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V. T. Cost Consulting, LLC
Control and Prevention of Cracking in Concrete Flatwork
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Tim Cost, PE, FACIV. T. Cost Consulting, LLC
69th Annual Concrete ConferenceDecember 5, 2019
Earle Brown Heritage Center,Brooklyn Center, MN
V. T. Cost Consulting, LLC
Owner expectations and cracking
My concrete should not crack. (unrealistic) Cracks should not be visible. Any cracks should not impair performance.
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The realities of cracking
By far the most common project complaint Frequent source of disputes and litigation Typical cracking is almost always avoidable!
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Discussion topics
• Review – basic causes of flatwork cracking• Cracking Influences, their relative significance, and
how they can be controlled or minimized Materials & mix designs Construction procedures Site conditions
• Best practices – cracking avoidance or management• Resources, suggested reading
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• It’s generally about volume change (shrinkage) vs. restraint, and the timing of related events Resulting tensile stresses that exceed strength Movements other than shrinkage can also be involved
(settlement, load-deflection, expansion, etc.)
• Restraint is usually from mechanical contact Also geometry / mass
• Timing factors: Time of setting Shrinkage onset and rate Rate of strength development
Why does cracking happen?
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There are many possible influences, but…Causes of shrinkage & influential factorsChemical shrinkageAutogenous shrinkageSubsidence & plastic shrinkageDrying shrinkagePlastic shrinkageCurling – differential shrinkageThermal changesLoad-related strain and deflectionCreepExpansion in aggressive environments Concrete water contentAggregate grading & sizeFine aggregate FM, impurities, PSDCement chemistry & finenessSCM chemistry, fineness, PSDAdmixturesEnvironmental conditionsDifferential temperature / moistureRapid cooling - unexpected rainfall
Sources of cracking-related restraintNormal subgrade dragContact with structuresSubgrade rutting / irregularitiesSteel reinforcementReinforcement / tiebars across a jointThickness variabilityPanel geometryDowel / tiebar details & placementDifferentials in temp across thicknessAngular granular subbasesRigid subbasesBond of overlays to base layersIneffective control joints
Late or shallow saw cuts Tooled joints too shallow
Adjacent panels placed previouslyPenetrations – manholes, inlets, etc.Rapid surface moisture loss
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• Influences on volume change Plastic shrinkage Drying shrinkage Thermal changes Environmental factors (evaporation rate) Curing effectiveness & timing Warping & curling influences
• Rate of strength gain• Construction or service issues
Restraint variables Jointing & reinforcement issues Excessive loads or fatigue Support settlement or erosion
These are the most common & significant
…and these influences can usually be managed!7
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• Shrinkage occurs as excess mix water evaporates
• Usually the most significant category of volume change
• Higher water content = greater drying shrinkage
• Both the amount and timing of evaporation influence crackingErika E. Holt, “Where Did These Cracks Come
From?”, Concrete International, Sept. 2000
Fundamental concrete volume changes
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After drying shrinkage, thermal and other volume changes generally cannot restore concrete’s original plastic volume
Fundamental concrete volume changes
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Stress from restrained shrinkage vs. strength
Cracking occurs when stresses from restrained shrinkage exceed the concrete’s tensile strength at that time.
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Cracks develop when stress from restrained shrinkage exceeds tensile strength (both change over time)
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• Various successful strategies for reduced cracking involve moderating or delaying shrinkage Lower paste content mixes Lower hydration heat Lower cementitious content Highly controlled curing Evaporation controls Shrinkage compensating
additives or cements
• High potential for benefits at reasonable costs
Reduction of total shrinkage
Delaying onset of shrinkage
Minimizing cracking – reduced or delayed shrinkage
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Concrete water content vs. ultimate shrinkage
Directly related!13
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• Aggregate top size• Combined aggregate grading• Fineness of sand and impurities
Fineness modulus Clay content
• Cementitious materials• Temperature
Concrete mix properties and “water demand”
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-500
-450
-400
-350
-300
-250
-200
-150
-100
-50
0
0 20 40 60 80 100 120Age [days]
Shrin
kage
Defo
rmati
on [x
10-6
]
CIP Mix #1
CIP Mix #2
CIP Mix #3
28% paste
32% paste
36% paste
Mixture paste content and shrinkage
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Paste content – inversely related to max agg size
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Aggregate grading effects on mixture paste content
Ideal combined grading
Total Aggregate Gradation
0%
5%
10%
15%
20%
25%
30%
1.5"
1.0"
3/4"
1/2"
3/8"
No.4 No.8No.1
6No.3
0No.5
0No.1
00 Pan
Change Limits
Typical “real world” grading
“Gap graded” aggregates (right) generally increase paste requirements and water demand
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Aggregate grading effects on mixture paste content
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Aggregate grading effects on mixture paste content
• Various graphical and other quick evaluation tools are used to optimize combined grading: 8-18 rule Workability-coarseness 0.45 power chart Mortar fraction
• These have become somewhat controversial –reported impacts on mix shrinkage vary.
