The effect of carbon nanotubes on the properties of asphalt concrete

. The method of nanomodification was reviewed and its effect on the properties of polymer-bitumen adhesives (PBA) and asphalt concrete prepared on their basis was studied. It was determined that nanomodified PBA is less sensitive to aging processes, which is the result of the peptization of asphaltene-resin complexes in the structure of modified adhesives and their construction with polymer matrix. It has been found that the nanotubes used as modifiers act as a tailoring agent and an inhibitor of aging processes in PBA. It has been determined that when nano-carbon tubes are applied in a complex form with MasterLife PVA 100, a surface-active agent, the adhesion capacity of bitumen increases and, accordingly, the operational properties of asphalt concrete improve. Studies have shown that the most important process in the application of nanomodifier is ensuring its uniform distribution in the volume of bitumen. The process of diffusion of carbon nanotubes in bitumen is carried out at a temperature of 100°C with the presence of a plasticizer. As a result of this operation, it is possible to achieve a uniform dispersion and distribution of nanoparticles in the volume of the carrier medium (plasticizer), which allows to the intensification of the processes of incorporating carbon nanotubes into the composite matrix. Achieving high performance of nanomodified PBA against layering and aging is explained by the processes occurring at the micro and nano levels. Along with the use of plasticizers, ultrasound is also used to introduce nanotubes into the bitumen to break up the nanotube agglomerates (entangled balls of hundreds of carbon nanotubes of different sizes held together by Van der Waals forces) and ensure their uniform distribution in the bitumen. The effect of nanomodified PBA asphalt concrete on strength and deformation properties was studied. It was found that the use of modified adhesives in asphalt-concrete mixtures increases the water resistance, heat resistance, and shear resistance of asphalt concrete.


Introduction
In modern materials science, nanotechnologies are part of the successful and progressive development of all areas of construction.Analysis of innovative processes in construction technology and materials science shows that materials and technologies based on knowledge in the field of nanotechnology will dynamically enter production practice in the next 10-20 years [1][2][3][4].Today, the application of nanotechnological compositions to improve the functional properties of building materials and products is an innovative direction in construction science and production.At present, asphalt concrete is used as the main structural material in the country's highways.Unfortunately, asphalt concrete pavements are not resistant to cracking, cutting, and breaking.This is due to their sensitivity to changes in the temperature of the external environment, which serves as a source of continuous structural changes that occur together with the mechanical effects of traffic flow.Such effects almost continuously lead to changes in the plasticity of asphalt concrete and its adhesive, including damage to the integrity of the structure -cracking, as well as loss of deformation stabilityslips, flows, breaks, etc., causes.
To date, numerous studies have been conducted in order to improve the quality of asphalt concrete coating [5][6][7][8][9].Analyzing these, it can be concluded that one of the most promising directions in improving the operating characteristics of asphalt concrete is the modification of raw materials and organic adhesives.Taking into account the role of bitumen as a solvent structure former in the composition of asphalt concrete and the dynamically increasing load on the coating, it is necessary to systematically approach the improvement of the quality of organic adhesives.This can be achieved by purposefully improving the properties of bitumen using various modifiers.Polymer additives, antioxidants, nanomodifiers, surfactants, and complex additives can be mentioned as modifiers [7][8][9].Researchers are more interested in nanostructured additives [1,10].This method allows the creation of new-generation technologies and materials.
The current research work is devoted to improving the properties of carbon nanotube organic adhesives and asphalt concrete with their application.

Materials and research methods
Here is BNB 50/70 brand oil bitumen TS AZ 3536601.242-2015(production of Baku oil refinery named after H.Aliyev); Europrene SOL T 6302 brand styrene-butadiene block copolymer -from SBS (produced by the Ravenna plant in Italy); MasterLife PVA 100 (manufactured by BASF) was used.Europrene SOL T 6302 styrene-butadiene block copolymer has a specific density of 0.94 g/cm3, a tensile strength of 20 MPa, Brookfield viscosity of 4.0 Pa.s.The density of MasterLife PVA 100 is 0.93 kg/l, pH=8, the boiling point is 3500C, flash point is above 1600C.Polycrystalline multi-walled carbon nanotubes (Taunit) in the form of black powder, brought from Tambov, and multi-walled carbon nanotubes (MWCN) synthesized from hydrocarbon gas raw materials in the 28th laboratory of the Institute of Catalysis and Inorganic Chemistry of ANAS were used as nanoparticles.
In laboratory conditions, mixing of the modifier with bitumen was carried out at a temperature of 160°С.PBY softening temperature is determined according to the К and S (GOST 11506-73) method.Brittleness temperature is measured according to Fraas (GOST 11507-78).The depth of needle penetration at 25°С is determined according to GOST 11501-78.Adhesion with mineral fillers is determined according to GOST 11508-74.The stability of bitumen, which is evaluated according to the change of quality indicators during long-term storage at high temperatures (1630С, 5 hours), is carried out according to GOST 18180-72.

