Hot Mix Technology: On Solid Ground
The current challenges in hot mix asphalt pavement-making in India is to manage sustainability, conserve energy and natural resources, eliminate waste yet achieve a pavement quality with competitive cost per unit length. EQUIPMENT INDIA trains its gaze on the emerging hot mix technology, detailing the pros and cons of drum type and batch type hot mix plants, with a focus on reduction of greenhouse gases, higher rates of recycling, use of specialised mix designs and operational safety.

Hot mix technology is evolving in India, and the pace is quite rapid. However, most of the hot mix producers in the country are struggling, since the equipment they own isn't geared to reap the benefits of modern hot mix technologies., says Blesson Varghese, Managing Director, Marini India. He adds, 'Marini has inherited the Fayat 5E philosophy, it is designed and supplied as a Future Fit machine. So, our users can very easily adapt and evolve to the challenges they face in the market. As responsible leaders in the field, congruent to our commitment to sustainable development, we have launched a new initiative called the Marini Future Fit, aimed at helping hot mix producers to ramp up their equipment and unleash the power and potential of modern-day hot mix technology.

According to Somnath Bhattacharjee, President 'Material Handling Solutions and Equipment & Project Solutions Business, TIL, the current challenges in hot mix asphalt pavement-making in India is to manage sustainability, conserve energy and natural resources, eliminate waste yet achieve pavement quality with competitive cost per unit length. Somnath says, 'Key government and regulatory agencies in India like MoRTH, NHAI, IRC,CPRI, have initiated significant steps by encouraging the recycling of existing asphalt in road-making and maintenance in order to reduce carbon emissions in road construction activity, besides capturing significant benefits in cost and energy saving. Research has shown that combining Warm-Mix Asphalt (WMA) and Reclaimed Asphalt Pavement (RAP) offers a significant improvement in pavement performance. The recycling of existing asphalt pavement material for making new pavements results in considerable savings in financial and material resources and energy.'

Says Sudhir R Hoshing, Chief Executive Officer, (Roads), Reliance Infrastructure, 'Road construction technologies have evolved over time to focus on efficient as well as sustainable quality of pavement construction. Some of the key developments in this regard have been advancements in hot mix plants, sensor pavers and intelligent compaction technologies.' He adds, 'A hot mix plant is supposed to receive aggregates and then, heat/dry/screen/measure and mix them with a measured quantity of filler and bitumen, to continuously produce a homogenous mix ready to be laid at the site. Advanced hot mix plants use automation to execute its function, in the process saving considerable amount of energy compared to conventional techniques. Unlike mechanical pavers, sensor pavers are provided with electronic sensor device for slope and grade control. Our road construction practices/methodologies/design mix have come of age with the use of HMP and sensor pavers, and can now conveniently adopt this technology. Even lenders are insisting on the use of environment-friendly construction methodologies.' ?Asphalt mix designs are conducted on based on American Asphalt Institute Standards and Specifications. Asphaltic concrete is a mixture of coarse aggregate, fine aggregate, mineral filler and bitumen,' states Divakar Marri, Head - Corporate Planning & PMO, Ramky Infrastructure. According to him, the desirable properties of asphalt mix are:

Stability - enough resistance to deformation under sustained or repeated loads.
Durability - Resistance to disintegration by weather or abrasive force of traffic.
Flexibility - Ability of a bituminous mix to bend repeatedly without cracking and to conform to changes in the shape of the base course.
Skid Resistance - Offers enough resistance to the skidding of tyres.
Impervious Layer - Should be highly impervious to water.
Some influential factors for a good asphalt mix are: selection of aggregates, selection of aggregate gradation, proportioning of aggregates, selection of bitumen. Some of the problems relate to integrated mix design structural design approach, damage considerations in pavement design and design of pavement with recycled/ marginal material, adds Divakar.
Selection Criteria
Hot Mix Asphalt (HMA) production is the first step in the construction of roads. Mixing plants are generally categorised as a) batch plants, which produce a series of asphalt batches and b) continuous mixing plants (these are generally of the drum mixing type), which produce a continuous flow of asphalt. This categorisation reflects not only in the mode of production but also highlights different quality control needs. Plants may be used and/or modified to cater for production of asphalt mixes using recycled asphalt. A number of variations of both batch and continuous drum mixing plants have been developed for recycling.

