Er Jagvir Goyal shares his views on the equipment for asphalt roads.
In the last two decades, there has been unprecedented growth in road traffic in India. Cargo transport has grown by 60 times during the last 60 years. Passenger traffic has risen by 70 times and number of vehicles by 85 times. In comparison, the road network, though under attention, has not grown in equal proportion. The National Highways constitute a small percentage; 70 per cent of the road network consist of village roads. Their condition is unimpressive. The situation demands a fresh focus and emphasis on quality control and better use of road construction machinery for faster and better work.
Indian roads are gradually turning into a network of rigid and flexible pavements. Concrete roads are being adopted more and more, yet as of date, the major percentage belongs to asphalt roads or flexible pavements. Each type of pavement has its own advantage and disadvantage. Flexible pavements have the great advantage that they can be strengthened and improved in stages with the growth of traffic. Another major advantage of these roads is that their surfaces can be milled and recycled. They are less expensive, too. Concrete roads, on the other hand, offer better skid resistance, need lesser maintenance, allow better vehicle mileage to road users, and do well when subjected to freezing and thawing conditions. They are 15-20 per cent costlier than asphalt roads but are being strongly recommended these days due to other advantages and lower maintenance requirements.
As the vehicular and passenger traffic is bound to increase further in India in an unabated manner, there is an immediate need to adopt the latest techniques for road construction and to maintain and widen the existing roads. It is therefore, time to take into stock the latest developments and adopt the latest equipment to the maximum possible extent.
The main equipment that can be listed for road construction is:
1. Equipment for compaction
2. Equipment for mixing
3. Equipment for paving
Compaction equipment can be listed as:
a) Smooth wheeled rollers
b) Sheep foot rollers
c) Pneumatic tired rollers
d) Vibratory rollers
e) Oscillatory rollers
Smooth wheeled rollers rely on weight only to compact the layers. Sheepfoot rollers are suitable for cohesive soils. Pneumatic tired rollers are suitable for bituminous roads, as well as earthwork. Proving superior than simple heavy-weights, vibratory rollers have seen greater improvement and use for compaction of base courses. These can be used for compaction of soil as well as asphalt surfaces. They act on the principle of eccentricity, and their main components are an eccentric weight and a rolling drum. The eccentric weight is made to rotate by a motor. Its connection to the roller drum produces vibrations in the drum which repeatedly lifts itself and bounces against the base, sending impact waves through it and thus, compacting it to a greater depth. Such rollers have proved highly effective in sandy soil. The compaction efficiency of vibratory rollers is higher than smooth wheel rollers. For compacting water-bound macadam, a vibratory roller can be used instead of a 10 tonne smooth wheel roller.
For proper compaction, the depth of layer to be compacted and the type of material needs to be considered. While hard stone aggregate needs a 10-12 tonne roller for compaction, brick aggregate can be compacted by a 6-8 tonne roller. In sandy soils, a 6-8 tonne roller will need to be given 8-16 passes. In clayey soils, a sheepfoot roller will need to be passed 8-16 times and a smooth wheeled roller 4-8 times. The thickness of the stone aggregate layer cannot be more than 90 mm and that of loose soil, not more than 25 mm.
Oscillatory rollers are an improvement over vibratory rollers. In these rollers, there are two eccentric weights instead of one, as in vibratory rollers. These two eccentric weights cancel the vertical vibrations of each other and thus, cause horizontal vibrations only. Due to the cancellation of vertical vibrations, the drum does not lift above the ground but causes oscillations in it, resulting in better compaction of upper layers of soil. Such rollers prove that even if they are 25-30 per cent lighter than vibratory rollers, they offer better and smoother compaction. Their fuel consumption is also lesser. These rollers are quite useful when an under-laid sewer or a nearby foundation is to be saved.
Laying of pre-mix on the roads has become a common term these days. A pre-mix can be the cold mix or hot mix type. Pre-mix laying has advantages over conventional and old methods of surface dressing and penetration methods of road construction, as it ensures easier quality control, controlled thickness, mass production, better compaction and uniform and proper coating of aggregate with bitumen. It also economises bitumen consumption and is suitable for heavy- duty roads.
While the cold mix plants involve mixing of cold or lukewarm aggregate with hot or cold bitumen, hot mixing involves mixing of hot bitumen with heated aggregate, the aggregate heated to near bitumen application degree.
Hot mix plants produce a mix of heated ingredients ready for laying at site. The choice of a hot mix plant mainly depends upon the quantum of work to be handled as it determines the capacity of the hot mix plant. Pollution norms and standards also become a compulsory factor as it is required to check whether the hot mix plant emissions fall within the permissible limits in force in the area where the plant is being used.
