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Fine Aggregate

Sand particles passing the #4 sieve, filling voids and improving workability

Fine aggregate (sand) consists of particles passing the #4 sieve (4.75mm) and retained on the #200 sieve (75 micrometers). Sand fills voids between coarse aggregate, provides workability, and enables finishing. It occupies 25-35% of concrete volume and significantly affects workability, finishability, and economy.

Why It Matters

Sand makes concrete workable and finishable. Without adequate sand, concrete is harsh and difficult to place. With too much sand, concrete requires excess water and cement, reducing economy and strength. Proper sand content and gradation create workable, economical concrete that finishes well.

Sand quality varies dramatically. Clean, well-graded sand produces good concrete. Sand with excessive fines, organic material, or salt creates problems. Verify sand source meets ASTM C33 specifications.

Technical Details

Sand types:

  • Natural sand: River or pit sand, rounded particles, good workability
  • Manufactured sand: Crushed stone screenings, angular particles, lower workability but available
  • Beach sand: Never use (salt content), requires washing and desalting

Gradation requirements:

  • Continuous gradation from coarse to fine
  • Moderate amount passing #50, #100 sieves
  • Limited fines (less than 5-7% passing #200)
  • ASTM C33 defines acceptable ranges

Fineness modulus:

  • Single number characterizing overall fineness
  • Calculated from sieve analysis
  • Range 2.3-3.1 typical
  • Higher number = coarser sand

Effects on concrete:

  • Workability: More sand = more workable to a point
  • Finishability: Adequate sand essential for finishing
  • Economy: Proper sand content minimizes cement
  • Pumpability: Adequate sand prevents blockages
  • Bleeding: Excess sand reduces bleeding

Typical proportions:

  • 40-50% of total aggregate for flatwork
  • 35-45% for structural concrete
  • Adjusted based on gradations and workability needs

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