Fine milling is increasingly being used on municipal and main roads, motorways and even runways today in order to prepare the surface for more extensive rehabilitation. Three practical on-the-job examples with cold milling machines from Wirtgen illustrate the importance of fine milling for optimally preparing the substrate so that thin cold layers can subsequently be applied.
Costly all-round rehabilitation is not always possible today when maintaining the German network of roads, due to the financial constraints of public budgets. Ruts, slippery surfaces and lateral displacements in the pavement of heavily frequented roads pose a serious threat to traffic safety. Road maintenance authorities have to find a quick and cost-efficient solution. In the case of motorways or main roads, it is also important to minimize the nuisance to traffic due to the rehabilitation work and the resultant traffic jams. Thin cold layers – also known as microsurfacing – are placed wherever the service life of a damaged asphalt top course is to be extended without replacing the complete pavement. The method has for many years been a quickly implemented and cost-efficient alternative which experts consider to be a reasonable temporary solution for road maintenance. The properties of the road surface, such as tyre adhesion, are restored or even improved. It is estimated that more than five million square metres of thin cold layer are placed in Germany every year when repairing roads.
Cold milling machines with fine milling drums do the main preparatory work for laying a thin cold layer in accordance with the requirements of the ZTV BEA-StB 98/03 regulations. Fine milling ensures that the deformations are removed from the carriageway surface and a uniform, level surface is produced instead. The typical fine-milled profile of a carriageway "roughened" in this way ensures optimum adhesion for the subsequent thin cold layer, for the "valleys" in the fine-milled profile firmly engage with the thin cold layer.
Preliminary treatment of the damaged pavement by fine milling also ensures that the thickness of the subsequent thin cold layer remains constant due to the defined milling depth and compensatory levelling technology of the cold milling machines. Future susceptibility to damage after rehabilitation is reduced in this way, as the load is spread equally over the entire, uniformly thick pavement layer. This reduces the incidence of deformation or undesirable structures.
After the preparatory fine milling, self-propelled mixing and laying machines produce the mixture for the thin cold layer directly on site and normally lay two layers on the pre-milled surface. The first layer acts as a profile, the second as surface course. The mixture is spread over the complete carriageway strip with Vario screeds which can be infinitely adjusted to the required working width.
One important point in this context is that the decision in favour of preparatory fine milling for subsequent placement of a thin cold layer does not increase the overall costs of the project. On the contrary: fine milling significantly helps to cut costs as considerably less of the mixture is needed when topping the fine milled surface with a thin cold layer, due to the low valley-to-peak depth of the fine milled surface. The savings achieved for the cost of the mixture may be greater than the costs additionally incurred per square metre for the actual fine milling work. The method's cost-efficiency is clearly proven.
Frost damage and ruts posed a danger to traffic on several sections of the A48 motorway between Koblenz and the Dernbach motorway merging point. A solution had to be found quickly, for the motorway is heavily used by heavy goods vehicles and transit traffic. More extensive damage to the underlying substructure also had to be prevented. The asphalt pavement was removed to a depth of between 0.8 and 1.2 cm in the affected areas by a W 2000 from Wirtgen and the reclaimed asphalt material (RAP) loaded directly via the machine's conveyor belt onto a truck travelling ahead of the machine. During the milling work, the surface was additionally cleaned by vacuum sweepers following immediately behind the milling machine so that the first layer of mixture could subsequently be laid onto a surface with optimum adhesion. An area of more than 3,000 m² altogether was fine-milled by a fine milling drum with a spacing of 6 mm x 2. Drums of this kind are equipped with 672 point-attack tools and produce a profile with a correspondingly good grip. Traffic was able to pass unhindered on the left-hand lane throughout the work. The next day, the first layer of mixture was placed on the fine-milled asphalt as a preliminary profile. The planeness of the rehabilitated carriageway was to a large extent assured by fine milling as the first step. Fine milling also eliminated any cross-sectional convexity so that the thin cold layer could subsequently be laid without further work and the associated costs.
In this particular case, the motorway could rapidly be reopened to traffic as a result of the following measures: both the fine milling work and placement of the thin cold layer took the form of travelling road works so that traffic could be diverted past the site. Traffic jams can also be reduced by undertaking such work at night on particularly heavily frequented sections of motorway. Traffic was able to flow over the fine-milled surface as soon as the W 2000 had passed on, even before the thin cold layer was placed a few days later. That would also apply if the thin cold layer were to be placed very much later, for fine milling in itself is sufficient to significantly improve driving comfort by minimizing ruts in the majority of cases.
