Fines
Very small particles in aggregate passing the #200 sieve, affecting workability and water demand
Fines are very small particles in aggregate passing the #200 sieve (75 micrometers), consisting of silt, clay, and rock dust. Moderate fines content (3-7%) improves workability and finishability. Excessive fines (greater than 10%) increase water demand, reduce strength, and cause durability problems. Insufficient fines (less than 2%) create harsh, unworkable mixes.
Why It Matters
Fines fill small voids between larger particles, improving cohesion and preventing segregation. They make concrete easier to finish. But fines coat aggregate particles, increasing surface area and water demand. Too many fines means weak, porous concrete. Too few means harsh concrete difficult to place and finish.
Most aggregate suppliers wash material to control fines. ASTM C33 limits fines to 3-7% depending on application. Verify your aggregate source meets specifications—excessive fines are a common cause of weak concrete.
Technical Details
Fines measurement:
- Wet sieve analysis through #200 sieve
- Material passing weighedexpressed as percentage
- ASTM C117 test method
Sources of fines:
- Clay and silt in natural deposits
- Rock dust from crushing operations
- Weathered or soft aggregate particles
- Contamination from soil
ASTM C33 limits:
- Fine aggregate: 5-7% fines maximum depending on application
- Coarse aggregate: 1% maximum for most applications
- Tighter limits for some specialized concretes
Effects of excess fines:
- Increased water demand
- Higher shrinkage
- Reduced strength
- Increased permeability
- Frost susceptibility
Effects of insufficient fines:
- Harsh, unworkable mix
- Segregation
- Difficulty finishing
- Poor pumpability
- Bleeding
Clay vs. rock dust:
- Clay fines worst (swells when wet)
- Rock dust better (inert)
- Test distinguishes types
- Clay limits much tighter
Related Terms
- Aggregate - Fines are smallest aggregate particles
- Fine Aggregate - Contains most fines
- Workability - Affected by fines content
Learn More
- How to Mix Concrete - Material selection
- Concrete Basics - Mix components
- Concrete Calculator - Calculate volumes