Transcript Floor Cracking: How, What, Where?

Floor Cracking:
How, What, Where?
Fred Goodwin, FICRI
Fellow Scientist
BASF Construction Chemicals
Beachwood OH
Outline
How, Why, Where, and When Does Concrete
Crack
• Tensile failure
– Restraint of internal and external volume changes
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Plastic Cracking
Hardened Cracking
Cracking Potential
Deterioration Cracking
Avoiding Cracking
Crack Repair
Does Concrete Crack?
How does concrete crack?
The Simple Answer Is:
CRACKING TENDENCY
Stress (i.e.,Shrinkage)
Tensile Stress Capacity
(i.e. Tensile Strength)
Start of Crack =
Stress + Strain
Relief
TIME
Why does concrete crack?
The Simple Answer Is:
Internal
Restraint
External
Restraint
Where does concrete crack?
The Simple Answer Is:
PORES
Micro
CRACKS
Defects
Control or Contraction Joints:
If it’s gonna crack, then at least we can
compromise with the concrete as to
where (usually).
VOIDS
TRANSITION
ZONES
Early Cracks Caused by
• Setting shrinkage
– Plastic shrinkage
– Drying shrinkage
• Construction movement
– Sub grade movement
– Form movement or premature form removal
• Settlement
– Such as when rebar too close to surface
Early Cracking
Dampen Base if No Vapor Retarder
Avoid Use of Under Slab Vapor Retarder
Use Moisture Retaining Coverings/Evaporation Retarders
Wind, Sun, Temperature, RH, Mix Design
Postpone Finishing Steps
• Plastic Shrinkage
H 2O
Early Cracking
• Plastic Shrinkage
Settlement Shrinkage
• Occurs within the concrete paste itself as
air voids collapse and aggregates wet out
• Cracks may form
over areas of
restraint (i.e., rebar)
• Settlement may
also create pockets
under rebar and
aggegates.
Settlement Shrinkage
Areas of stress concentration are prone to
Cracking
Movement of
Formwork
• Reentrant corners
• Sudden change in placement depth
Movementofofthe
the Sub-grade
Settlement
Surrounding
structures and
conditions
From Structural Condition Assessment, Robert Ratay, Wiley & Sons, 2005
Thermal Cracking
Crazing Cracking
•Caused by Minor Surface Shrinkage
•Surface Effect Mostly Cosmetic
To Avoid:
Cure Immediately After Finishing
Avoid Water >20F Cooler Than
Slab
Avoid Wetting/Drying Cycles
Do Not Over-Consolidate
Do Not Over-Finish
Do Not Dust With Cement
Do Not Finish With Water
Use Clean Aggregates
Avoid Excessive Fines
Hardened Cracking
• Drying shrinkage
• Curling
• Applied loads
– Too early
– Impact
– Earth movements
• Deterioration
Premature
Loading
Drying Shrinkage
Drying Shrinkage
• Decrease in volume due to the loss of free
moisture from concrete through evaporation
• Stresses caused by volume differences from
variations in moisture loss and restraint
Drying Shrinkage Cracking:
Reducing Drying Shrinkage Cracking
• Low Water to Cement Ratio
– Less Water to Evaporate, Usually Excess for Hydration
OR ACTUALLY
– Less Paste (cementitious and water)
• Avoid:
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Restraint
High Early Mixes,
High Cement Fineness,
High Cement Factors
High Alkali Cement
Dirty & high fines in aggregate
• Use Shrinkage Reducing Admixtures
• Slow & Thorough Curing
– Controlled Uniform Water Evaporation
Two Methods for NO DRYING SHRINKAGE CRACKING
• Place Underwater or Keep Wet Forever
– No Drying = No Drying Shrinkage
• Post Tensioning and Shrinkage Compensating Concrete
– Always Under Compression
PostTensioning
Example
Post
Tensioning
Shrinkage Compensating Concrete
Drying Shrinkage
Drying of 4” Slabs to MVTR = 3 Lb/1000 sq. ft.
Drying from ONE side
Bottom side moist
Drying from TWO sides
No external
humidity
Higher
W/C dry
slower.
If bottom of slab is wet,
harder to dry.
Kanare, H. Concrete Floors & Moisture, Eng. Bulletin #119 PCA/NRMCA, 2005
Drying & Curling of Concrete Floor
Drying Rate
→
Stage 1 Bleed water on surface evaporates
Stage 2 Water evaporates from pores refilled
from within concrete = settlement
Time→
Stage 3 Water evaporates from within as vapor
= drying
Stage 4 Top drys & shrinks more than bottom
Curling occurs lifting edges of slab.
