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Concrete

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Title: Concrete


1
Concrete
Prepaid by 130670106098
130670106099
2
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)

3
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

4
Further Defined
  • Concrete exists in three states
  • Plastic
  • Curing
  • Hardened

5
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

6
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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8
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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10
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

11
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

12
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

13
w/c 0.5
Source Mindess, Young, and Darwin, 2004
14
Source Mindess, Young, and Darwin, 2004
15
Source Mindess, Young, and Darwin, 2004
16
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17
High Permeability(Capillary Pores Interconnected)
Capillary Pores
C-S-H Framework
Neville
18
Low-Permeability Capillary Pores Segmented and
Only Partially Connected
Capillary Pores
C-S-H Framework
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21
Dimensional Range of Solids and Voids in Hydrated
Cement Paste
Source Mehta and Monteiro, 1993
22
Source Mindess, Young, and Darwin, 2004
23
Source Mindess, Young, and Darwin, 2004
24
Source Mindess, Young, and Darwin, 2004
25
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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28
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

29
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

30
Air-Void System
ASTM C 231 and C 173
  • Stereo Microscope ASTM C 457
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