Improving the in-service performance of asphalt pavement on roads and on the bridges of the transport infrastructure, through the “dry method” of polymer modification.
Introduction
In recent years the quality of materials and of pavement construction works in the countries of Europe, Asia, the CIS, America and Africa has improved significantly. This is due to a large extent to the introduction of new technologies and standards and to the use of modern construction materials that deliver durability and reliability of the pavement. However, given climate change and the growing intensity of traffic loading, the question of securing the stability and the temperature resistance of asphalt pavements is becoming ever more pressing.
The problems associated with the physical and chemical properties of the bitumens used as the binding element in asphalt mixtures call for deeper research and for new technological approaches. Modifying bitumen with polymers and other additives can significantly raise the resistance of asphalt pavement to external effects such as temperature swings, the action of moisture and mechanical loading. This question has been raised repeatedly in various scientific publications, such as “Bitumen and Bitumen Modification: A Review on Latest Advances” (Applied Sciences) and “Исследование особенностей взаимодействия битумов с полимерами” (D. A. Ayupov, L. I. Potapova) and many others. However, as scientific research shows, the particular character of bitumens produced by distillation either considerably complicates their modification or makes it practically impossible. The inconstancy of bitumen indicators and their divergence from the declared characteristics likewise call for a search for alternative solutions to obtain a quality asphalt pavement.
The aim of the work
The aim of this work is to draw the attention of experts and specialists in the road construction industry to an alternative technology for modifying asphalt mixtures, one that makes it possible to reach stable in-service characteristics of the pavement irrespective of the instability of the properties of the original bitumen. Particular attention is given to the use of polymer additives capable of compensating for the shortcomings of modern bitumens, especially those obtained by the distillation method, and in particular for their low capacity to be modified with polymers. This technology is intended to secure the stability and the length of service of road pavements under significant temperature swings and increased loading from traffic.
The research is directed at finding an optimal polymer modification process, one that will deliver the necessary physical and mechanical characteristics of asphalt concrete, wear resistance and a long service life of the pavement structure. Beyond that, a principal aim is to simplify the modification process and to reduce the costs of producing quality pavements.
The main body of the research
The active development of the road construction industry in developing and developed countries alike, and the growing volume of road building, calls for a significant improvement in the quality of the materials used in laying asphalt pavements. Among the main components that determine the durability of the pavement, the asphalt mixture plays no small part. It has to withstand not only the dynamic loading from traffic but also to show resistance to sharp changes of temperature, to the effect of precipitation and to other aggressive factors of the external environment.
The key element determining the properties of asphalt mixtures is bitumen, which performs the function of the binding material, providing adhesion between the particles of the mineral components. However, as scientific research in this field has deepened, it has been found that the quality of the original bitumen does not always meet the necessary standards. This follows not only from the characteristics of the raw material itself but from the particulars of its production, especially where bitumens obtained by distillation at oil refineries are used.
As a colloidal system, bitumen consists of a variety of chemical compounds which have particular properties and form its structure. Depending on the quantitative ratio of these components, bitumen can have a different physical structure. The most widely accepted model of bitumen describes it as a colloidal solution in which large molecular structures (micelles) are linked to one another by weak hydrogen bonds. The micelles are formed from asphaltene molecules, which are the hardest part of the bitumen, while their outer surface is stabilised by resins. This provides the structural stability of the bitumen in the colloidal phase.
On the basis of this model, three main structural types of bitumen are distinguished: “sol”, “sol-gel” and “gel”. Each of these types has its own particular physical and chemical characteristics. For example, bitumens of the “sol” type are characterised by a high asphaltene content, which makes the structure of the bitumen more rigid and brittle. Bitumens of the “gel” structure, by contrast, have a thinner consistency, since they have a high content of oils and resins. The most suitable for use in producing polymer-modified bitumen (PMB) are bitumens of the “sol-gel” type, since they combine the best properties of both types and have balanced physical and mechanical characteristics.
Despite this, the process of modifying bitumens with polymers often runs into a number of difficulties. One of the main problems is the competition between the asphaltenes and the polymers for the maltene fraction of the bitumen, which is the principal dispersion medium of the modification. During this process the polymers introduced into the bitumen have to acquire a stable dispersed phase in a medium already occupied by asphaltene molecules. This significantly complicates the swelling of the polymer and reduces the effectiveness of the modification. It shows itself particularly in the case of bitumens obtained by the distillation method, which have a lower asphaltene content, and this makes them less suited to modification.