Cracking as slab no longer supported by subbase
Thickness Drying Factors
64oF
4” Thick 0.5 W/CM
RH 60%
2 weeks rain, 2 weeks moist
Dry to 90% RH
Two Side Drying
Thickness
4” = 1
6” = Twice as Long
7” = 2 ½ Times as Long
8” = 2.8 Times Longer than 4”
10” = 3 ½ Times Longer
Thinner Sections Dry Faster than Thicker
Swedish Concrete Association, 1997
Avoid Restraint
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Subbase Friction or Unevenness
Doweling
Reentrant Corners
Lack of / Or Improper Joints
Recommended layout
External Restraint
Permaban Floor
Solutions
Avoid Restraint
COLUMNS
•Reinforcement Tie In to Columns, Walls, Etc.
•Reinforcement Continuing Through Joints
Dissimilar Materials or
Placement Sections
Reducing Drying Cracking
Shrinkage
Tensile Capacity
NO Cracking if Shrinkage is Low Enough
TIME
Reducing Drying Cracking
Shrinkage
Tensile Capacity
NO Cracking if Tensile Capacity is
High Enough to Overcome Shrinkage
Stress
Extremely
Strong
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TIME
Reducing Drying Cracking
MODULUS EFFECTS
Modulus = dy/dx= slope in linear portion
TENSILE STRAIN/Time
Reducing Drying Cracking
Lower Modulus Shifts the Intersection of
Shrinkage Stress and Tensile Capacity
Where Cracking
ModulusOccurs.
= dy/dx= slope in linear portion
Shrinkage stress
Crack Occurs
But a Low Modulus
is Like “Bubblegum”
TENSILE STRAIN/Time
Reducing Drying Cracking
CREEP EFFECTS
Tensile Stress From
Restrained Shrinkage
CREEP
Or at 10000F
INTERNAL
ABSORPTION OF
SHRINKAGE STRESS
= “COLD FLOW"
TIME
Combined Material Properties
If only
Modulus
we had a test method
Tensile
Strength
Cracking
Potential
Shrinkage
Tensile
Creep
for all these properties
simultaneously.
Volume Stability
ASTM C1581
Cracking 23 ± 2 °C (73.4 ± 3 °F)
Resistance
50 ± 4% RH
Steel Ring & Strain
Gauges
Inner and Outer
Steel Ring for Mold
Cast Repair Donut
Strip off Outer Steel
Ring
Wax Top Surface
√ Shrinkage
Shrinkage Happens
√ Tensile
Compresses Steel Ring
Strength
Steel Ring Resists
√ Tensile Creep & Specimen Cracks
Tensile Modulus
®
Mast er Bui lders
Technologies
Ring Test Graph Example
Ring Test Graph Example
Volume Stability
ASTM C1581 Cracking
Resistance
LOW Cracking Potential
Moderate Cracking
Potential
HIGH Cracking Potential
Deterioration
• Interior Restraint
– AAR
– Sulfate Expansion
– Reinforcement Corrosion
– F/T Cycle Deterioration
AAR=Alkali Aggregate Reaction
a.k.a ASR or ACR
Some aggregates react with
alkali
(Na, K) causing expansion
Reacting Aggregate
Select non-reactive
aggregates, low
alkali cement,
mitigating
admixtures
Sulfate Attack
• Sulfates react with aluminates in the
cement to form ettringite
• Some shrinkage
compensating
concretes use the
same reaction
• Use sulfate resistant
cements and pozzolan
admixtures
Steel Reinforcement Corrosion
• The carbonation reaction lowers the pH
• If pH of concrete surrounding steel falls
below 8.5, corrosion will occur
• Cl- ion accelerates corrosion
• Steel must be properly embedded
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Cl
Corrosion
Cracks
Corrosion
No
Corrosion
Steel
O2
Concrete
Air Entraining Agents
• Provide small, correctly sized & uniformly
distributed air bubbles that provide the
freezing water a place to expand into.
Frost damage, concrete not air entrained
Air entrained concrete
Detecting Cracks
• Visually – dampening substrate helps
• Magnification
• Pulse velocity devices – measure cracks’
effect of the velocity of sound waves
• Impact echo – short duration pulse is
reflected by a flaw
Classification of Cracks
Directional cracks indicate restraint
perpendicular to the crack direction
– propagate from reentrant corners
– parallel companion cracks
– penetrations through the concrete
Classification of Cracks
• Classified by direction, width & depth
• Hexagonal pattern of short cracks Surface had more restraint than the
concrete interior or substrate
Active and Dormant Cracks
• Active cracks continue to grow after the
concrete has hardened.
