Two of the most commonly mixes used to mitigate cracking in pavement surfaces have been studied in this research: sand-asphalt (S-A) and stone mastic asphalt (SMA) (9, 10). All mixes were manufactured with the same filler (cement) and had the same aggregate type (limestone). Thus, this study focused on the evaluation of the effect of the two types of binder used: (i) polymer-modified bitumen (BM3b); (ii) high-viscosity rubber-modified bitumen via the wet process (HVRMB). The characteristics of the mix components are listed in Tables 1 and 2.

The grain sizes used to manufacture the S-A and SMA mixes are shown in Figure 1. It should be underlined that the mineral skeletons of the HVRMB and BM3b mixes were the same and that the mixes only differed in the type and content of the binder. Table 3 lists the properties of the mixes in regards to moisture sensitivity and plastic deformations.

Mix resistance to fatigue cracking was evaluated with the 4-point bending test. The dimensions of the test specimens were 410 × 50 × 50 mm. The experimental device used had a high-resolution image capturing system as well as an optic sensor, which monitored deformations and thus facilitated the analysis of the cracking process in the specimens. In this way, the force and deformation values occurred during each load cycle as well as the propagation of the macro-crack, was recorded inside a representative volume where the cracking process takes place. Furthermore, two movement indicators were placed near the macro-crack (on both sides of the crack and underneath it), which are used to measure the deformations produced in the lips of the crack (Figure 2).

The test specimens had a triangular notch located in the center (10 mm high and 10 mm long at the base). This made it possible to induce the appearance of the macro-crack in the area monitored by the camera (Figure 2). The failure criterion signaling the end of the test was when the macro-crack attained a length of 25 mm. When this value was reached, the propagation of the crack became unstable and it was no longer possible to track its behavior. Macro-crack propagation was monitored by digitally measuring the crack as captured in sequential images. For this purpose, the area where the crack appeared and subsequently spread was painted white.

For a more in-depth study of mix response to cracking, the experiment was carried out inside a climate chamber. The test conditions for the evaluation of the mixes were a load amplitude of 0.5 mm, a frequency of 10 Hz, and a constant temperature of 20 °C. For a more realistic assessment of the performance of the mixes, the healing effect during the fatigue processes of the materials was taken into account (11–13). For this reason, rest periods were included during the sinusoidal cyclic loading applied as part of the test. Accordingly, there were 1500 loading cycles with a rest period of 150 seconds (the same time period that it took to apply the 1500 cycles). For each type of mix and condition, two specimens were tested.

Figure 3 shows the results obtained for the length and width of the crack when the Sand-Asphalt mixes manufactured with BM3b and HVRMB bitumen were tested. As reflected in the figure, in both cases, there is a correspondence between these measurements. More specifically, for the temperature studied, the propagation of the macro-crack is in direct relation to the deformations generated by the crack opening due to the stresses suffered (see Figure 4). For the same loading conditions, the crack width in the HVRMB mix was found to be greater than in the BM3b mix.

The left panel in Figure 5 shows the amount of energy dissipated during the experiment. The values reflect that as the crack propagated throughout the material, less energy was needed for the crack to continue spreading. This occurred because the material decreased in strength as the macro-crack became longer and wider. Consequently, the crack spread more rapidly. The graphs indicate that the BM3b mix was found to be more resistant to fatigue cracking because more energy was needed for crack propagation than in the HVRMB mix.

As can be observed in Figure 5 (right panel), there is a linear relation between the length of the macro-crack and the amount of energy dissipated during the experiment. This indicates the correspondence between dissipated energy and the damage to the material. Thus, the amount of dissipated energy can be regarded as optimal design criteria. Within the context of a conventional phenomenological approach (in which loading conditions are related to the number of loading cycles resulting in the failure of the material), the mechanical response of both mixes is similar. Nonetheless, from the perspective of dissipated energy, the BM3b mixes with SBS polymers were found to be more crack-resistant than the HVRMB mixes with crumb rubber. Figure 6 shows images of the macro-crack propagation in both materials.

As with the previous mixes, the opening of the macro-crack was also linearly related to propagation length. However, in this case, the mix with the highest crack resistance was the one with the largest crack opening. Consequently, the width of the crack is not directly related to crack propagation in the material (Figures 7 and 8).

Once more, the results confirmed that during the cracking process, there was a progressive decrease in the amount of dissipated energy (Figure 9, left panel). This occurred because more energy was consumed when the crack first appeared in comparison to the energy consumed during its stable propagation phase. As in the case of the other mixes, there is a close relation between the cumulative dissipated energy and crack propagation throughout the specimens (Figure 9, right panel). Since the energy required for the crack to appear and subsequently spread throughout the BM3b SMA mix is greater, this signifies that that this mix is more resistant to fatigue cracking.