LESSON 5

• 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

  Shape and Texture of Aggregates
• Water Content


  Adding water

  ↑ workability, ↓ STRENGTH
• Maximum size of aggregate


  ↓ surface area to be wetted, ↑ water in medium
• 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

  1. Coarse particles tend to separate out since they settle more than the fine particles
  2. Segregation on wet mixes (separation of water + cement) from the mix
• FRESH CONCRETE DEFECTS

  Bleeding (Water Gain)

  Laitance
• 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

  1. Internal Vibration
  2. External Vibration
  3. 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 is product of binding and hardening of the concrete mixture which contains ingredients quantified in the appropriate proportions.
• Ingredients in Hardened Concrete are quantified in the appropriate proportions.

  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


  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 - 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 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.
• Combination of Stresses - it is the causes of failure when the concrete fails.
• The durability of concrete can be defined as its resistance to deterioration resulting from external and internal causes.
• DURABILITY

  The external causes include the effects of environmental and service conditions to which concrete is subjected such as weathering, chemical actions and wear.
• DURABILITY

  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).
• Cracks are formed in concrete due to many reasons but when these cracks are very deep, it is unsafe to use that concrete structure. Various reasons for cracking and improper mix design, insufficient curing. omission of expansion and contraction joints, use of high slump concrete mix, unsuitable sub-grade etc.