Yes, there exists a 50 lane highway in China and it merges to 4 lanes!!!
It is a 50-lane parking lot on the G4 Beijing-Hong Kong-Macau Expressway, one of the country’s busiest roads.
An aerial view from Google maps shows that the G4 Expressway is typically a 4-lane highway. The road expands to the width of approximately 50 cars when it approaches the Zhuozhou Toll Gate, but before and after this toll checkpoint it is only a 4-lane road.
Here’s an aerial view of the toll we stitched together from Google Maps. Note how both the the northbound and southbound portions of this highway are merely 4-lane roads after they leave the toll area:
Highways are classified according to their functions in terms of the service they provide. The classification system facilitates a systematic development of highways and the logical assignment of highway responsibilities among different jurisdictions. Highways and streets are categorized as rural or urban roads, depending on the area in which they are located. This initial classification is necessary because urban and rural areas have significantly different characteristics with respect to the type of land use and population density, which in turn influences travel patterns. Within the classification of urban and rural, highways are categorized into the following groups:
• Principal arterials
• Minor arterials
• Major collectors
• Minor collectors
• Local roads and streets
Freeways are not listed as a separate functional class since they are generally classified as part of the principal arterial system. However, they have unique geometric criteria that require special design consideration.
Functional System of Urban Roads
Urban roads comprise highway facilities within urban areas as designated by responsible state and local officials to include communities with a population of at least 5000 people. Some states use other values, for example, the Virginia Department of Transportation uses a population of 3500 to define an urban area. Urban areas are further subdivided into urbanized areas with populations of 50,000 or more and small urban areas with populations between 5000 and 50,000. Urban roads are functionally classified into principal arterials, minor arterials, collectors, and local roads.
A schematic of urban functional classification is illustrated in Figure 1 for a suburban environment.
Urban Principal Arterial System
This system of highways serves the major activity centers of the urban area and consists mainly of the highest-traffic-volume corridors.
It carries a high proportion of the total vehicle-miles of travel within the urban areaincluding most trips with an origin or destination within the urban area. The system also serves trips that bypass the central business districts (CBDs) of urbanized areas.
Fig 1 : Schematic Illustration of the Functional Classes for a Suburban Road Network
All controlled-access facilities are within this system, although controlled access is not necessarily a condition for a highway to be classified as an urban principal arterial.
Highways within this system are further divided into three subclasses based mainly on the type of access to the facility: (1) interstate, with fully-controlled access and gradeseparated interchanges; (2) expressways, which have controlled access but may also include at-grade intersections; and (3) other principal arterials (with partial or no controlled access).
Urban Minor Arterial System
Streets and highways that interconnect with and augment the urban primary arterials are classified as urban minor arterials. This system serves trips of moderate length and places more emphasis on land access than the primary arterial system. All arterials not classified as primary are included in this class.
Although highways within this system may serve as local bus routes and may connect communities within the urban areas, they do not normally go through identifiable neighborhoods. The spacing of minor arterial streets in fully developed areas is usually not less than 1 mile, but the spacing can be 2 to 3 miles in suburban fringes.
Urban Collector Street System
The main purpose of streets within this system is to collect traffic from local streets in residential areas or in CBDs and convey it to the arterial system. Thus, collector streets usually go through residential areas and facilitate traffic circulation within residential, commercial, and industrial areas.
Urban Local Street System
This system consists of all other streets within the urban area that are not included in the three systems described earlier. The primary purposes of these streets are to provide access to abutting land and to the collector streets. Through traffic is discouraged on these streets.
Functional System of Rural Roads
Highway facilities outside urban areas comprise the rural road system. These highways are categorized as principal arterials, minor arterials, major collectors, minor collectors, and locals. Figure 2 is a schematic illustration of a functionally classified rural highway network.
Fig 2 : Schematic Illustration of a Functionally Classified Rural Highway Network
Rural Principal Arterial System
This system consists of a network of highways that serves most of the interstate trips and a substantial amount of intrastate trips. Virtually all highway trips between urbanized areas and a high percentage of trips between small urban areas with populations of 25,000 or more are made on this system.
The system is further divided into freeways (which are divided highways with fully controlled access and no at-grade intersections) and other principal arterials not classified as freeways.
Rural Minor Arterial System
This system of roads augments the principal arterial system in the formation of a network of roads that connects cities, large towns, and other traffic generators, such as large resorts. Travel speeds on these roads are relatively high with minimum interference to through movement.
Rural Collector System
Highways within this system carry traffic primarily within individual counties, and trip distances are usually shorter than those on the arterial roads. This system of roads is subdivided into major collector roads and minor collector roads.
Rural Major Collector System
Routes under this system carry traffic primarily to and from county seats and large cities that are not directly served by the arterial system. The system also carries the main intracounty traffic.
Rural Minor Collector System
This system consists of routes that collect traffic from local roads and convey it to other facilities. One important function of minor collector roads is that they provide linkage between rural hinterland and locally important traffic generators such as small communities.
Rural Local Road System
This system consists of all roads within the rural area not classified within the other systems. These roads serve trips of relatively short distances and connect adjacent lands with the collector roads.
Every pavement, other than those on bridges, self-evidently includes soil. The most basic design requirement of any pavement is that the underlying soil is adequately protected from applied loads. Thus, no pavement engineer can avoid the need to understand soil. The following list features some key facts.
Soils vary from heavy clays, through silts and sands to high-strength rocky materials.
Soils are not usually consistent along the length of a road or across any pavement site.
Soils are sensitive to water content to differing degrees.
Water content will vary during the life of a pavement, sometimes over quite short timescales, in response to weather patterns.
Some soils are highly permeable; some clays are virtually impermeable.
All this leads to one thing – uncertainty. However clever one tries to be in understanding and characterising soils, it is quite impossible to be 100% sure of the properties at a given time or in a given location.
