460.4 Paving Equipment

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The following sections give important and useful information about the paving equipment. The inspector should become familiar with all of the equipment used on the project.

Contents

460.4.1 Distributor

This information applies to tack coats, prime coats, seal coats, scrub seals, and fog seals. Each of these will be discussed in more detail in articles on Tack Coat, Prime Coat, Seal Coat, and Surface Treatments. The purpose of a distributor is to apply the bituminous material in a uniform and continuous spread and at the rate required by the applicable Standard Specifications. The bituminous material may be an asphalt emulsion, a cutback asphalt, or a PG binder depending on the type of coat or seal being applied. Proper application is dependent upon interference and proper pump speed, spray bar height, and nozzle operation.

The distributor will be calibrated in accordance with EPG 106.4.2.6 Bituminous distributors. After calibration, the distributor and the dipstick will be issued a calibration number.

460.4.1.1 Interference

The nozzles on the spray bar must be adjusted so that the spray patterns do not interfere with, or distort, each other. Normally, each nozzle opening is adjusted to an angle of approximately 30 degrees with the centerline of the spray bar. A 30 degree angle will allow the passage of air currents without undue distortion of the fan pattern.

460.4.1.2 Pump Speed

The highest possible pump speed, which does not cause distortion of the spray pattern, should be used. A pump speed that is too low will result in streaking and non-uniform discharge of material from the nozzles. A pump speed that is too high will result in excessive distortion of the spray pattern. The ideal spray pattern should be a solid, triangular sheet of bituminous material from each nozzle onto the surface.

460.4.1.3 Spray Bar Height

The spray bar height should be set at a height that provides for uniform, sufficient application of the bituminous material on the surface. Overlapping of the bituminous material varies with the type of coat or seal used and is governed by the applicable Standard Specifications. It should not be set so low that blotches and/or voids of material occur because of air turbulence. Some method must be in place to ensure that the spray bar height will remain constant as the amount of material in the distributor decreases.

460.4.1.4 Nozzle Operation

The nozzles on the spray bar should be fully open, functioning, and clean. Most clogged nozzles are a result of allowing the spray bar to cool between applications. This allows the material remaining in the nozzle to harden and act like a plug. Before each application, hot material should be circulated through the spray bar until the temperature of the bar reaches the temperature of the material. This procedure will usually soften the hardened material. The material must be heated within the spraying temperature ranges in Standard Specification Section 1015.6.

460.4.2 Haul Trucks

The truck beds must be clean and free of foreign material. Also, the bed should be reasonably smooth without major indentations or depressions where release agent or mix can accumulate. A minimum amount of lime solution or an approved release agent, without runoff, should be used. The idea is to coat the inside of the bed to prevent mix from sticking to it. If a release agent is used, it should be diluted no less than the minimum rate specified and applied with equipment recommended by the manufacturer. Also, the release agent shall be in accordance with Standard Specification Section 1071.3. Diesel fuel, fuel oil, or other detrimental products will not be allowed for cleaning truck beds, as a release agent, or to dilute the release agent. The use of these products is considered as contamination of the mix.

If necessary, the truck beds should be insulated in order to keep the mix within the desired temperature range for proper placement. The purpose of a tarp is to insulate the mix and protect it from the weather during transport from the plant to the jobsite. All trucks must be equipped with a tarp that completely covers the top of the load and extends over the sides and tailgate of the truck. The tarp must be water repellent, of sufficient weight and strength to resist tearing, and in good condition with no holes or tears. There should be enough tie-down points to secure the tarp and prevent it from flapping in the wind during delivery.

If the tarp does not completely cover the load or is not properly secured, the rate of cooling of the mix will increase because of airflow under the tarp. The rate of cooling depends on the mix temperature, ambient air temperature, the efficiency of the tarp, and any insulation in the bed. Cooling results in the formation of a crust on top of the mix. The extent of crusting depends on whether or not the load was tarped and how long the mix has aged. In a sense, crusting is beneficial because it helps insulate and reduces the rate of cooling of the mix underneath. However, the crust decreases the workability of the mix and may have adverse effects on the mat.

