Pavement (roads)

This article is about the American English usage of pavement as the durable surfacing of roads and walkways. In British English, pavement is usually taken to mean a footpath next to a road, the same as sidewalk in American English. See Pavement (disambiguation).

Pavement in American English refers to the durable surface for an area intended to sustain traffic, which can be either vehicular traffic or foot traffic. The most common modern paving methods are asphalt and concrete. In the past, brick was extensively used, as was metalling. Today, permeable paving methods are beginning to be used more for low-impact roadways and walkways. See Pavement marker for information on that topic.



Metal or metalling has had two distinct usages in road paving. Metalling originally referred to the process of creating a carefully-engineered gravel roadway. The route of the roadway first would be dug down several feet. Depending on local conditions, French drains may or may not have been added. Next, large stone was placed and compacted, followed by successive layers of smaller stone, until the road surface was a small stone compacted into a hard, durable surface.

Road metal later became the name of stone chippings mixed with tar to form the road surfacing material tarmac. A road of such material is called a "metalled road" in British usage, and is still a common modern usage. The word metal is derived from the Latin metallum, which means both "mine" and "quarry", hence the roadbuilding terminology.

Asphalt paving

Asphalt (specifically, asphalt concrete) has been widely used since 1920-1930, though in ancient times asphalt was already used for road-building. The viscous nature of the asphalt binder allows asphalt concrete to sustain significant plastic deformation, although fatigue from repeated loading over time is the most common failure mechanism. Most asphalt pavements are built on an imported gravel base which is generally at least as thick as the asphalt layer, although some 'full depth' pavements are built directly on the native subgrade. In areas with very soft or expansive subgrades such as clay or peat, thick gravel bases or stabilization of the subgrade with Portland cement or lime can be required. The actual material used in paving is termed HMA (Hot Mix Asphalt), and it is usually applied using a free floating screed.

Advantages of asphalt roadways include relatively low noise, relatively low cost compared with other paving methods, and ease of repair. Disadvantages include less durability than other paving methods, less tensile strength than concrete, the tendency to become very slick in the case of a mild oil spill, and a certain amount of hydrocarbon pollution to soils and waterways.

Concrete (cement) paving

Concrete pavements (specifically, Portland cement concrete) are created using a concrete mix of Portland cement, gravel, and sand. The material is applied in a freshly-mixed slurry, and worked mechanically to compact the interior and force some of the thinner cement slurry to the surface to produce a smoother, denser surface free from honeycombing. Cement concrete can be either reinforced or non-reinforced. Non-reinforced pavements will typically have joints at a 5 meter interval. Reinforced concrete pavements can have a much longer joint spacing, or no built-in joints at all. Typical reinforcement used includes "rebar" (reinforcing bar) or wire mesh or both. Vertical misalignment of the joints, known as joint faulting, can be caused by differential settlement of the slabs and is a major source of driver annoyance. A common failure mode of concrete pavements is loss of support of the slab edges or corners due to erosion of the foundation material. If this condition is caught before it leads to breakup of the slab, support can be restored by filling the void with grout or foam in a process known as 'mud jacking' or 'slab jacking'.

Advantages of cement concrete roadways include that they are typically stronger and last longer than asphalt concrete pavements. They also can easily be grooved to provide a durable skid-resistant surface. Disadvantages are that they have a higher initial cost, are more difficult to repair, and are also generally noisier and less smooth.

Bituminous Surface Treatment (BST)

Bituminous Surface Treatment (BST), also known as chip seal or seal coat, is used mainly on low traffic roads, but also as a sealing coat to rejuvenate asphalt pavement. It consists of one or more layers of sprayed-on asphalt emulsion followed by a thin layer of aggregate, which is embedded in the asphalt using rubber-tired rollers.

Other paving methods

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Pavers, generally in the form of pre-cast concrete blocks, are often used for aesthetic purposes, or sometimes at port facilities that see long-duration pavement loading. Pavers are rarely used in areas that see high-speed vehicle traffic.

Brick, cobblestone and wood plank pavements were once common in urban areas throughout the world, but due to their high manual labor requirements they are typically only maintained for historical reasons. Likewise, macadam and tarmac pavements can still sometimes be found buried underneath asphalt concrete or Portland cement concrete pavements, but are rarely constructed anymore.

Stress injury to pavement

As pavements primarily fail due to fatigue (in a manner similar to metals), the damage done to a pavement increases exponentially with the axle load of the vehicles traveling on it. Heavily loaded trucks can do more than 10,000 times the damage done by a normal passenger car. Although tax rates for trucks are higher than those for cars in most countries, they are generally not high enough to make up for this discrepancy.

Several pavement design methods have been developed to determine the thickness and composition of pavement required to carry predicted traffic loads for a given period of time. Among these is the Shell Pavement design method.

The physical properties of a stretch of pavement can be tested using a falling weight


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