1984
Energy & Fuels 2005, 19, 1984-1990
Influence of Crumb Rubber Concentration on the Rheological Behavior of a Crumb Rubber Modified Bitumen F. J. Navarro, P. Partal,* F. Martı´nez-Boza, and C. Gallegos Departamento de Ingenierı´a Quı´mica, Universidad de Huelva, Facultad de Ciencias Experimentales, Campus del Carmen, 21071, Huelva, Spain Received November 22, 2004. Revised Manuscript Received June 23, 2005
Bitumen is widely used in construction. Its applications go from binder for road asphalts to built-up roofing membranes and other waterproofing uses. The use of crumb tire rubber as a binder modifier may contribute to solving a waste disposal problem and to improving the quality of road pavements. This paper deals with the effect that rubber concentration exerts on the thermorheological properties of rubber-modified bitumens. The results obtained confirm that the rheology and storage stability of these modified binders are largely dependent on rubber concentration. The linear viscoelastic measurements carried out revealed an improvement of both low and high in-service properties of the binder. The viscous flow measurements showed an important increase in viscosity and a noticeable reduction in temperature susceptibility. A swelling process of the crumb rubber by light components of the maltenic fraction was confirmed by modulated differential scanning calorimetry (MDSC). The stability tests, performed on samples stored at 180°C, demonstrated that the storage stability of rubber-modified bitumen was improved as rubber concentration increased.
Introduction Bitumen is usually defined as a dark-brown to black material, natural or manufactured, mainly composed of high molecular weight hydrocarbons, which can be classified in two main groups in terms of polarity and solubility in n-heptane: asphaltenes and maltenes.1 Bituminous binders have been widely used in construction applications, mainly for flexible road pavements, waterproofing, roofing, joint sealant, etc.2 In general, a binder must remain flexible enough to withstand sudden stresses without cracking at low temperatures during winter, but it must also resist permanent deformation or viscous flow at high in-service temperatures. In this way, bitumens should resist stresses due to traffic loads and low temperatures in pavement application, as well as extensions and contractions in roof construction.3 To achieve and improve the desired in-service properties, bitumens are usually modified, mainly with polymers, increasing the cost of the final product. In this way, waste rubber from discarded tires, from environmental and economical perspectives, can provide an alternative source of elastomeric polymer material for bitumen modification, being a substitute to virgin polymers.4-7 This could favor the development of a high * To whom correspondence should be addressed. Phone: +34 959219989. Fax: +34 959219983. E-mail:
[email protected]. (1) Corbett, L. W. Anal. Chem. 1968, 41, 576-579. (2) Akmal, N.; Usmani, A. M. Polym. News 1999, 24, 136-140. (3) Ait-Kadi, A.; Brahimi, H.; Bousmina, M. Polym. Eng. Sci. 1996, 36, 1724-1733. (4) Takallou, H. B.; Hicks, R. G.; Esch, D. C. Transp. Res. Rec. 1986, 1096, 68-80.
potential market for optimum environmental recycling of scrap tires and further the use of crumb rubber in pavement structures. This application is also in line with the European directive that will prohibit all landfill of scrap tires by 2006. In this sense, the European Tire Recycling Association (ETRA) has stressed the increasing importance of material recycling, such as the use of recycled postconsumer tires in asphalt pavement mixtures, to the European Commission.8 Different studies have remarked on the relationship between the rheological characteristics of asphalt binders and field performance,9 illustrating the importance of understanding the rheological behavior of bitumen. Generally, for pavements and applications concerning building construction, the binder is assumed to sustain small deformations, and consequently, its linear viscoelastic properties can adequately describe its inservice behavior. Thus, these properties may define the resistance of the bitumen to traffic loading (rutting) and cracking due to fatigue.10 However, when road pave(5) Coplantz, J. S.; Yapp, M. T.; Finn, F. N. Review of Relationships between Modified Asphalt Properties and Pavement Performance; SHRP-A-631; Strategic Highways Research Program, National Research Council: Washington, DC, 1993. (6) Roberts, F. L.; Kandhal, P. S.; Brown, E. R.; Lee, D. Y.; Kennedy, T. W. Hot Mix Asphalt Materials, Mixture Design and Construction, 1st ed.; NAPA Education Foundation: Lanham, MD, 1991; pp 68382. (7) Airey, G. D.; Rahman, M. M.; Collop, A. C. Int. J. Pavement Eng. 2003, 4, 105-119. (8) European Tire Recycling Association (ETRA). Summary of the Post-Consumer Tire Market; France, 1999. (9) Anderson, D. A.; Christensen, D. W.; Bahia, H. U.; Dongre´, R.; Sharma, M. G.; Antle, C. E.; Buttom, J. Binder Characterization and Evaluation. 3. Physical Characterization; Strategic Highway Research Program, National Research Council: Washington, DC, 1994.
