1. Introduction
Asphalt is one of the most used construction materials on roads in Europe. Approximately 98% of pavements in Slovakia and the Czech Republic have asphalt mixtures in the covers. These roads are primarily in the rural area, with high traffic load, affected by climatic factors, and roads where winter maintenance is based on the application of chemical spreading materials. Several factors must therefore be considered when designing a road pavement. The first is the design of the type of pavement structure depending on the traffic load. Next is the choice of materials and technological procedures, from which the number of pavement layers and their dimensions is determined. When choosing materials for asphalt pavement, it is important to consider the temperature extremes and methods of road maintenance. Due to the viscous-elastic properties of bitumen, the asphalt mixture has a low viscosity at high temperatures and high viscosity at cold temperatures. The high temperature of pavement makes the asphalt softer; thus, the risk is high that heavy vehicles cause rutting due to the plastic deformation. As the ambient temperature decreases, the elasticity and stiffness of asphalt increase. At the same time, the relaxation capacity decreases. By preventing thermal shrinkage, so-called cryogenic tensile stresses are created during cooling, which at temperatures of −20 °C are so great that they exceed the tensile strength of the asphalt and lead to the formation of cracks near the surface. Furthermore, the cracks gradually penetrate downwards and reduce the service life of the pavement structure by the erosive action of the penetrating water. In winter, the covers undergo repeated stresses when the effects of freeze–thaw cycles and the effect of de-icing salt are added under constant traffic load.
It is known that the type of asphalt binder significantly affects the fatigue properties of the asphalt mixture. Some bituminous binders currently available on the Central European market contain a certain ratio of visbreaking residues, which, according to [
1], affect the properties of the binders in middle and elevated service temperatures.
Worldwide, many research tasks are devoted to the issue of the impact of temperature extremes and related processes on individual activities and sectors. The reason is the risks related to climate change, which have been at the forefront of current global environmental risks for several years. Following the Global Risk Report 2021 [
2], in the last ten years, these risks are beginning to appear in the top five, along with economic, social, and technological risks. In 2019, “extreme weather, climate action failures, and natural disasters” were in the top three. In 2020, only environmental risks were in the top five, in addition to “biodiversity loss and human-made environmental disasters”. Global warming forecasts point out that, by 2100, our planet could warm by an average of 3.5 to 7.5 °C. In Central Europe, and thus also in the Czech Republic and Slovakia, the average annual air temperature has risen by about 3.0 °C over the last 140 years. Significant changes are in the frequency of extremes—a rapid rise in extremes of maximum temperature and a rapid decrease in extremes of minimum temperature [
3,
4]. Manifestations such as strong winds, storms, torrential rains, and thunderstorms in the winter with a large volume of snow cover are more common. The consequences of these changes are immediate and intense and require adequate adaptation measures regulated in international [
5] and national [
6,
7,
8] documents.
These extreme weather events cause higher demands on the materials of the pavement structure layers and the construction technologies. A number of laboratory findings showed that the freeze–thaw cycle significantly causing a change in the load-bearing capacity of the pavement (load transfer) and the stiffness of materials (permanent deformation, fatigue) [
9,
10,
11,
12,
13,
14]. The demand for increasingly high-quality road construction material and the improvement of strength and deformation parameters of the mixture caused the use of various other materials in asphalt mixtures. An example is the use of various additives [
14,
15] that have been standardly used in several countries around the world since the 1990s or the application of materials such as crushed rubber, sulfur, which have been used in asphalt mixtures since the 1970s, which allows the long-term monitoring of their effect on the strength, fatigue life, and durability of different asphalt mixtures [
13,
16]. However, it is essential to consider the actual rationale for the use of the additive or technology. Even without the use of additional components, the asphalt mixture should be designed with sufficient parameters so that the layer and, consequently, the entire roadway can withstand the influencing factors throughout its lifetime.
