Masonry Magazine October 1976 Page. 11
BIA Technical Notes
on Brick Construction
Brick Institute of America 1750 Old Meadow Road, McLean, Virginia 22101
INTRODUCTION
MORTAR FOR BRICK MASONRY
SELECTION AND CONTROLS
Since the introduction of engineered brick masonry in early 1966, the trend of masonry specifications nationwide has been towards the use of higher cement content, so-called "stronger", mortars. The result of this trend has been more expensive masonry construction that is not necessarily higher in performance characteristics. It has also been noted that many masonry specifications require inordinately expensive provisions for control or no control for the batching of the ingredients and mixing of the mortar.
This issue of Technical Notes discusses the selection of the proper mortar dependent upon its use. It also discusses some of the methods and procedures suggested for the control of proportion batching and mixing of mortar at the project site.
This Technical Notes does not discuss mortar ingredients or specific mortar specifications. Information on these subjects can be found in Technical Notes 8 and 8A of this series.
SELECTION OF MORTAR
Since the contemporary bearing wall of engineered brick masonry has become widely accepted and used, the trend in masonry specifications has been toward higher cement content, higher compressive strength mortars. It appears that many architects, engineers and specifiers believe inherently in the adage that "stronger is better". This could not be farther from the truth for mortar for brick masonry.
As stated in previous issues of this Technical Notes series, mortar is the bonding agent that integrates a masonry wall. It must be strong, durable, capable of keeping the wall intact, and it must create a water-resistant barrier. It could also have been stated that the mortar must have certain resilient properties and be both economical and easy to use by the mason.
The specifications and requirements for use of high compressive strength mortars can, in fact, work contrary to the above desirable properties and performance. It is of no value if a mortar has huge compressive strength and does not bond to the masonry units or cannot be properly and easily placed by the mason. Neither is it acceptable if the mortar has high cement content, high compressive strength and shrinkage characteristics that will cause separation cracking between masonry units and mortar, resulting in wind-driven rain penetration of the wall elements.
Selection
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July/Aug
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Selection of a particular mortar for a particular project is the function of balancing the needs of the finished element with the properties of the various available mortar mixes. For example, for areas of high winds, high lateral strengths may be required and, hence, a mortar that develops high tensile bond strength. For heavily loaded loadbearing walls, high compressive strength may be required, and hence high compressive strength mortar. Other considerations may be durability (below grade or retaining wall use), color uniformity, flexibility (parapet walls and exterior wythes), and other considerations. For these reasons, the selection of a mortar requires the careful evaluation of the properties of each type of mortar and the selection of that mortar which will best meet the particular in-use performance requirements for the project.
The Basic Rule. No single type of mortar is best for all purposes. The basic rule for the selection of a mortar for a particular project is: Never use a mortar that is stronger (in compression) than is required by the structural requirements of the project. Always select the weakest (in compression) mortar that is consistent with the performance requirements of the project.
This general rule must, of course, be tempered with good judgment. For example, it would be uneconomical and unwise to continuously change mortar types for various pieces or parts of a structure. However, the general idea of the rule should be followed, using good judgment and economic sense.
General Properties of Mortar Types
The following are the recommended general uses for various mortars to be applied, using the basic rule as stated above. (Also, see Table 1.)
Type N Mortar. Type N mortar is specifically recommended for exterior walls wherever they are subject to severe exposure, for chimneys and for parapet walls. It is a medium strength mortar suitable for general use in exposed masonry above grade.
Type S Mortar. Type S mortar is recommended for use in reinforced masonry, for unreinforced masonry where maximum flexural strength is required, and for use where mortar adhesion is the sole bonding agent between facing and backing. It is a reasonably high compressive strength mortar, which tests indicate has a high tensile bond strength with most brick units.