Asphalt batch mix plant A machine laying asphalt concrete, fed from a dump truck
Asphalt concrete (commonly called asphalt, blacktop, or pavement in North America, and tarmac or bitumen macadam or rolled asphalt in the United Kingdom and the Republic of Ireland) is a composite material commonly used to surface roads, parking lots, airports, as well as the core of embankment dams. It consists of mineral aggregate bound together with asphalt, laid in layers, and compacted. The process was refined and enhanced by Belgian inventor and U.S. immigrant Edward de Smedt.
The terms asphalt (or asphaltic) concrete, bituminous asphalt concrete, and bituminous mixture are typically used only in engineering and construction documents, which define concrete as any composite material composed of mineral aggregate adhered with a binder. The abbreviation, AC, is sometimes used for asphalt concrete but can also denote asphalt content or asphalt cement, referring to the liquid asphalt portion of the composite material.
As shown in this cross-section, many older roadways are smoothed by applying a thin layer of asphalt concrete to the existing portland cement concrete, creating a composite pavement.
Mixing of asphalt and aggregate is accomplished in one of several ways:
Hot-mix asphalt concrete (commonly abbreviated as HMA) This is produced by heating the asphalt binder to decrease its viscosity, and drying the aggregate to remove moisture from it prior to mixing. Mixing is generally performed with the aggregate at about 300 °F (roughly 150 °C) for virgin asphalt and 330 °F (166 °C) for polymer modified asphalt, and the asphalt cement at 200 °F (95 °C). Paving and compaction must be performed while the asphalt is sufficiently hot. In many countries paving is restricted to summer months because in winter the compacted base will cool the asphalt too much before it is able to be packed to the required density. HMA is the form of asphalt concrete most commonly used on high traffic pavements such as those on major highways, racetracks and airfields. It is also used as an environmental liner for landfills, reservoirs, and fish hatchery ponds. Asphaltic concrete laying machine in operation in Laredo, Texas Warm-mix asphalt concrete (commonly abbreviated as WMA) This is produced by adding either zeolites, waxes, asphalt emulsions, or sometimes even water to the asphalt binder prior to mixing. This allows significantly lower mixing and laying temperatures and results in lower consumption of fossil fuels, thus releasing less carbon dioxide, aerosols and vapors. Not only are working conditions improved, but the lower laying-temperature also leads to more rapid availability of the surface for use, which is important for construction sites with critical time schedules. The usage of these additives in hot mixed asphalt (above) may afford easier compaction and allow cold weather paving or longer hauls. Use of warm mix is rapidly expanding. A survey of US asphalt producers found that nearly 25% of asphalt produced in 2012 was warm mix, a 416% increase since 2009. Cold-mix asphalt concrete This is produced by emulsifying the asphalt in water with (essentially) soap prior to mixing with the aggregate. While in its emulsified state the asphalt is less viscous and the mixture is easy to work and compact. The emulsion will break after enough water evaporates and the cold mix will, ideally, take on the properties of an HMA pavement. Cold mix is commonly used as a patching material and on lesser trafficked service roads. Cut-back asphalt concrete Is a form of cold mix asphalt produced by dissolving the binder in kerosene or another lighter fraction of petroleum prior to mixing with the aggregate. While in its dissolved state the asphalt is less viscous and the mix is easy to work and compact. After the mix is laid down the lighter fraction evaporates. Because of concerns with pollution from the volatile organic compounds in the lighter fraction, cut-back asphalt has been largely replaced by asphalt emulsion. Mastic asphalt concrete, or sheet asphalt This is produced by heating hard grade blown bitumen (i.e., partly oxidised) in a green cooker (mixer) until it has become a viscous liquid after which the aggregate mix is then added. The bitumen aggregate mixture is cooked (matured) for around 6–8 hours and once it is ready the mastic asphalt mixer is transported to the work site where experienced layers empty the mixer and either machine or hand lay the mastic asphalt contents on to the road. Mastic asphalt concrete is generally laid to a thickness of around 3⁄4–1 3⁄16 inches (20–30 mm) for footpath and road applications and around 3⁄8 of an inch (10 mm) for flooring or roof applications. High-modulus asphalt concrete, sometimes referred to by the French-language acronym EMÉ (enrobé à module élevé) This uses a very hard bituminous (penetration 10/20), sometimes modified, in proportions close to 6% on the weight of the aggregates, and a proportion of mineral powder also high, between 8–10%, to create an asphalt concrete layer with a high modulus of elasticity, of the order of 13000 MPa, as well as very high fatigue strengths. High-modulus asphalt layers are used both in reinforcement operations and in the construction of new reinforcements for medium and heavy traffic. In base layers, they tend to exhibit a greater capacity of absorbing tensions and, in general, better fatigue resistance.
