CE303Lecture

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CE-303-Lecture 1

• Fundamentals of Concrete

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Objectives

• To explain the basic concepts of concrete
• To explain briefly the properties of freshly
  mixed concrete
• To explain briefly the properties of hardened
  concrete

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Concrete

• A mixture of two components
• Paste (25-40)
• Aggregates (60-75) and

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Paste

• Comprises of
• a binder (cement)
• Water
• Entrapped air or purposely entrained air

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Aggregates

• Comprises of
• Sand (fine aggregates)
• Gravel or crushed stone (coarse aggregates)

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• When paste and aggregates are mixed together the
  particles of aggregates get coated with the
  paste.
• Because of the chemical reaction of the cement
  and water the paste hardens and binds the
  aggregates together to form a rocklike mass
  called Concrete

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Components of Concrete
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Concrete
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Volume Distribution of Materials in Concrete
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Quality of Concrete

• Aggregate Requirement
• Paste Requirement

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Aggregate Requirement

• Aggregates should consist of particles with
  adequate strength against compression, impact,
  and abrasion.
• Aggregate particles should have resistance
  against aggressive exposure conditions such as
  highly fluctuating temperature and humidity

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Aggregate Requirement

• Aggregates should not contain deleterious
  materials such as chloride, sulfate, etc. that
  will cause deterioration of concrete.
• The size, shape and particle size distribution
  (gradation), of the aggregates should be such
  that the least amount of paste is required to
  fill the spaces between aggregate particles.

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Paste Requirements

• Quality of paste greatly affects the quality of
  the concrete
• Paste should be sufficient enough to completely
  coat each particle of aggregate and to fill
  spaces between aggregate particles completely.

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Paste Requirements

• The quality of hardened concrete is determined by
  the amount of water used in relation to the
  amount of cement
• (W/C ratio of the paste)

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Cement-paste cylinders
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Advantages of reducing the W/C ratio of the paste

• Increase compressive strength and flexural
  strength.
• Lower permeability, thus increased
  water-tightness and lower absorption.
• Increased resistance to weathering.
• Better bond between successive layers and between
  concrete and reinforcement.

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Advantages of reducing the W/C ratio of the paste

• Less volume change from wetting and drying
• Reduced shrinkage cracking tendencies.

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Effect of W/C ratio on Compressive Strength of
Concrete
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Uses of Admixtures to Improve the Quality of
Concrete

• Adjusting setting time or hardening in abnormal
  conditions of concreting such as cold and hot.
• Reducing water demand
• Increasing workability in case of a very low w/c
  ratio
• Intentionally entrained air in concrete

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Mixing Requirements

• All the ingredients of concrete should be mixed
  properly so as to get a homogeneous mixture.
• The sequence of charging ingredients into the
  mixer plays an important part in the uniformity
  of the finished product.
• Size of the batch in relation to the size of the
  mixer

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Mixing Requirements

• The elapsed time between batching and mixing.
• Design, configuration, and condition of the mixer
  drum and blades.

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Consolidation Requirements

• Consolidation of the freshly placed concrete
  removes voids in concrete and improves the
  quality of concrete on account of increased
  density.
• For a highly workable concrete the consolidation
  should be done by hand rodding.

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Consolidation Requirements

• For stiffer mixtures having low w/c ratio the
  consolidation should be done by vibration.
  Vibration sets into motion the particles in
  freshly mixed concrete, reducing friction between
  them and thereby increasing the mobility of
  concrete which helps in removing the voids even
  at low w/c ratio.

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Consolidation Requirements

• Thus consolidation by vibration permits the lower
  w/c ratio which in turn results in improved
  quality.
• Vibration of highly workable concrete should be
  avoided as it can cause segregation of the
  concrete.

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Effect of Consolidation
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Curing Requirements

• Curing of a placed and finished concrete is done
  for maintaining a satisfactory moisture and
  temperature in concrete for some defined period
  after placing and finishing, to allow the
  complete hydration of the cement.

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Curing Requirements

• Following are various methods commonly used for
  concrete curing
• Ponding or immersion
• Spraying or fogging
• Wet covering
• Plastic sheets and impervious paper coverings
• Membrane-forming curing compounds
• Steam curing

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Advantages of curing

• More strength and abrasion resistance
• More watertigtness
• Less volumetric changes more volume stability.
• More resistance to freezing and thawing and
  deicer salts.
• More durability

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Effect of curing on Strength
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Properties of Freshly Mixed Concrete

• Workability
• Hydration
• Setting time
• Hardening

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Workability
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Properties of Hardened Concrete

• Strength
• Unit Weight
• Resistance to freezing and thawing
• Permeability and water tightness
• Abrasion Resistance
• Volume Stability
• Control of cracking

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Compressive Strength
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Freezing and Thawing