Concrete One Definition of Portland Cement Concrete Portland

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, reprecipitation, gelation and solidification through a continuous and co-existent series of crystalline, amorphous, colloidal and cryptocrystalline states and ultimately subject to thermoallotriomorphic 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 PCC… § A mixture of: § Portland Cement § Fine Aggregate § Coarse Aggregate § Water § Air § Cement and water combine, changing from a moist, plastic consistency to a strong, durable rocklike construction material by means of a chemical reaction called “hydration”

Further Defined… § Concrete exists in three states § § § Plastic Curing Hardened

Mix Design § Combination of materials to provide the most economical mixture to meet the performance characteristics suitable for the application § Developed in laboratory - produced in a batch plant § Mix proportions will typically vary over a range for a given job § Required strength and exposure conditions § Mix consistency must be ensured to guarantee concrete performance

Mixture Design Concepts § Cement content § Sacks/yd 3 or lbs/yd 3 § To a point, increasing cement content increases strength and durability § Too much cement is uneconomical and potentially detrimental § Amount of water § Proper selection of aggregate and grading § Admixtures?

Water-to-Cement Ratio § The ratio of water-to-cement, or w/c, is the single most important parameter with regards to concrete quality § Theoretically, about 0. 22 to 0. 25 is required for complete hydration § Practically, the useful limit is around 0. 33 § Reducing the water for a given amount of cement will move the cement particles closer together, which in turn densifies the hydrated cement paste § This increases strength and reduces permeability § It also makes the concrete more difficult to work § In combination, the w/c and degree of hydration control many of the properties of the hardened concrete

Voids in Hydrated Cement § Concrete strength, durability, and volume stability is greatly influenced by voids in the hydrated cement paste § Two types of voids are formed in hydrated cement paste § Gel pores § Capillary pores § Concrete also commonly contains entrained air and entrapped air

Voids in Hydrated Cement Paste § Gel Pores § Space between layers in C-S-H with thickness between 0. 5 and 2. 5 nm § Includes interlayer spaces, micropores, and small isolated capillary pores § Can contribute 28% of paste porosity § Little impact on strength and permeability § Can influence shrinkage and creep

Voids in Hydrated Cement Paste § Capillary Voids § Depend on initial separation of cement particles, which is controlled by the w/c § It is estimated that 1 cm 3 of anhydrous portland cement requires 2 cm 3 of space to accommodate the hydration products § Space 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

w/c = 0. 5 Source: Mindess, Young, and Darwin, 2004

Source: Mindess, Young, and Darwin, 2004

Source: Mindess, Young, and Darwin, 2004

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

Low-Permeability Capillary Pores Segmented and Only Partially Connected Capillary Pores C-S-H Framework

Dimensional Range of Solids and Voids in Hydrated Cement Paste 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 Zone § Zone between the aggregate and bulk paste § Has a major impact on the strength and permeability of the concrete § The 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 surface § § § Greater porosity and few unhydrated cement grains Microcracking commonly exists in transition zone Results in shear-bond failure and interconnected macroporosity, which influences permeability § Modification of transition zone is key to improving concrete

Entrained Air § Provides the path for water to migrate from larger voids to smaller voids § Water in smallest capillary/gel pores won’t freeze § For adequate protection § 6 -8% air by volume § Entrained air spacing factor = 0. 2 mm

Entrained Air Measurement § Proper air entrainment is one of the most critical aspects of producing durable concrete § Air entrainment affects § § § Strength Freeze-Thaw durability Permeability Scaling Resistance Workability § Air content must be measured accurately at the job site

Air-Void System Stereo Microscope ASTM C 457 ASTM C 231 and C 173

Curing Concrete § Extremely important § Concrete will not achieve its potential strength unless it is properly cured § Concrete will crack if not properly cured § Curing should be started immediately after final set § Curing includes providing both moisture and temperature

Curing § Concrete must not dry out, especially at a young age § Preferably water is applied after the concrete has set § Steam curing applies both heat and moisture, accelerating hydration § Often, waterproof barriers are used to hold mix water in…not as good as wet curing

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

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

Common Types of MRD § Physical Mechanisms § Freeze-thaw Deterioration of Hardened Cement Paste § Deicer Scaling/Deterioration § Freeze-Thaw Deterioration of Aggregate § Chemical Mechanisms § Alkali-Aggregate Reactivity § Alkali-Silica and Alkali-Carbonate Reactivity § Sulfate Attack § External and Internal Sulfate Attack § Corrosion of Embedded Steel

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

Summary § Concrete is an immensely complex material that will perform to its potential only if treated properly during the entire construction phase § § Mix design and proportioning Transporting Placing and consolidating Finishing and curing § As 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