In addition to the asphalt and aggregate, additives, such as polymers, and antistripping agents may be added to improve the properties of the final product.
Asphalt concrete pavements—especially those at airfields—are sometimes called tarmac for historical reasons, although they do not contain tar and are not constructed using the macadam process.
A variety of specialty asphalt concrete mixtures have been developed to meet specific needs, such as stone-matrix asphalt, which is designed to ensure a very strong wearing surface, or porous asphalt pavements, which are permeable and allow water to drain through the pavement for controlling stormwater.
An airport taxiway, one of the uses of asphalt concrete
Different types of asphalt concrete have different performance characteristics in terms of surface durability, tire wear, braking efficiency and roadway noise. In principle, the determination of appropriate asphalt performance characteristics must take into account the volume of traffic in each vehicle category, and the performance requirements of the friction course. Asphalt concrete generates less roadway noise than a Portland cement concrete surface, and is typically less noisy than chip seal surfaces.
Because tire noise is generated through the conversion of kinetic energy to sound waves, more noise is produced as the speed of a vehicle increases. The notion that highway design might take into account acoustical engineering considerations, including the selection of the type of surface paving, arose in the early 1970s. With regard to structural performance, the asphalt behaviour depends on a variety of factors including the material, loading and environmental condition. Furthermore, the performance of pavement varies over time. Therefore, the long-term behaviour of asphalt pavement is different from its short-term performance. The LTPP is a research program by the FHWA, which is specifically focusing on long-term pavement behaviour.
Asphalt damaged by frost heaves
Asphalt deterioration can include crocodile cracking, potholes, upheaval, raveling, bleeding, rutting, shoving, stripping, and grade depressions. In cold climates, frost heaves can crack asphalt even in one winter. Filling the cracks with bitumen is a temporary fix, but only proper compaction and drainage can slow this process.
Factors that cause asphalt concrete to deteriorate over time mostly fall into one of three categories: construction quality, environmental considerations, and traffic loads. Often, damage results from combinations of factors in all three categories.
Construction quality is critical to pavement performance. This includes the construction of utility trenches and appurtenances that are placed in the pavement after construction. Lack of compaction in the surface of the asphalt, especially on the longitudinal joint can reduce the life of a pavement by 30 to 40%. Service trenches in pavements after construction have been said to reduce the life of the pavement by 50%, mainly due to the lack of compaction in the trench, and also because of water intrusion through improperly sealed joints.
Environmental factors include heat and cold, the presence of water in the subbase or subgrade soil underlying the pavement, and frost heaves.
High temperatures soften the asphalt binder, allowing heavy tire loads to deform the pavement into ruts. Paradoxically, high heat and strong sunlight also cause the asphalt to oxidize, becoming stiffer and less resilient, leading to crack formation. Cold temperatures can cause cracks as the asphalt contracts. Cold asphalt is also less resilient and more vulnerable to cracking.
Water trapped under the pavement softens the subbase and subgrade, making the road more vulnerable to traffic loads. Water under the road freezes and expands in cold weather, causing and enlarging cracks. In spring thaw, the ground thaws from the top down, so water is trapped between the pavement above and the still-frozen soil underneath. This layer of saturated soil provides little support for the road above, leading to the formation of potholes. This is more of a problem for silty or clay soils than sandy or gravelly soils. Some jurisdictions pass frost laws to reduce the allowable weight of trucks during the spring thaw season and protect their roads.
