Four layers Roads are made up of four layers.

What is the top layer of a road called?

The wearing course is the upper layer in roadway, airfield, and dockyard construction. The term ‘surface course’ is sometimes used, however this term is slightly different as it can be used to describe very thin surface layers such as chip seal, In rigid pavements the upper layer is a portland cement concrete slab.

In flexible pavements, the upper layer consists of asphalt concrete, that is a construction aggregate with a bituminous binder. The wearing course is typically placed on the binder course which is then laid on the base course, which is normally placed on the subbase, which rests on the subgrade,

There are various different types of flexible pavement wearing course, suitable for different situations. Stone mastic asphalt is a type of flexible pavement wearing course which is typically used for heavily trafficked roads.

What is the base layer of a road made of?

The Base Layer Forms the Foundation for the Road Surface – The bottom layer essentially consists of an unbound mixture of coarse and fine crushed stone as well as crushed sand to achieve the desired bearing capacity and absorb traffic loads so that the underlying subgrade is not deformed.

The base layer forms the foundation for the road surface. Depending on the expected load, the road structure is composed of different layers of different thicknesses to withstand different weather conditions and to remain functional for decades. A Gravel Layer Protects against Frost Roads are exposed to particularly high stresses when the water contained in the pavement structure begins to freeze.

Water expands when freezing, which can lead to frost damage that sooner or later will also have an effect on the road surface. This is prevented by means of a frost protection layer, which usually consists of a mixture of gravel and sand supplemented by crushed mineral aggregate. Hamm compactors perform extremely hard jobs in earthworks and – as seen here – when compacting the cement-treated base layer.

What is the standard thickness of road?

Why thickness measurement in road construction is essential for the quality of the road In road construction, there is an infinite number of pinwheels to make the process as perfect as possible. Everything has to be considered and taken into account, starting with the planning, the tendering process, the specification of machines and staff, the choice of material and of course the paving process.

• Looking at the process, I would like to go a little deeper into the different asphalt layers, because they have a significant impact on the resilience and many other characteristics that are often marginally mentioned.
• The different asphalt layers and their tasks First, I would like to give a short introduction into the different layers and their functions.

Every layer is important for the reliability of the road. I. Base course The base course, as the name implies – along with the binder course – is the load carrying element of the road and gives the binder and top layer a consistent hold. Furthermore, the base course protects quickly and effectively from rainfall and other environmental conditions.

• Exposed traffic forces are removed and evenly distributed on the base.
• The layer thickness should be selected in such a way that the required compaction is achieved at each point of the street.
• Often, with base course thicknesses of more than 15cm, the layer is paved in two production steps. II.
• Binder course The binder course ensures that shear stresses are absorbed and thus deformations in the road are prevented.

Uneven surfaces remaining from the base course are further optimized in the binder installation, so that the perfect smoothness is achieved as soon as possible after the binder installation. The layer thicknesses stagger depending on the material and type of the road between 5cm – 12cm.

• III. Top layer The top layer is the most heavily used layer of the road, because it is subject to the direct influence of traffic, weather and thawing agents.
• Thus, the top layer must be particularly resistant to wear and deformation.
• Durable evenness, grip, environmental friendliness and low noise are other properties required by the top layer.

In addition, the layer must be designed with structures, textures, colors and shapes. The specified layer thicknesses can be very different, depending on the requirement and the type of material. The classification of the road in its different asphalt layers and their components has an economic as well as a technical background.

Every single layer has its own special task as described above. Its position, strength and material composition make it a part of the load-bearing capacity and resilience of the finished road. The individual layers are combined into a compact structure in order to fulfill the desired specification, which is essential for the durability of the entire construction.

Not to be forgotten is the infrastructure aspect of every street, because the users of the road are also very interested in a stable road that meets the requirements. How do unstable or even too low layer thicknesses affect the overall road? 1. Lifetime of the road Unstable layer thicknesses prevent the individual layers from fulfilling their function.

