Solving pavement water damage

New research is looking for affordable methods of waterproofing our road surfaces. 
RICHARD SILCOCK explains a four-year research project in progress.

WITH ROAD FREIGHT volumes increasing by 2.5 percent annually and extreme rainfall events perhaps becoming more prevalent, the New Zealand Transport Agency and local authorities are looking for new and innovative ways to futureproof and maintain the affordability of the roading infrastructure.

On average $1.3 billion is spent each year by the NZTA and local and regional authorities on road maintenance – that can be largely attributed, either directly or indirectly, to water damage. According to NZTA figures, almost 90 percent of the road network is susceptible to water damage.

Under a four-year research project being carried out by Opus Research at its laboratory in Petone, work is focused on a non-permeable membrane which restricts water entry while also providing a skid resistant surface.

The $2.7 million research project which started in October 2015 is being largely funded by the Ministry of Business, Innovation and Employment with assistance from NZTA – and in consultation with a number of contractors around the country, including Downer, Higgins and Fulton Hogan. The research is also being carried out in collaboration with the University of Auckland and with some assistance from ARRB Group (Austroads research division).

Phil Herrington, a senior road pavement scientist with Opus Research who is leading the research, says one year of research has certainly confirmed that roading seals used in the past have not been particularly effective in their water-proofing function.

“That is, they [the chip seals] leak,” says Phil. “Water damage of our roads manifests itself in several ways: deformation and potholing, which leads to the loss of bonding between the chip seal and the aggregate, and ‘flushing’ of the seal. This ‘flushing’ causes the bitumen surface layer to become slick and unsafe to drive on.

“We are looking at alternative ways to create a far better bond and seal. Our research so far is leading us towards applying a ‘primer’ to encourage a far superior bond between the aggregate base course and the surface layers which, along with a water impermeable membrane layer, should prevent water from entering the base course.

“In essence we are researching methods of modifying the chip surface to prevent water ‘disbonding’ the bitumen sealer.

“At this stage we are looking at placing a membrane layer under the chip seal and above the aggregate. We are testing and evaluating different membranes for their waterproofing capability, subjecting them to extreme temperature differences and intense sunlight, and looking at application and measuring wear tolerance.

“We are also looking to find new and innovative construction methods.

“Our aim is for the primer, membrane and method of application to become the accepted industry standard utilised by the Transport Agency, local authorities and contractors and other road construction authorities overseas.

“Once we have completed the research on types of membranes we will evaluate them using our in-lab accelerated pavement wear testing machine and would expect to do field trials on sections of road during the last year of the project.

“This will be done in collaboration with NZTA in different parts of the country and under various climatic and traffic conditions.”

John Donbavand, national pavements manager with NZTA, says the agency has been promoting the use of primers to provide a more water-resistant layer but they are not a complete fix.

“We rely on unbound granular base course to provide the structural component of the pavement and it is this type of pavement that is particularly susceptible to water ingress. “Currently our base course layers use a relatively large stone size to provide an open grading, so when water does get in, it can drain away.

“However, these large stones tend to break down in service which reduces the drainage capacity and can trap water in the structure making it susceptible to large pore pressures under loading from heavy vehicles.

“This leads to the potential for pavement shear and deformation. In winter there is also the possibility of ‘swelling pressure’ where, if the water freezes, a phenomenon known as frost heave occurs.”

The economic benefits from this research project are two-fold. There are predictions that the net cost reductions for NZTA and local authorities could be in the region of $81 million a year within five to 10 years of the research being completed. Additionally, the waterproofing technologies developed are estimated to at least double pavement life and as a result reduce the need for frequent road maintenance.

Phil Herrington says the research has the potential to revolutionise the way in which future road pavements are constructed.

“Whereas current chip seal has a life expectancy of around eight to 10 years, and asphalt 10 to 15 years, we have an expectation of developing a solution that will provide for up to 30 to 40 years of life.

“In addition expensive premium base course materials and current cement and foamed bitumen stabilisation techniques could be replaced with much cheaper materials and changes in road construction design.

“We could perhaps even see quarried ‘waste material’ used in some instances,” he says.