This uncertainty makes life considerably harder. Nevertheless, it is necessary to categorise each soil type encountered in as realistic a way as possible, and there are two fundamental areas in which soil behaviour affects pavement performance. These are :
stiffness under transient (i.e. moving wheel) load
resistance to accumulation of deformation under repeated load, likely to be related to shear strength.
Granular material
Granular material is unbound material with relatively large particle sizes, and includes natural gravel, crushed rock and granulated industrial by-products such as slag from steel production. Soils are technically granular materials, albeit often with a very small particle size (2 mm or less for clay), but the key difference is that a soil is not, in general, ‘engineered’ in any way.
A granular pavement layer, on the other hand, will be selected and quite possibly deliberately blended to give a particular combination of particle sizes. It can also be mixed with a predetermined amount of water. One would
therefore naturally expect that much of the uncertainty inherent in soil properties is removed in the case of a granular material.
However, it may still be difficult to predict performance accurately, as different material sources, most commonly different rock types, might be expected to exhibit slightly different properties due to their different
responses to crushing or their differing frictional properties.
Nevertheless, a granular material will be a much more controlled and predictable component than the soil.
Even the water-content variation of a granular material will be a little more predictable, in both magnitude and effect, than in the case of soil.
However, the properties of granular material of interest to the pavement engineer are actually more or less the same as those of soil, namely
stiffness under transient load
resistance to accumulation of deformation under repeated load, related to shear strength.
Hydraulically-bound material
Nowadays, the availability of Portland cement, and substitutes such as fly ash or ground granulated blast-furnace slag, means that it can be economical to use such a binding agent to strengthen a granular material. These binders are known as ‘hydraulic’ binders, as they require the presence of water for the cementing action to take place.
Cement technology is a vast subject in its own right and involves several different chemical reactions, the most important of which are the conversion of tricalcium silicate (c. 50% of Portland cement) and dicalcium silicate (c. 25%) into hydrates (forming strong solids) by reaction with water, also generating calcium hydroxide and heat. The
first reaction is rapid; the second is slower. The reader should refer to specialist literature for details.
Hydraulically-bound materials, including so-called pavement-quality concrete (hereafter referred to as PQC) at the upper end of the strength spectrum, introduce a quite different type of behaviour and totally different design requirements. They possess a key property that is lacking in soils and granular materials, namely the ability to withstand tension.
Individual particles are rigidly bonded together by the binding agent, and a definite tensile force is required to break that bond. In the case of a strong concrete, all the large particles are well bonded into a continuous matrix of fine aggregate and cement paste, and the whole material is solid and rigid. It has a stiffness that is still partly
governed by the contacts between the large particles, but which is also heavily dependent on the qualities of the surrounding cementitious matrix.
In the case of a weaker hydraulically-bound material, the binding effect may be less complete and there may be many particle contacts that remain unbound, giving a certain freedom of movement
within the material and a reduced stiffness and strength.
Nevertheless, even a weak hydraulically-bound material will remain as a solid, with negligible permanent deformation
until the bonds are fractured, that is until the tensile strength is overcome. The key properties for the pavement engineer are, therefore
stiffness
tensile strength.
One further property that could arguably be added is fatigue resistance, that is, the resistance of the material to failure under repeated load at a stress level less than its failure strength. However, the relationship with tensile strength is so close that it is hardly a separate property.
It would also have been possible to add curing rate (the rate of strength gain), as this certainly affects the construction process and economics significantly, and thermal expansion coefficient, as this property strongly influences the tendency of a hydraulically-bound material to crack under day–night temperature variation, requiring the introduction of movement joints in concrete pavements
Bitumen-bound material
This is a material almost unique to pavement engineering, amaterial whose beneficial properties were discovered almost by accident, but a material which is now very much at the centre of pavement technology. There are countless stories as to when bituminous products were first used on roads, such as the accidental spillage of tar outside Derby iron works in 1901.
Although mastics including natural asphalt had been used on footways since the 1830s, they were not stable enough for roads, and it was not until around 1900–1901 that the first usages of tar-bound stone occurred at approximately similar dates in the USA and Europe. Lay (1990) gives further information.
While proportions differ around the world, typically some 90% of paved highways have a bitumen-bound surface layer; whatever the make-up beneath the surface, bitumen and bitumen-bound materials (referred to hereafter as asphalts) currently play a major role.
And asphalt is quite different from concrete or any hydraulically-bound material. Bitumen is a binding agent, like Portland cement and the other hydraulic binders, but it has very different properties. Whereas hydraulic binders create a rigid material that cannot deform appreciably unless it first cracks, bitumen remains a viscous liquid at
normal in-service temperatures. It therefore has the ability to ‘flow’.
An ability to flow may seem a rather undesirable quality in a material that is aiming to bind rock particles together, and it does indeed lead to the possibility that an asphalt can deform – hence the phenomenon known as ‘rutting’ or ‘tracking’. However, it also overcomes some of the difficulties encountered with rigid hydraulically-bound materials.
For a start, the expansion and contraction with day–night temperature variation is accommodated simply by a small viscous strain within an asphalt, meaning that no movement joints are required, and that thermally-induced cracking will only occur under the most extreme temperature conditions (continental winters, deserts).
Asphalts are also able to accommodate any moderate movement within the foundation, for example, minor differential settlement in an embankment, movement that might lead to the fracture of a rigid concrete slab. Furthermore, the tendency of asphalt to flow can be controlled by proper mixture design such that rutting is avoided.
However, despite the flexible nature of asphalt, it can still crack. It is impossible to define a tensile strength, as this will vary with temperature and rate of loading; the relevant parameter is the ‘fatigue characteristic’, defining resistance to cracking under repeated load. The key properties required for design are therefore:
stiffness
resistance to deformation under repeated load
fatigue characteristic.