Results and discussions
In the experiments, based on previous studies, PBA obtained by applying Europrene SOL T 6302 styrene-butadiene block copolymer was used [7][8][9].MasterLife PVA 100 was used as a surfactant to ensure that PBA had high adhesion.By applying these modifiers, the most rational and effective binding compositions were selected, and the modified PBA production technology was developed using a carbon nanotube-based nanomodifier.In general, the most important process in the application of a nanomodifier is to ensure its uniform distribution in the composition of the mixture.The process of dispersing nanoparticles in bitumen is carried out at a temperature of 100°C with the presence of a plasticizer.As a result of this operation, uniform dispersion and distribution of nanoparticles in the volume of the carrier medium (plasticizer) is possible and allows for to intensification of the processes of incorporating carbon nanotubes into the composite matrix.
One of the main properties of bitumen to improve many operational properties of asphalt concrete is to increase its adhesion to fillers.To study the effect of nanocarbon tubes on bitumen adhesion ability, bitumen is filled in a glass equipped with a heater and thermometer and melted at 120oC.Then the mixer is turned on and the plasticizer is added to the bitumen along with the necessary amount of nano additives.Mix at this temperature for 40 min.during mixing and the adhesion of prepared samples to mineral, fillers is determined.
Adhesion is aimed at reducing the surface energy of the system as a result of interphase interaction.The quantitative measure of the adhesion process of bitumen with the surface of mineral material is called adhesion work.The higher the price of adhesion work, the higher the adhesion of bitumen with mineral material [11].
The Dupré-Young equation can be used to calculate the work of adhesion [11].
Here: Wa -work of adhesion, mN/m2; σ1.2 -bitumen air surface tension, mN/m2; ϴ -the wetting angle of the glass plate.To determine the wetting angle, a drop of bitumen was placed on a glass plate and its picture was taken with the help of a microscope [12][13][14].As can be seen, the wetting angle of bitumen increases significantly when SAS is added together with nanocarbon tubes.This contributes to the increase in adhesion.The composition and adhesion properties of the bitumen samples are given in Table 1.As can be seen from Table 1, the adhesion performance of bitumen is slightly increased when adding MWCN to bitumen, but when Masterlife PAV 100 SAS is added together with MWCN, the adhesion of PBA to the mineral part of asphalt concrete increases significantly.
Two samples of modified PBA, as well as a control series without nanomodifiers, were prepared to verify the theoretical initial conditions and the obtained laboratory dependences.The composition of PBA is given in Table 2.In the research process, a comparative assessment of the effect of nanotubes was carried out, and the standard indicators of the quality of nanomized PBA, and their resistance to stratification and aging were studied (Table 3).Also, the effect of purchased PBA on the quality of asphalt concrete was studied (Tables 4, and 5).When examining the quality indicators of PBY (see Table 3), it was determined that control sample No. 3 prepared without nanomodifier does not meet the requirements of GOST R 52056-2003 according to indicators such as softening temperature at 0°C, penetration (conditional viscosity), elongation and elasticity.In the control samples, there is a tendency to delamination (after the delamination test, the difference in the upper/lower brittleness temperature of the pipe is 8°C, and the softening point is 13°C).PBA is characterized as a low-quality adhesive due to the insufficient amount of polymer required for the formation of a solid spatial polymer network.
However, the quality of PBA prepared with the use of nanomodifiers of similar compositions significantly exceeds the control sample and meets the requirements of GOST and EN for all parameters.Important indicators such as resistance to stratification and aging can be specially noted.
The results of all studies conducted on samples 1 and 2 showed that PBA with added carbon nanotubes meets all quality indicators the standard.
