Being one of the oldest road construction materials, the asphalt enables high performance, long lasting, safe and environmentally friendly roads to be constructed. Today, 95% of the 3.63 million km long coated roads in the USA and the all of the coated roads in Germany, one of the most developed countries of Europe, are asphalt coated. Today, 72% of the motorways with a total length of 12.000 km in Germany are asphalt coated, and the hot and cold mixture facilities for the light and heavy duty traffic roads and several surface coating types make the asphalt the most densely used material for road coating. Also, the surfaces of the concrete roads are coated with asphalt in order to increase the comfort of the ride, the structural endurance and slipping resistance.

According to asphalt statistics of the year 2000, 298,5 million ton and 590 million ton hot mixture containing bitumen in Europe and the USA, respectively, are used for coating the roads. In Turkey, all state and provincial roads as well as motorways are built with asphalt and ten million ton hot mixture containing bitumen is used per year. The statistics show that the asphalt layers used the flexible pavement construction that distribute the traffic loads to the ground and are resistant to the deformation shall continue to be an indispensable material of road building in the future as well.

The drivers, as the users of the roads, demand comfortable roads with high slipping resistance, ensuring fast access, reducing the transportation cost, visible with its marks and reduced noise levels while the road builders target roads that are durable, require minimum maintenance, are resistant to the permanent deformation with high wearing resistance and a long service life.

The advantages offered by the asphalt which is able to answer all requirements with its improved properties and new types can be summarized as follows:

Quick and easy construction:

Since asphalt coatings do not require cure like the concrete coating, the layer construction can be completed in a few hours and the road can be opened to the traffic. Due to quick construction technique, it is possible to coat the sub-layers quickly and that eradicates the likelihood of the layers' losing their bearing power due to contact with the air. Also, in case of roads with no alternative and urban application, it can be applied safely, quickly and even with the night shifts without causing any traffic congestion, thus, the roads are opened to the traffic soon.

Thinner layers are adequate:

Due to the fact that the asphalt distributes the traffic load to the ground due to the flexibility of the asphalt pavements and the asphalt mixtures have high durability, the asphalt coatings may be applied with thinner layers as compared to the concrete, particularly in case of light traffic conditions. Due to the advanced performance features of the asphalt, today, super thin asphalt coatings with a thickness of 1-3 cm can be applied as the surface layer.

Its ability to more robust pavement:

Since asphalt layers prevent the water from penetrating into the pavement, it protects the base of the pavement from freezing, thereby, creating higher pavement durability. The asphalt pavements have higher long term performance since they are elastic and more resistant to the permanent deformations. In recent years, performance-based asphalt specifications have been developed for the purpose of applying asphalt layers suitable for different traffic and climatic conditions. The asphalt mixtures with high performance, low noise levels and high slipping resistance such as Stone Mastic Asphalt-SMA and Porous Asphalt with free drainage were introduced and the typical deteriorations such as settlement on the wheel trace, exhaustion and thermal cracks were prevented.

The leading problems of the concrete coatings the shrinkage cracks that come into being as a result of the reaction of the cement with the water and the severe deteriorations that appear due to high sensitivity against the heavy loads. Also, the concrete coatings are affected by the settlement and swelling in the base more than the asphalt pavement. In the past, low-durability granular foundation layer with cement binders used below the asphalt layers in the motorways of our country caused shrinkage and foundation cracks, which was a typical rigid pavement deterioration, and, thus, use of this type of layer was given up and granular foundation without binders was preferred, which is still in use.

Low requirement of maintenance:

Well-designed and high quality asphalt roads require minimum maintenance. The initial performance of the asphalt pavements may be retained for several years only through regular maintenance. The maintenance cost of the asphalt coating is low. The defective part may be repaired quickly and locally. When the asphalt needs to be strengthened over the time, addition of a 4-5 cm thick layer is adequate. In comparison to the asphalt roads, the regular maintenance of the concrete roads requires more time, money and labor. Depending on the severity and density of the deterioration, the maintenance procedure turns into a re-building procedure in several cases. Also, hot-mixed asphalt is required for repairing the deteriorated concrete roads and such procedure is common all around the world.

