Assessing the Effect of Specimen Preparation Methods on DSR Test Results of Bitumen Using Factorial Design Analysis

A two-level three-factor factorial design experiment was conducted to study the influences of three critical specimen preparation parameters on the measurement results of bitumen by a dynamic shear rheometer (DSR). The investigated factors were (1) the pre-heating temperature (HT) for manufacturing the specimen, (2) the bonding temperature (BT) onto the rheometer, and (3) the trimming (Trim) operation for preparing the specimen after bonding. The analysed data were the measured shear modulus |G*|, phase angle δ, and the characteristic temperatures of bitumen's specific stiffness TX with corresponding phase angle δ according to the European standard EN 14770:2023. Five types of bitumen were tested, including three penetration grades and two modified bitumen specimens (with polymer and wax additives). In addition, a repeatability evaluation of the test results was conducted. We found that the trimming operation for preparing the specimen has a noticeable impact when using smaller plates (PP08) for the DSR measurement. At higher test temperatures when using larger plates (PP25), the trimming operation does not significantly impact the measured parameters, in contrast to the HT and BT. Except for bitumen type 70/100, modified binders are more susceptible to variation in the analysed parameters than unmodified ones. The three-way interaction Trim:BT:HT tends to cause relatively little variation in measured data. Interactions between two factors Trim:BT, Trim:HT, and BT:HT contribute more to the fluctuation in δ value than in TX and |G*|. The variation employed in this study affects the test repeatability of wax-modified bitumen significantly; however, for unmodified binders the repeatability of TX and δ are within 0.4-2.1 °C and 0.3-3.1°, respectively.