Aggregates are inert granular materials such as sand, gravel, or crushed stone that, along with water and dust is used for the preparation of sub base and base course materials, along with water and Portland cement, are used in concrete production. Aggregates must be of varying sizes as per gradation requirement for specific uses. In general, for the production of aggregates for road construction, sub base course materials, base course material, aggregates, dust etc. are used to prepared at crusher plant. However, as per availability of river or stream nearby to the highway, sand can also be produced. The production of aggregates starts from the source of raw materials. To identify the raw materials best for use, sampling and series of laboratory test are mandatory. The raw materials shall be selected in such a way that, the properties of the produced aggregates could meet the specification requirement of the work. For the efficient production of aggregates in construction sites, quarry having sufficient raw materials must be selected. The production of aggregates involves following steps:
a. Extraction of Raw Materials
Most materials for aggregate production come from bedrock or unconsolidated deposits available near the construction site. The vast majority of are obtained from surface- mined stone quarries or from sand and gravel pits. The quality of materials depends on the method of extraction. The materials must be extracted in such a way that, loose and unwanted materials will be removed as possible as much can. The breaking of stones, pulling of gravels, sand etc. can be done with the help of heavy equipment. The mostly used heavy equipment are; Excavator, Backhoe, Hydraulic breaker etc. Blasting is also another option for the breaking of bed rock. The broken and pulled out materials must be stock piled at the suitable location as per site demand and requirement of construction. There may require different materials having different properties at different location of site. So, in order to produce the materials with different properties, the stockpiling must also be at different place. The mixing shall also be done based on the requirement of quality of work.
b. Transportation of Raw materials
The extracted and stockpiled materials then shall be transported to the crusher plant for the production of required aggregates. The raw materials can be transported by using transporting equipment such as trippers, trucks, dumpers etc. The transported materials could be then stockpiled or put directly on the hopper of crusher plant as per site condition.
c. Crushing of aggregates
The transported materials from the quarry must be crushed in the crushed plant. During crushing the raw materials is placed in the inlet hopper of crusher. The hopper is provided with gravity flow of raw materials up to towards the scalping unit. The over coarser materials are broken at the crushing unit and transferred towards the scalping unit. The scalping unit if fitted with the sieve of variable sizes. The separation of coarse and fine particles will be held in the sieve. The fines are transported towards the fine storage unit and coarser will be moved towards the primary crushing unit. It takes place on Jaw Crusher. The primary crushing produces the primary aggregates. The scalping may again require to separate the fine and coarser particles. After separation, the coarser particles are again transferred towards the cone crusher to produce the most desired shaped and sized aggregates. The aggregates of desired shape can be obtained by changing the variable sieve fitted in crusher plant. The crushed aggregated are then stockpiled below the exit of conveyor belts or at the construction site.
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