For bitumens to be successfully modified with polymers, the original bitumen has to have optimal properties. In particular it matters, by way of example, that the needle penetration depth at a temperature of 25 °C should be between 60 and 90 mm⁻¹ for the climate of Eastern Europe. However, the realities of the oil refining industry force producers to extract the light fractions of the oil to the maximum, leaving bitumen as a residual product, which often leads to a deterioration in its quality.
Oil refining technologies are constantly being improved, making it possible to extract to the maximum from the raw material the valuable light petroleum products — petrol, diesel fuel, aviation kerosene — which are far more costly than bitumen. Modern methods such as deep refining, hydrocracking and thermal cracking increase the yield of motor fuels considerably. A side effect of this process, however, is a deterioration in the quality of the bitumen, since it remains a residual product with altered structural characteristics.
Formerly bitumen was obtained predominantly by straight-run distillation, which gave it a balanced composition and stable in-service properties. Today destructive processes prevail in production, in which the most valuable components are removed from the oil, and the bitumen that remains is characterised by a low content of high-molecular compounds, an unstable structure and an increased tendency to age. Beyond that, oil refineries increasingly use raw material with a high paraffin content, which further reduces the elasticity of the bitumen and worsens its adhesive properties. Bitumens obtained by the distillation method without subsequent oxidation are particularly problematic. They interact poorly with polymers such as SBS, since they contain an insufficient quantity of the asphaltenes needed to form a stable network structure. As a result, polymer modification calls for additional plasticisers and stabilisers, an increased consumption of polymers, or combined and expensive solutions in order to reach the required in-service characteristics. In substance this makes the modification of bitumen extremely complicated and economically unjustified, since the financial outlay and the technical risks are not covered by the possible result.
In these conditions road authorities and producers of asphalt mixtures are obliged to look for effective additives and technologies that compensate for the shortcomings of modern bitumen and secure the durability of the pavement.
The market and the results of the research
In recent years demand for road bitumen in the countries of Europe, the CIS and Central Asia has grown significantly, especially in connection with the delivery of large-scale infrastructure projects such as the building of new roads and the reconstruction of existing transport arteries. If in 2017 the total requirement of the road industry for bitumens in one country alone – Ukraine – was around 350-400 thousand tonnes a year, then by 2021 this figure had grown to 1.2-1.4 million tonnes. The growth in demand for bitumen led to active imports of distillation bitumens from the countries of Southern Europe and Asia. These bitumens, however, often have properties that differ significantly from the requirements of the standards, especially during modification with polymers.
Over the course of 2021-2024 a group of competent experts carried out more than 90 tests of various samples of bitumen produced in Italy, Spain, Greece, Poland, Azerbaijan, Iran and other countries. The generalised results of this research revealed serious problems with the correspondence of the declared characteristics of the bitumen to the actual indicators, in particular for the softening point and the penetration. Moreover, no clear dependence was established between the initial characteristics of the bitumen and the results of the modification. Importantly, some samples did not undergo modification at all, even when the quantity of polymer and the time of the modification process were increased. Beyond that, the finished asphalt mixture sometimes showed unpredictable physical and mechanical properties, which called for constant correction of the modification process.
A further problem arising in the production of polymer-modified bitumen is the separation of the bitumen during storage and transport. This leads to a situation where, when the bitumen is used for laying the pavement, its properties may differ significantly from the original ones, which tells negatively on the quality of the asphalt mixture. Worst of all, staff at asphalt plants and laboratories very often have to take difficult decisions quickly, without having enough time for tests and laboratory trials.
An alternative to modifying the bitumen is the technology of modifying the asphalt mixture, as the end product, by the “dry-add” method, with a polymer additive such as “MPC” VELETON™ added directly into the asphalt mixer, which meets the requirements of ДСТУ 8959-2019, ГОСТ Р 58406.2-2020, СТ РК 2373-2019 and others.
