Prediction of load-CMOD curves for HMA mixtures at intermediate temperatures subjected to mixed mode loading

Highlights

• Load-CMOD response of asphalt mixtures is obtained by means of CZM simulations

• Crack growth under mixed mode loading and viscoelastic behavior of the material.

• Effects of mixture’s constituents on crack propagation mechanism were investigated.

Abstract

This paper discusses the effects of mixed mode loading on cohesive zone parameters obtained from load-crack mouth opening displacement (CMOD) curves in asphalt mixtures at intermediate temperature. To this end, a coupled viscoelastic numerical-experimental approach has been used to capture the cohesive strength and fracture energy in the fracture process zone utilizing asymmetric semi-circular bending test. Three mode mixities (i) pure mode I (ii) mixed mode I/II with higher fraction of tensile deformation and (iii) mixed mode I/II with higher fraction of shear mode were tested and simulated. The load-CMOD curves for the investigated mode mixities and for different asphalt mixtures were predicted successfully using the aforementioned computational framework. The results showed that the binder type and loading mode have significant effect on both cohesive strength and cohesive fracture energy values of asphalt mixtures. Aggregate gradation was only effective on the cohesive strength values. Considering the interactions of the mixtures’ constituents and loading, the binder- loading mode and binder-aggregate gradation were significantly effective only on cohesive fracture energy. The stiffer an asphalt binder, the more cohesive strength and fracture energy values were observed. Decreasing the distance between the supports which is equivalent to higher fraction of the shear mode in the ASCB test contributes to greater cohesive fracture energy.

Introduction

Cracking is the dominant mode of distress in pavements at low and intermediate temperatures. Different types of cracks may form in flexible pavements due to the combined effects of load and temperature. The bottom-up or fatigue cracking is the most common type of cracking at intermediate temperatures. It is well-known that when the applied tensile strain/stress exceeds the strength of the asphalt concrete layer, the crack initiates from the bottom of the asphalt layer [1]. Further research studies suggested that radial truck tires cause tensile stresses at the surface of the pavement which can lead to crack initiation and propagation at intermediate temperatures [2], [3]. Regardless of the crack initiation mechanism in asphalt concrete pavements, the crack propagation stage is a mixed mode process due to heterogeneity of the asphalt concrete, non-uniformity distribution of the tire-pavement contact pressure and load positioning relative to the crack location [4], [5], [6].

Mechanistic approaches for investigating fatigue cracking in asphalt pavements can be divided into three broad categories: (i) the dissipated energy (DE), (ii) the continuum damage mechanics (CDM) and (iii) the fracture mechanics approaches. In the first two approaches, the fatigue life of asphalt materials is determined based on some damage parameters applied on the entire specimen [7], [8], [9], [10], [11], [12], [13], [14]. They also have promising results of fatigue characteristics of asphalt materials and good correlation between laboratory results with the accelerated load facilities (ALF) and field measurements [15], [16], [17], [18]. But these approaches are limited to micro-crack formation and propagation and do not deal with the initiation and propagation of macro-cracks [12].

The fracture mechanics approach, on the other hand, has the merits of dealing with both stationary and moving cracks. The fracture properties of different asphalt mixtures under the assumptions of linear elastic fracture mechanics (LEFM) were investigated by stress intensity factors (SIFs) which are valid only at very low temperatures [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29]. The J-integral based approach has been used to investigate cracking resistance of asphalt mixtures at intermediate temperatures [30], [31], [32], [33], [34], [35], [36]. The bituminous mixtures in pavements undergo a wide range of temperatures during seasonal variations or even a single day. The viscoelastic nature of the binder/mastic phase of the asphalt mixture also limits the application of LEFM and even the J-integral based approaches. The cohesive zone model (CZM) may be a good surrogate to overcome these limitations. The CZM provides an efficient and powerful computational tool applicable on finite element method (FEM), discrete element method (DEM) and finite volume method (FVM) in asphalt materials [37], [38]. The CZM has proved to be applicable to a wide range of materials’ behavior including brittle, semi-brittle and ductile. This characteristic has made CZM a suitable coupled computational-experimental tool for simulating the rate and temperature dependent damage evolution of asphalt mixtures [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56]. Tensile crack initiation in pavement at intermediate temperatures can also be occurred under loads which are too heavy for the pavement layers [57]. This type of fracture cracking alongside with the time consuming and variability nature of fatigue tests have emerged researchers to investigate the feasibility of substitution the monotonic fracture tests. They not only can provide quantitative measures such as fracture energy but also showed to correlate well with fatigue data measurements [57], [58], [59], [60], [61], [62]. The SCB test has gained more interest from the research community and extra fracture parameters are introduced by this testing setup; being able to characterize the fracture behavior and categorize different types of asphalt mixtures yet conforming with fatigue performance data [33], [34], [63], [64], [65], [66]. The crack growth behaviors of different types of bituminous mixtures at intermediate temperatures were analyzed by means of CZM simulations and calibrating with SCB experimental results. Elseifi et al. [67] studied the crack propagation mechanisms of different asphalt mixtures including conventional, polymer modified binder (PMB) and reclaimed asphalt pavement (RAP) on the basis of the viscoelastic behavior of asphalt mixtures at the temperature of 25 °C. They considered opening mode fracture for crack initiation [67]. The CZM provided excellent results in terms of load versus load line displacement and could distinguish different asphalt mixtures performance. Furthermore, FE simulations showed the damage propagation is the resultant of vertical and horizontal stresses while the shear effects are negligible [67]. In another study the crack propagation behavior of different asphalt mixtures prepared according to different mix design criteria were investigated using the computational framework of CZM. The entire behavior of the asphalt concrete (AC) mixtures was considered viscoelastic at the intermediate temperature of 21 °C. The CZM simulations were in good agreement in terms of load displacement curves of SCB specimens with different initial notch lengths [68]. The CZM- SCB simulations were adopted to characterize the pure mode I and pure mode II fracture of fine aggregate matrix (FAM) assuming linear elastic behavior of the FAM mixture under monotonic loading [69]. Other experimental studies utilized SCB testing configuration to capture the mixed mode fracture energy of different types of asphalt materials at intermediate temperatures [70], [71]. These researches reported the area under the global load displacement curve as the fracture energy. Since the energy dissipated consist other sources of energy such as plastic work and/or viscoelastic behavior (viscosity) of the bulk material, it may not represent an accurate condition of fracture process zone [72]. To that end, some attempts have been made to characterize the mode I crack growth of fine aggregate matrix at low and intermediate temperatures using force- crack tip opening displacement (CTOD) curves of the SCB tested specimens [73], [74].

The current research study aims to investigate the tensile fracture behavior of asphalt mixtures at intermediate temperature under mixed mode loading conditions. The data acquisition system is based on force – crack mouth opening displacement (CMOD) measurements to capture only the opening or tensile fracture mode of the asphalt mixtures. To provide mixed mode loading, the semicircular bending (SCB) test was adopted and the bulk behavior of the material is considered viscoelastic. This type of mixed mode loading inevitably induces shear force or in-plane deformation. But due to the importance of tensile loads in pavements as outlined above, only crack opening measurements and the associated fracture process zone simulations are simulated and discussed in this research. Four different asphalt mixtures with varying asphalt binder and aggregate gradations are used to investigate the effects of mixture’s constituents on the overall fracture properties of the HMA. The cohesive zone modeling is utilized to calculate the peak traction and cohesive fracture energy of each loading mode to get a better understanding of the fracture mechanisms in the fracture process zone (FPZ).