400.4.3 Material Transfer Vehicle (MTV)

The most common type of MTV is essentially a surge bin on wheels. The trucks dump the mix into the hopper on the front of the MTV. The mix is carried by the augers in the hopper to a conveyor, which delivers the mix into the storage chamber where it is reblended. From here, the mix is carried by another conveyor and dropped into the paver hopper. This second conveyor can rotate so that the MTV can operate directly in front of the paver or off to one side. This allows for greater mobility in confined areas. Also, the MTV eliminates the problem of trucks bumping into the paver, which causes bumps in the mat.

One advantage of using this type of MTV is that it reblends the mix reducing segregation and temperature variations allowing for proper compaction and uniform density. The second advantage is that it allows for continuous paving because of its storage capacity, as long as mix is continuously supplied from the plant. In turn, a constant head of material can be kept in front of the screed, producing a smoother mat.

There is another type of MTV that transfers mix from the trucks to the paver. The mix is dumped from the trucks into the hopper, carried up a conveyor, and dropped into the paver hopper. The storage capacity of this type of MTV is significantly less than the type discussed above. Since this type of MTV does not reblend the mix, any possible segregation in the mix may or may not be affected. This type of MTV does, however, provide a smoother mat by allowing for the possibility of maintaining a constant head of material in front of the screed and by eliminating the problem of trucks bumping into the paver. A material pickup machine may sometimes be used as an MTV.

460.4.4 Pavers

The purpose of the paver is to place the mix to the desired width, grade, cross slope, and thickness and to produce a uniform mat texture. The paver should also be able to place the mix in a manner that results in improved ridability and smoothness of the roadway. There are two types of pavers, track and rubber-tire. The track paver is generally used when paving over a soft or yielding base. The rubber-tire paver is generally used when paving over well-compacted bases or existing pavements. The paver consists of two main units, the tractor and the screed.

400.4.4.1 Tractor Unit

The tractor unit provides motive power to the paver and pushes the haul trucks if a material transfer vehicle (MTV) is not used. The tractor also tows the screed unit. On the track pavers it is important that both tracks are properly adjusted so that they are snug but not too tight. If they are loose, the tractor may have a rhythmic bumping movement. This movement is reflected in the screed and causes a ripple effect in the mat. This rippling effect frequently cannot be removed by rolling and remains in the finished pavement. On rubber-tire tractors, it is important that the tires are uniformly and properly inflated to provide maximum traction and smooth screed operation. The tractor unit includes the push rollers and the material feed system consisting of the receiving hopper, slat conveyors, material flow gates, and distribution augers.

460.4.4.1 Push Rollers

Push rollers are located on the front of the paver hopper and are used to maintain contact with the haul truck. The push rollers must rotate freely. If they do not, the truck tires will slide on the rollers increasing the load on the paver making it more difficult for the paver to push the truck. If one roller rotates freely and the other does not, the paver may be more difficult to steer.

460.4.4.2 Receiving Hopper

The receiving hopper is used for temporary storage of mix. An insert with a larger capacity is available for use with an MTV. If mix is delivered directly to the paver, the hopper must be wide enough to allow the body of the haul truck to fit inside it. Also, the hopper must be low enough to allow the truck bed to be raised without putting weight on the hopper. The front of the hopper is designed to minimize spillage of mix.

460.4.4.3 Slat Conveyors

The purpose of the slat conveyors is to carry the mix through the paver from the hopper to the augers. The slat conveyors operate independently from each other so that more or less mix can be fed to one side of the paver without affecting the other. In some cases, the slat conveyors operate at speeds independent from the speed of the paver and augers.

460.4.4.4 Material Flow Gates

There is usually a material flow gate over each slat conveyor that regulates the amount of mix delivered through the tunnels and back to the corresponding auger. The flow gates can be adjusted vertically to allow more or less mix to be carried back to the augers. Flow gates are not used on pavers with conveyors and augers that operate independently.

460.4.4.5 Distribution Augers

The mix deposited into the auger chamber by the slat conveyors is distributed across the full width of the paver by the augers.

The augers on either side of the paver operate independently from each other. However, the auger on one side of the paver usually operates in conjunction with the slat conveyor on the same side. The auger gearbox is located in the center of the paver. Typically, a reverse auger or paddle is adjacent to the gearbox on either side. These are used to place mix under the gearbox so that sufficient mix is present at the screed directly behind the gearbox.

