Issue 43

M. Fakhri et alii, Frattura ed Integrità Strutturale, 43 (2018) 113-132; DOI: 10.3221/IGF-ESIS.43.09 115 air void percentage, on fracture energy have been studied at two different intermediate temperatures (5°C and 25°C). Identifying fracture properties of asphalt mixtures becomes more complicated when viscoelastic behavior is taken into account at intermediate temperatures. At such conditions, the fracture energy might be a better representative parameter for describing the fracture behavior of HMA. Although a portion of the energy monitored by calculating the area below the experimental force-displacement curves is considered as energy dissipation due to the viscoelastic behavior of the matrix[49]. Different researchers have tried diverse criteria based analysis and computation methods to model the fracture behavior of different materials [11, 50-54]. Im et al. [55] investigated the fracture characteristics of asphalt mixtures subjected to different loading rates, at different temperatures, using fracture-energy values from the force–NMOD-COG curves, force–NTOD-DIC curves, and cohesive- zone modeling. As a result, loading rate didn’t affect the fracture energy obtained at -10°C, while the fracture energies at 0°C to 30°C obviously change at the different loading rates. They considered low rate-dependency of AC at -10°C, reasonable since the behavior of asphalt mixture at low temperatures is considered linear elastic. By increasing test temperatures to 0°C, they noticed that asphalt mixture behavior changed to viscoelastic and more energy was dissipated and the magnitude of fracture energy became greater under all applied loading rates. However other researchers reported that; by increasing loading rate, fracture energy decreases at sub-zero temperatures [2, 20, 56]. This research is trying to find out if this trend can be approved at higher and wider range of temperatures in asphalt mixtures with different characteristic specifications. At small range of intermediate temperatures (21°C to 30°C), fracture energy increases as the loading rates become higher which shows that fracture behavior of AC depends strongly on test loading rate [2]. This trend has been observed in several studies which were trying to characterize the rate-related fracture behavior of adhesive and polymeric materials [49, 57]. In the following sections, the difference in fracture energy trend by test loading rate alteration at near zero (5°C) and intermediate (25°C) temperatures will be reported and discussed. In summary, most of other researchers have investigated the fracture behavior of asphalt mixtures with specific characteristic specifications and also, many AC fracture toughness studies have been performed generally at subzero temperatures. As it is highlighted in this study, the variations of asphalt characteristic specifications have incontrovertible influences on the fracture behavior of hot mix asphalt mixtures at intermediate and low temperatures. Authors have selected different combination of variables to perform the test as: three testing temperatures (i.e. 5, 15 and 25 °C), four loading rates (i.e. 1, 5, 10 and 50 mm/min), using load line displacement measurement tools for asphalt mixtures with different characteristic specifications (air void content and aggregate type). The primary objective of this research was to investigate the effect of test loading rate on fracture energy of asphalt mixtures with different characteristic specifications at two temperatures (i.e. 5 and 25°C) under three loading mode mixities. Moreover, an excessive testing temperature was chosen (15°C) to study the effect of temperature on fracture energy as well. However, in this research strain energy as the total energy consumed in specimen to overcome the fracture resistance of asphalt mixtures, is assumed to investigate characteristic specification of various asphalt mixtures under different loading rates. E XPERIMENTAL STUDY hree different parts are explained separately in this section: (i) materials used for manufacturing the HMA, (ii) the procedure of manufacturing mixed mode I/II fracture samples, (iii) conducting the fracture experiments at different intermediate temperatures, under various mixed mode loading conditions. Materials The HMA mixture contains at least three main ingredients: aggregates, bitumen and air voids. Since the weight percentage and type of each parameter can affect significantly the fracture properties of asphalt concrete, they should be studied separately. In this study, lime and silica aggregates with nominal maximum aggregate size (NMAS) of 12.5 mm (which corresponds to grading No. 4 according to national Iranian paving code 234) and two different bitumen types PG64-22 and PG52-28 were used to manufacture the fracture test samples made of HMA. The mentioned aggregate and bitumen types are frequently used in the composition of asphalt mixtures all around the world. The aggregates and bitumen were provided from Asbcheran mine near Tehran province and Tehran Oil Refinery Company, respectively. The main specifications of the used aggregates and bitumen are presented in Tabs. 1 and 2. T