Issue 43

M. Fakhri et alii, Frattura ed Integrità Strutturale, 43 (2018) 113-132; DOI: 10.3221/IGF-ESIS.43.09 119 R ESULTS AND DISCUSSION he primary output of this investigation is: fracture energy, expressed as Kilojoules per square meter (KJ/m 2 ), derived from the load-displacement curve to evaluate the overall cracking potential of asphalt mixture under various loading conditions. The work-of-fracture method proposed for concrete by Hillerborg and coworkers [62, 63] and later evolved to RILEM’s three-point bend fracture test method [64] is reliable for measuring the fracture energy of quasi-brittle materials. Bazant [65], Elices et al. [66], Guinea et al. [67] and Planas et al. [68] presented detailed discussions of the work- of-fracture method using an objective test and calculation procedure. To calculate fracture energy load displacement curve from SCB fracture energy test has been used. Researchers in civil engineering domain apply this conventional formula [29, 69]: f f lig W G A  (1) f G = fracture energy (J/m 2 ) P = applied load (N) f W =  Pdu , work of fracture (J) r =specimen radius (m) U = average load line displacement (m) t = specimen thickness (m)   lig A = r - a × t , ligament area (m 2 ) a = notch length (m) The calculated fracture energy with this conventional formula can serve as an order-of-magnitude estimator, as the result is specimen dependent and is actually not the fracture energy of the material. Therefore, the parameter G f calculated this way is assumed as fracture work with which fracture resistance of different asphalt mixtures can be studied. However, using strain Energy as the total energy consumed in global system on specimens to overcome the fracture resistance of asphalt mixes is accepted in this research and used to investigate characteristic specification of various asphalt mixtures under different loading conditions. To compare different AC specimens under various loading conditions, by using the fracture energy as paragon, mode of fracture should be studied first of all. The fracture energy of each sample was determined from Eq. 1 using the area under load-displacement curve until peak load obtained from each fracture test. In the upcoming sections, variations of fracture energy with various physical characteristics in different loading conditions is studied and the effect of loading rate on AC fracture behavior is discussed. The effect of aggregate type on fracture energy under pure mode I loading condition Fig. 5 shows that the fracture energy of asphalt mixtures manufactured with lime aggregate is more than those manufactured with silica under mode I loading condition. This finding is in agreement with the reports of other researchers as well [70- 72]. By increasing the loading rate, the trend of fracture energy change differs under different temperature conditions. At 25°C, the fracture energy increases continuously by increasing the loading rate but, at 5°C G f increases to a maximum value under loading rate of 5 mm/min and drops afterward, which shows the simultaneous effect of temperature and loading rate on fracture behavior of asphalt mixtures. The reason is that at lower temperatures (i.e. 5°C) asphalt mixtures are somehow brittle and the crack does not have enough time to heal under high loading rates so the fracture energy drops after reaching its maximum value. However at higher temperatures (i.e. 25°C) the bitumen in asphalt mixture behaves differently such that the asphalt mixtures have enough time to heal and fracture energy increases by increasing the loading rate. The same trend was observed for both asphalt mixtures manufactured with silica and lime aggregate. It should be noted that loading rate alteration has less influence on fracture energy of asphalt mixtures manufactured with silica aggregate compared to those manufactured with lime aggregate at 5°C. Moreover, under high loading rates the fracture resistance of two mentioned asphalt mixtures are approximately the same, showing the fact that under high loading rates (which is seen in highways with high speed limits for passing vehicles), aggregate type does not affect the fracture energy of AC specially at low temperature climates (i.e. 5°C). Fig. 6 shows the fact that by increasing the test temperature to an intermediate level (i.e. 25°C), the loading rate results in significant increase of G f , but still the fracture energy of asphalt mixtures manufactured with the lime aggregate is higher than those manufactured with silica aggregate. T