Frequently Asked Questions (FAQs)
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Frequently Asked Questions > Lightweight
concrete masonry
Q: What is lightweight concrete
masonry and is it suited for structural applications?
A: The standard for concrete masonry
is ASTM C90, Specification for Load-bearing Concrete Masonry Units.
C90 defines three weight classifications for masonry: lightweight
is less than 105 pcf (1680 kg/m3), medium weight is defined
as 105 to less than 125 pcf (1680 to 2000 kg/m3), and
normal weight is 125 (or more) pcf (2000 kg/m3). (Properties
are tested with methods outlined in ASTM C140, Test Methods for
Sampling and Testing Concrete Masonry Units.) Because the block
has a lower density, designers and specifiers may wonder if it has
adequate compressive strength for structural applications and the
answer is yes.
 Some
lightweight aggregate (LWA) particles have hard shells while others
have surfaces with open voids. Aggregate used to cast lightweight
block is typically expanded shale, clay, or slate, but other materials
are also available. Information about structural lightweight aggregate
is available from the Expanded Shale, Clay, and Slate Institute.
Many LWA are strong enough for structural uses and lead to concrete
masonry units with compressive strength of 3000 psi (about 21 MPa)
or higher. At that strength level, the use of an appropriate mortar
will result in masonry construction with a compressive strength
of at least 2000 psi (about 14 MPa).
In addition to good compressive strength, lightweight block has
excellent fire resistance, acoustical properties, and thermal conductivity.
For additional information, refer to PCA’s Concrete
Masonry Handbook for Architects, Engineers, Builders (EB008)
and National Concrete Masonry Association’s TEK Sheet 2-6,
Density-Related Properties of Concrete Masonry Assemblies
at www.ncma.org
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