There are several research tasks and programs in Central Europe that deal with the effect of increasing temperatures on the physical-mechanical properties of asphalt pavement materials. A few of them are aimed at assessing extreme negative temperatures or the increasingly frequent alternation of extremes of positive and negative temperatures. The studies [
17,
18] documented the behavior of asphalts at high and low temperatures, including the effect of freeze–thaw cycles. As confirmed by studies [
19,
20,
21,
22], the long-term application of sodium chloride (NaCl) has negative consequences on soils and the water environment. Heavy metals are released and chlorides increase. However, despite the above-mentioned negative effects of sodium chloride on the environment, it will continue to be used for its availability, applicability, and economic simplicity. Based on the mentioned experience, it is assumed that the repeated effects of repeated freeze–thaw cycles and saline solution on the strength and deformation characteristics are negative. However, according to the authors [
23] and their experience, salt water shortens the time the mixture is in contact with frozen water, which is one of the most harmful effects on asphalt. When the mixture is exposed to cycles, the saline solution has a protective effect on the samples that remain immersed in it. Salt in water protects the bituminous putty, maintains mechanical strength, and increases the number of load cycles for any range of stresses. The results of their research point to the fact that temperature has a detrimental effect on the mechanical properties, but samples immersed in salt water achieve better results than their analogs, which are immersed in distilled water. However, it must be emphasized that the authors used only five freeze–thaw cycles, which may be a number characteristic of Spain, but not suitable for conditions in the Czech Republic and Slovakia.
In general, current issues of road de-icing adjustments in former Czechoslovakia have been more actively addressed at various research institutes since the 1970s. The questions are mainly concerned with the necessary development of chemical spreading materials with increased efficiency and more favorable environmental impacts. In addition to standard materials, research was also carried out on sources of cheap secondary raw materials. The most extensive research program on improving winter maintenance with chemicals was carried out in 1978 at the Transport Research Institute in Žilina (Žilina, Slovakia) and the branch in Brno (Brno, Czech Republic). The issues of winter maintenance overlapped with the maintenance programs of other professional and scientific workplaces in Žilina and Bratislava, as well as with the activities of motorway maintenance centers and district road administrations. From the 1970s to the 1990s, reports on the effect of chemical sprinklers on road surfaces, road equipment, bridge structures, reports specifying criteria for the protection of roadways against the effects of freeze–thaw cycles, etc. have been prepared [
24]. An extensive information base and working materials from world road congresses were developed, which made it possible to shape the direction of progress in the modernization of applications of chemical de-icing agents. Whereas in the past the production of de-icing agents was only possible through the importation of chloride compounds, in the last 25 years, there have been several possibilities for the production of chloride de-icing agents even at home. Very interesting for the decision-making process can be the computer decision tree for the application of de-icing agents [
25], which could increase the efficiency of winter maintenance processes in our conditions.
The authors aim to point out the need to introduce and implement freeze–thaw cycles with the action of salt solution during standard asphalt testing in the conditions of both countries. The issue of the impact of winter maintenance on the physical and mechanical properties of asphalt has not received much attention in Slovakia for a long time. It is not included in the standards and technical regulations for testing asphalt. However, in the period before the adoption of EU standards, freezing cycles were also used in some tests and, in general, more attention was paid to the impact of chemical de-icing materials on asphalt pavements [
26]. Currently, the resistance to freeze–thaw cycles is obligatorily assessed only for concretes and cement-bonded mixtures. The situation is slightly different in the Czech Republic, where the CSN 73 6161 standard for determining the adhesion of asphalt binders to aggregates defines this property even under the conditions of freeze–thaw cycles in NaCl solutions.
The next part of the article is devoted to the analysis of the legislation of both states in the given issue, which is also one of the important bases for setting up the experimental part. The setup of the entire experiment was based on the mentioned Czech standard CSN 73 6161 and TP 170—regulation for the asphalt pavement design, as well as, due to the very similar topographic and climatic conditions of both countries and the mutual long-term existence of common standards and technical regulations during the existence of a common state. The results of the laboratory measurements were the basis for drawing conclusions and further recommendations.
