modified bitumen. Modified polymer-bitumen binders: a new round in the evolution of the roadway. The term "polymer bitumen"
To supplements whose ultimate function is to improve quality roadbed, include the following types of materials:
1. Bitumen modifiers;
2. Adhesive additives to bitumen;
3. Stabilizers SCHMAS (crushed stone-mastic asphalt mix);
4. Asphalt mix modifiers;
5. Structuring additives in asphalt mix;
An even more difficult factor is the growth in freight traffic. Our road system is filled with trucks. Modifier Types Modified asphalts are often commonly referred to as "polymer modified asphalts". Polymers are probably the most common type of modification, but modern modified asphalts can be made in several ways. According to the Asphalt Institute's Asphalt Handbook, modifiers and additives used to improve performance include polymers, chemical modifiers, fillers, oxidizers and antioxidants, hydrocarbons, and anti-dust additives.
At low temperatures bitumen becomes hard and brittle, at high temperatures it becomes soft and fluid. Bitumen is modified with various elastomers, the use of which is intended to improve the performance of bitumen at extreme temperatures. When bitumen is modified with styrene butadiene styrene, the polymer bitumen mixture becomes soft and more flexible at low temperature, and more viscous at high temperature.
"Polymers" cover a wide range of modifiers, with elastomers and plastomers being the most commonly used types. Styrene-butadiene rubber and styrene-butadiene-styrene often use elastomers. These modifiers are used to reduce gauge and increase resistance to fatigue and thermal cracking.
Rubber rubber is an elastomer made from dirt tires. There are several technologies for using a rubber tire. This material is used primarily to solve the rut problem. Plastomers are used to improve the high temperature properties of modified materials. Low density polyethylene and ethylene vinyl acetate are examples of plastomers used in asphalt modification.
The use of various bitumen additives is aimed at improving a particular property of bitumen: adhesion with the mineral component of the asphalt concrete mixture or plastic fluidity at low temperatures. There is an additive that prevents bitumen oxidation and, as a result, the formation of microcracks on the surface.
SCMA stabilizers are usually understood as fiber additives that are used to prevent bitumen from running off the mixture during transportation and installation. Stabilizer fibers are evenly distributed in the mixture during mixing, acting as a microscopic frame that reinforces the bitumen.
The most commonly used chemical modifier is polyphosphoric acid. This modifier can be used in combination with polymers to increase high temperature stiffness. Other modifiers that may be used include asphalt aggregates and hydrocarbon materials. Hydrocarbons can give either hardening or softening effects. Materials can be added to increase crack resistance. Emollients or rejuvenators are used to reduce the viscosity of compressed asphalt binders in blends containing recycled asphalt pavement.
Also on the Russian market there are asphalt concrete modifiers based on crumb rubber. Similar modifiers are introduced into the mixer during the preparation of the asphalt concrete mixture, in contrast to bitumen modifiers, the improvement of bitumen by which is a laborious and energy-intensive process. Under the structuring additives understand the mineral powder or cement.
The positive effectiveness of modified asphalts is widely recognized and demonstrated in laboratory tests, but there was little documentation available to improve fieldwork. The study by the Asphalt Institute shown here is an example of one of the efforts to document performance benefits. Although this study only dealt with polymer-modified asphalts, it provides documentation for the performance benefits of this type of modifier and how other types can be evaluated and documented.
Properties and characteristics of modifiers
SBS (styrene-butadiene-styrene) and other styrene copolymers (SBR, SEBS, SIS, SEPS, etc.)
Highly elastic polymers (synthetic rubbers) that give bitumen flexibility at low temperatures. SBS - coatings have excellent adhesion and high resistance to sudden temperature changes (with the transition through 0°C). In addition, SBS-materials are highly elastic, frost-resistant, and also easily follow the shape of the surface on which they are laid. Its heat resistance is somewhat worse than APP, however, when using a high-quality SBS modifier, it can reach 100°C. SBS is the most common bitumen modifier.