This technology has long been established and is actively used by the road construction industry of the leading countries of Europe and Asia, and of the USA as well, and has certain advantages over conventional bitumen modification, namely:
- The quality and the physical and mechanical properties of the end product – the asphalt mixture – do not depend significantly on the quality of the bitumen or on its capacity to be modified with polymers. The reason is that a “dry-add” polymer additive works with the components of the mixture, specifically with the coarse aggregate, so the asphalt mixture gains strong polymer bonds between its components, which can be pictured as a 3D polymer lattice.
- The process of producing the asphalt mixture is significantly simplified, because no complex and costly equipment for polymer modification of bitumen is required, such as a colloid mill and high-speed mixers. The producer also saves a significant amount of time, because the bitumen can be used immediately, without the usual 8-12 hours of modification (as with additives of the SBS type).
- There is no need to heat the bitumen significantly (above temperatures of 165-170 °C), which does not lead to its rapid oxidation and ageing.
- Results from using the “dry-add” MPC additive are always predictable and calculated mathematically, depending on the type of asphalt mixture and the amount of polymer in it.
Table 1 — analysis of the use of the “dry-add” additive on the example of Ukraine in the period 2020-2024 shows the following results:
| Indicator | Asphalt mixture without polymer | Additive dosage 2.2 kg per tonne of mixture | Additive dosage 2.5 kg per tonne of mixture |
|---|---|---|---|
| Compressive strength R20 | 2.6 MPa | 5.2 MPa | 6.1 MPa |
| Compressive strength R50 | 1.24 MPa | 2.05 MPa | 2.4 MPa |
| Rut depth over 20,000 passes, Hamburg wheel method | 7.5-8 mm | 2-2.5 mm | 1.2-1.5 mm |
Table 2 — results of the comparison of experimental and theoretical data, determination of the number of load cycles to failure of the studied asphalt concrete samples with the additive. Temperature: T = +20 °C.
| Asphalt concrete designation | Number of cycles to failure Nт (theoretical) | Number of cycles to failure N (experimental) | Δ, % |
|---|---|---|---|
| B-20-6.0% | 413 | 478 | 13.7 |
| Additive dosage 2.2 kg/tonne of mixture | 3843 | 4438 | 15.4 |
| Additive dosage 2.5 kg/tonne of mixture | 6379 | 7368 | 15.5 |
The data obtained (Table 2) indicate that the theoretical calculations agree with the experimental data. This shows that these dependencies can be used in practical calculations in order to secure or to forecast the durability of asphalt pavement on roads and on bridges under the effect of temperature and of the loading from vehicles, and also to forecast its changes during service over a given period of time.
Experience of use by leading road construction companies in the countries of Europe, the CIS and Central Asia has shown high and, most importantly, repeatable results, high resistance to loading, frost resistance and shear resistance of asphalt pavements. The test results show that the asphalt mixture meets the requirements of ДСТУ 8959-2019, ГОСТ Р 58406.2-2020, СТ РК 2373-2019 and others on every indicator.
Examples of the active use of dry-add additives
Turkey, the D-100 motorway, the Istanbul — Izmit section
On this section the technology of direct (dry) addition of the polymer additive into the asphalt mixer without prior modification of the bitumen was used. This made it possible to:
- Reduce rut formation at an air temperature of +45 °C to less than 2 mm (on the results of dynamic testing);
- Raise the shear strength of the pavement by 60–70% compared with sections where conventional bitumen modification was used;
- Save 7000–9000 euros a week through reduced energy consumption and the elimination of the bitumen homogenisation stage.
The technology was delivered by one of the leading contractors in Turkey and approved by the national road laboratory. The experience confirmed that it applies in regions with increased temperature loading and high traffic intensity.
Georgia, road sections in Tbilisi, “Avlabari” square
The technology of “dry-add” of the “MPC” VELETON™ polymer additive into the asphalt mixer without prior modification of the bitumen was used, on the basis of the JEO GROUP plant. This made it possible to:
- Reduce rut formation at a surface temperature of up to +65 °C to 1.5 mm;
- Raise the shear strength of the pavement by 60–70% compared with sections where conventional bitumen modification was used;
This section, with the polymer additive applied by the “dry-add” method, has been under the observation of specialists from the Municipal laboratory for more than 3 years, with no remarks on the pavement.
