Nanoclay-modified asphalt mixtures for eco-efficient construction
Abstract:
Polymeric nanocomposites are among the most exciting and promising classes of materials discovered recently. A number of physical properties are successfully enhanced when a polymer is modified with small amount of nanoclay on condition that the clay is dispersed at nanoscopic level. In this research, comparative rheological tests on binders and mechanical tests on asphalt mixtures containing unmodified and nanoclay modified bitumen were carried out. Two types of nanoclay were used: Nanofill-15 and Cloisite-15A. Rheological tests on binder were penetration, softening point, ductility and aging effect. Mechanical tests on asphalt mixture were Marshall stability, indirect tensile strength, resilient modulus, diametric fatigue and dynamic creep tests. Test results show that nanoclay can improve properties such as stability, resilient modulus and indirect tensile strength and result in superior performance compared to that of unmodified bitumen under dynamic creep. Nanoclays do not seem to have beneficial effects on fatigue behaviour at low temperatures. Optimum binder content and void in total mixture (VTM) increase by adding nanoclay to bitumen.
6.1 Introduction
Temperature susceptibility characteristics and physical properties of asphalt binder at high and low field operating temperatures can affect final performance of the mixture. To improve the performance of bitumen and asphalt concrete mixtures, addition of modifiers such as polymers has become popular in recent years. Polymeric nanocomposites are one of the most exciting materials discovered recently and physical properties are successfully enhanced when a polymer is modified with small amounts of nanoclay on the condition that the clay is dispersed at nanoscopic level (Pinnavaia and Beall, 2000).
Many research studies have been undertaken on nanoclay modified polymers; however, relatively little published information is available about nanoclay modified bitumen. Material variables which can be controlled and can have a profound influence on the nature and properties of the final nanocomposite include the type of clay, the choice of clay pre-treatment, the selection of polymer component and the way in which the polymer is incorporated into the nanocomposites (Pinnavaia and Beall, 2000).
Common clays are naturally occurring minerals and are thus subject to natural variation in their constitution. The purity of the clay can affect final nanocomposite properties. Many types of clay are alumina-silicates, which have a sheet-like (layered) structure, and consist of silica SiO4 tetrahedron bonded to alumina AlO6 octahedron in a variety of ways. A 2:1 ratio of the tetrahedron to the octahedron results in mineral clays, the most common of which is montmorillonite (Fig 6.1). The thickness of the montmorillonite layers (platelets) is 1 nm and aspect ratios are high, typically 100–1500 (Grim, 1959). The degree of expansion of montmorillonite is determined by their ion (e.g., cation) exchange capacities, which can vary widely. A characteristic number of these types of clay is the cation exchange capacity (CEC), which is a number for the amount of cations between the surfaces. The CEC of montmorillonite ranges from 80 to 120 meq/100 g (milli-equivalents per 100 grams), whereas kaolinite has CEC values ranging from 3 to 5.
The expansion pressure of montmorillonite in which sodium ions constitute the majority of the adsorbed cations (called Na-montmorillonite) is very high, leading to the exfoliation and dispersion of the crystal in the manner of fine particles or even single layers. When Ca2 +, Mg2 + and ammonium are the dominant exchangeable cations, the dispersion is relatively low and the size of the particle is relatively large. Separation of the clay discs from each other will result in a nanoclay with an enormous large active surface area (it can be as high as 700–800 m2 per gram). This helps to have an intensive interaction between the nanoclay and its environment (bitumen in our case). The process to realize the separation (surface treatment) is dependent on the type of material to be mixed (Lan et al., 1995).
