a220 week2 lecture web cmu

8/2/2019 A220 Week2 Lecture Web CMU

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Concrete Masonry Unit (CMU) Construction

. General: Concrete block, concrete brick. As specified in ASTM C90.

. Virtues (advantages) of CMU buildings:

a. Durable - These buildings will endure the test of time. b. Self contained - CMU building materials can act as the structure, walls, foundation and other components of

the building.c. Fire resistant - Suitable for the most stringent fire ratings.d. Local Labor - Practically any contractor is capable of building with CMU.e. Attractive - Huge variety of available textures, patterns, etc.f. Low maintenance - Build it and forget about it.

3. Possible disadvantages of CMU buildings:

a. Expensive labor - CMU construction is labor-intensive. Depending on localities, labor CAN be very expensive. b. Heavy - Masonry buildings weigh more than comparable steel-framed and wood-framed buildings.c. Absorbent - CMU, like any other cementitious material is absorbent to water penetration and must be weather-

proofed.d. 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.e. 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.

4. Types (shapes) of CMU Block:

a. Stretcher block

b. Header blockc. Corner blockd. Control joint blocke. Bond beam block

of 8Concrete Masonry Unit (CMU) Construction

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f. Split-face blockg. Split-ribbed blockh. Many, many more…

5. Grades of CMU Block:

a. Grade "N" - Suitable for use above or below ground and exposed to weather. b. Grade "S" - Only for above ground, not exposed to weather.

6. CMU Block Modular Sizes:

a. HEIGHT - Nominal 8" high (actual = 7 5/8") b. LENGTH - Nominal 16" long (actual = 15 5/8")c. WIDTH - Nominal 4", 6", 8", 10", 12" (actual = nominal - 3/8") The nominal 8" wide CMU block is most

common.

7. CMU Block Terminology:





a. Block course - Horizontal "layer" of blocks. 3 blocks stacked on top of each other = 3 couses. b. Wythe - Vertical "layer" of masonry. A 2 whythe wall may consist of an 8" CMU block wall tied to a wall

of face brick.c. Block (and brick) positions:

d. Typical types of block (or brick) bonds:





a.e. Typical mortar joint types:

8. Mortar and Grout:

a) Mortar - used to bond masonry products together. Composed of portland cement, sand, lime andwater. 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. Specifiedas either fine-grained or coarse-grained. Conforms to ASTM C476.

9. Expansion & Control Joints:

a. Purpose - to allow movement in masonry walls due to thermal stresses, soil settlement, external (wind or seismic) forces, moisture absorption, etc.

b. Expansion Joint - Used within straight lengths of the same wall. Examples:





c. Control Joint - Used to isolate one major building component with another (example - new buildingabutting an existing building). Examples:





d. Joint construction examples:

1. Sash block with compressible preformed gasket2. Control joint block with preformed gasket & sealer

0. CMU Unit Ties:

a. Purpose - Tie the masonry back to the building structure and to tie multi-whythe walls together. Ties usually sitin horizontal mortar joint.

b. Typical types of unit ties:

i. Corrugated metal flat tieii. Adjustable channel slot with dove-tail anchors (tying to structural steel)

iii. Flat strap anchor iv. Weld-on "Z" anchor clip

1. Horizontal (Joint) CMU Wall Reinforcement:

a.





i. Purposes:To strengthen the wall against "bowing" in due to lateral pressure (earth, wind, seismic)ii. To make the wall more ductile (i.e., less brittle) and hold it together in extreme events such as

earthquake or hurricane.

b. Horizontal joint reinforcement consists of heavy wire welded together to take the shape of a ladder (or russ), and is usually selected as follows:

i. 10 Gage wire - for light duty interior or exterior applicationsii. 9 Gage wire - standard duty

iii. 8 Gage - heavy duty for use in seismic or other high-stress applicationsiv. 3/16" diameter wire - extra heavy duty for extreme conditions

c. Horizontal joint reinforcement placed in horizontal mortar joints as follows:

i. Placed in every CMU course if used for foundation wallii. Placed every 2 or 3 courses for above-ground walls (or more if necessary)

2. Vertical CMU Wall Reinforcement:

a. Purpose - Greatly strengthen the wall to accommodate larger vertical loads as well as resist lateral loads. b. 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.





ast updated: December 2, 2008


a220 week2 lecture web cmu

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