Title: Floor Cracking: How, What, Where?
1Floor Cracking How, What, Where?
- Fred Goodwin, FICRI
- Fellow Scientist
- BASF Construction Chemicals
- Beachwood OH
2Outline
- Early cracking
- Plastic shrinkage
- Reduce wind, raise humidity, lower temperatures
(concrete ambient) - Narrow with depth, go around aggregate
- Dampen base if no vapor retarder
- Avoid use of vapor retarder
- Moistue retaining coverings
- Postpone finishing steps
- Crazing
- Due to minor surface shrinkage / shallow mud
cracking - Cure immediately after finishing
- Curing water gt20F cooler than concrete
- Avoid alternate wetting / drying cycles
- Do not overuse consolidation or finishing
- Do not prematurely float of finish
- Do not dust with cement
- Dirty aggregates
- Blessing during finishing
- Settlement around reinforcement or embedment
- Tensile failure
- Restraint of Volume Change
- Internal
- External
- Factors
- Drying shrinkage
- Thermal contraction
- Rapid change is worse
- Curling
- Settlement of soil support system
- High clay or sulfate content in subgrade
- Applied loads
- Too early
- Impact
- Earth movements
- Prevention
- Joints
- Contraction joint Spacing
- Sawn Deep enough
3Outline
- How, Why, Where, and When Does Concrete Crack
- Tensile failure
- Restraint of internal and external volume changes
- Plastic Cracking
- Hardened Cracking
- Cracking Potential
- Deterioration Cracking
- Avoiding Cracking
- Crack Repair
4Does Concrete Crack?
Where
When
Why
How
YES!
?
5How does concrete crack?
The Simple Answer Is
The Tensile Strength is Exceeded
6 CRACKING TENDENCY
Stress (i.e.,Shrinkage)
Tensile Stress Capacity (i.e. Tensile Strength)
TENSILE STRESS
TENSILE STREGTH
Start of Crack Stress Strain Relief
TIME
7Why does concrete crack?
The Simple Answer Is
RESTRAINT
Internal Restraint
External Restraint
8Where does concrete crack?
The Simple Answer Is
Through the weakest part
Defects
Control or Contraction Joints If its gonna
crack, then at least we can compromise with the
concrete as to where (usually).
9Early 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
10Early 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
H2O
H2O
11Early Cracking
12Settlement 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.
13Settlement Shrinkage
- Areas of stress concentration are prone to
- Cracking
- Reentrant corners
- Sudden change in placement depth
Movement of Formwork
Movement of the Sub-grade
14Surrounding structures and conditions
From Structural Condition Assessment, Robert
Ratay, Wiley Sons, 2005
15Thermal Cracking
16Crazing Cracking
- Caused by Minor Surface Shrinkage
- Surface Effect Mostly Cosmetic
To Avoid Cure Immediately After Finishing Avoid
Water gt20F 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
17Hardened Cracking
- Drying shrinkage
- Curling
- Applied loads
- Too early
- Impact
- Earth movements
- Deterioration
Premature Loading
Drying Shrinkage
18Drying 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
19- Drying Shrinkage Cracking
20Reducing Drying Shrinkage Cracking
- Low Water to Cement Ratio
- Less Water to Evaporate, Usually Excess for
Hydration - OR ACTUALLY
- Less Paste (cementitious and water)
- Avoid
- 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
21Post- Tensioning Example
22Post Tensioning
Shrinkage Compensating Concrete
23Drying 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
24Drying Curling of Concrete Floor
Stage 1 Bleed water on surface evaporates
Stage 2 Water evaporates from pores refilled from
within concrete settlement
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
25Thickness Drying Factors 4 Thick 0.5 W/CM 64oF
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
26Avoid Restraint
- Subbase Friction or Unevenness
- Doweling
- Reentrant Corners
- Lack of / Or Improper Joints
External Restraint
Permaban Floor Solutions
27Avoid Restraint
WALLS
- Reinforcement Tie In to Columns, Walls, Etc.
COLUMNS
- Reinforcement Continuing Through Joints
Dissimilar Materials or Placement Sections
28 Reducing Drying Cracking
Tensile Capacity
TENSILE STRESS
NO Cracking if Shrinkage is Low Enough
TIME
29 Reducing Drying Cracking
Tensile Capacity
TENSILE STRESS
NO Cracking if Tensile Capacity is High Enough to
Overcome Shrinkage Stress
?
TIME
30 Reducing Drying Cracking
MODULUS EFFECTS
Modulus dy/dx slope in linear portion
High Modulus
TENSILE STRENGTH/Time
Low Modulus
TENSILE STRAIN/Time
31 Reducing Drying Cracking
Lower Modulus Shifts the Intersection of
Shrinkage Stress and Tensile Capacity Where
Cracking Occurs.
