RHEOLOGICAL AND PERFORMANCE ANALYSIS OF UNMODIFIED AND POLYMER MODIFIED ASPHALT BINDERS

Abstract

The 'Performance Grading' (PG) proposed by the SHRP program consists of criteria known as rutting (|G*|/sinδ) and fatigue cracking (|G*|.sinδ) criteria to grade the asphalt binder. Here, the rheological parameter remains constant while the temperature at which the parameter must be achieved varies in respect of PG grade. This thesis examines the utility of the rheological parameters, such as phase angle (δ), sinδ, complex modulus (|G*|), |G*|/sinδ, |G*|sinδ, frequency of oscillation (), etc., for unmodified and polymer modified asphalt binders (PMBs). The effective utility of thermoplastics in combination with styrene butadiene styrene (SBS) is also explored based on the binder and mix analysis. In the PG grading, the rutting specification was set as the temperature at which |G*|/sinδ ≥ 1000/2200 Pa for unaged/rolling thin film oven (RTFO) aged binders at 10 rad/s  and 10 % strain (γ). While fatigue specification was set as the temperature at which |G*|.sinδ ≤ 5000 kPa for pressure aging vessel (PAV) aged binder at 10 rad/s and 1% γ. The results show that for unmodified binders at true PG upper limiting temperature (TU), the rutting criterion can be equated to the viscosity of the binder. The equivalence of |G*|/sinδ and viscosity is valid over a wide range of testing conditions in oscillatory and rotation shear. Consequently, the correlation of |G*|/sinδ and viscosity with rutting in asphalt pavements was similar. The fatigue cracking criterion is based on the energy dissipating capacity (loss modulus Gʺ = |G*|.sinδ ≤ 5000 kPa) of the 'RTFO + PAV' aged binders. At true PG intermediate temperature (TI), the loss and storage modulus values of 'RTFO + PAV' aged binders were similar as the δ values were close to 45°. Therefore, Gʺ as the fatigue criterion will not provide any particular benefit in predicting the fatigue performance of RTFO + PAV aged binders. Furthermore, using δ to forecast fatigue performance may lead to inaccuracies, as fatigue cracking and δ show opposite trends after aging in asphalt binders. The results provide insights into the possible reasons for the not-so-good correlation observed between the two PG criteria and pavement performance. PMBs have been the most successful among various modified binders. Several researchers have found PG grading is inadequate for PMBs. In the PG grading, 10 rad/s was selected to analyze the rheological behavior that relates to a loading time of 0.1s. SBS polymer has a significantly different molecular structure and molecular weight than asphalt binder. Hence, the response to applied stress/strain, and  will be different. Thus, frequency sweep measurements were performed from 100 to 0.01 rad/s in the linear viscoelastic region at 60 °C. The rheological signature of the SBS polymer in the binder is primarily observed at lower frequencies (≤ 0.1 rad/s). The SBS molecules primarily respond at longer time scales of measurement due to their sluggish dynamics. Hence, the difference among SBS-MBs, as a function of SBS content and SBS structure, increases as  decreases. Notably, the correlation of rheological variables with rut depth improves significantly at frequencies ≤ 0.1 rad/s. On the other hand, at higher frequencies (≥ 10 rad/s), the brittle-like response from the polystyrene segments and the smaller units of the SBS molecule dominate the rheological signal. Therefore, at frequencies ≥ 10 rad/s, the evidence of varying SBS content and SBS structure was inadequate. The results reveal that the upper service temperature rheological properties of SBS-MBs are better quantified at lower frequencies (≤ 0.1 rad/s). Similar to unaged, RTFO-aged SBS-MBs also exhibited a strong dependence on the applied . The difference in the rheological properties among the aged MBs with varying SBS dosage increases as the  decreases from 100 to 0.01 rad/s. Gel permeation chromatography (GPC) exhibited degradation of SBS molecules because of elevated temperature and the presence of oxygen. The comparison among unaged and RTFO aged SBS-MBs exhibited two types of viscoelastic responses. At higher frequencies,  reduces after aging, whereas it increases at lower frequencies. Irrespective of SBS and binder type, the divergence increases as  decreases. For effective grading, quality control, and good correlation with rut depth, a rheological parameter at a  ≤ 0.1 rad/s is essential in PMBs. Commercially, various additives such as sulphur, waxes, aromatic oil, poly-phosphoric acid (PPA), etc., are used depending on the requirement. The increment in sulphur dosage showed only a 25 % reduction in  and 150 % increment in complex viscosity at a higher , while the difference increased to 60 % and 450 % at frequencies < 1rad/s. A similar trend was also observed for waxes, aromatic oil, PPA, and crumb rubber. Shenoy parameter also exhibited a strong dependence on the applied  The change in binder type or reduction in SBS dosage has a lesser effect on the rheological response of SBS modified binders. At higher frequencies, the presence of additives in 3.5 wt.% SBS modified binder is undetectable compared to modified binder without additives. In contrast, substantial divergence is observed in complex viscosity and phase angle at frequency < 1 rad/s. A comprehensive comparison among SBS and several thermoplastics (TPs) modified binders was also carried out based on conventional, rheological, morphological, and asphalt mix performance. The result shows that TPs modified binders (TP-MBs) may have similar viscosity and modulus values to SBS-MBs. However, the % elastic recovery (ER) of ungrafted TP-MBs was < 10 %,  values were close to 80°, and the softening point was 15 to 20 °C lower than SBS-MBs. MA-PE only gave marginal benefits, and most of the drawbacks remained. Similarly, the properties of the mixes prepared using TP-MBs were far lower compared to SBS-MBs. The TP MBs can enhance the Marshall stability and rut resistance in mixes by 20-40 % compared to the base binder. However, improvement was not observed in moisture susceptibility, indirect tensile strength, and fatigue crack performance. TPs are merely dispersed as discrete particles within the asphalt binder and fail to develop the required elasticity, cohesion, and interaction within the binder and mixes. Hence, mixes with TP-MBs are highly susceptible to failure by tensile stress, fatigue cracking, and moisture-induced damage. Analysis limited to viscosity and modulus of the MB and Marshall stability and rutting in mixes can falsely indicate that TPs can enhance the overall performance of asphalt binders and mixes. The study also explored alternative processes to utilize TP polymer, drawbacks of using chemically grafted polyethylene (g-PE) and demonstrated how g-PE can be used in combination with SBS. The ungrafted and g-PE produces modified binders exhibit highly inferior properties, and the polymer phase separates within a few hours of preparation. % ER of the ungrafted and g PE MBs was < 30 %, and the softening point was 10 to 25 °C lower than SBS-MBs. The '50/50' g PE/SBS blend also resulted in inferior MBs. On the other hand, the '25/75' g-PE/SBS blend resulted in MBs with properties similar to SBS-MB. Chemical grafting, sulphur content of 0.21 wt.%, and MFI value > 2 are the essential factors that resulted in acceptable '25/75' g-PE/SBS modified binders. The strategy illustrated in this study can be adopted for low-cost polymers and the effective utilization of waste plastics.