Road markings perform a unique function for proper control and regulation of both vehicular and pedestrian traffic. They not only channelize or guide the traffic into proper positions on the roadway, but also supplement other regulations and warnings displayed by signs and signals. Other functions performed by markings are to act as barriers for opposing traffic, as warning devices for restrictive sight distance or passing distance and provide supplementary information for turning movements. In general, road markings aid motorists to drive safely without diverting their attention from the roadway.
In India, the use of road markings has been rather limited so far. Consequently there are innumerable roads that are unmarked, only partially marked or in need of remarking. In the interests of safety and efficiency, all our important roads require to be provided with road markings. Thus selection of the marking material to be used, how it is to be applied and when it needs to be replaced are important decisions that must be made after most careful consideration of all aspects.
In India, road markings are in general being provided with ordinary paints con- forming to grade 2 (having wear resistance of 1 to 6 hrs) as per IS: 164-1981. Experience shows that markings with these paints hardly last for 2 to 3 months and need repainting very frequently. Of late, use of hot applied thermoplastic paints for road markings has been specified for some externally funded highway projects on National Highways. The numerous advantages claimed for the thermoplastic paints over the conventional paints are longer life (2 to 3 years), excellent visibility both during day and night, quicker drying time, improved skid resistance, good abrasion resistance, non cracking and stability on laying, good bondage to the surface and no change in colour due to atmospheric changes.
Thermoplastics are materials that change their physical state (reversibly) with changes in temperature. Thermoplastic marking materials are solids at ambient temperatures and liquids at elevated temperatures. They are applied at elevated temperatures by spray or extrusion equipment and cool rapid lyon the road to form solid marking materials.
Thermoplastic marking materials are a mixture of resins glass beads pigments and fillers. Long life traffic lines can be installed by manually propelled thermoplastic machines for small applications such as crosswalks and stop lines or by large self propelled machines with large tanks for heating the thermoplastic material. In either case, the machine must heat the material to the specified temperature and must also have supplemental equipment such as bead storage and dispensers for, spraying reflecting glass spheres onto the hot lines. Skipping devices are a necessity for lane line and centre line applications.
Experience abroad reveals that use of proper application temperature is crucial to the successful performance of thermoplastic marking materials. Most specifications call for an application temperature of 218°C which provides near optimum bond strength for these materials. Some specifications also call for an atmosphere and pavement tem- perature of 10°C to 16°C.
Spray applied thermoplastics are often applied at thicknesses between 60 and 90 mils (1.5 and 2.3 mm) whereas extrusion applied thermoplastics are often applied at thicknesses betweem 90 and 120 mils (2.3 and 3 mm). There are two basic methods used to extrude thermoplastic marking materials. One method is called the 'dragging die' or shoe method. By this method hot material pours into a die or shoe that has a gate. When the gate is opened to a preset gap, it forms the width and thickness of the line as the applicator drags the shoe forward. This is the most widely used method of extruding thennoplastic. The other method is the 'ribbon extrusion' method. in which a ribbon of moltenthennoplastic from a moving application machine is allowed to lie on the pavement Although extrusion methods of pavements are slower than spraying, they provide lines with sharper, clearer edges than those obtained with spray equipment.
It is necessary that thennoplastics should be applied to clean and dry pavements. When such markings are applied to portland cement concrete surfaces, they should be preceded by a surface treatment with a two part epoxy primer.
When purchasing thermoplastics on a contract basis, the purchaser wants to make sure that the full thickness of material contracted for is applied The applicator wants to apply what is called for and no more. It is difficult and expensive to measure film thickness of thermoplastics after they are applied. Film thickness measurements of materials being applied are normally done by placing metal plates in the path of the application machine and then picking them up and measuring the film thick- ness after the application machine has passed over them and applied the thermoplastic.
The successful performance of thermoplastics is closely tied to proper application. It is, therefore, essential to give more attention to crew training and better supervision. Records of materials used contractors employed and application conditions should also be kept so that they can be evaluated and necessary improvements made when remarking becomes necessary.
Thermoplastic marking materials cost five to six times more than paints, but they last for a period of about three to five years. Even though the materials have higher initial cost, their longer life and other advantages such as lower maintenance and better visibility, especially at night, more than make up for the difference. It would, therefore, be a good practice to insist on the use of such materials for road markings on all-important highways.
Pavement markings using hot applied thermoplastic paints have been laid in Bombay City as far back as in 1981. These have also been provided along National Highways in Nasik and Pune in May/June, 1984 and in recent years on NH 45 in Tamil Nadu. As reported these markings have stood the rigours of heavy vehicular traffic and climatic fluctuations and performed well.
Research work for indigenous development of thermoplastic paints and charac- terisation of glass beads is at present in progress by Central Road Research Institute and National Physical Laboratory, New Delhi respectively as part of Ministry of Surface Transport (Roads Wing)'s Research Project R-40. The results of some of the formulations developed are quite encouraging. Final formulations might be available for commercial exploitation in about a year's time.
At present, the bulk of thermoplastic marking material as well as the machines required for installing it are being procured from abroad There are also very few contractors carrying out work of markings with hot applied thermoplastic marking material. There is a need to substantially increase its usage especially on all the high traffic density corridors on our primary road network. This would encourage indigenous production of thermoplastic marking material as well as the related application machines and bring down substantially the initial cost of such markings. |