3. Results
3.1. Asphalt Binder Properties
The test results of asphalt binders [
34,
35,
36] are in the graphs of
Figure 3,
Figure 4 and
Figure 5. The values of the penetration and softening point of the 50/70 and 70/100 paving grade bitumen and PMB 25/55-60 polymer-modified bitumen follow the requirements of the standards. For the 50/70 binder, the softening point corresponded to the requirement of the standard, but the detected penetration was lower by two penetration units. However, this inconsistency does not affect the experiment.
The results’ analysis of the empirical characteristics of asphalt binders showed that the change in penetration occurs after the first two days in the saline solution. For the 70/100 binder, the penetration of the new binder and the binder two days in saline solution decreased by six penetration units, i.e., the change was about 8%. For the 50/70 binder, there was a decrease of four penetration units, i.e., also the change was about 8%. For a polymer-modified bitumen, this decrease is minimal. It was a change of one penetration unit, i.e., the change was about 3%. For all three binders, the further effect of the saline solution did not change the penetration. The penetration after seven days in the solution was identical to the penetration after two days in the solution.
The softening point results have different behavioral tendencies. Gradually, there is a slight increase in the softening point between the second and seventh day of the effect of the salt solution. For the binder 70/100, the total change was 1.1 °C and 1.2 °C, respectively, a change of 2.5%. For the binder 50/70, there was an increase of 2.3 °C, a change of 4.4%. For the PMB binder, in one case, there was an increase of 0.6 °C and a decrease of 0.8 °C in the other case. It can be stated that the saline solution does not affect this property of the PMB binder.
Elastic recovery was determined only for the PMB binder. After two days in the saline solution, there was a decrease of 3.1%, after another five days a further 2.5%. The total change in elastic recovery was 5.6%, i.e., the change was about seven percentage points.
In conclusion, the evaluation of the characteristics of asphalt binders revealed that there is a maximum change of 8% in the properties after seven days in the saline solution.
Additionally, in
Figure 6, the change in the shape of the test samples (elastic recovery test) due to the effect of saline solution can be seen.
3.2. Strength and Deformation Characteristics of Asphalt Mixtures
Marshall samples were into four groups divided: air (reference: 0 freeze–thaw cycles), 25 freeze–thaw cycles, 50 freeze–thaw cycles, and 75 freeze–thaw cycles. All four groups had approximately the same bulk specific gravity (
Figure 7). Air void value was 2.0% for the “Air” and “75 freeze–thaw cycles” groups of samples and the value of 2.2% for the “25 and 50 freeze–thaw cycles” groups of samples. The increasing number of freeze–thaw cycles increases the absorption value from 1.5% to 3.4% (
Figure 8): “Air” group samples—values of 1.1–1.9%; “25 freeze–thaw cycles”—values of 2.3–2.4%, “50 freeze–thaw cycles”—values of 2.8–3.3%; and “75 freeze–thaw cycles”—values of 2.9–4.2%. Relationship between the absorption of asphalt and the number of freeze–thaw cycles is in
Figure 9. Marshall stability, flow value, and Marshall stiffness [
38] (
Figure 7 and
Figure 10) were measured, and the change in stiffness modulus [
39] is shown in
Figure 11,
Figure 12,
Figure 13 and
Figure 14. The results of both tests show identical trends. The increasing number of freeze–thaw cycles negatively affects the deformation characteristics of the asphalt mixtures. The Marshall stability decreased from the original value of 10.8 kN to 9.07 kN due to freeze–thaw cycles, which is a decrease of about 16%. The flow value has the opposite trend. The Marshall stiffness decreased from the original value of 3.13 kN/mm to 2.00 kN/mm. It is a decrease of about 36%.
