Size effect analysis of mode I fracture performance of hot mix asphalt

Abstract

The diameters of asphalt concrete cylindrical specimens typically measure either 100 mm or 150 mm, as these are the standard sizes for almost all conducted tests. However, the impact of specimen size on the fracture performance of asphalt mixtures has not been extensively explored. This study aimed to address this gap by performing laboratory tests on semi-circular bending (SCB) specimens. These tests were designed to apply the Size Effect Law (SEL) in evaluating the fracture performance of two asphalt mixture types (AC10 and AC16) at −10 ℃. The experiment selected four different diameters (76 mm, 100 mm, 125 mm, and 150 mm) and two notch-radius ratios (α0 = 0.2 and 0.4) to investigate the relationship between the specimen dimensions and both the nominal stress and fracture toughness (KIC). Results indicated that the nominal stress and KIC adhered to the SEL. It was observed that both aggregate gradation and notch-radius ratio (α0) significantly influenced the SEL’s effect on nominal stress. An increase in α0 from 0.2 to 0.4 shifted the fracture failure towards a more linear elastic fracture behavior. Specifically, the coarser gradation in AC16, compared to AC10, predisposed the fractures towards linear elastic behavior. Moreover, an increase in α0 notably augmented the brittleness number. For specimens with identical dimensions and α0, AC16 exhibited higher D/D0 values, indicating a stronger alignment with Linear Elastic Fracture Mechanics (LEFM) principles for the tested specimen sizes compared to AC10. As the size of the specimen increased, there was a gradual rise in KIC, with the enlargement from 76 mm to 150 mm resulting in KIC increases of 25.2 % and 12 % for α0 values of 0.2 and 0.4, respectively, in the AC10 mixture. Conversely, the AC16 mixture saw respective enhancements of 15.8 % and 7.6 % for the same α0 values. The influence of α0 on the apparent fracture toughness became less pronounced with its increase; KIC gradually decreased for AC10, yet increased for AC16 with rising α0 levels. For the same asphalt mixture type, KIC was found to escalate with both D/D’0 and α0. At an α0 of 0.4, the disparities between KIc and KIcm, – expressed as (KIcm- KIc)/ KIcm, – stood at 13.9 % and 9.2 % for AC10 and AC16, respectively, with a diameter of 150 mm. Based on these findings, it is recommended to utilize an SCB specimen with a diameter of 150 mm and an α0 of 0.4 for fracture performance testing, as these parameters appear to provide the reliable assessment of the materials’ fracture behavior.