“Overall, the test results complied with the specifications, and no distress or failure was recorded after 7 years of being under service. This could indicate that plastomers and the dry method can be considered as reliable alternatives for high quality asphalt pavement production.”
(source researchgate.net)
Kazakhstan, Astana, Tlendiyeva Street
On this road section the road services carried out medium repair with milling at least 2 times a year, because the left lane is a braking section for heavy goods traffic ahead of the lights and the design loads exceed the permissible ones by more than 2 times. The technology of “dry-add” of the polymer additive into the asphalt mixer without prior modification of the bitumen was used for the lower layer (dense, coarse-grained, A-40) and the upper layer (dense, SMA-20). This made it possible to:
- Not carry out medium repair of this section for more than two years now;
- Raise the shear strength of the pavement by 70–90% compared with sections where conventional bitumen modification was used;
Ukraine, Kyiv, the “Northern” bridge
Previously the road services carried out routine repair of the upper layers on this site at least once a year. This is connected with the high loading on the carriageway and the increased temperature of the upper layer (in the summer period).
The technology of “dry-add” of the polymer additive into the asphalt mixer without prior modification of the bitumen was used for the upper layer (dense, fine-grained A-20).
At present the site has been in service for more than three years without any visible defects and is under quarterly observation by the city's road services.
Economic and time costs
In modern road construction, economic and time costs play a key role, especially in the work of an asphalt plant. The cost of materials, energy and logistics bears directly on the profitability of the project, and delays in production can lead to paving schedules being missed and to a rise in the costs of plant and labour.
Optimising production processes, choosing economically effective modifiers and managing resources rationally make it possible to reduce the cost of the mixture without loss of quality. Under tight construction deadlines it is important to minimise the time spent heating the bitumen, dosing the components and transporting the asphalt concrete, so as to secure its laying in the optimal temperature regime and to avoid overconsumption of materials.
1. Energy consumption and the modification technology
Modification of bitumen with SBS polymer or a similar one. When SBS polymer (or a similar one) is used, the bitumen is first heated to 180–190 °C (sometimes higher) and the polymer is introduced in mixing plant. This calls for significant energy consumption to heat the equipment (the reactor, the mixers, the bitumen storage) and to hold the high temperature throughout the modification process. Additional time and energy are also often required to cross-link (homogenise) the SBS with the bitumen. The polymer-modified bitumen (PMB) thus obtained then has to be stored at raised temperature, which continues to “consume” energy and creates a risk of thermo-oxidative degradation of the polymer during a long hold.
“Dry” modification of the asphalt mixture. With “dry-add” the polymer (in granule form) is added directly into the mixer during production of the asphalt mixture, together with the mineral materials. The temperature of the bitumen does not change relative to ordinary asphalt (without polymers) and is around 155–160 °C, and no separate process of homogenising the bitumen with the polymer is required, since the “dry-add” polymer additive is mixed evenly through the asphalt mixture, melts on contact with the heated stone materials, and then binds all the components of the asphalt mixture to one another.
This has a very positive effect on the properties of the bitumen, since no oxidative or thermo-destructive processes take place, and the aromatic hydrocarbons remain in the composition of the bitumen rather than “burning off”, as they do during modification with additives of the SBS type. This makes it possible to preserve the ductility and the plasticity of the bitumen.
From the energy point of view there is no additional cycle of high-temperature mixing of the bitumen with the polymer. The total outlay on holding temperatures and on mixing is therefore significantly lower than in the classic PMB variant.
2. Time costs
Modification of bitumen with SBS polymer or a similar one. A separate stage is needed at the plant (or within an external installation) to prepare the PMB: heating the bitumen (4-5 hours), dosing, homogenisation (6-8 hours), holding/thermal stabilisation (4-6 hours), quality analysis. All these actions lengthen the technological cycle by no less than 8-12 hours and require exceptionally highly qualified staff. If the PMB is stored badly (the temperature regime not observed), separation and loss of properties are possible.
