Masonry Magazine February 1993 Page. 23
WATER RETENTION
In accordance with ASTM C91, this test was developed to measure the ability of a mortar to retain its mix water under the suction action of masonry units to which mortar is applied. A certain amount of absorption of water by the unit is beneficial, but too much may be detrimental.
Water retention is determined in the laboratory by measuring its "initial flow," and its "flow after suction." Initial flow is "the percent increase in diameter of the base of a truncated cone of mortar when it is placed on a flow table and mechanically raised 1/2 inch and dropped 25 times in 15 seconds." The same procedure is used to determine flow after suction. However, the test is performed on the mortar sample after some of its mix water has been removed by an applied vacuum. This simulates the suction action of masonry units on mortar. The water retention is the ratio of flow after suction to initial flow, expressed in percent. ASTM C270 requires minimum water retention of 75% for mortars complying with the property specifications.
AIR CONTENT
As noted earlier, compliance with the Property Specification of ASTM C270 includes a limitation on the air content of portland cement-lime mortars, and also on masonry cement mortar if structural reinforcement is incorporated in the masonry cement mortar. For this specification, air content is determined in accordance with ASTM C91, as modified. ASTM C780 also covers procedures for determining the air content of mortars using a pressure or volumetric method. Either of these ASTM C780 methods is used in repetitive tests to show the effects of changes in mixing time, mixing efficiency, and other variables.
OTHER TESTS
ASTM C780 covers other tests which are intended primarily to evaluate mortars during actual construction. The tests can be used to verify results of preconstruction testing and to monitor batch-to-batch variations during construction. These test methods include:
* Consistency by cone penetration
* Consistency retention by cone penetration
* Mortar water content
* Mortar-aggregate ratio
* Splitting tensile strength of mortar cylinders
MORTAR CEMENT
The Uniform Building Code has introduced a new standard for cement for use in masonry mortars. Its designation is UBC Standard No. 24-19 (1991), "Specification for Mortar Cement." It is very similar to ASTM C91 on masonry cement, with a few exceptions. For instance, the UBC Standard has a lower maximum air content for standard mortar than ASTM C91. Also, UBC Standard No. 24-19 includes a minimum requirement for flexural bond strength for standard mortar cement. Required bond strength varies with mortar cement type. Compliance testing for flexural bond strength is conducted in accordance with UBC Standard No. 24-30 (1991), "Standard Test Method for Flexural Bond Strength of Mortar Cement." This method uses what are termed, "standard masonry units," assembled in a prism. Mortar bond is determined by calculating the modulus of rupture based on wrenching units from the prism using a "bond wrench." The Standard prescribes detailed requirements on aggregates, mix design, manufacturing, size, curing, and moisture content of the "standard" concrete masonry units used to determine compliance.
SUMMARY
As discussed in ASTM C270, Appendix X1, although portland cement concretes and masonry mortars contain some of the same principal ingredients, they differ from each other in working consistency, methods of placement, and in the curing environment.
Concrete is usually placed in non-absorbent metal or wooden forms or otherwise treated so that most of the water will be retained.
Mortar is usually placed between absorbent masonry
Table 1. General Effect of Mortar Properties on Mortar Performance
| | Compressive Strength | Bond Strength | Durability | Workability | Water Retention |
|-----------------------|----------------------|---------------|------------|-------------|-----------------|
| Increase in Water Content | Reduction | Increase | None | Reduction (above optimum) | Increase |
| Increase in Cement Content | Increase | Increase | Increase | Increase | Reduction |
| Increase in Air Content | Reduction | Reduction | Increase | Increase | Increase |
| Increase in Plasticizer Content | Reduction | Reduction | Reduction | Increase | Increase |