Heterogeneity effects on mixed‐mode I/II stress intensity factors and fracture path of laboratory asphalt mixtures in the shape of SCB specimen

Abstract

Edge cracked semi-circular shape specimen subjected to three point bend loading is a favourite test specimen for determining fracture toughness of asphalt mixtures. However, in the vast majority of previous experimental works, the homogeneous medium assumption has been considered for determining the stress intensity factor and geometry factors of asphalt mixtures tested with this test configuration. As a more realistic model and in order to consider the effects of heterogeneity on corresponding values of stress intensity factors, the asphalt mixture was modelled as a two-phase aggregate/mastic heterogeneous mixture and its fracture behaviour was investigated using numerical models of asymmetric semi-circular bend (ASCB) specimens. The generation and packing algorithm was employed to randomly distribute the aggregates with different shapes and sizes inside the mastic part. The effect of the mechanical properties of asphalt mixture (elastic modulus and the Poisson's ratios of aggregates and mastic), coarse aggregates distribution and crack length were studied on modes I and II geometry factors by means of extensive twodimensional finite element analyses. Moreover, the effect of the elastic modulus of asphalt mixture components was evaluated on the fracture path using the maximum tangential stress criterion. It was shown that crack tip location, elastic modulus of aggregates and mastic are the most important affecting parameters on the magnitude of modes I and II geometry factors. It was also shown that the geometry factors are not sensitive to the Poisson's ratios of aggregates and mastic. In addition, fracture cracking path is affected by the elastic modulus of the asphalt mixture components such that, depending on the difference between the stiffness of stiffer coarse aggregates and softer NOMENCLATURE: K I , K II , stress intensity factors of modes I and II; K Ic , K IIc , critical stress intensity factors of modes I and II; Y I , Y II , geometry factors of modes I and II; P, applied load; P c , critical fracture load; R, radius of the SCB specimen; t, thickness of the SCB specimen; E, Young's modulus; E agg , Young's modulus of aggregates; E mastic , Young's modulus of mastic; E interface , Young's modulus of interface; D, distribution of aggregates in the SCB sample; Xi, Yi, Cartesian coordinate of new generated aggregate; R i , radius of new generated aggregate; X j , Y j , Cartesian coordinate of generated aggregate in the previous steps; R j , radius of generated aggregate in the previous steps; a, crack length; S 1 , S 2 , bottom support distances; S 1 /R, S 2 /R, bottom support distances over the radius ratios; A ij , area of aggregate j in group i; A i max , maximum summation area of aggregates in group i; A i min , minimum summation area of aggregates in group i; SCB, semicircular bending sample; MTS, maximum tangential stress; HMA, hot mix asphalt; SIF, stress intensity factor

KIcandKIIcmeasurement for hot mix asphalt mixtures at low temperature: Experimental and theoretical study using the semicircular bend specimen with different thicknesses

“…A possible reason for this observation can be related to the decreasing in the viscosity and increasing the stiffness of bitumen material at subzero temperatures, which results in increasing the stiffness of whole HMA mixture at low testing temperatures. Furthermore, according to some analyses available in the literature, the SIF becomes more by increasing the stiffness or elastic modulus of bitumen used in the composition of two‐phase aggregate/mastic composite structure 52,53 . As it is well understood and has been reported in previous papers, 8,9,12,13,31,54–58 the asphalt binder as a viscoelastic material becomes more stiff at subzero temperatures up to the lower PG limit of the bitumen that is related to the glass temperature of binder material.…”

“…Furthermore, according to some analyses available in the literature, the SIF becomes more by increasing the stiffness or elastic modulus of bitumen used in the composition of two-phase aggregate/mastic composite structure. 52,53 As it is well understood and has been reported in previous papers, 8,9,12,13,31,[54][55][56][57][58] the asphalt binder as a viscoelastic material becomes more stiff at subzero temperatures up to the lower PG limit of the bitumen that is related to the glass temperature of binder material. Hence, for the bitumen material used in the current research with PG , it is expected to observe an enhancement on the overall strength and load carrying capacity of binder and consequently the whole asphalt mixture.…”

Heterogeneity effect on fracture parameters of a multilayer asphalt pavement structure containing a top‐down crack and subjected to moving traffic loading

“…But for the sake of simplicity, we assumed circular shape for the geometry of distributed aggregates inside the texture of heterogeneous HMA overlay. However, the authors were investigated in their previous works the influence of aggregate shape on the fracture parameters 34 . They examined different shapes such as hexagonal, diagonal, square, and circular configurations for the aggregates distributed inside the mixture of asphalt concrete and concluded that the SIFs and the T ‐stress values are not affected noticeably by the shape of aggregates.…”

“… where A i min and A i max are minimum and maximum summation area of aggregates, respectively, in group i which is determined based on the asphalt mixture aggregate gradation and A ij is the area of aggregate j in group i . More details about this algorithm can be found in the work by Aliha et al 34 The aggregate gradation No. 4 (according to the Iranian Paving Standard Code‐234 42 ) was considered for running the AGP algorithm.…”

“…They also accurately estimated the crack propagation path in the investigated SCB samples. In another study, Aliha et al 34 also simulated asymmetric SCB (ASCB) sample using 2D FEM modeling and employing the AGP algorithm for random distribution of aggregates inside the FAM. They showed that the SIFs of modes I and II and their different mixities in the heterogeneous modeling of asphalt mixture are very different from the values obtained for the homogeneous model in the ASCB specimen.…”