A necessary prerequisite for successful formation of polymer-clay nano-composite is therefore alteration of the clay polarity to make the clay ‘organophilic’. To achieve fine dispersion, mechanical forces alone are not enough; there should be a thermodynamic driving force as well to separate the layers into the primary silicate sheets. This thermodynamic driving force is introduced by inserting a certain coating of surfactants (an agent such as detergent which reduces surface tension) on each individual layer (Theng, 2012). These surfactant molecules increase the layer distance, improve the compatibility with the polymer and can give an increase in entropy because they can mix with the polymer. Organophilic clay can be produced from normally hydrophilic clay by ion exchange with an organic cation. The organic reagents are quaternary ammonium salt with alkyl chains such as 12-aminododecanoic acid (ADA), octadecanoic alkyl trimethyl quaternary ammonium salt. The reaction process is described as:
Addition of a positively loaded surface active material, a kind of ADA, will in this case form an ADA layer around each clay disc. The clay disc in this case changes from a hydrophilic disc into a hydrophobic disc. These modified clay discs will separate automatically in water and can be used as nanoparticles. The correct selection of modified clay is essential to ensure effective penetration of the polymer into the interlayer spacing of the clay and result in the desired exfoliated or intercalated product. In intercalate structure, the organic component is inserted between the layers of the clay such that the interlayer spacing is expanded, but the layers still bear a well-defined spatial relationship to each other. In an exfoliated structure (Fig. 6.2), the layers of the clay have been completely separated and the individual layers are distributed throughout the organic matrix (Nguyen and Baird, 2007).
With dispersing nanoclay in a thermoplastic material (a material that is plastic or deformable, melts to a liquid when heated, and freezes to a brittle, glassy state when cooled sufficiently), stiffness and tensile strength, tensile modulus, flexural strength and thermal stability will increase (Manias, 2001).
The Structures of bitumen and polymers are different as bitumen is a very complex polymer and not stable. The structure of asphaltenes on bitumen depends on the chemical composition of the binder and on temperature. In gel type, the asphaltenes are highly associated to each other, but in sol type, they are not associated to each other and have poor network and lower asphaltenes proportions and need a different approach to clay and bitumen interaction which probably limits the successes obtained in bitumen-nanoclay modifications.
6.2 Research on nanoclay-modified asphalt mixtures
Many studies have been carried out on nanoclay-modified polymers, but little published information is available about nanoclay-modified bitumen. Many research studies have been performed on bitumen modification by polymer materials such as SBS (styrene butadiene styrene block copolymer), SBR (styrene butadiene rubber latex) and EVA (ethyl vinyl acetate). Chen et al. (2002) showed that SBS improved the rheological properties of asphalt binder due to the formation of a polymer network in the binder. This network forms in two stages: at low polymer concentrations, the SBS acts as a dispersed polymer and does not significantly affect properties; at higher concentrations, local SBS networks begin to form and are accompanied by a sharp increase in the complex modulus, softening point temperatures, and toughness.
Radziszewski (2007) studied mechanical properties of asphalt mixtures containing elastomer, plastomer and fine rubber modified binders. His study showed that while being exposed to simulated short-term ageing and long-term ageing, asphalt mixtures behave differently in terms of rutting and creeping. Ageing causes higher stiffness with unmodified binder mixtures than with polymer or rubberized bitumen modified binder mixtures. Permanent deformation depends on the type of asphalt mixture and the type of binder used. Asphalt concrete with rubberized bitumen, asphalt concrete with 7% polymer modified binders and SMA and Superpave mixtures with unmodified binders appeared to be most resistant to permanent deformations after long-term laboratory ageing (Radziszewski, 2007).
Recently, nanoscale inorganic fillers have drawn increasing interest as it is theoretically possible to significantly improve the properties of pristine polymers such as bitumen with a relatively small percentage of additive (Lan and Pinnavaia, 1994; Lan et al., 1995, Kornmann et al., 2001; Zerda and Lesser, 2001; Becker et al. 2002, Liu et al., 2003). Nanoclays are micro-scale fillers which would make polymers efficient as filler reinforcements. Ghile (2005) performed mechanical tests on asphalt mixture modified by Cloisite. The results showed that nanoclay modification can improve mechanical behaviour properties of the mixture such as indirect tensile strength, creep and fatigue resistance. Chow (2003) investigated surface modified montmorillonite nanoclay and compatibilizer, and found that the strength and stiffness of polyamide polypropylene nanocomposites improved due to the synergistic effect of surface modified montmorillonite nanoclay and compatibilizer. Yasmin et al. (2003) found that the addition of Nanomer I.28E and Cloisite 30B into some pure epoxy polymers produced materials with higher elastic modulus than that of the pure epoxy.
