Masonry Magazine August 1970 Page. 17
Selection of Mortar Type
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 5. 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.
Many building codes acknowledge that the strength of masonry construction is dependent on both the strength of the concrete masonry unit and the strength of the applied mortar. If design is based on concrete block of a given strength, the allowable stress range per unit area of completed masonry is dependent upon the mortar strength employed. Conversely, the allowable stress for masonry completed from mortar of given strength is dependent upon the selected block strength. The designing architect or engineer therefore has a means of varying the thickness of masonry sections by selecting certain combinations of block and mortar strengths.
Properties of Plastic Mortars
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 qualitative or non-precise definitions to some extent.
Workability is the property of mortar characterized by the smooth, plastic consistency which makes it easy to spread. Workable mortar holds the weight of concrete blocks when placed and makes alignment easy. It adheres to vertical masonry surfaces and readily squeezes out of mortar joints. Water affects the workability of a mix by influencing the consistency.
| 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 con- struction, 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. |
| K | Interior non-load-bearing partitions where low compressive and bond strengths are permitted by building codes. |
Table 5. Mortar Types for Classes of Construction
A well graded, smooth aggregate improves workability as previously discussed. Air entrainment adds to workability through the action of the minute air bubbles which function like ball bearings in the mixture. An increase in lime content also improves workability by increasing the capacity for water retentivity.
Water retentivity is a measure of the ability of a mortar to retain its plasticity when in contact with an absorbent unit. This gives the mason time to place and adjust the block before the mortar stiffens. Water retentivity is related to the flow value noted in the section describing mortar composition. Mortar flow is measured by a laboratory test (ASTM C-109). A comparison of the flow obtained before and after suction gives an indication of the water retentivity. Water retentivity is improved through higher lime content, air entrainment, or addition of fines within allowable gradation limits.
Hardening of plastic mortar relates to the initial set indicated by resistance to deformation. A consistent rate of hardening assists the mason in tooling the joints at the same degree of hardness to produce a uniform joint color. Portland cement and masonry cement are required by ASTM specifications to have initial setting times which, at normal temperatures, provide adequate opportunity for tooling masonry joints.
Properties of Hardened Mortar
Properties of hardened mortars which affect the performance of the finished concrete masonry include compressive strength, bond strength, and durability. As can be noted by comparing tables 3 and 4, the compressive strength of mortar increases as the cement content is increased, but decreases as lime is increased. Increases in air entrainment also reduce compressive strength. Compressive strength is an important factor when the structure is loaded in compression. Structural failures due to compressive loading are rare, however, and other properties such as bond strength are generally more critical.
Bond is the property of hardened mortar that holds the masonry units together. Bond strength is possibly the most significant single property of hardened mortar. It also is the most capricious and unpredictable. Since bond strength is very much less than compressive strength, mortar joints subjected to relatively small tensile stresses are likely to fail. For example, mortar having a laboratory test compressive strength of 2500 psi could develop a bond strength of only 50 to.
| Mortar Type | Average Compressive Strength at 28 Days, psi |
|---|---|
| M | 2500 |
| S | 1800 |
| N | 750 |
| 0 | 350 |
| K | 75 |
Table 4. Compressive Strength of Cubes for Mortar Types (ASTM C-270)