Pre- and post-peak toughening behaviours of fibre-reinforced hot-mix asphalt mixtures P.J. Yoo a * and I.L. Al-Qadi b a Korea Institute of Construction Technology, 2311 Daehwa, Ilsan, Goyang, Gyeonggi 411-712, Republic of Korea; b Department of Civil and Environmental Engineering, University of Illinois at Urbana – Champaign, 205 N. Mathews Avenue, Urbana, IL 61801, USA (Received 25 October 2012; accepted 27 August 2013) Recycled plastic fibre-reinforced hot-mix asphalt (HMA) mixtures have better fatigue resistance than plain HMA. The toughening effects of recycled plastic fibre-reinforced HMA were characterised using direct tensile loading tests. Adding a small quantity of recycled plastic fibres to HMA was found to significantly increase the mixture’s fracture energy and toughness, which were calculated using the pre- and post-peak stages of tensile force – displacement curves. A theoretical model representing the pre-peak behaviour of fibre-reinforced HMA with direct tension-softening curves for various fibre contents is presented here. The enhanced toughness through post-peak analysis was also observed using toughness indices associated with fibre-bridging effect after the pre-peak composite stress. The pre-peak fracture energy model and post-peak toughness indices appeared to be governed by the direct tensile toughening of fibre-reinforced HMA’s enhanced fibre-bridging effects. The pre-peak fracture energy model demonstrates the effect of fibre content on the strain energy density during the pull-out process within the pre-peak composite stress region. The maximum pre-peak fracture energy of a coarse-graded HMA mixed with recycled plastic fibres is achieved at a fibre content of 0.4% of the total weight of the HMA. The increases in the toughness indices within the post-peak composite stress region indicate that the fatigue resistance of fibre-reinforced HMA is at least 30% greater than that of control HMA. Keywords: hot-mix asphalt; fibre; fatigue; fracture energy; toughness 1. Introduction In ancient times, natural resources such as rice straws and reeds were used to reinforce building materials and thereby enhance their structural integrity. However, materials such as those are not sufficient to overcome inconsistent durability, inadequate strength or quality assurance problems in reinforced structures. Synthetic plastics overcome some of the drawbacks of natural reinforcing materials and have been used in some civil engineering applications in the form of geosynthetics. Most geosynthetics, such as geogrids and geomembranes, are made of polypropylene (PP), polyethylene, nylon or glass fibres. These materials are mainly used in geotechnical applications to reinforce backfills and slopes (Trottier and Banthia 1994). In recent times, fibrillated micro- or macro-fibres have become widely used in concrete structures to control micro- or macro-cracks. Fibres are mixed into concrete, for example, to enhance toughness. Although fibres have been mixed with roadway pavement materials such as concrete and hot-mix asphalt (HMA) to enhance toughness, most fibre applications have been in concrete structures such as buildings and bridges (Trottier and Banthia 1994). The major drawback of HMA is its weakness in tension. One of the promising methods of improving the tensile toughness of HMA is to reinforce it by either using a woven fabric or mixing in randomly oriented short fibres. HMA with some types of fibres, such as polyester, PP, glass and nylon, have been reported to be superior to plain HMA in terms of toughness, indirect tensile strength, shear strength and fracture energy. Improved toughness and fracture energy may increase the fatigue life of an HMA (Trottier and Banthia 1994, Zahran and Fatani 1999, Bueno et al. 2003, Lee et al. 2005). The toughening effects of fibre composites are typically characterised by conducting direct tensile tests to develop force–displacement curves or tension-softening curves, which describe the pre- and post-peak toughening of the tensile behaviour of a ductile material. However, the randomness of the distribution and orientation of reinfor- cing fibres necessitate making some assumptions, such as probability density functions, to calculate composite stresses while accounting for the fibres’ bridging forces, the interface conditions between the fibres and matrix, and the mechanical behaviour of the fibres themselves (Li et al. 1991, Bolzan and Huber 1993). Wang et al. (1988) found that the inclusion of a small quantity of synthetic fibres significantly enhanced the tension-softening behaviour of cementitious composites. Although different types of fibres lead to different tension- softening curve shapes, the failure mechanism is governed by the fibres’ pull-out characteristics, which depend on the q 2014 Taylor & Francis *Corresponding author. Email: pjyoo@kict.re.kr International Journal of Pavement Engineering, 2014 Vol. 15, No. 2, 122–132, http://dx.doi.org/10.1080/10298436.2013.839789