Other materials
The fourmaterial types introduced so far represent the basic building blocks available to the pavement engineer. However, it is worth referring here to a couple of materials that do not fit so easily into any of the four categories.The first is block paving. Blocks are often made of concrete and so could have been introduced under ‘hydraulically-bound materials’.
On the other hand, they can be cut fromnatural stone or may comprise fired clay bricks.Moreover, the discontinuous nature of block paving means that the properties of the parent material are less important than the effects of the discontinuities. The blocks themselves may have the properties of concrete, for example, a stiffness modulus of some 40 GPa, but the effective layer modulus once the discontinuities are taken into account may be as little as 500MPa.
The second special case is a hybrid material, known in the UK as grouted macadam and in the USA as resin-modified pavement; it is also sometimes known as ‘semi-flexible’ material. This too does not fit neatly into any of the previous categories, as it combines an asphalt skeleton with a cementitious grout, filling the voids in the asphalt mixture.
It therefore utilises both bituminous and hydraulic binders. Having a two-stage production process the material tends to be expensive, and is used in particular heavyduty applications such as bus lanes and industrial pavements. As will be demonstrated later, it actually resembles an asphalt much more than a concrete, but it is nevertheless
distinct.
Block paving and grouted macadam are bulk-use materials at the expensive end of the range.
There are also specialist products that are only used in small quantities to strengthen, or in some way improve, a pavement layer.Here one could include steel reinforcement of concrete.
There are also reinforcing products designed for asphalt; some are steel, others polymeric or made of glass fibre.Asimilar range of products is available for the reinforcement of granular materials. Generically, these products are known as geogrids and their use is widespread in some areas, for example, as a means of stabilising roads over very soft ground.
Geotextiles comprise a closely related range of products, produced in various ways and forming continuous layers separating two different pavement materials (commonly the soil and a granular layer). These too can have a reinforcing function, but their most common use is simply as a separator, ensuring that fine soil particles do not migrate up into the pavement and that stones from a granular layer do not lose themselves in the soil.
The entire spectrum of geogrids and geotextiles is known under the collective name of geosynthetics and, although geosynthetics are specialist products, it is the responsibility of the pavement engineer to understand how (and whether) they work in particular applications rather than relying solely on the, sometimes not unbiased, opinion of a supplier.
It is impossible to know where or when the wheel was invented. It is hard to imagine that Stone Age humans failed to notice that circular objects such as sections of tree trunk rolled.
The great megalithic tombs of the third millennium BC bear witness to ancient humans’ ability to move massive stones, and most commentators assume that tree trunks were used as rollers; not quite a wheel but a similar principle!
However, it is known for certain that the domestication of the horse in southern Russia or the Ukraine in about 4000 BC was followed not long afterwards by the development of the cart.
It is also known that the great cities of Egypt and Iraq had, by the late third millennium BC, reached a stage where pavements were needed. Stone slabs on a rubble base made an excellent and long-lasting pavement surface suitable for both pedestrian usage and also traffic from donkeys, camels, horses, carts and, by the late second millennium BC, chariots.
Numerous examples survive from Roman times of such slabbed pavements, often showing the wear of tens of thousands of iron-rimmed wheels. Traffic levels could be such that the pavement had a finite life.
Even in such ancient times, engineers had the option to use more than simply stones if they so chose – but only if they could justify the cost! Concrete technology made significant strides during the centuries of Roman rule and was an important element in the structural engineer’s thinking.
Similarly, bitumen had been used for thousands of years in Iraq as asphalt mortar in building construction. Yet neither concrete nor asphalt was used by pavement engineers in ancient times, for the excellent reason that neither material came into the cheap, high-volume category. As far as the pavement engineer was concerned, economics dictated that the industry had to remain firmly in the Stone Age.
Even in the days of Thomas Telford and John Loudon Macadam – the fathers of modern road building in the UK – the art of pavement construction consisted purely of optimising stone placement and the size fractions used.
Times havemoved on; themassive exploitation of oil has meant that bitumen, a by-product from refining heavy crude oil, is now much more widely available. Cement technology has progressed to the stage where it is sufficiently cheaply available to be considered in pavement construction. However, there is no way that pavement engineers can contemplate using some of the twenty-first century’s more expensive materials – or, at least, they can be used only in very small amounts. Steel can only be afforded as reinforcement in concrete and, even in suchmodest quantities, it represents a significant proportion of the overall cost.
Plastics find a use in certain types of reinforcement product; polymers can be used to enhance bitumen properties; but always the driving force is cost, which means that, whether we like it or not, Stone Age materials still predominate.
Roads are exposed to tremendous loads that will sooner or later leave their marks on them. A time will come when every road will be in need of a general overhaul. But no two damage patterns are alike.
Which rehabilitation methods offer a cure for distressed roads? What are the differences between them? Which are suitable to be carried out as mobile roadworks?
Replacing the pavement
Replacing the pavement is a standard procedure when repairing roads. The challenge is to ensure that only the damaged layers of the road structure are removed – and to avoid disruptions to traffic at the same time. Under these conditions, cold milling is the only viable option for many construction projects
The tools that cold milling machines use for removing road layers were originally developed for the mining industry. So-called point-attack cutting tools, fitted to a rotating milling drum on the underside of the machine, bite into the road at precisely the specified depth.
Fine milling
Fine milling is an alternative to time-consuming and expensive complete rehabilitation. This method is used above all when traffic safety is severely compromized by undulations, ruts or a slippery surface.
A WIRTGEN cold milling machine roughening the runway at Cambridge Airport, UK.
Many countries are investing less money in maintaining their road network despite increasing traffic loads. The result is a growing demand for fast and economically efficient solutions that are capable of taking the edge off hazardous stretches of road.
