Aroad surface (British English) orpavement (North American English) is the durable surface material laid down on an area intended to sustain vehicular or foottraffic, such as aroad orwalkway. In the past,gravel road surfaces,macadam,hoggin,cobblestone andgranite setts were extensively used, but these have mostly been replaced byasphalt orconcrete laid on a compactedbase course. Asphalt mixtures have been used in pavement construction since the beginning of the 20th century and are of two types: metalled (hard-surfaced) and unmetalled roads. Metalled roadways are made to sustain vehicular load and so are usually made on frequently used roads. Unmetalled roads, also known as gravel roads or dirt roads, are rough and can sustain less weight. Road surfaces are frequentlymarked to guide traffic.
Today,permeable paving methods are beginning to be used for low-impact roadways and walkways to prevent flooding. Pavements are crucial to countries such asUnited States andCanada, which heavily depend on road transportation. Therefore, research projects such asLong-Term Pavement Performance have been launched to optimize the life cycle of different road surfaces.[1][2][3][4]
Pavement, in construction, is an outdoor floor or superficial surface covering. Paving materials includeasphalt,concrete, stones such asflagstone,cobblestone, andsetts,artificial stone,bricks,tiles, and sometimes wood. Inlandscape architecture, pavements are part of thehardscape and are used onsidewalks,road surfaces,patios, andcourtyards, among others.
The termpavement comes from Latinpavimentum, meaning a floor beaten or rammed down, through Old Frenchpavement.[5] The meaning of a beaten-down floor was obsolete before the word entered English.[6]
Pavement, in the form of beatengravel, dates back before the emergence ofanatomically modern humans. Pavement laid in patterns likemosaics were commonly used by the Romans.[7]
Thebearing capacity andservice life of a pavement can be raised significantly by arranging gooddrainage by an openditch or covereddrains to reduce moisture content in the pavementssubbase andsubgrade.
Wheeled transport created the need for better roads. Generally, natural materials cannot be both soft enough to form well-graded surfaces and strong enough to bear wheeled vehicles, especially when wet, and stay intact. In urban areas it was worthwhile to build stone-paved streets and, in fact, the first paved streets appear to have been built inUr in 4000 BC.Corduroy roads were built inGlastonbury,England, in 3300 BC,[8] and brick-paved roads were built in theIndus Valley Civilisation on theIndian subcontinent from around the same time. Improvements inmetallurgy meant that by 2000 BC stone-cutting tools were generally available in the Middle East andGreece allowing local streets to be paved.[9] Notably, in about 2000 BC, theMinoans built a 50 km paved road fromKnossos in northernCrete through the mountains toGortyn andLebena, a port on the south coast of the island, which had side drains, a 200 mm thick pavement ofsandstone blocks bound withclay-gypsummortar, covered by a layer ofbasalticflagstones and had separateshoulders. This road could be considered superior to anyRoman road.[10] Roman roads varied from simple corduroy roads to paved roads using deep roadbeds of tamped rubble as an underlying layer to ensure that they kept dry, as the water would flow out from between the stones and fragments of rubble, instead of becoming mud in clay soils.
Although there were attempts to rediscover Roman methods, there was little useful innovation in road building before the 18th century. The first professional road builder to emerge during theIndustrial Revolution wasJohn Metcalf, who constructed about 290 kilometres (180 mi) ofturnpike road, mainly in the north of England, from 1765, whenParliament passed an act authorising the creation ofturnpike trusts to buildtoll funded roads in theKnaresborough area.
Pierre-Marie-Jérôme Trésaguet is widely credited with establishing the firstscientific approach to road building in France at the same time as Metcalf. He wrote a memorandum on his method in 1775, which became general practice in France. It involved a layer of large rocks, covered by a layer of smaller gravel.
By the late 18th and early 19th centuries, new methods of highway construction had been pioneered by the work of two British engineers:Thomas Telford andJohn Loudon McAdam. Telford's method of road building involved the digging of a large trench in which a foundation of heavy rock was set. He designed his roads so that they sloped downwards from the centre, allowing drainage to take place, a major improvement on the work of Trésaguet. The surface of his roads consisted of broken stone. McAdam developed an inexpensive paving material of soil and stone aggregate (known asmacadam). His road building method was simpler than Telford's, yet more effective at protecting roadways: he discovered that massive foundations of rock upon rock were unnecessary, and asserted that native soil alone would support the road and traffic upon it, as long as it was covered by a road crust that would protect the soil underneath from water and wear.[11] Size of stones was central to McAdam's road building theory. The lower 200-millimetre (7.9 in) road thickness was restricted to stones no larger than 75 millimetres (3.0 in).
