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Sand and gravel are loose mixtures of sand and gravel, and are the most widely used materials in infrastructure construction, applied in various fields such as building, transportation, and railways. They can be classified in various ways, but the core classifications can be based on formation method, particle size, quality grade, and application.
This classification is mainly based on the formation of sand and gravel, and is divided into two major categories: natural sand and gravel and artificial sand and gravel. The two differ significantly in characteristics and application scenarios, and it is also the most commonly used classification method in the industry.
Formed naturally from rocks through weathering, erosion, water scouring, transportation, and deposition, without artificial crushing or processing, the particles are mostly naturally rounded. They can be further subdivided based on their origin:
River Sand: Originating from rivers and streams, it has undergone long-term water erosion, resulting in rounded particles, uniform gradation, low mud content, and high purity. It is the most commonly used sand in construction projects, suitable for concrete, mortar, and various other applications. However, over-exploitation can lead to exposed riverbeds and ecological damage.
Sea Sand: Originating from coastal areas, it has rounded particles and a high salt content. It requires desalination (salt removal) before use in construction projects; otherwise, it will corrode steel reinforcement and affect concrete strength. It is mostly used in low-grade projects or non-load-bearing structures in coastal areas.
Mountain Sand: Originating from weathered rock layers in mountainous areas, it has sharp-edged particles, higher mud and impurity content, poorer gradation, and relatively lower strength. It is mostly used for base backfilling, non-load-bearing wall construction, and other applications where high-quality sand is not critical. Strict screening and impurity removal are necessary before use.
Pebbles: Naturally formed round or oval crushed stones with a particle size greater than 2mm. They are formed by the long-term grinding of rocks by water flow. They are hard and wear-resistant, and are often used as coarse aggregate in concrete and road subbase. They can be further classified according to particle size.
Manufactured sand is produced manually using crushing, screening, and shaping equipment to process raw materials such as rocks (e.g., limestone, granite), construction waste, and mine tailings. It allows for precise control of particle size distribution and quality, and is currently the main alternative to natural sand and gravel, accounting for approximately 70% of construction sand and gravel. It is mainly divided into two categories:
Manufactured sand: Sand produced purely through mechanical crushing and screening, with a particle size less than 4.75mm. The particles are multi-faceted and have a rough surface, resulting in good adhesion to cement. Different finenesses can be produced by adjusting the process, making it suitable for high-performance concrete, precast components, and other applications. The new national standard has added a definition and related technical requirements for flaky particles in manufactured sand.
Mixed sand: Sand made by mixing manufactured sand and natural sand in a certain proportion. It combines the advantages of both natural and manufactured sand, with a reasonable gradation and good workability. The mixing ratio can be adjusted according to project requirements. The new national standard for “Construction Sand” has explicitly added a definition and classification for mixed sand.
Machine-made crushed stone: Made from mechanically crushed rocks, with distinct angular particles, high strength, and controllable gradation. Depending on the type of raw rock, it can be divided into granite crushed stone, limestone crushed stone, etc. It is one of the main sources of coarse aggregate for concrete. The new national standard has revised and improved its indicators such as mud powder content and needle-shaped and flaky particle content.
This classification is based on the particle size of sand and gravel, and is mainly used to distinguish the specifications of sand and crushed stone, which facilitates engineering selection and proportioning, and conforms to industry standards.
Sand is classified into four grades according to its fineness modulus (an indicator of sand’s coarseness). The new edition of “Construction Sand” adds a fineness modulus range for extra-fine sand, as follows:
Coarse Sand: Fineness modulus 3.7-3.1, average particle size above 0.5mm. It has relatively coarse particles and high porosity, commonly used for coarse aggregate filling in concrete and road base backfilling. It needs to be mixed with fine sand to adjust the gradation.
Medium Sand: Fineness modulus 3.0-2.3, average particle size 0.5-0.35mm. It has uniform gradation and is the most commonly used sand in construction engineering. It is suitable for most scenarios such as concrete mixing, mortar preparation, and wall construction, offering the best cost-effectiveness.
Fine Sand: Fineness modulus 2.2-1.6, average particle size 0.35-0.25mm. It has relatively fine particles and good workability, but lower strength. It is mostly used for mortar preparation and wall plastering, and is not suitable for use alone in high-strength concrete.
Extra-fine sand: Fineness modulus 1.5-0.7, average particle size below 0.25mm, extremely fine particles, mud content easily exceeds the standard, requires special treatment before use, mostly used in low-strength mortar, base leveling and other scenarios, the new version of the national standard clarifies its fineness modulus range.
In the industry, “particle size range” is commonly used to indicate specifications for ease of screening and engineering applications. Common specifications are as follows:
5-10mm (5-1 gravel): Particle size between 5-10mm, smaller particles, commonly used for fine aggregate in concrete, mortar aggregate, road surface leveling, and can also be used to adjust coarse aggregate gradation and fill voids.
10-20mm (1-2 gravel): Particle size between 10-20mm, the most widely used, commonly used for coarse aggregate in concrete, road base courses, bridge engineering, etc. Suitable for most high-strength concrete applications, it is the most widely used crushed stone specification in engineering.
16-31.5mm (1-3 gravel): Particle size between 16-31.5mm, larger particles, higher strength, commonly used for large concrete components, road base courses, foundation backfill, and other applications requiring high strength. It needs to be mixed with fine-particle crushed stone to adjust the gradation.
10-15mm (2-4 aggregate): Corresponds to 1/4-1/2 inch in international standard. With a moderate particle size, it is commonly used in small to medium-sized concrete projects, wall construction, and subbase paving. It is highly adaptable and can be flexibly combined with other materials.
15-20mm (4-6 aggregate): Corresponds to 1/2-3/4 inch in international standard. With uniform particle size, it is commonly used in concrete mixing, road subbases, and backfilling projects. Its reasonable gradation can reduce cement usage and improve concrete density.
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