On some pavers, the height of the augers can be changed. Generally, the augers are set according to the thickness of the lift and as low as possible to minimize the amount of mix carried in the auger chamber.

460.4.4.6 Screed Unit

The screed unit’s function is to establish the thickness and provide the texture and initial compaction of the mat. The screed unit is attached to the tractor unit at the tow (or pull) points, one on either side of the tractor. The tow points allow the screed leveling arms (also called side arms or pull arms) of the screed to rotate or pivot around these points. Therefore, the screed “floats” over the mix, averaging out any changes in the grade or elevation experienced by the wheelbase of the tractor unit. This allows for more mix to be placed in low spots and less mix to be placed in high spots. In order for the floating-screed principle to work properly, both tow points must be set at the same level.

Screed Strike-offs. Sometimes the screed is equipped with a strike-off. The purpose of a strike-off is to control the amount of mix passing under the nose of the screed and to reduce the wear on the leading edge of the screed. The exact location of the strike-off depends on the make and model of the paver being used and on the thickness of the mat. No compaction of the mix occurs under the strike-off.

Screed Heaters. Screed heaters are used to heat the plates on the bottom of the screed to the temperature of the mix in order to prevent the mix from sticking to the screed. If mix does stick to the screed, the mat will tear, causing a rough texture. If the screed is properly heated, a more uniform mat texture and consistent mat thickness will be achieved. If the screed is overheated, permanent warping of the screed plates may occur.

Screed Crown Control. The screed can be angled at its center to provide for positive or negative crown. The crown can also be introduced into the screed extensions, rigid and hydraulic, at the points where the extensions are attached to the main screed. The lead crown (front of the screed) should be set slightly above the tail crown (back of the screed) so that the mix will flow easily under the screed and provide a more uniform mat texture.

Screed Vibrators. The screed vibrators may or may not be used. The amount of compaction imparted to the mix by the screed is a function of the properties of the mix, the amount of bearing pressure applied to the mix by the screed, the amount of mix passing under the screed, and the paver speed. The frequency (number of vibrations per minute) and the amplitude (amount of force) of vibration within the screed itself also affect the degree of compaction.

The frequency of vibration is controlled by the speed of rotation of the vibrator shaft and can be adjusted. Changing the frequency changes the compactive effort. In general, the highest frequency setting should be used to obtain the greatest compactive effort possible. The amplitude is determined by the location of the eccentric weights on the shaft, which can be altered to change the compactive effort. Low amplitude should be used for thinner lifts and high amplitude should be used for thicker lifts.

Rigid Screed Extensions. Rigid screed extensions are used to increase the width of the paver. The width of the rigid extensions added to the screed should be equal on both sides, if possible, in order to keep the paver in balance. Extensions should be securely attached to the main screed at the same elevation and angle of attack as the main screed. Auger and auger chamber extensions of the same length as the screed extensions should also be used. If a strike-off is used on the main screed, a strike-off must be added to the extensions at the same elevation.

Hydraulically Extendable Screeds. Hydraulically extendable screeds may be in front of or behind the main screed. It is important that the elevation and angle of attack of the extendable screeds are properly set. If they are not properly aligned with the main screed, a difference in thickness, surface texture, and compaction will result. For front-mounted extendable screeds, the elevation is usually set slightly below the main screed. For rear-mounted extendable screeds, the elevation is normally set the same as the main screed with a greater angle of attack. The forces that act on the extendable screeds and the main screed are similar.

If a strike-off is used on the main screed, strike-offs should be used on the extendable screeds. If a front-mounted extendable screed is to be used in a fixed position for a long period of time, the paver should be equipped with auger and auger chamber extensions to distribute mix across the width of the mat and help maintain a constant head of material. If a rear-mounted extendable screed is used, auger extensions should not be used because an excess of mix will occur in front of the extendable screed resulting in non-uniform texture and variable mat thickness. The mix will flow out to the end of the extendable screed without assistance from the augers. With either front- or rear-mounted extendable screeds, the flow control sensors should be mounted on the end plate so that enough, but not too much, mix is delivered to the ends of extendable screeds.

Automatic Screed Control. Automatic screed controls, consisting of grade references and slope controls, are used to keep the elevation of the tow points on the paver constant. Keeping the elevation of the tow points constant allows the screed to maintain a consistent angle of attack, which helps to produce a smooth mat. As the tractor unit moves over the existing grade, the tow points move up and down less than they would if a reference was not used. Deviations in the underlying surface are averaged out over the length of the reference.

