Outside Storage

The outside storage series, the C-Series, remained outside in Santander (Spain) for one year, from 20 December 2016 to 20 December 2017, and was protected from precipitation. The maximum and minimum daily temperatures during this period were collected by the Spanish Meteorological Agency AEMET (Figure 3).

**Figure 3.** Maximum and minimum temperature per day in Santander for the year 2017.

## *2.3. Tests Programme*

2.3.1. Indirect Tensile Strength (ITS) and Water Sensitivity Test (ITSR)

These tests have been done to determine the mechanical strength and how the aggregate-binder adhesive is influenced by the action of water. As established by Spanish Standard PG-3 [29], the test procedure was the "Method A" of the UNE-EN 12697-12: 2009 standard [30], with a compaction of the specimens by impact (UNE-EN 12697-30: 2013) [31], with 50 blows per side. Four dry specimens and another four wet specimens for each series were evaluated.

The dimensions of the specimens are 63.5 mm in height and 101.6 mm in diameter. This test is done for all specimens.

#### 2.3.2. Wheel Tracking Test

This test has been carried out to determine the susceptibility of the specimen to deformation when a moving vertical load is applied. The test is done in accordance with the standard UNE-EN 12697-22: 2008 + A1 "Procedure B in air" [32], compacted by roller compactor (UNE-EN 12697-33: 2006 + A1) [33]. The limits established by PG-3 depending on the heavy tra ffic categories are 0.07 and 0.10 mm/10<sup>3</sup> cycles of Wheel Track Slope (WTS).

The specimens are 410 mm long by 260 mm wide by 50 mm high. The Wheel Tracking Test is only done for the A-Series and C-Series. For these series, two specimens of each one were tested.

#### 2.3.3. Dynamic Modulus Test

Since bituminous mixtures are a viscous material, this test was done to provide insight into the viscosity properties of the mixtures. The test was carried out as established by the standard UNE-EN 12697-26: 2012 "Annex B" four-point bending test [34]. Six specimens have been evaluated for each series, except for the C-Series, where eight specimens have been tested due to the long period that these specimens have been outdoors.

The specimens are 410 mm long by 60 mm wide by 60 mm high. The Dynamic Modulus Test was done for all specimens.

## 2.3.4. Fatigue Test

This test provides an estimate of durability of the bituminous mixture by load–unload cycles. The test procedure has been the UNE-EN 12697-24: 2013 "Annex D" four-point bending test [35]. The same number of specimens, as in the case of the Dynamic Modulus Test, was evaluated.

The dimensions of the specimens are the same as the ones of the Dynamic Modulus Test. The Fatigue Test is done for all specimens.

#### **3. Results and Discussion**

#### *3.1. Indirect Tensile Strength (ITS) and Water Sensitivity Test (ITSR)*

Table 6 shows the results of Indirect Tensile Strength and Water Sensitivity Test for all the series. The same as Hassan et al. [14] indicated, the values of ITS are similar or even better for mixtures exposed to salt than those submerged in distilled water.

The results obtained in the A-Series for ITS and ITSR are the regular ones for a hot mix asphalt. There are no noticeable di fferences between the series submerged in distilled water (A1) and the series submerged in salt water (A2). This result was expected due to the fact that the A-Series was not submitted to temperature changes.

In the series subjected to five freeze–thaw cycles (B-Series), although the result of ITS for dry specimens (B0) shows no significant di fference when compared to the dry reference specimens (A0), that di fference exists for the wet series. This variation is lower between specimens submerged in salt water (B2 and A2) and greater between specimens submerged in distilled water (B1 and A1). These

results show that the greatest damage to the hot mix asphalt is not the temperature changes but the amount of time that the mixture is submerged in frozen water. The B2 series obtained a value of ITRS, which is close to the limit established by PG-3 (Spanish Regulation) for surface layers (85%).


