ACKNOWLEDGEMENTS N3TC – Philip du Plessis Multiserve – Nick Jones, Koos Jan van Brouwershaven Ilanga Aspahlt – Jan van der Merwe Levy Company – John Yzenas Rand Roads – Mike Kohlberg Illinois Department of Transportation USA Steel Slag Association Australian Steel Slag Association
INTRODUCTION WHY DO WE NEED AN ALTERNATIVE PRODUCT?? Changes in traffic spectrum; Changes in traffic loading; Heavy traffic damage (rutting and shoving); Moisture damage (Wet environments); Gradient (e.g. Van Reenens Pass); and Temperature ranges (Summer / Winter variation).
INTRODUCTION WHAT ARE WE HOPING TO ACHIEVE?? Improved Deformation Characteristics i.e. strength and stability; Improved Durability i.e. moisture sensitivity; Improved Skid resistance i.e. polished stone value, SCRIM values
STEEL SLAG AGGREGATE WHAT IS STEEL SLAG?? It is a by product of the steel making process, It has a complex chemical structure, comprising oxides and silicates that are formed through the oxidation of impurities within the steel by the addition of lime and fluorspar; On completion of the steel making process, the liquid slag is separated and poured into purpose built hot slag carriers. The slag is then cooled before further processing, which includes weathering, crushing, and screening; The weathering process is an essential part of the handling of steel slag, as it enables the hydration of any uncombined free lime (CaO) or free magnesia (MgO) present in the material.
STEEL SLAG AGGREGATE WHAT IS STEEL SLAG?? Its chemical composition is primarily calcium silicates together with oxides and compounds of iron, manganese, alumina and other trace elements. Steel slag is mildly alkaline with a solution pH generally in the range of 8 to 10 (highest measured at 12,6).
STEEL SLAG AGGREGATE AGGREGATE PROPERTIES The properties that have to be evaluated for the suitability of slag as an aggregate relate to the chemical, physical and mechanical nature of the material. Hardness / toughness, resistance to abrasion; –The high Fines Aggregate Crushing Test (10% FACT); –Low Aggregate Crushing Value (ACV); –Low Los Angeles Abrasion Value; –Indicate high density and crushing strengths of the steel slag aggregate.
STEEL SLAG AGGREGATE AGGREGATE PROPERTIES Particle shape and surface texture, stability and skid resistance; –Cubical shape; –Rough surface texture rough; –The Flakiness Index (FI) of the steel slag aggregate is markedly lower than those for the dolerite and quartzite aggregates; –A Polished Stone Value (PSV) of 63 for the steel slag aggregate indicates a high resistance to the surface of the aggregate polishing under the action of traffic.
AGGREGATE PROPERTIES Specific gravity and water absorption –Particle density of the steel slag aggregate compared to natural aggregates (dolerite & quartzite) is more dense and heavier material by approximately 20%; –Steel Slag typically has a higher water absorption than dolerite and quartzite;
STEEL SLAG AGGREGATE AGGREGATE PROPERTIES Durability, resistance to stripping; –Steel slag, with a pH of between 8 and 10, has a strong affinity to bitumen and therefore displays a greater degree of binder retention. – Rough surface texture, improves stripping resistance. –This is very important in the prevention of stripping of the binder and moisture damage.
STEEL SLAG ASPHALT MIXTURE MIX DESIGN Continuously graded mix Aggregate used: –13,2 mm –9,5 mm –-3 mm Polymer modified binder –SBS binder used for high seasonal temperature fluctuations and high stability & flexibility mixes for high traffic.
STEEL SLAG ASPHALT MIXTURE MIX PROPERTIES Strength and Stability –the cubical shape of the aggregate enables the formation of strong interlocking structures within the asphalt mixture, which exhibit high stiffness and excellent resistance to permanent deformation –Stability of 15,1 kN; –Flow of 3,7 mm; –Stability / Flow Ratio of 4,1 –Indirect Tensile Strength (ITS) of 1288 kPa;
STEEL SLAG ASPHALT MIXTURE MIX PROPERTIES Durability –Attributable to such things as air voids in the mixture, binder film thickness and moisture sensitivity. However, one of the superior properties of steel slag is its ability to retain the binder coating; –Steel slag, with a pH of around 10, has a strong affinity to bitumen and, therefore, displays a greater degree of binder retention; –Voids In Mix (VIM) designed to be 4,1%; –Voids in Mineral Aggregate (VMA) of 16,8%; –Voids Filled with Binder (VFB) of 72%; –Air permeability below 1,0 x 10 6 cm 3 / second. –Immersion Index of > 85% (well above required 75%); –Film thickness 6,8 to 7,2 micron @ 5,2% binder.
STEEL SLAG ASPHALT MIXTURE MIX PROPERTIES Skid Resistance –A polished stone value of 63 classifies the steel slag as a high specification aggregate and is generally used for the higher risk sites; –It is usual for an aggregate having a PSV in this range to have a poor abrasion value; –Steel slag, however, also has excellent resistance to abrasion; –An unusual combination of properties, making steel slag a safe and durable alternative to natural aggregates. Grip tester friction measurements.
STEEL SLAG ASPHALT MIXTURE ADDITIONAL TESTING Gyratory compaction; –Average voids 3,1% after 300 gyrations (minimum 2,5% after 300 gyrations); Wheel tracking using MMLS3; –The MMLS performance test results indicate that the steel slag asphalt has better resistance to deformation than the Dolerite asphalt and SMA samples evaluated. Soluble deleterious material test; –Unwashed sample 85%; –Washed sample 100%
ENVIRONMENTAL CONSIDERATIONS ADVANTAGES Reduces landfill; Reduces the need to quarry; Preserves our natural resources.
CONCLUSIONS Steel slag produces a high quality aggregate which can benefit asphalt materials in a number of ways. Steel Slag aggregate can out perform natural aggregates in terms of: –Shape; –Resistance to crushing; –Skid resistance; and –Durability; and –Adhesion. Steel Slag in asphalt mixtures improves properties of the mix in terms of: –Stability; –Resistance to permanent deformation; –Durability; and –Skid resistance. To date approximately 90,000 tons placed on N3.