Specifying Mortar for Loadbearing Masonry

Masonry construction can be used for all kinds of loadbearing applications, ranging from relatively simple single storey structures to low to mid-rise multi-storey structures in seismic zones. An important component of masonry assemblages is the mortar which can be specified using two different paths of compliance.

A common mistake is to interchangeably use references to property specifications such as mortar strength with proportion specified mortars. The purpose of this page 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.

This page is part of a larger series of specification examples meant to highlight cases where vague or poorly written items have lead to issues on masonry jobs across Canada.

Disclaimer

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.

If there are any questions or concerns regarding masonry design or construction, CMDC offers technical assistance to our supporting members, as well as the design community.

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How the specification typically appears:

X.Y      Mortar for all interior and exterior above grade:

X.Y.a   Loadbearing: Type S based on Property specifications. Minimum compressive strength to match compressive strength of concrete masonry units shown on the structural drawings at 28 days.

Points to Consider when Specifying Mortar for Loadbearing Masonry

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. In this case, a minimum 28-day mortar cube strength of 8.5 MPa and 12.5 MPa is required for jobsite prepared and laboratory prepared mortars, respectively. The lower strength requirements for jobsite mortar accounts for the effects of increased water content to achieve higher flows, effects of retempering and age of mortar on the board, and 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. There is no benefit to higher masonry strengths for design unless accompanying 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 and make it more difficult for the masons to place it properly and hence increasing the risk of bond problems between the mortar and the masonry units.

Using the strengths given by the mortar property specifications detailed CSA A179-14 Table 6 has been shown to meet masonry compressive strengths detailed in Table 4 of CSA S304-14. The best way to increase the 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 then that may only be achieved through the use of prism tests per clause 5.1.2 of CSA S304-14.

Designers may be mistaking 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 the 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, which is why the standard will only recognize prism testing as a means to exceed tabular values for masonry strength.

Matching strengths of mortars or grouts to blocks has no theoretical or rational basis within the CSA S304-14 design standard.

Suggested changes to the specification:

X.Y Mortar for all interior and exterior above grade:

X.Y.a   Loadbearing: Use pre-packaged, pre-bagged, or silo products. Type S mortar complying with CSA A179 Property Specifications. Minimum compressive strength on jobsite of 8.5 MPa at 28 days.

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