Feature: Asphalt and concrete mixing technology
By Chris Sleight19 July 2011
Asphalt and concrete plants may provide fundamentally different materials, but there are many common themes running through their design. The first is that they have to be economical to run, and able to cope with a wide range of mix recipes. The second is that when they are owned by contractors, they have to be quick and easy to transport and set up.
One of the big trends in the asphalt sector over recent years has been the move towards warm mix asphalt (WMA). Instead of production temperatures of 140°C to 180°C for traditional hot mix asphalt (HMA), today's warm mixes are created at 90°C and 120°C.
Despite these lower temperatures, tests have shown that roads built with these asphalt mixes perform as well, if not better, than those featuring HMA.
The advantage is that less energy is required to mix the asphalt, cutting bills as well as CO2 emissions. It also cuts down on the distinctive smell associated with asphalt, which becomes more noticeable at temperatures of 130°C or more. This is important in reducing the nuisance that road work can sometimes cause, particularly in built-up areas.
With the popularity of WMA has come the challenge of producing this type of asphalt with existing plants, which were not necessarily designed to work in these temperature ranges and with this exact type of material. A large part of this involves introducing additives into the bitumen, which means building new storage bins, feeders and metering systems into plants.
If finances allow, a better option may be to invest in new equipment that was designed for WMA, such as Fayat's new generation of Top Tower plants from its specialist subsidiary Marini.
These offer a two stage mixing process - the first with low temperature aggregates around 120°C and high penetration bitumen, and the second with low penetration foamed bitumen. Although this means having two bitumen tanks and additional feed circuits, foam ramps and software to manage the process, it does mean mixing can be carried out at low temperatures using high proportions of recycled asphalt pavement (RAP).
Another method of foaming bitumen in the Top Tower plants is to introduce wet sand at ambient temperatures to bitumen and hot aggregates.
With the energy savings that can be achieved using warm mixes, along with better performance of the finished road surfaces, Marini believes warm mixes are the future of asphalt mixing. "Nowadays in Europe and throughout the world, 'asphalt' means hot mix, but in the space of just a few years 'asphalt' will come to mean warm mixes and, when this happens, asphalt plants are ready and waiting to take part in the green revolution," the company said.
An interesting point about the Top Tower's design is that each module has its own electrical cabinet - what the company describes as a "decentralised design." This means there is less wiring involved when it comes to setting up the plant, speeding the process up.
Portability is a key feature of Terex's new E110P asphalt plants, with all of the key components - counterflow drum mixer, baghouse, cold-feed bins, discharge and aggregate conveyors, scalping screen and control house - placed on a single chassis for transportation. Terex says the entire plant can be relocated in just two days, and as well as making transportation easier, its compact dimensions give it a small footprint when erected on site.
The plant can make virtually any mix, and is capable of outputs of 90 tonnes per hour or more. Options include Terex's warm mix asphalt system, RAP systems and mineral filler silos to increase flexibility in the range of mixes that can be produced.
Also new from Terex is its Impulse II control package for asphalt plants. This features a programmable logic controller and Windows computer operating system to give plant owners and operators precise control over asphalt mix designs, thanks to features like automatic proportioning of mix materials and burner control.
Plant operators enter mix recipes using screen menus or import them from spreadsheets, and can assign a specific name to each mix.
The system stores a virtually unlimited number of mix designs on the computer's internal hard drive, and operators can switch from one to another using a screen menu. The Terex control package offers automatic on-the-fly mix changes, and this helps to reduce waste when starting-up and shutting-down the asphalt plant at either end of the day.
Impulse II also offers advanced, detailed diagnostics for trouble-shooting, so any issue can be quickly fixed to limit downtime.
Beyond mixing asphalt and diagnostics the system gives companies the data they need to improve operating efficiencies and lower mix costs. The software automatically calculates, displays and records the real-time cost-per-tonne of each mix. In addition to tracking standard material, fuel and power costs, the mix cost analysis programme can be expanded to track other related costs such as labour and maintenance.
New from Asphalt Drum Mixers (ADM) meanwhile is the SPL line of hot-mix asphalt plants, which the company describes as an affordable option for contractors with low production requirements. Again, portability is a key consideration, with all the drum mixer components fitting on a single trailer.
Available in 60, 110 and 160 tons per hour (55, 100 and 145 tonnes per hour) production rates, standard features of the SPL range include a drying/mixing drum, wet wash, cold feed bin and surge system. With all models contractors have the option of either a portable or stationary configuration, as well as automated or manual operation.
Like all ADM equipment, the SPL range comes pre-wired and pre-tested. In addition, the company puts an emphasis on its products being low maintenance and easy to use. Only one plant operator and one loader operator are needed to run SPL plants for example, and repairs can generally be done by on-site personnel.
Among the new equipment from Ammann, which builds both asphalt and concrete plants, is the new EcoMix concrete plant. The plant has designed to be flexible and compact, but it can still deliver up to 190 m3/hour of concrete when fitted with a 4 m3 twin-shaft mixer from the company's Amix range.
The galvanised steel supports mean the plant can even be set up in areas of seismic activity, while the optional climate control system means it can be used in any climate, be it in a confined urban site, or on a more remote project.
New from Imer, meanwhile, is the ORU MD twin-shaft mixer, which incorporates several new ideas from the Italian manufacturer. These include an oversized and reinforced drum, which is lined with bolt-on wear-resistant linings. Other features include hatch seals that guarantee water tightness, an automatic greasing system and weld-on wear resistant plates on the steel shafts.
Imer says this has improved the mixing characteristics of the unit, as has an improved gearbox, which is designed to prevent overheating in heavy-duty applications. There is also an automatic high-pressure washing system.
The developments in the asphalt and concrete sector alike illustrate how important fast set up times and ease of transportation are for the construction industry. But there are other trends too. The increase in computerisation of plants helps to makes mixes more traceable as well as produce material with complex recipes and a range of ingredients.