The full report of this study, "Quantification of the Effects of Polymer Modified Asphalt for Paving Distress Reduction" is available from the Asphalt Institute as a technical report. An abridged version is also available in Information Series 215. The disasters included in study performance predictions and comparisons are spalling and fatigue cracking. Cross cracking was also included. Comparison of thermal cracking and stripping was not included as too little data was available.
When modifying bitumen with SBS, a polymer matrix is created, which in this case is a three-dimensional network formed due to the interaction of polystyrene blocks into the so-called polystyrene domains. Bitumen is distributed inside this elastomeric network in the form of a fine dispersion.
The production of high-quality SBS-modified bitumen is extremely difficult. technological process. To do this, it is necessary to use SBS-compatible bitumen, which is characterized by a high content of aromatic compounds. Actually, SBS can be used either in the form of a finely ground powder or in the form of granules.
Although the benefits of using modified asphalts are widely recognized, not all modifications asphalt mixes or treatments need to be modified. Each application must be evaluated to determine if the traffic load, life expectancy, environmental conditions, and desired performance justify the use of modifiers. Modified asphalts can be a good investment.
Rheological evaluation of polymer modified asphalt binders. Currently, the most used polymer for asphalt binder modification is thermoplastic elastomer styrene-butadiene-styrene and aromatic oil, usually added to mixes to improve their compatibility. This article proposes the use of sedimentary shales as a compatibilizer for a polymer-modified asphalt binder. The mutual elastic behavior of the samples confirmed the results for the classical properties and varied depending on the morphology and composition of the sample.
In the first case, it is possible to obtain high-quality mixtures on conventional mixers. In the case of using granulated SBS, it is necessary to have a homogenizer - a device that "grinds" the polymer with bitumen. Without a homogenizer, the mixture is inhomogeneous (non-homogeneous). The heat resistance of such an inhomogeneous mixture may sometimes even be somewhat higher, but the flexibility in the cold will be significantly worse and will begin to deteriorate steadily over time.
Modification of road bitumen
Thus, shale oil can be successfully used as a compatibility agent without loss of properties or even as a substitute for aromatic oil. Asphalt binders are widely used in pavement 1-5 and their viscoelastic properties depend on their chemical composition. The use of synthetic polymers to change the characteristics of binder asphalt and extend the life of bridge structures dates back to the early 70s 6-8. These polymers are thermoplastic rubbers and have a two-phase morphology, a glassy phase of the polystyrene terminal blocks, with a glass transition around 100°C, and an elastomeric phase due to the polybutadiene center blocks.
As experience shows, it is flexibility that can serve as an indirect criterion for the quality of an SBS material. Materials in which a high quality SBS modifier is used (for example, from SHELL) achieve flexibility down to -30°C. At the same time, materials with flexibility worse than -20°C either have an insufficient concentration of polymer in bitumen, or they use bitumen incompatible with SBS. In practice, such materials break down rather quickly.
On the other hand, since the molecular weights of the polymer chains are higher or similar to molecular weights asphaltenes, they compete for the solvency of the maltene fraction and phase separation can occur if there is an imbalance between the components. Phase separation indicates incompatibility between asphalt and polymer and can be prevented by adding aromatic oils 7, 12 or compatibilizers to the mixture.
The need to modify bitumen with polymers
Aromatic oil is a petroleum fraction supplied by refineries and its availability may be reduced in the next few years as there is a high demand for lighter petroleum fractions and therefore must be cracked to increase the production of high value petroleum products. Currently, non-petroleum sources are considered alternative routes for the production of petroleum products. Sedimentary shales contain bituminous materials, which are released as oil-like liquids when the stone is heated in chemical process pyrolysis.
modified bitumen
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Materials in road construction
modified bitumen
Steady growth in the number of trucks and axle loads creates challenging operating conditions highways, especially those built on the basis of organic binders, and require appropriate countermeasures. The main problem arising from high axial loads and high traffic intensity, - deformation asphalt concrete pavement highways built using conventional bitumen.
