OPEN-GRADED ASPHALT MIXES: HYDRAULIC PERMEABILITY AND MECHANICAL CHARACTERISTICS

Skid and hydroplaning induced by water film on pavement are critical problems for road safety. Porous pavements are considered to potentially be effective solutions. A new low cost and user friendly piece of equipment for measurement of Darcy hydraulic permeability of pavement is presented in this study. The reliability and repeatability of measurements have been verified. The main points used to verify the strength of the equipment, recently patented in Italy, follow: (i) the measurement is developed before pavement construction in the laboratory and it is possible to optimize the mix design, (ii) Marshall samples were used for hydraulic measurements, unless destruction or perturbation was necessary, for subsequent mechanical testing, (iii) standard cores extracted from existing pavement can be inserted into the instrument to measure the current hydraulic permeability, (iv) the equipment measures permeability, assuming homogeneity and isotropy as a hypotheses, (v) the equipment is low in cost, both for materials and for technology, (vi) no electric or other power is needed, it works under hydraulic load, (vii) the equipment works both at constant and at variable hydraulic load. Finally, the experimental outcomes of a research project are discussed, in order to assess the correlation between the hydraulic permeability of pavement and bitumen content, which voids percentage and aggregate grading. Very cautious standards for mix design are imposed in order to have adequate permeability, and this is validated after pavement development. The final permeability obtained respecting those standards is always higher than the needed one. It induces relevant economic and environmental costs. After verifying in the laboratory the hydraulic permeability and the mechanical resistances of the 24 pavement samples, the effectiveness of an experimental procedure for the optimization of the mix in terms of hydraulic and mechanical standards has been assessed. Outcomes show that permeability always decreases if Young’s Modulus increases. Aggregate grading influences this relationship and the procedure of making of samples influences the mechanical and hydraulic final characteristics. Relevant economic and environmental benefits are a reasonable expectation.