TrafficInfraTech Magazine Linking People Places & Progress
Warm Mix Asphalt (WMA): Warm mix asphalt has conquered the asphalt industry – it is not a stretch to say that in the very near future most, if not all, asphalt mixes in most countries will be warm mix asphalt. In WMA, the lowering of viscosity of asphalt at a lower temperature enables the mixing and compaction at lower temperatures, reduction in energy and smoke, as well as a host of other emissions to which road workers are exposed everyday. The long list of benefits include reduced emissions, reduced fuel usage, capability to pave in cool weather and still obtain density, haul-mix longer distances and still have workability, improved compaction, facilitation of deep patches, ability to use more RAP, reduced worker exposure as well as less short term aging of mixes during production.
Soil Engineering: Developments in soils related to pavement engineering include the use of geocells and nano-engineered materials for soil stabilisation and dust control. Geocell is a type of a “three dimensional” structure formed by welding strips of high density polyethylene into a honeycombed structure. Different types of thickness of the material, as well as height and aspect ratio are used for geocells, and they can be backfilled with aggregates. Geocells are utilised as effective means of improving soil confinement and hence, the bearing capacity, for example in poor quality subgrade. New generations of dust control additives are being developed on the basis of nanotechnology. These additives are environmentally safe, biodegradable, cure quickly and can be applied without modification of conventional spraying equipment. Such products can be used in military applications such as landing areas for helicopters and civil airports, and can be used for unpaved roads and in industrial areas for dust control.
Multi-scale and Precision Engineering
The scale of things that we are dealing with in pavements is mind boggling – it ranges from 10-10m (molecular level) to 1m (concrete cylinder). The trend in research in both academia and the industry is “precision engineering” materials with molecular dynamics, nano technology, atomic force microscopy and advanced modelling to obtain the most desirable materials. The idea is to understand the interaction of materials at a molecular level so that their properties in the micro and macro levels can be precisely controlled. The idea of self healing pavements is not very far-fetched – it will happen in the near future. Self healing pavements refer to such technologies in which special polymers in asphalt mixes will be able to heal (close) cracks in asphalt pavements and very tiny hollow fibres will release sealants to seal cracks in concrete pavements.
Developments in soils related to pavement engineering include the use of geocells and nano-engineered materials for soil stabilisation and dust control. Geocell is a type of a ‘three dimensional’ structure formed by welding strips of high density polyethylene into a honeycombed structure.
Designing methods – empirical and mechanistic empirical
Design is a very complicated word, and we have realised that simply going to charts and plucking a number will not be sufficient to get economic, good performing and long lasting pavements for the buzzword in today’s technology is optimisation. Pavements are engineered structures and just like aeroplanes, we need to make them safe and durable and hence, they should be designed properly. The recent trend is to move away from empirical methods to mechanistic empirical methods of design where we use sound principles to predict stress and strain, and utilise the acquired real information regarding traffic and climate. By incorporating the process in pavement evaluation systems, we develop a comprehensive structural design procedure which can be used to predict various forms of distresses in different years, as well as the alteration in pavement material properties with a change in climatic conditions. The system requires local calibration but we hope to reach a day when we can predict with good certainty how many years a pavement would last, and hence, optimise our pavement construction and management system.
Although composite pavements, with cement treated base and relatively thin asphalt mix surface layer, have been used in South Africa for many years, it is only of late that other parts of the world have exhibited interest in them. They are attractive primarily because of their ability to carry heavy loads, to survive long times with appropriate maintenance and to reduce the amount of processed or mix aggregates that is needed for constructing equivalent all-asphalt or all-concrete pavements.
Construction – conventional vs. modern methods
What is interesting is that along with developments in the mix design of various types of materials and mixes have come the very practical developments in equipment and plant that make the implementation of modern concepts feasible. “Heated Dump Body”, for example, refers to a transport vehicle that is capable of diverting engine exhaust and transmitting the heat evenly throughout the dump body to help keep the porous friction course (PFC) at the desired temperature. Another example is of vertical storage tanks. They are often used in place of conventional horizontal tanks because of the increased efficiency of agitation and product circulation during the storage of modified asphalt binders.
