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HMA Construction Program Module 5 – HMA Placement.

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  • Slide 1
  • HMA Construction Program Module 5 HMA Placement
  • Slide 2
  • This module will cover all the aspects of placing HMA. The primary piece of equipment, the paver, will be discussed in detail, along with the operational techniques for producing a quality HMA mat. Again, focus on the balance aspects of the paving operationforces on the paver must be in balance to produce a uniform, dense, and smooth mat. Principles of paver operation are applicable to a variety of HMA mix types, including dense-graded, Superpave, and SMA.
  • Slide 3
  • Module 5 Objectives Objective of HMA PlacementObjective of HMA Placement Components and Function Tractor and Screed UnitComponents and Function Tractor and Screed Unit Operational Principles of ScreedOperational Principles of Screed Grade and Slope Control SystemsGrade and Slope Control Systems Types Functions Capabilities
  • Slide 4
  • Present the objectives of this module.
  • Slide 5
  • Module 5 - Objectives (cont.) Yield-Thickness-Smoothness RelationshipYield-Thickness-Smoothness Relationship Types of Paving and When to Use ThemTypes of Paving and When to Use Them Night PavingNight Paving Paver MaintenancePaver Maintenance Operating TechniquesOperating Techniques
  • Slide 6
  • (Objectives continued)
  • Slide 7
  • Push Rollers Tractor Hopper Tow Point Side Arm Conveyors Screed Depth Crank Augers
  • Slide 8
  • The purpose of the paver is to place the HMA to the desired width and thickness and to produce a satisfactory mat texture. The paver consists of two primary components: the tractor unit and the screed. The tractor unit provides all of the power for the paver, and carries the mix from the hopper back to the screed. The screed is towed by the tractor unit, and provides the initial density and smoothness to the mat.
  • Slide 9
  • Tractor Unit Drive SystemsDrive Systems Push Rollers and Truck HitchesPush Rollers and Truck Hitches HopperHopper Slat ConveyerSlat Conveyer
  • Slide 10
  • This list gives the major components of the tractor unit and the subject areas that will be covered during this part of the module.
  • Slide 11
  • Tractor Unit (Cont'd) Conveyer Flow GatesConveyer Flow Gates AugersAugers Materials Feed SystemMaterials Feed System Tow PointsTow Points MaintenanceMaintenance
  • Slide 12
  • (List continued)
  • Slide 13
  • Slide 14
  • The tractor unit has its own engine that provides the power to move the pave forward. Pavers travel on either rubber tires or tracks. Rubber tired pavers can be moved around more readily. The have a faster travel speed than a track paver. The rubber tires are inflated to between 240 and 585 kPa. The tires are also ballasted with a calcium chloride solution (between 1700 and 3800 liters) to reduce bounce in the paver.
  • Slide 15
  • Slide 16
  • Many rubber tire pavers also include a front wheel assist option on the steering wheels. The power assist can increase the tractive power by up to 50 percent.
  • Slide 17
  • Slide 18
  • Pavers also come with track drive systems. The tracks spread the weight of the paver over a larger area. These types can be more effective on paving grades. The tracks can be all steel, steel with rubber pads, or flexible bands with steel shoes and rubber pads. Ask: Who has a preference between rubber tired and track pavers? Why?
  • Slide 19
  • Slide 20
  • On the front of the paver are the push rollers and the truck hitches. The truck rests against the push rollers as the paver is pushing it while paving. The rollers must be kept clean and be free to rotate while pushing the truck.
  • Slide 21
  • Slide 22
  • Truck hitches are used to keep the truck in contact with the paver, to keep it from rolling away. The paver operator controls the truck hitches. Ask: Any reasons not to use the truck hitches?
  • Slide 23
  • Slide 24
  • If the rollers are not moving freely, the truck tires will slide on the rollers, and the weight of the truck will bear more directly on the paver.
  • Slide 25
  • Slide 26
  • The paver hopper is where the mix is received from the truck or pickup machine. The hopper must be wide enough to allow the bed of the truck to fit inside its edges. The hopper must be low enough to allow the truck bed to be raised. Ask: What is the actual purpose of the wings being able to fold? (To reduce the paver width during transport.) Ask: How often should the wings be folded? (Minimally, to reduce segregation.) The wings should be folded while the hopper is relatively full of mix.
  • Slide 27
  • Slide 28
  • This wedge of steel keeps the mix from building up in the corners of the wings, reducing the need to fold them during paving. The coarse aggregates tend to congregate at the edges of the wings. Frequent dumping produces the chevrons (or gull wings) in a finished mat. With the advent of Superpave, mixes will typically become coarser, and the segregation caused by frequent wing folding will be a greater concern.
  • Slide 29
  • Slide 30
  • Flashing or overflow guards on the front of the hopper keep mix from spilling onto the pavement.
  • Slide 31
  • Slide 32
  • The flaps are a thick, flexible material. Most commonlyrubber or polymer. To be effective, the flashing must be replaced when it is worn.
  • Slide 33
  • Slide 34
  • At the bottom of the hopper are slat conveyors. They transport the mix from the hopper back through the tunnel in the tractor unit to the spreading screws. The slat conveyors move independently, each of them feeding one side of the screed. Ask: Why would one conveyor need to be moving faster than the other? (If one side was being paved wider, as with a turnout, driveway, taper, etc.) Above the slat conveyors at the back wall of the hopper are flow gates.
  • Slide 35
  • Slide 36
  • The flow gates are adjustable to regulate the amount of mix carried to the spreading screws. The goal while paving is to have the flow gates set so that the slat conveyors are running continuously. One paver manufacturer has replaced the slat conveyors with auger screws running from the hopper back to the screw augers.