• F or example, if a road has changing thicknesses in the binder layer, it will absorb the resulting shear stresses better and worse.
• Early road damage is the result.
• However, if the binder course is made uniformly and consistently, the building structure will be evenly stressed and will be able to maintain its originally specified lifespan.

If there are irregular layer thicknesses in the top layer, premature road damage will be the result. Especially in the top layer you need to have a consistent layer thicknesses in order to avoid cracks or waves. If you are able to ha ve perfectl y leveled layers, stable thicknesses can much easier be achieved. It gets more critical if the layer thicknesses are irregular across all layers. Critical points over the entire course of the road are the result. There will be spots where the road will take early damage and not reach the desired lifespan. In the following sketch you can see, even as a layman, that something is wrong.

What is not critical in the base course, changes dramatically with the binder and even more in the top layer. If the binder course is irregular and even too thin in some places, it can no longer fulfill its task as a tension absorber. This problem gets even bigger in the base course, because in this case the problem becomes visible first.

Cracks and holes are the result of too thin and irregular layers. The durability of the road decreases significantly. 2. Economic and infrastructure aspects The economic aspect is especially important for the contractor because he has a great interest that the completed project achieves the quality to reach the specified and planned lifespan. The contractor will not have any warranty claims.

• Especially contractors, but also road owners and we as the tax-payers are interested in roads with high quality and a long lifespan.
• One particularly essential basis for this are constant layer thicknesses, which are paved exactly as required.
• Furthermore, by working on the tolerance limits, the contractor gets the opportunity to save material and thus costs.

If one considers that with a material price of 80.00 EUR per ton, a paving scope of 100km with an average screed width of 6m can result in a saving of 80,000.00 EUR for every saving of 0.1cm layer thickness, the cost savings for a year can easily be calculated.

Even if the guidelines and specifications all over the world vary: At some point the road construction project and thus the paving material has to be paid. At the end of the day, after each project, there are people using the street. No matter if it´s on two-wheeled vehicles, cars, trucks or others. Whether business or private purposes.

We all have a great interest in traveling safe and relaxed but still quick to reach our destination. : Why thickness measurement in road construction is essential for the quality of the road

What is the minimum thickness of road?

Discussion :: Highway Engineering – Section 3 ( Q.No.21 ) –

Sanjay said: (Apr 18, 2017)
It should be 15 cm.

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Shanaya said: (Apr 24, 2017) What is the right answer? please tell.

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Aadil said: (Jun 8, 2017) I think it is 25.

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Vivek said: (Jun 16, 2017) It should be 25 cm.

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Jp Dadhich said: (Jun 27, 2017) Can you explain why it should be 15cm? @Sanjay.

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Jeet said: (Oct 3, 2017) Wearing course 10 cm. Base course 30 cm.

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Garry said: (Oct 15, 2017) As per IRC 37. Minimum thickness of base course for traffic up to 2 msa is 225mm. Minimum thickness of base course for traffic greater than 2 msa is 250 mm.

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Ankush said: (Dec 26, 2017) It should be 15cm.

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Naveengowda said: (Jul 23, 2018) Given answer is correct. Surface course -> 25 – 50mm. Binder course -> 50 – 100mm. Base course -> 100 – 300mm. Sub base course -> 100 – 300mm. Subgrade -> 150 – 300mm.

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Dhaval Darji said: (Jul 31, 2018) Surface-10cm. Base- 200mm to 300mm. Sub-base-75mm to 150mm. Subgrade -depth 600mm to the water level.

