Mortar Compressive Strength
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Q: Our field tests are not meeting the specified
compressive strength for masonry mortar, even though we used the
mix specified by the architect. What’s wrong?
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| Compressive strength specimens for mortar
are 2-in. cubes |
A: Unfortunately, this is an all-too-common
occurrence: trying to compare a lab-specified strength with a field-tested
mortar. Quite simply, what is wrong is your expectation: for a number
of reasons that are outlined here, the field compressive strength
is not expected or required to meet the specified mortar strength.
Masonry mortars are specified by ASTM C 270, Specification for
Mortar for Unit Masonry. Mortars are evaluated by ASTM C 780, Preconstruction
and Construction Evaluation for Mortars for Unit Masonry. The two
documents are designed to work together, but have distinct purposes.
The two mortars (field vs. lab) would not be expected to exhibit
the same strength. The ONLY way to make any comparison between the
two is to run preconstruction tests on the (C270) mortar, recognizing
that this mortar will be proportioned with a higher water content
(higher flow) in actual field conditions.
Based on compression strength tests alone, it’s not possible
to say that your particular mortar meets or does not meet the project
specification. However, comparing C 270 requirements and C 780 results
is NOT acceptable. The approach is flawed.
In a lab, C 270 mortar is mixed to a drier mix consistency than
you would have in the field. A wetter mix, C 780 as mixed on the
job, has a higher water content and higher water-cement ratio, which
lead to lower strength results on mortar specimens, especially since
the field mortar is being placed into non-absorbent molds. On the
actual project, mortar is being placed between absorbent masonry
units that will draw water out of the mortar, lowering its water
content and increasing its strength. In addition, the aspect ratio
(height to thickness) of a specimen affects its apparent/measured
strength. A 2-in. cube will exhibit a lower strength than a 3/8-in.
joint even when made from the exact same material. The bottom line
is that field mortars are frequently just as strong if not stronger
than the lab mixes they are based on.
There are numerous documents written to address this exact misinterpretation
of the use of C 270 and C 780. ASTM developed a guide to the correct
use of C 270 and C 780. That document, C 1586, is entitled Guide
for Quality Assurance of Mortars. The flow chart below shows that
C 270 is used to establish mortar mix proportions and that C 780
is used to evaluate site proportioning. It bears repeating that
if compressive strength testing is used:
- it's recommended to have preconstruction testing to establish
baseline values
- compressive strength is not to be used for compliance but only
mortar uniformity, and quite specifically
- the field compressive strength is not required to meet C270
minimum compressive strength.
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| After curing, cubes are crushed to determine
strength |
Both C 270 and C 780 contain statements to discourage their improper
use. In C 270, Section 3.1, “Specification C 270 is not a specification
to determine mortar strengths through field testing.” And also
in Section 3.3, “The compressive strength values resulting from
field tested mortars do not represent the compressive strength of
mortar as tested in the laboratory, nor that of the mortar in the
wall. Physical properties of field sampled mortar shall not be used
to determine compliance to this specification and are not intended
as criteria to determine the acceptance or rejection of the mortar.”
In C 780, Section 1.2, “This test method… is also used
to establish baseline values for comparative evaluation of field
mortars.” And in Section 1.4, “The test results obtained
under this test method are not required to meet the minimum compressive
values in accordance with the property specifications in Specification
C 270.”
The same issues are covered in a continuing education section on
mortar quality assurance, RP415,
highlighted under the heading "Avoid the Most Common Testing
Mistake.”
Consider using the mortar aggregate ratio test as a more timely
and effective job site quality control test than compressive strength
of mortar.
Additional resources:
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