The reuse of reclaimed asphalt makes economic, ecological and constructional sense. Against the background of increasingly scarce building materials, selective milling makes an important contribution to the sustainable use of our resources. With this method, the valuable building material can be removed in such a way that it can be recycled to the highest possible quality - as prescribed by law. This is not witchcraft, as long as some technical boundary conditions are taken into account during the tendering, planning and execution.

The aim of selective milling is to remove the individual asphalt layers separately. In this way, each raw material can be reused "in a high-quality manner according to its type, composition, quantity and ingredients". This is what the Closed Substance Cycle Waste Management Act requires. For asphalt recycling this means: the different asphalt layers must be "kept and treated separately for recycling". In short: layer-by-layer milling of surface, binder and base courses.

Modern cold milling machines can do this with the aid of automatic levelling. Usually, the thickness of the layer to be removed, the milling depth, is determined roughly in the tender and in detail on site. To ensure that the milling machine removes the pavement to exactly the required thickness, mechanical or optical sensors on the cold milling machines scan a reference plane, also known as the reference surface. The automatic system then ensures that the milling depth is maintained. Which sensor technology and which sensor arrangement is suitable depends on the conditions on site. In any case, the necessary equipment is available at Europe's milling service providers.

Notes in the tender help the milling service providers to offer or plan for suitable machines and technologies. However, the various sensors and levelling methods are differently complex in preparation and application and deliver differently precise results. However, a separation of the layers for separate recycling is possible in most cases. Information on levelling methods can be found on the website of the VESF (www.vesf-ev.com), the Association of European Road Milling Companies.

As a rule, the support provided by the client's decision-makers also has a positive effect on the quality of the milled material. When milling begins on site, it often becomes apparent that the actual layer thicknesses deviate from the planning data. The site management has to decide ad hoc on site which milling depth to use in order to obtain the best possible graded milled material. For this reason, the milling service providers welcome it when appropriately authorised employees accompany the construction work - especially when milling heavily deformed pavements.

In order to achieve a high degree of purity, especially when removing the surface layers, the milling depth must be selected in such a way that - even with variable layer thicknesses - no material from the underlying layer is taken up. This is because, although milling machines can precisely maintain the specified milling depth, they do not have "X-ray eyes". They cannot recognise the height of the individual layers. As a rule of thumb, if the milling depth is 5 mm less than the layer thickness specified in the ceiling book, the milled material is graded.

In the Netherlands, the 5 cm thick surface layers of drain asphalt in particular have been milled separately for some time. The experience has been consistently good. In general, it has been shown that a clean condition survey in advance, which allows reliable statements about the layer thickness, has a positive effect on the quality.

Incidentally, the time required for selective removal is only slightly higher than for full removal in one pass, as the machines run much faster when milling thinner layers. The main reason for the increased time requirement is that the machines pass over each spot twice or three times instead of just once and have to reset again and again. Experience shows that the milling contractors need approx. 10 - 15 % more time for milling in 2 layers (e.g. 12 cm surface and binder course and 18 cm base course) with the same number of machines as for full-depth paving.

The additional costs for layer-by-layer milling are also manageable. Compared to the full-scale construction, they are 15 - 20 %, which are mostly compensated for in the costs for the overall process due to the savings in new aggregate. The overall economic efficiency is therefore given.

Truck drivers are pleased with a completely different aspect: the milling edges are less high. As a result, the trucks can drive and manoeuvre much more easily to remove the milled material on the construction site.

Last but not least, the tendering authorities have it in their hands to strengthen this approach. Together with the road construction companies as direct clients of the milling service providers, they can create the conditions for a high recycling rate through consistent tendering and implementation. The technology and know-how are available in the market. So there is nothing technically standing in the way of increased selective milling for the purpose of recycling the high-quality reclaimed asphalt - it just needs to be commissioned and implemented.

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