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Ravishankar V L said: (Apr 26, 2019) Sub-base materials comprise natural sand, gravel, laterite, brick metal, crushed stone or combinations thereof meeting the prescribed grading and physical requirements. The sub-base material should have a minimum CBR of 20 % and 30 % for traffic upto 2 msa and traffic exceeding 2 msa respectively. Sub-base usually consist of granular or WBM and the thickness should not be less than 150 mm for design traffic less than 10 msa and 200 mm for design traffic of 1:0 msa and above. Base The recommended designs are for unbounded granular bases which comprise conventional water bound macadam (WBM) or wet mix macadam (WMM) or equivalent confirming to MOST specifications. The materials should be of good quality with minimum thickness of 225 mm for traffic up to 2 msa an 150 mm for traffic exceeding 2 msa. Bituminous surfacing The surfacing consists of a wearing course or a binder course plus wearing course. The most commonly used wearing courses are surface dressing, open graded premix carpet, mix seal surfacing, semi-dense bituminous concrete and bituminous concrete. For binder course, MOST specifies, it is desirable to use bituminous macadam (BM) for traffic upto o 5 msa and dense bituminous macadam (DBM) for traffic more than 5 msa.

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Prajwol Pratap said: (Feb 2, 2020) 10 cm for Base course and 15 cm for Sub-base Course.

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Bjpegu said: (Nov 28, 2020) The thickness of the base ranges from 15- 30cm. Not only 10cm.

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Engineer Johny Sins said: (Jun 22, 2021) Thickness of the base ranges from 10cm – 30cm. Here the question asked is minimum thickness, So, The answer is 10cm.

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Naveed Kamal Marwat said: (Sep 19, 2021) Surface course -> 25 – 50mm. Binder course -> 50 – 100mm. Base course -> 100 – 300mm. Sub base course -> 100 – 300mm. Subgrade -> 150 – 300mm.

Civil Engineering – Highway Engineering – Discussion

What is WBM layer?

What is WBM Road? – Full form of WBM is Water Bound Macadam, It is the layer of broken stone aggregates bound together by stone dust or screening material and water applied during construction and compacted by heavy smoothed wheel roller, It is the oldest type of highway pavement used in modern times is known as Macadam after the name of Johan Macadam, A Scottish Engineer.

What is road floor called?

This article is about road surface pavement material. For other uses of the term “pavement”, see Pavement (disambiguation), Construction crew laying down asphalt over fiber-optic trench, in New York City A road surface ( British English ), or pavement ( American English ), is the durable surface material laid down on an area intended to sustain vehicular or foot traffic, such as a road or walkway,

1. In the past, gravel road surfaces, hoggin, cobblestone and granite setts were extensively used, but these have mostly been replaced by asphalt or concrete laid on a compacted base course,
2. 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, are rough and can sustain less weight. Road surfaces are frequently marked to guide traffic, Today, permeable paving methods are beginning to be used for low-impact roadways and walkways.

What is bitumen layer?

What’s the difference between Asphalt and Bitumen? Should I use Bitumen or Asphalt? – Bitumen is actually the liquid binder that holds asphalt together. The term bitumen is often mistakenly used to describe asphalt. A bitumen-sealed road has a layer of bitumen sprayed and then covered with an aggregate.

What is subgrade layer in road?

Subgrade is that portion of the earth roadbed which after having been constructed to reasonably close conformance with the lines, grades, and cross-sections indicated on the plans, receives the base or surface material.

What is a road capping layer?

A capping layer helps to create a construction platform where the subgrade is not viable. It is granular product from a crushed rock quarry, and often incorporates recycled materials. Capping layer materials help to reduce costs and to protect the subgrade from rainfall and weathering.

What are the 4 major types of roads?

The following photos and information illustrate the four major road function classifications: Interstates, Other Arterials, Collectors, and Local roads. The amount of mobility and land access offered by these road types differs greatly.

How many levels are in the road?

How many levels in a road hierarchy? Introduction In this note I will use the terms “road hierarchy (RH)” and “functional classification (FC)” as if they mean the same thing – which essentially they do. The apparent difference is that RH by definition says that some roads are more important than others, whilst in FC people assume that some roads are more important than others.