A similar project on the B83 main road between Hameln and Hehlen in May 2004 proved that fine milling in combination with placement of a thin cold layer can be thoroughly profitable. Due to extensive rutting, the asphalt pavement on both sides of the carriageway had to be durably rehabilitated here in order to cope with heavy traffic loads in the coming years. A total area of almost 23,000 m² was rehabilitated over a length of roughly 3 km. Preparatory fine milling is particularly effective on sites of this size and with the type of damage found there: the savings and the repairs made to the damaged asphalt pavement confirm the cost-efficiency and quality of the method selected. Here too, the tried-and-tested W 2000 with fine milling drum took on the job and completed the milling work in just two days so that the thin cold layer could be applied.
Carriageways of every kind, including airport runways which are exposed to extreme load pressures due to the weight and speed of aircraft, can profit from the placement of thin cold layers with preparatory fine milling. Rapid completion of a planned construction project is particularly important during ongoing airport operation and the durability of the pavement must be guaranteed for several years to come. The purpose of the work at Jever airport in northern Germany was to restore the skid-resistance of the runway and foreseeably extend the service life of the old surface courses.
Here too, it was perfectly clear that the quality-boosting fine-milling method would be applied in order to assure a level surface overall. And safety naturally takes top priority at an airport. As a result, the optimum bonding between rough fine-milled surface and thin cold layer once again proved the unbeatable technical advantage of fine milling at Jever airport.
The projects outlined here clearly show that the various possible uses of thin cold layers combined with preparatory fine milling open up enormous economic potential for future road repair projects, not only for specialized road milling contractors, but also for the construction authorities inviting tenders for the work. This method indisputably constitutes a cost-efficient alternative with high quality standards.
It goes without saying that a high-quality result also requires machinery which fits the bill for the job in hand. The cold milling machines of type W 2000 used in the three projects outlined above were equipped with fine milling drums which produced a profile with optimum intermeshing structure for placement of the new pavement. The special milling drums are produced in this precise version and high quality by the market leader Wirtgen in a unique manufacturing process at its plant in Windhagen. The optimum tool system with high-grade point attack tools and toolholders for equipping the milling drums has been developed here together with the systems partner Betek, a manufacturer of specialized tools in Aichhalden. Through years of experience and know-how in the road milling business, Wirtgen GmbH has been able to expand its competence in cutting technology to such an extent that fine milling drums with a spacing of less than 8 mm have been produced in top quality for the past five years. Many German contractors specializing in road milling now have a large selection of fine milling drums with spacings of 4, 5, 6, 7 or 8 mm which they use to prepare for the placement of a thin cold layer, for example. Depending on requirements, these drums can be flexibly installed in the cold milling machines equipped with Wirtgen's patented quick-change drum system known as FCS (Flexible Cutter System). A road milling contractor with FCS can fit his machine with different milling drums with different spacings and working widths. The practical advantages are clear: one machine can be used for a whole variety of applications and its owner can considerably boost its capacity utilization.
Michaela Adams, Mario Linnemann
Press and Public Relations
Phone: +49 2645 / 131-0
Fax: +49 2645 / 131-499
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Sophisticated precision: the special fine milling drums from Wirtgen are equipped with high-grade point attack tools from Betek and produced in a unique manufacturing process with spacings of less than 8 mm.
After milling: the typical fine-milled profile is skid-resistant, with rough valleys ensuring optimum interlocking of the thin cold layer. Result: a strong bond between layers as proof of quality.
Less can be more: mixing and laying machines spread the mixture for the thin cold layer over the fine-milled surface with Vario screeds. Preparatory milling cuts the costs otherwise incurred for laying expensive pavement material.
Jever airport: the skid-resistance of the runway was economically restored by placement of a thin cold layer on a prepared fine-milled surface. The method is particularly advantageous for use on large areas.
A48 motorway near Koblenz: rapid rehabilitation of the carriageway surface without major traffic jams. The pavement was fine-milled by a W 2000 on a travelling site.
B83 near Hameln: the reclaimed asphalt pavement is loaded onto the truck by the W 2000 in a single pass without interrupting the work. Fine milling eliminates deformities in the road surface.