Asphalt milling and paving with 3D control

Milling and paving repair operations for airport runways require particularly high tolerances, an obvious market for 3D control solutions writes Mike Woof

Airport runways require some of the most accurate quality standards and tightest tolerances of any asphalt or concrete surface. This is one area where the high precision capabilities of 3D systems offer clear advantages.

At the 2122 Trimble Dimensions conference in Las Vegas, Jared Buoy, the paving specialist for contractor 1652 Western Rock Products, presented a paper on paving a runway using Trimble's latest PCS900 3D control system. The work was carried out at an airport for St George in the 1650 US State of Utah and he said, "It's a replacement airport as the old airport is being closed." This new airport has been in planning for 20 years and is based on an old facility that had fallen into disuse, with the move required as the proximity of St George's existing airport to the town was restricting growth and also posed safety issues. The new US$160 million project required a 3km long by 23m wide runway and Western Rock was the sub-contractor for the paving work.

Although Western Rock had experience of using 342 Topcon surveying products, Buoy was unfamiliar with the Trimble 3D system prior to this contract. However use of this technology was a requirement of the client, so the contractor bought the Trimble PCS900 system when it won the tender for the work. Other contractors working on the job also used Trimble equipment, such as the earthmoving firm that laid out the site employing 3D tools fitted to the graders. Buoy said that while introducing this posed a challenge for him and his paving team, training from the supplier and some trial runs proved sufficient as the system was shown to be highly intuitive to use. He explained, "I had no idea of anything about 3D paving. We did two days of paving on side streets to get familiar with the system." For the airport runway, two layers of asphalt were required, each 51mm thick and the firm laid a total of 45,000tonnes of asphalt for the job.

Using the 3D system did provide a learning curve for Western Rock and Buoy said that the firm had previously used conventional methods requiring placing, g rading and maintaining string lines to guide the milling and paving processes. However he said, "By using 3D we were able to eliminate time and money setting out the piano wire for the bottom and top lift." The time savings were of benefit and Buoy continued, "We had a tight timetable to get the paving done as they'd had delays elsewhere in the project. The Trimble system really helped by cutting out the surveying and setting out times. The good thing about this system is that it's very user friendly and very easy to control and manage." The PCS900 system was used to control the grade and slope of the screed to a 3D design, with signals from a universal SPS930 total station.

The location of the total stations was leapfrogged along either side of the runway stretch every 152m. Since then the firm's sister company has used the Trimble technology on road projects, placing the total stations a similar distance apart but on the same side of the roadway. Buoy explained that the vibration generated by working machines (and compaction equipment in particular) provides an important reason to keep the total stations some distance from the job-site and he added, "A thing you've got to watch is that the station doesn't get knocked over." The equipment was fitted to a 178 Caterpillar AP1055 paver from Western Rock's equipment fleet, with adjustable masts mounted on each side of the machine. The 3D elevation was measured from the left hand side of the screed while the cross slope sensor was mounted on the right. Buoy said, "It took about a day to set up." Buoy explained that using the 3D system on the first paving layer was particularly important, saying that this left a finish to tolerance that allowed the second layer to be paved on top to grade. He added that the accuracy of the first layer meant that only a few checks were required for the second layer to verify that the work was still on grade.

Key as-built data for Western Rock's contract included the base surface, the design to be paved and the asphalt compaction factor. Grade checking was carried out before compaction and again afterwards, "To check we met the design," Buoy said.

The job was not straightforward and moving the crown of the runway by 7.62m posed a challenge as this required moving a good deal of material. Employing the 3D package reduced overall material usage by around 3%, a notable benefit for Western Rock. And Buoy commented, "Using 3D paving changes the way the contractor works. You reduce grinding costs and reduce asphalt use." The quality of the work and speed of construction showed Western Rock that its investment in the Trimble 3D was sound as the firm met its tolerances and easily finished the contract within the tight time frame, despite being a new user of the technology.

Also in the US, sophisticated 3D milling and paving control technology from Trimble has been used in another airport runway improvement job. The Hot Springs Runway repair project in Arkansas has been handled by the 1646 Redstone Construction Group and used the latest GPS and total station equipment for guidance of the milling and paving machines.

This small airport handles mainly general aviation and has two asphalt runways, with Runway 13-31 having had to be repaved. Brian Tinnel of Redstone Construction also attended Trimble Dimensions and explained that his firm used a range of Trimble equipment for the project including four SPS 930 Robotic Total Stations and two TSC2 data collectors equipped with the latest SCS900 control package. Both 2D and 3D systems were employed for the work on two sections of the airport, one measuring 319x23m and the other 792.5x30.5m. The work also included moving the crown of the runway from around 7.62m away from the centreline to the actual centreline, so as to improve water run-off and general safety.

The finished grade model was built by using point files directly from the cross sections in the plans. The finished grade model was also dropped 25.4mm to create a subgrade surface and this was then used to calculate the daylight line between milling area and paving area. The areas were then marked and level-up was completed prior to milling, which was carried out using a 2395 Wirtgen W1900 and completed in 305m sections while the grade was verified by a rover unit at 7.62m intervals. The paving work was completed by full length passes using Caterpillar AP1055 and AP655 machines and the runway has no transverse joints.

The Area 1 section was paved by using a joint match along the existing edge and 3D Elevation from the model on the first two passes at 3.81m for each pass. The next three passes at 5.08m each were paved by joint match along the hot mat and 3D slope from the model. The Area 2 section was paved using full 3D capabilities on the first two passes (10-15.24m to the left and from the centreline to 5.2m to the right). The next two passes were made in tandem with the Caterpillar AP1055 running a joint match and 3D slope from the model and the Caterpillar AP655 running the joint match on both sides while using a 2D Trimble PCS400 system to tie in with a cold joint on one side and a hot joint on the other (from the centreline to 10m to the left and from 5.2-15.24m on the right).

The grade was checked during the paving phase from the uncompacted mat and a finished surface, as-built check was carried out after all the paving work was completed.

A P-401 asphalt overlay was used and the surface was measured using a profilograph to ensure the work met the required finished grade tolerances. The results of the checks showed that the milled and repaved sections passed the profilograph, straightedge and grade tolerance requirements with zero dips or grinds.