Moderntarmac was patented by British civil engineerEdgar Purnell Hooley, who noticed that spilled tar on the roadway kept the dust down and created a smooth surface.[12] He took out a patent in 1901 for tarmac.[13] Hooley's 1901 patent for tarmac involved mechanically mixing tar and aggregate prior to lay-down, and then compacting the mixture with asteamroller. The tar was modified by adding small amounts ofPortland cement,resin, andpitch.[14]
Asphalt (specifically,asphalt concrete), sometimes called flexible pavement since its viscosity causes minute deformations as it distributes loads, has been widely used since the 1920s. The viscous nature of thebitumen binder allows asphalt concrete to sustain significantplastic deformation, althoughfatigue from repeated loading over time is the most common failure mechanism. Most asphalt surfaces are laid on a gravel base, which is generally at least as thick as the asphalt layer, although some 'full depth' asphalt surfaces are laid directly on the nativesubgrade. In areas with very soft orexpansive subgrades such asclay orpeat, thick gravel bases or stabilization of the subgrade withPortland cement orlime may be required.Polypropylene and polyestergeosynthetics are also used for this purpose,[15] and in some northern countries a layer ofpolystyrene boards are used to delay and minimize frost penetration into the subgrade.[16]
Depending on the temperature at which it is applied, asphalt is categorized as hot mix, warm mix, half warm mix, or cold mix. Hot mix asphalt is applied at temperatures over 150 °C (300 °F) with afree floating screed. Warm mix asphalt is applied at temperatures of 95–120 °C (200–250 °F), resulting in reduced energy usage and emissions ofvolatile organic compounds.[17] Cold mix asphalt is often used on lower-volume rural roads, where hot mix asphalt would cool too much on the long trip from theasphalt plant to the construction site.[18]
An asphalt concrete surface will generally be constructed for high-volume primary highways having an average annual daily traffic load greater than 1,200 vehicles per day.[19] Advantages of asphalt roadways include relatively low noise, relatively low cost compared with other paving methods, and perceived 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 ofhydrocarbon pollution to soil andgroundwater orwaterways.
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In the mid-1960s,rubberized asphalt was used for the first time, mixing crumb rubber from used tires with asphalt.[20] While a potential use for tires that would otherwise fill landfills and present a fire hazard, rubberized asphalt has shown greater incidence of wear in freeze-thaw cycles in temperate zones because of the non-homogeneous expansion and contraction with non-rubber components. The application of rubberized asphalt is more temperature-sensitive and in many locations can only be applied at certain times of the year.[21] Study results of the long-term acoustic benefits of rubberized asphalt are inconclusive. Initial application of rubberized asphalt may provide a reduction of 3–5 decibels (dB) in tire-pavement-source noise emissions; however, this translates to only 1–3 dB in total traffic-noise reduction when combined with the other components of traffic noise. Compared to traditional passive attenuating measures (e.g., noise walls and earth berms), rubberized asphalt provides shorter-lasting and lesser acoustic benefits at typically much greater expense.[citation needed]
Concrete surfaces (specifically,Portland cement concrete) are created using a concrete mix of Portland cement,coarse aggregate,sand, and water. In virtually all modern mixes there will also be various admixtures added to increase workability, reduce the required amount of water, mitigate harmful chemical reactions, and for other beneficial purposes. In many cases there will also be Portland cement substitutes added, such asfly ash. This can reduce the cost of the concrete and improve its physical properties. The material is applied in a freshly mixed slurry and worked mechanically to compact the interior and force some of the cement slurry to the surface to produce a smoother, denser surface free from honeycombing. The water allows the mix to combine molecularly in a chemical reaction calledhydration.
Concrete surfaces have been classified into three common types: jointed plain (JPCP), jointed reinforced (JRCP) and continuously reinforced (CRCP). The one item that distinguishes each type is the jointing system used to control crack development.
