## 5 Optimization of the WMA Design  
Based on the previous results, the A2 additive appears be appropriate for use in manufacturing low energy asphalt mixtures, since it shows mechanical performance that is comparable to conventional  
Figure 4: Triaxial results for the Hot Mix Asphalt (HMA) and Warm Mix Asphalt (WMA) with different additives. Figure 3: Stiffness modulus at 20 \({}^{\circ}\)C for the mixtures with different additives. HMA, obtaining higher tensile strength, water resistance, bearing capacity, and resistance to plastic deformations than the case of the WMA used with other additives. The additive A2 was therefore selected as the most appropriate for studying the optimization of the WMA design, with a view to its later use in a real asphalt plant. Figure 5 compares the workability of WMA with 0.5% of additive (the quantity commonly used) and 0.05% (the quantity defined to assess the possibility of reducing the amount of additive used), taking the conventional HMA as a reference. The results show that for both manufacturing temperatures (145 \({}^{\circ}\)C and 120 \({}^{\circ}\)C, in Figure 5a,b, respectively), the quantity of additive exerts little influence, since both dosages generated rather similar curves of the evolution of the air void content in the asphalt mixtures. Regarding the effect of temperature, it appears that it is possible to obtain similar values of air void content to those measured for the HMA at the manufacturing temperature of 145 \({}^{\circ}\)C, which indicates the ability of this additive to improve the workability of the material despite the decrease in temperature. Moreover, despite the reduction in additive effectiveness at lower temperatures (around 120 \({}^{\circ}\)C), the air void content for such mixtures was lower than around 6%, obtaining density values higher than 98% over the conventional HMA. Thus, these results indicate that the use of this additive allows for appropriate workability and compaction of the asphalt mixture at low temperatures (within 120-145 \({}^{\circ}\)C), presenting little effect of the dosage of additive utilized (between 0.05% and 0.5% in this study). In addition, Figure 6 shows that although the decrease in additive dosage could lead to a slight increase in water sensitivity of the mixture, using a low quantity of this chemical agent (around 0.05% over the bitumen mass) allows for comparable (or even higher) indirect tensile strength to both the conventional HMA and the WMA with a higher amount of additive (0.5%). This fact indicates that despite the lower value of ITSR that is measured in WMA [14], the values of tensile strength under wet conditions make it acceptable for application when compared with the mixtures used as a reference in this study. In addition, the decrease in additive dosage was shown to be more effective in reducing the effect of manufacturing temperature (within 120-145 \({}^{\circ}\)C) on the behaviour of the asphalt mixture under tensile effort and water action, since quite similar values of ITS and ITSR were recorded for both temperatures when 0.05% of additive was used. In contrast, applying a higher amount of additive (0.5%) led to a decrease in tensile strength of the asphalt mixture when the manufacturing temperature was reduced to 120 \({}^{\circ}\)C. This fact could be associated with the nano-composition of this additive (A2),  
Figure 5: Densification curves for the HMA and WMA with different quantities of additives and manufacturing temperatures. (**a**) 145 \({}^{\circ}\)C vs. 165 \({}^{\circ}\)C; (**b**) 120 \({}^{\circ}\)C vs. 165 \({}^{\circ}\)C.  
which acts at the microscopic aggregate/bitumen interface [13], thereby improving performance when low quantities are used since it is possible to avoid the formation of agglomerations [14]. Similarly, Figure 7 shows that reducing the quantity of additive seems to be more effective in yielding a higher stiffness modulus (compared with using 0.5% additive) under the same manufacturing temperatures (producing a similar reduction in bitumen ageing).