Grading Of Aggregates for Concrete

—Grading Of Aggregates for Concrete

  • Aggregate comprises about 85 % volume of mass concrete—
  • —Concrete contains aggregate upto a maximum size of 150 mm

    Grading of Aggregates
    Grading of Aggregates
  • ——Way particles of aggregate fit together in the mix, as influenced by the gradation, shape, and surface texture—
  • —Grading effects workability and finishing characteristic of fresh concrete, consequently  the properties of hardened concrete
  • —Good grading implies, sample of aggregates containing all standard fractions of aggregate in required proportion such that the sample contains minimum voids
  • —Well graded aggregate containing minimum voids will require minimum paste to fill up the voids in the aggregate
  • —Minimum paste means less quantity of cement and less quantity of water, hence increased economy, higher strength, lower-shrinkage and greater durability

Gap Grading

  • —Voids created by higher size filled up by immediate next lower size—
  • —Lower size may not be accommodated in the available gap due to small voids left out which can reduce density
  • —Voids created by a particular size can accommodate second or third lower size only—
  • —For example voids created by 40mm can accommodate 10mm & 4.75mm but not 20mm, this concept is called Gap Grading
—Gap-Graded Mix
—Gap-graded mixes contain aggregate retained on a 19mm or 37.5mm sieve
—Fines passing the No. 4 sieve (4.75mm)
—Used to obtain uniform textures for exposed-aggregate concrete
—Prone to Segregation, controlled by FA %
—Rounded used, by 25%
— Air entrainment usually is required to improve the workability
Advantages of Gap-Graded Mix
  • —Increase strength and reduce creep and shrinkage—
  • —Requirement of sand reduced by 26 to 40%—
  • —Specific area of total aggregates will be reduced due to less sand—
  • —Requires less cement as net volume of voids is reduced

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