Fine Aggregate
Sand particles passing the #4 sieve, filling voids and improving workability
Fine aggregate is the sand in concrete: natural or manufactured particles that pass the #4 sieve (4.75 mm) and are mostly retained on the #200 sieve (75 µm), graded to ASTM C33. In concrete, fine aggregate fills the voids between the gravel or crushed stone (coarse aggregate), makes the mix workable, and lets the surface be finished. 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
Fine Aggregate Size: ASTM C33 Grading
ASTM C33 sets the grading limits for fine aggregate used in concrete. A sieve analysis (ASTM C136) must fall inside these bands:
| Sieve | Opening | Percent Passing |
|---|---|---|
| 3/8 in | 9.5 mm | 100 |
| #4 | 4.75 mm | 95–100 |
| #8 | 2.36 mm | 80–100 |
| #16 | 1.18 mm | 50–85 |
| #30 | 600 µm | 25–60 |
| #50 | 300 µm | 5–30 |
| #100 | 150 µm | 0–10 |
C33 also requires that no more than 45% pass one sieve and be retained on the next, and that the fineness modulus fall between 2.3 and 3.1. Material finer than the #200 sieve is capped at 3% for concrete subject to abrasion and 5% for other concrete (5% and 7% for manufactured sand free of clay).
Fine Aggregate vs. Coarse Aggregate
| Fine Aggregate | Coarse Aggregate | |
|---|---|---|
| Size | Passes #4 sieve (under 4.75 mm, about 3/16 in) | Retained on #4 sieve, up to 3/4–1 1/2 in for most flatwork |
| Common material | Natural sand, manufactured sand | Gravel, crushed stone |
| Role in the mix | Fills voids, workability, finishability | Bulk volume, strength, lower shrinkage |
| Share of total aggregate | 40–50% for flatwork | 50–60% for flatwork |
| Governing spec | ASTM C33 fine aggregate | ASTM C33 coarse aggregate size numbers |
Fine Aggregate Tests
The standard tests that qualify a sand for concrete:
| Test | ASTM Standard | What It Checks |
|---|---|---|
| Sieve analysis | C136 | Grading and fineness modulus |
| Material finer than #200 | C117 | Silt and clay "fines" by washing |
| Organic impurities | C40 | Organic matter that retards setting (color comparison) |
| Relative density and absorption | C128 | Specific gravity and water absorption for mix design |
| Clay lumps and friable particles | C142 | Soft particles (C33 limit: 3%) |
| Total moisture content | C566 | Water in the sand, to adjust batch water |
| Alkali-silica reactivity | C1260 | Potential for ASR expansion |
Concrete Fines
"Fines" in concrete are the smallest particles in the sand, those passing the #200 sieve (75 µm): silt, clay, and rock dust. A small amount improves workability and finish, but excess fines raise the water demand, which increases shrinkage and can contribute to surface dusting and crazing. That is why ASTM C33 caps them at 3–7% depending on exposure and sand type. See fines for more.
Frequently Asked Questions
What is fine aggregate in concrete? Fine aggregate is the sand portion of concrete: particles smaller than 4.75 mm (#4 sieve) that fill the spaces between the coarse aggregate. It makes up about 25–35% of the concrete volume and 40–50% of the total aggregate in a typical flatwork mix.
What size is fine aggregate? Under ASTM C33, 100% passes the 3/8-inch (9.5 mm) sieve, 95–100% passes the #4 (4.75 mm) sieve, and no more than 3–7% passes the #200 (75 µm) sieve. The fineness modulus must be 2.3–3.1.
Is fine aggregate the same as sand? In practice, yes. Fine aggregate for concrete is natural sand, manufactured sand (crushed stone), or a blend, washed and graded to ASTM C33. Not every sand qualifies: play sand and beach sand are usually too uniform or too salty, and mortar sand (ASTM C144) is graded finer than concrete sand.
What is the difference between fine and coarse aggregate? The #4 sieve (4.75 mm) is the dividing line. Fine aggregate passes it; coarse aggregate is retained on it. Coarse aggregate supplies bulk and strength, while fine aggregate supplies workability and a finishable surface.
Related Terms
- Aggregate - Fine is one type
- Coarse Aggregate - Larger particles
- Fines - Smallest particles in sand
Learn More
- How to Mix Concrete - Material proportioning
- Concrete Basics - Mix components
- Concrete Calculator - Calculate volumes