Development and Evaluation of Hot and Warm Sand-Sulphur-Bitumen Paving Mixes

Abstract

The economic growth of a country leads to urban expansion and infrastructure development importantly in pavements, which eventually increases the demand for stone aggregates. The uncontrolled extraction of construction aggregates has resulted in severe depletion of existing natural stone reserves. Keeping this issue in mind, the use of locally available and waste materials as a substitute for stone aggregates, particularly in road construction has become a major research interest. Contemporarily, sand is available abundantly in some places, such as in coastal deltaic plains, beaches and desert areas. Hence, researches involving utilisation of sand as an alternative to coarse aggregates are worth taking up. In this direction, few studies reported sand-bitumen mix to be weak and unstable (low stability and high air void content), and hence unsuitable for pavement construction. However, the properties of sand-bitumen mix could be improved with inclusion of sulphur which is a by-product of coal processing, and petroleum and gas refining processes. When Sulphur is added as third material along with sand and bitumen, the mix is typically termed as sand-sulphur-bitumen (SSB) mix. This research work addresses the development and evaluation of SSB mixes in order to utilise locally available sand as an alternative to coarse aggregates for paving purposes. This study basically investigates two types of mixes namely, hot and warm SSB mixes. For preparation of hot mixes, VG 30 bitumen was used whereas for warm mixes, VG 30 bitumen was modified with sasobit so as to facilitate mixing and compaction at lower temperature. First of all, a suitable mixing method was developed to prepare SSB mixes that involved selection of critical mixing parameters such as mixing sequence, mixing temperature, mixing time and compaction effort. Concurrently, gaseous emissions were monitored to assess the potential hazards due to toxic gases released during mix preparation. Next, the effects of varying proportions of constituents such as sand, sulphur and bitumen were considered to determine the optimum mix compositions based on Marshall properties. Further, crusher dust in varying concentrations and/or various types of fillers such as fly ash, stone dust and cement were considered for the mix, and the Marshall properties were studied to determine the best proportions of ingredients. In addition to Marshall properties, the selected mixes were evaluated in terms of other performance characteristics such as indirect tensile strength, moisture susceptibility, resilient modulus, fatigue performance and rutting characteristics. Furthermore, an attempt was made to study the internal structure of SSB mixes with the help of effective and quick non-destructive 3D (three-dimensional) high resolution technology namely X-Ray micro computed tomography coupled with digital image analysis techniques. Various air void (AV) parameters such as volume, size and shape of voids, connected air void content, and tortuosity of compacted specimens were assessed to describe the geometric and topological properties of voids. The impacts of variation in the mix constituents on AV parameters were examined to understand their effects on void structure and their properties. Additionally, probabilistic analysis was performed to model the void size distributions in SSB mix specimens. The permeability of SSB mixes was also measured using an asphalt permeameter and correlation analysis was performed to establish the relationships between permeability and AV parameters. In addition, predictive models for permeability were developed based on the fundamental mix properties. Experimental results revealed that for hot SSB mixes, the optimum mixing sequence included mixing of VG 30 bitumen with sand-molten sulphur blend at 145°C, whereas for warm SSB mixes the mixing sequence involving mixing of molten sulphur with sand-modified binder at 125°C produced the best results. The gaseous emission analysis showed that the concentration levels of various harmful emissions near the place for SSB mix preparation (in the laboratory) were found to be much below the prescribed limits specified in different standards for hot SSB mixes. The emissions were found to be much less for warm SSB mixes. With regard to mix composition, the concentrations of sand and sulphur by weight of mix were found to be 85% and10% respectively for both hot and warm SSB mixes. Substantial improvements in Marshall properties were noticed with the replacement of sand by 30% crusher dust. For all filler types, optimal filler content was found to be 5% by weight of aggregate. For each mix type, bitumen contents of 4.5 and 5% by weight of each mix were chosen based on the Marshall test results. The SSB mixes with selected mix compositions were found to exhibit satisfactory performance characteristics. The microstructural characterisation of SSB mixes revealed that the mix constituents, mainly incorporation of crusher dust, had significant impacts on the AV parameters. A two-parameter loglogistic distribution and a four-parameter stable distribution were found to be the best fit models for void size distribution of hot and warm mixes respectively. Further, permeability of SSB mixes had significant correlations with the geometric and topological void properties. Despite having high air void content, the SSB mixes exhibited a high degree of impermeability which is attributed to the less connectivity of air voids and higher tortuosity of the mixes. Predictive models developed using neighbourhood component analysis (NCA) regression approach for prediction of permeability was found to be accurate (R2 = 0.99) for both hot and warm SSB mixes. Furthermore, the life cycle cost analysis of typical SSB mixes revealed that there is a substantial economic benefit as compared with conventional bituminous mixes. In summary, the SSB mixes exhibited essential performance characteristics as required for a normal bituminous pavement. Overall, this study showed an effective way to use SSB mixes as a bituminous layer with complete exclusion of conventional coarse aggregates which are scarce in some locations.