Masonry Magazine October 1976 Page. 12
Type M Mortar
Type M mortar is specifically recommended for unreinforced and reinforced masonry below grade and in contact with earth, such as foundations, retaining walls, walks, sewers and manholes. It has high compressive strength and excellent durability.
Type O Mortar
Type O mortar may be used for loadbearing walls of solid masonry where compressive stresses do not exceed 100 psi, provided that the exposure is not severe. It is a relatively low compressive strength mortar suitable for limited exterior use and general interior use in loadbearing and non- loadbearing masonry. Type O mortar should not be used where it will be subject to freezing in the presence of moisture.
Special Mortar Applications
Cavity Walls. In general, Type N mortar is suitable for cavity walls, except where wind velocities are expected to exceed 80 mph. For high wind areas, use Type S mortar.
Tuck-Pointing Mortar. Use only prehydrated mortars. To prehydrate mortar, thoroughly mix all ingredients except water; then mix again, adding only enough water to produce a damp unworkable mix which will retain its form when pressed into a ball. After 1 to 2 hr, add sufficient water to bring it to the proper consistency; that is, somewhat drier than conventional masonry mortars.
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, aluminum tristearate, calcium stearate or ammonium stearate may be added to the construction mortar in an amount not to exceed 2 percent of the weight of the portland cement.
Where maximum dirt resistance is desired, such as in glazed structural clay unit installations, use mortar or grout consisting of 1 part portland cement, s 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.
MIX CONTROLS
Mortar is perhaps the last building material in history that is still manufactured at the project site. Almost all other materials, with the possible exception of plaster, are manufactured in highly sophisticated, often automated, quality controlled industrial plants or factories. Yet many contractors, architects and engineers often entrust the manufacturing of this product; i.e., mortar; to untrained and often unqualified personnel, utilizing measuring systems that are inaccurate by their very nature; i.e.. the shovel.
TABLE 1
Guide for the Selection of Masonry Mortars"
| Location | Building Segment | Mortar Type |
|---|---|---|
| | | Recommended | Alternative |
| Exterior | Loadbearing walls | N | S or M |
| Above grade | Non-loadbearing walls | N | Of or S |
| | Parapet walls | N | S |
| Exterior | Foundation walls, | | |
| At or below | retaining walls. | | |
| grade | manholes, sewers, | M | S |
| | pavements, walks | | |
| | and patios | | |
| Interior | Loadbearing walls | N | 0. S or M |
| | Non-loadbearing | 0 | Nor S |
| | partitions | | |
"This table does not provide for any specialized mortar uses, such as high bond and acid resistant mortars.
Alternatives are presented as suitable for use where design conditions or exposures are other than normal.
Type O mortar is recommended for use where the masonry is unlikely to be frozen when saturated or unlikely to be subjected to high winds and/or other significant lateral loads. Type N or S mortar should be used in other cases.
Masonry exposed to weather in a nominal horizontal surface is extremely vulnerable to weathering. Mortar for such masonry should be selected with caution.
After the proper mortar has been specified, it may be reasonably assumed that the proper materials (cement, lime, sand) are delivered to the project site, properly stored, and are available at the mortar mixing station. It is not unusual for specifiers to require that each of these materials be subjected to rigorous tests that will certify their conformance with standard specifications (see Technical Notes 8 and 8A). The specifier may then divorce himself of further responsibility by entrusting the manufacture of this product to possibly untrained and often poorly equipped personnel.
BATCHING CONTROLS
The method of measuring and batching materials shall be either by volume or by weight, such that the specified proportions for the mortar can be accurately controlled and maintained. For material weights and recommended proportions, refer to Technical Notes 8 and 8A of this series. Measurement of sand exclusively by shovel should not be permitted.
The cementitious materials (cement and lime) should be placed in the mixer in whole or minimum of half bags. This will require the mixer to be of suitable size, depending upon the project requirements and the size of the mason crew. The primary problem comes from the addition of sand, generally measured by the shovels full which is an unacceptable procedure. The following illustrations, taken from actual projects, show some examples of batching and measurement control methods that are both economical and accurate.
Example 1.
Figure 1 is a photograph of a project in Lawrence.