Says Divakar, 'The choice of a batch or drum mix plant depends upon business factors such as purchase price, operating costs, production requirements and the need for flexibility in local markets. Basically, a batch plant is the best choice for an operation where the plant will be called upon to make various blend mixes in the same operational run, or if the plant will be doing a lot of starting and stopping in the course of the day. A drum plant is the correct plant for a company not concerned with switching back and forth to various mixes and one that is able to afford the luxury of long production runs. Another consideration is the fact that batch plants utilise many more moving parts than drum plants, therefore they are more expensive per tonne to operate and year-end maintenance costs are higher, too.'

Blesson states, 'Both the drum and continuous plants deliver good asphalt mixes, provided they have an efficient gradation and precise metering of aggregates, fillers and bitumen. Without proper systems in place, neither of the plants can deliver good results. Heating of aggregates and control on emissions are equally important. A batch plant is more versatile when it comes to producing different mixes. It also has the advantage of making some correction with its secondary hot screening of aggregates. The decision to opt for drum / batch type, therefore, should be made only on the basis of the final mix quality, metering precision and conformance to emission standards.'

According to Sudhir, a batch hot-mix plant has better control over the ratio of aggregates, fillers and bitumen as compared to a drum mix plant. In view of the above, a batch type hot-mix plant is preferred over a drum mix plant with its added advantages such as low maintenance, energy saving, compact design, easily transportable and compact modular units, easy and quick onsite installation with pre-assembled stackable modular units. He adds 'In a nutshell, the ability to vary the mix formula and batch size from one batch to another makes the batch facility quite versatile.'

Says Blesson, 'Marini is India's most environmentally friendly asphalt plant. Marinis are never measured on spm levels but on the total impact on the environment. VOCs, COx, NOx,Sox are constantly monitored to ensure that Marini's impact on the environment is green. We have proved it several times over, in the country. The best example is the only plant in the heart of Delhi, producing over one million tonne of asphalt over a short period, and that was a few years ago. At Marini, we are committed to sustainable development and all our plants are equipped to meet standards way beyond the normal standards in the country.'

He adds, 'Compared to asphalt plants, a Marini ensures massive savings on carbon emission. A Marini producing regular hot mix delivers carbon savings equivalent to 16, 000 trees. With the Profimix technology, we are now recording massive savings equivalent to 22,700 trees each year.'

According to Divakar, almost all the batching plants are equipped with baghouses. The baghouse receives the exhaust gas stream from the dryer/mixer unit and it ?lters out dust and small particulate (?nes) carried by the exhaust stream. The baghouse fines are returned to the asphalt mixing plant, some producers (probably less than ten per cent) with excess dust dispose of it by sluicing it to a settling pond or returning it to the quarry. Silent generators are also in wide use to avoid noise pollution. The choice of a specific brand depends upon the vendor, who puts us first and can provide us with a plant matching our exact needs as well as great after sales service.

Speaking on the use of RAP, Divakar had this to say. 'Typically, asphalt is produced using 95 per cent rock and sand from rock quarries, and five per cent binder oil that must be shipped from oil refineries. Rather than purchasing one hundred per cent of the oil needed to produce asphalt, or using hundred per cent virgin materials that have been extracted from hillside quarries or out of river beds; using special equipment, we can incorporate recycled road millings that are already coated with binder oil from old asphalt roads that need to be rebuilt or resurfaced. Re-using already coated rock, leads to a reduction in the dependence on petroleum products, shipping of those products, keeping this material out of our landfills, and such.'

Says Sudhir, 'Our contractors work with batch type hot mix plants conforming to the specifications laid down by the Ministry of Road Transport and Highways (MoRTH). The plants are energy efficient, low in noise generation and help minimise air pollution. Moreover, we regularly conduct air and noise tests near the plants as well as frequent sprinkling water to control dust pollution. We opt for internationally acclaimed premium brands to ensure quality and environment-friendly construction.'

Blesson sums up the scenario on a positive note. 'With millions of kilometres of unpaved roads and the need for upgradation in India, the future of the HMA industry is quite bright. Proper planning and execution of the road development programme in India has very high potential.'