The four facets are explained below. These four facets completely define the characteristics of the block, according to the standard, and no further clarification is required.

The first facet indicates the solid content of the units. Hollow units, whose solid content is generally between 50 and 55%, are designated by the letter “H”; semi-solid units, with a solid content exceeding 75%, are designated by the letters “SS”; and solid units, the net cross-section of which is equal to 100% of the gross section (without voids), are designated by the letters “SF”.

The second facet indicates the specified compressive strength of the units: 10 for 10 MPa, 15 for 15MPa, 20 for 20MPa, etc.

For a concrete masonry unit to meet the requirements at a given specified strength, there is a statistical formula that must be satisfied within the block standard, CSA A165.1-14, that is based on 95% confidence of exceeding that specified strength. For example, a specified 15 MPa block would test around 18 MPa on average, and required to be stronger than that depending on the scatter of the testing results. Manufacturers are also required by CSA A165 to have compression testing for each mix design done within the last year, serving as an additional measure for quality assurance that is built into the standard.

Designers should note that higher strength units are not inherently “better” than the lowest strength that otherwise meet the structural design requirements. By simply specifying CMU following the 4-facet system and referencing CSA A165.1-14, the provisions discussed here are automatically included and there is no need to add further conservatism by further increasing the specified compressive strength.

The third facet indicates the density and maximum water absorption of the units: “A” for a normal density (greater than 2000 kg/m³) with a maximum absorption of 175 kg/m³; “B” for a density of 1800 to 2000 kg/m³ and a maximum absorption of 200 kg/m³; etc.

Units are typically specified as either Type A (“normal”) density, or Type C (“lightweight”) density. Depending on the concrete mix used to achieve the lightweight, Type C density classification, there may be increase in the fire resistance rating that can be considered using the equivalent thickness pathway within the National Building Code of Canada.

The fourth facet indicates whether the units are moisture-controlled (type “M”) or whether there is no limit to the moisture content (type “O”). Moisture-controlled units have different humidity limits depending on the total linear drying shrinkage. It is generally recommended not to specify moisture controlled units and to accommodate shrinkage through the regular placement of movement joints.

A concrete block can be specified as follows: H/15/A/O which would translate to a hollow unit, with a specified compressive strength of at least 15 MPa, normal density, and not requiring additional moisture control measures.

These four facets completely define the characteristics of the block, according to the standard, and no further clarification is required. The standard allows producers flexibility to meet these and other requirements of the standard through different modes of production. For example, the standard does not distinguish between a block that is cured at room temperature or high temperature, or at normal atmospheric pressure or autoclaved, or if the blocks are produced with new technology. In all cases, the minimum strength, shrinkage and absorption limits, and dimensional and cracking tolerances (and other aesthetic aspects) are the same, according to the standard, for all blocks designated with the same four facets. Therefore, all blocks that comply with CSA A165.1 and are designated with the same four facets are considered aesthetically and structurally equivalent, under the standard, including for differential movements.