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

Many designers and spec writers are very familiar with concrete construction and as a result, commonly use concrete specifications for masonry grout. This is incorrect and should be avoided to ensure compliance with CSA masonry standards. While the basic materials of masonry grout are the same as concrete (aggregate, cement, water), the environment, purpose, and pouring conditions are vastly different. As a result, masonry grout requires material properties that are very different than concrete.

A common mistake when specifying masonry grout is not understanding the differences that are associated with masonry grout in comparison to grout used for other applications. The purpose of this article is to explain the differences and offer insight on how grout can be specified to comply with the CSA Masonry standards and provide suitable structural performance.

This page is part of a larger series of specification examples meant to highlight cases where vague or poorly written documents have led to issues with masonry construction across Canada.

Proportion or Property specified grouts – both are acceptable.

Masonry grout is to follow the requirements of CSA A179-14: Mortar and grout for unit masonry. Under this standard there are two sperate compliance paths to specify grout: proportion specifications and property specifications. Proportion specified grout is for jobsite mixing where CSA approved materials are to be mixed to the volume-based proportions given in Table 5 of the Standard. Property specified grouts allow for manufacturers to use innovative materials to enhance the properties and/or behaviour of masonry grout but must follow strict testing requirements to ensure acceptable performance.

Aggregate size for masonry grouts do not typically match the acceptable gradations for concrete. Aggregates for concrete are typically more coarse and harsher than aggregates for masonry grout. Most aggregate suppliers will therefore carry ‘mortar sand’ or ‘masonry sand’ to allow for the batching of CSA-compliant masonry grouts. Additionally, the aggregate gradations are also different between fine and course grouts. Gradation requirements for aggregates in masonry grouts are found in Table 1 of the CSA A179-14 Mortar and grout for unit masonry.

Both grout types are adequate for masonry construction, but each has its own advantages. A coarse grout will require less cement per volume, and it is a generally cheaper alternative to fine grout, while fine grout is required in heavily reinforced or otherwise congested masonry. For instance, when both vertical and horizontal reinforcement are present, especially at lap splice locations, a fine grout can ensure a complete bond between the grout, reinforcement, and masonry units, while reducing the probability of voids.

There are no minimum strength requirements for proportion specified grouts, because they have a proven record of performance. Masonry design strengths found in CSA S304-14, Table 4 are based on proportion specified grouts. Proportion specified grouts will typically test to a minimum compressive strength of 10 to 12 MPa cast in non-absorbent cylinder molds. However, once again, there is no minimum strength required if the grout is mixed to the proportion specification.

The minimum compressive strengths for property specified grouts are 10 MPa and 12 MPa, for fine and coarse grout, respectively. Designers familiar with concrete construction, or with grouts in general, may think that this strength is low. However, the compressive strength of grout used in masonry construction is affected by its consistency at the time of pouring, by the size of the voids it will need to fill, and by the level of absorption of the masonry units in contact with the grout. Consider these key properties the differentiate masonry grouts from traditional concrete:

• Masonry grout must be fluid enough to fill all voids within the masonry cells and fully encapsulate the reinforcement, which requires a grout with high slump (required between 200 mm and 275 mm) and consequently a high water-to-cement ratio.
• A high water-to-cement ratio is also necessary to help replace the moisture absorbed by the surrounding masonry. Grouts are not “cast” into a non-absorptive form, they are poured into masonry units that are absorptive and will draw free water from the grout.
• The compressive strength of grout cylinders does not provide a realistic comparison to the in-situ conditions. A grout cylinder is cast in a non-absorbent mold, where the in-situ pouring of grout is into the cells of concrete masonry units. These are not the same conditions for curing. The masonry units absorb some of the moisture from the grout reducing the water-to-cement ratio, and hence increasing the in-situ actual compressive strength of the grout. This absorption process also plays an important part in providing a strong bond between the grout and the masonry units. For masonry design, the in-situ compressive strength of grout can be estimated as 1.5 times the 28 d grout cylinder strength, or roughly taken simply as 20 MPa for proportion specified grout, unless a more detailed determination of the in-situ grout strength is provided.

The other factor to consider with these “low” grout strengths is that higher grout strengths show very minimal increase in the strength of the masonry assemblage. An experimental program conducted by Drysdale and Hamid^[1] showed a very minimal increase in assemblage strength (dotted line) when grout was increased from 15 MPa to 45 MPa.

While it might be tempting to specify a stronger grout to match the strength of the masonry unit, decades of testing and experience have shown diminishing returns with higher grout strengths. The compressive strengths that engineers rely upon for their design calculations take the relatively low strength and high slump into account and can be seen in CSA S304-14: Table 4, Note (4). Cylinder tests as low as 10MPa for a proportion specified grout are compliant with even 30 MPa specified unit strengths. It is far more important that the grout be workable and fill all voids, bonding reinforcement to the units, than have a higher strength with a lower slump at the time of pouring.

The required slump of grout for unit masonry is between 200 mm and 275 mm and is typically achieved by mixing grout with a high water/cement ratio (reducing the cylinder strength of the grout). High strength and high slump grouts are possible with the use of plasticizers or water reducing agents; however, these materials can increase the viscosity of the grout to a point where it will “stick” to mortar fins and unit webs creating voids within the wall. Plasticizers can be effectively used, but it is recommended that the use of plasticizers is limited to property specified grouts with a proven track record of performance and reliability rather than adding plasticizer on the jobsite to achieve a certain slump. The inside of the cells of unit masonry can be quite congested; mortar fins, rebar can be in multiple direction (especially a concern at lap splices), offset webs from courses to course, flares in the unit webs and face shells, etc. Due to this congestion, the most important property of a masonry grout is the flowability to ensure there are no voids.

In summary, masonry grout is different than concrete or grouts used in other aspects of construction. Masonry grouts should be specified under the requirements for either proportion or property specifications in CSA A179, where high strength requirements should be avoided, and a high slump is required.

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[1] R. G. Drysdale and A. A. Hamid, “Behaviour of Concrete Block Masonry under Axial Compression”, ACI Journal, Vol. 76, No. 6, 1979, pp. 707-721.

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