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The global trend towards increasingly higher and stringent specifications for concrete structures, and concrete and asphalt roadway construction demands the production of high-quality aggregate materials with good shape characteristics usually expressed in terms of cubicity or flakiness index.

Investigations by civil engineering and construction industry research organizations have shown that significant strength and overall cost savings are achievable by using aggregates with high cubicity (low flakiness) values in the construction of roadways and concrete structures as the result of improved mechanical properties, improved wear resistance, reduction in bitumen content in hot mix materials, concrete with better flowability and

strength characteristics to form complex profiles and many more benefits.

The economic life of civil structures can be increased and the construction and maintenance costs can be significantly reduced by the judicious use of materials that are inherently cubical in shape. The use of materials with higher flakiness values which have a greater disposition to breakage under the imposition of loads will require more frequent, costly, and disruptive repair work to be undertaken.

Hence, there is general agreement that a cubical stone is ideal for use in concrete and asphalt constructions. The more elongated and flaky the material is, the more water, cement, or binder is required. A cubical aggregate allows to reduce the amount of cement and water, while still producing a strong concrete product, so it is better and the benefit is similar for asphalt.

The shape factor of aggregate plays a vital role in the design and performance of mix and it is mainly influenced by feed rock properties, the type of crusher, and the way in which the crusher is operated.

Feed rock properties: The strength of some sedimentary rocks is considerably lower in one direction than in another due to shale partings, mineral cleavage, or similar weak layers that occur in parallel planes. When crushed, these rocks may yield a large number of flat and elongated particles. Some strong, fine-grained, homogeneous rocks such as flint may also produce flaky particles when crushed. The particle shape of crushed igneous rocks is primarily affected by the mineralogical composition of the rock.

The right materials are hard and competent rocks that generally fragment into cubical products. Igneous rocks, such as basalt and granite, are an excellent source of the aggregate because they are hard, tough, and dense. Sedimentary rocks, such as limestone, are more challenging than igneous rocks to crush.

In a sedimentary deposit such as limestone, the strength of some rock is lower in one direction than in another due to mineral cleavage and other weak layers that occur in the parallel planes. Rocks with these characteristics, when crushed with conventional compression type crushers they tend to fracture into flat or elongated pieces. If the material is non-abrasive, both horizontal and vertical impact crushers are the preferred equipment of choice for these rocks. Impact crushing, more specifically with rock-on-rock action, helps such a stone to break along its natural lines, rounding off weak edges and sharp angles so that the particle obtained is cubical. However, high-performance cone crushers can still achieve product share specification at a lower cost per tonne and high product yield.  

Hence, to a degree, geology dictates the type of crusher required to produce cubical aggregate products. HSIs and shoe-and-anvil vertical shaft impact crushers (VSIs) are preferred for softer, less abrasive stones. Autogenous VSIs and cone crushers are better with a more abrasive, harder stone. Ultimately, for product shape, an autogenous VSI will produce the best quality product. 

Crusher type: It has been argued that rock-on-rock crushing produces the most cubical product. Impact crushers are the first crusher types that come to mind for rock-on-rock crushing, whether they are horizontal shaft impact crushers (HSIs) or vertical shaft impact crushers (VSIs). Impact crushers have two primary disadvantages, they can be expensive to operate and often generate a lower yield of the product.

Modern, high-speed cone crushers using a fine cone liner configuration also will provide some rock-on-rock crushing, as long as they are kept choke-fed. Whenever the right materials are present to create the required particle shape, It is recommended to use the more efficient and lower maintenance jaw and cone crushers. Cone Crushers are the best option to achieve great particle shape and yield at the lowest cost per ton. Selecting the right cone crusher is critical as is the successful operation with the chamber, load and feed gradation.

However, in need of extra shaping to meet the cubicity requirements for the more challenging materials, a Vertical Shaft Impactor (VSI) is often added to the crushing circuit as a supplemental machine to the cone crushers.

Crusher operation: Correct particle shape requires more than one crushing stage. Jaw- or gyratory-type primary crushers, because they're compression-type machines, produce more flat and elongated particles than impact crushers. Additional crushing is needed to increase the percentage of cubical particles. A primary crusher surge pile helps operators to regulate the feed to the secondary crusher. If the secondary crusher hopper isn't kept full (choke fed) during operation, problems with flat and elongated particles will be magnified. Adjustments in the reduction ratio also help to control particle shape. As noted in the article, decreasing the reduction ratio generally produces a more nearly cubical aggregate shape. Plant automation is another operational step that can improve particle shape by ensuring a steady feed of material through the crusher.

While feed rock properties and crusher type determine achievable limits on particle shape, crusher operation is still the most important single factor affecting shape