Fine milling is such a method, and is predominantly used where bumps and wheel ruts, or slippery surfaces pose an acute danger to traffic safety.
Paving thin layers hot
It is not necessary to replace the entire pavement if only the surface of the roadway shows signs of damage. The method known as “paving thin overlay hot on hot” is a particularly economical and eco-friendly alternative.
Perfect paving of resource-saving low-noise thin layers.
Rehabilitation methods that can be carried out both quickly and with economic efficiency are becoming increasingly important worldwide. Paving thin layers in hot application is such a method, offering an exceptionally economical alternative to full pavement rehabilitation.
Paving thin layers in hot application is eminently suitable for roads in need of rehabilitation but with damages limited to the surface, or poor grip, or pronounced surface irregularities. The surface properties of worn-out roads, such as grip, evenness and noise reduction, can be improved significantly for extended periods of time.
Paving thin layers cold
“Thin overlays paved cold on cold” – better known abroad as “micro-surfacing” – are another quick and cost-effective alternative to replacement of the complete pavement.
Mixing and laying machines spread the mixture for the thin cold layer over the fine-milled surface with Vario screeds.
Traffic safety is jeopardized when the road surface gets slippery, when wheel ruts have dug into the pavement, or when the road is covered with bumps and deformations. Paving thin layers in cold application is a method that is increasingly used for restoring road surfaces to good evenness and skid resistance. Known as “microsurfacing” in many countries around the globe, paving thin layers in cold application prolongs the service life of damaged asphalt roads without the need for replacing the entire road pavement.
InLine Pave
InLine Pave has become established as an official construction method according to German regulatory frameworks. The binder and surface courses are paved “in line”, i.e. one after another “hot on hot” in a single pass. Since the machines are just 3 m wide, traffic can continue to flow without obstruction on the remaining lanes.
Hot recycling
Porous and deformed surface courses can be rehabilitated by hot recycling, a process which is exclusively applied in the form of a mobile construction site. An intact layer structure is essential here. Hot recycling improves all relevant properties of the surface and roadway profile, as well as the composition of aggregate fractions in the surface course.
The panel heating machine travels ahead, gently heating the bituminous courses.
The surface course is heated up to 150 °C by a panel heating machine with gas-fired infrared heating panels so that the hot recycler can then scarify, remove, process and repave the softened asphalt. In this way, the road’s non-skid properties can be restored, water can run off again and ruts are eliminated. The potential savings are enormous.
Cold recycling in-situ
Cold recycling in-situ is just the right technology when it comes to building a new road from old. It is mostly carried out as mobile roadworks. What are the advantages of “in-situ” cold recycling? How does the method work?
The cold recyclers stand out thanks to their ease of operation with new automatic functions, main control panels that can be positioned as required, and an on-board diagnostic system.
The sub-base of roads exposed to major stresses due to heavy goods traffic is often damaged. To remedy this damage, the complete road structure must be repaired. Cold recycling restores the roads’ stability.
The difference between “in plant” and “in situ” cold recycling is that, in the latter case, the entire process takes place in a single operation.
This is what happens: special cold recyclers granulate the defective layers – usually the surface and binder course, as well as part of the base course – mix the reclaimed asphalt pavement with fresh binder and replace it immediately. The advantages of this method are short construction times and high cost efficiency.
Cold recycling in-plant
When cold recycling road pavements, contractors can choose between processing the milled material “in-situ”, meaning on the job site, or “in-plant”, meaning in a cold mixing plant. Their decision is influenced, however, not only by the damage patterns of the road to be repaired. What are the advantages offered by “in-plant” cold recycling? How does it work? What kinds of damage patterns can cold recycling “in-plant” be used for?
A typical application: the KMA 220 is positioned in the direct vicinity of the material storage area.
One speaks of cold recycling “in-plant” when the reclaimed asphalt material of roads in need of rehabilitation is recycled in a nearby mixing plant, transported back to the job site, and then placed again by road pavers. The method is often used with roads that are exposed to high loads by heavy traffic, and with damages extending all the way into the pavement subgrade, but where site conditions do not allow the operation of an “in-situ” cold recycling train.
A paver (paver finisher, asphalt finisher, paving machine) is a piece of construction equipment used to lay asphalt on roads, bridges, parking lots and other such places. It lays the asphalt flat and provides minor compaction before it is compacted by a roller.
All machines consist of two basic units; the tractor unit and the screed unit.
1. Tractor Unit
1.1 General:
The tractor unit provides moving power for the paver wheels or tracks and for all powered machinery on the paver. The tractor unit includes the receiving hopper, feed conveyor, flow control gates, distributing augers (or spreading screws), power plant (engine), transmissions, dual controls, operator’s seat, and wheels or tracks.
When in operation, the tractor unit power plant (engine) propels the paver, pulls the screed (leveling) unit, and provides power to the other components through transmissions. Hot mix is deposited in the hopper from where it is carried by the feed conveyor through the flow control gates to the distributing augers (spreading screws).
The augers distribute the mix evenly across the full width of the paver for uniform placement onto the roadway surface. These operations are controlled by the paver operator by means of dual controls within easy reach of the operator’s seat. Refer to Figure 1. Many pavers have hydrostatic drive systems that permit an unlimited number of speeds within the operating range and once set, will automatically maintain the desired speed.
1.2 Pneumatic Tires and Crawler Tracks:
The tractor unit may be equipped with either rubber tires or steel tracks. If the paver is equipped with pneumatic tires, tire condition and air pressure must be checked. It is particularly important for the pressure to be the same in tires on both sides of the paver.