• Dormant cracks remain unchanged
– Plastic cracks
– Cracks formed by temporary overloading of
the concrete
• Crack movement monitored by glued-inplace crack gauges, optical comparators
http://www.avongard.com/whatisit.htm
Crack Width
• Smaller cracks less problematic than wide
– Autogenous healing
• Requires moisture and continued cement hydration
– Aggregate Interlock
• Load transfer can occur at crack widths <0.035” (0.89mm)
[PCA Concrete Floors on Ground]
– Bridging with elastomers
– Bridging and distribution with fibers
Crack Repair Selection
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Purpose of the structure
Active or dormant
Structural or non-structural concrete
Number of cracks
Isolated crack or part of a pattern
Crack depth
Crack Repair Selection
• Location of the crack
– On the surface, underneath, or near a joint
• Crack orientation relative to the structure
– transverse or longitudinal
• Is weather resistance required?
• Is chemical resistance required?
• Must the repair be waterproof?
Structural Crack Repair
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Repair the cause not the symptoms
Structural integrity must be maintained!
Anticipate crack propagation & movement
Expansion joints may be necessary
Structural Crack Repair Techniques
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Epoxy Resin Injection
Stitching & Doweling
Bandaging
Post Tensioning
Structural Repair with Epoxy Injection
Cracks must be clean
and free from debris
Install entry ports
Install cap seal
Continue injection Remove cap seal &
Start injection at
ports
until refusal
widest segment of
crack
http://www.concrete.org/COMMITTEES/CommitteeDocuments.Asp?Committee_Code=000E706-00
Epoxy Resin Injection
• ASTM C 881 2-K epoxy
injected through plugs
• Excellent cohesive
strength
• Not successful if
movement occurs
• Not practical if cracks
are wet or too numerous
Crack filled using epoxy
injection process
Structural Repair with Stitching &
Doweling
• Steel reinforcement to restore strength
• Metal staples or ‘stitching dogs’ across
cracks, legs anchored in epoxy-filled holes
• Number, size & spacing of staples
determined by necessities of tensile
strength restoration
• Cracks will occur elsewhere if movement
continues
Steel Dowel Reinforcement
• Steel reinforcement bars or dowels are
embedded across crack
• Number and
location as
determined by
engineering
requirements
Cross-Stitching Method
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Holes drilled ~35o angles through the crack
Steel bars embedded into holes with epoxy.
Used in roadways and airport runways
No Joint Movement
– Similar to cracking pattern of misaligned dowels
Bandaging
• Surface seal or bandage is used when the
crack will remain active
• Flexible strip placed across crack with edges
attached
• Wearing course or aggregate broadcast in
traffic areas
• Movement in more than one plane
http://www.wbacorp.com/downloads/DataSheets/Arch/twinseam_data.pdf
Structural Strengthening with FRP
• Epoxy primer/putty/adhesive/fiber/adhesive/
topcoat composite
• Carbon/Aramid/Glass Fibers
Post Tensioning
• A compressive force is applied across the
crack using reinforcing tendons.
• External
• Internal
• Bonded
• Unbonded
Non-structural Repair
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Routing and Sealing
Injection and Vacuum Sealant Application
Gravity-Soak Technique
Overlays and Toppings
Hydraulic Cement Based Crack Repair
Autogenous Healing
Routing and Sealing
• Groove routed and filled with sealant
Crack
Crack routed
Sealant
Routing and Sealing
• Not dynamic cracks – Epoxy compounds
• Active cracks – Elastomeric polysulphide
& polyurethane sealants
• Flexible sealant repair should use bond
breaker at bottom of routed groove
Routed and sealed crack
Bond breaker, backer rod
Vacuum Sealant Application
• Vacuum pulled through ports, pulls sealant
into concrete
• Viscosity of sealant depends on cracks
– Microcracks require low viscosity
– Gel or foam required for larger cracks
• Higher pressure injection allows deeper
penetration but can widen cracks
Gravity Soak
• Polymers applied onto horizontal surface
• Squeegeed on, allowed to soak in
• Easier and cheaper than injection and
vacuum, but limited depth of penetration
• Epoxy, MMA, HMWM, & urethane used
• Unsuitable if crack runs to underside
Healer Sealer Application
• Crack Sealer poured onto concrete
• Workers moved material around deck with solvent
resistant rollers on extension polls.
• This material applied at ~100 square feet per gallon.
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Crack Sealer
Crack pre-treatment
•Surface preparation removes contaminants that inhibit penetration
•Also exposes additional cracks that were not previously visible.
Resin is mixed & poured into crack
Distributed by brush or roller.
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Crack Sealer
Vacuum Injection
• Vacuum pump and plastic tube
circuitry used to inject resin into
cable sheathing.
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Outline
How, Why, Where, and When Does
Concrete Crack
• Tensile failure
– Plastic Cracking
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Hardened Cracking
Cracking Potential
Deterioration Cracking
Avoiding Cracking
Crack Repair
?
Questions?
THANK
YOU !
Fred Goodwin
Fellow Scientist
BASF Construction Chemicals
Beachwood, Ohio