Aggregates
Aggregates are granular materials such as sand, gravel, crushed stone, and
recycled concrete that are mixed with a binding medium (like cement or
asphalt) to produce concrete or mortar. Because they constitute roughly 60% to
80% of the total volume of concrete, their properties profoundly dictate the
strength, durability, and cost of the final structural material.
Classification of Aggregates
Aggregates are classified using several criteria, primarily by their particle
size, their geological or manufacturing source, and their bulk density.
Fine vs. Coarse Aggregates
The primary classification is based on size. Fine Aggregates (like sand) are
those that pass through a No. 4 (4.75 mm) sieve. Coarse Aggregates (like
gravel or crushed stone) are those that are predominantly retained on a No. 4
sieve.
Use the simulation below to explore how sieve analysis determines the
gradation of a given aggregate sample. Gradation directly impacts the void
content and consequently the paste required in a concrete mix.
Aggregate Gradation Curve
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Interpretation:
A well-graded soil has a good representation of particle sizes over a wide range. This leads to high density and stability as smaller particles fill the voids between larger ones. Best for structural fill and base courses.
Source Classification
The origin of the aggregate affects its shape, texture, and chemical
composition.
Checklist
- Natural Aggregates: Sourced directly from the earth with minimal processing (e.g., river sand, natural gravel, crushed rock from quarries).
- Artificial/Manufactured Aggregates: Created through industrial processes or heating (e.g., expanded shale, clay, slate, and blast-furnace slag).
- Recycled Aggregates: Sourced from crushed waste concrete or asphalt, promoting sustainable construction practices.
Density Classification
The density of the aggregate directly controls the unit weight of the
resulting concrete, dictating its structural application.
Checklist
- Lightweight Aggregates: Bulk density less than (e.g., pumice, perlite, expanded clay). Used to reduce the dead load of structural concrete elements or for insulation.
- Normal Weight Aggregates: Bulk density between and (e.g., standard sand, gravel, crushed limestone). Used for standard structural concrete.
- Heavyweight Aggregates: Bulk density greater than (e.g., barite, magnetite, steel punchings). Primarily used for radiation shielding in nuclear facilities or medical bunkers.
Key Properties and Laboratory Testing
Aggregates must be rigorously tested to ensure they meet the specific
requirements of the concrete or asphalt mix design.
Sieve Analysis and Gradation
Gradation refers to the particle size distribution of the aggregate. Proper
gradation minimizes the void space between particles, which reduces the amount
of expensive cement paste required, improving both the economy and durability
of the concrete.
Fineness Modulus (FM)
An empirical index used to quantify the relative coarseness or fineness of a
fine aggregate sample.
Fineness Modulus (FM)
Calculated by adding the cumulative percentage of the sample retained on a standard series of sieves and dividing the sum by 100.
Variables
| Symbol | Description | Unit |
|---|---|---|
| Fineness Modulus | dimensionless | |
| Summation over the standard sieves | - |
Standard Sieves
The standard sieves strictly used for calculating the FM of fine aggregate
are: No. 4, No. 8, No. 16, No. 30, No. 50, and No. 100. A typical FM for fine
aggregate ranges from 2.3 to 3.1. A higher FM indicates a coarser aggregate.
Moisture States, Specific Gravity, and Absorption
Aggregates contain internal pores that can hold water. The moisture state of
the aggregate critically impacts the water-cement ratio of the concrete mix.
The four moisture states are:
Checklist
- Oven Dry (OD): All moisture is removed from the pores by heating.
- Air Dry (AD): Pores are partially filled with water, surface is dry.
- Saturated Surface-Dry (SSD): All pores are completely filled with water, but the surface is completely dry. This is the ideal reference state for mix design.
- Wet (or Damp): Pores are full, and there is excess free moisture on the particle surface.
Bulk Specific Gravity (SSD)
The ratio of the weight in air of a given volume of aggregate (including the
permeable pores filled with water) to the weight of an equal volume of water.
This is the most commonly used specific gravity for concrete mix design.
