Microwave-heating healing porous asphalt empowered by carbon nanotube polymer coated aggregates

Abstract

Porous asphalt (PA) mixture refers to open-graded asphalt concrete with a high air void content (typically 18-22%). It has been widely used as a pavement surfacing material in populated cities like Hong Kong due to its various benefits, including reducing traffic noise, efficient water drainage, improving driving safety, and reducing urban heat island effect. However, as a functional road surfacing material, PA concrete often experiences early-age raveling issues due to the debonding and dislodgement of aggregate particles from the pavement surface, significantly compromising its durability and shortening the service life of the pavement. To address the raveling problem and enhance the durability of PA pavement, the objective of this study is to develop an innovative microwave-heating healing system within PA concrete. This system has been designed to heal microcracks quickly and repeatedly in mastic and asphalt-aggregate interfaces, i.e., efficiently repairing cohesive and adhesive microcracks. To achieve this objective, four research tasks have been conducted as elaborated below. The first task involves preparing a novel functional aggregate by dip-coating the raw aggregate surface with functional carbon nanotube polymer (CNP) ink. Following this, the properties of the functional aggregates are characterized, focusing on surface properties, bonding behavior between aggregates and asphalt, and microwave-heating characteristics. The second task involves experimentally characterizing PA concrete incorporating functional aggregates on a bench-scale, in terms of moisture susceptibility, indirect tensile strength, cracking resistance, and microwave-heating properties. The third task focuses on investigating the microwave-heating healing performance of functional aggregate-based PA concrete and exploring the corresponding healing mechanisms, aiming to repair the microcracks at the aggregate-asphalt interfaces. In the final task, the strategic construction of dual-response microwave-heated healing systems in PA concrete has been undertaken. This involves the combined usage of microwave-sensitive fillers and functional aggregates, with the aim of further enhancing the microwave-heating healing performance of the PA concrete. Upon completion of this study, an innovative thermal healing system within PA concrete by coating aggregates with specially designed functional CNP ink has been developed. The properties of functional aggregates, as well as the mechanical, microwave-heating, and microwave-heating healing properties of functional aggregate-based PA concrete, have been systematically investigated. Additionally, the microwave-heating healing performance of PA concrete with the combined usage of functional aggregates and microwave-sensitive fillers has been explored. The key findings of this study include: i) A uniform coating of the functional CNP film on the surface of the raw aggregates has been achieved by using a dip-coating method, resulting in the outstanding microwave-heating ability of the functional aggregate; ii) Functional aggregate-based PA concrete has exhibited remarkable heating efficiency and uniformity when exposed to microwave radiation, with negligible mechanical strength loss; iii) The healing index (HI) of the functional aggregate-based PA concrete decreased to about 30% after six damage-healing-damage (D-H-D) cycles, with a suitable rest time of 9 hours; iv) The optimized PA concrete formulation incorporating functional aggregates and ferrite powder (PA-FAg-100FP) has demonstrated remarkable microwave-heating healing efficiency, consistently reaching around 70% after three D-H-D cycles, with only a marginal 4% reduction in cracking resistance. The outcomes of this research are expected to establish a knowledge foundation for a novel preventative maintenance method, improving the durability of PA pavement while reducing maintenance frequency, associated costs, and carbon emissions.