Extending the Lifespan of Porous Asphalt Concrete

Abstract

Porous Asphalt (PA) concrete is widely used as a surfacing layer on highways in the Netherlands. The service life of PA wearing courses is limited because of the fact that it is vulnerable to raveling. The possibilities of applying preventive maintenance to PA wearing courses by means of spraying rejuvenation products on the pavement surfaces are being investigated with high interest in the Netherlands. A material which has the ability to penetrate into the bituminous binder and soften (rejuvenate) the hardened bituminous binder in the mortar is defined as a rejuvenator. Spraying rejuvenators on asphalt pavement surfaces before raveling starts (during the initiation of damage) might influence the characteristics of the hardened bituminous binder such that the service life of the PA surface layer is extended. Considering the fact that the rejuvenation effectiveness of potential rejuvenation products is not clear, they are called Surface Treatment Products (STP) in this research. The main objectives of this research are the investigation of the effect of surface treatments on the microstructure and performance of PA wearing courses and the assessment of their rejuvenation effectiveness. Three surface treatment products were applied on two field trials of 5 year old PA pavement surfaces, respectively on the A50 and A73 highways. Computed Tomography (CT) scanning with a conventional medical CT scanner was performed on the PA concrete cores that were obtained from the field trial sections to measure volumetric properties like air voids and mortar contents. It was found that the difference between the mean values of the air voids contents for the PA concrete cores from the reference (untreated) and treated sections is not statistically significant in most of the cases. CT scanning with a high-resolution industrial CT scanner was used to detect the surface treatment products. Two parameters, the mortar film thickness and the surface treatment product content, were used to quantify changes in the microstructure due to the surface treatments. The treated PA concrete samples showed an increased mortar film thickness, which indicates the presence of bituminous binder added by the surface treatment products. Visual inspections on PA concrete thin slices with an optical microscope were performed to investigate the microstructure of the treated PA concrete samples. Micro cracks were observed in the reference PA concrete samples but no micro crack was found in the treated PA concrete samples. To determine the rejuvenation effectiveness of surface treatment products, artificially aged PA 0/16 concrete and mortar specimens were produced in the laboratory and the surface treatment products were applied to the specimens. The indirect tensile stiffness modulus and indirect tensile strength were measured on the PA concrete specimens. The results did not show a difference between untreated and treated specimens. For the mortar a special 3D finite element was developed before dynamic shear testing to study the influence of rejuvenation (diffusion inside the mortar) on mortar columns by simulating dynamic shear tests. From the results clear effects on the rheological behavior was shown. Based on the finite element results dynamic shear tests were used to measure the rheological and shear fatigue properties of mortar specimens. The results for the treated mortar specimens did not show a change in complex shear modulus and phase angle or shear fatigue properties compared to the reference mortar. It was concluded that no rejuvenation of the test samples did occur. Another finite element model was created for analyzing the stresses and strains as a result of traffic loads in a reference and treated PA pavement surface. The results showed that because of the added binder, the surface treatment does reduce the stresses, strains and dissipated energies in the mortar bridges at and close to the surface of the pavement, which will result in a higher fatigue life. In addition, the Rotating Surface Abrasion Test (RSAT), three-point being tests and nano-indentation tests were performed on the PA concrete cores taken from the reference and treated sections. The RSAT results for the treated PA concrete cores seem to indicate a lower stone loss than the reference PA concrete cores. The treated PA concrete beams showed higher mean stiffness values than the reference beams. From the nano-indentation tests, the presence of bituminous binders added by the surface treatment products to the mortar of the PA concrete was proven. Based on all the results, it was concluded that no rejuvenation took place. The product will have an effect. But that is because of added binder by the product. Furthermore, it was recommended that preventive maintenance interventions should be based on optical microscopy and nano-indentation tests on thin slices from small cores (?40 mm) rather than visual surveys of the pavement surface.