Laboratory Evaluation of Secondary Aggregates in Bituminous Mixtures

摘要

With a greater understanding of the need for sustainabledevelopment, the use of primary aggregates in asphalt mixtures for roador airfield pavements is seen as a wasteful use of a finite naturalresource. Therefore the reuse of primary aggregates and/or the use ofwaste (secondary) materials are seen as being of benefit to society. Ofthe various waste streams, the by-products of the iron and steel makingindustries (blast furnace and steel slags) and recycled crushed glass(cullet) can be considered sensible alternative sources of aggregate forasphalt mixture production. These secondary aggregates have similarphysical properties to conventional, primary aggregate and can beprocessed, crushed and screened into practical sizes for easy batchinginto both surfacing and base asphalt materials.This paper assesses the mechanical performance and durability of arange of both base and surfacing materials incorporating differentcombinations, size fractions and percentages of two primary aggregates(limestone and gritstone) and three secondary aggregates (basic oxygensteel slag, blast furnace slag and glass cullet). The mechanical propertiesof the asphalt mixtures have been measured using the suite of tests(stiffness modulus, resistance to permanent deformation and resistance tofatigue cracking) possible with the Nottingham Asphalt Tester (NAT).The durability of the primary and secondary mixtures has been assessedby subjecting the materials to simulative long-term laboratory aging andmoisture susceptibility conditioning using recognised testing(conditioning) procedures and protocols.The results indicate that the use of glass cullet fine aggregate in a 28mm continuously graded base material only marginally reduces thestiffness modulus of the secondary aggregate modified mixture. Themoisture susceptibility of the material was also shown to be less thanwhat would be expected for a smooth surface textured aggregate such asglass, with and without the use of an anti-stripping agent. Thereplacement of primary aggregate with glass cullet also significantly reduced the aging susceptibility of the mixture, while the permanentdeformation resistance, although inferior to that of a primary aggregatemixture, was still acceptable with the fatigue performance beingcomparable to the control mixture.The use of basic oxygen steel slag and blast furnace slag secondaryaggregates significantly increases the mixture density and stiffnessmodulus compared to primary aggregate mixtures. The moisturesusceptibility of these secondary aggregate mixtures was also found to besimilar to that of the control mixtures, although the slag mixtures didshow an increased susceptibility to age hardening. Overall the permanentdeformation resistance and fatigue performance of the slag mixturestended to be similar to that of the limestone and gritstone base andsurfacing control mixtures.