Is the compressive strength of mortar important?

ANSWER:  

Compressive Strength

In load-bearing masonry, mortar compressive strength may have some importance, but mortar only contributes a small amount to masonry assembly compressive strength (f’m).  For ASTM C90 concrete block, the assembly compressive strength only increases 250 psi when the mortar is changed from Type N to Type S[1] (from an f’m of 1750 psi to 2000 psi for CMU units with 2000 psi compressive strength).  The compressive strength of the masonry units is the largest factor, by far, in masonry assembly compressive strength.

In masonry veneer, mortar compressive strength is not important.  The veneer only has to support its self-weight, which results in axial compressive stress on the order of 10 psi for a 10-foot tall veneer.  For ASTM C216 clay face brick placed in Type N mortar, the assembly compressive strength is at least 1220 psi[2] (for clay brick units with 3000 psi compressive strength) and for ASTM C1634 concrete face brick   placed in Type N mortar, the assembly compressive strength is at least 2276 psi[3] (for 4-inch tall concrete brick units with 3500 psi compressive strength).  Thus, the veneer compressive capacity far exceeds the actual compressive stress to which the veneer is subjected, even with type N mortar.

Bond and Workability

Bond is the most important property of hardened mortar and workability is the most important property of plastic mortar[4].  Bond has three facets:  strength, extent, and durability.  Bond strength affects the ability of the masonry to resist cracking and bond extent affects the ability of the masonry to minimize penetration of water.  The tensile and compressive strengths of mortar far exceed the bond strength between the mortar and the masonry units[5]. That is why when cracking occurs, it usually appears at the interface of units and mortar. There are many variables that affect development of bond between mortar and units, but workability is one of the most significant factors.

Mortar having good workability can be picked up on a trowel, adhere to the trowel as the mortar, is evenly spread on units, and adheres to masonry units as they are placed into the wall. Workable mortar supports the weight of masonry units when they are placed and facilitates alignment.  “Good workability is essential for maximum bond with masonry units.” and “Complete and intimate contact between mortar and masonry unit is essential for good bond,” per ASTM C270, Appendix X1.

Good workability affords the mason the best opportunity to achieve “complete and intimate contact”, which is essential for achieving good extent of bond.  Workability is affected by the component materials in the mortar.  Water improves workability, to an extent, and the mason is permitted to add as much water to the mortar as the mason feels is appropriate.  The mason knows that adding too much water makes the mortar unable to support the weight of masonry units as they are laid and, therefore, makes the mortar unworkable.  In portland cement/lime mortar, the lime gives the mortar its workability.  In masonry cement mortar and mortar cement mortar, plasticizing materials and air entrainment give the mortar its workability.  High air entrainment reduces bond strength, however, which is reflected in the reduced allowable flexural tensile stress (modulus of rupture strength) values in TMS 402[6] for masonry cement mortar and air-entrained portland cement/lime mortar in unreinforced masonry.

In summary, there is usually too much focus on mortar compressive strength and not enough on workability. In ASTM C270, the proportion method is the default specification for mortar and is most commonly used.  When mortar is specified by proportions, testing to evaluate mortar compressive strength is not required and is not recommended. Refer to the FAQ “When should mortar cube testing be performed?”  Decades of experience have demonstrated that mortars mixed according to ASTM C270 perform appropriately, whether they are specified by proportions or by properties.

 

Resources:

[1] 2016 TMS 602, Specification for Masonry Structures, Table 2
[2] 2016 TMS 602, Specification for Masonry Structures, Table 1
[3] 2016 TMS 602, Specification for Masonry Structures, Table 2
[4] ASTM C270, Appendix X1
[5] ASTM C270, Appendix X1
[6] 2016 TMS 402, Building Code Requirements for Masonry Structures, Tables 8.2.4.2 and 9.1.9.2

 

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