Absorption Capacity
The maximum amount of water an oven-dry aggregate can absorb, expressed as a
percentage of its dry weight. It represents the transition from the OD state
to the SSD state.
Absorption Capacity
The percentage calculation of absorbed water relative to dry weight.
Variables
| Symbol | Description | Unit |
|---|---|---|
| Weight of Saturated Surface-Dry aggregate | g or kg | |
| Weight of Oven Dry aggregate | g or kg |
Durability, Shape, and Texture
Beyond size and density, physical characteristics determine how the aggregate
behaves under stress and environmental weathering.
Abrasion Resistance (Toughness)
The aggregate's ability to resist degradation from rubbing, grinding, or
impact. It is most commonly tested using the Los Angeles Abrasion Test (ASTM
C131/C535), where aggregates are tumbled in a steel drum with steel spheres.
The percentage of mass lost determines its suitability, especially for
pavements subjected to heavy traffic wear.
Soundness
The aggregate's resistance to weathering, specifically freeze-thaw cycles. It
is evaluated by repeatedly soaking the aggregate in a sodium or magnesium
sulfate solution and drying it, which simulates ice crystal expansion within
the pores (ASTM C88). High mass loss indicates poor durability in harsh
climates.
Shape and Surface Texture
Particle shape (round vs. angular) and surface texture (smooth vs. rough)
significantly impact a concrete mix. Angular, rough aggregates (like crushed
stone) create stronger mechanical bonds with the cement paste, yielding higher
concrete strength. However, they reduce the mix's workability compared to
smooth, rounded river gravel.
Deleterious Substances in Aggregates
Impurities and deleterious substances in aggregates can severely compromise
the strength, durability, and appearance of concrete. Strict limits are placed
on these materials by standards such as ASTM C33.
Organic Impurities
Organic matter, such as decaying vegetation or humus, can delay or completely
prevent the hydration of cement, leading to weak concrete. The presence of
organics is tested using the Colorimetric Test (ASTM C40).
Clay Lumps and Friable Particles
These weak particles can easily break down during mixing or under weathering,
causing popouts on the concrete surface and reducing overall strength and
durability (ASTM C142).
Silt and Clay Coating
Fine dust or clay coatings on coarse aggregates can interfere with the bond
between the cement paste and the aggregate particle, significantly reducing
concrete strength (tested via ASTM C117 - Material Finer than No. 200 Sieve).
Alkali-Aggregate Reactivity (AAR)
Alkali-Aggregate Reactivity (AAR) is a severe chemical reaction between the
reactive silica or carbonates in certain aggregates and the highly alkaline
pore solution of the cement paste.
Alkali-Silica Reaction (ASR)
The most common form of AAR. Reactive forms of silica (e.g., opal, chalcedony,
chert) in the aggregate react with alkalis (sodium and potassium) from the
cement to form an expansive alkali-silica gel. When this gel absorbs moisture,
it swells, causing internal pressure that leads to characteristic "map
cracking" or "pattern cracking" on the concrete surface.
Mitigation of ASR
To prevent ASR, engineers specify non-reactive aggregates, use low-alkali
cement, or incorporate supplementary cementitious materials (SCMs) like fly
ash, slag, or silica fume, which consume the alkalis before they can react
with the aggregate.
Key Takeaways
- Classification: Aggregates are primarily divided into fine (passing No. 4 sieve) and coarse (retained on No. 4 sieve), but also categorized by geological source and bulk density.
- Gradation: Proper particle size distribution minimizes void space. The Fineness Modulus (FM) numerically quantifies the coarseness of fine aggregate based on a standard sieve analysis.
- Moisture States: Understanding the transition from Oven Dry (OD) to Saturated Surface-Dry (SSD) is critical for controlling the actual water-cement ratio in concrete mix designs via the Absorption Capacity.
- Physical Properties: Toughness (abrasion resistance), soundness (weathering resistance), particle shape, and surface texture all dictate the ultimate strength, workability, and durability of the concrete.