Equipment for paving
In order to have the best road surface, excellent finish and excellent riding quality, the use of slip form pavers has become mandatory in developed countries. Though India has also been using pavers now for quite some time, the usage is still mostly infrequent. Pavers are wonderful machines that can give utmost accuracy in road construction and should be used for maximum road work; this can be possible by making their use mandatory in tender specifications. Pavers can be used both for concrete as well as bituminous roads, they are both crawler type and wheel-mounted, the former used for larger road widths.
The pavers are continuously fed by the mix through a hopper in the front. The mix received by the hopper is distributed over the operating width and to the required thickness by use of conveyors. The pavers are enabled to adjust themselves to the changed width of road where necessary, by adjustment of hydraulic screeds. The laid mix is required to be compacted by use of rollers though the paver may compact the layer to some extent. The conveyor belts are the self-cleaning type and don’t allow the mix to stay on them. The screeds can be controlled independently to adjust them for the road width; standard and extending screeds enable paving operations at widths ranging from 0.5-16 m.
Automatic controllers control the longitudinal level and transversal slope of roads. Electronic level sensors are mounted on the pavers for longitudinal level checking and these check the material thickness with respect to a fixed datum. Transversal slope controllers are mounted on crossbeams and send signals as and when there is a change in slope. In addition to these electronic controllers, manual controls are kept stand-by. Nowadays, microwave technology and laser systems are being inducted in road construction equipment control.
Equipment for recycling
Recycling of road material is an aspect extremely important to a country like India which cannot spend heavily on relaying altogether new roads along the worn-out ones. Patchwork is of no use in India; after one shower or two, roads regain their deteriorated condition. Laying of a seal coat over the worn-out road again proves useless as it is unable to provide strength and stability to the road. Moreover, relaying of seal coats not only proves expensive but raises the level of the roads thus disturbing the drainage system, side-walks and the berm position. Cold recyclers mill the broken road surface, add cement or bitumen emulsion to it and relay the course in a single cycle. A lot of saving is thus achieved and road level is also maintained. The Mumbai Municipal Corporation have used cold recyclers to repair their asphalt roads and results have been quite satisfactory.
Essential features of recyclers
The essential features of a good recycler are as under:
1. It should be able to do milling in any direction while moving in a forward direction.
2. The power output control should be automatic.
3. The milling and mixing drum should cater to all applications.
4. Volume of mixing chamber should get adjusted automatically according to the quantity of material to be processed.
5. The recycler should be able to use all binders, bitumen emulsion or cement or both.
Traffic markings such as centre lines, zebra crossings, edge bars, stop bars, direction arrows, bicycle lane markings and parking lines on the roads play an important role in the regulation of traffic. Often, these get rubbed off and need frequent re-painting. This problem has assumed more significance with the increase of traffic and the advent of radial tyres, which have a larger contact area with the road surfaces. Some permanent solution needs to be found for this recurring problem of traffic markings getting rubbed off repeatedly.
Another important requirement is to ensure visibility of road markings during night time. In general, this aspect is not paid due attention. Usually, adequate light is available at the crossings but not along the routes. It is not possible to provide fluorescent paint markings all along the road lengths because of cost restraints and high glare problems. Fluorescent paint glare may sometimes blind the vehicle drivers, resulting in their losing balance and causing mishaps.
Road-marking machines are hand-operated or self-operated. Equipped with spray nozzles and air compressors, these are connected to the paint tank and have the mechanism to draw paint and spray it in a controlled manner. Now, machines with cold plastic coating material that becomes hardened through a chemical reaction and serves as a permanent marking paint, are being used. This cold setting marking paint is thixotropic and carries refractive glass beads in it, which shine during the night without causing any glare to the drivers and make the markings visible during night. The paint hardens within 20-30 minutes of its application and thus, can be used without much hindrance to road traffic.
Produced in both the colours used for roads, ie, white and yellow, it has two components which are mixed and stirred well before applying. As the setting time of the mixed paint is 20 minutes, the mixture need to be used within this period. Thus, machine marking is preferred over manual marking for better efficiency, good and accurate marks and optimum coverage.
The road-marking machines these days, carry an auto glass bead application system. The glass bead container has a capacity of about 10 kg and the flow of glass beads from it can be increased or decreased through a knob provided for the purpose. Glass bead application activates automatically as soon as paint application begins. The bead container level indicator ensures addition of more quantity of beads to the container well in time. Aquarius Technologies produces versatile road marking machines in India.
Paint coverage capacity of road-marking machines is quite good and the paint consumption ranges between 1.5-2.0 kg per sq m per mm thickness. The paints used should be weather-resistant, skid-resistant and tested for day and night visibility. White paint should have the property of non-yellowing with time.
* Author, Technical books; Columnist, Technical journals; Deputy Chief Engineer Civil and Recipient, TIET Distinguished Alumni Award 2005.