• Experience with specific materials is advised.
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Mix design shrinkage limits
• Specified shrinkage limits for the submitted concrete mix design(s)
• Testing via ASTM C157 length change test• Also ASTM C1581 restrained shrinkage ring test
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• Concrete sets while hot and is expanded – then it shrinks• Temperature peaks within the first 12 hours• Air temperature often drops at the same• Combined affect can be significant• All while concrete
is very weak
Thermal shrinkage influences
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Limiting thermal shrinkage and gradients
• Control initial concrete temperature• Limit cement content• Use fly ash and/or slag cement• Protect concrete from thermal shock
Time placements to stagger ambient temps and hydration heat peaks
Use insulation and/or active cooling in extreme conditions
• Mass concrete protocols & controls
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Airport apron paving with cracking from sudden thermal shock (afternoon rain storm)
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Colorado highway paving project
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Subgrade restraint variables
• Granular materials vs. fine grained soils
• Vapor barriers & slip sheets• Subgrade surface influences
Wheel ruts Grade beams Integral footings, other structural
features• Inconsistent compaction• Bond to rigid subbases
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Evaporation rate influences
One of the most critical and elusive variables affecting cracking behavior (!)
Drives drying shrinkage rate and ultimate shrinkage
May cause plastic shrinkage
Critical factors: Wind Relative humidity Differential temps
Threshold of plastic shrinkage cracking
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• Cracks appear during finishing• Surface evaporation exceeds the
concrete’s bleeding rate• Cracks often parallel, shallow,
discontinuous• Causes: excessive evaporative
influences (wind, low humidity, extreme thermal differentials)
• More common where there is no protection from surface winds
• Excessive drying shrinkage also more likely
Plastic shrinkage cracking
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• Surface evaporation influences: Wind direction & surface exposure to wind Concrete / air temperature differentials Humidity
• Bleeding influences: Mix water content, paste factor Admixtures & proportions Concrete set time & temperature Fine particle content (microsilica, etc.) Reinforcement (fibers) Dry subgrade? Vapor barrier?
ASTM C 232 bleed test
Plastic shrinkage cracking – influences
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Beware of Wind• Wind results in significantly
greater drying shrinkage, both short and long term
• Affects both plastic and drying shrinkage and cracking
• Calls for extreme and immediate curing measures!
Figures: Erika E. Holt, “Where Did These Cracks Come From?”, Concrete International, Sept. 2000
Wind @ 2.5 m/s = 5.5 mph
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Severe PSC devastated this otherwise successful paving project
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Concrete placement for a convenience store parking lot on a windy day
4/10/2009 - 12:30 pm
Plastic shrinkage cracking is predictable!
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4/10/2009 - 5:00 pm
Plastic shrinkage cracking is predictable!
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V. T. Cost Consulting, LLC
4/10/2009 - 5:00 pm
Plastic shrinkage cracking is predictable!
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Weather APP for smart phones (weatherapp.us)
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Hand-held evaporation rate meter
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EvapoRATE by Eric Anderson, PE, KDOT Bridge Section
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Fogging a bridge deck placement during finishing
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CURING
The most overlooked tool for control of cracking!
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• Curing is anything done to maintain saturation
• Curing delays shrinkage• Must begin immediately
after finishing• During PSC conditions
something must also be done during finishing
• Rate of application of curing compound must be ≥ that called for by manufacturer!!
Delaying onset of shrinkage
Curing cannot be over-emphasized…
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Dry Wet
• Can result from moisture gradients created by surface drying while the slab bottom remains wet
• Can also result from differential temperature
Curling / warping of slabs
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• Place concrete on “normally dry” subgrades
• Thicker slabs• Shorter joint spacings• Internal curing, enhanced
long-duration curing
Measures for reducing curling / warping
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Some specifications call for the use of a granular fill placed over the vapor retarder, serving as a “blotter” to equalize moisture loss from slab top and bottom. This must be carefully monitored!
Vapor retarders can worsen the problem
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• Distributed steel reinforcement is not needed or recommended in most types of flatwork, and can actually cause more cracking
• The more steel and the longer the panel, the more cracks• In typical use, steel should be cut at all joints• Example – consider continuously reinforced pavement:
CRC is heavily reinforced, without joints, and develops stable transverse cracks every 3-6 feet.
