Masonry Magazine October 1995 Page. 19
Bond Strength of Masonry
OND STRENGTH of masonry is continually mentioned during discussions of masonry performance. The term "bond strength" has different connotations to the designer, materials supplier and contractor and maintenance personnel. Technically, bond strength of masonry is the adhesion developed by masonry mortar to a masonry unit. Compatibility of unit and mortar is sought. An infinite array of mortar, unit and service conditions influence compatibility, but most unit masonry should be recognized as a weathering facade with excellent reputation for longevity.
By AL ISBERNER
Consulting Materials Engineer
From a materials viewpoint, masonry mortars vary with composition and proportions. Likewise, masonry units vary with composition and physical properties dependent on manufacturing process, be it a fired clay/shale unit or a steam cured concrete unit. Thus, any specific combination of masonry materials producing a masonry mortar and a specific masonry unit have the potential for developing a bond strength unique to that combination.
The exposure conditions further expand the number of variables affecting the adhesion between mortar and units. Caution must be exercised while discussing what material influences are on bond strength.
From a structural viewpoint, bond strength is related to tension, shear, and flexure. Present structural design criteria have been related to past wall tests, and are associated with mortar type, composition and other controlled parameters. Because of their size, walls are built and stored within laboratory rooms where temperature and humidity controls are partially controlled variables.
It should be recognized that with the present state-of-the-art, as regards bond strength and wall performance, it's not possible to predict with any degree of accuracy the performance of a masonry wall basing the calculations on materials-mortar and unit-laboratory testing. This article addresses those considerations based on research that should promote bond strength of masonry.
Masonry Mortar Influences
Now, individual masonry materials are tested in accordance with a product specification under laboratory conditions. The combined materials yielding mortar can be laboratory and construction site tested. For example, ASTM C 91 addresses masonry cement, as a product specification requiring certain performance; ASTM C 270 addresses masonry mortar, as a product, prequalification specification for mortars required to conform to the property specification.
Additionally, ASTM C 270 standardizes mortar preparation at the construction site. Bond strength is not mentioned in either specification.
Bond strength developed between mortar and masonry unit has undergone considerable research during the past and research continues today. From this past research, gen-
MASONRY-SEPTEMBER/OCTOBER, 1995 19