Reducing the rate of rutting
A new prediction tool for asphalt deformation, enabled Nynas to quantify the benefit of using polymer modified bitumen (PMB) in asphalt pavements. A reduced rutting rate means less maintenance.
Rutting of asphalt surfaces is a major traffic safety concern and one of the main reasons for road maintenance work. Mostly caused by repetitive shear deformations due to traffic, the degree of rutting is influenced by the properties of asphalt mixtures.
“This is especially true at high temperatures and under relatively long loading duration when the properties of the mixtures are dominated by the viscous character of the material,” explains Xiaohu Lu, Senior Technical Specialist at Nynas.
Xiaohu Lu and his colleagues have studied how the use of polymer modified bitumen (PMB) can enhance the resistance to rutting in asphalt pavements.
PEDRO predicts the ruttingDeveloped by the Swedish National Road and Transport Institute (VTI) and used in a joint project within the strategic innovation programme InfraSweden2030, PEDRO is short for PErmanent Deformation of asphalt layers for ROads. It is a model that can be used as a tool for predicting the rutting performance of asphalt pavements.
Contrary to other similar models, PEDRO uses two separate components – volume and shape or shear – to predict the effect of the post-compaction or volume change and shear flow in the bituminous mixture. The material properties used as input for the PEDRO model include asphalt viscosity at maximum phase angle. This enabled the Nynas team to quantify the effect of new materials, such as polymer modified bitumen.
“We have conducted laboratory investigations on different asphalt mixtures made with a reference conventional bitumen (70/100 pen) and polymer modified bitumen (Nypol 76-28) in terms of resistance to permanent deformation,” says Xiaohu Lu.
Wheel tracking tests were used to evaluate a dense Swedish asphalt concrete (ABT11) at 60 °C. The scientists were able to demonstrate that with respect to rutting resistance, the asphalt with the polymer modified binder performed almost ten times better than the one with the reference bitumen.
In order to further investigate the asphalt samples, the scientists carried out dynamic shear modulus tests using a wide range of temperatures and loading frequencies. These tests enabled them to work out the viscosities of the asphalt material at maximum phase angles.
That viscosity data is used as a material property input for a newly developed tool for predicting the rutting performance of asphalt pavements (see fact box).
“We observed that the asphalt material with the polymer modified binder displayed more than 20 % higher viscosity than the unmodified one,” says Xiaohu Lu.
The increased viscosity increases the rutting resistance of the asphalt material.
“By inputting our results in the prediction model, we could predict that a road section with PMB shows about 20 % less deformation compared with a section made with unmodified binder”.
In other words this means that if a section of road paved with asphalt made with PMB needs maintenance work to be carried out after a period of 20 years, a reference section of asphalt made with an unmodified binder would need to be maintained four years earlier, after about 16 years of use.
“This is a significant difference,” says Xiaohu Lu. “Not only is it significant in terms of the costs involved, it will also benefit the environment as it promotes a more sustainable use of materials and energy.”
Xiaohu Lu, started at Nynas in 2002. He joined Nynas then from a post as senior lecturer at KTH Royal Institute of Technology in Stockholm, where he had previously obtained his doctorate in the area of Road Technology. In his role as Senior Technical Specialist, Xiaohu Lu is in charge of several Nynas R&D projects in the areas of bitumen and bituminous materials.