Concrete

One Definition of Portland Cement Concrete…One Definition of Portland Cement Concrete…
 Portland cement concrete (PCC) is a heterogeneous system ofPortland cement concrete (PCC) is a heterogeneous system of
solid, discrete, gradiently sized, inorganic mineral aggregates,solid, discrete, gradiently sized, inorganic mineral aggregates,
usually plutonic or sedimentary-calcareous in origin, embeddedusually plutonic or sedimentary-calcareous in origin, embedded
in a matrix compounded of synthesized polybasic alkaline andin a matrix compounded of synthesized polybasic alkaline and
alkaloidal silicates held in aqueous solution and co-precipitatealkaloidal silicates held in aqueous solution and co-precipitate
dispersion with other amphoteric oxides, this matrix beingdispersion with other amphoteric oxides, this matrix being
originally capable of progressive dissolution, hydration, re-originally capable of progressive dissolution, hydration, re-
precipitation, gelation and solidification through a continuousprecipitation, gelation and solidification through a continuous
and co-existent series of crystalline, amorphous, colloidal andand co-existent series of crystalline, amorphous, colloidal and
cryptocrystalline states and ultimately subject to thermo-cryptocrystalline states and ultimately subject to thermo-
allotriomorphic alteration, the system when first conjoined beingallotriomorphic alteration, the system when first conjoined being
plastic during which stage it is impressed to a predeterminedplastic during which stage it is impressed to a predetermined
form into which it finally consolidates, thus providing a structureform into which it finally consolidates, thus providing a structure
relatively impermeable and with useful capacity to transmitrelatively impermeable and with useful capacity to transmit
tensile, compressive, and shear stresses.tensile, compressive, and shear stresses.
(source unknown)(source unknown)

A Real Definition of PCC…A Real Definition of PCC…
 A mixture of:A mixture of:
 Portland CementPortland Cement
 Fine AggregateFine Aggregate
 Coarse AggregateCoarse Aggregate
 WaterWater
 AirAir
 Cement and waterCement and water
combine, changing from acombine, changing from a
moist, plastic consistencymoist, plastic consistency
to a strong, durable rock-to a strong, durable rock-
like construction materiallike construction material
by means of a chemicalby means of a chemical
reaction called “hydration”reaction called “hydration”

Further Defined…Further Defined…
 Concrete exists in threeConcrete exists in three
statesstates
 PlasticPlastic
 CuringCuring
 HardenedHardened

Mix DesignMix Design
 Combination of materials to provide the mostCombination of materials to provide the most
economical mixture to meet the performanceeconomical mixture to meet the performance
characteristics suitable for the applicationcharacteristics suitable for the application
 Developed in laboratory - produced in a batchDeveloped in laboratory - produced in a batch
plantplant
 Mix proportions will typically vary over a rangeMix proportions will typically vary over a range
for a given jobfor a given job
 Required strength and exposure conditionsRequired strength and exposure conditions
 Mix consistency must be ensured to guaranteeMix consistency must be ensured to guarantee
concrete performanceconcrete performance

Mixture Design ConceptsMixture Design Concepts
 Cement contentCement content
 Sacks/ydSacks/yd33
or lbs/ydor lbs/yd33
 To a point, increasing cement contentTo a point, increasing cement content
increases strength and durabilityincreases strength and durability
 Too much cement is uneconomical andToo much cement is uneconomical and
potentially detrimentalpotentially detrimental
 Amount of waterAmount of water
 Proper selection of aggregate and gradingProper selection of aggregate and grading
 Admixtures?Admixtures?

Water-to-Cement RatioWater-to-Cement Ratio
 The ratio of water-to-cement, orThe ratio of water-to-cement, or w/cw/c, is the single, is the single
most important parameter with regards to concretemost important parameter with regards to concrete
qualityquality
 Theoretically, about 0.22 to 0.25 is required forTheoretically, about 0.22 to 0.25 is required for
complete hydrationcomplete hydration
 Practically, the useful limit is around 0.33Practically, the useful limit is around 0.33
 Reducing the water for a given amount of cementReducing the water for a given amount of cement
will move the cement particles closer together,will move the cement particles closer together,
which in turn densifies the hydrated cement pastewhich in turn densifies the hydrated cement paste
 This increases strength and reduces permeabilityThis increases strength and reduces permeability
 It also makes the concrete more difficult to workIt also makes the concrete more difficult to work
 In combination, theIn combination, the w/cw/c and degree of hydrationand degree of hydration
control many of the properties of the hardenedcontrol many of the properties of the hardened
concreteconcrete

