Evaluating Torsional Recovery test for interlinkage with deformation-recovery measurement metrics of polymer modified bitumen

Elastic recovery of polymer modified bitumen (PMB) is considered an effective parameter to evaluate its capacity to resist permanent deformation through recovering some part of load-related deformation. Conceptually, it is represented as the ratio of recovered strain (measured under zero loading after a fixed duration) to maximum strain (under loading) on a bitumen sample, using various devices and loading types. Recovery tests also indicate the presence of polymer as well as degree of polymerization. Hence, such tests can be useful as rapid quality control and verification tool during field inventory inspections by procuring agencies. Traditional method, that is, ductilometer-based Elastic Recovery (ER-DB) relies on an unwieldy large device, delicate sample preparation process, and variable true strain rate. On the other hand, Multiple Stress Creep Recovery (MSCR) test-based Percent Recovery (%R) utilizes creep-recovery from application of shear-stress on a small bitumen sample for obtaining similar information, but it requires a sophisticated and expensive device. Torsional Recovery (TR), which uses an easily portable, manually operable, quick and inexpensive setup, also relies on shear deformation and angular recovery. In the present study, TR was evaluated for its correlation with these established methods using a total of 15 PMBs, prepared with 5 types of SBS and one base bitumen. The results showed strong linear relationships, with linear regression $R^2$ values exceeding 0.85 in many cases, especially for PMBs made with the same polymer. Notably, TR and ER-DB exhibited $R^2$ more than 0.90, especially within SBS-type specific cases, while global correlation was found to be 0.76. In addition, it was found that TR can be reliably used for preliminary quantification of SBS-dosage with $R^2$ of 0.88. Interestingly, Torsional recovery values show good global correlation with Dynamic Modulus values and associated parameters with $R^2$ $\ge 0.8$. In fact, SBS dosage, Torsional Recovery and Dynamic Modulus parameters also displayed excellent PMB specific correlation and reasonable global correlation. Other factors and correlations also indicate that TR results align well with existing test methods and hence, can be used for preliminary assessment of PMB quality and quantitative presence of polymer, with particular suitability for site deployment.