Crush for Cubical Aggregate
BY Kelly Graves
As the nation studies higher-performing bituminous mixes and pavements, attention remains focused on the need to optimize the percentage of cubical-shaped aggregate particles in the mix, and limit to low single digits the amount of particles that are flat or elongated.
During crushing and screening, cubical-shaped aggregate particles are created, along with so-called flat or elongated shaped particles, and fines. Since the advent of Superpave mixes in the late 1990s, mix designs have limited the percentage of flat or elongated particles allowed in a mix. That’s because cubical, or angular, and rough-textured aggregates have much greater particle-to-particle contact than rounded and smooth-textured aggregate. The ease of movement of one aggregate particle relative to another is related to the number of contact points between the aggregate particles.
If the aggregates are flat and/or elongated, they can fracture, thus creating a void in the mix. Cubical-shaped products lock together better as a skeleton within the matrix, providing a better-performing mat under compaction, while reducing voids in the mix.
The cubical shape is a better product for both bituminous asphaltic concrete and Portland cement concrete (PCC), as the cubicity suppresses potential voids that might occur, and provides a better finish for both materials.
In fact, elongated pieces in PCC can protrude from the surface of the freshly placed slab, and if your finisher strikes it, he may break or move it, creating a void in the surface. Both the power or hand finisher can strike the elongated particle, pulling it up and out of the surface and creating a void while ruining the finish.
Quite simply, the cubical shape is strong; cubical aggregates lock together, creating a strong pavement, which is why a very high cubical aggregate content appears in virtually every modern standard pavement spec.
Aggregate characteristics such as particle size, shape and texture influence the performance and rideability of HMA pavements. Crushed or broken surfaces are defined as facets of stone produced by crushing or breaking by natural forces, and are bound by sharp edges.
- A totally crushed or broken particle is one with more than 90 percent of its surface crushed or broken.
- A crushed or broken particle has more than 50 percent of its surface crushed or broken.
- A rounded particle has 50 percent of its surface crushed or broken.
- A totally rounded particle has more than 90 percent of its surface rounded.
The percentage of crushed and broken surfaces affects interparticle friction, and hence the shear strength of a mix. It also affects friction and surface texture for aggregates used in pavement surfaces.
When trying to optimize a crushing circuit to produce mostly cubical aggregate, it’s important to consider the material being processed and the gradations an operator wants to produce. You have to consider the entire operation in order to achieve one production goal, that of optimized cubical aggregate. You have to start from the very beginning: How you blast material, how raw shot material is conveyed at the plant, what its scalping capability is, and how the equipment delivers material to the primary, secondary, tertiary and even quaternary crushing. Surge control points—such as bins, hoppers and stockpiles—can smooth the flow of material throughout the plant.
Know the composition of your material, understand how it breaks, and then choose the right crushing and screening process needed to produce the finished product in the shape or gradation that your state or application requires. The right type of crushing will optimize creation of a cubical product.
To begin, we are looking for a deposit with minimal clay or deleterious material. Uniformity of feed encourages creation of cubical aggregates and consistency of gradations and shape.
For a feed with deleterious materials a jaw crusher will work without clogging, but jaws tend to produce elongated particles. If you have minimal clay or other deleterious material, a horizontal shaft impact crusher as a primary, and certainly as a secondary, will produce the highest ratios of cubical aggregates.
The type of material plays a big part in how you’re going to process the material. Different rock types favor cubical aggregate production. Chert or flint from a sand and gravel pit will fracture into sharp slivers. On the other hand, cubical shape can be created from granite or limestone or natural gravel or dolomite.
When you get into the slates or cherts or caliche, that kind of material will create more issues with optimizing cubical vs elongated product.
Granite can make a really good cubical shape if you use the right crushers, as well as limestone. When you get into the softer stones, like sandstone, caliche and slate, those can be a little more difficult to make cubical due to their inherent softness.
Jaws vs. Impactors
Crushing in an impact crusher takes place when the stone is struck by the spinning impact bars, and by that stone being flung against an apron. When impact crushing is used, the striking of the aggregate by the spinning rotor’s blow bars knocks off or breaks the elongated fractures. This process is repeated by an impactor in the secondary stage.
By contrast, with compression crushing via a primary jaw crusher, the material is smashed between two opposing surfaces, resulting in long slivers and more elongated product. That’s why primary impact crushing always will provide a more cubical shape than primary compression crushing with a jaw.
Compression crushing with a higher speed cone crusher is much different. Compression crushing will result in a cubical shape in a cone crusher in a secondary or tertiary application, due to the rock-on-rock compression that takes place within the cone as well as being “choke” fed. With the cones, especially the high-speed cones, material is retained in the crushing chamber longer, allowing rock-on-rock action, which produces a cubical product.
For an abrasive, tough material like an igneous rock, a primary jaw/secondary cone set-up works well, while jaw/impact and impact/cone crusher sets work well for the softer stones. Thus it is possible for cubical product to use a jaw as primary crusher, and then use an impactor or cone as a secondary. Tertiary crushers can be a cone or horizontal shaft impactor (HSI) or vertical shaft impactor (VSI); any one of those three; or a combination of both.
Impact crushers in either primary or secondary position—in particular, HSIs—are ideal for producing a high percentage of cubical aggregates. If you use an impactor as primary, you start the process of having a cubical shape right there, and the cubical proportion is increased as material moves to the secondary or tertiary crushing stages.
Screens Have Limitations
The crushers are the determinant of the percent cubical aggregate processed. The best option is to feed crushed, cubical material to the screen, and let the screen decks size the different particles into the desired cubical gradations. The crusher needs to be the device that controls the amount of elongated product in the feed, not the screen.
Still, choosing the right type of screen media will help control the amount of elongated particles in the product.
For example, any screen with slots in it—allowing material to turn on its side and drop through—will encourage the percent of elongated material produced. Slotted screens may have opening widths of a half-inch wide, but the slots will be 2 to 3 inches long, allowing elongated material to pass through the slot.
Square mesh screen media help keep elongated material out of the mix. Square openings in mesh also will allow elongated particles to pass, but it’s less likely that those elongated pieces will pass through, compared to slotted screen media.
Small as it is, the square opening is square, not elongated. Elongated particles can pass through a square media vertically, but if positioned horizontally, as most are, as the feed flows down the inclined screen, elongates will roll off and either be re-crushed via the return circuit, or go into a different, non-state spec product like base or fill material, but not asphalt or PCC mixes. With base or fill product, elongated material is acceptable.
When material goes to a screen, you are using the screen to size a finished product into individual gradations. Once flat or elongated materials are in the product flow, it’s very difficult to separate elongated material from the cubical material. The best scenario is to make a good cubical product at the crusher, and then allow the screen to size the material out to the proper sizes. Send good cubical material from crusher to the screen and let the screen separate the sizes into the finished gradations.
That means you want to have the right crushers up front to make the cubical shape in the feed before sending it to the screen plant. Have the right type of crusher based on the raw material, and the finished product sizes that are required. Make sure you choose the right type of crushing to minimize elongated material and maximize cubical material.
Kelly Graves is director of sales for North America, Kleemann div. of Wirtgen Group