If the paver moves on tracks (crawlers), the tracks should be checked to be certain they are snug but not tight, and the drive sprockets should be checked for excessive wear. Low tire pressure or loose crawlers can cause unnecessary movement of the paver, which when transmitted to the screed unit results in an uneven pavement surface. There should be no build-up of material on tires or on tracks. Excessively worn parts should be replaced.
1.3 Governor :
The governor on the engine must be checked to be sure there is no periodic surge in the engine RPM. If it is not working properly, there can be a lag in power when the engine is loaded. Such a lag causes temporary failure of the vibrators or tamping bars in the screed unit, resulting in a stretch of pavement that is less dense or contains slightly less material than the immediately adjacent area. After rolling, such an area shows up as a transverse ripple in the pavement. A power lag can also interfere with the smooth and consistent operation of electronic screed controls.
1.4 Hopper, Flow Gates and Augers :
The hopper, the slats on the feed conveyor, the glow gates, and the augers should be checked for excessive wear and observed to be certain they are operating properly. Necessary adjustments should be made by the contractor to ensure these components are functioning as designed and are able to deliver a smooth flow of mixture from the hopper to the roadway.
All of the machines have adjustable flow control gates, which regulate the amount of the paving mixture, which is carried on the slat feeders from the receiving hopper to the distributing augers. The slat feeders should be operating most of the time (80% to 90%) and the flow control gates should be set to keep the augers at least two-thirds covered with material. It is very important that the level of the material in front of the screed be kept fairly constant.
If the level of material is allowed to intermittently rise and fall and thereby flood or starve the screed, a rough mat, segregation of the material, and imperfections in the surface will result. Some pavers are equipped with automatic controls to maintain a uniform depth of material ahead of the screed; the adjustment of the sensing device and the flow gates should be coordinated so the slat feeders operate most of the time as stated above.
Quarter-line cracking or raveling of the mat is believed to be caused by worn interior augers, paddles, and baffles that tend to starve the screed near the center of the paver. Such equipment should be regularly inspected, and if excessively worn, should be replaced or rebuilt to original dimensions.
Feeders, gates, and augers should be checked for excessive wear. They should be observed while operating without mixture to be assured they are functioning properly and in synchronism with each other.
2. Screed Unit
2.1 General:
The screed unit strikes off the mix, controls thickness, imparts smoothness, and provides initial compaction of the mixture. A typical screed unit is comprised of the following: screed tow arms, screed plate, hearing unit, tamping bars or vibratory attachments, and controls.
The screed unit is towed by long arms attached to pivot points located forward on the tractor unit, permitting the screed to operate on a floating principle which tends to compensate or dampen irregularities in the base that affect the tractor unit. Mat thickness is adjusted manually by tilting the screed up or down around a pivot pin just above the screed.
When operating an automatic grade control the screed compensates for irregularities in the base by adjusting the screed arm at or near the pivot point of the screed arms. As the manual thickness controls are adjusted, the screed seeks a new level, up or down, as the machine moves forward, but the total effect of the change may not be realized until the machine has moved several feet. Consequently, the machine should be allowed to move 50 feet before any further adjustments are made.
The sensitivity of the controls differs between makes of machines and consequently the maximum amount of adjustment that should be made at any time differs with the machine. The amount of change in thickness produced by any given adjustment on the controls depends on the mixture being placed, so it is impossible to state that a particular adjustment will change the mat thickness by a definite amount.
Refer to Figure 1. Screeds with tamping bars or vibratory mechanisms are designed to strike off and then compact the mixture slightly as it is placed. There are two purposes to this screed action. It achieves maximum leveling of the mat surface, and it ensures minimum distortion of the mat surface, and it ensures that minimum distortion of the mat surface will occur with subsequent rolling. Because the different screed compaction systems function differently, they are discussed separately below.
2.2 Tamping Bar Type:
Tamping bar type screed compactors compact the mix, strike off the excess thickness, and tuck the material under the screed plate for leveling. The tamper bar has two faces; a beveled face on the front that compacts the material as the screed is pulled forward and a horizontal face that imparts some compaction. The horizontal face primarily strikes off excess material so the screed can ride smoothly over the mat being laid.
The adjustment that limits the range of downward travel of the tamping bar is the single most important adjustment affecting the appearance of the finished mat. At the bottom of its stroke, the horizontal face should extend 0.016″ – about the thickness of a fingernail – below the level of the screed plate. If the bar extends down too far, mix builds up on the screed face that tends to scuff the surface of the mix being placed.
In addition, the tamping bar will lift the screed lightly on each downward stroke, often causing a rippling of the mat surface. If the horizontal face of the tamping bar is adjusted too high (either by poor adjustment or due to wear of the bottom of the horizontal face), the bar does not strike off excess mix from the mat. Consequently, the screed plate begins to strike off the material, which results in surface pitting of the mix being placed as the leading edge of the screed plate drags the larger rocks forward.
Therefore, the tamper bar should always be checked before operating the paver. If necessary, the contractor should adjust it, and before it approaches knife-edge thinness it should be replaced. Refer to Figure 2. Clearance between the rear of the tamper bar and the leading edge of the screed frequently is overlooked.
Properly adjusted, there should be just enough clearance to allow free movement of the tampers – approximately 0.010″ to 0.020″. Refer to Figure 2. Screed plates will wear out first about 4″ to 6″ in from the trailing edge. The first indications will be either an indentation or an actual ripple in the surface of the screed plate. These should be checked periodically and replaced as needed.
2.3 Vibratory Type:
he operation of vibratory screeds is similar to that of tamping screeds, except that the compactive force is generated not by a tamping bar, but by either electric vibrators, rotating shafts with eccentric weights, or hydraulic motors that vibrate the screed plate.
