Composite grid reinforcement for long-term pothole prevention
Geogrid composites provide crack control and pavement reinforcement for a long-term solution to road repairs
More than just a bump in the road
The bill to UK taxpayers for repairing potholes has risen to more than £1billion, with the number of potholes being filled up by about a fifth on last year according to research published in March by car servicers Kwik Fit.
Despite spending on road maintenance increasing, with an extra £420million pledged to deal with the problem in the last budget, the Asphalt Industry Alliance (AIA) - whose members supply much of the materials used for repairing holes - claims one in five local roads in England and Wales is in a poor condition and £8bn is needed to carry out a one-time, thorough fix of potholes in England. More than 24,400 miles of road is identified as needing essential maintenance in the next year.
What Causes potholes to form?
Road surface damage and instability is usually a result of a combination of factors including poor soil conditions, poor drainage, weathering effects, increased weight of traffic and the age of the surface.
High vehicle weight and axle loads of traffic can cause load associated cracking with shear and bending forces exceeding the fracture strengths of the existing asphalt (see figure 1).
Figure 1: Water weakens the underlying ground, and the weight of traffic then fatigues and breaks the poorly supported asphalt surface, with aggregate moving both laterally and vertically under loading.
How potholes are typically repaired
The most common repair solution is to construct asphalt inlays (see figure 2). Inlays are economically practical and convenient, however old cracks under the new overlay can rapidly propagate back through to the new surface causing repeated failures. This phenomenon is called reflective cracking (see figure 3).
Because asphalt inlays are otherwise the most practical pothole repair option, research and development has focused on how to prevent reflective cracking.
Figure 2: Asphalt inlay
Figure 3: Reflective cracking
Solving the problem with Geogrids
Unsurprisingly in light of the road safety and cost issues caused by the UK pothole problem, cracking in asphalt pavements is now recognised as one of the biggest problems faced by highway maintenance engineers and focus has turned to finding a longer-term, more effective repair including using geosynthetics such as reinforced geogrids.
High-strength glass fibre or steel reinforced grids are backed with a polyester geotextile. The benefits of installing the geosynthetic interlayer between the old pavement and new overlay include:
- Waterproofing the pavement
- Delaying the appearance of reflective cracks
- Lengthening the useful life of the overlay
- Added resistance to fatigue cracking
- Saving thickness
Asphalt is confined and compacted within and above the geogrid reducing both vertical and lateral movement. Having a composite sandwich layer with reinforcement grid enables a more efficient means by which to transfer load onto underlying layers.
This approach has resulted in significant whole life cost savings:
- Maintenance cost reduction
- Significant extension of road life over conventional surfacing
- Reduction in asphalt thickness, in some circumstances, saving on material costs, tipping & planing
- Reduced environmental impact associated with longer maintenance intervals
- Reduced hidden costs to businesses and the general public through delays caused by road closure and traffic restrictions.
It is strongly advised that a competent specialist contractor is employed to undertake the installation. Installation by the asphalt surfacing contractor is not usually satisfactory because the operatives will be unfamiliar with the process and they are unlikely to have the necessary skills or qualifications or the correct equipment to ensure correct installation. Installation of the geogrid will usually be scheduled to take place immediately prior to the asphalt surfacing.
Case Study: Heavy Trafficking Trial, Rotaflex, Meltham Mills Road, UK
Kirklees Council Asset Management Policy aims to maintain the 1900km of roads in its area by maximising customer satisfaction levels and to look for the most efficient whole life costs possible. The council assessed the state of the heavily trafficked Meltham Mills Road to be unacceptable for safe passage due to a very stressed road surface. The road receives some of the highest loading in the area taking approximately 1800 HGV vehicles per week either passing or delivering to the local factories. In addition as a bypass to Meltham town centre it takes approximately 6000 light traffic movements per week. Using the guidelines from the West Yorkshire Combined Authority (WYCA), the council decided on a 60mm binder course with 40mm SMA overlay with a predicted lifetime of 15 years including first light maintenance at 5 years.
Whilst the design was to the required guidelines of Kirklees Council and in line with their policy of reducing whole lifetime costs, they were also looking for any improved techniques which could extend the maintenance periods still further. The site runs along the length of ABG Geosynthetics Ltd own offices and being heavily trafficked and easily monitored presented an ideal site for a trial.
ABG proposed a Rotaflex asphalt reinforcement geocomposite comprising a glass fibre geogrid backed with a polyester geotextile. The geogrid reinforces the road surface preventing propagation of cracks through the surface course. By stopping these cracks it delays the breakdown of the road surface considerably extending its life.
In 2015, one year before planned road maintenance period, the UNREINFORCED section of the carriageway showed premature breakdown and potholing. Note the Rotaflex REINFORCED half of the road remains in an unstressed state.
The polyester geotextile binds the bitumen used in the installation process sealing the subsurface and avoiding damage caused by water penetration.
The trial included Rotaflex reinforced and unreinforced areas which were to be monitored over the following 5 years up to the first predicted maintenance point. At first the finish was even across the whole road surface but after only 4 years in 2015, following several harsh winters and high traffic volumes the unreinforced areas were showing distinct signs of distress. Depressions, potholes and delamination started to occur. These areas had to be patched prematurely.
The reinforced Rotaflex geocomposite areas showed no signs of stress. The conclusion is that the trial is a success showing Rotaflex helps to strengthen new surfacing layers, prevents cracking and deformation. Rotaflex asphalt reinforcement geosynthetic provides a cost effective solution to road maintenance by extending the maintenance intervals and reducing whole life costs.
ABG Rotaflex geogrid system for asphalt reinforcement.
Kirklees Borough Council Highways Department with ABG have carried our a credible and long term closely monitored trial on the Meltham Mills Road. It is normally very hard to monitor a road due to resources and accessibility along with the risk of ad hoc maintenance caused by utility companies which can obscure results. SCANNER results (figure 4) confirm that the areas which are now showing distinct distress are the unreinforced areas of the road. Comparison Rotaflex reinforced sites have supported the findings at Meltham Mills Road showing extended maintenance periods above those normally experienced in the Borough. This then established a likely maintenance regime using a Rotaflex reinforced road leading to a cost analysis concluding that over a 26 year period a saving of at least 21% on maintenance costs has been achieved based on 2019 prices alone. A total saving of more than 50% on overall costs is forecast.
The use of Rotaflex inlay reinforced road represents a significant lifecycle cost saving.
Figure 4:SCANNER crack survey mapped to Rotaflex installation (Note: no reading over the Rotaflex protected area for either carriageway)