Got a photo? Identify rocks instantly with the Rockby app
Open the appDescription
Concrete aggregate is a granular material, typically composed of crushed stone, gravel, or sand, that is mixed with cement and water to form concrete. It can be classified as 'coarse aggregate' (retained on a 4.75 mm sieve) or 'fine aggregate' (passing a 4.75 mm sieve, typically sand). The properties of the aggregate significantly influence the strength, durability, and workability of the resulting concrete. Common source rocks include granite, basalt, limestone, dolomite, sandstone, and quartzite. The material is selected based on its physical and chemical properties, such as strength, durability, shape, texture, and absence of deleterious substances.
How to Identify
- Color
- Highly variable, depending on the source rock. Can be grey, black, white, brown, red, or a mix of colors.
- Luster
- Dull to vitreous, depending on the mineral composition of the source rock.
- Texture
- Rough, angular, sub-angular, sub-rounded, or rounded. Crushed aggregates tend to be angular, while natural gravels are more rounded.
- Crystal Form
- Not applicable for aggregate as it is a collection of rock fragments, not a single crystal. Individual mineral grains within the aggregate may exhibit their characteristic crystal forms if visible.
- Cleavage
- Not applicable for aggregate as a whole. Individual mineral grains within the aggregate may exhibit cleavage (e.g., feldspar, mica).
- Geological Environment
- Aggregates are sourced from quarries (for crushed stone) or natural deposits (for sand and gravel) such as riverbeds, glacial deposits, or ancient marine terraces. The original geological environment of the source rock can be igneous, sedimentary, or metamorphic.
Key Facts
- Hardness: Variable, depending on the source rock and its constituent minerals (e.g., quartz 7, calcite 3, feldspar 6-6.5 on Mohs scale).
- Specific Gravity: Typically 2.5 to 2.9 for normal-weight aggregates. Lightweight aggregates can be as low as 1.1, and heavy-weight aggregates can exceed 4.0.
- Crystal System: Not applicable for aggregate as a whole. Individual mineral grains within the aggregate will have their own crystal systems.
- Color: Highly variable, reflecting the source rock.
- Luster: Variable, from dull to vitreous.
- Transparency: Opaque to translucent, depending on the individual rock fragments and minerals.
- Fracture: Conchoidal, uneven, or splintery, depending on the source rock and mineral composition.
- Cleavage: Not applicable for aggregate as a whole. Individual mineral grains may exhibit cleavage.
- Composition: A mixture of various minerals and rock fragments, commonly including quartz, feldspar, mica, calcite, dolomite, amphibole, pyroxene, and various rock types like granite, basalt, limestone, sandstone, and quartzite.
Quick Check
- Color: Variable (grey, black, brown, white, red, mixed)
- Luster: Dull to vitreous
- Streak: Not applicable (aggregate is a mixture of rock fragments)
Physical Characteristics
- Crystal Habit: Not applicable for aggregate. Individual mineral grains may show characteristic habits.
- Cleavage Type: Not applicable for aggregate. Individual mineral grains may exhibit perfect, good, or poor cleavage.
- Fracture Type: Variable (conchoidal, uneven, splintery, hackly) depending on the constituent rock and mineral types.
- Tenacity: Brittle (for most common rock types used as aggregate).
- Luster Type: Variable (dull, earthy, vitreous, greasy) depending on the constituent minerals.
Formation
Concrete aggregate is formed by the mechanical crushing and/or natural weathering and sorting of various types of bedrock (igneous, sedimentary, metamorphic) or unconsolidated sediments (gravel, sand). The material is then screened and washed to meet specific size and quality requirements for use in concrete production.
Usage
Primarily used as a filler material in concrete, asphalt, and other construction applications. It provides bulk, strength, and stability to these composite materials. Also used in road bases, railway ballast, and drainage systems.
Age Distribution
Modern (processed material), but source rocks can be of any geological age.
Where to Find
Quarries
Locations where bedrock is extracted and crushed to produce aggregate. Found globally wherever suitable rock formations exist.
Gravel Pits
Deposits of naturally occurring sand and gravel, often found in river valleys, floodplains, glacial outwash plains, and ancient beach ridges.
Construction Sites
Aggregates are delivered to construction sites for immediate use in concrete, asphalt, or other civil engineering projects.
Finding Tips
Source Rock Identification
To understand the aggregate, identify the dominant rock types present. This requires basic knowledge of common igneous, sedimentary, and metamorphic rocks.
Particle Shape and Texture
Observe the shape (angularity) and surface texture (roughness) of the particles. Crushed aggregates are typically angular with rough surfaces, while natural gravels are more rounded and smoother due to abrasion.
Size Distribution
Note the range of particle sizes. Aggregates are typically graded into specific size fractions for different concrete applications.
Presence of Deleterious Materials
Look for any undesirable components such as clay lumps, organic matter, soft particles, or reactive minerals (e.g., chert, opal, volcanic glass) that could negatively impact concrete performance. These are often identified through laboratory testing.
Similar Rocks
Crushed Stone
Varies based on source rock
Also known as: Quarry Rock, Macadam
Gravel
Varies based on source rock
Also known as: Pebbles, River Rock
Sand
Mainly Quartz (SiO2)
Also known as: Construction Sand, Silica Sand
Scientific Classification
- Mineral Class
- Not applicable (aggregate is a rock product, not a single mineral).
- Group
- Not applicable (aggregate is a rock product).
- Crystal System
- Not applicable (aggregate is a rock product).
- Chemical Formula
- Not applicable (aggregate is a mixture of various minerals and rock fragments).
- Composition
- Heterogeneous mixture of silicate minerals (e.g., quartz, feldspar, mica, amphibole, pyroxene), carbonate minerals (e.g., calcite, dolomite), and various rock fragments (e.g., granite, basalt, limestone, sandstone, quartzite).
Explore Concrete Aggregate
Identify rocks anywhere
Free on iOS, Android, and Web. Photo identification, full database, and your personal collection.