K.Sitaramanjaneyulu, Head, Pavement Evaluation Division, CSIR - Central Road Research Institute, New Delhi
Smartness describes self-adaptability, self-sensing, and multiple functionalities of the materials. These characteristics provide numerous possible applications of such materials in manufacturing and civil infrastructure systems. Self-adaptation characteristics of smart pavements are a great benefit that utilizes the embedded adaptation of smart material. By changing their properties, smart materials can detect faults and cracks and therefore are useful as a diagnostic tool.
Pavements consist of a combination of layers of engineered materials that generally provide all-weather access to vehicles to travel in a safe and economical way. The layers of materials used are selected and engineered to provide a structure that can withstand the applied vehicular loads under a range of environmental conditions for a defined minimum life. Typically, materials deeper in the pavement structure will be less affected by the application of vehicular loads and environmental variations, while materials forming the surfacing of the pavement are directly exposed to the environment as well as the applied vehicular loads. It is the intention of a good pavement design to enable vehicles to travel safely and economically.
Monitoring of the current and long-term behavior of a pavement would lead to enhanced safety during its life since it would provide early warning of structural problems at a stage where minor repairs would enhance durability. This would influence the life costs of such pavements by reducing upfront construction costs, and by extending their life. Durable, long-life roads and highways with low maintenance would be one of the priorities for the future society. To bear the increasing traffic loads, new materials/emerging technologies and products will have to be developed. Managing the behavior and performance of asphalt mixtures will be the greatest challenge for the coming decades. Many innovative methods and applications are emerging in India and throughout the globe for improving the life of the pavements like emulsified bituminous mix, foam bituminous mix, fiber reinforced bituminous mix, composite pavements, perpetual pavements etc.
There are several materials which can be characterized as 'smart' and are used for pavement construction. Self-Healing material, for instance, has the ability to heal damages automatically and autonomously, that is, without any external intervention. Incorporation of self-healing properties in man-made materials very often cannot perform self-healing action without an external trigger. Nanotechnology is focused on materials in the nano-scale, while civil engineering infrastructure, especially road pavements, is focused on the macro scale.
There are a number of smart materials, some of which are quite common now. Examples:
Currently, most industrial materials rely entirely on passive protection mechanisms; which are readily applicable and universal for different materials systems. However, they will always stay passive, and, therefore, their lifetime and functionality is limited and related to the amount of protective additives and the intensity of their consumption. So, better, and preferentially active process for the protection/repair of damaged materials self-repairing processes were developed, and which need to be developed further. Although it sounds futuristic and which also affects the directions of research; self-healing of material systems already exists in all sorts of systems of materials or functionalities. Self-healing is an intrinsic property of bitumen. This technology helps in conservation of material resources since the usual over-design of materials is no longer required. The major objective of the self-healing pavement technique is to develop a smart bituminous pavement that can assess, anticipate and heal by itself in time, without any human intervention, and substantially return to its original state. For this, the following areas are needed to be explored and studied:
- Development of multiple healing processes which are sufficient for the complete design life.
- To develop a mechanism that initiates/stimulates the healing action on demand.
- To develop a method to quantify the success of self-healing action.
The clay nano-particles are the primary materials applyed in asphalt construction. Adding nano-particles like nanoclay, nanosilica, and nanotubes in asphalts normally increase the viscosity of asphalt binders and improves the rutting and fatigue resistance of asphalt mixtures. Using nanoclay as the second modifier in polymer modified asphalts can improve the storage stability and the aging resistance of polymer modified asphalts. Various Atomic Force Microscopy (AFM) techniques (e.g. tapping mode imaging, force spectroscopy, and nano-indentation) as well as X-ray diffraction (XRD) experiments can be conducted on modified asphalt binders to characterize the micro or nano-scale structures of nano-asphalts. Through the reasonable selection of nano-materials used in asphalt, nano-modified asphalt can offer many benefits in cold regions.
