RAP handling and preparation
Old becomes New
The road building industry is aware, like no other, that building roads requires raw materials and energy that burdens the environment. Looking for an optimal reuse of materials from the ones they made before has been part of researching projects for years.
This report shows the practical handling and preparation of RAP, from removed asphalt to the most processed controlled grading of RAP. Process controlled grading of RAP results in the same accuracy for asphalt design and production limitations as raw materials.
Today, collecting old asphalt often results in one big inhomogeneous stock of various types and ages of RAP. That is why reusing is very limited to small amounts and seen as a risk for new asphalt mixes. This process means that partial recycling is still limited mostly to about 40 % in the base and bind layers and 25% in the surface layers. New materials are used to technically reach the conditions that meet the regulation requirements. The moment that the secondary aggregate materials become original aggregates with the same quality level after a lifetime in asphalt, they should be upgraded technically and used as defined aggregates . Then all the old asphalt can be reused when needed and defined. The worldwide accepted covenant (Paris and Hannover principals) to make efforts to positively influence energy consumption and CO2 emissions needs this way of approach. One of the major CO2 users is the urban mining and transport of raw materials, because they have to be imported from abroad. Materials such as stone and sand have great shap stability and do not change their functionality whilst used in asphalt. Certainly for these materials, a repeated use is very popular. Because there are only a few types of gradation regulations and , sorting out upper and binder layers can already be very effective.
In this field, breaking and sieving methods (as in quarries who make new raw minerals), is very effective, because the accuracy is exactly the same. The starting point is not to leave a residual product. The goal is 100% reuse of aggregates and bitumen. Any undesirable substances contained in the old asphalt can be filtered out eg tar-containing bitumen, contaminants. Bringing together already existing know-how in different techniques and combining them leads to best results. For example, the stiffness of asphalts can be increased, by using different crushing processes. The processing can lead to a better quality of minerals to reuse.
It brings information to:
- The technical improvement of a parallel heating system (needed above 40%),
- The layout of a future asphalt plant
- The energy consideration and the laboratory activities
- The political choices and vision towards future normalities
- Implementation of materials also in concrete and rail construction
- Financial balanced situations in project costs relating to the use of energy and CO2 profit
The existing breaking method uses the hardness of the stone to break, but also to rub against each other. With this method, sand and bitumen particles will be divided to the sand part. With sufficient abrasion the result is that the stones are quite bitumenless because the bitumen concentrates in the dust / sand fraction. An additional advantage with this method is that the stones become more angular in quality, which is required more and more in road construction to increase the stiffness of the asphalt mix. The two most experienced techniques are described below. Working together on one line of the two systems has led to extra use because all grading types can be graded very precisely. The low energy load in these breaking systems support the extra handling. Basically breaking and scraping the stone is based on the same principle . The RAP material must rub against each other, whereby the bitumen film is separated from the mineral by sanding. The stones are forced against each other. These tests have been tried via various breaking arrangements. After leaving the crushing plant, the material can be sieved, leaving the fine fraction with a lot of bitumen and the stone fraction as bare as possible. The crushing plants that have been used are the following. A Prall tec crusher and a mobile Hazemag rotor crusher (existing equipment).
With the existing installations (Prall and rotor breaker) many tests have been done to optimize the breaking process. The Hazemag Rotor Crusher breaks the asphalt chunks forward into manageable granules. New types of breaking systems (turbine breaker), including both techniques, are available
The type of breaker also naturally influences the abrasion behavior of the mineral. One of the types that seems best suited to take the bitumen into the sand part seems to be a turbine breaker. The mineral remains in the abrasive phase for a long time because it passes through the entire cochlea of the cup, the effect is then greater than in a normal breaking process. With this technique, the output can really be increased. The breaker can be used with and without slats. The version with slats has been used in practice.
The optimized way of scraping ensures that minerals end up in a higher stone class (100% broken) and therefore get more value. This means that less exploitation of quarry material is required.
The separation of the various components ensures better utilization in recycling percentage, as a result of which asphalt types that are now partially recycled or not are given the addition of these recycled materials.
This can be achieved in a real operational setup, with the right attitude and control.
In the partial use of the old asphalt, a high environmental gain can be obtained in connection with the already existing techniques. Applying partial recycling can be increased to between 70 and 100%. Here the improvement is mainly in making minerals applicable and keeping them, which extends the life cycle of raw materials.
Financial reduction of about 50% of costs when old material replaces new in asphalt mixes, means investment in machinery can be gained back in less than five years.