Figure 12 and
Figure 13 show the relationship between the decrease in stiffness modulus [
38] and the number of freeze–thaw cycles at the test temperatures of 15 °C and 40 °C. The trend of results is the same. As the number of cycles increases, the stiffness modulus decreases. From the basic value of 10,219 MPa, after 75 freeze–thaw cycles, there was a decrease of about 32% of the stiffness modulus at 15 °C and a decrease of about 31% at a temperature of 40 °C. The relationship between the number of freeze–thaw cycles, the absorption, and the stiffness modulus show
Figure 14. When exposed to chemical de-icing agents, it can be assumed that a higher number of freeze–thaw cycles applied will result in higher absorption and thus lower strength and deformation parameters. As can be seen, for samples loaded with 0 freezing cycles, there are absorption values with a maximum value of approx. 1.5% and the average value of the stiffness modulus is 10,219 MPa (15 °C) and 979 MPa (40 °C). After the application of 75 freeze–thaw cycles, there is an increase in absorption to an average value of 3.4% and a decrease in the stiffness modulus of 68% for both temperatures. Such a relationship can also be assumed, in general, for other types of asphalt mixtures.
3.3. Application of Results—Pavement Assessment
The obtained data can be used to assess the pavement structure and evaluate the impact of winter chemical maintenance on shortening the theoretical life of the pavement structure. Two pavement structures were assessed using the TP 170 methodology [
30]. The influences of the decrease in stiffness modulus over time due to the effect of salt on the pavement surface course (theoretical service life) was determined. It is a certain simplification. Firstly, in the sense of the effect of chemical de-icing agents on the asphalt pavement layer only from the upper surface. Second, the binder ages. This can compensate for the decrease in stiffness modulus due to chemical de-icing agents.
Two catalogue pavement structures were selected for illustration: a motorway pavement structure (DO-N-1-PIII) and a second-class road pavement structure (D1-N-2-PIII) (
Table 5). The used asphalt mixtures are according to different quality requirements. For No. 1, asphalt concrete AC “S” was used, with mixtures with increased resistance to permanent deformations; for the highest traffic load classes (TLC) with an average daily intensity of more than 7500 heavy goods vehicles (HGVs); for wearing and binder course compacted in the laboratory design by 2 × 75 blows in Marshall compactor and 2 × 50 blows for the use in the base course. For No. 2, asphalt concrete “+” was used, with mixtures compacted in the laboratory design by 2 × 50 blows of Marshall compactor; for wearing and binder course for TLC II to IV (HGV = 101–3500) and for a base course with TLC V to VI (HGV = 15–100).
Only an 8-year period was assessed using the TP 170 methodology, for which data on the decrease in stiffness modulus depending on the number of freeze–thaw cycles were available. The reference structures were those for which a constant value of the surface course stiffness modulus was assumed for the whole eight years of the assessment period. This is a value that was experimentally determined on test samples not affected by saline solution.
The TP 170 methodology evaluates the theoretical life of a road structure by relative failure of D
cd. Every load generates a stress (relative strain or stress) in the structure. In the compacted layer, depending on the load intensity, the failure of the layer occurs, while in the subsoil, permanent deformation occurs. The accumulation of failures and permanent deformations leads to pavement failures. The reliability of the design of the pavement structure must be appropriate to the traffic volume and the traffic significance of the road, which is based on Miner’s hypothesis, i.e., the accumulation of partial damages is linear, the damage is cumulative, and the running order does not matter. Pavement damage occurs when the sum of the partial relative failure is D
cd ≤ 1. The total relative failure is then given by the superposition of the partial relative strain:
where D
cd presents the total relative failure during the design period, m
i presents the number of different categories of load sets, and m
j presents the number of different conditions.
In principle, this is the ability of the road structure to transfer traffic loads. This is expressed by the maximum horizontal relative deformation at the asphalt critical layer and by the vertical relative deformation at the subsoil of the road structure. Both deformations are compared with the effects of heavy vehicles over the entire theoretical life of the road.
