When the carriageway of the Dartford Crossing western tunnel failed, innovative material and contracting solutions were required. Access was limited so high application speeds were critical and the materials had to be finished to a high specification and durable, given the large volume of traffic, particularly HGV’s, on this section of the M25.
Following trials Connect Plus, the DBFO consortium for the M25, selected the Eurovia Flexiplast system as the waterproofing layer. But it was critical that for this, and the whole success of the new carriageway, the planed surface had to be finished to a very high specification; very smooth and very dry. Power Plane recommended the use of the new Wirtgen fine milling drum to achieve the optimal finish for the waterproofing layer, ensuring the success of subsequent layers.
The Innovative Concept
Conventional planing with a normal milling drum generally produces a finish of 5-6 mm texture depth. The fine milling drum (left) has a quadruple helix with a total of 672 picks at 6 mm spacing, compared with 168 picks at 15mm spacing on a conventional drum. It produces a surface with shallower texture of less than 1 mm, and leaves a much finer finish for immediate trafficking or to accept a new thin surfacing.
The twin cutting tool per line design enables the planer to move at a much faster forward cutting speed than a normal drum. It also cuts any protruding aggregate softly, compared with a drum with fewer picks, which can catch the aggregate and flick it out of the surrounding material.
At the start of 2010, Power Plane recognised the opportunity to introduce fine milling as an innovative means to save materials and reduce highway maintenance costs in the current environmentally conscious and economically tough climate.
Fine milling has been used in Germany for 10+ years, extensively for retexturing.
Power Plane trained their operatives and conducted their own trials at RAF Lakenheath in May 2010.
A major trial was carried out in July last year, finalising the selection of materials and work methods for the Dartford Tunnel resurfacing contract, and Power Plane’s proposal to use the fine milling drum was accepted. The Highways Agency agreed to close the tunnel on fourteen Friday and Saturday evenings, starting in October 2010.
Each shift was run as a military operation, beginning at 8pm. Five planers worked to remove the old layers, coarse milling down to the waterproofing layer, before changing to the fine milling drum to texture the concrete base.
Following milling, the surface was swept, washed and dried using a combination of a jet engine dryer and hand lances. Because of the fine texture there was no pooling of water and the surface dried very quickly. This was a critical stage in the process because for the Flexiplast to properly key to the base and succeed, the surface had to be completly smooth and dry.
The Flexiplast waterproofing membrane was then applied. The low texture ensured that an even coverage was achieved at the required thickness of 3mm, even at the top of the ridges, with no material wastage or extra coverage required. Within an hour the Flexiplast was ready to be overlaid with the asphalt.
Pre-contract and progress meetings during the contract ensured the operation went smoothly and to schedule, particularly as there were so many people and so much plant in a confined workspace. Power Plane’s operatives were all equipped with Pelter headsets, ensuring clear communications in the high-noise environment.
The milled surface was checked and recorded using sand patch tests, undertaken by both the main contractor and waterproofing contractor, measuring a texture depth of between 0.4mm to 0.7mm.
Wider Benefits and Future Potential
The shallow texture depth gives fine milling wider opportunities for use within the UK highway maintenance industry to level uneven carriageways, to improve skid resistance, or for fine milling carriageways that are to be trafficked immediately after milling, whether they are, or not, to be ultimately overlaid. Benefits to industry and the wider society include:
- High productivity and less traffic disruption
Fine milling of a surface is a fast, efficient process that produces a base surface with a shallow texture of less than 1mm, leaving a, smoother finish ready for immediate trafficking or for a new surfacing course. Fine milling is achieved more quickly than conventional milling. The Dartford Crossing contract has demonstrated what can be achieved in a very limited time window.
- Improved reliability of the overlay
There are potential problems achieving a proper key with a thin overlay on a conventionally planed surface, with the ridges often visible through the surfacing, affecting durability. Fine milling ensures the specified depth of the overlay is achieved, even on the ridges.
- Cost Savings
The minimum spread rate of the new surface can be applied because the grooves do not need to be filled in order to achieve the required nominal depth on the top of the ridges.
The use of a fine milling drum gives the client more options and more flexibility for surface preparation and overlay.
- An innovative maintenance solution with wider potential
Fine milling can be used to extend the life of an existing structurally sound concrete surface by 5 to 10 years by restoring skid resistance and thereby saving costs.
It can also be used pre surface dressing to even out wheels ruts and extend the life of the new surface dressing, negating the need for expensive regulating patching.
An asphalt road can be re-shaped by fine milling, retaining a lot of the wearing course, saving costs and materials.
Ultimately, fine milling is an innovative, sustainable and cost-effective highway maintenance solution, extending the life of surface, or in some cases replacing the need for a new surface.