On some pavers, both the frequency (number of vibrations per minute) and the amplitude (range of motion) of the vibrators can be adjusted. On others, the frequency remains constant and only the amplitude can be adjusted. Frequency and amplitude are set in accord with the type of pavers, speed of the paver, thickness of the mat, and the characteristics of the mixture. Once set, frequency and amplitude do not normally need adjustment until mat thickness or mixture change.
Some pavers have a “pre-strike-off unit” or a curved strike-off blade at the leading edge of the screed for use with “critical” material. It is attached to and receives vibration from the screed plate itself. It meters the amount of material going under the screed plate and can be adjusted vertically. Some other pavers have a round-nose strike-off bolted directly to the screed frames and welded to the screed plate so no adjustment can be made.
In these, the screed shield assembly is the only part of the screed that is adjustable. It can be moved up for normal material flow or down for dense mixes and thin lifts. As with tamping screeds, the screed plates will begin to wear first about 4″ to 6″ in from the trailing edge. Those with excessive wear should be replaced.
2.4 Oscillating Type:
Some pavers are equipped with both an oscillating screed and a vibrating compactor. The oscillating screed, beveled for initial compaction, operates at 600 strokes per minute, with 1/4″ transverse strokes, striking off the material for the desired depth. The vibrating compactor, mounted to the rear of the oscillating screed, supports the screed unit, imparts additional compaction, and irons out the surface of the mat.
The most critical adjustment is positioning the oscillating screed relative to the vibrating compactor. The bottom rear edges of the oscillating screed and the bottom rear edges of the vibrating compactor must be parallel. The compactor should be sloped or tilted so that a projection of the plane of the bottom will intersect at a point 0.4″ above the bottom of the screed. Minor adjustments in this differential may be necessary to obtain a uniform appearance in the mat. Refer to Figure 3.
2.5 Crown Adjustment :
All machines have provision for adjustment of crown in the screed. Usually it is desirable to provide a slight amount of crown in the screed to avoid the appearance of the mat being low in the center of the lanes.
The usual crown allowed per lane on rural-type pavements is 0.10″; however, urban cross sections may require special adjustment. Also, the amount of crown at the front edge of the screed is generally increased somewhat from that required at the back, with at least a 0.08″ increase usually being recommended.
The amount of this differential may be varied with the particular material being used, and is sometimes helpful in reducing and eliminating non-uniformity of the surface texture across the paved width. Too much crown in the leading edge of the screed will crease an open texture along the edges. Too little crown in the leading edge will create an open texture down the center of the mat.
2.6 Automatic Screed Controls:
The paver shall be equipped with an approved automatic control system, which controls longitudinal grade and transfer slope, except when paving miscellaneous areas or when the engineer finds the use of this system impractical.
The specifications require the use of such longitudinal control, unless the engineer permits its omission on the final surface course. Exceptional conditions could arise, however, where omission of the longitudinal grade control in the interest of a smoother ride can be permitted.
The engineer may discontinue use of the automatic equipment and require manual control when it appears better results may be obtained thereby. Such might be the case on work which includes sections with an urban-type cross section with non-uniform crown, or on intersections, interchanges, or similar areas.
Pavers not equipped with automatic controls may be used when the engineer determines the use of such controls is impracticable, as on small specialized projects, jobs entirely urban with variable crown, or jobs having frequent intersections or other features which are not adaptable to such use.
When the automatic grade control fails, the paver can be operated under manual control for only the remainder of the working day on which the automatic control system broke down. The automatic control systems use electronic sensors to control grade and to control transverse slope. Refer to Figure 3. The sensor gets its information from a sensing device riding on a fixed stringline, a mobile stringline, or a traveling straightedge.
The specific type of sensing device used on the initial lane of a layer or course is subject to the engineer’s approval. For paving subsequent lanes of the course or layer, the paver may use a shoe or straightedge riding on the adjacent lane as a sensor.
There are several grade sensor types and transverse slope sensor types. Some systems make screed adjustments by raising or lowering the pull arm pivot points with hydraulic rams activated through solenoids. Some make adjustments by varying the angle of hinged pull arms through electrically driven screws. Others adjust the thickness control screws with electric servomotors.
In operation, once the screed is set for the desired depth of spread, the automatic system takes over to produce a smooth mat. Transverse slope is controlled by a pendulum that acts through switches to activate the appropriate piston. The sensitivity of the controls is critical to the smoothness of the mat. The sensors should be properly nulled as provided in the manufacturer’s instructions.
2.7 Heaters :
The screed assembly is equipped with heaters to prevent the mix from sticking to the screed plate. They are used to heat the screed plate at the start of paving operations or on a cool, windy day. Heaters should never be used to heat mix being delivered to the paver. Heaters should be observed while lit to assure they produce sufficient heat.
2.8 Screed Extensions:
Extensions should be attached properly to the main section of the screed. They should be, as their name implies, an extension of the plane of the tamper bar and the screed section if uniform compaction behind the paver is to be attained.
Pavers with a vibratory main screed and vibratory side extensions should be checked by the inspector for satisfactory frequency. Use of a vibrating reed tachometer held against the main screed unit and each extension will give quick, reliable, and measurable results to be compared against manufacturer’s authentic data.
The accuracy of variable vibratory frequency control supplied on some models could also be checked with the tachometer.
2.9 Maintenance :
An important item of paver operation, often overlooked, is proper clean up of the paver at the end of the working day. While the machine is still warm from its day’s operations, the hopper, feeder augers, tamper bars, and screed plates should all be cleaned and sprayed with a light oil to assure smooth start-up the next day of use.
Road safety audit is the formal examination of existing roads, future roads or various sorts of traffic projects by any independent group of trained expertise. They examine the deficiencies in road safety. There are various stages of road safety audits. Number of stages depends on the number of stages in a road project before completion.
Feasibility study phase
Preliminary design phase
Detailed design phase
Pre-opening phase
In service phase
Feasibility Study
In feasibility study phase, trained specialist study and evaluate the results of following questions;
What is the scope of this project?