  • Slide 37
  • Slide 38
  • At the back of the tractor unit are screw augers. The mix is brought from the hopper by the slat conveyors, and the augers spread the mix laterally in front of the screed. As with the slat conveyors, the augers operate independently.
  • Slide 39
  • Slide 40
  • The augers reside directly between the tractor and screed units, in the auger chamber. This photo shows the proper head of material in the auger chamber and in front of the screed. The proper head of material is at the center of the auger shaft. The mix should not cover the augers, nor should the bottom of the augers be visible. It is important that the head of material is consistent across the screed, to keep forces on the screed constant.
  • Slide 41
  • Slide 42
  • In the center of the augers is a gear box. Kick back paddles are used to tuck mix under the gear box. If the paddles were not there, the proper amount of mix would not get under the gear box. These are a high wear item, and must be replaced when they are worn.
  • Slide 43
  • Slide 44
  • Reverse direction screw augers are also used in lieu of kick back paddles.
  • Slide 45
  • Slide 46
  • Pavers with extendable screeds can be equipped with hydraulically extendible augers. These make sure the mix gets to the outer edges of the screed. Auger extensions can also be hard extensions (added onto the end of the main augers.)
  • Slide 47
  • Hopper Wings Slat Conveyors Hopper Gates Augers Screed Plate Proportional Feed Control Material Flow
  • Slide 48
  • The previously described components comprise what is also called the material feed system. The goal of the material feed system is to get a constant head of material in front of the screed. If the material feed system is set and operating properly, the slat conveyors and augers on each side of the paver will rarely shut off. This is accomplished by setting the proper position on the hopper flow gates and using the correct speed on the slat conveyors and augers. If the conveyors and augers are constant speed (only on or off), the flow gate settings control the flow. For variable speed augers and conveyors, material flow is controlled by all three.
  • Slide 49
  • Slide 50
  • Many material feed systems have automatic feed control. A sensor measures the head of material and controls the slat conveyors and augers.
  • Slide 51
  • Support Mount Off On Control Arm Extendable Wand Stop Limit Switch Control Arm Clamp
  • Slide 52
  • The feed control sensors can be used with an on-off switch to control the conveyors and augers, or they can be used with a variable speed system. This sensor has a control arm that physically touches the mix to sense the depth. As the mix depth decreases, the arm moves down and activates the conveyors and augers. As the depth comes back up, the conveyor and auger are slowed or stopped.
  • Slide 53
  • Material Retaining Plate Auger Active Material
  • Slide 54
  • The proper position of the feed control sensor is as close to the end of the augers as possible. In addition to control arms that touch the mix, ultrasonic, infrared, and laser sensors are used to measure the amount of mix in the auger chamber.
  • Slide 55
  • Target in the Center Area Active Material Auger
  • Slide 56
  • Ultrasonic, infrared, and laser sensors send and receive signals to measure the mix level and control the slat conveyors and augers.
  • Slide 57
  • Tow Point
  • Slide 58
  • The last part of the tractor unit is the tow points (or pull points). The tow points are a pin connection where the screed is attached to the tractor unit. The tow point is the only place the tractor and screed units are connected together. The screed pivots around the tow point. (The forces acting on the screed and how the tow point is positioned are discussed later.)
  • Slide 59
  • Slide 60
  • The tow point can be raised or lowered as necessary. Hydraulic cylinders move the position of the tow point. The cylinders can adjusted manually or automatically with a grade and slope control system. (More on those later). The tow point is typically centered when paving begins, and then is adjusted as necessary.
  • Slide 61
  • Slide 62
  • As with any equipment, maintenance is an important consideration to make sure that the paver operation continues smoothly. If the paver stops, the whole operation comes to a halt. Ask: How many companies have an extra paver waiting in the yard, just in case there are problems.
  • Slide 63
  • Paver Maintenance Washing downWashing down Wear checkWear check Storage of electrical equipmentStorage of electrical equipment ChecklistsChecklists
  • Slide 64
  • Pavers must be kept clean to operate effectively. Excess HMA is easier to remove when the surfaces are still warm. Kerosene is typically used, but use it sparingly. Worn or loose parts will affect the quality of the mat. There are specific wear points on pavers that need checking. Various parts (sprockets, chains, steel plate, augers, etc.) are checked with different frequency. Electrical equipment (sensors, grade and slope control components) in particular must be carefully protected.
  • Slide 65
  • Slide 66
  • Manufacturers can supply wear check manuals. This two-page paver maintenance check is contained in the participants manual.
  • Slide 67
  • Screed Unit Screed plateScreed plate Strike-offStrike-off Crown controlCrown control Extensions and end platesExtensions and end plates
  • Slide 68
  • The second major part of the paver is the screed unit. The screed unit is towed by the tractor unit and consists of these parts.
  • Slide 69
  • Screed Unit (continued) Thickness Control ScrewsThickness Control Screws Screed ArmScreed Arm Pre-Compaction SystemPre-Compaction System Heating SystemsHeating Systems MaintenanceMaintenance
  • Slide 70
  • (List of screed components, continued)
  • Slide 71
  • Slide 72
  • The screed unit establishes the thickness of the asphalt layer and provides the initial texture of the HMA surface. The screed also provides the initial compaction to the mat with its weight and through vibratory action or with a combination of vibration and tamping. The free floating screed was developed in the 1930s. The concept allows the screed to level out the changes in grade experienced by the wheel base of the tractor unit.
  • Slide 73
  • Slide 74
  • The bottom of the screed unit is the formed steel screed plate.
