LESSON 212 min read
Aggregates & Gradation
Fine aggregate gradation, Fineness Modulus (FM), and aggregate moisture corrections.
In this lesson: Fine aggregate gradation, Fineness Modulus (FM), and aggregate moisture corrections.
Aggregates: 70–80% of Concrete Volume
Aggregates are the inert granular fillers that give concrete its volume and dimensional stability. Though chemically inert, their gradation, shape, and moisture content profoundly affect concrete workability, strength, and durability.
Fine Aggregates & Gradation
Fine aggregates (sand) pass the #4 sieve (4.75mm). Their gradation — the distribution of particle sizes — determines workability and bleeding. Well-graded sand fills voids between coarse aggregate particles, reducing paste demand.
The Fineness Modulus (FM) is a single number representing the overall coarseness of a fine aggregate. It's calculated by summing the cumulative percentages retained on standard sieves (#4, #8, #16, #30, #50, #100) and dividing by 100.
The Fineness Modulus (FM) is a single number representing the overall coarseness of a fine aggregate. It's calculated by summing the cumulative percentages retained on standard sieves (#4, #8, #16, #30, #50, #100) and dividing by 100.
Info · Fineness Modulus Range
FM 2.3–3.1 is typical for concrete sand. FM < 2.3 = fine sand (high water demand, sticky mix). FM > 3.1 = coarse sand (harsh mix, prone to bleeding). FM is NOT a measure of gradation quality — two sands with the same FM can have very different gradations.
Aggregate Moisture Corrections
Aggregates in stockpiles contain moisture. This affects concrete mix design in two ways:
1. Free moisture adds water to the mix — if not accounted for, the W/C ratio increases and strength drops. The batch water must be reduced by the free moisture weight.
2. Absorption is the moisture filling aggregate pores. Saturated surface-dry (SSD) aggregates neither absorb nor contribute water. If aggregates are below SSD, they absorb mix water, reducing workability.
Moisture states: Oven dry (OD) < Air dry (AD) < Saturated Surface Dry (SSD) < Wet. The difference between SSD and actual moisture = free moisture (positive if wet, negative if dry).
1. Free moisture adds water to the mix — if not accounted for, the W/C ratio increases and strength drops. The batch water must be reduced by the free moisture weight.
2. Absorption is the moisture filling aggregate pores. Saturated surface-dry (SSD) aggregates neither absorb nor contribute water. If aggregates are below SSD, they absorb mix water, reducing workability.
Moisture states: Oven dry (OD) < Air dry (AD) < Saturated Surface Dry (SSD) < Wet. The difference between SSD and actual moisture = free moisture (positive if wet, negative if dry).
Safety · Batching Error
Failing to correct for aggregate moisture is the #1 cause of out-of-spec concrete. A 5% free moisture in sand that's not corrected adds 25 lbs of water per cubic yard — reducing 4000 psi concrete to ~3500 psi. Always test aggregate moisture daily and adjust batch tickets.
FM = (Σ cumulative % retained on #4, #8, #16, #30, #50, #100 sieves) / 100. Example: if cumulative retentions are 0, 5, 15, 40, 70, 90, then FM = (0+5+15+40+70+90)/100 = 2.20. This is a fine sand.