## 1 Introduction  
Asphalt mixtures are the most widely used material in the construction of pavements for roads and highways around the world [1]. However, their use is associated with significant environmental pollution that is generated during manufacturing, placing and maintenance--namely the release of a large volume of greenhouse gases caused by the high manufacturing temperature (around 160 \({}^{\circ}\)C). This also leads to high levels of energy consumption, and hence, construction costs [2, 3]. Thus, in order to limit these problems [4], a cleaner production of bituminous mixtures is required to decrease the manufacturing temperature without reducing their mechanical behaviour. One possible alternative is the use of Warm Mix Asphalt (WMA), which is manufactured at 20-40 \({}^{\circ}\)C lower than the conventional Hot Mix Asphalt (HMA) [5]. These mixtures were developed to significantly reduce harmful emissions and energy consumption without compromising the mechanical performance of the material [6, 7, 8]. In particular, a number of studies [9, 10, 11] have demonstrated that the use of WMA in pavements for roads allows for lower emissions, fumes and odors; a reduction of ageing of the bitumen; a decrease in fuel consumption in plant; a more rapid turnover of traffic; and an increase in haulage distances among other economic and environmental advantages, due to the reduction in manufacturing and paving temperature. As a result of these potential benefits, WMA has attracted a considerable amount of attention in the USA as well as in various European countries in recent years [1; 9]. The reduction in WMA temperature is the consequence of recent technologies that can be divided into different categories depending on the agent used, and includes organic additives, chemical additives, and foaming (water-based or water-containing) [5]. Despite this qualification, all of these technologies share the same objective--an improvement in mixture workability and aggregate coating at lower temperatures by reducing bitumen viscosity or modifying its surface tension for better aggregate wetting [12]. Among these technologies, the use of chemical additives (which are liquid surfactants that act at the microscopic aggregate/binder interface to reduce internal friction during manufacturing) is one of the most common solutions, since these additives require minimal changes to the manufacturing process. Further, they act as anti-stripping agents that increase the adhesion between aggregates and bitumen, thereby reducing problems associated with WMA such as stripping and traveling [13]. Moreover, in contrast to other techniques such as the use of foam bitumen, their use does not require special investment or modifications in the manufacturing plant. As a result, new WMA surfactant additives that require further investigation have been emerging, and the choice of these materials is often a subjective decision based on the skill of the engineer. Therefore, in-depth studies are required to provide useful knowledge about the suitability of these additives for application in pavement construction as well as their capacity to improve the mechanical performance of asphalt mixtures manufactured at low temperatures [3]. Moreover, most of the work on WMA with chemical additives has focused on laboratory analysis, with relatively few studies conducted in a real life field setting [14; 15; 16; 17]. The latter type of study is essential to demonstrate the effectiveness of reproducing these WMAs in real asphalt plants for their application in pavements for roads. Given this lack of knowledge and its potential to reduce environmental issues associated with asphalt mixtures, the current paper set out to examine the effectiveness of various additives in improving the behaviour of WMA at a range of manufacturing temperatures (145 \({}^{\circ}\)C and 120 \({}^{\circ}\)C). To do this, the performance of each WMA was compared with that of conventional hot mix asphalt (manufactured at 165 \({}^{\circ}\)C) without additives, which was used as a control.