The Structure of Concrete

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The Structure of Concrete

• DEFINITIONS
• SIGNIFICANCE
• COMPLEXITIES
• STRUCTURE OF THE AGGREGATE PHASE
• STRUCTURE OF HYDRATED CEMENT PASTE
• TRANSITION ZONE IN CONCRETE
• Siddharth shankar

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DEFINITIONS

• The type, amount, size, shape, and distribution
  of phases present in a solid constitute its
  structure.
• The gross elements of the structure of a material
  can readily be seen, whereas the finer elements
  are usually resolved with the help of a
  microscope.
• The term macrostructure is generally used for the
  gross structure, visible to the human eye.
• The limit of resolution of the unaided human eye
  is approximately one-fifth of a millimiter (200
  µm).
• The term microstructure is used for the
  microscopically magnified portion of a
  macrostructure.

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Modern electron microscopes

• The magnification capability of modern electron
  microscopes is of the order of 105 times thus
  the application of transmission and scanning
  electron microscopy techniques has made it
  possible to resolve the structure of materials to
  a fraction of a micrometer.

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SIGNIFICANCE

• Progress in the field of materials has resulted
  primarily from recognition of the principle that
  the properties of a material originate from its
  internal structure.
• The properties can be modified by making
  suitable changes in the structure of a material.

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COMPLEXITIES

• From examination of a cross section of concrete,
  the two phases that can easily be distinguished
  are aggregate particles of varying size and shape
  and the binding medium, composed of an incoherent
  mass of the hydrated cement paste (henceforth
  abbreviated hcp).

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Macroscopic level

• At the macroscopic level, therefore, concrete may
  be considered to be a two-phase material,
  consisting of aggregate particles dispersed in a
  matrix of the cement paste.

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Microscopic level

• At the microscopic level, the complexities of the
  concrete structure begin to show up. It becomes
  obvious that the two phases of the structure are
  neither homogeneously distributed with respect to
  each other, nor are they themselves homogeneous.
• For instance, in some areas the hcp mass appears
  to be as dense as the aggregate while in others
  it is highly porous.

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Three-phase theory

• Three phases
• Aggregate
• Hardened cement paste (hcp)
• Transition (interface) zone??

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THREE PHASES OF CONCRETE
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STRUCTURE OF THE AGGREGATE PHASE
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AGGREGATE PHASE

• The aggregate phase is predominantly responsible
  for the unit weight, elastic modulus, and
  dimensional stability of concrete.
• These properties of concrete depend to a large
  extent on the bulk density and strength of the
  aggregate, which, in turn, are determined by the
  physical rather than chemical characteristics of
  the aggregate structure.
• In other words, the chemical or mineralogical
  composition of the solid phases in aggregate is
  usually less important than the physical
  characteristics such as the volume, size, and
  distribution of pores.

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Aggregate particles

• Natural gravel has a rounded shape and a smooth
  surface texture.
• Crushed rocks have a rough texture depending on
  the rock type and the choice of crushing
  equipment, the crushed aggregate may contain a
  considerable proportion of fault or elongated
  particles, which adversely affect many properties
  of concrete.
• Lightweight aggregate particles from pumice,
  which is highly cellular, are also angular and
  have a rough texture, but those from expanded
  clay or shale are generally rounded and smooth.

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STRUCTURE OF HYDRATED CEMENT PASTE

• Anhydrous portland cement is a gray powder that
  consists of angular particles typically in the
  size range 1 to 50 µm.
• It is produced by pulverizing a clinker with a
  small amount of calcium sulfate, the clinker
  being a heterogeneous mixture of several minerals
  produced by high temperature reactions between
  calcium oxide and silica, alumina, and iron
  oxide.

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STRUCTURE OF HYDRATED CEMENT PASTE

• The chemical composition of the principal clinker
  minerals coresponds approximately to C3S, C2S,
  C3A, and C4AF in ordinary portland cement their
  respective amounts usually range between 45 and
  60, 15 and 30, 6 and 12, and 6 and 8 percent.

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Hydration process

• Setting Solidification of the plastic cement
  paste
• Initial set beginning of solidification Paste
  become unworkable loss in consistency - not lt
  45 min.
• Final set Time taken to solidify completely
  Not gt 375min.
• Hardening Strength gain with time after final
  set

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Crystal formation of Cement
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Calcium silicate hydrate

• The calcium silicate hydrate phase, abbreviated
  C-S-H, makes up 50 to 60 percent of the volume of
  solids in a completely hydrated portland cement
  paste and is, therefore, the most important in
  determining the properties of the paste.
• The fact that the term C-S-H is hyphenated
  signifies that C-S-H is not a well-defined
  compound the C/S ratio varies between 1.5 to 2.0
  and the structural water content varies even more.

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Calcium silicate hydrate

• The morphology of C-S-H also varies from poorly
  crystalline fibers to reticular network. Due to
  their colloidal dimensions and a tendency to
  cluster, C-S-H crystals could only be resolved
  with the advent of electron microscopy.
• Although the exact structure of C-S-H is not
  known, several models have been proposed to
  explain the properties of the materials.
  According to the Powers-Brunauer model, the
  material has a layer structure with a very high
  surface area.

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Calcium hydroxide

• Calcium hydroxide crystals (also called
  portlandite) constitute 20 to 25 percent of the
  volume of solids in the hydrated paste. In
  contrast to the C-S-H, the calcium hydroxide is a
  compound with a definite stoichiometry?????,
  Ca(OH)2.
• It tends to form large crystals with a
  distinctive hexagonal-prism morphology. The
  morphology usually varies from nondescript to
  stacks of large plates, and is affected by the
  available space, temperature of hydration, and
  impurities present in the system.

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Calcium hydroxide

• Compared with C-S-H, the strength-contributing
  potential of calcium hydroxide due to van der
  Waals forces is limited as a result of a
  considerably lower surface area.
• Also, the presence of a considerable amount of
  calcium hydroxide in hydrated portland cement has
  an adverse effect on chemical durability to
  acidic solutions because of the higher solubility
  of calcium hydroxide than C-S-H.

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Mono-Sulfoaluminate Ettringite
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Ettringite
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Calcium sulfoaluminates

• Calcium sulfoaluminate compounds occupy 15 to 20
  percent of the solids volume in the hydrated
  paste and therefore play only a minor role in the
  structure-property relationships.
• It has already been stated that during the early
  stages of hydration the sulfate/alumina ionic
  ratio of the solution phase generally favors the
  formation of trisulfate hydrate, C6AS3H32, also
  called ettringite, which forms needle-shaped
  prismatic crystals.

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Reaction rate C3A gt C3S gt C4AF gtC2S
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Heat of hydration (Cal/g)
Compound 3 days 90 days 13 years
C3S 58 104 122
C2S 12 42 59
C3A 212 311 324
C4AF 69 98 102
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Three-phase theory-Transition zone
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Three-phase theory-Transition zone

• A thin shell layer (10-50 µm thick) around large
  aggregate.
• Formation Water films around large aggregate
  during mixing.
• Characteristic Larger CH crystals more porous
  framework relatively weak.?? ??

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Transition Zone
Miexhta and Monteiro Concrete
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Transition zone -Influence on concrete
properties

• Fraction of transition zone in size is much
  smaller than other two phases, its influence on
  concrete properties is far greater.
• It lower the strength
• It increase the permeability
• It prompt non-linear behavior
• It favorites crack formation

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Microstructural improvement

• Use of silica fume
• reduce the porosity of the ITZ geometrical effect
  (no space) reduces the amount of CH due to
  pozzolanic reaction