Low temperature rheology of polyphosphoric acid (PPA) added bitumen
Highlights
► The effects of polyphosphoric acid (PPA) on bitumens from different sources were investigated. ► Dynamic Mechanical Analysis (DMA) at low temperatures was adopted. ► PPA shifts, towards lower values, the bitumen glass transition. ► PPA seems to improve low temperature bitumen behavior. ► Experimental results suggest the existence of an optimal PPA concentration.
Introduction
Bitumen is a primary engineering material, often employed as a binder in road construction and roofing systems, thanks to its thermoplastic nature, water resistance and adhesion to many other materials. It is characterized by rheological properties strongly dependent on temperature, governed by the chemical-physical interactions of their individual constituents [1].
During the last few years the research into bituminous materials has focused on low and high temperature performance, because the mechanical properties of bituminous binders play an important role in the performance of the corresponding asphalt mix [2]. Bitumen has to be hard enough at high temperature to avoid permanent deformation of the pavement (rutting) and soft enough at low temperatures to avoid fractures due to a lack of flexibility (cracking) [3], [4]. It is usually difficult to obtain materials that could work properly in a wide temperature range, therefore there is a different paving grade, suitable for specific applications. As a consequence, nowadays, there are many types of bitumens with different mechanical properties, also owing to the different sources that affect their composition, and, therefore, their rheological properties.
Depending on the source, bitumen can be rich in waxes or almost wax free and the presence of these components can improve the sensitiveness to cracking or create plastic deformations after the laying of asphalts. At high temperature waxes act as a plasticizer, reducing viscosity, whereas at low temperature, owing to crystallization phenomena, they increase the stiffness making the bitumen more susceptible to cracking.
Moreover, properties of bitumen containing wax are also affected by ageing processes such as chemical ageing and physical hardening (also referred to as steric hardening). The former occurs either during asphalt mix preparation at high temperature (short ageing) or during the service life of the material (long term ageing) and is due to chemical modifications; the latter occurs with time and it is a heat reversible phenomenon potentially due to the strengthening of the asphaltene network with time. Both of them yield, as a final consequence, to material hardening which is particularly evident at low temperatures [3], [5].
In order to widen the operative temperature range and reduce the ageing issues, many additives, such as polymers and acids, are currently used as bitumen modifiers; among them polyphosphoric acid seems particularly interesting [3] because, in small amounts, it can improve bitumen rheological properties in a significant way both for high and low temperatures [5].
Edwards et al. (2006) [5] observed a slight reduction of both glass transition temperature (Tg) and complex shear modulus when adding up to 1% PPA to different 160/220 bitumens obtaining a higher stiffness at intermediate and high temperatures.
Nevertheless, the PPA added bitumens showed an increased sensitivity to permanent deformation, when compared with a mixture without additive [5], evidencing potential breaking problems.
Even though there is great interest in PPA addition for commercial applications, data on rheological behavior of added bitumen at low temperature are still scarce. In the present paper the rheological behavior of different PPA added bitumens was investigated by Dynamic Mechanical Analysis (DMA) aiming at a better understanding of the effects of different PPA levels on bitumen behavior at low temperature (⩽5 °C).
Section snippets
Materials
Four plain bitumens from three different sources (Venezuela, Saudi Arabia and Russia) were used in this study. Venezuelan (N) and one Saudi Arabian (C) binders were 70/100 penetration grade bitumens, while the others, from Saudi Arabia (P) and Russia (M), were 50/70 penetration grade bitumens.
Polyphosphoric acid, 83.3% P2O5, or otherwise stated 115% H3PO4 equivalent [6], was provided by ICL Performance Product LP (St. Louis, MO, USA).
Asphaltenes were isolated from bitumens according to the
Results and discussion
The Fraass points for all samples are shown in Table 1, Table 2. The data evidence some differences among considered materials. As can be seen, sample N is characterized by the lowest temperature because of the low wax content [3], whereas similar values are observed for the other unmodified materials. When added samples are considered, the most relevant effects can be observed on samples N1–N2 probably because, owing to the low wax content, the breaking point is significantly affected by the
Conclusion
Four types of asphalt have been studied and modified by the addition of poly-phosphoric acid. Dynamic data, obtained by applying a heating thermal ramp to bitumen initially cooled down to −30 °C, were used to estimate a rheological glass transition temperature (Tg) for all tested samples; it was observed that, generally, Tg decreases with PPA content and the complex modulus (E∗) increases, even though a non-monotonous trend was observed with the largest effects at a concentration ranging between
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