The damage a vehicle causes is proportional to the axle load raised to the fourth power, so doubling the weight an axle carries actually causes 16 times as much damage. Wheels cause the road to flex slightly, resulting in fatigue cracking, which often leads to crocodile cracking. Vehicle speed also plays a role. Slowly moving vehicles stress the road over a longer period of time, increasing ruts, cracking, and corrugations in the asphalt pavement.
Other causes of damage include heat damage from vehicle fires, or solvent action from chemical spills.
The life of a road can be prolonged through good design, construction and maintenance practices. During design, engineers measure the traffic on a road, paying special attention to the number and types of trucks. They also evaluate the subsoil to see how much load it can withstand. The pavement and subbase thicknesses are designed to withstand the wheel loads. Sometimes, geogrids are used to reinforce the subbase and further strengthen the roads. Drainage, including ditches, storm drains and underdrains are used to remove water from the roadbed, preventing it from weakening the subbase and subsoil.
Good maintenance practices center on keeping water out of the pavement, subbase and subsoil. Maintaining and cleaning ditches and storm drains will extend the life of the road at low cost. Sealing small cracks with bituminous crack sealer prevents water from enlarging cracks through frost weathering, or percolating down to the subbase and softening it.
For somewhat more distressed roads, a chip seal or similar surface treatment may be applied. As the number, width and length of cracks increases, more intensive repairs are needed. In order of generally increasing expense, these include thin asphalt overlays, multicourse overlays, grinding off the top course and overlaying, in-place recycling, or full-depth reconstruction of the roadway.
It is far less expensive to keep a road in good condition than it is to repair it once it has deteriorated. This is why some agencies place the priority on preventive maintenance of roads in good condition, rather than reconstructing roads in poor condition. Poor roads are upgraded as resources and budget allow. In terms of lifetime cost and long term pavement conditions, this will result in better system performance. Agencies that concentrate on restoring their bad roads often find that by the time they’ve repaired them all, the roads that were in good condition have deteriorated.
Some agencies use a pavement management system to help prioritize maintenance and repairs.
A small-scale asphalt recycler
Asphalt concrete is 100% recyclable and is the most widely reused construction material in the world. Very little asphalt concrete — less than 1 percent, according to a 2011 survey by the Federal Highway Administration and the National Asphalt Pavement Association — is actually disposed of in landfills.
There is asphalt recycling on a large scale (known as in-place asphalt recycling or asphalt recycling performed at a hot mix plant) and asphalt recycling on a smaller scale. For small scale asphalt recycling, the user separates asphalt material into three different categories:
Blacktop cookies Chunks of virgin uncompacted hot mix asphalt which can be used for pothole repair. The use of blacktop cookies has been investigated as a less expensive, less labor-intensive, more durable alternative to repairing potholes with cold patch. In a program in Pittsfield, Massachusetts, workers purchased new hot mix asphalt and spread it liberally on the ground to produce approximately 25 lb. wafers. Once cooled, the wafers could be stored until reheated in a hotbox to make minor road repairs. Blacktop cookies may also be produced from leftover material from paving jobs. Reclaimed asphalt pavement (RAP) Chunks of asphalt that have been removed from a road, parking lot or driveway are considered RAP. These chunks of asphalt typically are ripped up when making a routine asphalt repair, man hole repair, catch basin repair or sewer main repair. Because the asphalt has been compacted, RAP is a denser asphalt material and typically takes longer to recycle than blacktop cookies. Asphalt millings Small pieces of asphalt produced by mechanically grinding asphalt surfaces are referred to as asphalt millings. Large millings that have a rich, black tint indicating a high asphalt cement content are best for asphalt recycling purposes. Surface millings are recommended over full depth millings when choosing asphalt millings to recycle. Full depth millings usually contain sub-base contaminants such as gravel, mud and sand. These sub base contaminants will leach oil away from original asphalt and dry out the material in the recycling process. Asphalt milled from asphalt is better than asphalt milled from concrete. When milling asphalt from concrete the dust that is created is not compatible with asphalt products because it is not asphalt.
