Masonry Magazine August 1976 Page. 17
Tuck Pointing
For best results, duplicate the original mortar proportions. When in doubt, use prehydrated Type N mortar. For a more complete discussion of tuck pointing, see Technical Notes 7, "Dampproofing and Waterproofing Masonry Walls".
Dirt-Resistant Mortar. Where resistance to staining is desired, add aluminum tristearate, calcium stearate or ammonium stearate to the construction mortar in the amount of 3 percent of the weight of the portland cement.
Where maximum dirt resistance is desired, use mortar or grout consisting of 1 part portland cement, 1/8 part lime, and 2 parts graded fine (80 mesh) sand, proportioned by volume. To this add aluminum tristearate, calcium stearate or ammonium stearate equal to 2 percent of the portland cement by weight.
SUMMARY
General. Mortar requirements differ from concrete requirements, principally because the primary function of mortar is to bond masonry units into an integral element. For many years, portland cement, lime, sand and water have been the principal constituents of mortars. The most predictable mortars are still made with these ingredients.
"Standard Specification for Portland Cement-Lime Mortar for Brick Masonry", Technical Notes 8A, is the recommended standard for mortars to be used with brick masonry. It specifies materials, proportions and properties.
Properties of both plastic and hardened mortars are important. Plastic properties determine construction suitability; hardened properties determine performance of finished elements. No one combination of ingredients provides a mortar which is highest in all desirable properties. Factors that improve one property may do so at the expense of others. When selecting a mortar, evaluate all properties, then select the mortar providing the best compromise for the particular requirements.
Water Content. Water requirements of mortars are not similar to those for concrete. Use the maximum water compatible with workability and permit retempering.
Workability. No laboratory test exists for work-ability. Rely on water retentivity and aggregate gradation requirements to assure workability. To increase workability:
1. Use a well graded sand of which approximately 10 percent passes a No. 100 sieve.
2. Use highly plastic lime (Type S lime or lime putty).
3. Increase air content, but not to exceed 12 percent.
Flow and Water Retentivity. Flow is the percent increase in diameter of a test specimen, when it is placed on a flow table and then mechanically dropped 25 times in 15 sec. Flow after suction is determined by the same test performed on a specimen which has had some water removed. Water retentivity is the ratio of initial flow to flow after suction, expressed in percent. Construction mortars usually require greater flow and water retentivity than those for laboratory mortars. In general, to increase water retentivity:
1. Use a well graded sand.
2. Use highly plastic lime (Type S lime or lime putty).
3. Increase air content, but not to exceed 12 percent.
Bond. Bond is probably the most important single property of mortars. Currently the crossed-brick couplet assembly is used to test tensile bond strength (ASTM E 149). Many variables affect bond. In general, to increase tensile bond strength:
1. Mix mortar to the maximum flow compatible with workmanship. Use maximum mixing water and permit retempering.
2. Use brick whose suctions are less than 20 g when laid. Control high suction by wetting.
3. Use Type S mortar. (Increasing the cement content increases bond up to a cement:lime ratio of 1:½.)
4. Keep air content to a minimum.
5. Use mortars having high water retentivity.
6. Bond mortar to wire cut or roughened surfaces rather than to a die skin.
Durability. To increase the durability of mortar cubes:
1. Increase portland cement content.
2. Add water repellents (stearates).
Extensibility. For greatest resiliency and flexibility, although not necessarily maximum ultimate strength, increase lime content of mortars.
Volume Changes. Volume changes are caused by hardening, cyclic wetting and drying, temperature change, and unsound ingredients. Only the latter two appear significant. Provide for expansion and contraction in design; avoid unsound ingredients.
Strength. Compressive strength is measured by forming and testing cubes and standard briquets of mortar (ASTM C 109 and C 348). Compressive strength is less important than bond and is not directly related to most other mortar properties. However, because tests are simple and reliable, strength is one basis for comparing mortars. To increase compressive strength:
1. Increase cement content; decrease water; use a low water-cement ratio.