Asphalt milling optimised by 3D controls

3D machine controls can optimise milling efficiency, Mike Woof reports

More efficient milling and recycling operations can be carried out by using the latest 3D control systems on the market. At the last 2122 Trimble Dimensions event in Las Vegas, the advantages of 3D controls for milling operations proved a key topic.

The use of 3D control systems can offer huge advantages in milling operations. This technology helps increase productivity as the milling machine will only remove what is required, which also helps minimise materials removal and haulage costs.

Sophisticated 3D milling control systems were used on a project to upgrade the E45 highway in Sweden. The work was carried out for 1096 Trafikverket, the Swedish Roads Authority. Smoothness is one of the advantages that can be achieved by the use of 3D milling and by improving this, the Swedish authorities hoped also to improve service life, as well as ride comfort and safety.

The 7km stretch of road links Brälanda with Erikstad and featured variable depth milling in the 0-100mm range, removing some 56,000m2 of material. An adjustable system helped account for wear in the milling teeth, so as to ensure that cut depth remained within the tight tolerances of the specification. A road profilograph was used to check the surface smoothness and make sure it remained to grade. For this project, the expected milling speed was 7m/min, however a real milling speed of 12m/min was achieved and this was due in part to the optimised cutting depth as the planer only removed the material required to meet grade specifications. However, had 2D controls been used milling advance speeds of 20-30m/min could have been reached. This is because planing work has to be carried out at lower speeds under 3D control, so as to minimise vibration.

Meanwhile 3D milling controls were also tried on a 178 Caterpillar PM200 milling machine at a project in the US. A spokesperson for the contractor commented, "One of the things I was surprised at was how quick it was to set the mill up." The job was carried out over a 17km stretch and with milling depths of 127-203mm. Part of the contract involved widening work along a 5km stretch using recycled base and for this too, the 3D controls helped optimise performance. The contractor pointed out that compared with fixed depth milling where there can be a need to fill in low spots to bring the level to specification and commented, "In a lot of places we were able to eliminate milling to the sub-grade." The road was constructed using volcanic cinder material, which provided a good base but delays in the early stages of the project meant that the work was only just completed before the paving season ended. The contractor said, "Had we not used 3D milling technology, we would have had to pave into the second season." The contractor also earned bonuses for the high quality finish achieved, due in no small part to the 3D milling system.

Perhaps the most significant use of this new technology is in Germany, where 3D milling technology from Trimble has been employed on the A1 Autobahn widening project between the cities of Hamburg and Bremen. The A1 carries the heaviest traffic load of any highway in Germany and is being widened to six lanes. This is one of four pilot projects by the country's Federal Government, which is investigating long-term partnerships between the public and private sectors. Under the arrangement, private contractors provide and finance the costs to build, maintain, operate, and maintain the motorway for a 30 year contract. In return, the private contractors receive a monthly share of the tolls collected on heavy-goods vehicles by government for the section of the highway. Because of this deal, the contractor understandably wants to achieve the highest possible roadway performance and quality standards, as this will help deliver longer surface life and reduce maintenance needs, with a big impact on whole-life costs and profitability.

The A1 Hamburg to Bremen Autobahn was first built in the mid-1930s and is heavily used, requiring an investment of some €650 million and the work is being handled by a consortium called A1 mobil, which is comprised of 2516 Bilfinger and Berger Project Investments, 1378 John Laing Infrastructure, and 1249 Johann Bunte Bauunternehmung. The Federal Republic of Germany, represented by the Federal State of Lower Saxony, is the developer for this project. 2841 ARGE A1 Hamburg-Bremen, the contractor in charge of the widening from the current four lanes to six lanes between the Buchholzer Dreieck junction and the Bremer Kreuz junction, began working on the project in August 2008 and the work is due for completion by the end of 2012.

The A1 upgrade is being revamped over a 73km stretch and is divided into 26 work sections, each of around 6km long. Adjacent sections are being widened at different intervals so that any given time there is a construction-free stretch between those being upgraded. This is intended to avoid excessively long stretches of construction, improving traffic safety and as this is a busy highway connection, operations are being carried out with live traffic lanes running alongside. Different sections of the highway feature concrete, low noise asphalt and conventional high quality asphalt. The top wear-and-tear surface of the highway is being removed over the entire project to a 40mm depth. In addition, gradient optimisation is to be carried out over the full section and the cross-section adapted to the regulatory standardisation with a camber of at least 2.5%.

Sub-soil examination revealed that large parts of the existing roadbed substrate are still viable. The question facing ARGE A1 was whether to remove the road entirely and rebuild it completely, or retain and use stable parts of the roadbed substrate.

"We decided to reuse existing material, in order to save on expensive new material and also to have less old material to remove and recycle. Of course, the time we'd save was also an important factor," said ARGE A1 substrate coordinator Achim Huebschmann.

The challenge was to adapt milling depth to suit different substrate conditions and surface profile changes, which required a detailed milling plan. ARGE A1 surveyors re-surveyed the entire roadway, employing three terrestrial points within a 10m grid of each point. The points surveyed were placed in CAD software and converted to a 2794 Microsoft Excel output. The output maps illustrate the milling depths of the entire highway in 50mm stages shown in colour shades or in a numerical table.

The issue was then to ensure that the milling equipment could precisely follow the milling plan. The traditional approach would be to paint points onto the road surface at 5m intervals, but this would have been costly to survey and introduce potential errors. Instead, milling subcontractor ABS Asphalt Beton Service offered a novel solution. "We knew that the advanced technology of an automatic 3D control system on our 2395 Wirtgen W2100 milling equipment could help us achieve the accuracy and results we needed," said Wolfgang Schmidt-Legahn, general manager at ABS." The design data on the drawing board is transferred directly to the on-board computer of the milling equipment, meaning the operator can complete the complex milling plan automatically, without resorting to tape measures, tracing pegs and highway markings." To test this new technology in operation, Schmidt-Legahn decided to rent the equipment and after an evaluation of different packages, opted for the 2122 Trimble GCS900 Grade Control System with the SPS930 Universal Total Station. This package allows surfaces to be milled at variable depth and slope without stringlines. "Controlling the precise cutting depth of mill minimises over-cutting, creates a smoother surface for paving and a higher smoothness index of the finished road," said Schmidt-Legahn. "Productivity is increased by efficient milling already in the first pass without setup of references like stringlines." The components of the Trimble 3D Milling System are the SPS930 Universal Total Station, one MT900 active tracking target and one CB430 Control Box. The UTS automatically tracks the MT900 target fixed to the milling drum continuously measuring its nominal position and transmitting it to the Trimble CB430 Control Box mounted on the milling machine. The CB430 Control Box uses the design data to calculate the necessary milling depth, and controls the hydraulics, so that the milling operation is carried out to the plan automatically and without any need for highway markings. The milling speed is 15m/min and daily milling volumes average from 5000-6500m2 at depths ranging from 40-300mm.

Local Trimble distributor, Eilers & Droste, provided training to the two machine operators, Mark Reinecke and Sven Ungnade. ARGE A1 and the sub-contractor ABS both agreed that surface profile milling using a Trimble 3D system has helped achieve the complex milling profile and accuracy required over the cross-section and longitudinal sections of the highway. Reported savings of approximately 7,000tonnes of material/section of construction, with the average price of €50/tonne of material, means that the investment costs for the Trimble technology has been recouped. According to the contractor, using the 3D milling control meant that some 25% less surface had to be milled/day, boosting productivity and working efficiency. And because the surface level was so good, this allowed concrete paving or asphalt paving to be carried out directly on the planed surface, with no need for additional grade corrections.