In India, transportation infrastructure is rapidly expanding with the ambitious development of road network and flexible pavements constitute about 98 percent of the total road network. Flexible pavements are mostly preferred over rigid pavements as they can be strengthened and improved in stages with the growth of traffic and are less expensive with respect to life cycle cost. Bituminous mixtures are the main materials used to construct the bituminous layers of flexible pavements. Bituminous concrete is a composite material consisting of aggregate and bitumen. A bituminous concrete mixture must be designed, produced and placed in order to obtain the desirable mix properties such as, stability, durability, flexibility, fatigue resistance, skid resistance, impermeability and workability.
The stability of a mixture depends on internal friction and cohesion. Internal friction among the aggregate particles (inter-particle friction) is related to aggregate characteristics such as shape and surface texture. Cohesion results from the bonding ability of the bitumen binder. Bitumen binder is a viscoelastic, thermoplastic, complex material that behaves differently with temperature and loading time. It is purely viscous at high temperatures and/or under slow moving loads and the materials become prone to permanent deformation (Fig.1). It is also totally elastic and eventually brittle at low temperatures and high rapid loads and subsequently the materials become prone to the low-temperature cracking. However, within 10oC to 35oC in-service pavement temperatures, where the pavement is subjected to a considerable part of its repetitive traffic loads, the main mode of distress is fatigue cracking (Fig.2). These fatigue cracks make pathway for surface water to enter inside the pavement structure which affects the durability of bituminous pavements.
The durability of a bituminous pavement is its ability to resist factors such as changes in the binder (polymerisation and oxidation), disintegration of the aggregate, and stripping of the binder films from the aggregate (Fig.3) and pavement layers (Fig.4). These factors can be the result of weather, traffic, or a combination of the two. A dense gradation of sound, tough, stripping-resistant aggregate contributes to pavement durability in three ways. A dense gradation provides closer contact among aggregate particles which makes mixture impermeable. A sound, tough aggregate resists disintegration under traffic loading; and stripping-resistant aggregate resists the action of water and traffic, which tend to strip the binder film off the aggregate particles and lead to raveling of the pavement. Use of additives or modifiers can increase the resistance of a mixture to stripping.
Use nanomaterials in bituminous mixture have the following key benefits:
- Improvement in the storage stability of polymer modified bitumen
- Increasing the resistance to UV aging
- Reducing the moisture susceptibility
- Improving the properties of bituminous mixtures even at low temperature
- Improving the durability of flexible pavements
- Saving energy and cost, thus becomes sustainable material
- Decreasing maintenance requirements
A nano particle is often defined as a particle with at least one dimension that is less than 100 nm and is measured in nanometre (nm). The physics and chemistry of nano-sized particles differ from those of conventional materials, primarily because of the increased surface area-to-volume ratio of nanometre-sized grains, cylinders, plates, and because of the quantum effects resulting from spatial confinement. The clay nano particles are the primary materials that could have application in bituminous road construction based on a literature review of nano particles and nano materials. Montmorillonite and/or Bentonite clay, silica, alumina, magnesium, calcium, and titanium dioxide (TiO2) nano particles are widely used to improve bituminous mixture performance.
Nanoclay – is clay that can be modified to make the clay compatible with organic monomers and polymers. Addition of a small amount of nanoclay dispersed at the nano-scopic level improves the short and long term aging resistance of the bitumen and enhances the physical properties such as stiffness and tensile strength, tensile modulus, flexural strength and modulus thermal stability of the bitumen. Generally, the elasticity of the nanoclay modified bitumen is much higher and the dissipation of mechanical energy is much lower than in the case of unmodified bitumen (Jahromi and Khodaii 2009). A good dispersion of nanoclay particles in the bitumen binder mastic was observed by Scanning Electron Microscopy (SEM) image (Fig.5).
Nanosilica – Silica is an abundant compound over the earth that is largely em-ployed in industries to produce silica gels, colloidal silica, and fumed silica and so on. Silica nanoparticles have been used in the industry to reinforce the elastomers as a rheological solute and cement concrete mix-tures. The advantage of these nanomaterials resides in the low cost of production and in the high performance features (Lazzara et al. 2010). With the addition of nanosilica in the base bitumen binder, the viscosity values of nanomodified bitumen decreased slightly. Lower viscosity of the binder indicates that a lower compaction temperature or lower energy consumption of the construction process will be achieved. The addition of nanosilica improves the recovery ability of bitumen binders, and increases the resistance to aging and, fatigue cracking and rutting significantly (Yao et al. 2012).
Stability and Density
In order to understand the effect of nanosilica particles in bituminous mixture performance, Marshall Stability test at 60oC with loading rate of 50.8mm/minute was conducted on laboratory produced specimens. Aggregate gradation satisfying the bituminous concrete with nominal maximum particle size of 13.2mm was used. The specimens were prepared with viscosity grade (VG30) base bitumen modified by liquid nanosilica at various dosage levels. The results of stability and density are presented in Fig.6 and Fig.7 respectively. Nanomodified bituminous mixture has shown improved stability and increased density than the unmodified mixture, by satisfying MORTH guidelines. Optimum dosage of liquid nanosilica was obtained as 1.0% by weight of binder corresponding to the design air voids content of 4.0%, maximum stability and maximum bulk density.
Adding nanomaterials to the base bitumen improved the viscoelastic behaviour of the bitumen. Stability of the mixtures containing nanosilica, results in higher values than that of the unmodified bitumen mixtures. Among all types of mixtures prepared using nanosilica, the mixtures with 1.0% of liquid nanosilica gives improved mechanical behaviour. The durable bituminous pavements can be constructed by using nanomaterial as additives or modifiers. Thus it is concluded with quote given below.
“American roads are good not because America is rich, but America is rich because American roads are good.” – John F. Kennedy.
- Jahromi, S. G., and Khodaii, A. (2009). “Effects of Nanoclay on Rheological Properties of Bitumen Binder.” Construction and Building Materials, 23, 28942904.
- Lazzara, G., and Milioto, S. (2010). “Dispersions of nanosilica in biocompatible copolymers” Polymer Degradation and Stability, 95, 610-617.
- Yao, H., You, Z., Li, L., Lee, C. H., Wingard, D., Yap, Y. K., Shi, X., and Goh, S. W. (2012). “Properties and Chemical Bonding of Asphalt and Asphalt Mixtures Modified with Nanosilica.” Journal of Materials in Civil Engineering, DOI: 10.1061/ (ASCE) MT.1943 5533.0000690.