  • Slide 75
  • Slide 76
  • The screed plate is bolted to the screed, and works much like a hot iron to smooth out the surface of the mat. The screed plate is the only part of the screed unit that develops the initial texture of the mat.
  • Slide 77
  • Slide 78
  • In the front and middle of the screed, relief cuts are cut into the screed. These cuts allow the screed plate to be flexed or warped, so a crown can be placed into the mat.
  • Slide 79
  • Strike-off Correct Strike-off Wear Too High Strike-off Wear Too Low Rocks are Dragged Fines Accumulate
  • Slide 80
  • Typically, a strike-off (or pre-strike-off) plate is mounted in front of the leading edge of the screed. The strike-off regulates the amount of mix fed under the screed plate. It also reduces wear to the leading edge of the screed. If the strike-off is too high, too much material is fed under the screed, causing it to rise. The screed would have to be pivoted forward around the tow point to compensate, causing high nose plate wear. If the strike-off is too low, too little material goes under the screed. The screed would have to be pivoted backward, causing high tail wear. Again, stress the balance needed for quality paving.
  • Slide 81
  • Lead Crown Tail Crown Correct Relationship Maintained by Chain
  • Slide 82
  • The screed plate can be angled at its center to provide a positive or negative crown to the mat. The amount of crown can be varied. A turnbuckle device is used to flex the screed plate to its desired position.
  • Slide 83
  • A B C A - Lead crown too high Heavy matl flow in center B - Lead crown too low Heavy matl flow at edges C - Uniform mat texture and material flow from correct crown adjustment
  • Slide 84
  • Typically, the lead crown is 1 to 5 mm more than the tail crown to achieve the desired results. If the lead crown is too high, more material will be forced under the center of the screed, and the mat will be shinier there. If the tail crown is too high, more mix will be forced under the screed at the edges. A difference in texture will result from eitherthe loose mat texture is caused by the lower amount of material in that location. The angle of the screed is called the angle of attack.
  • Slide 85
  • Slide 86
  • Most paver screeds can be extended either hydraulically or with fixed extensions. Hydraulic extensions can be manipulated to create changes in slope. For hydraulic extensions, that portion of the screed is either in front of or behind the main screed. When paving a level surface, it is important that the extendable portion of the screed is set at the same angle of attack as the main screed. If the extensions are not properly aligned with the main screed, a longitudinal line or ridge will appear in the mat. Also, a difference in surface texture between the main screed and extension may occur.
  • Slide 87
  • Slide 88
  • The basic width of the screed (2.4 to 3 m) can be increased with rigid screed extensions. The extensions come in varied widths. Again, it is important that the extensions be set at the same elevation and angle of attack as the main screed. Auger and auger tunnel extensions should be added when using rigid extensions. Not used much in road paving now replaced by hydraulic screed extensions. Used in FAA airfield paving with rigid extensions set at 25 ft.
  • Slide 89
  • Slide 90
  • The hydraulic strike-off is an option for most screeds. These are not screed extensions, but extensions that can be used for brief periods to form turn-outs, ramps, etc. The hydraulic strike-off does not compact the mix as much as the screed, so it must be adjusted higher to place more mix than the main screed.
  • Slide 91
  • Slide 92
  • Using hydraulic strike-offs instead of rigid extensions on main line paving is not recommended. (The mat difference from the hydraulic extension is noticeable in this photo.)
  • Slide 93
  • Slide 94
  • The end plate is attached to the end of the screed to keep the mix contained behind the screed. While paving, the end plate is typically positioned tightly to the existing surface to retain the mix and control the width of the mat.
  • Slide 95
  • Slide 96
  • Various shaped edge plates are also available to form longitudinal joints.
  • Slide 97
  • Slide 98
  • The most common type is a wedge shaped joint. Many states have been conducting research in the styles and shapes of these edges to try and improve the performance of the longitudinal joints formed between paving lanes. This is a notched wedge joint under evaluation in Michigan. Very safe. Ask: Are wedge shaped joints used locally?
  • Slide 99
  • Slide 100
  • Cut-off shoes of various styles are available to reduce the width of the mat to less than the basic width of the main screed. The shoes typically come in widths of 0.3 or 0.6 m, and are adjustable in increments of 40 to 80 mm, depending on the manufacturer.
  • Slide 101
  • INCREASEINCREASE DECREASEDECREASE
  • Slide 102
  • The thickness of the mat can be controlled one of two ways: with the thickness control screws, or with the tow points. The thickness control screws rotate the screed about its pivot point. The rotation changes the screeds angle of attack; as the angle of attack changes, the screed rises or falls to balance the forces acting on the screed.
  • Slide 103
  • Hand cranks turned clockwise to thicken Tow point unchanged
  • Slide 104
  • To increase depth, the front of the screed is pivoted up, and the screed rises. The change in thickness is not instantaneous, but happens over a length of paving. (This is discussed in detail later in this module.) This change occurs with no change in the tow point.
  • Slide 105
  • Slide 106
  • The other way to increase thickness is to raise the tow point. The screed arm (or tow arm or leveling arm) attaches the screed to the tow point. Instructors note: Coarse graded Superpave and SMA mixes have more coarse aggregate and less sand size aggregate. Screed problems such as dragging of the screed and mat tearing can be more critical with these types of mixes. Some contractors have reported use of a heavier screed with Superpave mixes in order to improve compaction.
  • Slide 107
  • Tow point raised Hand cranks unchanged
  • Slide 108
  • As the tow point is raised or lowered, the angle of attack is raised or lowered, and the mat thickness is subsequently changed.
