Polyurethane prepolymer modified porous asphalt as a durable low-carbon paving material : performance enhancement mechanism and sustainability assessment

Abstract

Porous asphalt (PA) is widely used for its functional benefits, such as quick water drainage, noise absorption, and improved surface friction. Despite these benefits, PA often suffers from a shorter service life due to moisture-induced damage, which accelerates raveling and leads to structural degradation. Polymer modification of asphalt has emerged as an effective and practical solution to improve the durability and moisture damage resistance of PA. Among the various polymers, polyurethane (PU) has recently gained growing attention due to its superior mechanical properties after curing. However, conventional methods for preparing PU-modified asphalt encounter significant challenges, particularly due to the rapid reaction between the two PU components during preparation process. This reaction results in a rapid increase in asphalt viscosity, thereby significantly reducing the workable time of modified asphalt. Recent research has revealed that the PU prepolymer (PUP), one of the key raw materials in PU synthesis, can be independently used to modify asphalt. Unlike the conventional two-component PU system, PUP cures after the asphalt preparation process, thus mitigating the issue of rapid viscosity increase. Currently, limited research has been conducted on PUP-modified asphalt. Therefore, the modification mechanism of PUP as well as its effect on asphalt mixture, especially in PA, require further investigation. This study aims to address this gap by exploring the modification mechanism of PUP and thoroughly assessing its effect on the mechanical properties and moisture-induced damage of PA. To achieve the objectives of this study, the modification mechanism of PUP in asphalt mixtures was first explored. Subsequently, the effects of PUP on the adhesion between asphalt and aggregate, as well as the cohesive properties of asphalt mastic, were studied. The preparation process for PUP-modified PA (PU-PA) was then determined, followed by an investigation into its strength development. Additionally, the mechanical and functional performance of PU-PA were thoroughly evaluated. Afterwards, the moisture damage resistance of PU-PA was extensively investigated, with particular attention to the changes in its mechanical properties and internal void structure after experiencing moisture damage. Lastly, the economic and environmental impacts of PU-PA were assessed using life cycle cost analysis (LCCA) and life cycle assessment (LCA). The effect of PUP modification on the adhesion between asphalt binder and aggregate was first investigated. Through frequency sweep (FS), Fourier transform infrared (FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS) tests, it was found that the isocyanate groups on PUP chemically react with the hydroxyl groups at the aggregate surface. The results of the boiling water and pull-off tests further demonstrated that the chemical bonds formed between PUP and the aggregate significantly improve the adhesion between asphalt and aggregate, thereby mitigating moisture-induced damage at the asphalt-aggregate interface. Subsequently, the influence of PUP modification on the cohesive property of asphalt mastic was studied. The FTIR tests revealed that the curing of PUP in asphalt mastic involved reactions with hydroxyl groups at the filler surface and atmospheric moisture. The direct tensile (DT) and linear amplitude sweep (LAS) tests were conducted on PUP-modified asphalt mastic (PUAM) both before and after moisture damage. The results showed that PUP modification significantly improved the cohesive property and fatigue performance of asphalt mastic. Moreover, PUP modification effectively mitigated the detrimental effects of moisture damage on the mechanical properties of the asphalt mastic. For PU-PA, the PUP content was determined to be 10% by mass of base binder, based on the results of the Marshall stability test. The optimum mixing and compaction temperatures were determined to be 150 °C and 130 °C, respectively, using the viscosity-temperature curve of PUP-modified asphalt binder (PUMB) and compaction energy index. The results of the indirect tensile stiffness modulus (ITSM) test indicated that the strength development of PU-PA occurred in two stages. The first involved the reaction between PUP and hydroxyl groups at aggregate surface under high-temperature conditions, primarily from start of mixing to the completion of compaction. The second stage occurred after compaction, as moisture diffused into mixture and reacted with the remaining uncured PUP to further enhance the mechanical property of PU-PA. The evaluation of mechanical and functional performance of PU-PA demonstrated that PUP significantly enhanced the resistance of PA to raveling, rutting, and fatigue cracking while maintaining its permeability and noise absorption capability. Subsequently, the focus shifted to evaluating the moisture damage resistance of PU-PA by analyzing the changes in its mechanical properties and internal void structure after moisture damage. The findings revealed that PUP modification significantly enhanced the moisture damage resistance of PA, as it helped preserve the mechanical performance and internal void structure stability of PA after experiencing moisture damage. Finally, LCCA and LCA were conducted to assess the economic and environmental performances of PU-PA. The results showed that using PU-PA as a substitute for conventional SBS-PA offers substantial potential for both economic and environmental benefits, indicating PU-PA is a promising material for PA pavement.