Bitumen technology ideal for road repairs

Mike Woof discusses some novel developments relating to bitumen

In the developed countries of Western Europe there is an increasing shift away from new highway construction to maintaining and rebuilding existing roads. In Germany alone, a network of asphalt roads extending more than 600,000km will have to be maintained or repaired.

Highway maintenance techniques do vary between European countries but some commonalities exist. There are techniques that have been sidelined in the last few years but which now offer cost-competitive repair solutions with the development of new technologies. One of the most notable is for surface dressing, which in the past has been regarded as a cheap but comparatively low quality process. Surface dressing techniques have had a reputation for delivering road surfaces with higher than desirable running noise and loose chippings that can cause cracked windscreens (windshields) in an undesirably large number of vehicles.

However, according to UK-based specialist John Baxter, consultant director and secretary of the Road Surface Dressing Association (3294 RSTA), new technology means that surface dressing can now offer a competitive repair option. This is due to the development and widespread use of road binders that exhibit good cohesion and comparatively low temperature susceptibility. Baxter says that in the past as much as 80% of the binders used were of the unmodified cutback type or K170 bitumen emulsion. However he explains that the market has changed and cutback has been largely withdrawn from the European market, with up to 80% of binders used now being polymer modified types.

This is an important point as polymer modification improves the cohesion of the binders and reduces temperature susceptibility. As a result, the surface dressing performance is significantly improved and the greater cohesion means that chippings will be retained. Meanwhile the improvements in temperature susceptibility mean that the binder will not become brittle in winter weather conditions, further improving chippings retention. In a similar vein, in hot weather the road surface will not soften.

Tests were carried out by the University of Ulster in partnership with the 1441 UK Highways Agency to look at the issue of tyre noise. This research showed that surface dressings with noise levels similar to 14mm SMA can be achieved and that standards have since been developed to best practices can be carried out.

Another key factor for the use of surface dressings has been the development of sophisticated equipment using computer controls to provide a consistent and accurate application rate for the binder. A sufficient and consistent binder thickness is vital to ensure that the chippings are retained. In the past the application rate would be controlled by varying the speed of the tanker, based on the driver's readings of a calibrated speedometer and this led to somewhat haphazard material thicknesses. But the new generation equipment uses a computer to control material delivery by linking the spray bar pump with the vehicle's powertrain to provide a far more accurate flow rate. Proper training practices have also been developed along with quality assurance systems, further helping improve the reliability of surface dressing applications according to Baxter.

Using surface dressing can help seal a pavement and provide sufficient skid resistance, offering an efficient and low cost repair option and as it is a cold process, energy costs are also low. When the surface dressing needs to be removed, conventional cold milling can be used while the recovered material can also be recycled using either foam bitumen processes or through a hot mix plant. Using cold recycling with either foam bitumen or emulsion-based processes with surface dressing also allows early opening of the road to traffic, reducing traffic disruption caused by repairs. Loose chippings can be swept up from the newly dressed surface and as these are generally of a high quality stone, they can be recycled into an asphalt plant for wearing course mixes or rewashed and used in the next batch of surface dressings.

Wider options

There are other repair techniques too. A specialist contractor Allied Drilling, which focuses on airfield maintenance and runway resurfacing, is using a purpose built, self contained machine, brought in from America, for applying hot bituminous materials and sealing joints. The 6086 Cimline Magma 110 Melter/Applicator has proved successful on its first job in the UK, on an airport runway surface where a quality finish is vital. According to Allied Drilling, the machine has proven to be faster, more efficient, user friendly, safer and more cost effective than conventional sealing methods of pouring hot liquid from a hand held can or metal fabricated cones. The trailer-mounted Cimline Magma 110 has been towed behind a maintenance van, during night-time closure possessions of the airfield. Allied Drilling, based in Leigh, Lancashire, worked for main airfield lighting contractor ATG Airports, which was working alongside principle contractor Mansell Airports for Cornwall County Council. Allied Drilling bored large diameter holes for new runway lights and carried out saw cutting for cables. After the saw cutting operation, ATG Airports installed the new cables into the cable slots. Allied Drilling then followed with the Cimline Magma 110, sealing the chases with polymer-modified, hot bituminous Armourseal N2 JS joint sealing compound, using the machine's heated flexible hose, wand and adjustable nozzle. This work was done before the runway was resurfaced. Once the resurfacing was completed, Allied Drilling returned to the light positions and installed the seating ring housings for the runway lights. ATG Airports then installed the luminaire along with the necessary electrical switching units.

The new lighting is part of the Transition Project needed by Newquay Cornwall Airport to secure a Civil Aviation Authority Licence during the conversion of the military airfield at RAF St Mawgan to a civilian operated airport. The airfield's 2.75km long runway is capable of handling the largest aircraft, including the Boeing 747 and the double deck Airbus A380. Expansion of the arrivals and departure hall lounge and facilities to accommodate the increase in flights is also complete.

