Masonry Magazine October 1987 Page. 23
TABLE 2. Property Specification Requirements
| Mortar | Type | Average Compressive Strength at 28 Days, Min. psi | Water Retention, min, % | Air Content, max, % | Aggregate Ratio (Measured in Damp, Loose Conditions) |
|---|---|---|---|---|---|
| Cement-lime | M | 2500 | 75 | 12 | |
| | S | 1800 | 75 | 12 | |
| | N | 750 | 75 | 148 | Not less than 2% and not more than 3% times the sum of the separate volumes of cementitious materials. |
| | O | 350 | 75 | 148 | |
| Masonry cement | M | 2500 | 75 | C | |
| | S | 1800 | 75 | 0 | |
| | N | 750 | 75 | 0 | |
| | O | 350 | 75 | C | |
A Laboratory prepared mortar only. When structural reinforcement is incorporated in cement-lime mortar, the maximum air content shall be 12%. C When structural reinforcement is incorporated in masonry cement mortar, the maximum air content shall be 18%.
TABLE 3. Mortar Types for Classes of Construction
| ASTM Mortar Type Designation | Construction Suitability |
|---|---|
| M | Masonry subjected to high compressive loads, severe frost action, or high lateral loads from earth pressures, hurricane winds, or earthquakes. Structures below grade, manholes, and catch basins. |
| S | Structures requiring high flexural bond strength, but subject only to normal compressive loads. |
| N | General use in above grade masonry. Residential basement construction, interior walls and partitions. Concrete masonry veneers applied to frame construction. |
| 0 | Non-load-bearing walls and partitions. Solid load bearing masonry of allowable compressive strength not exceeding 100 psi. |
With masonry cement, fewer materials are handled and mixing is more convenient. The plasticity and water retention characteristics of masonry cement contribute to the workability and bonding properties of the mortar. A combination of portland cement and lime will produce mortars of equal quality. However, more care must be exercised in the proportioning of the ingredients and mixing should be closely controlled to insure that blending is complete. The decision to select either of the two materials should be based upon a consideration of the requirements of the particular project involved.
Instead of using the proportioning method of producing mortar just discussed, approved materials may be mixed in controlled percentages as long as the resultant mortar meets the minimum compressive strength requirements designated in ASTM C270 as shown in Table 2.
Mortar Selection
The performance of masonry is influenced by various mortar properties such as workability, water retentivity, bond strength, durability, and compressive strength. Since these properties vary with mortar type, it is important that the mortar type selected for a particular application is the one that best meets the end use requirements.
The strength of masonry usually increases with the strength of the applied mortar, but mortar strength achieved beyond that required for structural adequacy of the masonry is generally of little practical value. As a guide, the ASTM types of mortar may be used for the kinds of construction specified in Table 3. These recommendations are based on successful experience with concrete masonry construction, but are not a substitute for engineering evaluation or required compliance with building codes.
Masonry Mortar Properties
Many properties of mortar are not precisely definable in quantitative terminology because of a lack of definitive standards by which to measure them. Contractors and masons are inclined to rate properties of mortars on the basis of convenience in obtaining visually satisfactory mortar joints. Even engineers and other technically oriented persons must rely on non-precise definitions to some extent.
Dependent upon the particular circumstances for a given project, the criteria for mortar selection may be based upon the properties of mortar in the plastic state or of those of mortar in a hardened state. Both are equally important, so compromise is usually necessary to achieve a desired result.
Properties of Plastic Mortar
Workability is the property of mortar characterized by the smooth plastic consistency which makes it easy to spread. This is the property of most importance to the mason. A workable mortar spreads easy under the trowel; adheres to vertical surfaces during unit handling, placement, and bedding; supports the weight of the concrete masonry unit after being laid; maintains alignment as other units are positioned; and finishes with a closed tooled joint.
Once mix proportions have been established, the addition of water should be consistent with that required to improve trowelability without sacrificing supporting capacity. Such an addition promotes intimate contact between the unit and mortar, which is essential for satisfactory bond. While water content has the greatest influence on the workability of a mortar, cementitious materials, aggregate gradation, and air-entrainment also contribute to a somewhat lesser degree.
Water retention of mortar is a measure of the mortar's ability to retain its plasticity when subjected to the absorptive forces of a concrete masonry unit. Mortars with low water retention lose their water rapidly to adjacent units, making unit bedment difficult. Mortars with desired water retention characteristics allow the mason to lay a mortar bed two or three units ahead before placing subsequent units.
The time lapse between spreading mortar and placing block should be kept to a minimum, because the consistency will be reduced through suction of the block on which it is first placed. If too much time elapses before another block is placed, the bond could be reduced.
Since all mortar is not used immediately after mixing, evaporation of the mixing water accompanied by some cement hydration may require retempering the mortar to restore it to its original consistency. This is not harmful as long as it is done prior to hardening of the mortar. Mortar should be placed in final position within 2½ hours after the original mixing.