  • Slide 109
  • Vibratory Shaft With Weights
  • Slide 110
  • Screed vibrators provide initial compaction of the mix as it passes under the screed. The vibration is created with a rotating shaft that contains eccentric weights. As the shaft rotates, the weights create an up-and-down vibration. The degree of compaction is controlled by the speed of rotation (frequency) and position of the weights. Frequency is typically set at the maximum. Weight position (if changeable)more eccentricity for thicker lifts. Paver speed also affects initial compaction. Typically, initial compaction = 70 to 80% of maximum Some newer screeds-both vibratory and tamper bars.
  • Slide 111
  • Slide 112
  • Screeds also contain a screed heater, to heat a cold screed to about 150 C prior to the start of paving. A properly heated screed, particularly at the start of the day or after a prolonged shutdown, provides for a more uniform mat surface texture. Heating the screed prevents the mix from sticking to the screed plate and tearing the mat, causing a rough texture. Screed heaters cannot be used to raise the heat of the mix. (The mix is not under the screed long enough.)
  • Slide 113
  • Slide 114
  • Dont leave the screed heater on too long With the $10-11,000 price tag, this wouldnt make the boss too happy...
  • Slide 115
  • Slide 116
  • The biggest maintenance item with the screed is uneven wear on the screed plate. Heavy wear on the nose of the screed plate will result if the screed is too front down in the mat.
  • Slide 117
  • Pull Point Too High Abnormal Wear Abnormal Wear Pull Point Too Low Pull Point Correct Head of Material Level with Auger Shaft Normal Uniform Wear Slight Positive Angle of Attack
  • Slide 118
  • The proper angle of attack with the screed is where the nose is slightly above the tail. The wear on the screed will be uniform if the angle of attack is adjusted properly.
  • Slide 119
  • Operational Principles of the Screed Self-leveling ConceptsSelf-leveling Concepts Screed Response versus DistanceScreed Response versus Distance Forces Acting on a ScreedForces Acting on a Screed
  • Slide 120
  • The next item to discuss is the operational principles of the screed. This topic will cover...
  • Slide 121
  • Self Leveling - Rubber Tired Paver Rear Drive Tires Front Bogie Wheels Line of Pull
  • Slide 122
  • As the paver moves along the road, the tractor unit follows the ups and downs of the existing surface. While the tow point moves up and down, the screed reacts slowly to this up and down movement, so it moves along relatively unchanged. In effect, the screed averages out and levels itself over these changes. Hence the term, self-leveling screed. One further step toward producing a level surface is setting up a reference unattached to the paver. The tow point is guided by the reference, instead of the motion of the tractor unit, to keep the screed level. Use of the reference is guided by automatic grade controls.
  • Slide 123
  • Detail of Screed Path Tow Point Path Step Amplitude = 100% L = Leveling Arm Length Screed Path Direction of Paving 63% 87% 6L5L4L 3L 2L 1L0 96% 98% 99% L
  • Slide 124
  • The screed levels the surface due to the fact that the screed must travel about five lengths of the tow arm before fully reacting to the change of the tow point. Even without automatic paver controls, the leveling or averaging effect places more mix in the low spots and less mix in the high spots. For example, if the tow arm is 2.5 m long, the paver would have to move about 13 m before the screed fully reacted to a change in thickness input into the system.
  • Slide 125
  • 25mm movement at Tow Point 3mm movement at Leading Edge of Screed
  • Slide 126
  • The change in thickness command can be made with the thickness control screw, or can be made by adjusting the tow point. If the tow point is moved, the paver must still move forward about five lengths of the tow arm before the screed fully reacts to the change. A 25 mm change in the tow point will produce a 3 mm change in the leading edge of the screed.
  • Slide 127
  • Main Forces Acting on Screed 1. Speed of Paver 2. Head of Material 3. Angle of Attack 4. Other Forces Pre-compaction Screed Weight
  • Slide 128
  • The self-leveling action of the screed means it will respond to the various forces acting on it to find its balance or equilibrium. The three main forces acting on the screed and how thick it places the mat are...
  • Slide 129
  • Tow Point Screed Depth Crank Pivot Point F1 F2 F3
  • Slide 130
  • Ask: How does the speed of the paver affect mat thickness? If everything else is constant: Faster = thinner mat Slower = thicker mat If you change the speed of the paver, you will need to change the amount of mix delivered to the augers to compensate for the change. Automatic material flow control will adjust for these changes. Speeding up the paver will change the precompactive effort into the mat, requiring more compactive effort from the roller.
  • Slide 131
  • Superpave Coarser mixturesCoarser mixtures Modified asphaltModified asphalt Effect on Paving?Effect on Paving?
  • Slide 132
  • Ask: How will changes in these mix characteristics affect forces on the screed? A coarser mix and/or a modified asphalt mix will generally be tougher. Compared to a finer or unmodified mix, the screed would ride higher due to the increasing force required to push against the mix.
  • Slide 133
  • Constant Head of Material Volume Head of Material Volume Too High Head of Material Volume Too Low Correct Depth of Mat Maintained Screed Rises Due to Excess Material Forced Under Nose of Screed Screed Settles Due to Inadequate Supporting Material
  • Slide 134
  • Ask: How does the head of material affect mat thickness? As discussed earlier, the ideal head of material covers the center of the screw augers. If head of material is more than ideal, then the force on the front of the screed increases, and the screed compensates by moving upward. If head of material is less than ideal, then the force on the screed decreases, and the screed compensates by moving downward.
  • Slide 135
  • Correct Elevation Auger Overloaded Auger Underloaded
  • Slide 136
  • If the augers are underloaded, the thickness placed is too thin. If the augers are overloaded, the thickness placed is too thick. The head of material is the most important force acting on the screed. Some paving experts feel that 90 to 95 percent of paver-related problems can be solved by maintaining a uniform head of material during paving.