Allied Drilling is sawing about 15,000m of 15mm wide slots varying in depth from 100-255mm and sealing after the cables have been installed. The firm had been looking to replace hand applied sealing when it saw the Cimline, which has since proven faster, safer and easier to use than other methods. According to Allied Drilling, the job would have taken us twice as long using hand applied sealing and the new machine has saved the firm time and money. The company said that on some nights it has put over 1tonne of material through the machine and has already lined up other runway resurfacing projects. The firm also hopes to use the machine on motorways and trunk roads for sealing transverse and long horizontal joints.

The Cimline Magma 110 is one of a four-model range available in the UK through sole distributor Castellan Group in Milton Keynes. Safety is maximised as it means there is no longer a need to use propane gas-fired boilers and running back and forth with large watering cans full of hot bitumen to make a sealed joint. The technology is proven as the Cimline machines have been used in the US for around 20 years. The Magma 110 is a self contained, 415litre capacity, indirect heated diesel fired boiler, with a hydraulically driven pump and agitator. A diesel hydraulic drive system powers the material agitator and external pump, which circulates the temperature controlled, hot material through a 6m long heated flexible hose to a hand held lance with a variable flow nozzle controlled by the operator. The sealant comes in 15kg blocks, which are placed in the machine's heated mixing tank through angled splash-proof doors and can be ready for work within 40 minutes of initial start up. Emissions are low and the operator has greater control and accuracy of placing the very hot sealant directly where it is needed.

6088 Marathon Equipment 's branch in Iran is now repairing roads in the United Arab Emirates (UAE) using a novel method to tackle cracking. Cracking is a major cause of highway surface failures as is the primary mode of deterioration in asphalt cement pavement. Cracking occurs in a variety of forms: transverse, longitudinal, block, and alligator shape. However, whatever the form these have to be tackled quickly as they allow moisture and dust ingress into the pavement layers, which can cause serious damage. Left untreated, pavement cracking results in fast deterioration and problems such as potholes or base failures.

In the UAE's Alain Municipality, longitudinal and lateral cracks have been inspected in various city streets, revealing some 50,000 linear m of cracks. To repair these cracks hot applied sealant material is used, delivered in 15kg solid boxes, and loaded into a kettle with plastic cover/bags. The material is then heated up indirectly using hot oil in the oil jacket tank of the kettle prior to being injected into the routed crack through an oil jacketed hose. The material is injected at temperatures of 180-210ºC and the hose makes the job easier as there is no risk of suction or blockage.

According to Marathon Equipment, the crack sealing procedure involves a primary jobsite inspection followed by opening of the crack. The area can then be cleaned using a hot air lance and wind blower, prior to injection of the sealant material using the KERA 145 HD Kettle, with the final task being spreading of cement powder. Used properly, the technique is said to be highly productive and can be used to repair 1,000 linear m of crack/day, while the repaired road surface can be opened to traffic just 30 minutes after the work is carried out.

1194 Vögele is offering a purpose-built machine for this market in the shape of its SprayJet Module, which can be fitted onto its Super 1800-2 highway paver. Fitting this module allows contactors to use a sophisticated spray paver according to the firm, as the unit can spread bitumen emulsion and place asphalt wearing course in a single pass. Combining the Super 1800-2 with the SprayJet Module gives a package that suits paving thin overlays on spray seal, hot on hot. However it can also be employed by non-specialised companies for spraying tack coats and then carrying out conventional asphalt paving. The machine is said to offer an economical and clean alternative to competing units in spraying configuration but can be quickly reconfigured for conventional paving tasks.

The SprayJet Module can be used for widths of 5m and is equipped with a 2000litre emulsion tank as standard, however an extra emulsion tank is now available as an option for contracts requiring very large rates of spread. The extra tank holds 5,000litres and is mounted in the paver hopper, providing a total capacity of 7,000litres in all.

For these applications, the paver can be supplied with mix from the MT1000-1 transfer unit. The material flows through a receiving bin in the extra tank and onto the paver's conveyors. A stand-alone, diesel-fired burner unit with a heating capacity of 30kW is installed in the extra tank to ensure that the emulsion remains at the necessary temperature. In addition the extra tank has its own pump circulating the emulsion and maintaining it in a highly homogeneous state. Should the emulsion level in the standard tank drop below 1,000litres, the material is automatically delivered from the extra tank into the standard tank.

Another upgrade for this module is an optional spray bar system for paving widths up to 6m. The lateral spray bars are longer and feature two additional nozzles, with a total of 24 nozzles for spraying. Combined with a Vögele AB600-2 screed or AB500-2 screed with bolt-on extensions, the machine is can handle pave widths up to 6m. ·