Structural maintenance, repair and rehabilitation
Roads are exposed to tremendous loads that will sooner or later leave their marks on them. A time will come when every road will be in need of a general overhaul. But no two damage patterns are alike.
Which rehabilitation methods offer a cure for distressed roads? What are the differences between them? Which are suitable to be carried out as mobile roadworks?
Replacing the pavement
Replacing the pavement is a standard procedure when repairing roads. The challenge is to ensure that only the damaged layers of the road structure are removed - and to avoid disruptions to traffic at the same time. Under these conditions, cold milling is the only viable option for many construction projects.
The pavement is removed by cold milling machines. In most cases, machines of various sizes make up a team: the large milling machine removes the bulk of the pavement while the smaller machine takes care of such fittings as manhole covers and edgings.
Whether asphalt or concrete, the machines transfer the reclaimed asphalt pavement (RAP) directly onto trucks for removal in a single operation.
Fine milling is an alternative to time-consuming and expensive complete rehabilitation. This method is used above all when traffic safety is severely compromized by undulations, ruts or a slippery surface.
The process is a modified form of cold milling, the only difference being that the cutting tools are spaced more closely on the milling drum. The technique is called fine milling when the line spacing decreases to 8 mm or less. The aim is to produce a defined new surface texture. Although fine milling drums cannot repair any damage rooted deep in the road structure, they can create a level, non-skid surface. A single pass is all it takes - further steps, such as paving a new surface course, are normally not necessary.
Paving thin layers hot
It is not necessary to replace the entire pavement if only the surface of the roadway shows signs of damage. The method known as "paving thin overlay hot on hot" is a particularly economical and eco-friendly alternative.
A paver with integrated spray module is preferably used when paving the thin overlay. It automatically applies a tack coat of polymer-modified bitumen emulsion to seal the base. At the same time, it paves a thin asphalt surface course just 1.2 to 2.0 cm thick.
Dynamic compaction with oscillation is ideal, ensuring that the underlying cold layer is not damaged by compaction. Static compaction is also possible, but without vibration.
Paving thin layers cold
"Thin overlays paved cold on cold" - better known abroad as "micro-surfacing" - are another quick and cost-effective alternative to replacement of the complete pavement.
Cold milling machines with fine milling drums first prepare the road surface for application of the thin overlay. The resultant fine profile of the milled roadway creates an ideal substrate with which the micro-surfacing can engage. The "valleys" of the fine-milled profile securely engage the layers of the micro-surfacing, thus producing a perfect layer structure.
Thin layers paved cold on cold provide a cost-effective as well as quick solution. The thin wearing course can be paved in the form of a mobile site and the road reopened to traffic within a short space of time.
InLine Pave has become established as an official construction method according to German regulatory frameworks. The binder and surface courses are paved "in line", i.e. one after another "hot on hot" in a single pass. Since the machines are just 3 m wide, traffic can continue to flow without obstruction on the remaining lanes.
Very high precompaction of the binder course prevents the binder course material mixing with that of the surface course and ensures clear separation of the layers as well as an optimum surface seal. The bond between binder and surface course is outstanding. The very strong layer structure not only makes application of a bitumen emulsion unnecessary, but also additionally extends the useful life of the road.
Porous and deformed surface courses can be rehabilitated by hot recycling, a process which is exclusively applied in the form of a mobile construction site. An intact layer structure is essential here. Hot recycling improves all relevant properties of the surface and roadway profile, as well as the composition of aggregate fractions in the surface course.
The surface course is heated up to 150 °C by a panel heating machine with gas-fired infrared heating panels so that the hot recycler can then scarify, remove, process and repave the softened asphalt. In this way, the road's non-skid properties can be restored, water can run off again and ruts are eliminated. The potential savings are enormous.
Cold recycling in-situ
The sub-base of roads exposed to major stresses due to heavy goods traffic is often damaged. To remedy this damage, the complete road structure must be repaired. Cold recycling restores the roads' stability.
The difference between "in plant" and "in situ" cold recycling is that, in the latter case, the entire process takes place in a single operation. This is what happens: special cold recyclers granulate the defective layers - usually the surface and binder course, as well as part of the base course - mix the reclaimed asphalt pavement with fresh binder and replace it immediately. The advantages of this method are short construction times and high cost efficiency.
Cold recycling in-plant
A mobile cold recycling mixing plant is normally installed near the job site for "in plant" cold recycling. Trucks transport the reclaimed asphalt pavement directly from the site to the mixing plant. It essentially does exactly the same as the cold recycler in an "in situ" process: the reclaimed asphalt is recycled 100% by adding one or more binders. Trucks then transport the cold recycled mix back to the job site.
The cold mix need not be replaced immediately if it is mixed with foamed bitumen in the mobile cold recycling mixing plant. Such mixes can be "stockpiled" for longer periods of time. As a result, the mix need not stem from the same job site as that on which it is subsequently reused.