6.3 Material and methods
The aggregates used in this study were crushed limestone aggregates with gradation characterized by 12.5 mm nominal size (according to Pavement Guidelines in Iran) and limestone mineral filler. Physical properties of the aggregate, both coarse and fine, together with mineral filler are given in Table 6.1 and aggregate gradation shown in Fig. 6.3. The bitumen was a 60/70 penetration grade (AC-10) and its properties are shown in Table 6.2. Two types of common nanoclay used in this research were Cloisite-15A and Nanofill-15. Properties of nanoclays are shown in Tables 6.3.
Table 6.1
| Coarse aggregate (ASTM C127) | |
| Bulk specific gravity, g/cm3 | 2.698 |
| Apparent specific gravity, g/cm3 | 2.714 |
| Absorption, % | 0.33 |
| Fine aggregate (ASTM C128) | |
| Bulk specific gravity, g/cm3 | 2.683 |
| Apparent specific gravity, g/cm3 | 2.735 |
| Absorption, % | 0.62 |
| Filler (ASTM D854) | |
| Apparent specific gravity, g/cm3 | 2.743 |
| Los Angeles Abrasion, % (ASTM C131) | 23.57 |
| Polishing value (BS813) | 0.47 |
Table 6.2
| Softening point | 54 |
| Penetration grade at 5°C | 63 |
| Flash point | 243 |
| Penetration index | + 0.4 |
| Ductility at 25°C | > 100 cm |
| Fraass breaking point | 14 |
| Loss of heating | 0.05% |
| Density | 1.035 |
| Viscosity | |
| at 50°C | 250,000 |
| at 60°C | 100,000 |
| at 72°C | 20,000 |
| Maltens | 75% |
| Asphaltenes | 27.2% |
Table 6.3
| Treatment/properties | Cloisite-15A | Nanofill-15 |
| Organic modifier | MT2ETOH (methyl, tallow, bis-2-hydroxyethyl, quaternary ammonium) | Nanodispers layered silicate, long chain hydrocarbon |
| Base | Montmorillonite | Montmorillonite |
| Modifier concentration | 90 meq/100 g clay | 75 meq/100 g clay |
| Moisture | < 2% | < 3% |
| Weight loss on ignition | 43% | 35% |
| Anion | Chloride | Ammonium chloride |
| Particle sizes | ||
| 10% less than | 2 μm | 5 μm |
| 50% less than | 6 μm | 15 μm |
| 90% less than | 13 μm | 25 μm |
| Colour | Off-white | cream |
| Loose bulk, kg/m3 | 230 | 190 |
| Packed bulk, kg/m3 | 364 | 480 |
| Density, gr/cc | 1.66 | 1.88 |
| X-ray results | d = 31.5 Å | d = 28 Å |
| Plastic index | 88% | 85% |
Current test procedures used on modified and unmodified bitumen are empirical such as penetration, ductility, softening point. Marshall test and performance-based tests such as indirect tensile strength, resilient modulus test, fatigue resistance test and dynamic creep test were carried out on the mixture samples. Specimen preparation and compaction were conducted in accordance with ASTM D1559-89 (1994). All performance-based tests were done on Marshall-sized samples.
6.4 Rheological tests and results
Empirical rheological tests carried out on unmodified and modified bitumen with different nanoclay content were penetration, softening point and ductility tests. The modification of bitumen with nanoclay was performed at nanoscale level by thermodynamic driving force. The empirical tests were performed according to the standard test procedures. The nanoclay contents selected were 0.2%, 0.4% and 0.7% by weight of bitumen. The test results are shown in Fig. 6.4.
Nanofill-15 modification makes little change on penetration and softening point of the unmodified 60/70 pen bitumen. Addition of a few percent of Nanofill-15 increased penetration at 25 °C, however, further increasing Cloisite-15A content caused a decrease in penetration. Nanofill-15 has little effect on softening point; by adding 7% nanofill, softening point increased by only 3%. In contrast, Cloisite-15A has a relatively higher impact on penetration and softening point of bitumen. By increasing Cloisite-15A content, penetration decreases from 63 to 45 and softening point increases from 54 to 61. Also both nanoclays reduce ductility of binder but Cloisite-15A has a more pronounced effect in reducing ductility. This behaviour may be the result of chemical reaction and change in chemical structure, as pointed out by Ghile (2005).
