Laboratory Testing of Ferrochrome Slag as an Aggregate in Porous Pavements

Porous asphalt (PA) is a bituminous mixture that is preferred in surface wearing courses because of its comparably better skid resistance and reduced spraying and hydroplaning risk, leading to improved driving quality compared with dense-graded hot mix asphalt mixtures. Use of recycled materials with satisfactory strength and durability properties as an aggregate in PA mixtures would provide both economic and environmental benefits because natural resources would be preserved, and transportation costs, and consequently CO₂ emissions, would be reduced, and the stockpiled recycled material would be employed instead of occupying scarce landscape. However, because of the high air void content and exposure of a larger surface area to atmosphere in PA mixtures, the potential for raveling and rapid aging for any proposed aggregate material needs to be evaluated carefully. This study was conducted to investigate the performance of ferrochrome slag (FS), a by-product of ferrochromium industries, as an aggregate in porous asphalt. Air voids content, permeability, particle loss, tensile strength, and moisture susceptibility were measured on three different gradations of FS and the results were compared with the limits given in construction specifications. The gradation of the slag aggregates along with the bitumen content played a key role on performance of the PA mixtures. The FS specimens prepared at an optimum bitumen content of 6.5 % performed significantly better than limestone (LS) aggregate specimens at the same gradation; the air void content, permeability, and tensile strength of the FS mixtures were 2 %, 50 %, and 16 % higher than the LS mixtures, respectively.