• The note briefly describes examples of: • A 4-level road hierarchy • A 3-level road hierarchy • A 5-level road hierarchy • A 6-level road hierarchy • A 2-level road hierarchy • A many levels road hierarchy
• The note concludes with some comments.
• 4-level road hierarchy
• A Wikipedia page gives one answer to the question (). What the above figure, taken from the Wikipedia page, suggests is:

• There are only four levels in a road hierarchy (freeway, arterial, collector/distributor, local)
• Road hierarchy can be defined in terms of two topics. One topic is “access to property”, the other is “through traffic, movement and speed”
• So far as RH is concerned only these 4 topics are involved (traffic, movement, speed and access)
• “Through traffic”, “movement” and “speed” each have the same relevance to RH

The figure is most likely illustrative, and not based on measured values. Having said this however,

• The graph is a continuous curve – it does not show four discrete steps. The continuous curve implies that there can be any number of levels in a road hierarchy, not just four. Indeed, the accompanying text brings in additional road types which do not appear in the graphic (e.g. motorways, limited access roads, streets) and also introduces sub-classes (major and minor arterials, rural and urban arterials)
• There is no obvious reason why increasing through traffic should be directly related to increasing speed
• There is no quantification of the term “through traffic.” For example, perhaps a definition of through traffic on a motorway would be “any vehicle trip which is longer than (say) 100 km” – which would mean that any vehicle trip which partly uses a motorway and which is only 99 km long should be defined as “access traffic”.

1. 3-level road hierarchy
3. The USA’s Federal Highway Administration (FHWA) has a webpage on functional road classification (see ) from which the above figure is taken.
4. From the FHWA figure:

• There are only three levels in a functional road classification (arterial, collector, local)
• Functional road classification can be defined in terms of two topics. One is “land access”, the other is “mobility”
• The graph is a continuous curve – it does not show 3 discrete steps. This implies that there can be any number of levels in a road hierarchy, and just 3. Indeed, the accompanying text on the web page has a table 3.2 which shows four classes and a table 3.3 which shows eight classes.

From the accompanying text

• There is an indication that “mobility” refers to journey distance (“the traffic (i.e., local or long distance”) and to “the character of traffic service”
• The text implies mobility can also be equated with speed (“once the functional classification of a particular roadway has been established, so has the allowable range of design speed”) – and see also, which says “Mobility can incorporate a wide range of elements, but the most basic is operating speed or trip travel time”.

5-level road hierarchy Example 1 In 1991 the UK’s Overseas Development Association (ODA) published a book on the planning and design of roadworks, called “Towards safer roads in developing countries” (ref.617). In its section on road hierarchy, the document says: “Developed countries define road networks as a hierarchy in terms of road types, according to the major functions the road will serve.

• Primary distributors
• District distributors
• Local distributors
• Access roads
• Pedestrian streets

Example 2 Tanzania’s Road geometric design manual (2011 edition) (ref.294) was prepared through technical cooperration with the Norwegian Public Roads Administration. Table 2.1 of the manual refers to 5 levels in a functional road classification:

• Trunk roads
• Regional roads
• Collector roads
• Feeder roads
• Community roads

6-level hierarchy The East African Community has been working on the harmonisation of roadway geometric standards (ref.589). Page 11 of the document discusses FC and says: “Geometric design standards depend on the functional requirements of the road. However, the functional classification of the road system does not automatically lead to the selection of a design speed and cross section for a specific link in the network.”

• And
• “Roads may also be classified based on some other criteria but the classification based on speed and accessibility is the most generic one”. (own italics)
• The figure which accompanies this text (see below) describes six levels of functional classification.
• 2-level road hierarchy
• Example 1

Buchanan’s “Traffic in towns” (ref.602 para.108) says: “Basically, however, there are only two kinds of roads – distributors designed for movement, and access roads to serve the buildings” Example 2 Writing in 1989 (ref.358.7), Australia’s Ray Brindle said that: “Because the movement and access functions are seen as being continuous, the road classes are not clearly defined but merge from one to the other” He refers to four grounds for questioning the classical model of road hierarchy, including: “It creates a broad range of roads intermediate between major traffic routes and minor streets on which there is a conflict of access and movement functions, leading to lower than desirable levels of safety and amenity”

1. Brindle argues in favour of a ‘separate functions’ or ‘two categories’ road hierarchy, where a road has either an access function or a traffic function (see figure).
3. Many levels road hierarchy
4. Example 1

In 1996, A. Talvitie wrote that “For several reasons, a state or country may wish to create more than the three functional classes.” (ref.370). He also said that “. specialized functional classes may be desirable in some states or countries because of unique functions served.

