Concrete

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Concrete
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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)

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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
  rock-like construction material by means of a
  chemical reaction called hydration

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Further Defined

• Concrete exists in three states
• Plastic
• Curing
• Hardened

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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

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Mixture Design Concepts

• Cement content
• Sacks/yd3 or lbs/yd3
• 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?

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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

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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

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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

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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 cm3 of anhydrous portland
  cement requires 2 cm3 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

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w/c 0.5
Source Mindess, Young, and Darwin, 2004
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Source Mindess, Young, and Darwin, 2004
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Source Mindess, Young, and Darwin, 2004
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High Permeability(Capillary Pores Interconnected)
Capillary Pores
C-S-H Framework
Neville
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Low-Permeability Capillary Pores Segmented and
Only Partially Connected
Capillary Pores
C-S-H Framework
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Dimensional Range of Solids and Voids in Hydrated
Cement Paste
Source Mehta and Monteiro, 1993
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Source Mindess, Young, and Darwin, 2004
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Source Mindess, Young, and Darwin, 2004
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Source Mindess, Young, and Darwin, 2004
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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

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Entrained Air

• Provides the path for water to migrate from
  larger voids to smaller voids
• Water in smallest capillary/gel pores wont
  freeze
• For adequate protection
• 6-8 air by volume
• Entrained air spacing factor 0.2mm

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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

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Air-Void System
ASTM C 231 and C 173

• Stereo Microscope ASTM C 457

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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

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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 innot as good as wet curing

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Durability

• Concrete is inherently durable, having a history
  of exceptional long-term performance
• In 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 placed
• These distress manifestations are categorized as
  materials-related distress (MRD)

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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

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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

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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

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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

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