Experimental investigation on the bond strength between sustainable road bio-binders and aggregate substrates

Interest is growing on the application of bio-binders in road pavements. However, currently there is a lack of data concerning the adhesion between bio-binders and aggregates, which is a crucial aspect to ensure adequate performance and durability of bituminous mixtures, especially in the presence of water. In this regard, the present investigation focuses on the evaluation of the binder bond strength (BBS) between bio-binders, characterized by different percentages of a renewable wood bio-oil and different aging levels, and aggregate substrates (limestone and porphyry), in dry and wet conditions. Preliminarily, the binders were subjected to viscosity tests to determine BBS application temperatures. The main results show that the bio-binders studied exhibit a good adhesion with limestone both in dry and wet conditions as well as with porphyry in dry conditions, resulting in cohesive failures. For porphyry substrate, after wet conditioning, a progressive transition from adhesive to cohesive failures is observed as the bio-oil content increases, indicating that the bio-oil might improve the adhesion between bitumen and siliceous aggregates. Based on previous findings on the chemical characteristics of the bio-binders, the contribution of the bio-oil to the adhesion may be attributed to its high content of esters. Overall, the results suggest that the use of bio-binders in road pavements could lead to significant benefits in terms of performance and resistance to moisture damage.