Voids in Hydrated CementVoids in Hydrated Cement
 Concrete strength, durability, and volumeConcrete strength, durability, and volume
stability is greatly influenced by voids instability is greatly influenced by voids in
the hydrated cement pastethe hydrated cement paste
 Two types of voids are formed in hydratedTwo types of voids are formed in hydrated
cement pastecement paste
 Gel poresGel pores
 Capillary poresCapillary pores
 Concrete also commonly containsConcrete also commonly contains
entrained air and entrapped airentrained air and entrapped air

Voids in Hydrated CementVoids in Hydrated Cement
PastePaste
 Gel PoresGel Pores
 Space between layers in C-S-H with thicknessSpace between layers in C-S-H with thickness
between 0.5 and 2.5 nmbetween 0.5 and 2.5 nm
 Includes interlayer spaces, micropores, and smallIncludes interlayer spaces, micropores, and small
isolated capillary poresisolated capillary pores
 Can contribute 28% of paste porosityCan contribute 28% of paste porosity
 Little impact on strength and permeabilityLittle impact on strength and permeability
 Can influence shrinkage and creepCan influence shrinkage and creep

Voids in Hydrated Cement PasteVoids in Hydrated Cement Paste
 Capillary VoidsCapillary Voids
 Depend on initial separation of cementDepend on initial separation of cement
particles, which is controlled by theparticles, which is controlled by the w/cw/c
 It is estimated that 1 cmIt is estimated that 1 cm33
of anhydrous portland cementof anhydrous portland cement
requires 2 cmrequires 2 cm33
of space to accommodate the hydrationof space to accommodate the hydration
productsproducts
 Space not taken up by cement or hydration products isSpace not taken up by cement or hydration products is
capillary porositycapillary porosity
 On the order of 10 to 50 nm, although largerOn the order of 10 to 50 nm, although larger
for higherfor higher w/cw/c (3 to 5 mm)(3 to 5 mm)
 Larger voids affect strength and permeability,Larger voids affect strength and permeability,
whereas smaller voids impact shrinkagewhereas smaller voids impact shrinkage

Source: Mindess, Young, and Darwin, 2004
w/c = 0.5

Source: Mindess, Young, and Darwin, 2004

Source: Mindess, Young, and
Darwin, 2004

Capillary Pores
C-S-H
Framework
Neville
High PermeabilityHigh Permeability
(Capillary Pores Interconnected)(Capillary Pores Interconnected)

Capillary Pores
C-S-H
Framework
Low-Permeability Capillary PoresLow-Permeability Capillary Pores
Segmented and Only PartiallySegmented and Only Partially
ConnectedConnected

Dimensional Range of SolidsDimensional Range of Solids
and Voids in Hydrated Cementand Voids in Hydrated Cement
PastePaste
Source: Mehta and Monteiro, 1993

Interfacial Transition ZoneInterfacial Transition Zone
 Zone between the aggregate and bulk pasteZone between the aggregate and bulk paste
 Has a major impact on the strength and permeability of theHas a major impact on the strength and permeability of the
concreteconcrete
 The interfacial zone is 10 to 50 mm in thicknessThe interfacial zone is 10 to 50 mm in thickness
 Generally weaker than either the paste or aggregate due toGenerally weaker than either the paste or aggregate due to
locally highlocally high w/cw/c and the “wall effect” (packing problems) – inand the “wall effect” (packing problems) – in
some cases predominately large crystals of calcium hydroxidesome cases predominately large crystals of calcium hydroxide
and ettringite are oriented perpendicular to aggregate surfaceand ettringite are oriented perpendicular to aggregate surface
 Greater porosity and few unhydrated cement grainsGreater porosity and few unhydrated cement grains
 Microcracking commonly exists in transition zoneMicrocracking commonly exists in transition zone
 Results in shear-bond failure and interconnectedResults in shear-bond failure and interconnected
macroporosity, which influences permeabilitymacroporosity, which influences permeability
 Modification of transition zone is key to improving concreteModification of transition zone is key to improving concrete

Entrained AirEntrained Air
 Provides the path forProvides the path for
water to migrate fromwater to migrate from
larger voids to smallerlarger voids to smaller
voidsvoids
 Water in smallestWater in smallest
capillary/gel porescapillary/gel pores
won’t freezewon’t freeze
 For adequateFor adequate
protectionprotection
 6-8% air by volume6-8% air by volume
 Entrained air spacingEntrained air spacing
factor = 0.2mmfactor = 0.2mm

