The most important characteristic of fresh mortar is workability, generally defined as the ability to be easily spread on masonry units using a trowel. In the context of ASTM C270, workability is defined and measured very simply, in terms of flow. A standard-shaped, circular sample of mortar 4 in. in diameter is placed on a flow table, which is then dropped 25 times. The flow of that mortar is defined as the increase in diameter of the sample, divided by the original diameter and multiplied by 100. Thus, if the final diameter is 8 in., the flow is (8 – 4)/4, or 100. Laboratory mixed mortars have a flow of about 110 ± 5; field mortars, about 130 to 150. Field mortars should be retempered (water added) as necessary to maintain workability, but should not be used beyond 2-1/2 h after mixing. Workability can also be measured with a cone penetrometer.
According to ASTM C270, mortar can be specified by proportion (the default) or by property. If mortar is specified by proportion, the following characteristics of fresh mortar are controlled indirectly as a result of complying with the required proportions. If mortar is specified by property, they are controlled directly:
1. Retentivity: This is the ratio of the flow after suction to the initial flow. Flow after suction is measured using mortar from which some of the water has been removed using a standard vacuum apparatus. In one other specification, the mortar is spread on a masonry unit and allowed to sit for 1 min. According to ASTM C270, mortar is required to have a retentivity of at least 75 percent.
2. Air content: Percent air by volume (ASTM C91). Cement-lime mortar and mortar-cement mortar usually have a maximum permissible air content of 12 percent. Masonry-cement mortar usually has a maximum permitted air content of 18 percent if used in reinforced masonry.