Early-Age Stiffening of Cold Recycled Bituminous Materials Using Shear Wave Velocity

The characterization of cold recycled bituminous materials (CRMs) at a very young age, shortly after compaction, is inherently challenging due to the nature of the material. The granular aspect of CRM at this stage and its high-water content render the use of conventional mechanical techniques impractical. Following previous work, the use of a nondestructive technique based on the frequency analysis of mechanical elastic shear waves, piezoelectric ring actuator technique (P-RAT), has enabled assessment of the behavior of cold in-place recycled material treated with bitumen emulsion from 10 min after compaction to 30 days of curing. Emphasis on shear wave velocity (V _s) measurements during the early age confirmed the rapid stiffening of the mix along with the departure of water present in the mix. A 6 °C drop of the surface temperature is observed along with this rapid increase of V _s and water loss. The initial and final V _s values range from 287 to 330 m s⁻¹ and from 461 to 578 m s⁻¹, respectively. To assess the capabilities of P-RAT, specimens with different void contents were tested, mainly 12, 15, and 17 %. It was observed that in each tested specimens, a similar behavior was exhibited during the first few hours of curing. Based on these observations, hypotheses are put forth regarding the phenomena governing the increase in stiffness during this period. Finally, the influence of the compaction of the specimens on the V _s values is consistent and comparable with the information available in the literature for such CRM.