Road surface
- 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.
Metalling
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 was called a "metalled road" in British usage, although this would be very rare in modern usage. It would be more common to refer to a macadam road. 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 bitumen 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. In some countries with soft soils, a foundation of polystyrene blocks is used instead. 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 slick and soft in hot weather and a certain amount of hydrocarbon pollution to soil and groundwater or waterways.
Concrete 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 steel) or wire mesh or both.
Vertical misalignment of the joints, known as joint faulting, can be caused by differential settlement of the substrate, and are a 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 somewhat noisy if jointed, but unjointed concrete pavement is actually a method of roadway noise mitigation.
The record for first mile of concrete pavement to be laid in the United States is claimed by Michigan.
Bituminous Surface Treatment (BST)
Bituminous Surface Treatment (BST) is used mainly on low traffic roads, but also as a sealing coat to rejuvenate an asphalt concrete pavement. It generally consists of aggregate spread over a sprayed-on asphalt emulsion or cut-back asphalt cement. The aggregate is then embedded into the asphalt by rolling it, typically with a rubber-tired roller. BSTs of this type are described by a wide variety of regional terms including "chip seal", "tar and chip", "oil and stone" and "seal coat".
Other types of BSTs include micropaving, slurry seals and Novachip. These are laid down using specialized and proprietary equipment. They are most often used in urban areas where the roughness and loose stone associated with chip seals is considered undesirable.
Other paving methods
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 in some countries typically only maintained for historical reasons, while in other countries they are still common in local streets. They make maintenance of cabling and pipelines under the pavement easier but are also harder to walk on.
Likewise, macadam and tarmac pavements can still sometimes be found buried underneath asphalt concrete or Portland cement concrete pavements, but are rarely constructed anymore.
Pavement deterioration
As pavement systems primarily fail due to fatigue (in a manner similar to metals), the damage done to pavement increases exponentially with the axle load of the vehicles traveling on it. Civil Engineers consider truck axle load, current and projected truck traffic volume, supporting soil properties and sub-grade drainage in design. Passenger cars are considered to have no practical effect on a pavement's service life.
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. Pavement design methods are continuously evolving. Among these are the Shell Pavement design method, and the AASHTO 1993 "Guide for Design of Pavement Structures". A new mechanistic-empirical design guide has been under development by NCHRP since 1998, but has not yet been adopted by AASHTO.
According to the AASHO Road Test, heavily loaded trucks can do more than 10,000 times the damage done by a normal passenger car. Tax rates for trucks are higher than those for cars in most countries for this reason, though are not levied in proportion to the damage done. [1]
The physical properties of a stretch of pavement can be tested using a falling weight deflectometer.
Further research by University College London into pavements has led to the development of an indoor 80 sq metre artificial pavement at a research centre called Pamela (the Pedestrian Accessibility and Movement Environment Laboratory). It is used to simulate everyday scenarios, from different pavement users to varying pavement conditions. [2]