Small scale asphalt recycling will usually involve high speed on-site asphalt recycling equipment or overnight soft heat asphalt recycling.
Small scale asphalt recycling is used when wanting to make smaller road repairs vs. large scale asphalt recycling which is done for making new asphalt or for tearing up old asphalt and simultaneously recycling / replacing existing asphalt. Recycled asphalt is very effective for pothole and utility cut repairs. The recycled asphalt will generally last as long or longer than the road around it as new asphalt cement has been added back to the material.
For larger scale asphalt recycling, several in-place recycling techniques have been developed to rejuvenate oxidized binders and remove cracking, although the recycled material is generally not very water-tight or smooth and should be overlaid with a new layer of asphalt concrete. Cold in-place recycling mills off the top layers of asphalt concrete and mixes the resulting loose millings with asphalt emulsion. The mixture is then placed back down on the roadway and compacted. The water in the emulsion is allowed to evaporate for a week or so, and new hot-mix asphalt is laid on top.
Asphalt concrete that is removed from a pavement is usually stockpiled for later use as aggregate for new hot mix asphalt at an asphalt plant. This reclaimed material, or RAP, is crushed to a consistent gradation and added to the HMA mixing process. Sometimes waste materials, such as asphalt roofing shingles, crushed glass, or rubber from old tires, are added to asphalt concrete as is the case with rubberized asphalt, but there is a concern that the hybrid material may not be recyclable.
LaneThru lanes indicated by arrows on California CR G4 (Montague Expressway) in Silicon Valley.
In the context of traffic control, a lane is part of a roadway (carriageway) that is designated for use by a single line of vehicles, to control and guide drivers and reduce traffic conflicts. Most public roads (highways) have at least two lanes, one for traffic in each direction, separated by lane markings. On multilane roadways and busier two-lane roads, lanes are designated with road surface markings. Major highways often have two multi-lane roadways separated by a median.
Some roads and bridges that carry very low volumes of traffic are less than 15 feet (4.6 m) wide, and are only a single lane wide. Vehicles travelling in opposite directions must slow or stop to pass each other. In rural areas, these are often called country lanes. In urban areas, alleys are often only one lane wide. Urban and suburban one lane roads are often designated for one-way traffic.
Lane capacity varies widely due to conditions such as neighboring lanes, lane width, elements next to the road, number of driveways, presence of parking, speed limits, number of heavy vehicles and so on – the range can be as low as 1000 passenger cars / hour to as high as 4800 passenger cars /hour but mostly falls between 1500 and 2400 passenger cars / hour.The Ontario Highway 401 in the Greater Toronto area, with 17 travel lanes in 6 separate carriageways visible in the midground. Turning lane on the Rodovia BR-101 (Brazil) Play media Changing lanes, Gothenburg, Sweden Transfer lanes, connecting surface collector lanes with through lanes between two tunnels A left-turn merging lane in Germany, needing explanation by a crafted sign These usages lead to the phrases life in the slow lane and life in the fast lane, used to describe relaxed or busy lifestyles, respectively and used as the titles of various books and songs.
While in general, wider lanes are associated with a reduction in crashes, in urban settings both narrow (less than 2.8 m) and wide (over 3.1~3.2 m) lanes increase crash risks. Wider lanes (over 3.3~3.4m) are associated with 33% higher impact speeds, as well as higher crash rates. Carrying capacity is also maximal at a width of 3 to 3.1 metres (9.8 to 10.2 ft), both for motor traffic and for bicycles. Pedestrian volume declines as lanes widen, and intersections with narrower lanes provide the highest capacity for bicycles. As lane width decreases, traffic speed diminishes.
