I would like to require a minimum ultimate bond strength in my project specifications. What value for bond strength should I require?
No standard now sets minimum values for ultimate bond strength of masonry. Building codes list only allowable values for bond for use in design. Until some standard sets minimum values (ASTM is now attempting to do that), I suggest requiring a minimum 90 psi for Type S and 70 psi for Type N portland cement-lime mortars, tested according to ASTM C 1072, Standard Method for Measurement of Masonry Flexural Bond Strength. You can require somewhat higher values if higher bond strength is needed for structural reasons. Lower values might adversely effect wall permeability, mortar durability, or other important performance characteristics. I base these recommendations for minimum ultimate bond strength on an unpublished 1964 study by the Structural Clay Products Institute (now the Brick Institute of America). In the study, 135 tests were conducted using Type S portland cement-lime mortar and 27 different types of brick with initial rates of absorption (IRA) from 1 to 50. All samples were cured for 28 days in laboratory air. The average bond strength in the study was 123 psi with a standard deviation of 32 psi. Assuming a normal distribution, 85% of the tests would be greater than 90 psi. A similar procedure was used to determine a minimum value for Type N mortar. Though the 1964 study used what later became ASTM E 518, Test Method for Flexural Bond Strength of Masonry, C 1072 will give higher results than E 518.