Over recent years, a new term has come into usage in the asphalt vocabulary, namely warm asphalt mixes.
A general increase in environmental awareness means that a considerable amount of attention is now paid to this type of asphalt mix, mainly regarding the reduction of exhaust gases into the atmosphere, in accordance with the directives of the Kyoto protocol.
Recent developments into various production techniques of warm asphalt mixes have led to the release of a new range of products onto the market.
And this, in turn, has resulted in the redesign and modification of asphalt plants, to cater for these new methods and to guarantee that the quality of the final product is, at very least, equivalent to the quality of traditional hot mixes.
While the production temperature of traditional mixes ranges from 140°C to 180°C, new plant solutions and technical developments have now made it possible to produce asphalt mixes at a temperature between 90°C and 120°C, in complete compliance with work specifications.
Studies in this field were first carried out in 1995 and preliminary trials were conducted in 2000. Nowadays various, well-tested technologies are available which allow work to be effected at low temperatures without compromising plant performance. Also, the roads built with this type of product and methodology have now been tested over long periods of time and their behavior can certainly be considered similar to (or, in some ways, even better than) traditional asphalt mixes.
But why do we need to produce asphalt mixes with lower than standard temperatures?
First of all, less energy is required (lower burner consumption estimated in the range of 20% to 40%) with consequent reduction in the fumes volume, especially CO2 and COT emissions. As you can see in the following photo, plant working conditions are considerably improved, with almost complete elimination of emissions and a sensitive reduction in the odor of the asphalt (in fact, working at temperatures lower than 130°C considerably cuts down on fumes/odor problems).
Even more important are the advantages experienced on the work site. Working at low temperatures greatly improves conditions for paver finisher workers, the road works can be quickly reopened to traffic, production time and laying time can be lengthened, and features of production may be improved such as the reduction of oxidization with consequent bitumen aging. Then added to all this is the reduced environmental footprint, especially in urban areas.
Why is there a reduction in burner consumption?
The following diagram illustrates the linear increase in fuel consumption as the temperature increases.
When 100°C is reached a high burst of energy is experienced due to the heat of vaporization (this consumption is connected with aggregate moisture).
Once the aggregates are dried, the temperature continues to increase together with fuel consumption. Therefore, if the production temperature is reduced, let’s say from 160°C to 120°C, the 40°C reduction in final aggregate temperature results in a saving of around 1 kg of fuel per ton of asphalt produced.
The present state of the art envisages a series of additives which allow this type of work to be carried out using existing plants without having to make major alterations.
Solid additives may be fed into the mixer in the form of crystals which contains up to 20% of their weight in water, which is then released at temperatures exceeding 85°C, creating a ‘foam’ inside the mixer and encouraging correct bitumen adhesion to the aggregates, even at lower than standard temperatures.
To introduce these additives the plant must be fitted with a pneumatic metering system into the mixer. (Trials have also been carried out feeding the additives from sacks onto special chutes, a method which has not at present been optimized).
Different types of wax may also be melted into the bitumen. These are a kind of paraffin which is fully soluble in bitumen at temperatures over 115°C and reduce the bitumen viscosity, therefore allowing mixing at lower temperatures.
The asphalt plant must be equipped with a wax storage and feeding system into the bitumen circuit. (Alternatively, these waxes may be fed directly into the bitumen tanks but, again, this method has not been optimized, given the difficulty in managing the correct quantities of bitumen and additive).
Several liquid additives exist which allow bitumen viscosity to be reduced to work temperatures. With this solution, the plant must be fitted with a feed system for metering small amounts of additive into the bitumen weighing recipient.
There are further interesting technologies requiring considerable intervention on existing plants and/or completions on new plants, but which guarantee even better plant performance than other methods. In particular, we cannot fail to mention the new generation of TOP TOWER plants which are already fitted out for use with all available market technologies.
The double mixing method is especially interesting: the first mix is low temperature aggregates (around 120°C) with high penetration bitumen; the second mix involves low penetration bitumen ‘foamed’ on its special ramp.
This technology allows work to be carried out at very low temperatures and with the use of high percentages of RAP; this method does however require major plant adaptation, in particular with regard to bitumen storage (two tanks for the two special types of bitumen used), bitumen feed circuit, water, bitumen ‘foam’ ramp and software management for the whole process.
Another technology uses the introduction of wet sand into the mixer. This process again involves two successive mixing processes: firstly, for mixing large aggregates at 140°C with standard bitumen, then secondly for wet sands at ambient temperature whose water content creates a ‘foam’ effect and ensures the correct coating of aggregates even at low temperatures.
Excellent results are obtained with this methodology in terms of energy savings and emission reduction. However additional material must be added to the site for feeding the sand into the mixer and, above all, equipment is necessary to correctly monitor the whole operation.
Warm asphalt mixes are a response to an attempt to produce a more eco-friendly product and reduce the environmental footprint.
The difficulty in proving the validity of these products has now effectively been overcome; the main problem faced today is making the practical step of introducing these technologies into the work specifications of public authorities.
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. There are no general rules which are true for all sites and all clients, and so Marini is intent on working as a partner with each and every client, to analyze production requirements and to adapt the plant to suit every working situation.
Given the importance of this discussion, together with the fact that rising oil prices are making bitumen an increasingly precious product, many companies in the sector are now aiming at solving the problem, at least in part, by developing technological solutions which utilize both warm asphalt material and RAP, without compromising the high quality of the final product.
Consequently it is Marini’s intent, as market leader, to work alongside its clients in getting the most out of the plants and achieving the top in asphalt production, bearing in mind two key words: ‘recycling‘ and ‘temperature reduction’.