The Secret of Road Pavements: How to Strengthen the Contact Between Bitumen and Mineral Materials

Introduction: Why Do Roads Deteriorate?

Imagine our road as a giant sandwich where stone materials are the bread and bitumen is the butter holding everything together. The durability of the entire road surface depends on how firmly the "butter" adheres to the "bread". Interfacial contacts in bitumen-mineral systems are precisely the area where organic binder (bitumen) and mineral material (crushed stone, sand) meet. These contacts are one of the weakest links of the entire system; this is where thousands of microcracks begin, which later grow into macrocracks and lead to the destruction of the road surface ¹ .

Strengthening the bond between bitumen and mineral materials is not just a scientific task but a pressing necessity for creating durable roads. Research shows that it is particularly difficult to achieve good adhesion with siliceous materials (such as quartz sands), which are widespread and cheap but adhere poorly to bitumen ² . In this article, we will examine how modern science addresses this problem, opening paths to creating higher quality and more durable road surfaces.

Basic Concepts: What Are Bitumen-Mineral Systems?

World at the Interface

A bitumen-mineral system is a composite material consisting of a mineral skeleton (aggregate) and an organic binder (bitumen). Asphalt concrete used in road construction is a typical example of such a system. This is an anisotropic composite material consisting of particles of different sizes, irregular shapes, and random orientation immersed in a bitumen matrix ¹ .

Electrical Nature of Adhesion

Modern concepts of adhesion in bitumen-mineral systems go beyond simple physicochemical interactions. Research shows that adhesion has an electrical nature. At the phase interface in the "mineral-bitumen" system in the presence of moisture, there exists a double electrical layer, the structure of which is determined by the charge of the mineral surface and the dipole of polar binder molecules ² .

The destruction of adhesive contacts between bitumen and mineral surface represents a physical mechanism analogous to separating the plates of a capacitor formed by the double electrical layer ² . This fundamental understanding of the nature of adhesion has opened paths for new methods of strengthening interfacial contacts using electric and electromagnetic fields.

Microscopic structure of bitumen-mineral interface showing the complex interactions at the boundary

Theoretical Foundations: How Adhesion Works at the Molecular Level

Role of Mineralogical Composition

The mineralogical composition of the aggregate plays a key role in determining the strength of the bond with bitumen. Research shows that different minerals interact differently with bitumen. For example, diabase and basalt demonstrate better adhesion and moisture resistance, while granite shows the worst results ¹ .

Interestingly, in molecular modeling, simple oxides (SiO₂, Al₂O₃, CaO, MgO, and Fe₂O₃) have traditionally been used as models of mineral aggregates. However, this approach does not fully reflect the actual mineralogy of rocks since the contribution of the same SiO₂ to adhesion may differ depending on which mineral it is part of ¹ . Modern research is transitioning to using more complex models based on real minerals rather than simple oxides.

Molecular-Kinetic Theory

From the perspective of quantum mechanics, intermolecular interactions can be considered as the interaction of two force centers in a hydrogen-like approximation. The total bond energy between force centers when one of the interacting substances is activated equals the sum of the energies of covalent bonding of atoms in a binary approximation (E_cov) and the energy of ionic bonding (E_ion), and for dipole molecules it is also necessary to consider dipole-dipole interactions (E_dip) ² :

Calculations show that near the tribo-activated surface of SiO₂ particles, molecules of gaseous bitumen will decompose with a tendency to increase their adsorption on the substrate and form "pure carbon" layers ² . This explains why activation of mineral surfaces can significantly improve adhesion.

Detailed Analysis of Key Experiment: Adhesion Assessment by Rolling Bottle Method

Experiment Methodology

One of the most illustrative experiments in studying bitumen adhesion to various minerals is the rolling bottle test. This experiment, described in research, allows evaluating how firmly bitumen adheres to the mineral surface in the presence of water ¹ .

Results and Analysis

The experiment results clearly demonstrated the influence of mineralogical composition on bitumen adhesion. After 6 hours of rotation in water, Granite I showed the greatest loss of bitumen coating, followed by Granite II, Gravelite I, and Gravelite II. Diabase retained the most bitumen, while Basalt performed slightly worse ¹ .

With increased rotation time to 24 hours, almost all bitumen film on Granite II separated, while diabase and basalt maintained significant coating. After 72 hours of rotation, diabase and basalt still demonstrated the best performance, while gravelites and granites showed significantly worse results ¹ .

Scientific Significance

This experiment has important scientific and practical significance. It not only confirms the substantial influence of mineralogical composition on bitumen adhesion but also provides a reliable method for assessing the resistance of bitumen-mineral systems to moisture impact. Moisture sensitivity is a key factor in the durability of road surfaces, and the rolling bottle method allows quantitative evaluation of this characteristic for different bitumen-aggregate combinations ¹ .

The results of such experiments allow road engineers to predict the behavior of asphalt concrete pavements in real conditions and select optimal material combinations for specific climatic conditions. This is especially important in regions with frequent temperature fluctuations and high humidity, where the problem of moisture sensitivity is most acute.

Methods for Strengthening Interfacial Contacts: From Theory to Practice

Practical Applications

The developed methods have found application in real production. A technological model for producing bitumen-mineral mixtures was created using the operation of aggregate electrification in an ionized air environment, taking into account the interrelationship of technological parameters and quality indicators of the finished product .

Conclusion: Prospects for Strengthening Interfacial Contacts

Research in the field of strengthening interfacial contacts in bitumen-mineral systems opens new possibilities for creating durable and economical road surfaces. Modern science is transitioning from an empirical approach to targeted design of materials with specified properties based on a deep understanding of molecular and electrical processes at the phase interface.

A multiscale approach, combining molecular dynamics modeling, laboratory testing, and practical activation methods, allows creating materials with fundamentally new characteristics. This is especially important in light of tasks related to waste utilization and the use of local non-standard materials such as quartz sands and spent molding mixtures ² .