How many choices of routes available?
What will be the impacts on the existing transportation system?
Which design should be selected as a design standard for that road?
How long this route could be continued?
Which location will be the best location of interchanges?
Number of lanes required for managing maximum average daily traffic?
Where to provide the route terminals?
What will be the effects on the environment?
Control access
Technical team works on these questions step by step and at the end gives the most feasible solution possible.
Preliminary Design Stage:
Preliminary design is the second stage of road safety audits. In this stage designs of roads are carried out. As preliminary phase, therefore designing is not carried out in a very detailed. Following are road designs that are done in this phase;
Alignment of horizontal and vertical curves
Width of land and shoulder
Layout of intersection
Provision of super elevation and side slopes with pavements
Provision of overtaking lanes
Provision of separate way for pedestrians and cyclists.
Safety arrangements during construction on site
Provision of sign boards.
Design of Link roads
Space management
Detailed Design Stage
After the completion of 2nd stage, designs are carried out in detail. Following are the road designs that completes at the end of this stage;
Signals
Sign boards
Line marking
Lighting
Intersection details
Delineation
Provision of shoulders
Management of traffic during construction
Design of road drainage system
Provisions of way for road user groups. For example, pedestrians, cyclists, vans, trucks etc.
Provision of slopes.
Provision of road side objects
Pre-Opening Stage
In this phase technical audit team drive through the completed project. During drive, they observe the provision of safety level, quality, sign boards, road material and all other aspects that they took under consideration during the preliminary survey and detailed design phase. They came on the site during different weather conditions like during day time, night, etc… after the completion of survey they wrote a report on it and deliver it to the main authority.
In-service:
After the pre-opening examination, road is opened for public and during that still technical team remains active and they observe the working of safety features during heavy traffic.
Flexible pavement differ from rigid pavement in terms of load distribution. In flexible pavements load distribution is primarily based on layered system. While, in case of rigid pavements most of the load carries by slab itself and slight load goes to the underlying strata.
Structural capacity of flexible pavement depends on the characteristics of every single layer. While, the structural capacity of rigid pavements is only dependent on the characteristics of concrete slab. This is so, because of low bearing soil capacity of underlying soil.
In flexible pavements, load intensity decreases with the increase in depth. Because of the spreading of loading in each single layer. While, in case of rigid pavement maximum intensity of load carries by concrete slab itself, because of the weak underlying layer.
In flexible pavement deflection basin is very deep, because of its dependency on the underlying layers. While in case of rigid pavement, deflection basin is shallow, this is because of independency of rigid pavement on the underlying layers.
Flexible pavement has very low modulus of elasticity (less strength). Modulus of elasticity of rigid pavement is very high, because of high strength concrete and more load bearing capacity of the pavement itself. Than compared to flexible pavements.
In flexible pavements, underlying layers play very important role. Therefore, more role are playing only underlying layers. In case of rigid pavements, slight function of underlying layers. Maximum role is playing by the top layer (that is slab) by itself. Therefore, minute part is taking by sub layers.
A hill road may be defined as the one which passes through a terrain with a cross slope of 25% or more. There may be sections along hill roads with the cross slope less than 25%, especially when the road follows a river route. Even then these sections are also referred to as hill roads. Hence, to establish a hill road overall terrain must be taken into account.
The hilly regions generally have extremes of climatic conditions, difficult and hazardous terrains, topography and vast high altitude areas. The region is sparsely populated and basic infrastructural facilities available in plain terrain are absent. Hence, a strong stable and feasible road must be present in hilly areas for overall development of other sectors as well.
IRC:SP:73-2015 and IRC:SP:84-2014 have merged the Mountainous and Steep Terrain having Cross Slope more than 25%.
2. Design Issues in Hill Roads
Design and Construction of Hill roads are more complex than in plain terrain due to factors summarized below:
Highly broken relief with vastly differing elevations and steep slopes, deep gorges etc. which increases road length.
The geological condition varies from place to place.
Variation in hydro-geological conditions.
Variation in the climatic condition such as the change in temperature due to altitude difference, pressure variation, precipitation increases at greater height etc.
High-speed runoff due to the presence of steep cross slopes.
Filling may overload the weak soil underneath which may trigger new slides.
Need of design of hairpin bends to attain heights.
Need to save Commercial and Residential establishments close to the road.
Need to save the ecology of the hills.
3. Special Consideration in Hill Road Design
a – Alignment of Hill Roads
The designer should attempt to choose a short, easy, economical and safe comforting route.
b – General considerations
When designing hill roads the route is located along valleys, hill sides and if required over mountain passes.
Due to complex topography, the length of the route is more.
In locating the alignment special consideration should be made in respect to the variations in:
Temperature
Rainfall
Atmospheric pressure and winds
Geological conditions
Resettlement and Rehabilitation considerations
Environment Considerations
c – Temperature
Air temperature in the hills is lower than in the valley. The temperature drop being approximately 0.5° per 100 m of rising.
On slopes facing south and southwest snow disappears rapidly and rain water evaporates quickly while on slopes facing north and northeast rain water or snow may remain for the longer time.
Unequal warming of slopes, sharp temperature variations and erosion by water are the causes of slope failure facing south and southwest.
d – Rainfall
Rainfall generally increases with increase in height from sea level.
The maximum rainfall is in the zone of intensive cloud formation at 1500-2500 m above sea level. Generally, the increase of rainfall for every 100 m of elevation averages 40 to 60 mm.
In summer very heavy storms/cloud burst may occur in the hills and about 15 to 25% of the annual rainfall may occur in a single rainfall. The effects of these types of rainfall are serious and should be considered in design.
e – Atmospheric pressure and winds
Atmospheric pressure decreases with increase in elevation.