  • Slide 137
  • Travel Angle of Attack
  • Slide 138
  • We discussed earlier how moving the tow point or thickness control screw (and hence adjusting the angle of attack) changes the thickness. Increase the angle of attack, increase the thickness. Decrease the angle of attack, decrease the thickness.
  • Slide 139
  • Slide 140
  • Ask: What can cause the most problems during paving? (Stopping the paveras here, waiting for trucks). Ask: What can change in the screed forces if the paver stops for an extended length of time? The need to stop and start paver. Any mix in the paver (head of material, conveyors, hopper) can cool, requiring more force to move the paver. Screed can settle, causing bump in the mat. Re-starting adjustments to get paver in tune. Keep the forces on the screed in balance to maintain a uniform mat.
  • Slide 141
  • Screed Control Systems Whos Responsible for ChangesWhos Responsible for Changes Sticking the MatSticking the Mat Manual ControlsManual Controls Automatic ControlsAutomatic Controls
  • Slide 142
  • The next topic of discussion...
  • Slide 143
  • Slide 144
  • Ask: Who is responsible? The Screed Operator!
  • Slide 145
  • Pavement Profile Direction of Paving
  • Slide 146
  • Ask: If the screed operator stuck the mat this frequently on this project, what would most likely happen? The mat thickness would be adjusted too frequently. Ask: What part of the paver is trying to adjust for the unevenness? (The tow point.) If the operator makes adjustments, both the tow point and the thickness screw would be trying to compensate at the same time. The adjustments could counteract each other, or duplicate each other. Sticking the mat can be useful during on-the-job training to show how long it takes for the screed to react.
  • Slide 147
  • Detail of Screed Path Tow Point Path Step Amplitude = 100% L = Leveling Arm Length Screed Path Direction of Paving 63% 87% 6L5L4L 3L 2L 1L0 96% 98% 99% L
  • Slide 148
  • Again, the screed takes a distance of five times the tow arm length to react to a change in thickness. If the paver and thickness controls are being operated manually, the distance-response concept is even more important to understand. If a second change is made before the first one is accomplished, the first change will never be completed.
  • Slide 149
  • Slide 150
  • Has anyone had Charles Atlas as the screed operator? Under manual screed operation, the screed operators ability to produce a consistently smooth mat will depend on: the frequency with which the operator feels the need to adjust the thickness the roughness of the existing surface. The more the operator tries to assist the self-leveling action of the screed, the rougher the new surface will be.
  • Slide 151
  • Slide 152
  • Automatic screed controls are used to produce a smoother asphalt pavement. They function by using a reference point other than the wheelbase of the paver to maintain the tow point elevation. Even though the tractor unit may be moving up and down, the tow point stays tied to the reference outside the paver. Differences in pavement elevation are smoothed out over longer distances (such as a stringline or ski) than the length of the tractor wheelbase. The screed stays level instead of following the highs and lows, creating a smoother pavement.
  • Slide 153
  • Types of Grade Reference StringlineStringline Mobile referenceMobile reference Joint matching shoeJoint matching shoe Sonic sensorSonic sensor LaserLaser
  • Slide 154
  • Three types of grade control references. Grade references can be used alone on either side of the paver, or on both sides. One type can be used on one side (e.g. stringline) and one on another (e.g. ski).
  • Slide 155
  • Slide 156
  • Stringlines provide the longest grade control reference. Principal advantage: a predetermined grade can be matched very accurately. Drawbacks: Elevation needs to be set by survey crew. Difficult to use on horizontal curves. Easily disturbed on the job site. Not typically used unless smoothness or grade elevation compliance is extremely important.
  • Slide 157
  • Slide 158
  • A mobile reference (commonly called a ski) can come in various styles, although they function in the same manner. This style is a tube with a wire stretched between the ends. The tube rides directly on the surface, and the grade sensor rides on the wire to detect changes in elevation. The pipe is available in several lengths (up to 15 m) to increase the length of the reference.
  • Slide 159
  • Slide 160
  • Another mobile reference is the floating beam. A series of shoes are attached to the bottom of the beam. The grade sensor rides directly on the midpoint of the beam. Again, several lengths available. This type of mobile reference typically results in a smoother pavement. It is more likely to ignore isolated changes in grade, such as a rock on the pavement.
  • Slide 161
  • Slide 162
  • One manufacturer developed a mobile reference that bridges over the paver. One beam senses the grade of the existing surface in front of the paver. Another beam rides on wheels behind the screed, sensing the elevation of the newly placed mat. Intermediate beams connect the two reference beams, and the grade sensor rides on one of them.
  • Slide 163
  • Slide 164
  • The third type of grade reference is the joint matching shoe. This reference is a short (0.3 m) ski that rides directly on the adjacent surface or curb. For base or binder layers, the mobile reference will provide a longer reference. For surface courses, the joint shoe will provide a matching elevation across the longitudinal joint, but the mobile reference is still better paving practice. Laser technology has also been successfully used on paving projects.
  • Slide 165
  • Grade Sensor - Side Arm Mount
  • Slide 166
  • Ask: Where should the grade sensor be positioned on the paver? Typically, rely on the paver manufacturers recommendations. The general preference for the location of the grade control sensor is somewhere between the third points of the leveling arm.
  • Slide 167
  • Grade Sensor - Screed End Mount
  • Slide 168
  • Sometimes the grade sensor is located at the tow point. When paving wide widths (when the tow arm is far from the reference ski) it is more common to hang the grade reference near the screed. In all cases, the sensor should be in front of the screeds pivot point.
