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When the masonry is loadbearing, how do I know I’m specifying the right mortar?

Masonry construction can be used for all kinds of loadbearing applications, ranging from relatively simple single storey structures to low- and mid-rise multi-storey structures in seismic zones. An important component of masonry assemblages is the mortar which bonds masonry units together, distributes loads evenly over the surface of the masonry units, and helps accommodate construction tolerances. The first step is always to select the appropriate type of mortar for the application. It is also important to specify the selected mortar in a manner that is clear and in compliance with applicable standards.

Masonry mortar can be specified using two different paths of compliance within CSA A179-14: Mortar and grout for unit masonry: proportion specification, or property specification. A common mistake when specifying mortar for a project is to interchangeably use references to property specifications, such as mortar strength, with proportion specified mortars. The purpose of this article is to explain the differences and offer insight on how mortar can be specified in a way that is compliant with codes and standards, as well as suitable for loadbearing use.

Part of our Masonry Specification Series

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The information contained here is intended to serve as educational content for designers, specifiers, or contractors. It is not to be relied upon for formal technical advice, as masonry projects may have details and considerations that are unique to a particular project and may be beyond the scope of the content of this page.


Masonry mortar should be specified under the requirements for either proportion or property specifications in accordance with CSA A179-14: Mortar and grout for masonry.

In nearly all cases, simply specifying the type of mortar (Type S or Type N) as well as indicating that the mortar must meet the requirements of CSA A179-14 is sufficient for loadbearing applications without the need for specifying a minimum strength, even for assemblages using higher strength units.


Knowing which type of mortar is required

Table A.1 of CSA A179-14 provides a guide for the selection of mortars for modern unit masonry. According to the guide, Type S mortar should be specified for all loadbearing masonry applications requiring high compressive strength, as well as all exterior masonry below grade. Type N mortar may be used for interior and above-grade exterior loadbearing masonry if only low compressive strength is required.

How to specify mortar

Masonry mortar must meet the requirements of CSA A179-14: Mortar and grout for unit masonry. Under this standard there are two separate compliance paths to specify mortar: proportion specifications and property specifications.

Proportion specified mortar is for jobsite mixing where CSA approved materials are to be mixed to the volume-based proportions given in Table 3 or Table 4 of the Standard. Property specified mortars allow for manufacturers to use innovative materials to enhance the properties and/or behaviour of masonry mortar but must follow strict testing requirements to ensure acceptable performance.

The effect of these different types of mortar on the strength of a masonry assemblage is reflected in Table 4 of CSA S304-14: Design of masonry buildings. A higher assemblage strength, f’m, is assigned to masonry assemblages with Type S mortar compared to those with Type N mortar. The strength of masonry assemblages also increases with increasing specified strength of the units, however the specified properties of Type S and Type N mortar does not change depending on the units used. In general, specifying a mortar strength greater than the minimum required by CSA A179 will not provide any benefit to the specified strength of masonry for design.

Proportion Specification

Proportion specification in CSA A179-14 sets the proportion, by volume, of Portland cement, lime, and sand, in accordance with Table 3 or Table 4 of the Standard, to be combined to achieve the desired Type S or Type N mortar. Although testing of site-prepared mortar, including compression testing, may be used as quality control to verify the mortar is being mixed to the correct proportions, there is no minimum compression strength required for proportion specified mortar – even for loadbearing applications. Proportion specified Type S and Type N mortars have a long history of good performance, when mixed using the correct materials to the correct proportions, for loadbearing masonry applications.

Property Specification

Property specifications in CSA A179-14 allow the designer to require any 28-day mortar cube strength they may need, so long as it is not less than those listed in Table 6. For Type S mortar, a minimum required 28-day mortar cube strength of 8.5 MPa and 12.5 MPa for jobsite prepared and laboratory prepared mortars, respectively. The lower strength requirements for jobsite prepared mortar accounts for the effects of increased water content to achieve higher flows, effects of retempering and age of mortar on the board, among other job site conditions. Research indicates that specifying a higher strength mortar than those detailed in Table 6 of CSA A179-14 may only yield a marginal increase in the overall compressive strength of the masonry assembly, fʹm. This marginal increase is even less apparent in assemblages with higher strength units and when units are grouted. Increased mortar strength offers no benefit to masonry strengths for design unless accompanying masonry prism testing is being undertaken in order to exceed the prescriptive values for fʹm given in CSA S304 Table 4. Additionally, higher strength mortars are achieved by increasing the cement content of the mix which will decrease the workability of the mortar. A mortar with low workability is more difficult for the masons to place properly and may increase the risk of bond problems between the mortar and the masonry units.

For mortars manufactured off-site in a batching plant (like ready-mixed mortars), Property Specification should be specified, and a minimum compressive strength value may be specified but not lesser than the strength requirements in Table 6 of CSA A179-14. It should be noted that certain pre-packaged, pre-bagged or silo products of dry materials may simply contain the components of a proportion specified mortar, in compliance with Table 3 or Table 4 of CSA A179 and should be treated as such. However, other dry-mixed property specified mortar products contain admixtures and/or other components and must meet the requirements of CSA A179 through property specification.

When the designer requires on-site introduction of an admixture or other materials to improve the performance of the mortar, then Property Specification should be followed. In such cases, the designer should work with the masonry contractor to develop a mortar mix that meets the required properties.  A ratio of aggregate to cementitious material in the mortar may be established (monitoring batching, mixing, and handling procedures) or a minimum compressive strength value may be specified but not lesser than strength requirements in Table 6 of CSA A179-14.  Monitoring of properties (i.e., compressive strength of mortar cubes) is often used in lieu of monitoring batching, mixing, and handling.

Use of mortars with the strengths given by the property specifications detailed in CSA A179-14 Table 6 has been shown to result in masonry compressive strengths meeting or exceeding those detailed in Table 4 of CSA S304-14. The best way to increase the compressive strength, fʹm, of a masonry assemblage is to specify concrete masonry units with a higher compressive strength. If a masonry strength is required that exceeds those provided in CSA S304-14 Table 4, testing of masonry prisms in accordance with clause 5.1.2 of CSA S304-14 must be conducted.

Outdated Approaches

Designers occasionally mistakenly use historic approaches to estimate masonry strength by using empirical equations that relate block, grout, and mortar strengths to estimate the assemblage strength. Such equations predate limit states design of the CSA S304-14. Without prism testing there is no design benefit to specifying mortar properties that exceed those indicated in CSA A179-14. Furthermore, designers are reminded that mortar cubes, grout cylinders and concrete block units, as tested, do not represent in-situ wall properties and cannot serve as a rational basis to estimate assemblage strength. The loss of free moisture, confining effects of the units, and dimensional properties of mortar and grout in cube/cylinder form are completely different than those within a masonry assemblage. This is why the standard only recognises prism testing as a means to establish a masonry strength in excess of the values listed in Table 4 of CSA S304-14. Matching strengths of mortars or grouts to that of blocks has no theoretical or rational basis within the CSA S304-14 design standard.

Part of our Masonry Specification Series

Offering recommendations accompanied with background explanatory material to explain how these recommendations were formed. Click here to see the full series.

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