ml12153a412
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logyand
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Non-structural
Concre
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Concre
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Letter Type of Cracking SubdivisionMost Common
Location
Primary Cause
(excludingrestraint)
Secondary
Causes/Factors
Time of
Appearance
A Over reinforcement Deep sections
B Arching Top of columns
C Change of depthTrough and waffle
slab
D Diagonal Roads and slabs
E RandomReinforced concrete
slabs
F Over reinforcementReinforced concrete
slabs
Ditto plus steel near
surface
G External restraint Thick wallsExcess heat
Excess bleedingRapid early drying
conditions
Plastic shrinkage
Plastic settlement
Rapid early drying
Low rate of bleeding
Ten minutes to
three hours
Thirty minutes to six
hours
H Internal restraint Thick slabsExcess temperature
gradients
I Long-term drying shrinkageThin slabs (and
walls)Inefficient joints
Excessive shrinkage
inefficient curing
Several weeks or
months
J Against formwork"Fair faced"
concrete
Impermeable
formworkRich mixes
K Floated concrete Slabs Over troweling Poor curing
L NaturalColumns and
beamsLack of cover
M Calcium chloride Precast concreteExcess calcium
chloride
I Alkali-aggregate reaction Damp locations
Reactive aggregate
plus high-alkali
cement
More than five years
Crazing
Corrosion of reinforcement
Early thermal contraction Rapid cooling
Poor quality
concreteMore than two years
One to seven days,
sometimes much
later
One day or two or
three weeks
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Why does concrete shrink?
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Non-structural Cracks
Fresh
Settlement
Plastic shrinkage
Drying shrinkage
Thermal dilation
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Before Hardening
Early frost damage
Plastic
Shrinkage
Construction movement
Formwork
Sub-grade
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PLASTIC SHRINKAGE CRACKING
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Typical Plastic ShrinkageCracking
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Evaporation of SurfaceMoisture from Concrete
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logyan
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Codes
logyan
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Plastic Settlement
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Subsidence Cracking
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Resistance to subsidence bytop reinforcement
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Cracks usually appear alongreinforcement bars
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..but can also form fromdifferential settlement
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Resistance to subsidence byvoid tubes in hollow core
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After Hardening Physical
Shrinkable aggregates
Drying shrinkage
Chemical
Corrosion of rebar
Alkali aggregate reaction
Carbonation
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After Hardening Thermal
Freeze thaw cycles
External seasonal temperature variations
External restraint
Internal temperature gradients
Structural
Accidental overload
Creep
Design loads
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logyan
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Codes
logyan
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Codes
Drying Shrinkage
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Why cracks form
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Shrinkage and Cracking
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Drying Shrinkage Cracks Occur: In thin sections (low V/S ratio)
When: Several weeks after casting
Causes:
High paste content
Inefficient joints
Poor curing
Remedies:
Reduce water content of mix
Improve curing
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Cracking tendency dependson many factors
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Shrinkage is Size Dependent
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Theoretical ShrinkageStresses
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Mitigation of DryingShrinkage Cracks Aggregate
Content Size
Workability
Curing
Eliminate externalrestraints by allowing
joint movement
Ease of placement
Consolidation
Admixtures
Chemical
Mineral
Prov e crac contro
steel distribution
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The role of fibers Fibers will:
Reduce plastic shrinkage cracking
Reduce bleeding
Fibers can:
Bridge cracks
Distribute stresses and limit crack widths
Volume is critical!
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Tensile DeformationPlain Concrete
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Tensile DeformationSteel Reinforced Concrete
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Tensile Deformation
Fiber Reinforced Concrete
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logyan
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Codes
logyan
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Thermal Cracking
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ConcreteTechn
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Internal Thermal Restraint
HOT CENTERHOT CENTER
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Temperature Rise and
Cement Type
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ACI 224
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ACI 224
Tolerable Crack Widths
Exposure conditionTolerable crack
width, in.
Dry air or protective membrane
Humidity, moist air, soil
Deicing chemicals
Seawater and seawater spray;
wetting and drying
Water-retaining structures
0.016
0.012
0.007
0.006
0.004
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Crack Control
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Control of Cracking Joints
Isolation Contraction
Reinforcement
Cover
Size of crack
Frequency (numerous tight cracks)
Whats allowable (crack width)
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Take an
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Take an
example Sidewalk set
above pavement 500 ft long
avement stri s
= 6x10-6 in/in/F Approximately
0.7/100/100F
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Result Omission of full
depth isolation/expansion joint
Ex ansion of
adjacentpavement results
in cracking and
buckling of
concrete sidewalk
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Mid-Panel Cracking
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Crack Repair Fix large cracks prior to exposure
Use: MMA
Sealers
Routing and sealing
Effect of cover
Importance of curing on cracking
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Summary Many forms of cracking
Concrete is weak in tension Shrinkage!
o n ng
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