Extracting oil shale from pyrobitumen is more difficult than conventional oil extraction and is currently more costly. Due to its chemical nature, bitumen coming from shale oil obtained from pyrolysis is excellent material for the production of asphalt additives. It can be used to produce broad spectrum asphalt additives, provide adhesion, weather resistance and permanent deformation resistance, increase wear resistance and durability.
Shale oil is less volatile and has less aromatic hydrocarbons than aromatic oil, favoring the polymer binder mixture at high temperature. Shale oil contains basic nitrogen and a small amount of asphaltenes, which are more saturated compounds than aromatic oil. The classical and rheological properties of modified asphalt binders were determined to test whether any improvement could be achieved through the use of shale oil.
To improve the reliability and durability of coatings, it is currently recommended to use bitumen modified with polymers. The widespread use of modified bitumen instead of conventional bitumen is due to their improved properties. Polymeric bitumens have a wide operating temperature range (difference between softening temperature and brittleness temperature) - up to 100 °C (ordinary bitumen up to 60 °C).
All asphalt-polymer mixtures were prepared in a suitable container equipped with a high shear mixer. For the compatibilizer runs, shale oil or flavor oil was added to hot asphalt prior to polymer addition at 500 rpm and the mixture was mixed for 10 minutes.
Bitumen modification plant
All asphalt binder samples were analyzed using the following classical methods: permeability, ring and ball softening point, Brookfield viscosity, storage stability, and recovery of elasticity using a ductility tester. The elastic recovery test evaluates the ability of a binder to stretch and recover elastically.
Asphalt concrete prepared using polymeric bitumen has a high resistance to deformation due to the high elasticity of the bitumen used. In addition, the aging process of asphalt concrete is significantly slowed down. Studies by foreign scientists show that bitumen extracted from pavement, which have served for 10 years, there are no significant changes in viscosity.
Aging binders were analyzed using a dynamic shear rheometer to assess changes in their rheological properties. The deformation amplitude was limited by the linear viscoelastic reaction of the asphalt binder. It contains elastic and viscous components, designated storage modulus and loss modulus; respectively. These two components are related to the complex modulus and to each other through the phase angle δ, which is the phase or delay time between applied shear stresses and shear strains during testing.
To improve the properties of road bitumen (modifications), it is customary to use specially made artificial materials. Currently, due to the variety of artificial materials offered by petrochemical industries, there is a wide choice of polymers used for modification. Conventionally, they can be classified as thermoplastics (plastomers); elastomers and thermoelastic synthetic materials.
When the phase angle δ is 90°, the material can be considered purely viscous in nature, while a phase angle δ of 0° corresponds to purely elastic behaviour. Between these two extremes, material behavior can be considered viscoelastic in nature, with a mix of viscous and elastic responses.
The term "polymer bitumen"
All asphalt binder samples in this study were subjected to the same heat treatment to avoid any difference in their properties caused by the high temperature used during their preparation. There was no modification to the process conditions when the shale oil was added to the asphalt binder, and there was no change in the viscosity of the mixture.
Thermoplastics consist of linear or lightly branched polymers that soften when heated. When cooled, they become solid again. The addition of plastomers increases the viscosity and stiffness of bitumens at normal operating temperatures (from -30°C to 60°C). But plastomers do not affect the elasticity of modified bitumen.
The light areas in the former were much more uniform and transparent, demonstrating the superior efficiency of the shale oil in dispersing the polymer in the asphalt. This result suggests that the effect of shale oil composition on polymer swelling should be further investigated.
It can be seen that there is a decrease in penetration and an increase in softening point with the addition of polymer and a reciprocal effect on these properties with the addition of oil. As expected, the modified asphalt samples had reduced penetration and a higher softening point and elastic recovery, indicating an improvement in the stiffness and flexibility of the asphalt binder. Thus, these modified patterns are more suitable for paving as they will be more resistant to deformation, fatigue and plastic deformation.