It can be assumed that nanomodified PBA has high indicators against layering and aging, which is explained by the processes occurring at the micro and nano levels.Ultrasound used to introduce nanotubes into bitumen allows for to break up of nanotube agglomerates (entangled balls of hundreds of carbon nanotubes of different sizes held together by Van der Waals forces) and distribution them evenly in the bitumen.Also, the specific surface area of carbon nanotubes increases significantly.It is clear that the nanoparticles included in PBA enter the structure of asphaltene-resin complexes (ARC) and play the role of a kind of bridge when connecting dispersed aggregates of asphaltenes and polymer molecules.Thanks to this, it is possible to bring the particles closer to each other in the bitumen-polymer adhesive.
This hypothesis was reflected in the study conducted with an electron microscope (Fig. 2).As can be seen from the microstructure of the unmodified adhesive, it has a homogeneous granular structure composed of larger ARC resembling a nail (Fig. 2, c).When adding the nanomodifier, a unique reinforcement effect is observed.The taunite carbon nanotube structures ARC around itself, during which their peptization occurs (Fig. 2, a).At this time, the presence of a convex regular structure of ARC slows down the PBA aging process and minimizes the stratification process.A "comb-like" layered structure, which is not clearly observed, is formed when the MWCN nanotube modifier is added (Fig. 2, b).The formation of such a structure has a positive effect on the properties of PBA.The arrangement of ARC structure with nanoparticles increases PBA cohesion strength and flexibility, which determines the intermolecular interaction force.It is clear that the nanotubes act as an aging inhibitor and cross-linking agent resulting from ongoing ARC peptization processes in the PBA structure and their cross-linking with the polymer matrix.As a result, it becomes possible to obtain PBA with effective properties.It should also be noted that an interesting regularity was identified when comparing the results related to the properties of nanomodified PBA.Nanotubes have a strong influence on the heat resistance and low-temperature performance of the adhesive, which is reflected in the final product, asphalt concrete.
The results of the effect of PBA on the properties of asphalt concrete are given in Table 4.In order to eliminate the effect of the error in the dosage of mineral materials and to eliminate their inhomogeneity in the volume of one batch, crushed stone, slag, and mineral dust were separated into separate fractions, and then the mineral part of polymer asphalt concrete mixtures was compiled according to table 4. The content of the adhesive in all studied polymer asphalt concrete samples was 5.5%.
Asphalt-concrete samples were prepared based on the selected mineral composition and their physical and mechanical properties were studied.The results of the study are given in Table 5. Apparently, the use of nano-modified adhesives in polymer asphalt-concrete mixtures contributes to a significant increase in the waterproofing and deformation resistance of asphalt concrete.Without the use of additional adhesion-enhancing and cross-linking additives, the water resistance of samples No. 1 and 2 increases by 17.4% and 20.6%, respectively, compared to the control sample of binders.A clearer dynamic of changes in PBY water resistance can be observed after long-term water saturation.Thus, the indicator of long-term water resistance for sample No. 1 was 3.8%, for No. 2 -1.8%, and for the control sample No. 3, the decrease indicator was 12.2%.Thus, the obtained results show that the nanomodified polymer asphalt-concrete composition has high resistance to the aggressive effects of atmospheric precipitation and sewage.
The heat resistance of nanomodified polymer asphalt-concretes increases significantly, their sensitivity to temperature changes (R0/R50) decreases, and it is possible to control the strength properties of the composite.

Results
Thus, it was determined by the research conducted that polymer-asphalt concretes prepared using nano modifiers will have a set of indicators that help to significantly extend the service life of the road surface without premature degradation, which will make the roads safer for all users.

Fig. 1 .
Fig. 1.A nanocarbon tube on a glass plate and surface-active substance (SAS) added together with a nanocarbon tube bitumen wetting angle: a-nanocarbon tubes bitumen; b-bitumen with nanocarbon tubes and with 0.2% SAS, c-bitumen with nanocarbon tubes and with 0.4% SAS.

Table 1 .
Composition and properties of bitumen mixture

Table 2 .
Composition of PBY samples

Table 3 .
Physical and mechanical properties of PBA

Table 4 .
Granular composition of polymer asphalt-concrete mixture

Table 5 .
Properties of polymer asphalt-concrete