Ramesh Palagiri, Managing Director and CEO, Wirtgen India, elaborates on the paradigm shift in the use of road construction, rehabilitation methods and technologies in India.
The Ministry of Surface Transport has set an objective of building 40 km of roads a day from 2017-18 from the current level of 18 km a day. This has resulted in a paradigm shift in the use of road construction and rehabilitation methods and technologies. Current conventional construction practices are slowing getting replaced with new technologies and this is required for achieving good ride quality and durability, while achieving construction speed and keeping costs in control to achieve the target of 40 km per day.
Current construction practice
Currently, the pace of concrete pavement construction has picked up and 30-40 per cent of the road construction is concrete roads. However, the larger portion is still with flexible pavements. Considering the current scenario, the market will look at alternatives in the next three-five years to cater to speed of construction while looking at resources like aggregates required for construction.
While concrete pavements promise long-term performance, the challenge remains as far as construction quality is concerned. In contrast, flexible pavements have ease of construction, but the overall cost remains a challenge, especially due to fluctuating crude prices, which eventually determine bitumen prices.
At Wirtgen Group, we have been continuously innovating on how to build better roads. Here are some of the technologies already available and relevant for Indian conditions.
The following technologies would be a standard part of Indian highway construction programmes in coming years:
In-line paving of hot-mix asphalt layers: Combined paving of upper DBM and BC;
Mobile feeder: This will ensure seamless feeding of hot mix into paver, thus avoiding the joint-free paving while also ensuring perfect levels;
Road Scan: This system will ensure temperature of the paved asphalt material at the time of paving while recording the temperature of mix;
SprayJet paver: This application will ensure that the emulsion is sprayed just before the paving of hot mix asphalt, avoiding oxidation of tack coat and dust-free application of the same, even while bonding. The same paver does the tack coat and the bitumen layer;
Double layer pavement quality concrete application: The double layer paving of concrete pavements will allow the use of marginal/recycled/rap aggregates in base layer, while the top concrete layer will act as a concrete wearing layer. The dowel could be placed in the bottom layer to ensure proper position even in high slump mixes;
Cement stabilised bases/sub-bases: These not only offer a stronger foundation, but also reduce the requirement of fresh aggregates by 30 per cent. The stabilised bases will also reduce the consumption of asphalt layer in flexible pavements;
Cold recycling for rehabilitation of existing asphalt highways: Cold in situ recycling with foam bitumen will ensure structural strengthening of a damaged pavement by addition of 2 per cent bitumen and cement while reusing 100 per cent of pavement, which is re-granulated by milling and in situ or in site and rebounded by addition of bitumen foam and cement. This process would ensure huge savings of fresh resources and totally eliminate the heating cost of aggregates.
In-line pave hot-on-hot joint paving technology
Roads built using twin layer, hot-on-hot paving techniques can last considerably longer than those constructed using conventional methods. This is achieved by the elimination of the need for emulsion spraying between binder and wearing courses and with the use of thin layer wearing course designs. Market leader Joseph Vögele offers the technology of in-line paving of hot bitumen mixes, paving the binder layer (DMB) and wearing coat (BC) together in one pass application, thus completely allowing the cold joint.
The machines used for the in-line paving system are all based on standard units, but with modifications, which allow them to carry out the specialised processes. The Super 2100-2IP is the key to the job due to its specially modified, high compaction screed. To achieve the necessary 98 per cent pre-compaction of the binder course, the TP2 version of the AB600-2 screed is used. This features altered tamper stroke lengths, a special tamper geometry and special pulsed-flow hydraulics with two pressure bars for high compaction.
This is said to provide sufficient compaction of the binder course so that the second paver can operate directly behind on the newly laid mat without risk of sinking into the hot surface and also without the need for the use of rollers in between the two pavers.
The Super 1800-2 paver that lays the wearing course is a standard machine and its only special feature is a control system that ensures it maintains an optimum distance from the paver operating in front, to prevent bumping and also eliminate the risk of it operating too far away from the first machine. Meanwhile, the MT1000-1 material transfer vehicle is also a standard unit.
The machines are conventional to operate and an experienced paving crew needs only a relatively short training programme to understand the process. However, a key factor for hot-on-hot paving is the need for efficient on-site logistics. A reliable supply of trucks, transporting both binder and wearing course materials, is crucial.
SprayJet versus conventional paving
In many countries, paving binder and surface courses with conventional pre-spraying of emulsion is a standard practice when rehabilitating roads. Normally, an emulsion of bitumen and water is applied to the milled base by spray bars and the water is allowed to evaporate overnight. This is known as 'breaking' of the emulsion. A closed-textured bitumen film remains after the process, which is subsequently overlaid with asphalt. The considerable time and equipment required is a disadvantage of this conventional method. A further problem is that the surrounding areas, roads and curbs are fouled with emulsion by job site vehicles. These problems are now a thing of the past, thanks to SprayJet technology from Vögele. With the SprayJet, the fresh emulsion is directly overlaid with asphalt as soon as it is sprayed.