• Arterials: Motorways and other divided arterials;
• Principal arterials
• Minor arterials
• Collectors:
• Major collectors
• Minor collectors
• Locals:
• Public local roads
• Private local roads
• Other: Bicycle and pedestrian paths

Example 2 Lancashire County Council (UK)’s 2002 document on a “Functional road hierarchy strategy” (ref.306) has a road hierarchy which lists 13 categories (~ classes). Comment

Roads exist for many more reasons than just access and through traffic.

None of the examples quantify key terms such as “through traffic” or “access”.

The topics which RH focus on appear to be, essentially, access and speed.

The suggested, continuous curves are misleading. In the world of real roads, there is not an infinitely large number of possible values for speed, but only around 14. The reason is that posted speed limits are generally restricted to multiples of 10 km/hr,

Adopting Ray Brindle’s two-functions curve I would argue that at speeds from about70 km/hr and above, the “access” element is zero. Below 70 km/zhr the speed/access curve should be a stepped curve, not a continuous curve, with the steps at intervals of 10 km/hr.

The distance element of access (how many junctions or access points per km) should be based on the concept of road network mesh.

• There are so many requests for exception from the FC concept for special case roads that the FC concept itself begins to seem inappropriate. Examples of these special case roads include:
  • Mr. Talvitie’s examples (quoted above)

  Streets (see for example ref.446)

Quiet lanes (e.g. ref.306)

‘Lifeline’ roads (ref.400)

Terminology – different authors writing at different decades use the same term but not necessarily with the same precise meaning. Imprecise use of terminology for a desirably precise technology such as highway engineering is a bit dodgy.

If there are road classes for motor vehicles, there should also be road classes for other vehicle types such as pedestrians, cyclists, horse-riders etc – and in fact, examples of such NMT (non-motorised transport) hierarchies exist.

It seems to me that an air of respectability surrounds a concept (RH / FC) which – although it might be useful – nevertheless uses vague terminology, is not well-quantified, ignores practicalities from the real world and is described differently by different writers.

• References
• 294 – “Road geometric design manual, 2011 edition”, Ministry of Works, Dar es Salaam 2012
• 306 – “Functional road hierarchy strategy”, Lancashire County Council, England; October 2002
• 358.7 – Brindle, Ray “Living with traffic: (7) road hierarchy and functional classification” (1989) re-published Australia, ARRB special report 53, 1995

370 – Talvitie, A. “Functional classification of roads”, paper presented at the annual meeting of the TRB, Washington DC, USA; January 1996.

1. 400 – World Bank report, “Improving the Management of Secondary and Tertiary Roads in the South East Europe Countries”, Transport Unit, Sustainable Development Department; February 2008
2. 446 – Ribeiro, Paulo “A new perspective on street classification towards sustainability”, Proceedings of the 8th WSEAS conference; 2012
3. 589 – East Africa, Preparation of the East Africa Transport Facilitation Strategy, thematic area 1 – standards and specifications, “chapter 2: harmonisation of roadway geometric standards”
4. 602 – Buchanan, Colin “Traffic in towns”, London; HMSO 1963
5. 617 – “Towards safer roads in developing countries”, England, TRRL/ODA; 1991

: How many levels in a road hierarchy?

What are 4 way roads called?

Freeways: – Freeways are also called as access-controlled highways. Freeways are wide roads designed for fast-moving vehicles to travel long distances with higher speeds. These are generally designed in four lanes, two lanes in each direction. Traffic movement on freeways is continuous and unhindered because there are no railway or road intersections and no signals. You can see from the above image that there is no obstruction and free flow of traffic in each direction. Parking and Walking are strictly prohibited on freeways and they don’t have footpaths on either side of roads. The minimum speed limit and maximum speed limit varies from the country by country and it ranges between 45mph to 75mph.