At high altitudes, the wind velocities may reach up to 25-30 m/s and depth of frost penetration is also 1.5 to 2 m.
Intensive weathering of rocks because of sharp temperature variations.
f – Geological conditions
The inclination of folds may vary from horizontal to vertical stratification of rock. These folds often have faults. Limestone or sandstone folds may be interleaved with layers of clay which when wetted may cause fracturing along their surface. This may result in shear or slip fold.
The degree of stability of hill slopes depends on types of rock, degree of strata inclination or dip, occurrence of clay seams, the hardness of the rocks and presence of ground water.
When locating the route an engineer must study the details of geological conditions of that area and follow stable hill slopes where no ground water, landslides, and unstable folds occur.
g – Resettlement and Rehabilitation
Due to limited availability of flat areas and connectivity issues, most of the residential and commercial activity happens very close to the road leading to large scale R&R and becomes a challenge in alignment design.
h – Environment
Hills are ecologically sensitive areas relatively untouched by human activity. The alignment design must attempt to minimize tree cutting and large scale earth filling/cutting to minimize damage.
4. Route Selection
Hill road alignment may follow alignment at Valley bottom or on a ridge depending on the feasibility of the road. The first is called River route and the second is called Ridge route.
a – River route
Most frequent case of hill alignment as there is a great advantage of running a road at a gentle gradient.
Runs through lesser horizontal curvature.
Requirements for the construction of bridges over tributaries.
Construction of special retaining structures and protection walls on hill side for safe guarding the road against avalanches in high altitude areas.
Benefit of low construction cost and operation cost.
b – Ridge route
Characterized by the very steep gradient.
Large number of sharp curves occurs on the road with hair pin bends.
Extensive earthwork is required.
The requirement for the construction of special structures.
High construction and operation cost.
5. Engineering Data for Design
The design data includes:
The terrain classification all along the alignment – to be established through topographic data/ Contours of the area using Satellite Imagery.
All features like river course, streams, cross-drainage structures (for existing alignment), flooding areas, high flood levels, landslide areas, snow/avalanche prone areas etc.
River Morphology and Regime data.
Chainage wise inventory of the side slope material type i.e. soil with classification and properties, rock type and its structural geology of the area.
Hydrological data for all stream and river crossings.
Available material and resources that can be used in the road construction.
Geometric standards.
6 – Geometric Design Standards
a – Hill Road Capacity
Type of Road
Design Service Volume in PCU per day
As per IRC:SP:48-1998 and IRC:52- 2001
As per IRC:SP:73-2015 & IRC:SP:84-2014
For Low Curvature (0-200 degrees per km)
For High Curvature (above 0-200 degrees per km)
Level of Service ‘B’
Level of Service ‘C’
Single lane
1,600
1,400
–
–
Intermediate lane
5,200
4,500
–
–
Two Lane
7,000
5,000
9,000
–
Four Lane
–
–
20,000
30,000
b – Design Speed:
The design speed for various categories of hill roads are given below:
Road Classification
As per IRC:SP:48-1998 and IRC:52- 2001
As per IRC:SP:73-2015 & IRC:SP:84-2014
Mountainous Terrain
Steep Terrain
Mountainous and Steep Terrain
Ruling
Minimum
Ruling
Minimum
Ruling
Minimum
National and State Highways
50
40
40
30
60
40
Major District Roads
40
30
30
20
–
–
Other District Roads
30
25
25
20
–
–
Village Roads
25
20
25
20
–
–
c – Sight Distance:
Visibility is an important requirement for safety on roads.
It is necessary that sight distance of sufficient length is available to permit drivers enough time and distance to stop their vehicles to avoid accidents.
Design Speed (Km/h)
As per IRC:SP:48-1998 and IRC:52- 2001
As per IRC:SP:73-2015 & IRC:SP:84-2014
Mountainous and Steep Terrain
Stopping Sight Distance (m)
Intermediate Sight Distance (m)
Safe Stopping Sight Distance (m)
Desirable Minimum Sight Distance (m)
20
20
40
–
–
25
25
50
–
–
30
30
60
–
–
35
40
80
–
–
40
45
90
45
90
50
60
120
60
120
60
–
–
90
180
d – Minimum Radius of Horizontal curves
Classification
As per IRC:SP:48-1998 and IRC:52- 2001
As per IRC:SP:73-2015 & IRC:SP:84-2014
Mountainous terrain
Steep terrain
Mountainous and Steep
Area not affected by snow
Snow Bound Areas
Area not affected by snow
Snow Bound Areas
Ruling Minimum
Absolute Minimum
Ruling Minimum
Absolute Minimum
Ruling Minimum
Absolute Minimum
Ruling Minimum
Absolute Minimum
Desirable Minimum Radius
Absolute Minimum Radius
National Highway and State Highways
80
50
90
60
50
30
60
33
150
75
Major District Roads
50
30
60
33
30
14
33
15
–
–
Other District Roads
30
20
33
23
20
14
23
15
–
–
Village Roads
20
14
23
15
20
14
23
15
–
–
e – Typical Cross-sections – 2 lane carriageway (as per IRC:SP:73-2015)
f – As per IRC:SP:48-1998 and IRC:52- 2001
Road Classification
Carriageway Width (m)
Shoulder Width (m)
National and State Highways
i) Single lane
3.75
2 x 1.25
ii) Double Lane
7.00
2 x 0.9
Major District Roads and Other District Roads
3.75
2 x 0.5
Village Roads
3.00
2 x 0.5
i –Typical Cross-sections – 4 Lane Carriageway Widening Towards Valley Side (as per IRC:SP:84-2014)
j –Typical Cross-sections – 4 Lane Carriageway Widening Towards Hill Side (as per IRC:SP:84-2014)