When bitumen gets aged it becomes harder. Retained penetration (RP) and increase in softening point (ISP) values, as defined below, were used to find the ageing effect:
A lower RP value and higher ISP reflect more ageing of the binder. Long-term ageing was performed for 20 hours at 90 °C and atmospheric pressure. The retained penetration and increase in softening point were computed and are presented in Fig. 6.5. It can be observed that there are some improvements in the resistance to ageing in the long term due to the Nanofill-15 modification and therefore it will probably suffer less when in contact with hot air or hot oxygen.
6.5 Mechanical testing of asphalt mixtures
6.5.1 Specimen preparation
The specimens prepared for the different tests were Marshall tablets with an average height of 60–65 mm and 100–102 mm diameter (ASTM D1559-89, 1994). Dense mixture specimens were compacted by 75 blows applied on both sides. As mentioned before, Cloisite-15A reduces the viscosity of modified binder so that it is not fluid enough at the normal mixing temperature used for the standard binder (140 °C). Hence, a high mixing temperature was needed for the preparation of the modified mixtures and so the temperature was increased to 155 °C. The Cloisite-15A content was 0.2%, 0.4% and 0.7% by weight of bitumen.
During the preparation of the specimen, the modified binder had a different smell and became more viscous at 185 °C. Modified binder was relatively less sticky to the mixing pan and to the moulds as compared to the specimens of the standard mixes. All tests were performed in closed temperature-controlled cabinets. In addition, all specimens selected for the different tests were stored in a temperature-controlled cabinet to the target temperature for a minimum of 3 h before commencing any test. The loading control and input parameters used in testing are given in Table 6.4.
6.5.2 Marshal stability, flow and VTM
To compare the effects of different nanoclays in the mixes, Marshall stability and VMA test results are shown in Figs 6.6 and 6.7. The results show that, by adding nanoclay, Marshall stability and VMA increase.
Nanoclay is an active filler that improves strength properties of bitumen. By adding 2% Cloisite-15A, stability increased by 15% but Nanofill-15 increased it by only 6%. Because of the large surface area, in the nanoclay modified mixture, the optimum binder increased. Even 1% nanoclay increased optimum binder by almost 0.3–0.35% as compared to unmodified mixtures. Cloisite-15A reduced viscosity of modified binder as compared to Nanofill-15, so in compaction process, nanofill modification compacted better than cloisite modification as the VTM increases in the cloisite-modified mixture.
6.5.3 Indirect tensile strength test
The tests were conducted at three different temperatures (5°C, 25°C and 40°). The indirect tensile strength is computed from the maximum compressive force measured during the test at failure. The results in Fig. 6.8 show an increase in strength at different temperatures for comparison. Results show that modified specimens have higher strength at all test temperatures. By increasing Cloisite-15A content from 2% to 7%, indirect tensile strength values increase from 8% to 40% and the percentage of increase is larger for the higher testing temperatures. There seems to be no major difference in the effects of adding nanofill and cloisite when tested at 5 or 25°C, but at 40°C and specially when 7% nanoclay is added, Cloisite-15A had increased the IDT almost twofold compared to Nanofill-15.
The area under force versus vertical displacement curve in the ITS test represents the dissipated energy to crack or fracture the specimen. Two fracture energy values can be defined: fracture energy until failure, which is the energy dissipated before the specimen starts failing, and total fracture energy, which is the total energy dissipated to completely destroy the specimen.
Figure 6.9 shows that addition of nanoclay increases the total energy as defined above. This increase in total energy ranges between 55 and 95% for nanofill and 26 and 72% for cloisite. It can be seen that, at low temperatures (5 C), modified mixtures need more energy to start the crack initiation as compared to standard mixture, but when the cracks gets started, less energy is required to destroy the specimen. At high temperatures (40°C), fracture energy decreases because of the visco-elasto-plastic behaviour of bitumen.
6.5.4 Resilient modulus test
The resilient modulus (Mr) test method detailed in ASTM D4123 was used in this study. The specimens were tested at 5, 25 and 40 °C and the loading frequency applied at each temperature was 0.5 Hz. Pulse period and recovery time were set at 500 and 1500 ms, respectively. Resilient modulus depends on the test temperature and the loading frequency (ASTM D4123, 1995). Increase in modulus as plotted in Fig. 6.10 shows that nanoclay-modified mixture has a greater value than the unmodified mixture at all test temperatures. An increase in modulus due to the addition of 2–7% of nanoclay modification varies from 8% to 40% for Cloisite-15A and from 3% to 18% for Nanofill-15, depending on the test temperature.
