Concrete

One Definition of Portland Cement Concrete

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

A Real Definition of PCCA mixture of:Portland CementFine AggregateCoarse AggregateWaterAirCement and water combine, changing from a moist, plastic consistency to a strong, durable rock-like construction material by means of a chemical reaction called hydration

Further DefinedConcrete exists in three statesPlasticCuringHardened

Mix DesignCombination of materials to provide the most economical mixture to meet the performance characteristics suitable for the applicationDeveloped in laboratory - produced in a batch plantMix proportions will typically vary over a range for a given jobRequired strength and exposure conditionsMix consistency must be ensured to guarantee concrete performance

Mixture Design ConceptsCement contentSacks/yd3 or lbs/yd3To a point, increasing cement content increases strength and durabilityToo much cement is uneconomical and potentially detrimentalAmount of waterProper selection of aggregate and gradingAdmixtures?

Water-to-Cement RatioThe ratio of water-to-cement, or w/c, is the single most important parameter with regards to concrete qualityTheoretically, about 0.22 to 0.25 is required for complete hydrationPractically, the useful limit is around 0.33Reducing the water for a given amount of cement will move the cement particles closer together, which in turn densifies the hydrated cement pasteThis increases strength and reduces permeabilityIt also makes the concrete more difficult to workIn combination, the w/c and degree of hydration control many of the properties of the hardened concrete

Voids in Hydrated CementConcrete strength, durability, and volume stability is greatly influenced by voids in the hydrated cement pasteTwo types of voids are formed in hydrated cement pasteGel poresCapillary poresConcrete also commonly contains entrained air and entrapped air

Voids in Hydrated Cement PasteGel PoresSpace between layers in C-S-H with thickness between 0.5 and 2.5 nmIncludes interlayer spaces, micropores, and small isolated capillary poresCan contribute 28% of paste porosityLittle impact on strength and permeabilityCan influence shrinkage and creep

Voids in Hydrated Cement PasteCapillary VoidsDepend on initial separation of cement particles, which is controlled by the w/cIt is estimated that 1 cm3 of anhydrous portland cement requires 2 cm3 of space to accommodate the hydration productsSpace not taken up by cement or hydration products is capillary porosity On the order of 10 to 50 nm, although larger for higher w/c (3 to 5 mm)Larger voids affect strength and permeability, whereas smaller voids impact shrinkage

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

Source: Mindess, Young, and Darwin, 2004

Source: Mindess, Young, and Darwin, 2004

High Permeability(Capillary Pores Interconnected)Capillary PoresC-S-HFrameworkNeville

Low-Permeability Capillary Pores Segmented and Only Partially ConnectedCapillary PoresC-S-HFramework

Dimensional Range of Solids and Voids in Hydrated Cement PasteSource: Mehta and Monteiro, 1993

Source: Mindess, Young, and Darwin, 2004

Source: Mindess, Young, and Darwin, 2004

Source: Mindess, Young, and Darwin, 2004

Interfacial Transition ZoneZone between the aggregate and bulk pasteHas a major impact on the strength and permeability of the concreteThe interfacial zone is 10 to 50 mm in thickness Generally weaker than either the paste or aggregate due to locally high w/c and the wall effect (packing problems) in some cases predominately large crystals of calcium hydroxide and ettringite are oriented perpendicular to aggregate surfaceGreater porosity and few unhydrated cement grainsMicrocracking commonly exists in transition zoneResults in shear-bond failure and interconnected macroporosity, which influences permeabilityModification of transition zone is key to improving concrete

Entrained AirProvides the path for water to migrate from larger voids to smaller voidsWater in smallest capillary/gel pores wont freezeFor adequate protection6-8% air by volumeEntrained air spacing factor = 0.2mm

Entrained Air MeasurementProper air entrainment is one of the most critical aspects of producing durable concreteAir entrainment affectsStrengthFreeze-Thaw durabilityPermeabilityScaling ResistanceWorkabilityAir content must be measured accurately at the job site

Air-Void SystemStereo Microscope ASTM C 457ASTM C 231 and C 173

Curing ConcreteExtremely importantConcrete will not achieve its potential strength unless it is properly curedConcrete will crack if not properly curedCuring should be started immediately after final setCuring includes providing both moisture and temperature

CuringConcrete must not dry out, especially at a young agePreferably water is applied after the concrete has setSteam curing applies both heat and moisture, accelerating hydrationOften, waterproof barriers are used to hold mix water innot as good as wet curing

DurabilityConcrete is inherently durable, having a history of exceptional long-term performanceIn some instances, the structures service life has been adversely affected by the concretes inability to maintain its integrity in the environment in which it was placedThese distress manifestations are categorized as materials-related distress (MRD)

What is Materials-Related Distress?MRD is commonly associated with the durability of the concreteDurability is not an intrinsic material propertyDurability cannot be measuredConcrete that is durable in one application may rapidly deteriorate if placed in another applicationIt is not related to loading, although loading can exacerbate the distress

Common Types of MRDPhysical MechanismsFreeze-thaw Deterioration of Hardened Cement PasteDeicer Scaling/DeteriorationFreeze-Thaw Deterioration of AggregateChemical MechanismsAlkali-Aggregate ReactivityAlkali-Silica and Alkali-Carbonate ReactivitySulfate AttackExternal and Internal Sulfate AttackCorrosion of Embedded Steel

Important ConsiderationsThe concrete constituents, proportions, and construction all influence MRDWater is needed for deleterious expansion to occurSevere environments (e.g. freezing and thawing, deicer applications, high sulfate soils, etc.) make it worseStrength does not equal durability

SummaryConcrete is an immensely complex material that will perform to its potential only if treated properly during the entire construction phaseMix design and proportioningTransportingPlacing and consolidatingFinishing and curingAs billions are spent annually on concrete construction, the most sophisticated testing is used to ensure quality

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