Masonry Magazine October 1995 Page. 22
Conditions
Type M, S and N masonry mortars yielded average tensile bond strengths of 192, 171 and 120 psi respectively-the writer's first encounter where Type M mortar yielded better tensile bond strengths than Type S mortars. The test method allows comparisons of individual mortar and unit combinations, not necessarily related to performance in service.
Research
Bond strength of mortar to masonry units remains an evasive property of masonry mortars. Research of this mortar property should continue, more with the desire to obtain general relationships than to affix accept/reject numerical limits. A fertile research area is to identify the role of lime in masonry mortars.
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Lime Additions
Lime additions to portland cement mortars serve to drastically decrease the compressive strength. i.e., the higher the lime content the lower the cube compressive strength. Yet, this exceedingly fine powder increases the tensile bond strength of mortar to masonry unit. Increased water demand and water retaining characteristics might be responsible for more complete hydration of the portland cement within the mortar. Another fertile area for research is to join materials and structural interests so the role of each can be coupled toward analyzing significance of present tests being performed. Research should recognize these variables, and others, that affect bond strength:
Masonry Mortar
Composition-cement, Composition-lime. Composition-admixtures, Mix Proportions, Aggregate, Conditioning: age, temperature, humidity. Mortar water content Mixing time and temperature
Masonry Unit
Texture. Initial rate of Absorption, Absorption. Density. Moisture content,
Masonry Execution
Q/C during mixing. Rate of mortar usage, Retempering practice, Delay between mortar spreading and unit bedment. Work (force and distance) during placement, Extruded mortar cut-off, and Joint tooling.
Natural Effects
Temperature. Humidity. Carbonation, and Wind speed.
Masonry Execution-Bond
The combination of masonry mortar and masonry unit during construction of a masonry segment can have a significant effect on bond strength of mortar to unit. Significant factors include:
Conformance to Construction
Conformance to Construction Practices of ASTM C 270. Compliance with Industry Association recommendations, Method of unit bedment, and Protection of masonry. Continued on Page 41