2.2. Testing Plan
The testing plan followed in this study was divided into three main stages: (i) laboratory study (assessment of binders and design of the asphalt mixtures); (ii) analysis of mixture reproducibility in-plant; (iii) application of the mixture in a trail section.
The first stage, conducted in the laboratory, consisted of evaluating the characteristics of both types of binders as well as the properties of the designed mixtures. To assess the rheological behavior of the CRMB in comparison with the traditional PMB, complex modulus and phase angle were measured at different temperatures (10, 20, 30, 40, 45, 52, 58, 64, 70, and 80 °C, in order to perform a precise analysis of the rheological response of the two binders under severe thermal gradients) and a range of frequencies (from 0.1 Hz to 20 Hz), using a Dynamic Shear Rheometer (DSR) through a shear loading at a constant amplitude of 0.1% strain (using two specimens per binder). Similarly, this equipment was used to carry out the MSCRT (Multiple Stress Creep and Recovery Test) to evaluate the resistance of the binders to permanent deformations, along with their ability to recover the elastic strain under different levels of stress (0.1 and 3.2 kPa, applying loading cycles consisting of 1 second loads and a 9-second recovery phase) at various temperatures (45, 65, and 70 °C) and using two specimens per binder.
Regarding the design of the PMB-HMA and CRMB-HMA hot mixtures, the water sensitivity test [
19], wheel tracking test [
20], bulk density test [
21], and air void content test [
22] were conducted to determine optimal binder content. The water sensitivity test involves the manufacture of 6 test specimens with a diameter of 101.6 mm and a thickness of 60 mm, compacted with 50 blows on each side by a Marshall hammer. The specimens were subsequently divided into two sets of three specimens (a dry set and a wet set). The dry set was stored at room temperature in the laboratory (20 ± 5 °C), whereas a vacuum was applied to the wet set for 30 ± 5 min until a pressure of 6.7 ± 0.3 kPa was obtained. The specimens were then immersed in water at a temperature of 40 °C over a period of 72 hours. The next step was to perform an indirect traction resistance test on each of the cylinders (in both the dry set and the wet set). This was done at a temperature of 15 °C, and after a previous period of adjustment of 120 min to this temperature. The results of the experiment are expressed in terms of the retained strength of the test specimens after dividing the strengths of the wet specimens into the strengths of the dry specimens (ITSR, %).
The wheel tracking test involves the manufacture of two prismatic test specimens of 408 mm × 256 mm. Compaction was carried out by a roller compactor with a smooth steel roller to a thickness of 60 mm and a minimum density of 98% of the Marshall density. Two days after their compaction, both specimens were allowed to adjust to a temperature of 60 ± 1 °C, and then were tested at that temperature. The test itself involved the application of a load on the test specimen by means of the repeated passes of a loaded wheel. The load applied was 700 N and the number of passes was 10,000. The frequency of the device was 26.5 load cycles per minute. In each of the wheel passes, the resulting deformation on the test cylinder was measured. The objective of the test was to determine the wheel tracking slope (WTS, mm/103 load cycles) measured in the last 5000 load cycles.
Following this, and using the same design as that used for the CRMB-HMA, the workability of the CRMB-WMA at lower manufacturing temperatures (150 °C and 130 °C) was assessed in comparison with the conventional RMB-HMA (which was manufactured at 175 °C). For this purpose, the density of the mixtures as a function of the compaction energy at different temperatures was analyzed (using a gyratory compactor), while evaluating the stiffness modulus at 20 °C [
23] and loss of particles at 25 °C [
24] of the specimens obtained following the compaction process. Additionally, having selected the most appropriate manufacturing temperature of the RMB-WMA, its properties and mechanical performance were compared with the conventional hot mixtures PMB-HMA and RMB-HMA using the same tests described previously (water sensitivity, wheel-tracking, bulk density, and air void content).
Stiffness modulus was measured using the Indirect Tensile Stiffness Modulus Test (ITSM) as described in standard UNE-EN 12697-26 (Annex C). For the performance of the complete test, three test cylinders were manufactured for each of the mixtures studied. These specimens with a diameter of 101.6 mm and heights ranging from 35 mm to 75 mm were compacted with 50 blows on each side by a Marshall hammer. This test determined the stiffness modulus, based on a series of 15 load pulses with controlled strain and sinusoidal waveform of a three-second duration. The first ten pulses conditioned the equipment so that it could adjust to the size of the load and its duration. Values were obtained within the limits established by the standard (which establishes that the value of the test load factor should be between 0.5 and 0.7, the deformation value should neither be greater than 20 µm nor less than 3 µm, and the rise time should be 120–128 ms). The five subsequent pulses determined the stiffness modulus of the mix, which was the mean value of the five pulses. Once this value was calculated, the cylinder was turned to determine the modulus along the perpendicular diameter. This modulus should be 80–110% of the first value otherwise the test is not valid. The final value of the stiffness modulus of each specimen is the mean value of both diameters. The stiffness modulus of each mix is the mean value of the results obtained for the three test cylinders.
In the second study stage, following the design and evaluation of the mixtures at a laboratory level, a series of mixing processes were carried out in a real asphalt plant for each type of mixture (PMB-HMA, CRMB-HMA, and CRMB-WMA) to test their reproducibility. For this purpose, after their manufacture in-plant, the samples were taken to evaluate their mechanical response in the laboratory using the water sensitivity test [
19] and wheel-tracking test [
20]. Additionally, to assess their performance under severe climatic actions, the particle loss test [
24] was also carried out under the following conditions: the response to water action was tested by comparing the results at 25 °C after conditioning in hot water at 60 °C for 24 hours in comparison with those obtained when tested in dry conditions at 25 °C; the effect of temperature was tested by conducting the test at 10, 25, and 60 °C; and the effect of aging was tested after conditioning at 165 °C for 12 hours. Furthermore, the bearing capacity of the mixtures manufactured in-plant was assessed through the stiffness test [
19] at 5, 20, and 40 °C; cracking resistance was assessed at low temperatures using the TSRST (Thermal Stress Restrained Specimen Test, [
25]) and fatigue cracking was evaluated using the UGR-FACT (University of Granada Fatigue Asphalt Concrete Test) at 10, 20, and 30 °C [
26,
27].
In the third phase of the study, the mixtures were used in the rehabilitation of a section of pavement on the A-92 highway (in the province of Granada, Spain). The location of the trial section was selected according to environmental and technical criteria. Regarding the first of these criteria, the section was placed in a mountain pass in a natural park where the use of CRMB-WMA (manufactured with recycled materials and at low temperatures) would help to reduce the negative impacts caused by road construction (in addition, the type of mixture used could reduce noise levels due to traffic rolling). Regarding the second criterion, the mixtures were evaluated under extreme conditions on account of the high volume of traffic (more than 18,000 vehicles per day, with a daily average of more than 2600 heavy vehicles) and extreme climate conditions (the section was placed at more than 1400 m above sea level, with the presence of snow during winter, and high temperatures and many hours of solar radiation during summer) (
Figure 2). In addition, this section was ideal for evaluating the real workability of the CRMB-WMA mixture since the transit time from plant to worksite was approximately 1 hour (
Figure 3). The temperatures of the asphalt pavement operations were in a range between 12 and 28 °C.
Finally, to complete the assessment of the performance of the CRMB-WMA in reference to the conventional high-performance mixtures (RMB-HMA and PMB-HMA), a series of cores were obtained from the pavements to evaluate their bulk density [
21] following compaction of the sections, whilst the stiffness test [
23] and UGR-FACT (at 20 °C) were also carried out to evaluate their mechanical response.