Masonry Magazine October 2000 Page. 18
CHOOSING the RIGHT MORTAR
by John Melander
Mortar selection impacts both the construction process and the quality of the finished masonry product. Unfortunately, mortar selection is not always given proper consideration in planning for a masonry construction project. Selecting the right mortar requires an understanding of the materials (mortar and units), the process (masonry construction), and the product (masonry).
UNDERLYING PRINICPLES AND ISSUES
Mortar is used to lay brick, block, or stone units. Within that context, it must:
* facilitate the placement of units,
* contribute to the serviceability of masonry,
* provide required structural performance, and
* exhibit the desired appearance.
Consideration to each of these areas of performance needs to be given in the selection of mortar type and mortar materials.
Different members of the design and construction team have different perspectives on the relative importance of these areas of performance. The engineer focuses on the structural implications of mortar selection, the architect looks at appearance, and the mason contractor expects a workable product to facilitate productivity. The owner/builder wants a completed project, delivered on time and within budget, which will serve his needs. The relative influence different team members have on mortar selection varies from project to project. However, a balanced perspective is needed in the selection process. Basic principles to remember are:
* No one mortar type is best for all applications
* No one mortar type will rate the highest in all areas of performance
* No single mortar property defines mortar quality
18 MASONRY-SEPTEMBER/OCTOBER, 2000
MORTAR TYPES
ASTM Standard Specifications provide a means for specifiers to identify acceptable materials and products without limiting those items to specific brands or manufacturers. Project specifications should reference ASTM C 270, the Standard Specification for Mortar for Unit Masonry. Mason contractors and specifiers need to understand the provisions of that specification and available options.
ASTM C 270 defines four different types of mortars intended to address the variety of needs stemming from different masonry applications. Type N mortar is a general-purpose mortar that provides good workability and serviceability. It is commonly used in interior walls, above-grade exterior walls under normal loading conditions, and in veneers. Type S mortar is used in structural load-bearing applications and for exterior applications at or below grade. It also provides increased resistance to freeze-thaw deterioration. Type M is a high-strength mortar that may be considered for load bearing or demanding freeze-thaw applications. Type O is a low-strength mortar that is sometimes used for interior masonry or repointing.
PROPORTION SPECIFICATIONS FOR MORTAR
(Adapted from ASTM C 270)
| Mortar type | Portland cement or blended cement | Masonry cement or mortar cement type | Hydrated lime or lime putty | Aggregate |
|---|---|---|---|---|
| M | 1 | - | - | 4½ to 6 |
| | - | 1 | - | 2¼ to 3 |
| | 1 | - | ¼ | 2½ to 3½ |
| S | ½ | - | - | 3½ to 4½ |
| | - | 1 | - | 2½ to 3 |
| | 1 | - | Over ¼ to ½ | - |
| N | 1 | - | - | 2½ to 3 |
| | - | - | Over ½ to 1¼ | - |
| O | 1 | - | - | 2½ to 3 |
| | - | - | Over 1¼ to 2½ | - |
*The total aggregate shall be equal to not less than 2¼ and not more than 3 times the sum of the volumes of the cement and lime used.
ASTM C 270 provides two alternatives for specifying mortars: proportion specifications and property specifications. Under the proportion specifications, mortar ingredients must meet indicated product standards and be volumetrically proportioned within limits given in Table 1. No physical requirements are placed on the mortar itself. Under the property specifications, in addition to meeting the individual product standards, mortar materials mixed in the laboratory using job site proportions must meet certain property requirements of the specification Table 2. Either the proportion specifications or the property specifications should be selected, not both. If project specifications do not indicate which has been selected, the proportion specifications govern, unless data qualifying the mortar under the property specification are presented to and accepted by the specifier.