FRESH CONCRETE

• Fresh concrete


  A composite material made from readily available constituents such as cement, sand, gravel, and water
• Fresh concrete


  • It is one of the most commonly used building materials
  • It is a versatile material that can easily be mixed to meet a variety of special needs and formed to virtually any shape
• Workability


  It is desirable that freshly mixed concrete be relatively easy to transport, place, compact and finish without harmful segregation
• Factors affecting workability


  • Method and duration of transportation
  • Quantity and characteristics of cementing materials
  • Aggregate grading, shape and surface texture
  • Quantity and characteristics of chemical admixtures
  • Amount of water
  • Amount of entrained air
  • Ambient & air temperature
• Consistency


  The fluidity or degree of wetness of concrete, generally dependent on the shear resistance of the mass and a major factor in indicating the workability of freshly mixed concrete
• Test methods for measuring consistency

  • Flow test
  • Kelly-Ball test
  • Slump test
• Slump test


  The most widely used method for measuring consistency, related to the ease with which concrete flows during placement
• Slump test procedure


  1. The slump cone can be filled in three layers with equal volume, rodded 25 times each layer evenly

  2. Measure the slump by determining the vertical difference between the top of the mold and the displaced original center of the top surface of the specimen
• Segregation


  Separation of the components of fresh concrete, resulting in a non-uniform mix
• Factors affecting segregation


  • Larger maximum particle size (25mm) and proportion of the larger particles
  • High specific gravity of coarse aggregate
  • Decrease in the amount of fine particles
  • Particle shape and texture
  • Water/cement ratio
• Bleeding


  The tendency of water to rise to the surface of freshly placed concrete, caused by the inability of solid components of the mix to hold all of the mixing water as they settle down
• Undesirable effects of bleeding

  • The top portion becomes weak & porous (high water cement ratio), reducing resistance to freezing-thawing
  • Water rising to the surface carry fine particles of cement which weaken the top portion and form laitance, reducing resistance to abrasion
  • Water may accumulate under the coarse aggregates and reinforcement, leading to weak zones and reduced bond
• Mixing of concrete


  The aim is to blend all the ingredients to form a uniform mass and to coat the surface of aggregates with cement paste
• Consolidating concrete


  The process of compacting concrete to eliminate entrapped air, achieved by tamping, rodding or using vibrators
• Inadequate consolidation can result in

  • Honeycomb
  • Excessive amount of entrapped air voids (bugholes)
  • Sand streaks
  • Placement lines (Cold joints)
• Internal vibrators


  • The poker is immersed into concrete to compact it, and can be easily removed from point to point
• External vibrators


  • Clamp directly to the formwork, requiring strong/rigid forms
• Curing of concrete


  The procedures used for promoting the hydration of cement, controlling temperature and moisture movement
• Objectives of curing


  • Keep concrete saturated or as nearly saturated as possible
  • Hydration reactions can take place in only saturated water filled capillaries
• Curing methods


  • Methods supplying additional water to the surface (wet covers, sprinkling/ponding)
  • Methods preventing loss of moisture (waterproof plastics, liquid membrane-forming compounds)
  • Methods accelerating strength gain (steam curing, heating coils)
• Uniformity of concrete


  Checked by conducting tests on fresh and hardened concretes, to account for within-batch and batch-to-batch variations
• Tests for uniformity


  • Slump, unit weight, air content tests
  • Strength tests
• Due to the heterogeneous nature of concrete, there will always be some variations
• Types of variations

  • Within-Batch Variations: inadequate mixing, non-homogeneous nature
  • Batch-to-Batch Variations: type of materials used, changes in gradation of aggregates, changes in moisture content of aggregates