EFFECT OF SBS MOLECULAR STRUCTURE AND CONCENTRATION ON THE THERMO-OXIDATIVE PERFORMANCE OF MODIFIED BINDER AND ASPHALT MIXES

Abstract

Higher stress generation due to unrestrained traffic volume and significant temperature variation necessitates enhanced quality and durability of asphalt mixes on highways. Offering a plausible solution to cater this unprecedented need, styrene-butadiene-styrene (SBS) polymer modification has been broadly recognized owing to its effective history in improving the properties of asphalt mixes. Often the explicit differing performance is a consequence of the change in molecular weight, polarity and structure of SBS polymer. Despite the several advantages exhibited by SBS-modified binders (SBS-MB), the demonstration of poor thermal stability at higher temperatures often overshadows its performance. Past studies revealed that ageing has an ill effect on the performance behaviour of binders. A significant gap could be identified in the literature focusing on the thermal stability of SBS-modified binders under non-oxidative ageing. The efficacy of varying molecular structures of SBS copolymer-modified binders exposed to elevated storage temperature and the corresponding asphalt mixes were not performed by previous researchers. Additionally, less attention was given to identifying the impact of different SBS molecular structures on the oxidative ageing resistance characteristics of asphalt mixes. Thus, the use of commercial-grade SBS polymers with varying molecular structures may affirm the imperative role of polymer molecular structure in affecting the performance properties of the asphalt mixture. It should be assessed in consideration of the ageing process that happens from manufacturing to the service period of the pavement, which can give a clear indication of its suitable utilization in road construction. In this study, four types of SBS copolymer having linear SBS (L-SBS), branched SBS (B-SBS), high vinyl SBS (HV-SBS) and diblock SBS (DB-SB) were used along with VG-10 binder to develop the modified binder. Degradation in properties has been quantified through the change in performance of the modified binder as well as the corresponding asphalt mixture under thermo-oxidative ageing conditions. Moreover, chemical and morphological analysis of the binders has also been conducted to identify the chemical change in the behaviour of the aged binder with respect to the said condition. Firstly, the characterization of studied polymers has been carried out in terms of Gel permeation chromatography (GPC), Fourier Transform Infrared Spectroscopy (FT-IR), melt rheology, and Differential scanning calorimeter (DSC) analysis. GPC study reveals that B-SBS and DB-SB polymers have the highest and lowest molecular weights, respectively. FT-IR studies show contrast differences among the studied four polymers only in terms of % vinyl content. Asphalt mixture study acknowledged binder study where, B-SBS polymer exhibits maximum performance properties followed by HV-SBS, L-SBS and DB-SB. Results showed that as compared to widely used linear SBS polymer, B-SBS copolymer generates ≈ 33 %, 26 % and 18 % more fatigue resistance, rutting resistance and stiffness behaviour, respectively at a polymer content of 4.5 wt.%. On the other hand, L-SBS and HV-SBS polymers depict almost similar mixture performance due to their alike molecular structure. Whereas, the presence of a di-block structure, and lower molecular weight results in lower performance properties for mixes prepared with DB-SB polymer. The results clearly signify the dominance of polymer molecular structure in identifying the mixes’ performance properties at respective polymer content. This study also elucidates the important role of storage temperature and SBS concentration on the performance of modified binders and their corresponding mixes. SBS-MBs with varying SBS content (3 %, 4.5 %, and 7 wt.%) were first stored in closed metal containers at 150 °C and 180 °C. It was observed that the properties of the modified binder remained stable at 150 °C, but erosion was evident when stored at 180 °C due to SBS degradation. The conventional and rheological analysis demonstrated that the magnitude of property erosion was significant at an SBS content of 4.5 wt.% while, 3 % and 7 wt.% SBS content showed muted property erosion at such elevated temperatures. GPC and Florescent Microscopy (FM) studies clearly depicted that storage at high temperatures had a substantial effect on the dense polymer chain. Results also exposed a substantial drop in mixes’ Marshall stability, indirect tensile strength, stiffness modulus, fatigue, and rut resistance properties after using 4.5 wt.% SBS-MBs that were stored at 180 °C for 7 days. The consequential outcome of such property deterioration of asphalt mixes may impede pavement stability and service life. Meanwhile, only ≈ 9 % property reduction took place in asphalt mixes that were prepared using 3 % and 7 wt.% SBS-MBs stored at 180 °C. The findings from the study are practically significant while using SBS-MBs for pavement construction. This research has also focused on monitoring the properties of varied SBS-MBs (with four different SBS copolymers) that were stored at 180 °C and 150 °C for 14 days. SBS concentration was fixed at 4.5 wt.% since maximum deterioration was observed at that mentioned concentration range. Results demonstrated that SBS-MBs with L-SBS and B-SBS polymer displayed the maximal deterioration in stiffness and elastic response, while HV-SBS and DB-SB modified binders showed subdued property erosion. The performance of asphalt mixtures prepared with stored binders was scrutinized through Marshall stability, resilient modulus, water susceptibility, permanent deformation, and fatigue cracking behaviour. Results showed that asphalt mixes prepared with stored L-SBS and B-SBS modified binders depict a substantial drop in the mixes’ performance parameters. However, the use of HV-SBS and DB-SB polymers for preparing modified asphalt mixes demonstrated a marginal property reduction of ≈ 10 %. The plausible reason behind the exceptional thermal stability is the arrangement of butadiene chains in the SBS microstructure having a high vinyl content (> 35%) in its composition. One of the primary objectives of this research is to assess the performance of modified binders containing different SBS copolymers in consideration of the concurrent effect of stress levels, and test temperature in association with oxidative ageing conditions. Results highlight that variation of rutting performance and chemical properties of the modified binders after rolling thin film oven (RTFO) ageing condition was minimal and negligible stress and temperature susceptible behaviour were observed irrespective of different polymer structures. However, after pressure ageing vessel (PAV) ageing L-SBS and B-SBS showed higher stress and temperature susceptibility as well as higher polymer degradation index (PDI) as compared with HV-SBS and DB-SB modified binder. Fragmentation of the polymer network was detected through fluorescent microscopy (FM) images for PAV-aged modified binders containing L-SBS and B-SBS polymer. The performance properties in the mixture before and after laboratory ageing have been identified through abrasion study, fracture properties, fatigue and rut resistance behaviour. Studying the response of the varied SBS-modified asphalt mixture against laboratory ageing conditions, it may be understood that the short-term ageing (STA) had an insignificant effect on the asphalt mixture performance properties. Asphalt mixes prepared with B-SBS and L-SBS copolymer depict higher variation in the ageing index after long-term ageing (LTA) as compared with their counterparts. Results suggest that LTA causes polymer disruption for said polymers which were unable to protect the base binder from stiffening. FTIR test on extracted binders from the LTA asphalt mixes substantiates the adverse effect of long-term ageing on linear and branch polymers. The result resonated with the augmentation of carbonyl and sulphoxide compounds and the lowering of polybutadiene compounds in the extracted binders. Thus, this research is expected to help the research community and pavement industry to decide on the suitability of the SBS copolymer type for utilization in the construction of asphalt pavements and its efficacy in long-term performance properties.