PAVEMENT STRUCTURE
To support heavy vehicles moving at high speeds, a modem road is made up of several layers. Each layer helps the layers above it support the weight and pressure of moving traffic. Roads that carry more traffic at higher speeds, like highways, are built to stronger standards than roads that carry less traffic, such as rural roads.
The number of layers in a road often depends on the intended use of the road, but generally roads have three distinct layers. From bottom to top, the layers are the roadbed, the base course, and the wearing course.
Roadbed. The roadbed is the very bottom layer of a road. Natural soil is the most common roadbed material. The roadbed is shaped to make a smooth, level surface that will support the layers built over it. Engineers use bulldozers and other construction equipment to distribute soil evenly along the roadbed. If a road is planned through an area where the natural landscape is uneven, soil can be removed or filled in as needed to obtain a level surface.
Base Course. The base course rests directly on top of the roadbed and is often made up of compacted gravel. If the roadbed material itself is suitable, it may be treated, or stabilized, and used as the base. Soil can be stabilized by adding or mixing materials such as calcium chloride, bituminous material, lime, or portland cement to the soil. For very inexpensive, light-traffic roads, stabilized soil alone can suffice as the finished road surface. Drainpipes are usually installed within the base course to control rain and moisture drainage. Without adequate drainage, roads may buckle or collapse as water swells the ground underneath. Some roads include a second base layer, called the top course, for extra support.
Wearing Course. A road's top layer, which directly supports moving vehicles, is called the wearing course. It is made of a solid layer of pavement and is designed to be smooth and to withstand erosion from traffic and weather. Two main types of pavements are used - bituminous, or flexible, pavement and concrete, or rigid, pavement. Bituminous pavement is cheaper and easier to construct, but it requires more maintenance. Concrete pavement lasts for a very long time with minimal upkeep but is much more expensive and time-consuming to build.
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TYPES OF PAVEMENT
Two main types of pavements are used - bituminous, or flexible, pavement and concrete, or rigid, pavement. Bituminous pavement is cheaper and easier to construct, but it requires more maintenance. Concrete pavement lasts for a very long time with minimal upkeep but is much more expensive and time-consuming to build.
Bituminous pavements are made with by-products of petroleum, such as asphalt. Weather and seasonal changes can cause roadways and the earth below them to rise or fall slightly. As these natural shifts occur, bituminous pavements allow the road surface to bend or flex slightly without breaking.
Bituminous material softens when heated and can be prepared and applied in a wide range of concentrations. Thin layers of bituminous material are sometimes applied with a pressure sprayer to the base course or the top course. The bituminous material seeps into the crushed rock and penetrates the tiny spaces between the compacted rocks, binding the aggregate together.
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Asphalt, thick bituminous material, can be used directly as a pavement. Asphalt can be applied in thin overlays less than 2 cm (0.75 in) deep or in layers several centimeters deep, depending on the type of surface and its purpose. Asphalt is mixed with aggregates. The asphalt and aggregates are usually mixed and heated at a plant. The material is then transported to the construction site, where it is spread directly over the base course or top course and compacted. Asphalt surfaces are easy to construct and repair.
Concrete or rigid pavements are made from amix of portland cement and aggregate. Concrete pavements have along life and require little maintenance. Concrete is generally laid as asingle thick layer directlyover abase course. For heavily ' traveled roads, concrete layers can range in thickness from 20 to 36 cm (8 to 14 in). Concrete is usually laid in long sections or slabs of varying length. Metal bars or dowels inserted into the edges of the slabs help connect the joints where one slab ends and another begins.
Concrete is a strong material and can withstand compression, but it has poor tensile strength (resistance to being pulled). When the ground underneath expands and contracts from seasonal or weather changes, the concrete becomes prone to cracking. Cracks can occur at or near the joints where concrete slabs meet or on the slabs themselves. Deep cracks can allow the broken concrete slabs to move upward or downward, creating an uneven road surface. Metal bars or dowels inserted between the slabs help hold the slabs together. Reinforced concrete contains steel bars or mesh imbedded within the concrete layer. The steel helps hold concrete together over time, even if cracks occur. Unreinforced concrete may be used when cost is a factor, or where weather conditions are more mild. Unreinforced slabs have several shallow grooves cut into them, allowing the concrete to crack at defined points. The cracked slabs are kept in place by pressure and by the grainy texture of the concrete itself.
