Stålslagg i asfaltbeläggning: fältförsök 2005 – 2012

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Torbjörn Jacobson

During the manufacturing process of steel, lime and dolomite are added as fluxes. These then combine with some of the melted impurities and form a slag byproduct. In many countries steel slag is used in both bound and unbound road construction layers. This report discusses the use of the byproduct from the steel mill in Smedjebacken, EAF (Electric Arc Furnace) slag, as an asphalt aggregate. Good experiences with such a material can be found in Halmstad and Laholm, where EAF slag has been used in surface courses on high traffic volume roads since the beginning of the 1990s. In Denmark, EAF slag from Smedjebacken is frequently used in roundabout construction and on roads with intensive traffic. The slag from Smedjebacken has good mechanical properties. The aggregate, after crushing in several steps, has a cubic form but still retains a rough surface texture due to its porous nature. This results in a very stable asphalt road surface. The slag contains a suitable amount of free lime (<0.5%), so resistance against water permeability is very good. To help eliminate any risk of swelling, aggregate is stored outdoors for about a year. The slag from Smedjebacken has good volumetric stability. Metal content is separated off before the slag is crushed and graded into appropriate aggregate fractions. Aggregate size greater than 4 mm is normally used for asphalt surfacing materials. The slags high density (high content of iron oxide) and surface porosity must be carefully considered during the design of the surfacing material. Slag asphalt binder content is lower than conventional asphalt. A number of test sections, where steel slag was used as a surface course, ABS11 (SMA) aggregate, were constructed in Dalarna between 2005 and 2008. At Smedjebacken (2005) and Borlänge (2006), slag asphalt was used in roundabout construction. On Road 68, north of Horndal (2007), a 300 metre long test section was constructed. A larger project using steel slag, ABS11, was carried out on Road 50 between Grängesberg and Ludvika (2008). In this project, the asphalt mix contained 20 per cent porphyry aggregate. To reduce traffic noise emissions, steel slag asphalt (Swedrain8) was used in Skälbyvägen, Järfälla (2010,) and in a test section with drainage asphalt (ABD11 - upper layer in a double drain construction) on the E4, Husqvarna (2010). Another section, constructed with an ABT16 (dense-graded asphalt concrete) containing slag in all fraction sizes, has been tested on a mine access road, with a high volume of heavy traffic, at Garpenberg (2011). Many of the test sections are subjected to high traffic volumes or have high proportions of heavy vehicles. Test sections have been visually inspected for defects each year. On a few occasions, macro texture, rut depth and evenness have also been investigated. Laboratory test results, traffic data, and road information are also presented in the report. In summary, the inspections have shown that the slag asphalt has performed very well. Investigations of the asphalt mix and road surface have shown very good results in terms of stability, stiffness and durability. This makes the material particularly suitable for vulnerable areas. Even durability is deemed acceptable for Swedish winter conditions which involve the use of studded tyres. No defects, such as stone loss, cracking or other surface type defects were observed during the inspection surveys. The slag asphalt, as expected, provided good friction values. Noise reduction levels, for dense or semi-dense asphalt were about 1 dB lower when compared with similar sections with conventional asphalt surfacing.

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