In many countries there is no legislation limiting RAP employment in the asphalt mix; the choice is often up to the project manager or the contract specifications.
With the appropriate controls, the correct proportions of the ingredients and the employment of adequately equipped plants, it is possible to produce an excellent conglomerate, which in no way falls short of the mixture obtained using only virgin materials. Generally, it is recommended to use high RAP percentages in the lower layers of the pavement (with basic conglomerate or binder), while in the surface layers, which are easily ruined, it is preferable to use very low percentages.
Hence, using RAP implies:
Finally, during the removal phase, it is also very important to carefully position the RAP according to its extraction package and to ensure that it is covered, in order to keep the moisture content as low as possible; this is a determining factor in the employment of RAP. What has been described so far however is not enough on its own. The topic of RAP cannot be considered only in technical terms because nowadays the environment is becoming an an important question. Ultimately, the need arises for constructive and positive collaboration between public authorities, manufacturers and plant operators. Consequently, the question which has been raised and which requires an environmental, ethical and technical answer becomes the following:
on one hand, how can I utilize RAP to obtain a mixture which boasts the same quality as the traditional mix (preserving the bitumen in the RAP and optimizing the percentage used) while, on the other hand, how can I manage the emissions produced by the plant?
Now we will deal with the most frequent and worldwide spread techniques of RAP employment within the plant , paying particular attention to the aspects concerning the above mentioned friendliness to the environment.
In the production of hot asphalt mix, the mixture has to be heated to a temperature which will ensure satisfactory workability during the laying procedure, and RAP heating must preserve the characteristics of the bitumen and must reduce emissions. It is appropriate to distinguish two ways of heating the RAP, both implemented by different constructors with specific technical solutions.
The first case consists of RAP heating through direct contact with the virgin aggregates. The heat from the virgin materials makes the water in the cold RAP evaporate and then the correct temperature in the final mixture can be reached. Among these techniques we can differentiate between either feeding the RAP before the mixing phase or directly during the final mixing phase.
RAP introdotto nel cilindro essiccatore
To conclude, a high percentage of cold and humid RAP means a high temperature for the heating of the virgin material as well as an immediate and high production of VOC. Moreover, high temperatures damage both the bitumen in the RAP and the new binder.
For all these reasons, this technique is efficient and useful because it favours the use of very specific fractions in known quantities – before being fed into the mixer the RAP is always weighed However, the maximum limit of usage recommended is below 40% for the reasons mentioned above.
Finally, we will present the second way of heating the RAP, through a special dedicated drum, similar to the one for virgin aggregates. This technique allows the heating of high quantities of RAP to temperatures above 130°C. The reason why this method allows use of percentages even higher than 60-70% is immediately clear. During the final mixing phase it is necessary to integrate the mixture with smaller quantities of virgin materials, which are heated to the extent necessary for reaching the correct final mix temperature required.
The development of this technique was driven by the need to limit high emissions from plants equipped with these systems. In fact, the most recent applications are designed with systems which do not heat up the RAP through the direct exposure to the heat of the flame inside the parallel drying cylinder dedicated only the RAP, but through high volumes of air heated outside the drying cylinder.
In this way, the RAP will gradually reach the temperature of the final mix, and levels as high as 100% may be used. Obviously, the issue is always how to identify in what products it is actually appropriate to employ such a high percentage and such an expensive technology.
Obviously, adequate systems of exhaust, conveyance and treatment of emissions have to be matched with each different RAP usage in the production of hot asphalt mix. In order for these systems to be efficient, the exhaust has to operate in context with the fumes generation, it has to be related to the speed of exhalation and it also has be carried out as close as possible to the emissions points, paying due care to external currents. The emissions are therefore channeled air flow emissions and can be checked before being fed into the stack.
The most appropriate technical solution is closely related to and dependent on the characteristics of the production site, the working methods in operation and the costs/benefits ratio of the technical proposal, plus the ever-important emission “management”.
Finally, we will briefly examine RAP use in the production of low temperature mixes, with specially equipped plants.
This combination is extremely interesting; for example, the problems related to RAP packing disappear completely with production at low temperatures. This kind of production offers many advantages, which are primarily environmental but also economic: