Understanding the influence of filler type and asphalt binder content on the moisture and fatigue resistance of asphalt mortars

2.1 Materials

The materials employed in the manufacture of the mortars were B 35/50 penetration bitumen, limestone sand (with a maximum aggregate size of 2 mm and washed to remove filler particles contained thereon), and two different typologies of active filler: Portland cement and Calcium carbonate (Figure 1). The main characteristics of these materials are summarised in Tables 1 and 2.

In order to assess the influence of both the typology of filler and the bitumen content on the moisture resistance of asphalt mortars and their implication with the mechanical performance of this kind of material, four mortars were manufactured. Previously, a bitumen content of reference was established by following the methodology proposed in other studies (2727. Hidalgo, A.E.; Moreno-Navarro, F.; Tauste, R.; Rubio-Gámez, M.C. (2020) The influence of reclaimed asphalt Pavement on the mechanical performance of bituminous mixtures. An analysis at the mortar scale. Sustainability. 12 [20], 8343. https://doi.org/10.3390/su12208343.
, 3030. Tauste-Martínez, R.; Moreno-Navarro, F.; Sol-Sánchez, M.; Rubio-Gámez, M.C. (2021) Multiscale evaluation of the effect of recycled polymers on the long-term performance of bituminous materials. Road Mater. Pavem. 22 [sup1], S99-S116. https://doi.org/10.1080/14680629.2021.1906737.
, 3535. Cavalcanti De Sousa, P. (2010) Automated protocol for the analysis of dynamic mechanical analyzer date from fine aggregate asphalt mixes (Doctoral dissertation, Texas A & M University).
). This consisted of manufacturing the complete mixture intended for reproduction (in this case, a BBTM 11 B mixture (UNE 13108-2) (3636. EN 13108-2. (2007) Bituminous mixtures - Material specifications - Part 2: Asphalt Concrete for very thin layers. AENOR, Asociación Española de Normalización y Certificación, Madrid.
) with a bitumen content of 5.3% over the total weight of the mixture), which was then disaggregated after it had cooled down (Figure 2). The material was then sifted with the aid of metal balls, to separate it into four groups:

Bitumen extraction was performed by employing an ignition oven to determine the binder content in each group. The binder content to be used in the manufacture of the mortar was set using that obtained in Group 4, and was weighted with the values obtained in Group 3. Subsequent to this process, the binder content of reference to manufacture the mortars was set at 8% on the total weight of the asphalt mortar. This content was employed to establish the differences derived from the use of various kinds of filler.

Once the filler that led to the best performance was determined, two new mortars were manufactured to ascertain the influence of using a bitumen content higher and lower than that of reference. The composition and density of the various asphalt mortars studied herein can be consulted in Table 3.

The materials that compose the asphalt mortars (aggregates, filler, and bitumen) were heated up to a temperature of 165°C prior to the mixing process. During the mixing, the temperature was also monitored by using a thermal imaging camera. Cylindrical specimens with a size of 150 mm in diameter were then compacted with the aid of a gyratory compactor (EN 12697-31) (3838. EN 12697-31. (2020) Bituminous mixtures - Test methods - Part 31: Specimen preparation by gyratory compactor. AENOR, Asociación Española de Normalización y Certificación, Madrid.
) at a temperature of 155 °C.

In order to minimise the air void influence on the study of the aggregate/binder adhesiveness and its evolution subsequent to water action, the compaction process targets a void content close to 0% as pointed out in other studies (2727. Hidalgo, A.E.; Moreno-Navarro, F.; Tauste, R.; Rubio-Gámez, M.C. (2020) The influence of reclaimed asphalt Pavement on the mechanical performance of bituminous mixtures. An analysis at the mortar scale. Sustainability. 12 [20], 8343. https://doi.org/10.3390/su12208343.
, 3939. Leiva-Padilla, P.; Moreno-Navarro, F.; Iglesias-Salto, G.; Rubio-Gamez, M.C. (2020) Recovery capacity of electroconductive asphalt mortars under the influence of magnetic fields. Mater. Today Commun. 25, 101527. https://doi.org/10.1016/j.mtcomm.2020.101527.
). Once compacted, the specimens were cut in order to obtain the prismatic specimens of 8.5 x 8.5 x 50 mm that would subsequently be used in the DMA (Dynamic Mechanical Analyzer) characterisation.

2.2. Testing plan

The study has the main objective of assessing the influence of the type of filler and bitumen content (key elements in the formulation of both mortars and asphalt mixtures) on the behaviour of asphalt mortars in relation to the action of water by analysing their response to fatigue. To this end, a three-point bending test was employed with the aid of DMA equipment. This test enables bearing capacity, ductility, and fatigue resistance of the material to be characterised (3030. Tauste-Martínez, R.; Moreno-Navarro, F.; Sol-Sánchez, M.; Rubio-Gámez, M.C. (2021) Multiscale evaluation of the effect of recycled polymers on the long-term performance of bituminous materials. Road Mater. Pavem. 22 [sup1], S99-S116. https://doi.org/10.1080/14680629.2021.1906737.
, 4040. Tauste-Martínez, R.; Hidalgo, A. E.; García-Travé, G.; Moreno-Navarro, F.; Rubio-Gámez, M.D.C. (2022) Influence of type of filler and bitumen on the mechanical performance of asphalt mortars. Materials. 15 [9], 3307. https://doi.org/10.3390/ma15093307.
), thereby providing useful information regarding the influence of the components of the mortars on their mechanical performance and the changes thereof induced by moisture action.

In order to assess the susceptibility to water of bituminous mortars, this test procedure was carried out on two groups of samples of each type of mortar (following a methodology similar to that established in the water sensitivity test of bituminous mixtures (EN 12697-12)). Thus, the specimens manufactured were separated into two groups: a dry group of samples maintained at room temperature (20 ± 5 °C), and a wet group that was conditioned for 72 hours in water at 40 °C after applying a vacuum at a pressure of 6.7 ± 0.3 kPa for 30 ± 5 minutes (Figure 3).

For each group, nine specimens were considered, three for each level of strain studied (40, 60 and 80 μm, tested in strain-controlled mode). These present levels of load sufficient to reach the range of linear visco-elastic behaviour and to maintain it sufficiently for the evaluation of changes in stiffness and ductility before the fatigue failure occurs. Each sample was tested at a temperature of 35 °C (a demanding temperature to induce fatigue response, which enables a suitable characterisation and test durations to be attained) and 5 Hz until fatigue failure is reached (Figure 4).

The effect of moisture on the mortars was evaluated by comparing, for both groups, the various parameters obtained from the test (2727. Hidalgo, A.E.; Moreno-Navarro, F.; Tauste, R.; Rubio-Gámez, M.C. (2020) The influence of reclaimed asphalt Pavement on the mechanical performance of bituminous mixtures. An analysis at the mortar scale. Sustainability. 12 [20], 8343. https://doi.org/10.3390/su12208343.
):

where:

3.1 Influence of the type of filler

As can be observed in Figure 6, the use of cement (CEM 8) leads to a greater degree of rigidity and less deflection than does the use of calcium carbonate (CC8), regardless of whether the material has been subjected to water action. This means greater bearing capacity, and therefore higher resistance to permanent deformations.

In relation to the mortar manufactured with calcium carbonate (CC8), once the material is subjected to the action of the water, its stiffness does not vary significantly. However, it does increase the degree of deflection accumulated before failure, thereby proving this parameter to be more sensitive to moisture effect. As for Portland cement, its use presents smaller differences not only in terms of stiffness (as was the case with carbonate) but also in terms of ductility, thereby indicating a higher resistance to moisture.

Figure 7a shows the fatigue laws of both mortars before they are subjected to the effect of moisture. In this way, it can be deduced that, in line with that observed above in Figure 6, the higher rigidity and lower degree of ductility of mortars made with Portland cement (CEM8) also lead to a greater fatigue resistance than its counterpart made of Calcium carbonate (CC8) throughout the entire range of loads. The lower the load applied, the more marked the differences, which indicates a greater durability of mortars that use Portland cement as active a filler (since a higher level of load is required to obtain a similar fatigue life in the two materials, otherwise cement resistance is significantly higher).