  • Slide 169
  • Slide 170
  • The operation of the sensor should be checked regularly. When the paver is placing mix, the sensor indicator (either lights or a meter) should indicate movement. The tow point should also be moving in response to changes in elevation. Any changes should be smooth. The screed should not be constantly moving up and down.
  • Slide 171
  • Slide 172
  • Often, paving done with automatic screed controls is accomplished with a combination of grade control on one side of the screed and slope control determine the grade on the other side. Slope control is determined by a sensor attached to the cross beam between the two side arms of the screed.
  • Slide 173
  • Slide 174
  • Typically, the grade control sensor and its mobile reference is placed along the pavement centerline, and the slope sensor controls the slope at the outside edge. The grade sensor controls one side of the screed, and the slope control maintains the other side.
  • Slide 175
  • Slide 176
  • Adjustments in slope control must be done gradually, and only while the paver is moving.
  • Slide 177
  • Slide 178
  • For building a transition into a superelevated curve, it is good practice to supply the screed operator with a list of the station numbers and cross slopes, or have them visibly marked on the grade stakes. In superelevations, the grade sensor is operated automatically on one side of the machine, and the slope sensor is operated manually on the other. If automatic controls are being used on the paver, the screed operator should not change the angle of attack with the thickness control screws. The automatic controls will try to compensate with the tow point. Ask: Has anyone used automatic controls to construct a superelevated curve?
  • Slide 179
  • Yield-Thickness-Smoothness Base ConditionBase Condition Minimum ThicknessMinimum Thickness YieldYield SmoothnessSmoothness Controlling Yield versus Thickness or SmoothnessControlling Yield versus Thickness or Smoothness
  • Slide 180
  • Paving specs for HMA overlay projects are written a number of ways: yield, thickness, or smoothness A rough base can require more mix to achieve a minimum thickness. For minimum thickness spec, the mat is usually placed at a greater thickness than in the contract. A yield spec calls for a given amount of mix over the surface area (say, 60 kg per sq. meter). For this, the thick- ness requirement is an average depth, not a minimum depth. Smoothness specs are typically related to deviation from a straightedge (5 mm in 3 m), or in roughness (250 mm per 150 m of length). You cant control yield and thickness or smoothness. Establish beforehand which will be the controlling factor.
  • Slide 181
  • Types of Paving Planning PullsPlanning Pulls Mainline PavingMainline Paving Variable Width PavingVariable Width Paving TransitionsTransitions Temporary PavingTemporary Paving Echelon PavingEchelon Paving Night PavingNight Paving
  • Slide 182
  • There are various types of paving that will be encountered, along with concerns that need to addressed for each.
  • Slide 183
  • Slide 184
  • Ask: Who decides how to lay out the paving? (It can depend upon the specification.) Ask: When is the decision made about laying out the paving? (Well before the project begins.)
  • Slide 185
  • Slide 186
  • It is in both the contractors and agencys best interest to pave as wide as possible (within the given circumstances.) Having said that... The paving layout should be as practical as possible. The layout should keep the paving joints out of the eventual wheel paths. This longitudinal joint is in the wheelpath. Optimal joint spacing will place the surface course joint along the lane stripes. For placement of multiple lifts, the longitudinal joints should be staggered, or offset, from one another.
  • Slide 187
  • Slide 188
  • Mainline paving is typically 3.5 m wide, although greater widths are used to minimize paving joints.
  • Slide 189
  • Slide 190
  • The use of hydraulic strike-offs is not best practice on mainline paving. Texture and density must be the same as the rest of the mat. As mentioned earlier, the mat thickness must be higher on the strike-off area, since precompaction by the screed does not occur.
  • Slide 191
  • Slide 192
  • It is more common to see the shoulder paved with the driving lane, especially with hydraulically extendable screeds. One extension is used to pave the shoulder, and it can be set at a different cross slope than the main screed. It is good practice, in that it eliminates one longitudinal joint and an additional pass of the paver. One caution: If this combination is being used, careful attention must be given to the layout of the temporary and permanent striping.
  • Slide 193
  • Slide 194
  • With hydraulically extendable screeds, it is tempting to construct variable width sections on-the-fly. Be wary these changes affect the forces on the screed. The hydraulic strike-off may be used if texture and smoothness are matched. If it would replace handwork, it should be considered. For turnouts and tapers, do the mainline first to keep density and smoothness uniform. Then come back to do the tapers and turnouts. As mentioned earlier, transitions into superelevated curves need to be carefully planned. Temporary ramps and cross-overs need even more attention to grade and slope. Traffic usually goes too fast in construction zones, and these pavements often have sharper curves and slopes than normal.
  • Slide 195
  • Slide 196
  • Echelon paving is when two pavers are running next to one another. The cold joint between the two paving lanes is eliminated. Properly lapped and compacted, the longitudinal joint is usually difficult to see. Normally, the density in this joint will be similar to the density of the adjacent mat.
  • Slide 197
  • Slide 198
  • Night paving is becoming more common, and for urban area highways it is often standard practice. At night, everything typically needs more attention. Limited visibility, cooler weather, different paving and trucking crewsthese plus many other variables contribute to an increased need to attend to details. Sufficient supplemental lighting for all equipment is a big need, especially pavers and rollers. Is there a cost differential?
  • Slide 199
  • Slide 200
  • Safety is probably the biggest issue in night paving. Traffic and driving habits always a concern. Drunk drivers -- maybe even more so.
  • Slide 201
  • Slide 202
  • Equipment operators must be aware of all personnel moving around them. Equipment maintenance is particularly important, since parts and replacement equipment may not be available. Testing personnel need to be extra careful at night. A new mat looks quite different at night. Personnel must be aware what a quality pavement looks like under different lighting.
