Grading Of Aggregates for Concrete
- Aggregate comprises about 85 % volume of mass concrete
- Concrete contains aggregate upto a maximum size of 150 mm
- 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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