Laboratory investigation on adhesion properties and water susceptibility of bitumen-aggregate systems

Challenges concerning pavement engineering are nowadays mainly addressed to a low-cost and time-saving preservation of infrastructures. The need for innovative materials and technologies, combined with the necessity of guaranteeing adequate inservice performance, must face the growing distresses of pavements. Moreover, several works are not properly executed, due to inappropriate materials with not well-known properties. To this regard, the present paper focuses the microscopic mechanisms developed at the bitumen-aggregate interface in order to fully understand the bonding and adhesion factors that affect the asphalt concrete serviceability. Experimental tests were carried out to investigate surface texture of different kind of aggregates and its correlation with aggregate-bitumen adhesion strength. The external surface of aggregate was characterized in terms of microscopic structure using a 2D contact profilometer; then the bonding with different bitumens, also in the presence of water, was investigated through boiling water and peel tests. A 50/70 pen bitumen was selected to assess the adhesion characteristics with limestone, porphyry, basalt and steel slag aggregate. Samples were prepared both with smooth and rough aggregate surfaces (in order to simulate different physical conditions) and were characterized in dry and wet conditions. Experimental findings showed a strong correlation between the bitumen-aggregate adhesion and the different kind of tested aggregates. Such property seemed to be also influenced by surface conditions and bitumen-aggregate blending processes. In particular, bitumen-aggregate systems prepared with limestone and basalt manifested good water resistance, whereas the use of steel slag and porphyry led to worse performance, also according to roughness characteristics.