Filippo Merusi

A review of the fundamentals of polymer-modified asphalts: Asphalt/polymer interactions and principles of compatibility

During the last decades, the number of vehicles per citizen as well as the traffic speed and load has dramatically increased. This sudden and somehow unplanned overloading has strongly shortened the life of pavements and increased its cost of maintenance and risks to users. In order to limit the deterioration of road networks, it is necessary to improve the quality and performance of pavements, which was achieved through the addition of a polymer to the bituminous binder. Since their introduction, polymer-modified asphalts have gained in importance during the second half of the twentieth century, and they now play a fundamental role in the field of road paving. With high-temperature and high-shear mixing with asphalt, the polymer incorporates asphalt molecules, thereby forming a swallowed network that involves the entire binder and results in a significant improvement of the viscoelastic properties in comparison with those of the unmodified binder. Such a process encounters the well-known difficulties related to the poor solubility of polymers, which limits the number of macromolecules able to not only form such a structure but also maintain it during high-temperature storage in static conditions, which may be necessary before laying the binder. Therefore, polymer-modified asphalts have been the subject of numerous studies aimed to understand and optimize their structure and storage stability, which gradually attracted polymer scientists into this field that was initially explored by civil engineers. The analytical techniques of polymer science have been applied to polymer-modified asphalts, which resulted in a good understanding of their internal structure. Nevertheless, the complexity and variability of asphalt composition rendered it nearly impossible to generalize the results and univocally predict the properties of a given polymer/asphalt pair. The aim of this paper is to review these aspects of polymer-modified asphalts. Together with a brief description of the specification and techniques proposed to quantify the storage stability, state-of-the-art knowledge about the internal structure and morphology of polymer-modified asphalts is presented. Moreover, the chemical, physical, and processing solutions suggested in the scientific and patent literature to improve storage stability are extensively discussed, with particular attention to an emerging class of asphalt binders in which the technologies of polymer-modified asphalts and polymer nanocomposites are combined. These polymer-modified asphalt nanocomposites have been introduced less than ten years ago and still do not meet the requirements of industrial practice, but they may constitute a solution for both the performance and storage requirements.

Moisture Susceptibility and Stripping Resistance of Asphalt Mixtures Modified with Different Synthetic Waxes

Wax modification of asphalt binders represents an actual resource in warm-mix asphalt technology. Despite its benefits in reducing plant temperatures and atmospheric emissions, the presence of wax can be associated with side effects on the quality and durability of asphalt pavements. A simple procedure to evaluate the influence of wax on asphalt stripping resistance and moisture susceptibility is proposed. An unmodified asphalt binder was blended with waxes having different chemical structures (paraffinic and polyamidic); wax-modified asphalt mixtures were then prepared with various aggregate types. Moisture sensitivity was initially studied using conventional methods, such as indirect tensile strength measured after water conditioning. The stripping resistance of bitumen-coated aggregates was then quantitatively evaluated by identifying the stripped surface with a digital image analysis method. In contrast with expectations, results indicated that enhanced performances can be obtained in the presence of wax-modified asphalts. However, the type of wax was found to be decisive. In the same way, results of contact angle and adhesion measurements performed on bitumen–wax blends indicated that changes in asphalt–water affinity occur according to the type and content of wax.

Delayed mechanical response in modified asphalt binders. Characteristics, modeling and engineering implications

The research reported herein deals with the analysis of different rheometrical approaches to evaluate the damage-related properties of modified asphalt binders at high service temperatures. Static and repetitive creep experiments were performed to associate specific characteristics of the binders’ time-dependent response to the materials damage resistance in conditions of incipient failure. The model parameter η0 was found to be the most important factor, which controls and predicts the binder resistance to the propagation of a viscous flow. Further data show that this condition applies even if non-linear domains are considered. The concept of intrinsic resistance to non-reversible deformation was consequently introduced and advanced performance-based criteria for technical qualification of modified asphalt binders were proposed.

Structural transitions and physical networks in wax-modified bitumens

This research focuses on the characterisation and evaluation of wax-modified bitumens for use in warm mix asphalt (WMA). Wax-modified bitumens were produced by addition of different types of wax to a 50/70 unmodified bitumen. Five different waxes were used, including synthetic hydrocarbons Fischer–Tropsch wax, Montan waxes and amidic-functionalised waxes. A basic structural picture of wax-modified bitumens was initially derived from the analysis of the thermal properties and morphological arrangements. The mechanical properties of the binders were then evaluated by rheometry, which indicated that bitumen modification with synthetic and functionalised waxes provides significant alteration in the overall colloidal structure. A comprehensive analysis of linear viscoelastic spectra depicted the existence of a sequence of structural transitions and relative behavioural processes, which deeply alter the behaviour of the original binder at in-service temperatures. All the rheological dynamics were interpreted in the light of the concept of residual crystallinity. The presence of physical networks generated by contiguous microcrystalline segments was finally hypothesised to explain the gel-like behaviour of the materials.