10.1021/ef049699a CCC: $30.25 © 2005 American Chemical Society Published on Web 07/28/2005
Crumb Rubber Modified Bitumen
Energy & Fuels, Vol. 19, No. 5, 2005 1985
Table 1. Composition of the Crumb Rubber material
wt %
total rubber hydrocarbon (natural and synthetic rubber) carbon black THF extractable ash
50 32 11 4
ments and roofing membranes form ruts or crack, they are submitted to larger stresses and strains resulting in a nonlinear viscoelastic behavior. In addition, at high temperatures, when bitumen is handled and mixed with mineral aggregates, compacted, or laid, the flow properties of the binder drastically influence these operations. A large number of papers dealing with crumb tire rubber modified bitumens (CTRMBs) illustrate that they can improve the mechanical properties of the binder and the resulting asphalt mixes.4-7 Some authors postulate that the addition of crumb rubber to a bitumen may improve some mechanical properties of the asphalt pavement.4-6 However, several problems concerning rubber-modified bitumen still remain unresolved, and the rheological behavior of these systems has not been completely described. The main objective of this work was to study the influence of rubber concentration on the rheological properties and microstructure of CTRMBs, over a wide range of temperatures, to obtain information for road paving at low rubber concentration and for roofing applications at high rubber content. High-temperature storage stability was also evaluated. Experimental Section Materials. The rubber used, supplied by Alfredo Mesalles S. A. (Spain), was derived from ambient grind. In other words, the tire was torn at room temperature and then ground. The mean particle size of the particles, determined by screening, was 0.29 mm. The composition of the crumb rubber is shown in Table 1. A neat bitumen, 60/70 penetration grade and a softening point (ring and ball) of 52 °C, donated by Construcciones Morales (Spain), was used as the base material for polymer modification. The bitumen composition, determined by the procedures outlined in ASTM D3279 and ASTM D4124, was 5.8% saturates, 54.4% aromatics, 19.6% resins, and 20.2% asphaltenes. Preparation of Mixtures and Samples. Blends of bitumen and crumb rubber were prepared by mechanical stirring in an open low-shear batch mixer (80 mm diameter) using an IKA RW-20 agitator (Germany) equipped with a four-blade propeller (50 mm diameter). Crumb rubber particles were slowly added at the initial stages of the mixing process and special care was taken to avoid the production of lathers. Samples were processed for 1.5 h at 180 °C with a rotational speed of 1200 rpm. After the material was mixed, the resulting binder was poured into a small can and then stored in a freezer at -25 °C to retain the obtained morphology. The rubber concentration was varied from 5 to 40 wt %. Higher temperatures (210 °C) were necessary to prepare the modified bitumens with the largest rubber contents (30 and 40%). Methods of Analysis. Frequency sweep tests between 0.01 and 100 rad/s, in the linear viscoelasticity range, were performed in a controlled-stress Haake RS150 rheometer (Germany) using a serrated plate-and-plate geometry (10 and 20 mm diameter). Stress sweep tests, at the frequency of 1 Hz, were previously carried out on each sample to determine (10) Dongre´, R.; Youtcheff, J.; Anderson, D. Appl. Rheol. 1996, 6, 75-82.
the linear viscoelasticity region. Time sweep tests, at a constant stress and frequency, were also performed to confirm that no structural modifications occurred during the time required for each test. Measurements were done in a temperature range between -10 and 90 °C. Dynamic mechanical analysis tests at a constant amplitude, within the linear viscoelastic region (