Once mortars have been subjected to the action of water (Figure 7b), it can be observed how the difference between the fatigue lives of the two materials becomes more marked across the entire range of loads. This confirms the increased moisture resistance of the mortars that use Portland cement as already pointed out by the results of stiffness and ductility (Figure 6). It can therefore be said that the use of cement as active filler shows better performance than does calcium carbonate in terms of bearing capacity and resistance to both moisture and fatigue. Portland cement was hence chosen as the reference filler for the assessment of the influence of the bitumen content employed.

3.2 Influence of the binder content

Figure 8 shows the effect of the binder content on the stiffness and ductility of the asphalt mortars for both dry and wet groups. Regarding stiffness and by focusing on the mortars before water conditioning, it can be observed how the most rigid mortar is the reference mortar (with an intermediate binder content (CEM 8)) with a stiffness that ranges between 300-350 MPa. When the binder content is decreased (CEM 7), the level of stiffness achieved is similar, however, the deflection experienced is lower. This points towards a lower flexibility of the material when the binder content is reduced. On the other hand, when the bitumen content is increased (CEM9), the changes become more marked, with a significant reduction of stiffness (in a range of 200-250 MPa) and an increase of maximum deflection (approximately 1500 μm compared to the lower bitumen contents that stand at approximately 1000-500 μm for 8 and 7%, respectively). In this way, a higher percentage of asphalt binder in the mixture leads to the mortar becoming more ductile.

As regarding the effect of water on the mechanical behaviour of mortars, it can be observed that, regardless of the bitumen content, similar mechanical behaviour is reached. The maximum deflection of the samples of the wet group decreases although the stiffness of the material does not change significantly subsequent to the water action. Despite this general consideration, different degrees of deflection variations can be observed between the different contents of bitumen. Intermediate content (CEM8) shows a lower rate of change that is more marked when this percentage is reduced (CEM 7) and it becomes even more pronounced after increasing the bitumen content (CEM 9). This points towards a higher degree of sensitivity of this ductility parameter regarding moisture resistance.

Figure 9 shows fatigue life (Nf) in reference to the load applied in the test. According to the results, it can be observed how this methodology has remarkable potential to establish an optimal bitumen content for the binder tested, which considers both fatigue and moisture resistance since the three percentages analysed present different degrees of fatigue resistance and rate of change subsequent to water conditioning. As can be observed, the mortar manufactured with 8% of bitumen (CEM8) presents the highest degree of durability with levels of fatigue resistance superior to those achieved by the other bitumen contents, regardless of whether they have been subjected to the action of water.

In relation to moisture susceptibility, it can be observed that the reduction of deflection experienced after the water action (Figure 8) leads to even greater fatigue lives of the wet group. Nonetheless, this phenomenon changes for the other two contents. When the bitumen content is below the optimum (CEM 7), the fatigue life reached decreases despite the lower degree of ductility of the material. This could be related to a poorer bitumen/aggregate adhesion which is confirmed by the results obtained after the water action. According to these results, the fatigue resistance is reduced to a greater extent as the solicitation load level is higher.

When the bitumen content exceeds the considered optimum (CEM9), the differences between the levels of durability achieved before and after water conditioning are the lowest, which would confirm that a rise in the bitumen content is positive to the performance of asphalt materials against water (55. Hunter, R. N.; Self, A.; Read, J.; Hobson, E. (2015) The shell bitumen handbook (p. 789) London, UK: Ice Publishing.
). Nevertheless, the fatigue resistance of this material is significantly lower than the mortar manufactured with 8% of bitumen due to the reduction of stiffness and the increase of ductility (Figure 8) experienced once the optimum content of bitumen is exceeded. This proves the potential of the methodology proposed since, for the materials tested, it enables the effects of moisture on the asphalt mortars to be studied without limiting the assumption of a better performance to the differences found in a specific parameter before and after water conditioning. Instead, the joint consideration of stiffness, ductility, and fatigue evolution enables an optimal bitumen content to be established that considers the permanent deformation, fatigue, and moisture resistance of the material.