Advocates for safety of people walking and people on bikes, and many new urbanists disagree with traditional thinking in traffic engineering, saying that safety and capacity are not adversely impacted by reducing lanes widths to as little as 10 feet (3.0 m). Moreover, wider travel lanes also increase exposure and crossing distance for pedestrians at intersections and midblock crossings.assumed widths and heights in road design for Europe (in meters)
The widths of vehicle lanes typically vary from 9 to 15 feet (2.7 to 4.6 m). Lane widths are commonly narrower on low volume roads and wider on higher volume roads. The lane width depends on the assumed maximum vehicle width with an additional space to allow for lateral motion of the vehicle.
The maximum truck width had been 96 inches (2.438 m) in the Code of Federal Regulations of 1956 which matches with the width of eight-foot for shipping containers. This had been increased to 102 inches (2.591 m) in 1976 which explicitly states to be read as the slightly larger metric 2.6 metres (102.36 in) width respecting international harmonization. The same applies to standards in Europe which had increased the allowable size of road vehicles with a current maximum of 2.55 metres (100.39 in) for most trucks and allowing 2.6 metres (102.36 in) for refrigerator trucks. The minimum extra space had been 0.20 metres (7.87 in) and it is currently assumed to be at least 0.25 metres (9.84 in) on each side. For roads with a lower amount of traffic it is allowed to build the second or third lane in the same direction to an assumed lower width for cars like 1.75 metres (68.90 in), however this is not recommended as a design principle for new roads as changes in the amount of traffic could make for unnecessarily increased risks in the future.
The Interstate Highway standards for the U.S. Interstate Highway System uses a 12-foot (3.7 m) standard for lane width, while narrower lanes are used on lower classification roads. In Europe, as laws and road width vary by country, the minimum widths of lanes is generally between 2.5 to 3.25 metres (8.2 to 10.7 ft). The federal Bundesstraße interurban network in Germany defines a minimum of 3.5 metres (11 ft 6 in) for each lane for the smallest two lane roads with an additional 0.25 metres (9.84 in) on the outer sides and shoulders being at least 1.5 metres (59.06 in) on each side. A modern Autobahn divided highway will have two lanes per direction which are 3.75 metres (12 ft 4 in) wide with an additional clearance of 0.50 metres (19.69 in) on each side, while three lanes per direction are set at 3.75 metres (12 ft 4 in) for the rightmost lane and 3.5 metres (11 ft 6 in) for the other lanes. Urban access roads and roads in low-density areas may have lanes as small as 2.75 metres (9 ft 0 in) in width per lane with shoulders being at least 1 metre (3 ft 3 in) wide.Main article: Road surface marking A typical rural American freeway (Interstate 5 in the Central Valley of California). Notice the yellow line on the left, the dashed white line in the middle, and the solid white line on the right. Also note the rumble strip to the left of the yellow line.
Painted lane markings vary widely from country to country. In the United States, Canada, Mexico, Honduras, Puerto Rico, Virgin Islands and Norway, yellow lines separate traffic going opposite directions and white separates lanes of traffic traveling the same direction, but such is not the case in many European countries.
Lane markings are mostly lines painted on the road by a road marking machine, which can adjust the marking widths according to the lane type.
Traffic reports in California often refer to accidents being "in the number X lane." The California Department of Transportation (Caltrans) assigns the numbers from left to right. The far left passing lane is the number 1 lane. The number of the slow lane (closest to freeway onramps/offramps) depends on the total number of lanes, and could be anywhere from 2 to 8.
For much of human history, roads did not need lane markings because most people walked or rode horses at relatively slow speeds. Another reason for not using lane markings is that they are expensive to maintain.
When automobiles, trucks, and buses came into widespread use during the first two decades of the 20th century, head-on collisions became more common.
Without the guidance provided by lane markings, drivers in the early days often erred in favor of keeping closer to the middle of the road, rather than risk going off-road into ditches or trees. This practice often left inadequate room for opposing traffic.
The history of lane markings is connected to the mass automobile construction in Detroit. It resulted in the formation of the first Road Commission of Wayne County, Michigan in 1906 which was trying to make roads safer (Henry Ford served on the board in the first year). The commission would order the construction of the first concrete road in 1909 (the Woodard Avenue in Detroit) and it conceived the centerline for highways in 1911. Hence the chairmen of the Road Commission, Edward N. Hines is widely credited as the inventor of line markings.
