Summary
Highlights
Aggregates are extracted from underwater sources using dragline excavation or dredging, and from land sources through methods like truck drills, blasting bedrock, and crushing plants. Crushing plants are crucial for processing large rocks into smaller, specific sizes, including sand and various aggregate sizes.
Aggregates are mineral particles originating from rock, used as inert filler material such as sand and gravel in construction. They constitute 60-75% of normal concrete and up to 90% in bituminous concrete or asphalt. Aggregates can be naturally sourced from riverbeds and quarries or recycled from crushed concrete and demolition debris.
Aggregates are classified by shape into rounded, irregular/partly rounded, angular, flaky, and elongated. Rounded aggregates offer high workability due to minimal void spaces (32-33%) but result in weaker concrete due to poor interlocking. Angular aggregates contribute to higher strength due to better interlocking but are less workable and have more voids (38-40%). Recycled or crushed aggregates typically have angular shapes, while natural riverbed aggregates are generally rounded.
Aggregates are categorized as fine (less than 5 mm) or coarse (5 mm and above). Fine aggregates, like sand, are used for smooth finishes and make up 35-45% of total aggregates. Coarse aggregates, typically 9.5 mm to 37.5 mm, fill major voids, while fine aggregates fill smaller voids, and cement paste fills the smallest, ensuring a solid concrete mass.
Aggregates are derived from various rock types: minerals (e.g., silica, carbonates), igneous rocks (e.g., granite, basalt), sedimentary rocks (e.g., sandstone, limestone), and metamorphic rocks (e.g., marble, slate). The origin dictates the physical and chemical properties of the aggregate.
Critical properties of aggregates for construction include strength, hardness, impact value, resistance against weathering, shape, bitumen adhesion, specific gravity, and water absorption. These properties are tested using established standards such as ASTM (American Society for Testing and Materials) and AASHTO (American Association of State Highway and Transportation Officials).
The crushing strength test measures the load an aggregate can bear before failure. The abrasion test (using a Los Angeles abrasion machine) determines hardness. The impact test assesses resistance to sudden loads. The soundness test uses chemicals to gauge weathering resistance. The shape test evaluates aggregate shape for workability and strength. Bitumen adhesion tests the bond between bitumen and aggregates. Specific gravity and water absorption tests measure density and water intake, respectively, crucial for mix design.
Grading determines the particle size distribution of aggregates using sieves. Fine aggregates are graded with seven standard sieves (150 micrometers to 9.5 mm), and coarse aggregates with 13 sieves (1.18 mm to 100 mm). The fineness modulus (FM) calculates the average fineness of fine aggregates, indicating the concentration of aggregate sizes. A higher FM means coarser particles.
Key tests for sand quality on-site include organic impurity tests (to detect biodegradable components that weaken concrete), silt content tests (to measure fine waste particles like soil or clay, which should be minimized), particle size distribution/grading tests, and bulking of sand tests. Bulking refers to the increase in sand volume due to moisture content, which must be accounted for in concrete mix proportions.
Thorough testing of aggregates for properties like strength, hardness, and grading is crucial for ensuring the quality and durability of construction, especially in large-scale projects like high-rise buildings where material strength is critical. While small constructions may not adhere to strict testing, adherence to standards is paramount for structural integrity.