What are asphalt layers called?

What is Asphalt – EAPA Asphalt is a mixture of aggregates, binder and filler, used for constructing and maintaining roads, parking areas, railway tracks, ports, airport runways, bicycle lanes, sidewalks and also play- and sport areas. Aggregates used for asphalt mixtures could be crushed rock, sand, gravel or slags.

1. Nowadays, certain waste and by-products, such as construction and demolition debris, are being used as aggregates, which increases the sustainability of asphalt.
2. In order to bind the aggregates into a cohesive mixture a binder is used.
3. Most commonly, bitumen is used as a binder, although nowadays, a series of bio-based binders are also under development with the aim of minimising the environmental impact of the roads.

An average asphalt pavement consists of the road structure above the formation level which includes unbound and bituminous-bound materials. This gives the pavement the ability to distribute the loads of the traffic before it arrives at the formation level. Asphalt is produced in an asphalt plant. This can be a fixed plant or even in a mobile mixing plant. It is possible to produce in an asphalt plant up to 800 tons per hour. The average production temperature of hot mix asphalt is between 150 and 180°C, but nowadays new techniques are available to produce asphalt at lower temperatures. (See below). To be able to provide the best performance to different applications, a large variety of asphalt mixes can be used. Due to the different requirements (amount of traffic, amount of heavy vehicles, temperature, weather conditions, noise reduction requirements, etc.) the respective mix used needs to have an sufficient stiffness and resistance to deformation in order to cope with the applied pressure from vehicle wheels on the one hand, yet on the other hand, they need to have an adequate flexural strength to resist cracking caused by the varying pressures exerted on them.

1. Moreover, good workability during application is essential in order to ensure that they can be fully compacted to achieve optimum durability.
2. Hot Mix Asphalt (HMA) Hot asphalt mixes are generally produced at a temperature between 150 and 180 °C.
3. Depending on the usage, a different asphalt mixture can be used.

For more details of the different asphalt mixtures, go to Warm Mix Asphalt (WMA) A typical WMA is produced at a temperature around 20 – 40 °C lower than an equivalent Hot Mix Asphalt. Significantly less energy is involved and, consequently, less fumes are produced (as rule of thumb, a reduction of 25ºC produces a reduction of 75% of fumes emission).

1. In addition, during the paving operations, the temperature of the material is lower, resulting in improved working conditions for the crew and an earlier opening of the road.
2. Cold Mix Asphalt Cold mixes are produced without heating the aggregate.
3. This is only possible, due to the use of bitumen emulsified in water, which breaks either during compaction or during mixing.

Producing the coating of the aggregate. Over the curing time, water evaporates and strength increases. Cold mixes are particularly recommendable for lightly trafficked roads. An asphalt pavement consists of different asphalt layers. In general the asphalt layers are paved on a bound or unbound road base layer. Starting at the road surface, the first layer is called the surface course. The second layer is mostly called the binder course.

• The lower layers are the base courses.
• Surface course The surface course constitutes the top layer of the pavement and should be able to withstand high traffic- and environmentally-induced stresses without exhibiting unsatisfactory cracking and rutting.
• Its main mission is to provide an even profile for the comfort of the user, while providing enough texture to ensure minimum and safe skid resistance.

Depending on local conditions, functional characteristics such as skid resistance, noise reduction and durability are often required for wearing courses. In some cases, rapid drainage of surface water is desired while in other cases, the wearing course should be impermeable in order to keep water out of the pavement structure.

Asphalt Concrete (AC) Béton Bitumineux Mince (Thin Layer Asphalt Concrete – AC-TL) Asphalt Concrete Very Thin Layers (AC-VTL) Asphalt Concrete Ultra Thin Layer (AC-UTL) Stone Mastic Asphalt (SMA) Hot Rolled Asphalt (HRA) Porous Asphalt (PA) Double layered Porous Asphalt (2L PA) Mastic Asphalt (MA) Soft Asphalt (SA)

Double layered porous asphalt Binder course Binder courses are designed to withstand the highest shear stresses that occur about 50 – 70 mm below the asphalt surface. The binder course is therefore placed between the surface course and base course to reduce rutting by combining qualities of stability and durability.