The introduction as a common standard is connected to June McCarroll, a physician in Indio, California who started experimenting with painting lines on roads in 1917 after she was run off a highway by a truck driver. In November 1924, after years of lobbying by Dr. McCarroll and her allies, California officially adopted a policy of painting lines on its highways. A portion of Interstate 10 near Indio has been named the Dr. June McCarroll Memorial Freeway in her honor.black center line on an Autobahn in Germany (late 1930s)
The first lane markings in Europe were painted at an accident hotspot in the small town of Sutton Coldfield near Birmingham, England in 1921. The success of this experiment made its way to other hotspots and later standardization of white paint for line markings in Great Britain.
The first lane markings in Germany were used in Berlin in 1925 using white paint for line markings and road edge markings. When the standard for the new autobahn network was conceived in the 1930s it mandated the usage of black paint for the center line for each carriageway as black was better visible on the bright surface of the concrete roads.
By 1939, lane markings had become so popular that they were officially standardized throughout the United States. The concept of line markings spread throughout the world becoming standard for most roads. Originally the lines were drawn manually with normal paint which would bleach out quickly. After the war, the first machines for line markings were invented and a plastic strip was becoming standard in the 1950s which led to gradually find line markings on all roads.Main article: Right- and left-hand traffic
Crocodile crackingSealcoating a road on the University of California, Davis campus in 2013.
Sealcoating, or pavement sealing, is the process of applying a protective coating to asphalt-based pavements to provide a layer of protection from the elements: water, oils, and U.V. damage.
Sealcoat or pavement sealer is a coating for asphalt-based pavements. Sealcoating is marketed as a protective coating that extends the life of asphalt pavements. There is not any independent research that proves these claims.
Sealcoating may also reduce the friction or anti-skid properties associated with the exposed aggregates in asphalt.
Not all pavement sealcoat are created equal. For example, refined tar-based sealer offers the best protecting against water penetration and chemical resistance. Asphalt-based sealer typically offers poor protection against environmental chemical and harsher climates (salt water). Petroleum-based sealer offer protection against water and chemicals somewhere between the other two sealers. Another difference between coatings is in terms of wear. Again, refined tar-based sealer offers the best wear characteristics (typically 3–5 years) while asphalt-based sealer may last 1–3 years. Petroleum-based sealer falls between refined tar and asphalt.
There are concerns about pavement sealer polluting the environment after it is abraded from the surface of the pavement. Some states in North America have banned the use of coal tar–based sealants primarily based on United States Geological Survey studies. The industry group that represents sealcoat manufacturers has performed numerous research and reviews of the USGS and have found it to be erroneous, biased (citation and white hat, to name a few) and too generalized in order to draw the conclusions that the United States Geological Survey claims.
There are primarily three types of pavement sealers. They are commonly known as refined tar-based (coal tar based), asphalt-based, and petroleum-based. All three have their advantages but are typically chosen by the contractors’ preference unless otherwise specified.
Prior to application the surface must be completely clean and dry using sweeping methods and/or blowers. If the surface is not clean and dry, then poor adhesion will result. Pavement sealers are applied with either pressurized spray equipment, or self-propelled squeegee machines or by hand with a squeegee. Equipment must have continuous agitation to maintain consistency of the sealcoat mix. The process is typically a two-coat application which requires 24 to 48 hours of curing before vehicles can be allowed back on the surface. Once the surface is properly prepared, then properly mixed sealer will be applied at about 60 square feet per gallon per coat.The Sealcoating Process
Some studies that suggest that refined tar sealants are a significant contributor to polycyclic aromatic hydrocarbon levels in streams and creek beds and that the continual application of sealcoats may be a significant factor. As a result, a few municipalities in the United States have banned this material. The same studies also suggest that it can be harmful if ingested before curing and ingesting soil or dust contaminated by eroded coal tar sealant. It is also known to have effects on fish and other animals that live in water.
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