Stiffness modulus of Polyethylene Terephthalate modified asphalt mixture: A statistical analysis of the laboratory testing results Taher Baghaee Moghaddam ⇑ , Mehrtash Soltani, Mohamed Rehan Karim Center for Transportation Research, Department of Civil Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia article info Article history: Received 14 September 2014 Accepted 25 November 2014 Available online 10 December 2014 Keywords: Asphalt mixture Mixture stiffness Waste Polyethylene Terephthalate Environmental temperature Applied stress Response Surface Methodology abstract Stiffness of asphalt mixture is a fundamental design parameter of flexible pavement. According to liter- ature, stiffness value is very susceptible to environmental and loading conditions. In this paper, effects of applied stress and temperature on the stiffness modulus of unmodified and Polyethylene Terephthalate (PET) modified asphalt mixtures were evaluated using Response Surface Methodology (RSM). A quadratic model was successfully fitted to the experimental data. Based on the results achieved in this study, the temperature variation had the highest impact on the mixture’s stiffness. Besides, PET content and amount of stress showed to have almost the same effect on the stiffness of mixtures. The optimal amount of PET was found to be 0.41% by weight of aggregate particles to reach the highest stiffness value. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Stiffness of asphalt mixture is a fundamental design parameter of flexible pavement. It was found that there is a correlation between stiffness and other mixture properties such as rutting and fatigue, and thus it can be used as a criterion to evaluate Asphalt Concrete (AC) mixture performance [1]. As it is mentioned by Strategic Highway Research Program (SHRP) the stiffness value of AC mixture is very susceptible to environmental temperature and loading conditions [2]. Stone Mastic Asphalt (SMA) is gap-graded AC mixture which has been developed in Germany in 1916s. SMA consists of more course aggregate particles, mineral filler and asphalt binder. Due to inherited structure of SMA, it provides better permanent defor- mation (rutting) performance and durability compared to conven- tional dense-graded mixture [3,4]. Draindown is a common problem in SMA mixture because it contains higher amount of asphalt binder. Hence, to prevent from draindown in SMA mixture stabilizer additives, fibers and polymers are used. Using polymer in SMA mixture is very common. Utilizing polymer in SMA mixture prevents not only from the binder draindown but also it can enhance mixture performance [5,6]. In many cases, using polymers causes higher construction cost due to high cost of polymers. To overcome this disadvantage, many studies investigated using waste polymers in asphalt mixtures [6–8]. One of the important industrial plastic materials is Polyethylene Terephthalate (PET). PET is a semi-crystalline thermo plastic poly- mer material which is used in beverage and food industries for years. Nowadays, a large amount of waste PET is produced in the world and it is going to cause a serious environmental challenge due to non-biodegradability of PET [9]. Hence, some studies have been previously performed to evaluate the effects of using post- consumer PET as secondary materials in road pavement in order to find solutions to tackle with this potential environmental hazard and, moreover, to decrease construction cost imposed by applica- tion of polymers in asphalt mixture [6,10–12]. Statistical analysis is a precise and popular way to explore and present interactions between parameters affecting one phenome- non. Statistical analysis in pavement engineering has prominent utilization because it helps road engineers and designers to have better perspective about the pavement performance parameters. In this case, factorial Design of Experiments (DOE) which through the use of techniques such as Response Surface Methodology (RSM) – simultaneously consider several factors at different levels, and give a suitable model for the relationship between the various factors and the response came into popularity [13–15]. Aim of this study was examining the AC mixture stiffness at ele- vated temperatures and stress levels for the unmodified and PET modified mixtures following by finding interactions between these fundamental factors using RSM based on Central Composite Design (CCD). http://dx.doi.org/10.1016/j.matdes.2014.11.044 0261-3069/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +60 108927064; fax: +60 379552182. E-mail addresses: p.baghaee@gmail.com, payam_baghaei@siswa.um.edu.my (T. Baghaee Moghaddam). Materials and Design 68 (2015) 88–96 Contents lists available at ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/matdes