• Stability can be achieved by sufficient stone-on-stone contact and stiff and/or modified binders.
• Base course The base course is perhaps the most important structural layer of the pavement, which is intended to effectively distribute traffic and environmental loading in such a way that underlying unbound layers are not exposed to excessive stresses and strains.

This often implies comparatively high stiffness of the base course. Next to this the base course should also show adequate fatigue resistance. Unbound materials and foundation Since the formation and sub-soil often constitute relatively weak materials, it is of utmost importance that the damaging loadings are effectively eliminated by the layers above.

asphalt is the predominant material used for road construction and maintaining the road network in Europe?25% of the total world asphalt production takes place in Europe?the average amount of bitumen used in asphalt is 5% by weight?

Europe has over 4.000 asphalt production sides and over 9.500? companies are involved in the asphalt production and /or laying? some people still talk about tarmac of tarring a road. Since the 1990’s tar is not used anymore in road building. tar is totally different from bitumen.

They are both black, but tar comes from coal and bitumen from heavy crude oil. Europe has over 2.500 asphalt production sides and over 9.500 companies are involved in the asphalt production and / or laying? some people still talk about tarmac of tarring a road. Since the 1990’s tar is not used anymore in road building.

tar is totally different from bitumen. They are both black, but tar comes from coal and bitumen from heavy crude oil. : What is Asphalt – EAPA

What are the main layers of asphalt road?

Introduction – An asphalt pavement is made up of multiple layers, namely subgrade, sub-base, base, surfacing and wearing course. While there are design considerations involved in a pavement from the geometric, functional and drainage aspects, the structural design indicates estimation of appropriate thicknesses of the pavement layers.

1. Different kinds of materials are used to construct individual layers of a pavement structure.
2. These materials show complex response when subjected to load, temperature and moisture variations.
3. The engineering properties of a pavement material depends on the relative composition of its constituents.
4. The composition of a pavement material is finalized through appropriate mix design, so that the desirable levels of various engineering properties of the material are achieved.

However, sometimes a pair of desirable properties may show conflicting trends with the variation of a constituent of the mix. For example, with the increase of the asphalt binder content in the asphalt mix, the fatigue performance may improve, but the stiffness modulus value may start decreasing.

• From pavement design stand-point, both (i) good fatigue performance, as well as (ii) high stiffness modulus of the asphalt mix, are desirable.
• Thus, appropriate (i) choice of pavement material for the individual pavement layers, (ii) mix design for each of these materials and (iii) estimation of engineering properties (which are used as design input), are integral parts of the process of pavement design,

Environmental parameters used in pavement design primarily include variation of temperature and moisture conditions (including freezing and thawing situations) during the service period of the pavement. In the design guidelines such variations are accounted either (i) considering expected/equilibrium value of the temperature and the moisture content during the design period or, (ii) by dividing the total design period into certain time intervals and considering the effects of temperature and moisture content (in terms of incremental damage) for these time intervals,

• For design purpose, it is generally assumed that the temperature affects the stiffness of the asphalt layer and the moisture content affects the stiffness of the unbound granular layer and the subgrade.
• Further, it may be noted that there always exists a temperature profile and moisture content profile (that is, variation along the depth of the pavement).

For design purpose, a representative values of temperature and moisture content are chosen which carry the equivalent effects of such variations along the depth, Traffic parameters involve considerations of traffic volume, axle load spectrum, tyre contact pressure, wheel and axle configuration, lateral wander of wheel path, traffic growth rate etc.

Guidelines suggest various empirical factors or, computational approaches to take into account of these parameters so as to convert the estimated cumulative traffic volume (within the design period) to equivalent number of standard axle load repetitions, Sometimes, the damage caused by individual axle load groups are estimated separately.

The next section discusses the basic principles of asphalt pavement design and how different guidelines employ these principles to evolve respective design recommendations.