ncma continuing education services crack control in concrete masonry walls program # -- 0503 -01
TRANSCRIPT
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CRACK CONTROL IN CONCRETE MASONRY WALLS
PROGRAM # -- 0503 -01
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NCMAContinuing Education Services 1. Excessive deflection
2. Structural Overload
Causes of Concrete Masonry Cracks
TEK 10-1A
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3. Differential settlement
Causes of Concrete Masonry Cracks
TEK 10-1A
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4. Shrinkage
TE
K 1
0-3
• Moisture change
•Temperature
• Carbonation
Time (cycles)
Len
gth
chan
ge
General trend = shrinkage
Causes of Concrete Masonry Cracks
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Shrinkage Cracking
Restraint at bottom only
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Restraint at top and bottom
Shrinkage Cracking
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Accommodating Movement
• Control Joints– Used in concrete masonry construction
• Expansion Joints– Used with clay brick
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Purpose of Control Joints
• Relieve horizontal tensile stresses• reduce restraint and permit longitudinal
movement• Separate dissimilar materials
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Goal of Crack Control Provisions
Restraint at top and bottom
Limit crack widths to less than 0.02 in (0.5 mm)• able to be bridged by most coatings• prevents water penetration T
EK
10-
3
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NCMAContinuing Education Services • Included in C 90
– Compressive Strength
– Absorption
– Dimensional Tolerances
– Density Definitions
– Linear Drying Shrinkage
Specifying Concrete Masonry Units
TEK 1-1C
C 90Standard Specification for Loadbearing Concrete Masonry Units
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• NOT Included in C 90– Color
– Texture
– Density
– Water Repellency
– Fire Ratings
– Thermal Properties
– Sound Properties
Specifying Concrete Masonry Units
TEK 1-1C
Most architectural concrete masonry units are custom made. Work closely with producers to get exactly what you want.
C 90Standard Specification for Loadbearing Concrete Masonry Units
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Factors Influencing Concrete Masonry Drying Shrinkage
Shrinkage increases …
• as unit cement content increases• as aggregate hardness decreases• with increasing unit moisture content
ASTM C 90 maximumallowable shrinkage = 0.065%
TEK 10-1A
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ASTM C 426 Drying Shrinkage Test
Dis
tanc
e be
twee
n ga
ge p
oint
s
10.000
9.995
100%
10%
CM
U m
oisture content
Measuredshrinkage
70%
30%
Typicalfield
shrinkage
TEK 10-3
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Type 1 and Type 2 CMU
3.1 Types - Two types of concrete masonry units are covered as follows:3.1.1 Type 1, Moisture-Controlled Units - Units designated as Type 1 shall conform to the requirements of this specification.3.1.2 Type 2, Nonmoisture-Controlled Units - Units designated as Type 2 shall conform to the requirements of this specification with the exception of Table 1.
This section removed.Does not appear in C90-00.
TEK 1-1C
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50 % 30 %Should concrete block be wetted?Can concrete block be wet-cut?
Moisture Content Appearance
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Control Joint Spacing Recommendations
1.5 h maximum
So, for h = 8 ft, spacing < 12 feetfor h = 20 ft, spacing < 30 feet
25 feet
25 feet maximum
TEK 10-2B
h
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1. Control joint spacings are based on the use of horizontal reinforcement having an equivalent area of no less than 0.025 in2 / ft of height to keep unplanned cracks closed.
Sum of steel areaheight
TEK 10-2B
Control Joint Spacing Recommendations
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TEK 10-2B
Control Joint Spacing Recommendations
1. Control joint spacings are based on the use of horizontal reinforcement having an equivalent area of no less than 0.025 in2 / ft of height.
9 gage joint reinforcementevery other course (16 in.)
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TEK 10-2B
Control Joint Spacing Recommendations
1. Control joint spacings are based on the use of horizontal reinforcement having an equivalent area of no less than 0.025 in2 / ft of height.
Or, by use of….
#3 bars at 48 in. (4 ft.)#4 bars at 96 in. (8 ft.)#5 bars at 144 in. (12 ft.)
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The Role of Reinforcement in Controlling Shrinkage Cracking
Without joint reinforcement
With joint reinforcement
Length after shrinkageL
Length after shrinkage
TEK 10-2B
L
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Joint Reinforcement
•Primary function - control wall cracking associated with shrinkage
•Secondary Functions– metal tie system for bonding– structural reinforcement
where allowed by code
TEK 12-2A
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Joint Reinforcement
TEK 12-2A
Consists of two or more longitudinal wires connected with cross wires forming a truss or ladder configuration
Ladder Type Truss Type
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• Must be spliced a minimum of 6 inches per MSJC
Joint Reinforcement
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Control Joints
Backer rod
Sealant- polysulfide, urethane, silicone or epoxy. Avoid oil-based caulks or other materials that dry out rapidly or do not bond effectively to masonry.
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Types of Control Jointswith Shear Resistance
• Preformed gasket
• Grout shear key
TEK 10-2B
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Types of Control Jointswith Shear Resistance
TE
K 1
0-2B
• Dowels
• Special unit shape
Smooth dowel - sleeved or greased on one end
Horizontal reinforcement discontinuous at joint (except for structural bond beams)
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Eliminating Control Joints With Reinforcement
Control joints can be eliminated completely if enough horizontal reinforcement is provided.
ie., where As > 0.002 An
Maximum reinforcement spacing for 8 in. CMU (not solid grouted):
#4 bar - 24 inches#5 bar - 40 inches#6 bar - 48 inches
TEK 10-3
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4-hour Rated Control Joints
Ceramic fire blanket
TEK 10-2B
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Control Joint Locations
• Typically use above grade in concrete masonry walls– Shrinkage cracks are an aesthetic rather than
a structural concern
• Generally not used below grade– Less temperature and moisture variation
TEK 10-2B
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Control Joint Locations
TE
K 1
0-2B
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Control Joints at Openings
Openings less than 6 feet
TEK 10-2B
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Control Joints at Openings
Openings more than 6 feet
TEK 10-2B
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Dissimilar Materials
Bond-breaker as a slip plane + 1/4 control joint spacing each side of control joint
TEK 10-1A
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Control Joints for Multi-wythe Walls
If wythes are unbonded...
…consider control joint requirements separately for each wythe
If wythes are bonded...
…continue control joints from back-up through faceTEK 10-1A
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Impact of Mortar Strength
Weak mortar
Strong mortar
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Summary
• Better manage movement through masonry systems by– Proper design
– Using MSJC installation standards
– Proper location of control joints
– proper use of bond beams
– Proper placement of joint reinforcement