Brace for impact

Use of recycled materials has grown rapidly with no signs of slowing down. Acceptance of recycled materials such as concrete and asphalt as viable substitutes for virgin aggregates has been driven by opportunities to mitigate costs and improve profits while also complying with increasing regulations that create requirements and incentives to recycle.

These factors have caused many to change their views on aging roadways, bridges and structures, reconsidering them as sources of future building materials.

Concrete, asphalt and demolition materials can be processed using various technologies, including crushers and grinders. However, these materials typically are processed using horizontal impact crushers, also known as horizontal shaft impactors (HSIs). As the name implies, an HSI crushes by hitting these materials with a series of hammers that rotate at a high speed on a horizontal axis.

Like all equipment, an HSI has its pros and cons, but it has gained popularity given its simple design coupled with wide application diversity.

Fundamentally, an HSI provides high reduction ratios, high productivity and simple maintenance and operation. The design is more forgiving when processing sticky materials and metal contaminants, which means the same crusher can be used for asphalt pavement, millings, broken concrete or construction and demolition (C&D) debris.

In many cases, a single HSI can perform all the required crushing tasks, whereas other types of machines might require multiple processes to achieve the same net result. The trade-off is typically higher wear costs; however, careful analysis should be performed to account for opportunities to save fuel, labor, transportation and job site footprint.

Common considerations

While an HSI can process a wide variety of materials effectively, consideration must be given to the types of materials to be crushed.

Broken concrete, such as road pavement, typically is slabby and has a rebar matrix that must be liberated and separated from the finished aggregate. Accordingly, depending on the feed opening of the crusher, the operator generally will be well-served to precondition the material using a pulverizer or rock breaker before feeding it to the crusher. This will help material flow through the crusher more smoothly and with fewer interruptions caused by material bridging or plugging the feed opening.

Trimming off strands of rebar and removing as much metal contamination as possible using picking anchor plates are recommended.

Broken concrete also can contain a high percentage of dirt and soil, so some producers will scalp the material before feeding it into the crushing chamber.

While all the challenges that come with crushing broken concrete also apply to C&D materials, they present their own unique challenges. C&D material generally consists of brick and block, which are easily crushed. However, depending on the demolition crew’s process, this material also could include additional noncrushable materials, such as plastics, fabrics, metal and wood. These materials must be separated before crushing, and, in most cases, manual sorting remains the most effective method by which to do so.

Broken asphalt, like concrete, should be pulverized or broken into manageable sizes to prevent bridging and plugging of the crusher. In addition, the emulsion (oil) within the asphalt is generally very sticky, especially in warmer weather, and tends to build up on almost every surface it touches in the recycling process, including the HSI’s hopper walls and crusher liners and conveyor components.

On mobile plants using diesel engines, radiators and air cleaners could get plugged more rapidly than when processing other materials. Although it can be time-consuming, these components must be maintained to prevent catastrophic failures. Asphalt buildup on hopper walls and crusher surfaces must be removed, often daily. Screen openings must be cleared, and airflow through the radiators must be sustained. Release agents and other products have been designed for this task and their use should be integrated into daily maintenance checks.

Asphalt millings are processed by the milling machine. Depending on the desired final product size, about 80 percent of the milled material will not require additional crushing using an HSI.

Because the material often is finer, if the millings are not screened before being fed to the crusher, the buildup described above can be compounded significantly.

Accordingly, when processing recycled asphalt pavement millings, screening and removing any material that’s already been processed to the desired size before feeding larger material to the crusher is recommended. Not only does this help mitigate buildup, it also helps boost overall production and lowers production costs.

Final considerations

Depending on the scope of the project or type of business being operated, many types of processes are available for different applications, each with its unique benefits and challenges:

• stationary–Usually larger fixed plants where people haul and dump raw material to be processed and sold as an alternative to virgin aggregates. These plants are typically larger and provide higher production capacities and capabilities; however, they tend to be more labor-intensive to maintain and require more real estate and trucks to deliver raw materials and haul away fresh materials.
• portable–Transported on rubber tires, these plants can be moved to a specific job site. They can provide higher capacities compared with smaller mobile tracked systems; however, portable plans also require more time to set up and position and more people to operate and maintain. They also require a higher volume of material to absorb higher fixed costs.
• mobile–These plants typically are self-contained tracked units that can be moved on a lowboy and be operational within minutes of arriving on the job site. They are more economically feasible for smaller-volume jobs compared with portable plants, but usually are less productive, which limits the capabilities of smaller units.

What’s next?

While many variables must be considered when analyzing different applications, some general guidelines can be helpful:

1. If crushing concrete, C&D, millings, broken asphalt or any combination of these materials, an HSI crusher likely will provide the best solution because of its flexibility and relative forgiveness with contaminants and sticky materials.
2. Mobile tracked plants are useful for smaller volume projects or for producers providing on-site crushing services requiring frequent and rapid mobility.
3. Portable wheeled plants provide an attractive solution for projects that require a relatively large volume of material to be crushed within a short time frame.
4. Larger stationary systems are ideal for recyclers with fixed, permanent locations that allow the dumping, processing and resale of recycled materials.

If in doubt, seek out and speak to other recyclers. Unless you intend to become a direct competitor, most want to be good industry stewards and are willing to share their experience to help you succeed.

The author is North American sales manager for Rubble Master Americas Corp., Ennis, Texas. Email him at paul.smith@rubblemaster.com.