  • Slide 203
  • Slide 204
  • All personnel must wear safety vests and be clearly visible. Temperatures are an issue. Hard to find people who will work at night. If nondestructive testing is used for QC, may not have to come back. Ask: Has anyone had experience with night paving? How are things different? Good experiences? Bad experiences?
  • Slide 205
  • Operating Techniques Determining Paver SpeedDetermining Paver Speed Warming up MachineWarming up Machine Positioning on JointPositioning on Joint Nulling the ScreedNulling the Screed Initial SettingsInitial Settings Charging Hopper, Tunnels, ScreedCharging Hopper, Tunnels, Screed
  • Slide 206
  • The next three slides list the sequence of paving operations.
  • Slide 207
  • Operating Techniques (Cont'd) Pulling off JointPulling off Joint Re-checking SettingsRe-checking Settings Speed of PaverSpeed of Paver Checking YieldChecking Yield Truck ExchangesTruck Exchanges Folding Hopper WingsFolding Hopper Wings
  • Slide 208
  • (Sequence of paving operations, continued)
  • Slide 209
  • Operating Techniques (Cont'd) HMA Level in HopperHMA Level in Hopper Auger OperationAuger Operation Flow Gate PositionFlow Gate Position Raking and LutingRaking and Luting Traffic ControlTraffic Control Concluding PavingConcluding Paving
  • Slide 210
  • (Sequence of paving operations, continued)
  • Slide 211
  • Balancing Production Trucking Compaction PavingHMAFacility
  • Slide 212
  • As has been stressed, balance is the key to a quality paving project. Production at the facility, delivery to the job site, laydown by the paver, and compaction by the rollers must all be in tune to produce the highest quality pavement. Schedule ahead of time to balance all of these rates. Understanding the variables that can affect each rate is the key the adjusting the others when changes occurand be assured, changes will occur. Keeping the rates in balance will lead to a better pavement, and more production.
  • Slide 213
  • Slide 214
  • Use this form to complete a paver production problem. Other techniques are availableuse this one for consistency. This is a mainline paving problem. Other operations (short pulls, many transverse joints, more handwork) will decrease the paving rate, but schedules for slower operations are as important as high production jobs. Devote about 15 minutes to this problem.
  • Slide 215
  • Slide 216
  • The tractor must be warmed up and the screed heated before beginning paving. The screed should not be in contact with the mat during heating. This is a good time to give all of the equipment a thorough going-over to make sure everything is ready.
  • Slide 217
  • Starting Blocks
  • Slide 218
  • After the temporary joint is removed, cleaned, and tacked, the paver is ready to be positioned. Straightedge the cold joint to be sure the new joint will be level with the previous mat. Line up the screed far enough back for the strike-off to be a little behind the cold joint. Starting blocks must be placed under the screed. The blocks should be as thick as the roll down of the mat.
  • Slide 219
  • A good rule of thumb is to raise the screed 20 percent more than the compacted thickness.
  • Slide 220
  • A good rule of thumb is to raise the screed 20 percent more than the compacted mat thickness. SMA mixes will have only about half as much roll down change in height between compacted and uncompacted thickness as conventional HMA.
  • Slide 221
  • Slide 222
  • The paver must be properly aligned to begin paving. The first pass will determine the layout of all subsequent passes, so it must be effectively and visibly referenced. The steering guide (painted orange here) must be positioned so the operator can see the guide, any reference on the pavement, and some distance down the mat.
  • Slide 223
  • Slide 224
  • (Close-up of steering guide.)
  • Slide 225
  • Slide 226
  • The screed must be properly nulled prior to paving. Typically, this is done by turning the thickness control screws in both directions until the screw shows some free play. Nulling is done on one side of the screed, then the other, and then the first side is checked again. Freeing up one side often induces some force on the other side.
  • Slide 227
  • Slide 228
  • The initial angle of attack is induced into the screed. Typically, this is accomplished by turning the thickness control screws one full turn to increase depth. The angle of attack needs to be induced on both sides. Extensions also have to be adjusted, whether they are extended or kept at the main screed.
  • Slide 229
  • 1/4 Hydraulic Strike-Off With Vertical Blade
  • Slide 230
  • The strike-off must be in proper position. Manufacturers recommendations will determine its exact location.
  • Slide 231
  • Lead Crown - 3 mm Greater Than Tail Crown
  • Slide 232
  • If the pavement is to be crowned, the amount of crown must be induced into the screed. Typically, the lead crown is 3 mm greater than the tail crown. If hydraulic extensions are being used, any change in slope must also be made prior to starting.
  • Slide 233
  • Slide 234
  • If automatic material feed sensors are being used, they must be in place and operating properly. The proper location is as close to the end of the augers as possible, just inside the end gate. Mention again the primary key to a smooth pavement layermaintaining a level head of material in front of the screed.
  • Slide 235
  • Slide 236
  • All of the grade control sensors and reference systems must be properly set up prior to paving.
  • Slide 237
  • Slide 238
  • As mentioned earlier, the grade sensor is placed about one-half of the way up the tow arm in front of the screed. The tow point is typically started in the middle of the cylinder stroke. If thickness is being controlled manually, the tow points are pinned and changes are made with the thickness control screws.
  • Slide 239
  • Slide 240
  • Now the paver is ready to be charged with mix, but dont forget truck rule #1 When initially charging the paver, sometimes the second or third truckload is used because it may be hotter. Flood the hopper, and begin pulling mix to the augers with the slat conveyors.
