water to develop the bond. If it does not contain enough water, the mortar will have poor plasticity and workability and the bond will be weak and spotty. Sometimes you must wet the brick to control water absorption before applying mortar, but never wet concrete masonry units.
STRENGTH AND DURABILITY
7-10. The type of service that the masonry must give determines the mortar's strength and durability requirements. For example, walls subject to severe stresses or to severe weathering must be laid with more durable, stronger mortars than walls for ordinary service. Table 7-1 below gives mortar mix proportions that provide adequate strength and durability for the conditions listed. You can convert the unit volume proportions to weight proportions by multiplying the unit volumes given by the weight per cubic foot of the materials. Those specifications are:
Table 7-1. Recommended mortar mix proportions by unit volume
TYPES OF MORTAR
7-11. The following mortar types are proportioned on a volume basis:
Type M mortar consist of one part portland cement, 1/4 part hydrated lime or lime putty and, 3 parts sand or 1 part portland cement, 1 part type II masonry cement, and 6 parts sand. Type M mortar is suitable for general use but is recommended specifically for below-grade masonry that contacts earth, such as foundations, retaining walls, and walks.
Type S mortar consist of one part portland cement, 1/2 part hydrated lime or lime putty, and 4 1/2 parts sand, or 1/2 part portland cement, 1 part type II masonry cement, and 4 1/2 parts sand. Type S mortar is also suitable for general use, but is recommended where high resistance to lateral forces is required.
Type N mortar consist of one part portland cement, 1 part hydrated lime or lime putty, and 6 parts sand, or 1 part type II masonry cement and 3 parts sand. Type N mortar is suitable for general use in above- grade exposed masonry where high compressive and/or lateral strengths are not required.
Type O mortar consist of one part portland cement, 2 parts hydrated lime or lime putty, and 9 parts sand, or 1 part type I or type II masonry cement and 3 parts sand. Type O mortar is recommended for load-bearing, solid-unit walls when the compressive stresses do not exceed 100 psi, and the masonry is not subject to freezing and thawing in the presence of a lot of moisture.
STORING MORTAR MATERIALS
7-12. Store all mortar materials, except sand and slaked quicklime, in a dry place. Sand and lime should be covered to prevent excessive losses or gains of surface moisture.
7-13. When blending or mixing mortar, always use the best consistency for the job.
7-14. Mix large quantities of mortar in a drum-type mixer, like a concrete mixer. Mix a minimum of 3 minutes. Place all dry ingredients in the mixer first, mix them for 1 minute before adding the water.
7-15. Mix small amounts of mortar by hand in a mortar box (see Figure 7-3). Mix all ingredients thoroughly to obtain a uniform mixture. Mix all dry materials together first before adding water. Keep a steel drum of water close to the mortar box to use as the water supply. Use a second drum of water to store shovels and hoes when not in use.
MORTAR MIXING WITH LIME PUTTY
7-16. When machine mixing, measure the lime putty using a pail and place it into the skip on top of the sand. When hand mixing, add the sand to the lime putty. Wet pails before placing mortar in them and clean them immediately after emptying them.
7-17. Mixing water for mortar must meet the same requirements as mixing water for concrete. Do not use water containing large amounts of dissolved salts, because the salts will cause efflorescence and weaken the mortar.
7-18. The workability of any mortar that stiffens on the mortar board due to evaporation by remixing can be restored. Add water as necessary, but discard any mortar stiffened by initial setting. It is difficult to tell the cause of stiffening; a practical guide is to use mortar within 2 1/2 hours after the original mixing when the air temperature is 80°F or higher, and within 3 1/2 hours when the air temperature is below 80°F. Discard any mortar not used within these limits.
7-19. Do not use an admixture to lower the freezing point of mortar during winter construction. The quantity of antifreeze materials necessary to lower the freezing point of mortar to any appreciable degree is so large that it would seriously impair the mortar's strength and other desirable properties. Never use frozen mortar; freezing destroys its bonding ability.
7-20. Make a trial mix to find the percentage of calcium chloride that gives the desired hardening rate. Do not add more than 2 percent calcium chloride, by weight of cement to mortar, to accelerate its hardening rate and increase early strength. Do not add more than 1 percent calcium chloride to masonry cements. Calcium chloride should not be used for steel-reinforced masonry. You can also accelerate hardening rate in mortars with high-early-strength portland cement.
REPAIRING AND TUCK-POINTING
7-21. Use the mortar mixes given in Table 7-1 when repairing and tuck-pointing old masonry walls. Compact the joints thoroughly by tooling after the mortar partially stiffens.
|David L. Heiserman, Editor||
Copyright © SweetHaven
Revised: June 06, 2015