Today, use of technology has taken the standards of construction to a new, high level. Different types of procedures, methods and materials are used to attain a very good, sustainable and economic concrete construction. The process of self-healing of cracks or self-filling up of cracks due to bacterial reaction in the concrete after hardening, is known as Self-Healing Concrete. It can be observed that small cracks that occur in a structure of width in the range of 0.05 to 0.1mm gets completely sealed in repetitive dry and wet cycles. The mechanism of this autogenously healing is the width of 0.05-0.1mm that act as capillary and the water particles seep through the cracks. These water particles hydrate the non or partial reacted cement and the cement expands, which in turn fills the crack. The bacteria used for self-healing of cracks are acid producing bacteria. These types of bacteria can be in dormant cell and be viable for over 200 years under dry conditions; they acts as a catalyst in the healing process of cracks.
Temperature susceptibility characteristics and physical properties of bituminous binder at high and low field operating temperatures can affect the final performance of the bituminous concrete. To improve the performance of bituminous mixtures, the addition of modifiers to bitumen has become popular in recent years. Polymeric nano composites are one of the most exciting materials discovered recently and the 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. There are many emerging technologies such as warm mix asphalt, cool pavements, new generation stabilizers, cement grouted bituminous macadam, precast concrete pavements, geopolymer concrete, green pavements etc, some of which are discussed below.
Warm Mix Asphalt
Warm Mix Asphalt (WMA) is a green product that reduces temperatures, as it does not need as much energy to produce. It's been documented to lower energy costs by 30-60%. It reduces greenhouse gas and hydrocarbon emissions during the manufacture and construction of asphalt pavements. Lower temperatures make it safer for workers in the plant and at asphalt paving project job sites. WMA has been especially successful in areas where air quality is a problem.
WMA is manufactured, constructed and used with only minor adjustments. Several companies have developed technologies that increase workability at lower temperatures. There are chemical additives to improve workability and mechanical techniques to foam the asphalt binder. Special equipment is attached to the asphalt plant to create the foaming process, which coats the aggregates at lower temps. Some of the additives can be easily pre-blended with the liquid asphalt, or added at the asphalt mix plant. These additives will be specified in the job mix formula.
Cement Grouted Bituminous Macadam
Grouted Macadam consists of polymer-coated aggregates with 25-30% voids, in which is poured a cementitious grout. The grout is of improved cementitious binder, which allows greater penetration of the void structure of polymer coated aggregate (PCA), and hence, greater achievement of the theoretical packing density. The resulting material is typically used as a wearing course between 25mm and 75mm in thickness.
Over a conventional bituminous material, this type of grouted material offers benefits like high resistance to permanent deformation, fuel spill resistance, lower temperature susceptibility, higher stiffness modulus and improved fatigue performance. These properties are used to custom-design pavement structures that are comparable or better than conventional bituminous pavements. The use of Grouted Macadam as a wearing course material in a flexible pavement construction has resulted in extended life compared to construction incorporating conventional bituminous materials. It has been extensively field tried and tested and has gained wide market acceptance among several public/private sector organizations. The Grouted Macadam technology offers a novel cost-effective alternative in the field of road making and has the potential to significantly contribute to the country's infrastructural needs.
Precast Concrete Pavements
In recent years, many organizations have started investigating strategies for faster pavement rehabilitation and reconstruction and can produce long-lasting pavements. Expedient rehabilitation that results in a shorter pavement lifespan is no longer considered acceptable by most highway agencies. A promising alternative rehabilitation strategy is the effective use of modular pavement technologies, principally precast concrete pavement (PCP) systems, which provide for rapid repair and rehabilitation of pavements and result in durable, longer-lasting pavements. Rapid construction techniques can significantly minimize the impact on the driving public as lane closures and traffic congestion are kept to a minimum. Road user and worker safety is also improved by reducing their exposure to construction traffic. Over the last few years, many initiatives have been undertaken to develop better guidance for use of the PCP technology, and many repair and rehabilitation work has been done worldwide using this technology.
In view of the growing number of vehicles with ever increasing axle loads, the conventional materials may not perform as intended. So, use of non-traditional materials available in the region of construction will be beneficial. Technological innovations are being undertaken in all sectors. These technologies not only guarantee improvement in performance of pavements but also enhance safety, conserve natural resources, protect the environment, and also improve the social and economic conditions. Tapping the opportunities for advancing pavement engineering and management through forward-looking innovation, is, therefore, the need of the hour.