Entrained Air MeasurementEntrained Air Measurement
 Proper air entrainment is one ofProper air entrainment is one of
the most critical aspects ofthe most critical aspects of
producing durable concreteproducing durable concrete
 Air entrainment affectsAir entrainment affects
 StrengthStrength
 Freeze-Thaw durabilityFreeze-Thaw durability
 PermeabilityPermeability
 Scaling ResistanceScaling Resistance
 WorkabilityWorkability
 Air content must be measuredAir content must be measured
accurately at the job siteaccurately at the job site

Air-VoidAir-Void
SystemSystem
Stereo Microscope ASTM C 457Stereo Microscope ASTM C 457
ASTM C 231 and C 173ASTM C 231 and C 173

Curing ConcreteCuring Concrete
 Extremely importantExtremely important
 Concrete will not achieve its potential strengthConcrete will not achieve its potential strength
unless it is properly curedunless it is properly cured
 Concrete will crack if not properly curedConcrete will crack if not properly cured
 Curing should be started immediately afterCuring should be started immediately after
final setfinal set
 Curing includes providing both moistureCuring includes providing both moisture
and temperatureand temperature

CuringCuring
 Concrete must not dry out, especially at aConcrete must not dry out, especially at a
young ageyoung age
 Preferably water is applied after the concretePreferably water is applied after the concrete
has sethas set
 Steam curing applies both heat andSteam curing applies both heat and
moisture, accelerating hydrationmoisture, accelerating hydration
 Often, waterproof barriers are used to holdOften, waterproof barriers are used to hold
mix water in…not as good as wet curingmix water in…not as good as wet curing

DurabilityDurability
 Concrete is inherently durable, having aConcrete is inherently durable, having a
history of exceptional long-term performancehistory of exceptional long-term performance
 In some instances, the structure’s service lifeIn some instances, the structure’s service life
has been adversely affected by the concrete’shas been adversely affected by the concrete’s
inability to maintain its integrity in theinability to maintain its integrity in the
environment in which it was placedenvironment in which it was placed
 These distress manifestations areThese distress manifestations are
categorized as materials-related distresscategorized as materials-related distress
(MRD)(MRD)

What is Materials-RelatedWhat is Materials-Related
Distress?Distress?
 MRD is commonly associated with theMRD is commonly associated with the
“durability” of the concrete“durability” of the concrete
 Durability is not an intrinsic material propertyDurability is not an intrinsic material property
 ““Durability” cannot be measuredDurability” cannot be measured
 Concrete that is durable in one application mayConcrete that is durable in one application may
rapidly deteriorate if placed in another applicationrapidly deteriorate if placed in another application
 It is not related to loading, although loading canIt is not related to loading, although loading can
exacerbate the distressexacerbate the distress

Common Types of MRDCommon Types of MRD
 Physical MechanismsPhysical Mechanisms
 Freeze-thaw Deterioration of Hardened CementFreeze-thaw Deterioration of Hardened Cement
PastePaste
 Deicer Scaling/DeteriorationDeicer Scaling/Deterioration
 Freeze-Thaw Deterioration of AggregateFreeze-Thaw Deterioration of Aggregate
 Chemical MechanismsChemical Mechanisms
 Alkali-Aggregate ReactivityAlkali-Aggregate Reactivity
 Alkali-Silica and Alkali-Carbonate ReactivityAlkali-Silica and Alkali-Carbonate Reactivity
 Sulfate AttackSulfate Attack
 External and Internal Sulfate AttackExternal and Internal Sulfate Attack
 Corrosion of Embedded SteelCorrosion of Embedded Steel

Important ConsiderationsImportant Considerations
 The concrete constituents, proportions,The concrete constituents, proportions,
and construction all influence MRDand construction all influence MRD
 Water is needed for deleterious expansionWater is needed for deleterious expansion
to occurto occur
 Severe environments (e.g. freezing andSevere environments (e.g. freezing and
thawing, deicer applications, high sulfatethawing, deicer applications, high sulfate
soils, etc.) make it worsesoils, etc.) make it worse
 Strength does not equal durabilityStrength does not equal durability

SummarySummary
 Concrete is an immensely complex material thatConcrete is an immensely complex material that
will perform to its potential only if treatedwill perform to its potential only if treated
properly during the entire construction phaseproperly during the entire construction phase
 Mix design and proportioningMix design and proportioning
 TransportingTransporting
 Placing and consolidatingPlacing and consolidating
 Finishing and curingFinishing and curing
 As billions are spent annually on concreteAs billions are spent annually on concrete
construction, the most sophisticated testing isconstruction, the most sophisticated testing is
used to ensure qualityused to ensure quality

ASTM C 143-00 Standard Test Method for SlumpASTM C 143-00 Standard Test Method for Slump
of Hydraulic Cement Concreteof Hydraulic Cement Concrete

Concrete

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