One of the major advantages of concrete pavements is they are typically stronger and more durable than asphalt roadways. The surface can be grooved to provide a durable skid-resistant surface. Concrete roads are more economical to drive in terms of fuel consumption, they reflect light better, and they last significantly longer than other paving surfaces; but they have a much smaller market share than other paving solutions.[22] Modern paving methods and design methods have changed the economics of concrete paving so that a well-designed and placed concrete pavement will be cheaper in initial cost and significantly cheaper over the life cycle.[23] Another important advantage is that waterproof concrete can be used, which eliminates the need to place storm drains next to the road and reduces the need for a slightly sloped driveway to drain rainwater. Avoiding rainwater discharge by using runoff also means less electricity is needed (otherwise more pumps would be needed in the water distribution system) and rainwater is not polluted because it no longer mixes with polluted water. Rather, it is immediately absorbed by the earth.[24] A previous disadvantage was that they had a higher initial cost and could be more time-consuming to construct. This cost can typically be offset through the long life cycle of the pavement and the higher cost of bitumen. Concrete pavement can be maintained over time utilizing a series of methods known asconcrete pavement restoration which includediamond grinding,dowel bar retrofits, joint and crack sealing, and cross-stitching. Diamond grinding is also useful in reducing noise and restoring skid resistance in older concrete pavement.[25][26]
The first street in the United States to be paved with concrete wasCourt Avenue inBellefontaine, Ohio, in 1893.[27][28] The first mile of concrete pavement in the United States was onWoodward Avenue inDetroit, Michigan, in 1909.[29] Following these pioneering uses, theLincoln Highway Association, established in October 1913 to oversee the creation of one of the United States' earliest east-west transcontinental highways for the automobile, began to establish"seedling miles" of specifically concrete-paved roadbed in various places in theAmerican Midwest, starting in 1914 west ofMalta, Illinois, while using concrete with the specified concrete "ideal section" for the Lincoln Highway inLake County, Indiana, during 1922 and 1923.[30]
Concrete roadways may produce more noise than asphalt from tire noise on cracks and expansion joints. A concrete pavement composed of multiple slabs of uniform size will produce a periodic sound and vibration in each vehicle as its tires pass over each expansion joint. These monotonous repeated sounds and vibrations can cause afatiguing or hypnotic effect upon the driver over the course of a long journey.
Composite pavements combine a Portland cement concrete sublayer with an asphalt overlay. They are usually used to rehabilitate existing roadways rather than in new construction. Asphalt overlays are sometimes laid over distressed concrete to restore a smooth wearing surface.[31] A disadvantage of this method is that movement in the joints between the underlying concrete slabs, whether from thermal expansion and contraction, or from deflection of the concrete slabs from truckaxle loads, usually causesreflective cracks in the asphalt.
To decrease reflective cracking, concrete pavement is broken apart through abreak and seat,crack and seat, orrubblization process. Geosynthetics can be used for reflective crack control.[32] With break and seat and crack and seat processes, a heavy weight is dropped on the concrete to induce cracking, then a heavy roller is used to seat the resultant pieces into the subbase. The main difference between the two processes is the equipment used to break the concrete pavement and the size of the resulting pieces. The theory is that frequent small cracks will spread thermal stress over a wider area than infrequent large joints, reducing the stress on the overlying asphalt pavement. "Rubblization" is a more complete fracturing of the old, worn-out concrete, effectively converting the old pavement into an aggregate base for a new asphalt road.[33]
Thewhitetopping process uses Portland cement concrete to resurface a distressed asphalt road.
Distressed pavement can be reused when rehabilitating a roadway. The existing pavement is broken up and may be ground on-site through a process calledmilling. This pavement is commonly referred to as reclaimed asphalt pavement (RAP). RAP can be transported to an asphalt plant, where it will be stockpiled for use in new pavement mixes,[34] or it may be recycled in-place using the techniques described below.
Bituminous surface treatment (BST) orchipseal 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 asphaltemulsion or cut-back asphalt cement. The aggregate is then embedded into the asphalt by rolling it, typically with a rubber-tiredroller. This type of surface is described by a wide variety of regional terms including "chip seal", "tar and chip", "oil and stone", "seal coat", "sprayed seal",[38] "surface dressing",[39] "microsurfacing",[40] "seal",[41] or simply as "bitumen".
BST is used on hundreds of miles of theAlaska Highway and other similar roadways inAlaska, theYukon Territory, and northernBritish Columbia. The ease of application of BST is one reason for its popularity, but another is its flexibility, which is important when roadways are laid down over unstable terrain that thaws and softens in the spring.
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.
