Modern Asphalt Production Uses Reclaimed Asphalt
Summary: This paper reviews modern technologies for the production of asphalt. The re-use of reclaimed asphalt is a key element for effective use of resources, resulting in economic savings and the protection of the environment. Best practice solutions for the asphalt industry are shown.
- Sustainable Road Building
The necessity for thrifty use of limited raw materials and for a global reduction of CO2 emissions has been recognized by all countries worldwide. The road building industry has thus to fulfil 2 basic requirements: enabling society’s maximum mobility while preserving its living space. Road authorities, road builders and equipment manufacturers have to share this vision to make it real.
Asphalt roads are sustainable roads. European studies show that every ton of asphalt produced with traditional technologies generates 46 kg of carbon emissions. Direct emissions during production on site amount to 24 kg CO2 per ton while 22kg CO2 per ton are related to the generation of bitumen, aggregates and electricity prior to the mixing plant. Not included in this calculation are the CO2 emissions caused by transport (1).
Energy consumption is always related to CO2 emissions and energy consumption means production costs. A reduction of energy consumption in asphalt production reduces both the CO2 emissions and the production costs. Economic targets in the asphalt industry are thus automatically ecological targets and vice versa. Thanks to modern state-of-the-art technology it is possible to produce asphalt with 40% less new material resources and 25% less CO2. The economic benefit accounts to the same order as the CO2 emission reduction.
- Typical Asphalt Production Process
Asphalt basically consists of 2 types of components: various kinds of aggregates in different size distributions providing the load bearing performance and bitumen as the binder assuring mechanical and thermal flexibility.
Figure 1: Schematic modern batch-type Asphalt Plant
The two types of ingredients are mixed in a paddle mixer located on the mixer platform mid-height of a typical asphalt plant shown in Figure 1. The mixer is filled with material of 2 to 5 tons and mixing time to produce a batch of asphalt is around 1 minute. Such a batch-type plant is very flexible: Any kind of asphalt can be produced one batch after the other, from the top-layer of a highway to the playground of a school.
Key criteria for a quality asphalt mixture are good adhesion between agglomerates and the bitumen binder. Therefore the aggregates need to be dried and heated to the process temperature. This is done in a rotary drum on ground level equipped with a burner.
Afterward the hot minerals are lifted to the top of the plant, where a screen separates the various fractions and their size distributions into the hot aggregate silos above the mixing platform. The binder is pumped from the storage tanks to this platform as well. After mixing the asphalt batch is stored in the hot mix silo prior to loading onto trucks for transportation to the road construction site.
The drying and heating process of the raw material is the biggest energy consumer at the asphalt plant. The range of performance lies between 120 and 320 tons per hour, which requires 850 to 2’800 kg of fuel per hour (or any other equivalent fuel). Modern drying technology combined with easy-to-use plant control assures that this large amount of energy is used efficiently and burned cleanly. Poor performance with this high energy capacity has catastrophic impact on emission levels and production costs.
Another location to lower production cost and direct CO2 emissions is the bitumen tank farm. Traditional bitumen tank farms consist of horizontal tanks heated with thermal oil. Changing to an electrically heated, vertical tank farm as shown in Figure 1 results in significant advantages such as no emissions, no oil consumption and less use of electricity. Therefore electrically heated bitumen tanks have become standard in the whole of Europe and other parts of the world.
By far the biggest savings in production cost and in indirect CO2 emissions is the recycling of used asphalt. Reclaimed asphalt is not waste material but corresponds to a high-quality re-usable material.
The aggregates of reclaimed asphalt show little aging and are mechanically and geometrically within the quality ranges of new material. The aging of bitumen is limited as well and can be compensated by using small amounts of new bitumen. Figure 2 shows the potential cost savings (indicated of bitumen savings) and the indirect CO2 savings assuming the need of 1% fresh bitumen to reach new material standards. Overall cost savings using reclaimed asphalt include not only bitumen savings but aggregates savings as well. These significant economic advantages are linked with the ecological advantage of sustainable use of resources.
Figure 1 shows a second rotary drum on the top level of the plant where reclaimed asphalt is smoothly reheated for direct transfer to the mixer platform. For more than 20 years Ammann has parallel flow drums in use and today nearly all the asphalt mixing plants run with a parallel flow drum system.
In conclusion modern asphalt production technologies offer 2 types of economic and ecological advantages: energy savings due to improved plant and production processes, and material savings proportional to the degree to asphalt recycling.
Figure 2: Bitumen savings and reduction of indirect CO2 emissions as a function of the ratio of reclaimed asphalt used
- Recycling in Batch Plants
The usability of reclaimed asphalt, or recycling, is an absolute necessity of today. We are able to offer you custom-made solutions relating to recycling. Our modern plant technology guarantees you extremely high quality of asphalt using recycling asphalt (RAP).
- Feed of up to 60% hot recycled material via parallel dryer (below)
- Feed of up to 30% cold recycled material directly into the mixer (below)
- Individual feed options in the dryer drum (below)
Whatever your solution looks like, our experience in hundreds of installed systems guarantees you reliable functioning. Ammann RAP feeds are individually adapted to customer requirements.
Advantages at a glance
- Significant reduction of production costs due to lower costs of bitumen, minerals and transport costs
- Supported or promoted by statutory legislation (country specific)
- Less effect on natural resources (fewer oil, bitumen and mineral requirements)
- Reduction of expensive storage facilities
- CO2 reductions
The following conditions have to be met in order to be able to process recycled materials and thereby reduce costs:
- Sufficient RAP available on the market
- Suitable preparation of recycled materials (crushing, screening, homogenisation)
- Possibility of preparing the RAP to 0 – 32 mm granulate
Prerequisites for high RAP feed rates
- Suitable storage area for selective type-based management
- Suitable feed options in the asphalt mixing plant
- Sufficient RAP materials
- Laboratory-based monitoring
- The higher the RAP material quantity, the more important the quality of the RAP, as a smaller degree of new minerals can correct the grain characteristics
- Legislation must allow the application of RAP materials
On the way to 100% RAP feed
Ammann has set the target of increasing the reclaimed asphalt ratio from today’s maximum of 60% up to a maximum of 100%. The amount of reclaimed asphalt is increasing continuously and the market demands an environmentally-compatible and cost-aware reuse of natural resources. The new asphalt mixing plant concepts have been developed in this direction without forfeiting production capacity.
Cold-mix asphalt with bitumen foam
“Foam Asphalt,” as it is known, has become much more popular in recent years. This is a simple and highly effective method of producing cold asphalt. Water is sprayed through nozzles into the hot bitumen under pressure. The water expands explosively when the pressure is released, resulting in a fine bitumen foam that can easily be used to coat the cold aggregate substances. The quality of the cold mix can be influenced as desired by adding 100% RA and cement. This mix is laid conventionally with finishers.
- The Future
The asphalt-recycling technology moves ahead. The 100% asphalt-recycling asphalt plant has been developed and tested. The vision is to produce any asphalt using reclaimed asphalt in the range from 0% – 100%.
Another global trend in asphalt production points toward lower mixing temperatures. Numerous technologies are available or under investigation. We grouped them into 5 categories: special aggregates, special binders, additives, bitumen foaming and special mixing. The last two categories ask for adaptations at the plant, whereas the other technologies comprise additions to the mixture.
The conservation of living space and mobility are compatible when modern technologies are used.
To bring such technologies to the street, all participants involved need to cooperate closely: the road owners, the road engineers, the engineering standards, the asphalt producers and the manufacturers of the mixing plants. Today’s economic and ecological requirements will speed up such collaboration for the implementation of new technologies.