Table 6 and
Table 7 show the change in stiffness modulus due to freeze–thaw cycles and due to chemical de-icing agents. A comparison of the assessment of the reference pavement structure with pavement structures No. 1 and 2 leads to the following conclusions:
The length of the assessed period was eight years, and it corresponds to the average durability of the wearing course, which is stressed by 75 freeze–thaw cycles;
The use of chemical de-icing salt causes a decrease in the stiffness modulus of the wearing course, and the service life of road structures is shortened;
The service life of the road structure 1 is shorter by 5% for the asphalt-bonded critical layer during the eight-year design period, and by 11% for the subsoil; with a traffic intensity of 10,000 design trucks in 24 h, this is a reduction in the number of design trucks crossings for the assessed period of 8 years in the number of 1,460,000 HGVs for asphalt layers and 3,212,000 for subsoil;
The service life of the road structure 2 is shorter by 7% for the asphalt-bonded critical layer during the eight-year design period, and by 12% for the subsoil; with a traffic intensity of 440 design trucks in 24 h, this is a reduction in the number of design trucks crossings for the assessed period of 8 years in the number of 89,936 design trucks for asphalt layers and 154,176 for subsoil.
4. Conclusions
The article [
40], published in 1967, states that salt does not affect the change in the strength characteristics of asphalt mixtures. The results of this our study did not confirm this statement. The mentioned article does not state how many freeze–thaw cycles were performed before the Marshall test and what kind of solution was used at 25 freeze–thaw cycles, so the conclusions of both studies are not identical. The salt solution has the effect of changing the properties of asphalt binders and asphalt mixtures. The results of the characteristics of asphalt binders after seven days of stress in saline solution show a maximum change of 8%in the properties. The change in penetration occurs mainly after the first two days of stress, and the next stress is a minimal change in penetration. Between the second and seventh days of stress, a small increase in the softening point was recorded for 50/70 and 70/100 binders, and there was almost no change for PMB 25/55-60 binder. The final change in the elastic recovery of PMB 25/55-60 binder was 5.6%. The investigation of the behavior of asphalt binders was only marginal. The focus was on basic tests, which provided only basic information about the effect of salt on the binder. One of the next steps in the future will be to pay attention to low-temperature properties and rheological tests, which will provide a truer picture of the actual behavior of how the salt solution and repeated freeze–thaw cycles can affect the binder.
The decrease in strength and deformation characteristics occurs significantly after 50 and 75 freeze–thaw cycles. The increasing number of freeze–thaw cycles negatively affected the deformation characteristics of the asphalt mixture. Due to the freeze–thaw cycles, the Marshall stability decreased by about 16%, from the original value of 10.8 kN to 9.07 kN, and the Marshall flow value increased by about 30%—from the original value of 3.47 mm to 4.50 mm. The stiffness modulus decreased by about 32% at 15 °C, from the original value of 10,219 MPa to 6800 MPa, and the stiffness modulus decreased by about 31% at 40 °C, from the original value of 979 MPa to 666 MPa.
The conclusions from the stiffness modulus test show a relatively strong dependence between the absorbency of the test specimen, its voids, and stiffness. The high void content of the wear layer can negatively affect the service life of the road structure or the durability of the wear layer in localities where there is a more frequent fall of snowfall and repeated chemical winter maintenance. By expressing the relative failure of Dcd, it has been shown that, with decreasing strength characteristics, the service life decreases. These declines are significant and must not be forgotten.
The results of this study may be the basis for a change (decrease) in the air void requirement for the wear layers. It would be very interesting to perform a similar study on open asphalt mixtures, which can reduce rolling noise. Their high void content increases the consumption of salt, which has a higher negative effect on the deformation characteristics of asphalt mixtures and reduces their durability.