Page 102 - Design of Reinforced Masonry Structures

2.54 CHAPTER TWO

Prefabricated masonry panels are produced to conform to ASTM C901-01: Specification
for Prefabricated Panels. Prefabrication construction is covered by MSJC-08 Specification
for Masonry Structures [2.4].
Prefabricated masonry panels offer several advantages over conventional or in-place
masonry construction. A major advantage of prefabrication is the quality control that can
be exercised in a manufacturing plant to produce quality products. Since panels are made
indoors, they can be fabricated 24 h a day if necessary and in any kind of weather. Another
big advantage is that scaffolding is virtually eliminated since panels are installed by crane.
Panelization on some projects may save construction time. For some projects, it is possible
to build masonry panels as early as the groundbreaking for the project, thus keeping far
ahead of the in-place construction work to permit panel erection when needed.
As with any other construction methodology, prefabrication has some disadvantages as
well. The use of prefabricated panels is limited to certain types of construction. The use
of prefabricated masonry is limited primarily by transportation and erection limitations.
Architectural plan layout may, in some cases, preclude the use of prefabricated panels.
Absence of adjustment capabilities during the construction process is another disadvantage
of prefabricated masonry panels. In-place masonry construction allows the masons to build
masonry to fit the other elements of the structure by adjusting the joint thicknesses over a
large area so that it is not noticeable. This is not possible with prefabricated elements.

2.14 AUTOCLAVED AERATED CONCRETE

Autoclaved aerated concrete (AAC) is defined as low-density cementitious product cal-
cium silicate hydrates, whose material specifications are defined in ASTM C1386 [2.3,
2.4]. Also called autoclaved aerated concrete or autoclaved cellular concrete, AAC is an
innovative system of concrete blocks that are not molded. Unlike clay brick and concrete
cellular blocks used for reinforced masonry construction, AAC units are solid blocks, but
are much lighter than the conventional concrete masonry units. An AAC block weighs
2
about half a normal-weight concrete masonry unit—20 lb per ft versus 37 lb in an 8-in.-
thick wall. However, the unit-by-unit comparison shows that one standard AAC block
2
(8 in. wide × 10 in. high × 25 in. long) weighs 27 lb and creates 1.3 ft of wall area while
one standard concrete masonry unit block (8 × 8 × 16 in.) weighs 34 lb and creates 0.88 ft 2
of wall area. The AAC blocks are light because of their high air content—80 percent of
their volume.
A description of manufacturing and using the AAC blocks for buildings can be found in
Ref. [2.41]. AAC is made by mixing portland cement, silica-bearing sand, and water slurry
with a finely powdered aluminum. Some of the plants under construction will substitute
fly ash or mine tailings for sand. The aluminum powder reacts with other ingredients to

release millions of small gas bubbles causing concrete to expand to 5 times its original
volume. After the concrete sets initially, it is cut to size and then cooked in an autoclave to
fully complete the curing process. Because the autoclave completes the hydration process,
the AAC blocks do not experience the long term shrinkage problem associated with the
conventional concrete masonry units.
Design of AAC masonry is covered in Appendix A of the MSJC-08 Code. In normal
applications with AAC blocks, joint reinforcing is not required. AAC units are fully bedded
when laid, so they distribute load over a full block face to create a very strong joint. This
also eliminates problems of tension cracks and water penetration associated with joints
when conventional concrete masonry unit are used. U-shaped units are available where
bond beams are required. AAC can be easily cut with simple hand and power tools. The
air cells in the AAC block make it a good thermal insulator. According to Ref. [2.42],