Mobile feeder from Vögele: The contactless transfer
These high-performance material feeders are the ideal partners on every modern and well organised large-scale job site. The extremely high mix storage capacity and the innovative, heavy-duty transfer concept allow non-stop and non-contacting feed of the paver with mix. The equipment is capable of transferring as much as 1,200 tonnes of mix per hour. The trough-shaped and extra wide belt conveyor installed in the feeders provides for an accurate flow of mix and a clean job site.
Here are a few more advantages that these feeders confer:
Uninterrupted paving thanks to a total storage capacity (material feeder and paver) of 45 t with a conveying capacity of 1,200 t/h;
Non-contacting material transfer ensures maximum paving quality; Homogenised material in the receiving hopper of the material feeder due to conical augers;
Wide range of applications thanks to the pivoting and inclining conveyor.
RoadScan: Paving hot mix at hot temperature
Making quality measurable is one of the major challenges for contractors and clients worldwide. In road construction, it is particularly important to verify that a constant temperature of the freshly paved asphalt has been maintained, as this is a key criterion for ensuring the quality and durability of roads. As a result, the significance of area-wide temperature monitoring is currently on the rise in an increasing number of markets. With the non-contacting Road Scan temperature measurement system, Vögele provides an innovative and economical solution for the road construction sector.
The scanning and documentation of the freshly paved asphalt's temperature is already specified in tenders in several markets, and in others there are definitive plans to implement this. With its new RoadScan temperature measurement system, Vögele delivers the technology required for the job.
That's not all; the system helps road construction teams to continuously monitor the temperature of the supplied mix. After completion of the construction job, contractors can furnish detailed evidence that the temperature was maintained within the correct range ù thanks to the recorded GPS data, including precise tracking.
The high-precision infrared camera is the key component of the Vögele RoadScan system. It scans the asphalt pavement behind the screed over the entire area. The housing of the infrared camera also accommodates a high-precision GPS receiver. It records the exact position of the temperature data. The measuring width of 10 m comprises 40 grids of 25x25 cm-sized tiles. Each of these tiles contains up to 16 single measuring points which are used to calculate a mean value. The measurable temperature range lies between 0°C and 250°C with a tolerance of only °2°C.
Two-lift concrete pavement
Two-lift concrete paving involves the placement of two wet-on-wet layers instead of the homogenous single layer commonly placed in concrete paving. Two-lift paving can provide quality surface characteristics, reduce materials costs, and consume recycled aggregates (concrete and milled hot-mix asphalt). This scarcity of aggregates ù combined with advances in material knowledge and construction equipment and increasing demands for pavement surfaces that meet specific noise, durability, and safety objectives - is prompting many agencies and contractors around the world to consider two-lift paving as a sustainable solution for building concrete pavements.
Benefits of two-lift paving as a sustainable solution
Sustainable solutions are those that balance economic, environmental and social benefits. The two-lift process has great potential as a sustainable paving solution. It reduces the cost of materials and materials transportation, reduces the environmental impact caused by quarrying and importing aggregates, and increases social benefits by maintaining friction and low noise levels, thus providing a long-lasting pavement that does not need frequent reconstruction.
Economic benefits: In two-lift paving, the bottom layer is thick, typically 70 per cent of the total pavement thickness. The bottom lift generally contains locally available or recycled aggregates that may not be suitable as a wearing surface.
Environmental benefits: Two-lift paving can be optimised for recycling and reuse. Since the bottom lift is not subjected to as harsh an environment as the top lift, a variety of recycled aggregates, including those reclaimed from old concrete pavements, can be used in the bottom lift without sacricing durability.
An exposed aggregate surface is one example of optimised surface treatment. Results from Europe, where exposed aggregate surfaces are commonly used, suggest that these surfaces have good tyre pavement noise characteristics and demonstrate durable skid resistance, making these pavement surfaces both safer and more comfortable.
Cement treated sub-base (GSB) and bases
The aggregates are important ingredients of pavement structure. Good quality aggregates may not always be available in the vicinity of road construction sites. Transporting the aggregates from long distances may not be economically feasible. Under such circumstances, locally available inferior quality material like soil or gravel may not satisfy the requirement as pavement material. Therefore, engineering properties of these materials are modified by means of a process, known as stabilisation.
Advantages of stabilisation are:
Basic ingredients for cemented base/sub-base in pavements
Portland cement and lime are the most commonly used cementing materials for construction of cemented base with soil, sand and aggregates for pavements of roads and runways. Low grade or marginal aggregates with suitable proportioning of coarse and fine fractions, lime, and lime fly ash, can be used as a cemented base or sub-base. Natural soil, fly ash, sand, stone dust, riverbed materials, reclaimed asphalt pavement, low quality aggregates, etc., are considered ingredients of pavements with cemented base/sub-base.