Compressive Strength

The compressive strength of mortar is sometimes used as a principal criterion for selecting mortar type, since compressive strength is relatively easy to measure, and it commonly relates to some other properties, such as tensile strength and absorption of the mortar. The compressive strength of mortar depends largely upon the cement content and the water-cement ratio. The accepted laboratory means for measuring compressive strength is to test 2 in. cubes of mortar. Because the referenced test in this specification is relatively simple, and because it gives consistent, reproducible results, com pressive strength is considered a basis for assessing the compatibility of mortar ingredients. Field testing compressive strength of mortar is accomplished with Test Method C 780 using either 2 in. cubes or small cylindrical specimens of mortar. Perhaps because of the previously noted confusion regarding mortar and concrete the importance of compressive strength of mortar is over-emphasized. Compressive strength should not be the sole criterion for mortar selection. Bond strength is generally more important, as is good workability and water retentivity, both of which are required for maximum bond. Flexural strength is also important because it measures the ability of a mortar to resist cracking. Often overlooked is the size/shape of mortar joints in that the ultimate compressive load carrying capacity of a typical 3/8 in. bed joint will probably be well over twice the value obtained when the mortar is tested as a 2 in. (50.8 mm) cube. Mortars should typically be weaker than the masonry units, so that any cracks will occur in the mortar joints where they can more easily be repaired.
Compressive strength of mortar increases with an increase in cement content and decreases with an increase lime, sand, water or air content. Retempering is associated lith a decrease in mortar compressive strength. The amount of the reduction increases with water addition and time between mixing and retempering. It is frequently desirable to sacrifice some compressive strength of the mortar in favor of mproved bond, consequently retempering within reasonable time limits is recommended to improve bond.

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