Comfortable and safe surface:

The asphalt coatings provide a flat and constant surface with no pointing unlike the concrete coatings. Without a pointing, the desired surface flatness and inclination is gained. The surface slipping resistance of the asphalt coatings is provided at an optimum level by adjusting the mixture design, while concrete surfaces require an additional procedure to roughen the surface. The road signs are perfectly visible on the asphalt coating. Apart from application of the road line paint and other types of marking materials and rendering such signs permanent, the contract colors it creates in marking the critical lines and zones make the line or area stand out. The colorful asphalt recently developed enables esthetically good looking urban areas to be built. Since the asphalt attracts the heat due to its dark color and enables easy evaporation, the ice easily melts and the surface water dries rapidly. The Asphalt is not a reactive material like the concrete. Since it is resistant to the acid and sulphate, it does not give rise to corrosion, does not react with the other materials and ice melting chemicals do not harm damage the asphalt . Except for the superficial coatings, the noise level in the asphalt coated roads is lower than the concrete coated roads.

Reusable:

Unlike the cement, since the bitumen does not lose its binding properties, all bitumen and the aggregates contained in the asphalt mixtures can be reused. That is to say, the asphalt coatings are 100 percent recyclable natural material mixtures. By contrast, the concrete coatings can be evaluated merely as a resource to produce aggregates by the end of its service life . The bitumen used in the asphalt coating as binder is a leftover article from the crude oil refinement with the basic purpose of gaining petro-chemical products and using such materials in the asphalt coating renders the refinement more cost-effective and also protects the environment.

Suitability for gradual construction:

The asphalt coatings may be expanded and thickened since they can be done gradually. Any layer added increases the load bearing capacity of the pavement. The asphalt pavements are designed with a shorter useful life at the beginning due to lack of resources and then, the service life can be extended with additions. In our country where the grants allocated for road building are so limited, 79% of the roads (49.843 km) are covered with superficial coating containing bitumen that has no bearing power and merely protects the infrastructure and it is expected that these roads are covered with hot mixed asphalt layers when adequate grants are allocated. The concrete roads are not the road type that may answer the needs under the current economical conditions of Turkey due to high initial costs and its unsuitability for gradual construction.

Construction and maintenance cost:

The number one reason in choosing the asphalt coatings is the low cost caused by the structure and essence of these coatings. The reusability of the asphalt coatings is another important factor reducing the cost apart from the positive environmental effect. In comparison to the concrete, it is the type of coating with the lowest initial cost. When the maintenance, repair and operating costs are taken into consideration during the service life, majority of the road authorities state that the asphalt pavements with high performance are the cheapest coating type in the long run.

In terms of design, the asphalt pavements projected service life is 20 years and they have a permanent value by the end of such service life, thus, the asphalt pavement is a type of pavement that can be used forever with its maintenance, repair and replenishment procedures. . By the 21st century, the world asphalt industry has aimed permanent asphalt pavements that can be used for 50 to 100 years in consideration of the rapidly increasing traffic volume and loads. Therefore, particularly in the USA, efforts are made to spread the use of permanent pavements composed of asphalt layers which could serve more than 50 years through layer coating renewal in every 20 years. The maintenance of these types of pavements shall be somewhat like painting and retiling of the roof of a permanent building and the asphalt pavement that is still robust and has not lost its function shall answer the needs for several years.

As a result of the study conducted by Norway Asphalt Contractors' Association, to detect to what extent the rule of using concrete coatings in the international airports has been adopted in the world based on the experiences of Portland Cement Association, asphalt is a type of coating preferred in the airports as well. The results gained are shown in the following table.

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