Chromatic and Rheological Characteristics of Clear Road Binders

Clear asphalt pavements provide specific functional characteristics to improve safety and climatic conditions in urban areas. The technological development of clear pavements refers to the formulation of special binders with particular chromatic characteristics. This research evaluated three clear binders with different chemical formulations with the aim of establishing the effectiveness of their mechanical properties and chromatic characteristics. Mechanical properties were analyzed by rheological methods. All clear binders were found to have specific dependence on stress levels, time, and temperatures. Different behaviors emerged, depending on the chemical origin of the binders. Chromatic characteristics were quantitatively evaluated by means of a digital image analysis. Red–green–blue as well as hue, saturation, and brightness models were used to determine and compare the chromatic performances of the three binders. The stability of the chromatic characteristics was also estimated with a laboratory aging process. Relationships were found between the chemical origin of the clear binder and its chromatic performance.

Kerosene-Resistant Asphalt Binders Based on Nonconventional Modification

The development of bituminous binders with an intrinsic fuel resistance is required to limit pavement damage and maintenance operations in those areas, such as airport systems and industrial areas and filling stations, where the risk of fuel spilling exists. This paper focuses on the study of the interactions between bitumen and fuel to attain a composition of modified binders with enhanced fuel resistance. Several different modifiers, including polymeric materials, recycled crumb rubber, and synthetic waxes, were added to a base bitumen. All blends were studied with special reference to their chemical composition and subjected to a solubility test consisting of a controlled immersion in kerosene (jet fuel A-1). The results were interpreted on the basis of the compositional characteristics which determine the interactions occurring between bitumen and modifiers. Among the formulations with enhanced fuel resistance, those based on synthetic waxes appeared to be the most interesting ones.

Nanoclays as Asphalt-Binder Modifiers

Among the solutions to reduce failures of road pavements, polymer-modified asphalts (PMAs) represent the most effective one and have been adopted worldwide in the last decades. PMAs are blends of asphalt and a polymer, where the latter is swollen by the bituminous material, thus forming a network whose structure closely resembles that of gels. Unfortunately, only a limited number of polymers can be used as asphalt modifiers, being the most restrictive requirement of their compatibility with asphalt. Considering that asphalt is a complex organic material often described as a low molecular weight polymer, a recent approach is the addition of clays to both unmodified and polymer-modified asphalts. It is well known that polymer/clay nanocomposites may exhibit improved properties, if sufficient levels of clay exfoliation and silicate layers dispersion are achieved, but recent studies also underlined that clays may be used in polymer blends. In fact, an organoclay, though residing preferentially within the bulk of the most compatible polymer, may localize at the interfacial region as well, thus acting as a compatibilizer between the two polymeric phases. This is why the use of ternary systems asphalt/polymer/clay has a double potential advantage: (i) an improvement of polymer and asphalt properties and (ii) a polymer/asphalt compatibilizing effect.

Internal Structure of Bitumen/Polymer/Wax Ternary Mixtures

Polymer modified asphalts (PMA) and warm mix asphalts (WMA) are technologies widely adopted in the paving industry. The first one is well established, while the second one is relatively new, but rapidly growing since it guarantees economic and environmental advantages. Until now PMA and WMA have been used disjointedly, but it would be useful to combine them to keep the advantages of both. One of the adopted solutions to obtain a warm effect is the addition of waxes to the asphaltic binder. Therefore, a “warm mix polymer modified asphalt” may be potentially obtained with a ternary asphalt/polymer/wax system. However, the final warm effect and performances of the binder will depend on the interactions between the three components. A preliminary investigation was done by mixing asphalt, styrene-butadiene-styrene block copolymer and a wax chosen among the following three categories: paraffinic, partially oxidized and maleic anhydride functionalized. The morphological and calorimetric analyses and solubility tests allowed identifying different behaviors depending on the wax type, which may preferentially interact either with the asphalt or with the polymer, thus influencing the whole binder structure. With regard to the ternary mixes, it was found that: (i) the paraffinic wax preferentially resides in the polymer-rich phase, and slightly enhances the asphalt-polymer compatibility; (ii) the partially oxidized wax prefers the asphaltene-rich phase and reduces the compatibility; (iii) it is not clear where the functionalized wax is located, but it has a considerable compatibilizing effect and strongly alters the colloidal equilibrium of the asphalt-polymer blend.

Fuel resistance of bituminous binders

Abstract: The solubility of bituminous binders in oil-derived products is a primary cause of pavement damage in airport systems, filling stations, parking lots and industrial plants, where fuel spillage may occur. The chapter first introduces the solutions proposed and adopted in order to overcome this problem and the tests normally employed to quantitatively describe and measure solubility. Then, the effect of three common bitumen modifiers, i.e. thermoplastic polymers, crumb tyre rubber and waxes, on bitumen solubility is broadly described. In each of the three cases, the mechanism by which the modifier influences the binder solubility is hypothesized and discussed.