Investigating the use of moist aggregates in the production of low-temperature bituminous mixtures using foamed bitumen

Abstract

The use of moist/wet aggregate fractions in the asphalt mixtures with reduced production and compaction temperatures (HWMA - Half Warm Mix Asphalt) with foamed bitumen is aimed at obtaining an additional foaming effect of the binder. The paper presents results of investigations on the possibility of producing the asphalt concrete for the upper layers of the road pavement at reduced technological temperatures utilizing fine wet aggregates and foamed asphalt in opposition to specific patented solutions. This modification of the production process was evaluated in the scope of moisture content in the 0/4 mm aggregates in terms of mixture compactibility using the gyratory compactor and the assessment of related physical and mechanical properties of the asphalt mixtures. The analysed moisture contents resulted in a final 1% and 3% water contents in the whole mineral mix. In order to assess the effect of the addition of wet aggregates on the properties of the mixture produced at the reduced temperature with foamed bitumen, three identical asphalt concrete AC 16 mixtures in terms of composition were used, hot-mix HMAREF and warm-mix WMAREF, with differences only in technological temperatures of the mix constituents during their preparation and mix compaction, the form of the binder (liquid in HMAREF mixture, foamed with water in WMAREF mixtures) and half-warm mix HWMA with foamed bitumen additionally utilizing a portion of fine wet aggregates in the blends. The conducted studies have shown that the presence of the wet aggregates in the mixtures with foamed bitumen had a significant impact on the air void contents in the samples formed in the gyratory compactor. In a mixture with foamed bitumen, produced and compacted at a reduced temperature (HWMA), an improvement in the compaction in relation to the reference mixture WMAREF was found. Based on the results of indirect tensile strength tests and analysis of the ITSR indices, it was found that the use of wet aggregates had major, significant effects on the mechanical properties of the samples subjected to water and frost susceptibility testing. The mixture produced with wet aggregates obtained indices for resistance to the moisture and frost damage which were unsatisfying for the upper bituminous pavement courses as for Polish climatic conditions (ITSR<70%). This result was attributed to inadequate the aggregate coating and reduced adhesion of the binder. It was stated that the mixtures resembling HWMA, produced with moist or wet aggregates at reduced technological temperatures can be utilized in Polish and similarly harsh climatic conditions in the lower pavement structural layers, mainly due to their reduced moisture and frost resistance. The application of the tested mixtures to the upper structural layer would require using special treatments, e.g. chemical additives, in order to improve the adhesion of asphalt to aggregate or raise the technological temperature (as for WMA blends) to increase the dynamics of water evaporation from the damp aggregates.