Fracture resistance of asphalt concrete modified with crumb rubber at low temperatures

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scopus – master journals – JCR

ایمپکت فاکتور

۴٫۲۷۶ در سال ۲۰۲۰

شاخص H_index

۲۶ در سال ۲۰۲۱

شاخص SJR

۰٫۹۰۱ در سال ۲۰۲۰

شاخص Quartile (چارک)

Q1 در سال ۲۰۲۰

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Fracture resistance of asphalt concrete modified with crumb rubber at low temperatures

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Fracture resistance of asphalt concrete modified with crumb rubber at low temperatures


The main objective of this study is to obtain fracture toughness of asphalt concrete modified by Crumb Rubber (CR) and Sasobit at low temperatures. First,  Bending  Beam Rheometer  (BBR)  test was performed  on unmodified  binder (binder 60/70), binder 60/70 + 3% Sasobit and 20%CR + 3%Sasobit modified asphalt binder to find how each modifier affect asphalt binder stiffness and relaxation  rate at low temperatures. Mixed mode I/II fracture tests were conducted by cracked Semi-Circular  Bending (SCB) specimens and the critical stress intensity factors were calculated for pure mode I, mixed mode I/II and pure mode II conditions. Results of BBR tests indicated that 20%CR + 3%Sasobit reduces stiffness and the m-value increase at low temperatures. As a result, 20%CR + 3%Sasobit has positive effect on low temperatures performance by improving  thermal cracking resistance. Also, according to the fracture toughness test results, the Warm Mix Asphalt (WMA) mixture containing  ۲۰% CR, shows higher resistance against crack growth than WMA mixture. It was found that mixed mode I/II can be more detrimental than pure mode I and II conditions.

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  1. Introduction

With cost of energy and global warming issues rising, the demand of cleaner productions has been raised dramat- ically. Application of warm mix asphalt (WMA) additives leads to lower production  temperature of asphalt mixes between 20 and 40  C. These additives fall within three cat- egories including  organic, chemical, and water-based addi- tives.  WMA  can also be  achieved  employing   water- containing  foaming  processes. The advantages of using WMA are lowering bitumen viscosity, improved workabil- ity, reduction in harmful  gasses emissions and better mat workability [1].

Sasobit is a Fischer–Tropsch paraffin wax with an ali- phatic hydrocarbon derived from coal gasification and is the most widely-used additive  to produce WMA. It can easily dissolve in bitumen because of its low melting point (about 100  C) and can form a stable compound. Research- ers have suggested the amount of 1–۳ wt.% for consump- tion of this additive. The advantages of Sasobit regarding asphalt performance are: (i) permanent deformation  resis- tance improvement due to formation of a crystalline struc- ture in bitumen  after compaction  process and cooling down of the asphalt; (ii) decreasing bitumen aging.

Apart from these advantages, it has negligible negative impact on the low temperature performance grading (PG) of bitumen which means no noticeable  effect on thermal cracking after long-term   aging  [۲]. On the other  hand, Crumb Rubber (CR) has been known over decades  as one of the best asphalt binder modifiers. Various  studies show that application of  CR  (about 15–۲۰ wt.%  of

bitumen)  can considerably  improve  the performance of both asphalt and bitumen. It improves  the performance of bitumen by reducing thermal sensitivity, increasing rut- ting and thermal cracking resistance and fatigue life. How- ever, there are some operational  issues such as extreme blending  conditions  (heat and rotational  speed require- ments), lack of storage stability  for long term, and high density energy requirement [3–۵].

However, mixtures modified by CR require higher mix- ing and compaction temperatures than conventional ones and, therefore, more energy is consumed which is no longer environmentally friendly. Warm mix asphalt (WMA) tech- nology  offers promising solutions to mitigate CR draw- backs thanks  to the  use  of fluidifying additives  which were found to have the ability to lower viscosity of bitumen at mix production  temperatures without affecting bitumen performance at pavement service temperatures [6].

Asphalt mixture  is temperature  dependent  and may exhibit behavior falling within whether brittle  or quasi- brittle particularly at subzero  temperatures. The mixed mode fracture toughness determines the amount of energy required for crack propagation in asphalt concrete at low temperatures where it behaves like a brittle (quasi-brittle) material [7–۱۲]. There are various modes of crack propaga- tion including mode I (opening or tension mode), mode II (sliding or in-plane shear mode), mode III (tearing or anti- plane shear mode) or a combination of them (Fig. 1). The mechanism of the crack propagation has widely been stud- ied by many researches in the past [13–۲۶]. The criteria to investigate mixed mode fracture can mainly be categorized as stress-based [13–۱۸], strain  based [19–۲۴], and energy- based [25,26]. However, in all of the aforementioned  cate- gories, the fracture toughness is a key parameter describing fracture resistance of the material.

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