Aggregates are materials such as crushed stone, sand, and gravel. Along with water and cement, these are essential ingredients for cement. For a decent cement mix, it is required that aggregates be clean and robust materials, free of any chemicals or coatings that may deteriorate concrete.
Aggregates comprise around 70% of concrete composition and are presented in two forms: (1) fine and (2) coarse.
Fine aggregates are usually made of sand or crushed stone, whereas coarse aggregates can be up to 1.5 inches in diameter. One digs up sand or natural gravel from a lake, river, or seabed and then processes the aggregate. To ensure aggregate is cleaned correctly, the aggregate is crushed, washed, and screened. After being processed, the aggregate is stored to avoid any contamination.
Aggregates can heavily influence a concrete’s properties, proportions, and economy. Hence, the selection of aggregates is an important aspect. The consideration of sums is based on the following characteristics:
- Durability
- Particle shape and surface texture: Concrete mix is heavily influenced by the size and conditions of the particles.
- Voids and unit weights concern empty spaces between the aggregate particles.
- Grading: This is the percentage of fine aggregate passed from 600 microns sieve size. The quality of the fine aggregate can be assessed via the grading zone. There are different grading zones based on the sizing:
- Zone I: 15% to 34%
- Zone II: 34% to 59%
- Zone III: 60% to 79%
- Zone IV: 80% to 100%
- Abrasion and skid resistance
- Surface Moisture and Absorption: The acceptable aggregate density depends on the solid material and void content. One should measure the absorption rate before pouring water inside.
see also about the fine aggregate
Size and shape of aggregate matters
In freshly mixed concrete, the particle shape and surface texture influence the concrete’s properties. Angular particles or rough-textured particles require more water to produce workable and smooth Concrete. It also increases the amount of cement to keep the water-to-cement ratio. The void content between the particles affects the amount of cement paste required for a mix. Angular particles increase the void content, whereas well-graded aggregate decreases the void content. The amount of water is adjusted, considering the moisture conditions of a total. One should consider the aspects of abrasion and skid resistance for Concrete that will be dealing with a lot of abrasions.
Type of fine aggregates
Fine aggregates are classified based on the mode of origin, composition, and grain size.
- Fine aggregates based on the mode of origin
- Pit Sand: These are sharp and angular. Often clay covers such sand accumulation. It is best to wash and dry this aggregate before use.
- River Sand: This sand is found in river beds and is round in shape. This sand is commonly used in construction work.
- Sea Sand: This sand is found along beaches and sea shores. It is round and covered in salt, which is not easily separated. This type of sand is not preferred, and one should thoroughly wash it before use.
- Fine aggregates based on composition
- Clean Sand: A well-grained sand that comes in a variety of sizes.
- Clayey Sand: This is poorly graded sand with apparent clay fraction and plastic fines.
- Silty Sand: This is poorly graded and contains silts and non-plastic fines.
- Fine aggregates based on grain size
- Fine Sand: The sizes of the grain range from 0.25 – 0.15 mm. The degree of fineness is from 2.2 – 2.6.
- Medium Sand: The sizes of the grain range from 1 – 0.25 mm. The degree of fineness is from 2.6 – 2.9.
- Coarse Sand: The sizes of the grain range from 2 – 1 mm. The degree of fineness is from 2.9 – 3.2.
Depending on the type of work, different sizes of sand are necessary. That’s why there is the use of terms such as fine sand, medium sand, and coarse sand.
Role of fine aggregate in concrete
Fine aggregates are fillers that occupy the most volume in concrete. Fine aggregates’ size, shape, and composition can heavily influence the outputs. One can describe the role of fine aggregates as follows:
- The quality of fine aggregates affects the proportions and hardening properties
- Fine aggregates provide dimensional stability to the mixture
- Fine aggregates can affect the elasticity and abrasion level of the Concrete
- Fine aggregate properties have a significant impact on the shrinkage capacity of Concrete.
Use of fine aggregate
Fine aggregates are generally used in projects where the requirement is for a smooth and highly compact structure. They are ideal for underneath pavers, athletic infield material, and track fines, among other usages.
- Athletic Infield Material: Fine aggregates are excellent for baseball and softball fields. The fine material makes it best for sliding and performance.
- Soil Amendment: Farmers use fine aggregates to enrich the soil quality.
- Compacted Base: They are often used as a base for pathways. Often used as a base below a layer to provide a firm footing.
Coarse aggregate vs. fine aggregate
The differences between fine and coarse aggregates depend on the definition, size of particles, minerals, sources, surface area, and function in Concrete, among other things.
| Scopes | Fine Aggregate | Coarse Aggregate |
| Definition | These are small-size filler materials used in construction. | These are larger size filler materials used in construction. |
| Size of Particles | These aggregates should go through a 4.75 mm sieve and retain in a 0.075 mm sieve. | These aggregates remain on the 4.75 mm sieve. |
| Materials | Materials such as sand, stone screenings, and burnt clay are used. | Broken bricks, broken stones, gravel, and pebbles are used as materials. |
| Sources | River sand, crushed sandstone, and crushed gravel are sources for fine aggregates. | The sources for coarse aggregates are crushed gravel or stone and the natural disintegration of stones. |
| Surface Area | The surface area is high. | The surface area is less than fine aggregates. |
| Function in Concrete | The voids in between coarse aggregates are filled by fine aggregates. | These are used as filler material within concrete. |
| Uses | Used in mortar, concrete, plaster and filling for road pavement layers. | Mainly used in concrete and railway tracks. |
FAQs
What is the primary purpose of fine aggregates in Concrete?
Fine aggregates make the concrete compact. They also reduce the inclusion of water and cement and contribute to the strength of Concrete.
Any alternatives to fine aggregates?
Coal bottom fly ash, marble powder, ceramic powder, quarry rock dust, foundry sand, natural sand, and recycled sand are alternatives to fine aggregates.
What is the effect of fine aggregate on concrete strength?
The workability of concrete decreases with the increase of fine aggregates.
What is the effect of too much fine aggregate?
A large amount of fine aggregate increases the need for water, increasing concrete shrinkage and strengthening deterioration.
Why is it essential for aggregates to have a specific shape and texture?
An aggregate's particle shape and surface texture are essential for proper compaction, resistance, workability, and deformation.