  • Slide 241
  • Slide 242
  • Enough mix must be run back to provide a uniform head of material across the screed and just up to the auger shafts.
  • Slide 243
  • Gates too HIGH - augers overloaded Gates too LOW - insufficient material supply Correct adjustment - uniform material volume/flow
  • Slide 244
  • Not getting the head of material set properly in the beginning will only lead to problems later on. Problems with flow gate height are highlighted here. If extensions are being used, some mix may need to be shoveled to the corners of the end plates. Again the key (and it cannot be mentioned too much)create a uniform head of material and keep it constant.
  • Slide 245
  • Slide 246
  • Once the screed is charged, the sensors are switched from standby to on. Once all the settings are in place, the paver moves forward. Start fairly rapidly, and then move quickly up to paving speed. (Constructing the transverse joint is covered in the next section.) Note: typical operation would have more blocks.
  • Slide 247
  • Re-Checking Settings Is thickness okay?Is thickness okay? Is cross slope okay?Is cross slope okay? Is mat texture okay?Is mat texture okay?
  • Slide 248
  • Once you get moving, recheck all the settings, and adjust if necessary. Mat texture with SMA and Superpave mixes will look much more open than conventional mixes. However, still look for a uniform appearance. (The next few slides provide opportunities for discussion of these points.)
  • Slide 249
  • Slide 250
  • Is the slope correct? How is the mat texture? Some screed adjustments may be needed in this photo.
  • Slide 251
  • Slide 252
  • How about thickness? In this case, thickness is being changed by adjusting the position of the grade control sensor. Rememberthickness changes need time to fully occur in the mat.
  • Slide 253
  • Slide 254
  • Before you began paving, you calculated an ideal paver speed. This speed should be checked and maintained. Remember, balance of all the production rates is the key to quality pavement. Ask: If you have a number of trucks waiting, should you speed up the paver to empty them? (Not necessarily -- the roller operator may not be able to achieve compaction if the paver speeds ahead.) Rates of placement for SMA and Superpave designed mixes do not typically have to be any different than those for conventional HMA.
  • Slide 255
  • Check Yield Occasionally
  • Slide 256
  • If you are paving by yield specification, periodically check how far each truckload is going. Every truckload is typically too often; it may result in too many thickness changes. A longer distance (say, 300 m) may be more appropriate for checking yield.
  • Slide 257
  • Slide 258
  • As we discussed earlier, the truck exchanges are important to uniform production. Once the truck is empty, another should be ready to immediately back into the paver.
  • Slide 259
  • Slide 260
  • Ask: Once the truck leaves, should you immediately fold the wings? To minimize segregation, fold the wings as seldom as possible. Just enough to keep the mix in the wings from getting so cool that it wont go through the paver. How often will depend on delivery rate, mix temperature, and the air temperature. If you fold the wings, do so while the hopper is fairly full.
  • Slide 261
  • Slide 262
  • What you do not want to do is pull the hopper so low that you can see the slat conveyors.
  • Slide 263
  • Slide 264
  • Ask: What do you do if the next truck is not ready? (Stop the paver.) Do not slowly pull it forward and run the hopper empty. Wait with a full hopper. This keeps the mix and paver warm, and the screed charged with a full head of material. When the truck arrives, have the bed raised and ready to flood the hopper. Start the paver, getting it to paving speed as quickly as possible. When the truck is picked up by the paver, release the gate and flood the hopper. If the paver frequently waits, a paver speed adjustment may be necessary.
  • Slide 265
  • Slide 266
  • Remember, you want to keep a consistent head of material in front of the screed. The augers and conveyors should be operating almost continuously.
  • Slide 267
  • Slide 268
  • The flow gates may need adjusting to achieve the desired balance and uniformity. (This photo is for illustration of the flow gates, not to show a good mix level in the hopper.)
  • Slide 269
  • Slide 270
  • Ask: How much luting has to be done in this operation? Properly set up, the transverse and longitudinal joints will need minimal attention. (Joint construction will be covered in detail in the next module.)
  • Slide 271
  • Slide 272
  • Another project set up well, minimizing luting.
  • Slide 273
  • Slide 274
  • While it is not covered in detail in this course, traffic control is important to maximize the safety of everyone involved in the project, and to the motorists passing through the work zone.
  • Slide 275
  • Slide 276
  • Some contractors may be tempted to develop their own traffic control signs. It is best to follow the Manual of Uniform Traffic Control Devices (MUTCD).
  • Slide 277
  • Slide 278
  • The end of the paving results in a transverse joint, either temporary or permanent. (This will be covered in the next module.) Ask the contractors: What is a big issue when you are getting close to where you have to tie into the existing pavement? (Should we order just one more truckload of mix?) Yes and no answers both have consequences. The paver should be parked in a secure area, and cleaned of accumulated mix. Grease the paver while it is hot, and check for wear and loose parts.
  • Slide 279
  • Module 5 Objectives Review Objective of HMA PlacementObjective of HMA Placement Components and FunctionTractor and Screed UnitComponents and FunctionTractor and Screed Unit Operational Principles of ScreedOperational Principles of Screed Grade and Slope Control SystemsGrade and Slope Control Systems Types Functions Capabilities
  • Slide 280
  • Ask for specific responses to these questions. Use questions that will help draw out responses, such as: What do you remember? How is it related to what you know? How will you use this information in the future?
  • Slide 281
  • Module 5 Objectives (cont.) Yield-Thickness-Smoothness RelationshipYield-Thickness-Smoothness Relationship Types of Paving and When to Use ThemTypes of Paving and When to Use Them Night PavingNight Paving Paver MaintenancePaver Maintenance Operating TechniquesOperating Techniques
  • Slide 282
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