ConcreteConcrete

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One Definition of Portland Cement Concrete…One Definition of Portland Cement Concrete…

Portland cement concrete (PCC) is a heterogeneous system of Portland 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, embedded usually plutonic or sedimentary-calcareous in origin, embedded in a matrix compounded of synthesized polybasic alkaline and in a matrix compounded of synthesized polybasic alkaline and alkaloidal silicates held in aqueous solution and co-precipitate alkaloidal silicates held in aqueous solution and co-precipitate dispersion with other amphoteric oxides, this matrix being dispersion 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 continuous precipitation, gelation and solidification through a continuous and co-existent series of crystalline, amorphous, colloidal and and 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 being allotriomorphic alteration, the system when first conjoined being plastic during which stage it is impressed to a predetermined plastic during which stage it is impressed to a predetermined form into which it finally consolidates, thus providing a structure form into which it finally consolidates, thus providing a structure relatively impermeable and with useful capacity to transmit relatively 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 water Cement and water combine, changing from a combine, changing from a moist, plastic consistency moist, plastic consistency to a strong, durable rock-to a strong, durable rock-like construction material like construction material by means of a chemical by means of a chemical reaction called “hydration”reaction called “hydration”

Further Defined…Further Defined…

Concrete exists in three statesConcrete exists in three states PlasticPlastic CuringCuring HardenedHardened

Mix DesignMix Design

Combination of materials to provide the most Combination of materials to provide the most economical mixture to meet the performance economical mixture to meet the performance characteristics suitable for the applicationcharacteristics suitable for the application

Developed in laboratory - produced in a batch Developed in laboratory - produced in a batch plantplant

Mix proportions will typically vary over a range Mix 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 guarantee Mix consistency must be ensured to guarantee concrete performanceconcrete performance

Mixture Design ConceptsMixture Design Concepts

Cement contentCement content Sacks/ydSacks/yd33 or lbs/yd or lbs/yd33

To a point, increasing cement content To a point, increasing cement content increases strength and durabilityincreases strength and durability

Too much cement is uneconomical and Too 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, or The ratio of water-to-cement, or w/cw/c, is the single , is the single

most important parameter with regards to concrete most important parameter with regards to concrete qualityquality

Theoretically, about 0.22 to 0.25 is required for Theoretically, 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 cement Reducing 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, the In combination, the w/cw/c and degree of hydration and degree of hydration control many of the properties of the hardened control many of the properties of the hardened concrete concrete

Voids in Hydrated CementVoids in Hydrated Cement

Concrete strength, durability, and volume Concrete strength, durability, and volume stability is greatly influenced by voids in stability is greatly influenced by voids in the hydrated cement pastethe hydrated cement paste

Two types of voids are formed in hydrated Two types of voids are formed in hydrated cement pastecement paste Gel poresGel pores Capillary poresCapillary pores

Concrete also commonly contains Concrete also commonly contains entrained air and entrapped airentrained air and entrapped air

Voids in Hydrated Cement Voids in Hydrated Cement PastePaste

Gel PoresGel Pores Space between layers in C-S-H with thickness Space 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 small Includes 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 cement Depend on initial separation of cement

particles, which is controlled by the particles, which is controlled by the w/cw/c It is estimated that 1 cmIt is estimated that 1 cm33 of anhydrous portland cement of anhydrous portland cement

requires 2 cmrequires 2 cm33 of space to accommodate the hydration of space to accommodate the hydration productsproducts

Space not taken up by cement or hydration products is Space not taken up by cement or hydration products is capillary porositycapillary porosity

On the order of 10 to 50 nm, although larger On the order of 10 to 50 nm, although larger for higher for 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-HFramework

Neville

High PermeabilityHigh Permeability(Capillary Pores Interconnected)(Capillary Pores Interconnected)

Capillary Pores

C-S-HFramework

Low-Permeability Capillary Pores Low-Permeability Capillary Pores Segmented and Only Partially Segmented and Only Partially

ConnectedConnected

Dimensional Range of Solids Dimensional Range of Solids and Voids in Hydrated Cement and Voids in Hydrated Cement

PastePaste

Source: Mehta and Monteiro, 1993

Source: Mindess, Young, and Darwin, 2004

Source: Mindess, Young, and Darwin, 2004

Source: Mindess, Young, and Darwin, 2004

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 the Has a major impact on the strength and permeability of the concreteconcrete

The interfacial zone is 10 to 50 mm in thickness The interfacial zone is 10 to 50 mm in thickness

Generally weaker than either the paste or aggregate due to Generally weaker than either the paste or aggregate due to locally high locally high w/cw/c and the “wall effect” (packing problems) – in and the “wall effect” (packing problems) – in some cases predominately large crystals of calcium hydroxide some 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 interconnected Results in shear-bond failure and interconnected macroporosity, which influences permeabilitymacroporosity, which influences permeability

Modification of transition zone is key to improving concrete Modification of transition zone is key to improving concrete

Entrained AirEntrained Air Provides the path for Provides the path for

water to migrate from water to migrate from larger voids to smaller larger voids to smaller voidsvoids

Water in smallest Water in smallest capillary/gel pores won’t capillary/gel pores won’t freezefreeze

For adequate protectionFor adequate protection 6-8% air by volume6-8% air by volume Entrained air spacing Entrained air spacing

factor = 0.2mm factor = 0.2mm

Entrained Air MeasurementEntrained Air Measurement Proper air entrainment is one of Proper air entrainment is one of

the most critical aspects of the 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 measured Air content must be measured accurately at the job siteaccurately at the job site

Air-Void Air-Void SystemSystem

Stereo Microscope ASTM C 457Stereo Microscope ASTM C 457

ASTM C 231 and ASTM C 231 and C 173C 173