Paving

Paving - Paving contractors and suppliers in the United States

Asphalt
A black construction material like cement, varying from solid to semisolid at room temperature. When heated to the temperature of boiling water, it can be poured. Asphalt is used in surfacing roads, in lining the walls of water-retaining structures such as reservoirs and swimming pools, and in manufacturing floor tiles and roofing materials.

Asphalt should not be confused with tar, a substance made from coal or wood. Asphalt is found in natural deposits, but almost all of the asphalt used commercially is made from petroleum. Natural asphalt was used extensively in ancient times. Deposits occur in pits or lakes as residue from crude petroleum that has seeped up through cracks in the earth. The use of asphalt for street paving in the United States began in 1870.

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.

Bituminous Pavement
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. To provide traction for moving traffic, a thin layer of aggregate may then be spread over the bituminous material and compacted.

Asphalt, a 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 commonly mixed with aggregates for added strength and traction. The asphalt and aggregates are usually mixed and heated at a central location. 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 fairly easy to construct and repair.

Concrete, or rigid, pavements are made from a mix of portland cement and aggregate. Concrete pavements have a long life and require little maintenance. Concrete is generally laid as a single thick layer directly over a base 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.