Microsurfacing Treatment Modified with Bitumen Emulsion Containing Polyphosphoric Acid in Combination with Steel Slag Filler as an Alternative to Natural Material Filler: Experimental Assessment and Mixture Design

The present research seeks to investigate the modification of bitumen emulsion with polyphosphoric acid and study the feasibility of using steel slag powder as an alternative to conventional rock material filler in the design of microsurfacing mixture in an attempt to improve the ultimate performance of the mixture. For this purpose, firstly, polyphosphoric acid was used to modify the bitumen emulsion at 0.4, 0.8, 1.2, and 1.6 wt% (by weight of residual bitumen). Next, the optimally modified bitumen emulsion was used in different microsurfacing mixtures. Five different mixtures of micro surfacing containing steel slag filler at 0, 2.5, 5.0, 7.5, and 10.0 wt% (by total weight of aggregates), as an alternative to material passing through 0.075 mm sieve, were prepared. The asphalt mixtures were assessed by means of wet cohesion test, wet track abrasion test, loaded wheel-displacement, and loaded wheel-sand adhesion tests. Results of the bitumen tests showed that the use of the polyphosphoric acid tends to improve the bitumen properties in terms of decreased penetration, increased softening point, and reduced thermal sensitivity. With increasing the dosage of the polyphosphoric acid to up to 0.8 wt% by weight of the residual bitumen, significant changes were seen in the penetration and the softening point, which led to improved performance of the bitumen emulsion. Results of the asphalt tests on the samples showed that the mixtures containing steel slag filler at 7.5 and 10.0 wt% outperformed the other mixtures. Indeed, the mixture containing steel slag at 7.5 wt% could improve the cohesion, abrasion resistance, bleeding resistance, and vertical displace resistance by 29.2, 64.5, 18.6, and 44.0%, respectively. The corresponding figures for the mixture containing the slag at 10.0 wt% were 25.0, 64.3, 23.3, and 42.9 wt%, respectively.