6.5.5 Dynamic creep tests
Creep tests are used to evaluate the permanent deformation of the unmodified and modified mixtures at high temperatures. Accumulated permanent axial strain has three distinct stages with increasing number of cycles: primary stage, with a relatively large deformation during a short number of cycles; secondary stage, where the rate of accumulation of permanent deformation remains constant; and tertiary stage, which is the final stage where the rate of deformation accelerates until complete failure takes place. This stage is usually associated with the formation of cracks. The start of the tertiary stage is usually represented by the flow number, FN. This number is used as a rutting resistance indicator of asphalt mixtures (see Fig. 6.11).
In this test only modified mixture with 7% nanoclay was used and the results are compared to those of the unmodified mixture. The loading pulse was half sine with duration of 200 ms and a rest period of 800 ms. The specimens were tested at 40 and 60 °C and the results are shown in Figs 6.12 and 6.13.
At 40°C (Fig. 6.12), it can be seen that, for applied load levels of 100 kPa and 200 kPa, none of the modified and unmodified mixtures reached the tertiary stage before 6500 load repetitions. For applied load levels of 400 kPa, the unmodified mixture reached the tertiary stage at about 3200 pulses, whereas the modified mixture did not reach the tertiary stage before 6500 pulses. At 400 kPa, excessive deformation was shown in the unmodified mixture and specimens failed before the 6500 maximum pulse limit. The modified mixture did not show shear deformation failure till 6500 pulses and at all applied load levels, the primary deformations of unmodified mixture are bigger compared to those of the modified mixture samples.
At 60°C (Fig. 6.13), it can be seen that, after the 6500 pulses, none of the nanoclay modified mixtures reach the tertiary stage for applied load levels of 100 kPa and 200 kPa. All types of mixtures reached the tertiary stage if applied load was 300 kPa and the unmodified mixtures reached the tertiary stage after about 3000 pulses, whereas the Nanofill-15 modified mixtures reached the tertiary stage after about 4100 pulses, and Cloisite-15A modified mixtures reached the tertiary stage after about 5500 pulses. Unmodified mixture specimens had larger deformations in the primary stage and failed by excessive deformation at about 3800 pulse counts, whereas the Cloisite-15A and Nanofill-15-A modified mixture specimens did not fail completely at 5300 and 6500 pulse, counts respectively, at the 300 kPa loading.
6.5.6 Fatigue resistance test
Indirect tensile testing with diametric compressive loading was used to evaluate the fatigue resistance of unmodified and modified mixtures. A constant repetitive load was applied and the vertical deflection was measured in relation to pulse counts. The fatigue life is defined as the number of load repetitions at specimen fracture. As in the creep tests, only modified mixture with 7% nanoclay was used in this test to compare test results with those of the unmodified mixture. Fatigue resistance tests were performed at 5°C and 25 C. The results in Figs 6.14 and 6.15 show a linear fit between Nf and σ at 5 and 25 C. The R2 values are very close to 1 for all mixture types. The slope of the fatigue line at 5°C is larger than the slope of the fatigue line at 25°C for the modified mixture and the unmodified mixture.
Based on the result, shown in Fig. 6.16, at low temperatures (5°C) and almost for all loading conditions, the unmodified mixture performed better under fatigue compared to nanoclay modified mixtures. The average fatigue life ratio between fatigue lives of the modified and unmodified mixtures is about 93% for Nanofill-15 and about 80% for Cloisite-15A. At a low loading stress, the fatigue life ratio for the modified mixture is about 100% and at a high loading stress, the fatigue life ratio decreased to 85% (Fig. 6.16). At high temperatures (25°C), for all loading conditions, the modified mixture performed better under fatigue when compared to the unmodified mixtures (Fig.6.17).
The average fatigue life ratio of modified mixtures is about 1.70 for Cloisite-15A and about 1.45 for Nanofill-15. The fatigue life ratio depends on the stress level. At high stress levels, the fatigue life ratio decreases (see Fig. 6.17). This can be due to the rest period of loading applied in the tests at 25°C.