**Table 6.** Indirect tensile strength and water sensitivity.

For the series subjected to a year outdoors (C-Series), the results of ITS for dry and submerged specimens are lower than the ones of the reference series (A-Series). However, in the case of submerged specimens (C1 and C2), especially for the series submerged in distilled water, the values of the ITS test are lower than in dry specimens (C0). This result is due to the fact that remaining submerged for a year is a more damaging process for the mixture than remaining dry for the same period.

#### *3.2. Wheel Tracking Test*

The values for the Wheel Track Slope and the Ruth Depth at 10,000 cycles (RD) for all the series are shown in Tables 7 and 8. The results obtained from these tests comply with the Spanish regulations for most climate zones and traffic.



Although the results for all series are similar, the series that have remained a year outdoors have obtained WTS and RD values lower than the reference series, except for the series whose specimens were submerged in salt water (C2). These low values (C0 and C1) can be due to the fact that, after one year in outside storage, the binder has become stiffer.


**Table 8.** Rut depth.

#### *3.3. Dynamic Modulus Test*

As Table 9A,B indicate, the phase angles have no significant variation between the different series. Only in the case of those series which have been subjected to freeze–thaw cycles, the variation is slightly greater. This fact may be due to the larger temperature differences between the B-Series and the other series.




In the A-Series, very similar values were obtained for all the specimens. However, there exists a trend in which the dry series has higher modulus values and lower phase angles, due to the fact that it is more elastic than the two others.

However, for the series subjected to freeze–thaw cycles, a clear tendency appears; the dry specimens (B0) have greater modulus and phase angles for all of the test frequencies. In the case of the submerged series, B2 is more elastic than B1.

As in the case of the B-Series, for the C-Series, C0 has greater values of modulus, followed by C2 and finally by C1. However, in the case of the phase angles, the trend is di fferent; the greatest angle is C2 and the lowest is C0. For the C-Series, the values of the modulus are greater, and the values of phase angles are lower compared with the A-Series and B-Series, and this indicates that the specimens have su ffered a sti ffening process.

In general, for all of the series and frequencies tested, the modulus values are greater for the dry series. This was expected due to the fact that submerging specimens in water is more harmful for the mixture than keeping them dry during any temperature interaction. Likewise, for all of the temperature interactions, there exists a trend which indicates that the series that are submerged in salt water have greater values of modulus than their corresponding pair submerged in distilled water, even more for the series subjected to freeze–thaw cycles. This trend supports the idea that the largest damage to the mixture that is submerged in water is the amount of time that it remains in frozen water, and the specimens that are submerged in salt water remain in frozen water for a shorter period of time.

## *3.4. Fatigue Test*

The fatigue lines (Strain—Number of Load Cycles) obtained for all of the series are very similar, reaching notably high R<sup>2</sup> values; although, in the case of the C-Series, it is slightly lower. This may be due to the fact that the temperature interaction process is longer than in the A-Series and B-Series.

In the A-Series, for low values of strain, the submerged series, especially A1, obtain more load cycles than the dry specimens (Figure 4).

**Figure 4.** Fatigue test: specimens at constant temperature of 20 ◦C.

In the case of the series subjected to five freeze–thaw cycles (B-Series), the one that is submerged in salt water (B2) obtains more load cycles for any strain range than the other two series (Figure 5). This fact may be due to two main reasons. The first is that salt water o ffers a cushion e ffect on the temperature variations due to its thermal conductivity, which is smaller than that of distilled water [36]. The second is that the specimens of this series are in contact with frozen water for less time, which, as corroborated by the rest of the tests, is one of the most harmful e ffects for the mixture.

**Figure 5.** Fatigue test: specimens subjected to five freeze–thaw cycles.

The series that have been subjected to a year outdoors (C) obtain similar values for all specimens, being for C1 the number of load cycles slightly lower for high strain values (Figure 6).

**Figure 6.** Fatigue test: specimens subjected to one of year outside storage.
