CONCRETE

• FRESH CONCRETE


  The strength of concrete is affected by the degree of its

  compaction. It is vital, therefore, that the consistency

  (ability to flow) of the mix be such that, the concrete can be

  transported, placed and finished sufficiently, easily and

  without segregation.
• WORKABILITY


  Used to describe the ease with which concrete

  mixes can be compacted (completely compacted

  using the lowest possible w/c)
• Factors of Workability

  Water Content

  Maximum size of Aggregate

  Grading of Aggregates - poor grading reduces consistency

  Shape and Texture of Aggregates - smooth surface gives better workability
• Measurements of Workability

  (Indirect)

  Slump Test

  Compacting Factor Test

  Flow Table Test

  Vebe Test

  Kelly Ball Test
• SLUMP TEST


  Used in detecting variations in the uniformity of a mix of

  given nominal proportions.
• Stiff Mix

  zero slump
• Rich Mix

  slump varies but satisfactory
• Lean Mix


  shear or collapse slump
• Compacting Factor Test


  Measurement of the density ratio(actual recorded

  density to the density of the same concrete that is

  fully compacted)
• Flow Table Test


  Measures the diameter of the spread after

  vibration

  (Self ConsolidationConcrete/Self

  CompactingConcrete)
• Kelly Ball Test


  Measures the depth of penetration
• Segregation


  Separation of the constituents of a heterogeneous mixture

  so that the distribution is no longer uniform.
• TWO FORMS OF CONCRETE SEGREGATION

  Coarse particles tend to separate out since they settle more

  than the fine particles

  Segregation on wet mixes (separation of water + cement)

  from the mix
• Bleeding (Water Gain)

  Water from the mix tends to rise to the surface of

  freshly placed concrete. Bleeding can be expressed

  as the total settlement per unit height of concrete.
• Laitance


  a weak, friable layer on the surface of concrete

  and sand cement screeds which appears after

  curing.
• Compaction of Concrete


  Consists essentially of the elimination of

  entrapped air in concrete (ramming/vibration)
• Types of Vibration

  Internal Vibration

  External Vibration

  Vibrating Tables
• Curing of Concrete (Sample and Onsite)


  The creation of an environment for a period of

  time (suitable temperature, prevention of loss of

  moisture or providing moisture)
• Quality of Mixing Water


  Suitability of water for mixing and curing

  processes should be considered, quality of water is

  covered by saying water must be fit for drinking.
• Pumped Concrete


  A type of concrete which is designed to be pump able with

  relatively high slump (6-8")
• Underwater Concreting


  A special technique used to make concreting in

  water (Tremie Method)
• HARDENED CONCRETE

  Hardened concrete is product of binding and hardening of the concrete

  mixture which contains ingredients quantified in the appropriate proportions:
• HARDENED CONCRETE

  binder (cement)

  coarse aggregate (gravel)

  fine aggregate (sand, fraction <2mm)

  water and optional additives (> 5% by weight of binder)

  admixtures (<5% by weight of the binder).
• Compressive Strength


  The compression strength of concrete is a measure

  of the concrete's ability to resist loads which

  tend to compress it. It is measured by crushing

  cylindrical concrete specimens in compression.
• Flexural Strength


  Flexural strength is one measure of the tensile

  strength of concrete. It is a measure of an

  unreinforced concrete beam or slab to resist

  failure in bending.
• Tensile Strength


  There is no field test for direct determination of

  tension under axial loading. An indirect method

  called the splitting tensile test, in which a

  standard test cylinder is loaded in compression on

  its side.
• Shear, Torsion and Combined

  Stresses

  Shear is a loading in which a part of a member attempts to

  slide or shear along another part. Torsion, which is a

  twisting, is also complex and difficult to evaluate. When

  concrete fails, a combination of stresses causes the failure.
• Durability


  Besides its ability to sustain loads, concrete is also

  required to be durable. The durability of concrete can be

  defined as its resistance to deterioration resulting from

  external and internal causes. The external causes include

  the effects of environmental and service conditions to

  which concrete is subjected such as weathering, chemical

  actions and wear. The internal causes are the effects of

  interaction between the constituent material such as

  alkali-aggregate reaction, volume changes, absorption and

  permeability.
• External Causes

  Leaching out of Cement

  Actions of sulphates, seawater and natural slightly

  acidic water.

  Occurrence of extreme temperatures, abrasion and

  electrostatic action.
• Internal Causes

  Alkali-aggregate reactions

  Volume change due to difference in thermal

  properties of the aggregate and cement paste.

  Permeability of concrete.
• Shrinkage


  of concrete is caused by the setting of

  solids and the loss of free water from the plastic

  concrete (plastic shrinkage), by the chemical

  combination of cement with water (autogenous

  shrinkage) and by the drying concrete (drying

  shrinkage).