“Dry” modification of the asphalt mixture. In this technology the modification stage is built directly into the asphalt production process. The polymer is introduced into the mixture at the same time as the mineral components (the coarse aggregate and the filler) by means of a special high-precision doser. This makes it possible to do without a separate stage of pre-mixing the polymer with the bitumen. This approach answers the current requirements of the road construction industry, rests on modern technological solutions, excludes the influence of the human factor and reduces the risk of errors during modification. As a result, the process becomes more effective, the time and financial costs of modification are eliminated, and quality control becomes more stable and predictable.
Conclusion
At large-scale production (several hundred or thousand tonnes of asphalt concrete a day) the difference in energy consumption between modifying the bitumen (with additives of the SBS type or others) and “dry-add” of the polymer at one plant can reach 9,000 - 11,000 dollars a month, which over a construction season turns into a significant sum.
Beyond that, “dry” modification of the asphalt mixture, in which granulated additives are introduced directly into the mixer together with the mineral materials and the bitumen, makes it possible to simplify the production process at asphalt plants considerably. This approach removes the need for a separate stage of preparing modified bitumen, which in turn does away with the cost of additional equipment for introducing, dosing and homogenising the polymer in the bitumen. The need for prolonged mixing and holding of the bitumen at high temperature to dissolve the additive falls away.
This makes it possible to save a significant amount of time and to reduce operating costs. The plant does not need to keep separate bitumen tanks in reserve for different types of binder (ordinary and modified), which is particularly relevant for small and mobile production units. Energy consumption falls, and material losses in changing from one batch to another are reduced.
Applying the “dry method” of modification also tells positively on the laying properties of the asphalt mixture. Such a material compacts more easily under standard technological regimes thanks to the even distribution of the modifier right through the body of the mixture. This reduces the risk of voids, tears and uncompacted zones forming in the finished pavement, which bears directly on the durability and the strength of the road.
Beyond that, “dry” modification reduces the likelihood of technological errors which often arise when modified bitumen is prepared at the plant. In the conventional technology, problems such as these are possible:
- incomplete or uneven dissolution of the polymer in the bitumen;
- overheating and thermal destruction of the polymer structure;
- separation and instability of the resulting binder during storage;
- confusion in working with several types of bitumen.
With the “dry method” these risks are reduced to a minimum, since the modification takes place directly in the process of producing the asphalt mixture, without complex operations on the bitumen facility. This makes the technology more stable, more reproducible and more convenient for plants.
Summing up
The question of modifying bitumens and asphalt mixtures is becoming ever more pressing, since not all bitumens have a sufficient capacity to be modified with polymers or the necessary quality. This pushes producers to look for alternative methods of significantly improving the properties of asphalt concrete, taking the particulars of the components of the mixture into account. One of the proven and effective solutions is adding polymer additives directly into the asphalt mixer.
This method has a number of substantial advantages over conventional modification of bitumen with polymers. First, the quality of the modification becomes independent of the technology by which the bitumen was produced, which matters particularly when working with distillation bitumens that interact poorly with polymers. Second, time and energy resources are saved, since the need for prolonged heating and homogenisation of polymer-modified bitumen is eliminated. Third, the risk of thermal ageing of the bitumen falls, since the temperature to which it is heated does not exceed the working values in the asphalt mixer. As a result, producers obtain more predictable and stable properties of the asphalt mixture, which simplifies quality control in production and reduces the risk of pavement defects arising.
With the active support of leading producers of asphalt mixtures, of specialist research institutes and of the engineering community, the “dry-add” technology for polymer additives has not merely proved itself but has established itself as a full and highly effective method of bitumen modification — one that does not yield to, and on a number of key parameters surpasses, the classic bitumen modification technology. That is why it is applied ever more widely as the optimal solution, answering the industry's current requirements for quality, technological soundness and economic effectiveness.
“The dry process modification is characterized by simplicity, flexibility, low energy consumption, and low pollution, making it an ideal choice in pavement repair engineering.”
(source sciencedirect.com)
Co-authors of the scientific and practical work: Sergey Parfyonov, expert practitioner in the field of polymer modification, and Levan Gvelesiani, head of the central road laboratory (Tbilisi, Georgia).
The work was prepared with the participation of KazDorNII, the National Transport University, the National Association of Road Workers of Ukraine, SE “Dortsentr” (Kyiv), Veleton and AYITI.UZ (Uzbekistan).