Issue 43

M. Fakhri et alii, Frattura ed Integrità Strutturale, 43 (2018) 113-132; DOI: 10.3221/IGF-ESIS.43.09 114 I NTRODUCTION y changing the service temperature in the asphalt concrete layers, visco-elastic behavior of bitumen may result in different performances for asphalt mixtures. Especially at higher temperatures, the bitumen behaves as viscous material and failure (i.e. cracking), is more probable to occur at intermediate temperatures [1]. Considerable amount of costs is spent annually for the maintenance and rehabilitation of cracked pavements. Therefore, the need to study the affecting parameters on cracking behavior of asphalt mixtures at intermediate temperatures is essential. Many distresses in asphalt concrete (AC) pavements such as fatigue cracking, thermal cracking, and reflective cracking of the AC, can be investigated by the fracture mechanics approach. Cracking can be assumed to be the main responsible parameter in the pavement structure for long-term durability problems and final failure. Service life of AC pavements and, hence, the maintenance and managing the networks of pavements is influenced by the fracture resistance and crack growth characteristics of AC materials. Review of available paving codes and procedures, reveals that the fundamental fracture properties of the AC materials and proper characterization of the fracture process have not been adopted in the current pavement design-analysis procedures [2-6]. At low temperature conditions, the probability of brittle crack initiation and propagation in the AC layer becomes more due to the relative brittleness of pavements at these temperatures. Thus, many researchers have studied low temperature fracture behavior of asphalt mixtures using the principles of linear elastic fracture mechanic (LEFM) [7-12]. Various fracture models have been considered for different characteristics of asphalt mixtures, but such models are generally suitable for low temperature testing conditions, in which the type of fracture is dominantly linear and elastic. These models can not necessarily provide predictions for accurate inelastic nonlinear viscoelastic fracture behavior which usually occurs at intermediate temperatures [13-15]. Different researchers have utilized various fracture tests and analysis methods for better understanding of the fracture behavior and cracking mechanisms in AC materials. Single-edge notched beam (SENB) test [16-18], double-edged notched tension (DENT) test [19], disk shaped compact tension (DCT) test [20-22], Edge notched disc bend (ENDB) test [23-27], and semicircular bend (SCB) test [28-37], are to name a few. Disc-shaped compact tension (ASTM D7313-07a) and semi- circular bending (SCB) tests [29, 38-42] are among the frequently used fracture specimens for evaluating fracture behavior of asphalt concretes. Cylindrical samples obtained from Superpave Gyratory Compactor (SGC) or cores achieved from field and underservice AC layers can be used to build SCB samples which is known as low-cost test. Hence in this study the SCB specimen is selected to evaluate the cracking resistance of AC at intermediate temperatures. Simple three-point bending mechanism using crosshead movements is the main concept in the SCB test method. The experimental procedure used for determining intermediate-temperature cracking resistance of AC mixtures was developed by Wu et al. [31, 43]. They used a same SCB specimen geometry with different testing parameters (i.e. loading rate, temperature), apparatus (displacement measurement devices), and fracture energy calculation methods for both low and intermediate temperature fracture behavior evaluation of AC materials. Rate-dependent fracture behavior of asphalt mixtures have rarely been investigated by theoretical or empirical models. Cracking in hot mix asphalt (HMA) mixtures is one of the most challenging concepts in management of roads and pavements, since the complicated influence of ingredient characteristics and inelastic behavior of the asphalt mixtures contribute in this regard. Moreover, inelastic behavior of the asphalt mixtures is temperature sensitive and rate dependent. Therefore, linear elastic fracture mechanics (LEFM) may not be accurate enough to solve the cracking problem in asphalt mixtures because of these characteristics. LEFM can only predict the stress state close to the crack tips of damaged bodies if the fracture process zone (FPZ) around the crack tip is very small. But similar to the other concrete materials the FPZ in asphaltic materials might be large [2, 44-46]. In the past decades, many research works have been performed to study the fracture resistance of different asphalt mixtures using fracture energy or stress intensity factor concept by employing numerical analyses, simulation techniques and experimental data. However, the tests in most studies, are related only to pure mode I fracture case. However, because of multiaxial stresses, geometric complexity of pavement layers and different traffic load positions, mixed mode tensile-shear cracking that can be occurred in practice in AC pavements. Only very limited efforts have been done to characterize the mode II and mixed mode I/II fracture behavior of asphalt mixtures at intermediate temperature testing conditions [47, 48]. Many parameters including manufacturing process, composition of the ingredients, mix design, type of aggregate and binder, service loading rate and temperature can affect the properties, performance, durability and mechanical strength of HMA mixtures. Therefore, fracture energy concept has been used in this research, to study the effect of various loading conditions (i.e. temperature and loading rate). Also, the effect of asphalt characteristic specifications, including the aggregate type and B