Effects of steel reinforcement on cracking
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• Slow strength gain can be a cracking problem without effective curing Low temperatures Low cementitious content High SCM content Retarding admixtures
• Immediate, extended curing is critical for delay of shrinkage
Influence of rate of strength gain
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Unfortunately one of the most common causes of cracked flatwork
Poor subgrade support
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Uncontrolled cracking of flatwork is too often caused by poor jointing!
Objectives of jointing:• Control the location of expected
(normal) cracks• Provide constructability• Provide necessary load transfer at
all joints and cracks• Assure that random (unexpected)
cracks pose no performance problems
Jointing of flatwork for control of cracking
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Sawed joints must be made within 4-12 hours after final finishing
This joint was sawedsoon enough
This one was sawed too late
Timing of joint sawing
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Control joint
Construction joint
Isolation joint
• Cracking can be caused by using the wrong joint type or detail
• Most common mistakes: Keyways (not recommended) Isolation joints as regularly
spaced joints Staggered joints Isolation joints in high load
areas, no load transfer Reinforcement through joints Joints not connected to
structure or blockout corners Acute angles at slab edges
Joint type selection and proper detailing
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Keyways are not recommended as a joint detail for slab thickness < 11”!
Examples – poor joint details, related cracking
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• Monofilament or fibrillated polypropylene, polymers, steel, other synthetics, blends
• Various engineering properties achievable• Reduced cracking possible (especially PSC but also drying)
Fibers as secondary reinforcement
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• Shrinkage reducing admixtures Reduce rate and ultimate
drying shrinkage by lowering water surface tension & capillary tension
• Expansive cements and additives Type K cements Other expansive mix
additives Calcium aluminate systems
Special admixtures and cement types
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• Internal curing – replacing a portion of the mixture fine aggregate with lightweight aggregate (LWA) fines Usually around 15% of the FA depending on absorption, etc.
• LWA is porous, providing internal curing water storage Absorbed moisture within LWA is released over time for enhanced
curing Especially helpful for high performance concrete that is nearly
impermeable to externally applied curing moisture
• Shown to reduce or eliminate autogenous shrinkage, reduce drying shrinkage, increase strength development
Internal curing using LWA fines
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http://ciks.cbt.nist.gov/~bentz/ICnomographEnglishunits.pdf
Internal curing implementation guidelines (NIST)
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Hiperpav – cracking prediction software
www.hiperpav.com
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Pre-construction conference
• Cracking avoidance is only one of many project elements that can benefit
• Helps to define and communicate all project responsibilities
• Improves project quality • Saves time and money
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• Consider likely influences• Minimize mix shrinkage potential
Cement content & properties Water content Aggregates size, grading, fines content
• Minimize subgrade restraint, curling influences• Evaluate evaporation rate & mitigate if extreme• Immediate and effective curing• Cure longer for slow strength gain mixtures, cool
weather or high evaporative conditions• Proper jointing• Consider available special tools & techniques
Summary – management of cracking
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Suggested ReadingShort Articles
Holt, Erika E., “Where Did These Cracks Come From?,” Concrete International, Sept., 2000, pp. 57-60.
Hover, Kenneth C., “Avoiding Injury in Cold Weather – for Humans and for Recently-Cast Concrete,” Concrete International, Nov., 2002, pp. 31-36.
“Plastic Shrinkage Cracking”, CIP-5, Concrete in Practice Series, National Ready-Mixed Concrete Association, Silver Spring, MD, 1998, 2pp.
Tarr, Scott, “Demystifying Concrete Shrinkage – Achieving Better Slabs by Understanding a Mix’s Shrinkage Potential,” Concrete Construction Magazine, Sept. 2012, 4 pp.
Uno, Paul J., “Plastic Shrinkage Cracking and Evaporation Formulas,” ACI Materials Journal, Title no. 95-M34, July-August 1998, pp 365-375.
Industry Technical References
ACI Committee 305, “Guide to Hot Weather Concreting (ACI 305R-10),” American Concrete Institute, Farmington Hills, MI, 2010, 23 pp.
ACI Committee 306, “Cold Weather Concreting (ACI 306R-10)”, American Concrete Institute, Farmington Hills, MI, 2010, 26 pp.
ACI Committee 308, “Guide to Curing Concrete (ACI 308R-01, reapproved 2008),” American Concrete Institute, Farmington Hills, MI, 2001, 30 pp.
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Questions?
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Tim Cost, PE, FACIV. T. Cost Consulting, LLC
69th Annual Concrete ConferenceDecember 5, 2019
Earle Brown Heritage Center,Brooklyn Center, MN