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Re: Low mortor prism test results

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Although 800 psi is somewhat low for a field prepared Type S mortar, the
compressive strength of the masonry assemblage is more important.  The
commentary in ASTM C 270 (X1.6.3.2) has a passage that is particularly
relevant in these situations:

Perhaps because of the previously noted confusion regarding mortar and
concrete, the importance of compressive strength of mortar is
overemphasized. Compressive strength should not be the sole criterion for
mortar selection. Bond strength is generally more important, as is good
workability and water rententivity, both of which are required for maximum
bond.  Flexural strength is also important because it measures the ability
of a mortar to resist cracking.  Often overlooked is the size/shape of
mortar joints in that the ultimate compressive load carrying capacity of a
typical 3/8 in. (9.5 mm) bed joint will probably be well over twice the
value obtained when the mortar is tested as a 2 in. (50.8 mm) cube.  Mortars
should typically be weaker than the masonry units, so that any cracks will
occur in the mortar joints where they are more easily repaired.

Dave Woodham
Atkinson-Noland & Associates

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