2.2.2. Conformed of Bituminous Mixtures and Tests

Once the previous tests had been carried out, specific for each material and in accordance with the role that each material plays in the mixture, we proceeded to make the bituminous mixtures reflected in Table 2 with the materials detailed.


**Table 2.** Families of bituminous mixtures conformed with electric arc furnace slag, Hornfels aggregate, ladle furnace slag, calcareous filler and fibers from the papermaking industry.

As shown in Table 2, there are families of bituminous mixtures conformed with virgin materials and families conformed with waste. In this way, the qaulity of the incorporation of waste is easily comparable.

The materials that perform the function of aggregate, whether waste or natural aggregates, were dried and sieved by different sieves to obtain the desired grading curve. The grading curve used corresponds to the intermediate grading curve established by the grading envelope detailed in

the regulations of Circular Order OC 3/2019 [42]. The selection of this grading curve is motivated by an essential reason: to compare the difference between bituminous mixtures made with natural aggregates and those made with slag. To do so, they must have the same grading, thus avoiding secondary variables that could mask the final conclusions. In addition, cellulose fibers discarded from the papermaking industry were incorporated into all bituminous mixtures in a percentage of 0.5% in mass and regarding conventional aggregate (conventional aggregate density 2.65 t/m3), as indicated by various studies on this type of mixture. The grading curve for the three families of samples is shown in Figure 1.

**Figure 1.** Grading curve of the different families of bituminous mixtures (ACFC, ASFC and ASFC) type SMA.

Once the grading curve was defined, different groups of samples were conformed of the three types of bituminous mixtures detailed in Table 2. In order to be able to compare the results faithfully, and given that the electric arc furnace slag has a higher density than the hornfels aggregates, the percentage of bitumen by volume was proportioned. This proportioning by volume allows an objective evaluation of the bitumen absorption capacity of bituminous mixtures with slag, since if mass proportioning was done, the optimum bitumen percentage would not be comparable due to the high density of the slag.

Based on the comments above, the families of mixtures were manufactured with percentages of bitumen in volume and regarding aggregate from 15% to 18% in 0.5% increments. To this end, the aggregates (natural or waste) were heated in an oven to a temperature of 180 ± 5 ◦C for 1 h, as was the bitumen and cellulose fibers, and then mixed in an automatic planetary mixer (MECÁNICA CIENTÍFICA S.A., Madrid, Spain) for 10 ± 1 min. The resulting mixture was extracted and compacted by a Marshall compactor (MECÁNICA CIENTÍFICA S.A., Madrid, Spain) with 50 blows per side to each specimen (standard UNE-EN 12697-30) [60]. The conformed specimens were left at ambient temperature for 24 h for subsequent mechanical stripping. A total of 8 Marshall-type samples were made for each percentage of bitumen in each family.

Once the groups of samples with increasing percentages of bitumen from each family of samples had been obtained, the physical properties were characterized. The tests carried out were on the maximum density of the bituminous mixture (standard UNE-EN 12697-5) [61] and bulk density (standard UNE-EN 12697-6) [62]. In turn, the void characteristics of the bituminous mixtures obtained were calculated (standard UNE-EN 12697-8) [63].

The Marshall test was carried out to evaluate the mechanical resistance of the families of bituminous mixtures conformed (standard UNE-EN 12697-14) [64]. With this test, the plastic deformations that occur in each bituminous mix can be evaluated, this being an essential characteristic due to the high percentage of bitumen that SMA bituminous mixes have.

#### 2.2.3. Determination of Optimal Material Combinations and Comparison of the Results

Once the mechanical and physical properties of the three families of samples with different bitumen percentages had obtained, the optimum combination of materials was then obtained. This optimum combination of materials was calculated graphically, taking Marshall stability as the main property. In other words, the percentage of volume of bitumen which provided the highest Marshall stability of each family (ACFC, ASFC and ASFS) was calculated, provided that permissible values were obtained for the physical properties and deformation of the bituminous mixtures.

With the optimum combinations of materials for the three families of samples (ACFC, ASFC and ASFS), samples were again made to evaluate the physical and mechanical properties obtained graphically, thus corroborating the quality of the material selection. In turn, binder drainage tests UNE-EN 12697-18 [65] were carried out, to evaluate that the fibers fulfilled their function within the conformed bituminous mixtures and that there were no bleeding of bitumen due to their high percentage; wheel-tracking tests [66] were also conducted, to evaluate the durability of the mixture before the continuous passage of vehicles.

The results obtained from the different sample families for their optimal material combination were compared. In this manner, the influence of the use of electric arc furnace slag and ladle furnace slag in the manufacture of Stone Mastic Asphalt mixtures with cellulose fibers from the papermaking industry can be objectively evaluated.

## **3. Results and Discussion**

This section describes the results of the trials mentioned in the methodology, as well as the discussion about them. The series of trials logically ordered will condition the final conclusions, there being at all times a continuous process of feedback.
