Unit (CMU) Construction
Concrete block, concrete brick.
As specified in ASTM C90.
2. Virtues (advantages)
of CMU buildings:
3. Possible disadvantages
of CMU buildings:
Durable - These buildings will endure the
test of time.
Self contained - CMU building materials can
act as the structure, walls, foundation and other components of the building.
Fire resistant - Suitable for the most stringent
Local Labor - Practically any contractor is
capable of building with CMU.
Attractive - Huge variety of available textures,
Low maintenance - Build it and forget about
4. Types (shapes)
of CMU Block:
Expensive labor - CMU construction is labor-intensive.
Depending on localities, labor CAN be very expensive.
Heavy - Masonry buildings weigh more than
comparable steel-framed and wood-framed buildings.
Absorbent - CMU, like any other cementitious
material is absorbent to water penetration and must be weather-proofed.
Modular - Typical CMU has modular 8" x 8"
x 16" nominal dimensions, and is a bit difficult to have walls that have
odd dimensions or smooth curves.
Difficult to insulate - Block has a very low
"R" value and generally, walls must be insulated by adding width to them
- decreasing available floor square footage.
Control joint block
Bond beam block
Many, many more…
5. Grades of CMU
6. CMU Block Modular
Grade "N" - Suitable for use above or below
ground and exposed to weather.
Grade "S" - Only for above ground, not exposed
HEIGHT - Nominal 8" high (actual = 7 5/8")
LENGTH - Nominal 16" long (actual = 15 5/8")
WIDTH - Nominal 4", 6", 8", 10", 12" (actual
= nominal - 3/8") The nominal 8" wide CMU block is most common.
7. CMU Block Terminology:
Block course - Horizontal "layer" of blocks.
3 blocks stacked on top of each other = 3 couses.
Wythe - Vertical "layer" of masonry. A 2
whythe wall may consist of an 8" CMU block wall tied to a wall of face
Block (and brick) positions:
Typical types of block (or brick) bonds:
Typical mortar joint types:
8. Mortar and Grout:
a) Mortar - used to bond masonry products
together. Composed of portland cement, sand, lime and water.
Conforms to ASTM C270. Types M and S are used for exterior use, Types S
or N used for interior load-bearing walls. Type O used for
non-load-bearing interior walls.
b) Grout - similar to mortar, except used
as a filler, especially for vertically-reinforced walls. Specified as
either fine-grained or coarse-grained. Conforms to ASTM C476.
& Control Joints:
Purpose - to allow movement in masonry walls
due to thermal stresses, soil settlement, external (wind or seismic) forces,
moisture absorption, etc.
Expansion Joint - Used within straight lengths of
the same wall. Examples:
Control Joint - Used to isolate one major building
component with another (example - new building abutting an existing building).
d. Joint construction examples:
1. Sash block with compressible preformed gasket
10. CMU Unit Ties:
2. Control joint block with preformed gasket & sealer
Purpose - Tie the masonry back to the building
structure and to tie multi-whythe walls together. Ties usually sit in horizontal
Typical types of unit ties:
11. Horizontal (Joint)
CMU Wall Reinforcement:
Corrugated metal flat tie
Adjustable channel slot with dove-tail anchors
(tying to structural steel)
Flat strap anchor
Weld-on "Z" anchor clip
b. Horizontal joint reinforcement consists of heavy wire welded together
to take the shape of a ladder (or truss), and is usually selected as follows:
Purposes:To strengthen the wall against "bowing" in
due to lateral pressure (earth, wind, seismic)
make the wall more ductile (i.e., less brittle) and hold it together in
extreme events such as earthquake or hurricane.
c. Horizontal joint reinforcement placed in horizontal mortar joints
10 Gage wire - for light duty interior or
9 Gage wire - standard duty
8 Gage - heavy duty for use in seismic or
other high-stress applications
3/16" diameter wire - extra heavy duty for
12. Vertical CMU
Placed in every CMU course if used for foundation
Placed every 2 or 3 courses for above-ground
walls (or more if necessary)
Purpose - Greatly strengthen the wall to accommodate
larger vertical loads as well as resist lateral loads.
Vertical CMU wall reinforcement consists of
inserting steel rebar (usually #4 or #5 rebar) into open cores of the wall,
then filling those cores solid with a concrete-like grout.
Last updated: December 2, 2008