Modulus dy/dx slope in linear portion
High Modulus
TENSILE STRENGTH/Time
Low Modulus
Shrinkage stress
But a Low Modulus is Like Bubblegum
Crack Occurs
TENSILE STRAIN/Time
32 Reducing Drying Cracking
CREEP EFFECTS
Tensile Stress From Restrained Shrinkage
CREEP
TENSILE STRESS
Or at 10000F
INTERNAL ABSORPTION OF SHRINKAGE STRESS COLD
FLOW"
TIME
33 Combined Material Properties
If only we had a test method for all these
properties simultaneously.
Cracking Potential
34Volume Stability
ASTM C1581 Cracking Resistance
23 2 C (73.4 3 F) 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
v Shrinkage
Shrinkage Happens
v Tensile Strength
Compresses Steel Ring
Steel Ring Resists
Specimen Cracks
v Tensile Creep Tensile Modulus
35Ring Test Graph Example
36Ring Test Graph Example
37Volume Stability
ASTM C1581 Cracking Resistance
LOW Cracking Potential
Moderate Cracking Potential
HIGH Cracking Potential
38Deterioration
- Interior Restraint
- AAR
- Sulfate Expansion
- Reinforcement Corrosion
- F/T Cycle Deterioration
39AARAlkali Aggregate Reaction a.k.a ASR or ACR
Some aggregates react with alkali (Na, K)
causing expansion
Select non-reactive aggregates, low alkali
cement, mitigating admixtures
40(No Transcript)
41Sulfate 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
42Steel 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
Cracks
Corrosion
Steel
Concrete
43Air 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
44(No Transcript)
45Detecting 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
46Classification of Cracks
- Directional cracks indicate restraint
perpendicular to the crack direction - propagate from reentrant corners
- parallel companion cracks
- penetrations through the concrete
47Classification of Cracks
- Classified by direction, width depth
- Hexagonal pattern of short cracks -
- Surface had more restraint than the concrete
interior or substrate
48Active 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-in-place crack
gauges, optical comparators
http//www.avongard.com/whatisit.htm
49Crack Width
- Smaller cracks less problematic than wide
- Autogenous healing
- Requires moisture and continued cement hydration
- Aggregate Interlock
- Load transfer can occur at crack widths lt0.035
(0.89mm) PCA Concrete Floors on Ground - Bridging with elastomers
- Bridging and distribution with fibers
50Crack Repair Selection
- Purpose of the structure
- Active or dormant
- Structural or non-structural concrete
- Number of cracks
- Isolated crack or part of a pattern
- Crack depth
51Crack 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?
52Structural Crack Repair
- Repair the cause not the symptoms
- Structural integrity must be maintained!
- Anticipate crack propagation movement
- Expansion joints may be necessary
53Structural Crack Repair Techniques
- Epoxy Resin Injection
- Stitching Doweling
- Bandaging
- Post Tensioning
54Structural Repair with Epoxy Injection
http//www.concrete.org/COMMITTEES/CommitteeDocume
nts.Asp?Committee_Code000E706-00
55Epoxy 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
56Structural 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
57Steel Dowel Reinforcement
- Steel reinforcement bars or dowels are embedded
across crack
- Number and location as determined by engineering
requirements
58Cross-Stitching Method
- 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
59Bandaging
- 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/t
winseam_data.pdf
60Structural Strengthening with FRP
- Epoxy primer/putty/adhesive/fiber/adhesive/
topcoat composite - Carbon/Aramid/Glass Fibers
61Post Tensioning
- A compressive force is applied across the crack
using reinforcing tendons. - External
- Internal
- Bonded
- Unbonded
62Non-structural Repair
- Routing and Sealing
- Injection and Vacuum Sealant Application
- Gravity-Soak Technique
- Overlays and Toppings
- Hydraulic Cement Based Crack Repair
- Autogenous Healing
63Routing and Sealing
- Groove routed and filled with sealant
Crack
Crack routed
Sealant
64Routing 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
65Vacuum 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
66Gravity 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
67Healer 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.
67
04.11.2004
68 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.
68
04.11.2004
69 Crack Sealer Vacuum Injection
- Vacuum pump and plastic tube circuitry used to
inject resin into cable sheathing.
69
04.11.2004
70Outline
- How, Why, Where, and When Does Concrete Crack
- Tensile failure
- Plastic Cracking
- Hardened Cracking
- Cracking Potential
- Deterioration Cracking
- Avoiding Cracking
- Crack Repair
71(No Transcript)
72Questions?
?
THANK YOU !
Fred GoodwinFellow Scientist BASF Construction
Chemicals Beachwood, Ohio