Verifying Compressive Strength
Verifying Compressive Strength of Masonry: Unit
Strength Method vs. Prisms
For masonry under construction, we need to determine compliance
with the specified compressive strength of masonry. We have two
options for accomplishing this. One is the unit strength method
and the other is testing masonry prisms for compressive strength.
The unit strength method verifies the compressive
strength of the individual materials and then uses tables to determine
compressive strength of the assembly from that information. The
MSJC Specification in Section 1.4B, Compressive strength
determination, is one source of tables for the unit strength method
and the International Building Code (IBC) is another. They
are set up similarly. They have one table for clay masonry and one
for concrete masonry and each give the compressive strength of the
assembly based on the strength of the unit and the type of mortar.
If the wall is grouted, then the grout simply has to comply with
ASTM C 476, Specification for Grout for Masonry, or be the same
strength as the specified strength of masonry, but not less than
a minimum of 2000 psi.
If you do not use tables, you need to know about constructing prisms
to verify compliance with design compressive strength. These specimens
are built at the job site. Methods for this are outlined in ASTM
C 1314, Standard Test Method for Compressive Strength of Masonry
Prisms, which entails constructing the prisms, including grouting
if applicable, and bag curing them.
The construction will be deemed acceptable or not acceptable based
on the prism test results, so it’s important to do the job
right. Prisms are fabricated in moisture-tight bags. Large black
polyethylene bags, like heavy-duty trash bags, are common. Units
are mortared together, and the resulting prisms are left to cure
for 24 to 48 hours. If the construction is to be solidly grouted,
the prisms are grouted at this time. Following grouting, the bags
are resealed and cured for an additional 48 hours or longer. Prisms
are then strapped or clamped together to prevent damage during transport
to the testing laboratory. Then they are further cured, removed
from the bags two days prior to compressive strength testing, and
tested in compression at an age of 28 days or another designated
test age. This produces values for strength of masonry to determine
whether or not the as-constructed wall meets the design requirements.
| Masonry prisms for compressive
strength testing are constructed and then cured in plastic bags.
Following initial curing, they are shipped to the lab in a rig
to prevent damage during movement.(IMG15865)
Standard Test Method for Compressive Strength of Masonry Prisms,
ASTM C 1314, www.astm.org
Requirements for Masonry Structures and Specification for Masonry
and Commentaries, Masonry Standards Joint Committee, American
Concrete Institute, American Society of Civil Engineers, The Masonry
Society, Boulder, Colorado, 2005.
Building Code, International Code Council, Whittier, California,
2003. A newer version,
published in 2006, is also available. (Both versions are likely
to be used for the next several years.)