Experimental analysis of skeleton strength of porous asphalt mixtures

The internal structure is critical to the functionality and rutting resistance of porous asphalt (PA) mixture. The objective of this paper is to develop a new test (skeleton penetration test, SPT) to directly evaluate the skeleton strength and optimize the gradation of PA mixture. The optimized diameter (50 mm) of loading head and penetration rate (2.25 mm/min) were determined for SPT to obtain a reliable skeleton strength. The influence of percent passing of 9.5 mm (P9.5) (40%, 50%, 60%, 70%, and 80%), percent passing of 4.75 mm (P4.75) (10%, 20%, and 30%), and percent passing of 2.36 mm (P2.36) (5%, 15%, and 25%) were evaluated by the SPT and breakdown test. The porosity and durability of the optimized gradation were evaluated by the volumetric determination test and Cantabro loss test, and the correlation of skeleton strength to the flow number obtained from dynamic creep test was also evaluated. Test results show that skeleton strength decreases with the increment of P2.36. P4.75 has negligible influence on the skeleton strength, and P9.5 mm has the largest impact on the skeleton strength. Based on the results, 40% and 80% are recommended as the lower limit and upper limit of P9.5. 10% and 30% are recommended as the lower and upper limit of P4.75, and 25% is recommended as the upper limit of P2.36 for PA mixture. Finally, it is observed that the optimized gradation recommended by the SPT also meets the requirements of porosity and durability of PA mixture, and skeleton strength correlates well to the rutting resistance. Therefore, SPT has a potential to improve the mixture design of PA mixture.