There are three basic types of grade references: erected stringline, mobile reference, and joint-matching shoe. Each has a sensor that adjusts the elevation of the tow points as the reference reacts to changes in the grade. The location of the sensor, in relation to the reference, does make a difference in the reaction of the tow points and screed to the grade of the underlying surface and is different for the different types and manufacturers of the references. The purpose of the stringline and the mobile reference is to average out the deviations in the underlying surface over a distance that is greater than the wheelbase of the tractor unit. The purpose of the joint matching shoe is to match the grade of the adjacent lane.

The grade references can be used in any combination on either side of the paver. Using a reference on both sides of the paver will average out deviations in the grade on both sides of the mat. However, using grade references on both sides of the paver generally will not produce a uniform cross slope.

The grade reference is typically located on the centerline side of the paver and the slope control is typically located on the shoulder side. The slope controls may cause the paver to place mix at variable thicknesses on its side, depending on the condition of the underlying surface. More mix is placed in low spots and less mix is placed in high spots. However, a constant cross slope will be maintained.

460.4.5 Rollers

The size and type of compaction equipment varies with the type of mix being placed.

460.4.5.1 Static Steel Wheel Rollers

Steel wheel rollers should be equipped with scrapers or a wetting system to prevent mix from sticking to the drums. However, excessive amounts of liquids will not be permitted. Diesel fuel, fuel oil, and other detrimental products shall not be used.

The compactive effort applied to the mat by static steel wheel rollers depends on the weight of the roller and the contact area of the drums with the mix. Adding ballast can change the weight of the roller. The effective contact pressure over the contact area is the key variable and is dependent on the depth of penetration of the drums into the mix. On the first pass of the roller, the depth of penetration and the contact area are the greatest, so the compactive effort on the mix is less. On subsequent passes the mix becomes denser so that the drums penetrate less causing the weight of the roller to be applied over a smaller area and, therefore, the compactive effort to increase. Rollers with large-diameter drums have lower rolling resistance than smaller drums. Therefore, rollers with large-diameter drums are more efficient because they do not penetrate as far into the mix to develop the same contact area.

460.4.5.2 Pneumatic Tire Rollers

The compactive effort applied to the mix is dependent on the wheel load of the roller, the tire design and pressure, the contact area, and the depth of penetration of the tires into the mix. All of the tires on the roller should be the same size, ply, and pressure. Tire pressure should be adjusted depending on the type of mix and its position in the roller train. The greater the contact pressure between the tire and the mix, the greater the compactive effort applied by the roller. The wheels must oscillate (up and down movement), however wobble (side to side movement) wheel rollers are not to be used. The most effective practice in preventing roller pickup is to keep the temperature of the tires as close as possible to the temperature of the mix.

460.4.5.3 Vibratory Rollers

The vibratory roller can be operated in three modes: static (without vibration), with only one drum vibrating, and with both drums vibrating. Vibratory rollers should be equipped with a wetting system to prevent mix from sticking to the drums. However, excessive amounts of liquids will not be permitted. Diesel fuel, fuel oil, and other detrimental products shall not be used. The vibratory roller compacts by weight and impact, not by vibration. The impact is caused by a rotating eccentric weight inside the drum(s).

Vibratory rollers are equipped with controls to adjust both the amplitude and the frequency. The amplitude controls the amount of force applied to the mix by the movement of the eccentric weight. The amplitude setting is dependent on the thickness of the lift. However, it is recommended that the roller be operated in static mode for thin lifts. The amplitude should not be increased if density is not being achieved. Increasing the amplitude increases the applied force, which may cause the aggregate in the mix to fracture and the mat density to decrease. Instead, the number of passes made over the mat should be increased.

The frequency is the number of complete rotations of the eccentric weight per minute. The roller should generally be operated at high frequency in order to increase the number of impacts in a given distance at a given roller speed. The spacing of the impacts depends on the frequency and the roller speed. As frequency decreases and roller speed increases, the distance between impacts increases. As frequency increases and roller speed decreases, the distance between impacts decreases. Smaller impact spacing is preferred because the number of impacts in a given distance is increased which also increases the compactive effort.

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