When heated with plastomer-enhanced bitumen, there is a tendency to separate the bitumen and polymer phases, i.e. such bitumens are unstable to storage, therefore they must be prepared immediately before use in an asphalt concrete plant. The most commonly used plastomers are polyethylene and atactic (stereo random) polypropylene.
Elastomers consist of long polymer chains with wide branches. They are elastic in a wide range of temperatures: from low to 200 °C.
When elastomers are added to bitumen, its viscosity increases and elasticity improves. But these systems are also unstable during storage, constant mixing is required to prevent phase separation between bitumen and artificial material. Bitumen modified with elastomers can be called elastic filled bitumen. Natural or regenerated rubber and polybutadienes are commonly used as elastomers.
Thermoelastic plastic materials soften at temperatures above normal operating temperatures and deform well in this state.
Thermoelastic artificial materials have been used since 1965. The most famous representative of the group of thermoelastic plastics is styrene-butadiene-styrene (SBS). This artificial material is a block polymer consisting of blocks of styrene and polybutadiene.
The addition of this material to bitumen is typically 3 to 6% by weight. The required amount of added material depends on the dispersed state of the introduced substance: if SBS is introduced into bitumen in a finely dispersed form, then the consumption decreases, if in a coarsely dispersed form, then a large amount of modifier is required.
In addition to polymers, other modifiers can be used to improve the properties of bitumen: inorganic salts (manganese chloride), synthetic or natural resins, as well as natural asphalts.
Final Properties modified bitumen largely depend on the technology of introducing the additive.
Abroad, modified bitumen is produced using special technologies at oil refineries or in special enrichment plants with constant laboratory quality control of the product. Polymeric bitumen is a product ready for transportation, storage and processing.
In Europe, styrene-butadiene-styrene is most often used to modify bitumen. The polymer is administered as a solid (granules or powder) as well as a liquid (emulsion or solution). In any case, it is necessary to achieve homogeneity of the final material.
To obtain mixtures that are stable during storage, it is necessary to select the appropriate base bitumen. The mixture is suitable for storage if, during long-term storage of hot bitumen in the tank of the asphalt mixing plant, phase separation does not occur. Modern polymeric bitumen can be stored for up to 6 weeks.
Traditionally, two methods are used for the production of modified bitumen: - preparation of a bitumen-polymer dispersion in mixers with high shear forces (colloidal mills); – introducing the polymer into the bitumen chemically using slowly rotating agitators with low shear forces.
In the first case, sulfur and its compounds are used to stabilize the polymer-bitumen dispersion. Reactions take place between the polymer and sulfur, as a result of which new chemical compounds are formed, they remain evenly distributed in the bitumen due to their lattice structure.
In the second case, the polymer (eg SBS) is pretreated in such a way that it appears to be dissolved in the bitumen.
The disadvantage of colloid mills is the tendency to separate the macromolecules of the base material, so that eventually in the bitumen after processing there will be polymers with a lower average molecular weight than at the beginning. This is explained by the fact that large shear forces that occur in colloid mills lead to a change in the molecular structure of the polymer.
Low shear mixers achieve higher softening points and much more plasticity of the modified bitumen.
When using polymers that cannot be combined with the chemical system of bitumen (polyethylene, atactic polypropylene and natural rubber), plants are needed to prepare modified bitumen directly on asphalt plants so that the prepared material can be used for the preparation of asphalt concrete before phase separation.
For the processing of polypropylene and natural rubber or reclaimed rubber, low speed agitators are recommended. In this case, the mixing of bitumen with the polymer occurs due to the melting of the polymer. To obtain modifications based on polyethylene, agitators with high shear forces are used, which can provide a dispersed distribution of polyethylene in bitumen.
In the Republic of Belarus and in the conditions of Russia, the use of plastomer additives is not recommended due to climatic conditions. With strong cooling and prolonged frosts, asphalt concretes based on such binders are subject to severe cracking.
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