Aggregate Testing: Sampling, Grading, and Physical Properties
Why aggregates dominate concrete behavior
Aggregates make up the majority of concrete volume, so their grading, shape, and absorption drive workability, water demand, and durability. Candidates must know the standard test families: sieve analysis and fineness modulus, moisture content and absorption, bulk specific gravity (oven-dry, saturated-surface-dry, and apparent), unit weight and voids, and soundness or abrasion resistance for durability screening.
Grading and fineness modulus
A sieve analysis is run on a dried sample through a stacked series of sieves. Results are reported as percent passing or percent retained on each sieve. The fineness modulus is the sum of the cumulative percent retained on a defined set of sieves divided by 100; it is an index of coarseness, not a measure of gradation quality. A higher fineness modulus means a coarser fine aggregate. Traps include using percent passing instead of cumulative percent retained, and forgetting that fineness modulus is calculated only for fine aggregate.
Moisture, absorption, and specific gravity
Bulk specific gravity is the ratio of the aggregate's mass to the mass of an equal volume of water, with the volume defined by the condition chosen. Saturated-surface-dry (SSD) is the reference condition for batching because it represents aggregate that neither absorbs nor gives up water. Absorption is the moisture absorbed into permeable pores, expressed as a percentage of oven-dry mass. Free moisture above SSD must be added to the batch water calculation.
Worked example
An oven-dry sample of 500 g reaches 512 g in SSD condition. Absorption = (512 − 500) / 500 × 100 = 2.4%. If the field moisture is 4.0%, free moisture = 4.0 − 2.4 = 1.6% of dry mass, which must be counted as mixing water. Rule of thumb: always subtract absorption from total moisture to find water that actually affects the mix.