6.6 Conclusion
When bitumen is modified with small amounts of nanoclay, its physical properties are successfully enhanced on the condition that the clay is dispersed at nanoscopic level. Nanoclay materials have a larger aspect ratio and large surface area, and their particles are not uniform in size and arrangement. Nanofill-15 particles are smaller in size as compared to the Cloisite-15A particles. The plastic limit shows that nanoclay materials are the expansive type of clay. Adding low percentages of nanoclay to bitumen changes rheological properties, decreases penetration and ductility, and increases softening point and ageing. Tests performed on binders and dense asphalt mixtures show that the Cloisite-15A and Nanofill-15 modifications increase the stiffness and improve the rutting resistance, indirect tensile strength, resilient modulus and Marshall stability. However, fatigue performance decreases at low temperatures. Also optimum bitumen and VTM increase a little by adding nanoclay.
6.7 Future trends
Further investigations about nanoclay modification are needed in order to clarify several aspects, such as:
• use of different nanoclays on properties of bitumen mixture
• use of different nanomaterials and nanotubes on properties of bitumen mixture
• study of the effect of nanoclays on performance of different bitumen binders
• study of the effect of moisture and water on engineering properties of asphalt mixtures.
6.8 References
ASTM, D., Standard Test Method for Resistance to Plastic Flow of Bituminous Mixtures Using Marshall Apparatus 1559-89. American Society for Testing and Materials, West Conshohocker, PA, 1994.
Becker, O., Varley, R., Simon, G. Morphology, thermal relaxations and mechanical properties of layered silicate nanocomposites based upon high-functionality epoxy resins. Polymer. 2002; 43:4365–4373.
Chen, J., Liao, M., Tsai, H. Evaluation and optimization of the engineering properties of polymer-modified asphalt. Practical Failure Analysis. 2002; 2(3):75–83.
Chow, W. Development of Thermoplastic Nanocomposites Based on Blends of Polyamide and Polypropylene. University Sains Malaysia: Material and Mineral Resources Engineering; 2003.
Ghile, D. Effects of nanoclay modification on rheology of bitumen and on performance of asphalt mixtures. Delft University of Technology; 2005.
Grim, R. Physic-chemical properties of soils: clay minerals. Journal of the Soil Mechanics and Foundations Division, ASCE. 1959; 85(SM2):1–17.
Kornmann, X., Lindberg, H., Berglund, L.A. Synthesis of epoxy–clay nano-composites: influence of the nature of the clay on structure. Polymer. 2001; 42:1303–1310.
Lan, T., Pinnavaia, T. Clay-reinforced epoxy nanocomposites. Chem Mater. 1994; 6:2216–2219.
Lan, T., Kaviratna, P., Pinnavaia, T. Mechanism of clay tactoid exfoliation in epoxy–clay nanocomposites. Chem Mater. 1995; 7:2144–2150.
Liu, Y., Hsu, C., Wei, W., Jeng, R. Preparation and thermal properties of epoxy–silica nanocomposites from nanoscale colloidal silica. Polymer. 2003; 44:5159–5167.
Manias, E., Origins of the materials properties enhancements in polymer/clay nanocompositesGolovoy Amos, ed. Focus on Polypropylene/Montmorillonite Hybrids. Proceedings of Nanocomposites. ECM Publications, 2001. [Delivering New Value to Plastics,vol. 1,IL].
Nguyen, Q., Baird, D. Process for increasing the exfoliation and dispersion of nanoclay particles into polymer matrices using supercritical carbon dioxide. Virginia Polytechnic Institute and State University; 2007. [PhD dissertation].
Pinnavaia, T., Beall, G. Polymer–Clay Nanocomposites. Chichester: John Wiley & Sons; 2000.
Radziszewski, P. Modified asphalt mixtures resistance to permanent deformations. Journal of Civil Engineering and Management, XIII. 2007; 4:307–315.
Theng, B. Formation and Properties of Clay-Polymer Complexes, 2nd edn. Amsterdam: Elsevier; 2012.
Yasmin, A., Luo, J.J., Abot, J.L., Daniel, I.M., Mechanical and thermal behavior of clay/epoxy nanocomposites at room and elevated temperatures. Proceedings of ASC, 18th Annual Technical Conference. 2003.
Zerda, A., Lesser, A. Intercalated clay nanocomposites: morphology, mechanics and fracture behavior. Polymer Science, Part B, Polymer Physics. 2001; 39:1137–1146.