A thin membrane surface (TMS) is anoil-treatedaggregate which is laid down upon agravel road bed, producing a dust-free road.[42] A TMS road reduces mud problems and provides stone-free roads for local residents where loaded truck traffic is negligible. The TMS layer adds no significant structural strength, and so is used on secondary highways with low traffic volume and minimal weight loading. Construction involves minimal subgrade preparation, following by covering with a 50-to-100-millimetre (2–4 in) cold mixasphalt aggregate.[19] The Operation Division of theMinistry of Highways and Infrastructure inSaskatchewan has the responsibility of maintaining 6,102 kilometres (3,792 mi) of thin membrane surface (TMS) highways.[43]
Otta seal is a low-cost road surface using a 16–30-millimetre-thick (5⁄8–1+1⁄8 in) mixture ofbitumen and crushed rock.[44]
Gravel is known to have been used extensively in the construction of roads by soldiers of theRoman Empire (seeRoman road) but in 1998 a limestone-surfaced road, thought to date back to theBronze Age, was found atYarnton in Oxfordshire, Britain.[45] Applying gravel, or "metalling", has had two distinct usages in road surfacing. The termroad metal refers to the brokenstone orcinders used in theconstruction or repair of roads orrailways,[46] and is derived from theLatinmetallum, which means both "mine" and "quarry".[47] The term originally referred to the process of creating a gravel roadway. The route of the roadway would first be dug down several feet and, depending on local conditions,French drains may or may not have been added. Next, large stones were placed and compacted, followed by successive layers of smaller stones, until the road surface was composed of small stones compacted into a hard, durable surface. "Road metal" later became the name ofstone chippings mixed withtar to form the road-surfacing materialtarmac. A road of such material is called a "metalled road" in Britain, a "paved road" in Canada and the US, or a "sealed road" in parts of Canada, Australia and New Zealand.[48]
A granular surface can be used with a traffic volume where the annual average daily traffic is 1,200 vehicles per day or less.[citation needed] There is some structural strength if the road surface combines a sub base and base and is topped with a double-graded seal aggregate with emulsion.[19][49] Besides the 4,929 kilometres (3,063 mi) of granular pavements maintained in Saskatchewan, around 40% ofNew Zealand roads are unbound granular pavement structures.[43][50]
The decision whether to pave a gravel road or not often hinges on traffic volume. It has been found that maintenance costs for gravel roads often exceed the maintenance costs for paved or surface-treated roads when traffic volumes exceed 200 vehicles per day.[51] Despite this, different jurisdictions set and follow different guidelines; for example, the state ofVermont recommends considering to pave a gravel road only at 500 vehicles per day.[52]
Some communities are finding it makes sense to convert their low-volume paved roads to aggregate surfaces.[53]
Pavers (orpaviours), generally in the form of pre-cast concrete blocks, are often used for aesthetic purposes, or sometimes atport facilities that see long-duration pavement loading.Pavers are rarely used in areas that see high-speed vehicle traffic.
Brick,cobblestone,sett,wood plank, and wood block pavements such asNicolson pavement, were once common inurban areas throughout the world, but fell out of fashion in most countries, due to the high cost of labor required to lay and maintain them, and are typically only kept for historical or aesthetic reasons.[citation needed] In some countries, however, they are still common in local streets. In theNetherlands, brick paving has made something of a comeback since the adoption of a major nationwidetraffic safety program in 1997. From 1998 through 2007, more than 41,000 km of city streets were converted to local access roads with a speed limit of 30 km/h, for the purpose oftraffic calming.[54] One popular measure is to use brick paving - the noise and vibration slows motorists down. At the same time, it is not uncommon for cycle paths alongside a road to have a smoother surface than the road itself.[55][56]
Although rarely constructed today, early-stylemacadam andtarmac pavements are sometimes found beneath modern asphalt concrete or Portland cement concrete pavements, because the cost of their removal at the time of renovation would not significantly benefit the durabilty and longevity of the newer surface.
There are ways to create the appearance of brick pavement, without the expense of actual bricks. The first method to create brick texture is to heat an asphalt pavement and use metal wires to imprint a brick pattern using acompactor to createstamped asphalt. A similar method is to use rubber imprinting tools to press over a thin layer of cement to createdecorative concrete. Another method is to use a brick pattern stencil and apply a surfacing material over the stencil. Materials that can be applied to give the color of the brick and skid resistance can be in many forms. An example is to use coloredpolymer-modified concrete slurry which can be applied byscreeding or spraying.[57] Another material isaggregate-reinforcedthermoplastic which can be heat applied to the top layer of the brick-pattern surface.[58] Other coating materials over stamped asphalt are paints and two-partepoxy coating.[59]
Roadway surfacing choices are known to affect the intensity and spectrum of sound emanating from the tire/surface interaction.[60] Initial applications of noise studies occurred in the early 1970s. Noise phenomena are highly influenced by vehicle speed.
