Laboratory Evaluation of Stone Matrix Asphalt (Sma) Mixes With Partial Replacement of Natural Aggregates By Reclaimed Asphalt Pavement (Rap) Along With Rejuvenator

Abstract

Stone Matrix Asphalt (SMA) is a type of Hot Mix Asphalt developed in Germany in the 1960s to overcome the effect of studded tyres. SMA has mainly two parts - a coarse aggregate skeleton and a high binder content mortar. The coarse aggregate skeleton provides the mixture with stone-to-stone contact, giving it strength, while the high binder content mortar adds durability. A typical SMA composition comprises 70−80% coarse aggregate, 8−12% filler, 6.0−7.0% binder, and 0.3% fibre. SMA uses a gap-graded aggregate gradation because of which the binder mortar drains down from the mixture during production in the hot mix plant and transportation and placement in the field. The fibre acts as a stabilising additive to hold the bituminous binder in the mixture during the high production and placement temperatures. In the present study, SMA mixtures were as per prepared IRC SP 79 2008 using an aggregate gradation of 13 mm Nominal Maximum Aggregate Size (NMAS). A polymer-modified bitumen PMB 40 and conventional viscosity grade bitumen VG 30 were used to prepare the mixes. Three stabilising additives were used: areca fibre, coconut fibre and pelletized cellulose fibre when mixtures were prepared using VG 30 bitumen. Based on the results of the drain down tests, the Optimum Fibre Content (OFC) was determined to be 0.3% by weight of the mixture, and it was observed that the drain down decreased with an increase in the fibre content. Cylindrical specimens were cast at OFC to determine Optimum Bitumen Content (OBC) using the Marshall method of mix design and Superpave Gyratory Compactor (SGC). The OBC values obtained for the mixes with natural fibres were higher, possibly due to the higher surface area of the fibres and the possible absorption of bitumen by the fibres. The mixture with PMB 40 and coconut coir fibre exhibited the highest and lowest results with respect to Marshall Stability, indirect Tensile Strength, rutting, fatigue and moisture susceptibility, respectively. Further, the performance of the mixture with cellulose fibres in the form of pellets was comparable to that of the mixture prepared with polymer-modified bitumen. The results of the mixtures with natural fibres were comparable, and the performance of SMA with areca fibre was better than that of SMA with coconut fibre. The tests conducted on compacted specimens revealed that all the mixtures (SMA-PMB, SMA-PF, SMA-AF, SMA-CF) satisfied the viii requirements of SMA as per IRC SP 79 2008. Since areca fibres are abundantly available in South India and disposed of in empty spaces due to the lack of any applications, areca fibres can be used as stabilising additive in SMA mixes. Due to the increased construction activities, much demand is being exerted on natural resources such as aggregates and bitumen. Hence, to reduce the environmental impact and conserve natural resources, RAP materials are used as an alternative source of aggregates for the production of fresh bituminous mixtures. Since the RAP aggregates contain aged binder, the bitumen required for producing a fresh bituminous mixture is also lesser. When RAP is used, a rejuvenator is required, which helps rejuvenate the aged binder in the RAP material due to oxidation. Hence, an abundantly available and cost-effective liquid, a by-product of the Cahsewnut processing industry, called Cashewnut Shell Liquid (CNSL), is used as the rejuvenator. In the present study, SMA mixtures were prepared using a 13 mm Nominal Maximum Aggregate Size (NMAS) aggregate gradation. A conventional viscosity-graded VG 30 bitumen was used to prepare the mixtures. The natural aggregates were replaced by RAP in the mixture in percentages of 10%, 20%, 30% and 40% by weight by RAP. The mixtures containing RAP were prepared in accordance with the SMA wearing course specification as per IRC SP 79 2008. For each RAP content, CNSL was added at 0%, 3%, 6% and 9% by weight of the bitumen. The drain-down test conducted on all the mixtures showed that they were within the allowable limit of 0.3% by weight of the mixture. The ratio of VCAmix and VCADRC was found to be less than 1 for all the mixtures, confirming the presence of an aggregate skeleton. The OBC of the mixtures decreased with an increase in the RAP content. The mechanical properties of the mixtures such as Marshall Stability (MS), Indirect Tensile Strength (IDT), Fatigue, rutting and workability were determined. The moisture susceptibility of the mixtures such as Retained Marshall Strength (RMS), Tensile Strength Ratio (TSR) and stripping test on aggregates was evaluated. Though the results of the tests indicated that a mixture with a maximum RAP content of 40% and 3% CNSL provided the highest results, the mixture with a RAP content of 30% and 6% CNSL yielded the optimal results.