Use of a material transfer device between trucks and pavers to solve temperature segregation related problems are becoming routine, particularly in large projects. This device is a vehicle that receives the mix from the trucks, keeps it at an elevated temperature and also mixes it while storing, and then as required, transfers through a conveyor system to the paver – this way the paver does not need to stop (stopping of paver leads to non uniform flow of materials resulting in poor condition roads) because of non uniform supply of materials from trucks (because of reasons like congestion and traffic jam).
One weak point in building roads has always been the uncertainty about the level of compaction and density in the field. To solve this problem, many modern rollers with “intelligent compaction” have been introduced in the market. Examples: Rollers used to compact asphalt pavement on a bridge. The sensors keep monitoring and reporting the stiffness of the road as the roller keeps rolling and compacting the road – this is real-time stiffness monitoring as opposed to roll (compact), stop, check density/stiffness, and then again roll.
A recent trend in the pavement industry is the acceptance of the fact that joints in asphalt pavements need to be constructed properly and the most appropriate technique must be adopted to construct a watertight and adequately dense joint. These could range from notched joint to cutting edge of cold joint – along with the adoption of a proper specification.
Testing methods and equipment have seen major advancements. These range from non-destructive testing such as seismic and ground penetrating radar, to laboratory dynamic modulus, to overlay tester and micro level testing such as contact angle and surface energy measurements, and finally to rolling wheel deflectometer – the deflection testing equipment that evaluates pavement while travelling at 40 miles per hour.
Detection of problems and forensics : Sophisticated technology has been used to identify problems and provide solutions. For example, the use of infrared imaging has led to the identification of thermal segregation which adversely affects the quality of pavements. To reduce its adverse effect, material transfer devices have been increasingly used. Better guidelines have also been developed to conduct forensic investigation of failed pavements so that proper remedial measures can be taken.
Concrete Pavements : Significant improvements have been taking place in the concrete pavement area. Major technologies include the use of roller compacted concrete, an improvement on slipform paving technology which can now handle variable width pavements, along with automated dowel bar inserters, and stringless paving. Increasingly, non-Portland cements are being introduced such as those manufactured from fly ash and geopolymer cements, and the use of photocatalytic agents to oxidise material from concrete pavement surface and reduce pollutants is being researched. Diamond grinding is becoming common to improve rideability, and non-destructive testing such as maturity testing is makingfast track paving projects common.
Maintenance, Management and Human Resources
One tremendous advancement in pavement engineering has been through the realisation that pavements are resources that we utilise for our infrastructure and just like property or money that we could leave for our children, it is an asset that we need to protect and manage. Hence comes the concept of asset management – not a new concept – but new in its implementation in the case of pavements. Pavement preservation is being given high priority through the application of a host of techniques such as chip seals and thin lift paving – the key aspect being the identification of the right treatment at the right time for different types of pavements – with a focus on life cycle costing and the use of advanced methods and software for pavement management.
As budgets have become tighter everywhere, and the margin of error has decreased, we are increasingly resorting to real world or accelerated pavement testing to evaluate materials, processes and models, and finetuning them. This helps in adopting new technology and understanding the effect of new factors such as tyres. Such accelerated pavement facilities range from New Zealand to Scandinavia and Japan to the US, both for highways and airfield pavements.
The investment in human resources has increased. It has dawned upon us that however good the technology, if the engineers, technicians and workers are not properly trained and checked by certification, the money spent on it will be a waste. In the US, both regional and national level certification programmes are becoming common, just like the training in safety, cost and energy saving features is. In India, training of engineers is being conducted on a national and regional level by organisations such as IIT Madras but significant effort is needed for a comprehensive programme of training and certification of both engineers and technicians – to ensure good quality design and construction of pavements.
Rajib B MallickProfessor
Dept. of Civil Engineering Indian Institute of Technology, Madras
A Veeraragavan Professor Transportation Engineering Division Indian Institute of Technology, Madras