Roadway surface types contribute differential noise effects of up to 4dB, with chip seal type and grooved roads being the loudest, andconcrete surfaces without spacers being the quietest.Asphaltic surfaces perform intermediately relative to concrete andchip seal.Rubberized asphalt has been shown to give a 3–5 dB reduction in tire-pavement noise emissions, and a marginally discernible 1–3 dB reduction in total road noise emissions when compared to conventional asphalt applications.
As pavement systems primarily fail due tofatigue (in a manner similar tometals), the damage done to pavement increases with the fourth power of theaxle load of the vehicles traveling on it. According to theAASHO Road Test, heavily loadedtrucks 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 they are not levied in proportion to the damage done.[61] Passenger cars are considered to have little practical effect on a pavement's service life, from a materials fatigue perspective.
Other failure modes include aging and surface abrasion. As years go by, the binder in a bituminouswearing course gets stiffer and less flexible. When it gets "old" enough, the surface will start losing aggregates, andmacrotexture depth increases dramatically. If no maintenance action is done quickly on the wearing course,potholes will form. Thefreeze-thaw cycle in cold climates will dramatically accelerate pavement deterioration, once water can penetrate the surface. Clay and fumed silicananoparticles may potentially be used as efficient UV-anti aging coatings in asphalt pavements.
If the road is still structurally sound, a bituminous surface treatment, such as achipseal or surface dressing can prolong the life of the road at low cost. In areas with cold climate,studded tires may be allowed on passenger cars. In Sweden and Finland, studded passenger car tires account for a very large share of pavementrutting.[62]
The physical properties of a stretch of pavement can be tested using afalling weight deflectometer.
Several design methods have been developed to determine the thickness and composition of road surfaces required to carry predicted traffic loads for a given period of time. Pavement design methods are continuously evolving. Among these are theShell Pavement design method, and theAmerican Association of State Highway and Transportation Officials (AASHTO) 1993/98 "Guide for Design of Pavement Structures". A mechanistic-empirical design guide was developed through the NCHRP process, resulting in the Mechanistic Empirical Pavement Design Guide (MEPDG), which was adopted by AASHTO in 2008, although MEPDG implementation by state departments of transportation has been slow.[63]
Further research byUniversity College London into pavements has led to the development of an indoor, 80-sq-metre artificial pavement at a research centre calledPedestrian Accessibility and Movement Environment Laboratory (PAMELA). It is used to simulate everyday scenarios, from different pavement users to varying pavement conditions.[64] There also exists a research facility nearAuburn University, theNCAT Pavement Test Track, that is used to test experimental asphalt pavements for durability.
In addition to repair costs, the condition of a road surface has economic effects for road users.Rolling resistance increases on rough pavement, as does wear and tear of vehicle components. It has been estimated that poor road surfaces cost the average US driver $324 per year in vehicle repairs, or a total of $67 billion. Also, it has been estimated that small improvements in road surface conditions can decrease fuel consumption between 1.8 and 4.7%.[65]
Road surface markings are used on paved roadways to provide guidance and information to drivers and pedestrians. It can be in the form of mechanical markers such ascat's eyes,botts' dots andrumble strips, or non-mechanical markers such as paints,thermoplastic, plastic andepoxy.
The LHA also sponsored short concrete "Seedling Mile" object lesson roads in many locations (the first, built in the fall of 1914, was just west of Malta, Illinois). The "Seedling Miles," according to the LHA's 1924 guide, were intended "to demonstrate the desirability of this permanent type of road construction" and "crystallize public sentiment" for "further construction of the same character." Generally, the LHA worked with the Portland Cement Association to arrange donations of cement for the seeding mileage...The most famous "seedling" and one of the most talked about portions of the Lincoln Highway was the 1.3-mile "Ideal Section" between Dyer and Schererville in Lake County, Indiana. In 1920, the LHA decided to develop a model section of road that would be adequate not only for current traffic but for highway transportation over the following 2 decades. The LHA assembled 17 of the country's foremost highway experts for meetings in December 1920 and February 1921 to decide design details of the Ideal Section. They agreed on such features as: A 110-foot right-of-way; A 40-foot wide concrete pavement 10 inches thick (maximum loads of 8,000 pounds per wheel were the basis for the pavement design); Minimum radius for curves of 1,000 feet, with guardrail at all embankments; Curves superelevated (i.e., banked) for a speed of 35 miles per hour; No grade crossings or advertising signs; and A footpath for pedestrians.
... the cycle track is asphalt (...) and the lane for cars is brick ...
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