Multivariate Optimization of Recycled Road Base Cold Mixtures with Foamed Bitumen

The paper presents an approach for identifying the optimum range of foamed bitumen and Portland cement contents in recycled mineral-bitumen road base mixtures using simultaneous optimization of response variables (i.e. properties of mixtures estimated from statistical models) with the use of desirability functions. The 16 evaluated mixtures had a common aggregate composition comprising materials from existing pavement layers (reclaimed asphalt pavement and aggregates from recycled crushed stone base layer) and virgin material. The mixtures varied in foamed bitumen contents (2.0% - 3.5%) and Portland cement contents (1.0% - 2.5%). The investigated mixture parameters included: air void content (Vm), indirect tensile strength (ITS), tensile strength retained (TSR) and indirect tensile stiffness modulus (ITSM) at 25 °C. The use of the desirability functions and desirability index allowed to identify the effective and optimum range of binding agent dosing for the recycled mixture with respect to all of the measured mixture properties.