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 operation–forces on the paver must be in balance to produce a uniform, dense, and smooth HMA mat. Principles of paver operation are applicable to a variety of HMA mix types.
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. Make sure you are fully informed regarding safety rules, regulations, and procedures on the jobsite. Play it safe. Make sure you wear your safety vest at all times and other required safety equipment. Contact your organization's safety director if you are unsure.
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 HMA 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 HMA mat.
The tractor unit has its own engine that provides the power to move the paver 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 35-85 psi (240 and 585 kPa). The tires may be ballasted with a calcium chloride solution to provide additional traction and reduce bounce in the paver.
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.
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.
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.
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.
The paver hopper is where the HMA 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. How often should the wings be folded? As often enough to keep the HMA mix ‘alive’. Be consistent in this folding application – will be discussed more later.
Flashing or overflow guards on the front of the hopper keep HMA mix from spilling onto the pavement.
At the bottom of the hopper are slat conveyors. They transport the HMA mix from the hopper back through the tunnel in the tractor unit to the augers. The slat conveyors move independently, each of them feeding one side of the screed. 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.
The flow gates are adjustable to regulate the amount of HMA 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.
At the back of the tractor unit are screws or augers. The HMA is brought from the hopper by the slat conveyors, and the augers spread the HMA laterally in front of the screed. As with the slat conveyors, the augers operate independently. This set of augers can be raised or lowered depending on the type of HMA placed, normally set about 2 inches (50 mm) above the height of the screed. This option helps reduce centerline segregation.
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 approximately at the center of the auger shaft. The HMA 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. Once the level is set, the goal should be to keep the head of material level plus or minus 1 inch (25 mm) throughout the paving.
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.
The next item to discuss is the operational principles of the screed.
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.
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 9 ft (3 m) long, the paver would have to move about 45 feet (15 m) before the screed fully reacted to a change in thickness input into the system.
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 1 inch (25 mm) change in the tow point will produce a 1/8 inch (3 mm) change in the leading edge of the screed.
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 main forces acting on the screed and how thick it places the mat are a function of the items listed on this slide.
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 pre-compaction effort into the HMA, requiring more compaction effort from the roller.
How does the head of material affect HMA 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.
If the augers are under loaded, 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.
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.
What can cause the most problems during paving? (Stopping the paver–as here, waiting for trucks). 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 HMA in the paver (head of material, conveyors, hopper) can cool, requiring more force to move the paver. Screed can settle, causing bump in the HMA mat. Re-starting adjustments to get paver “in tune.” Keep the forces on the screed in balance to maintain a uniform HMA mat.
We have just spent a considerable amount of time discussing the proper operation of the screed with a focus on quality, consistency, and uniformity. What happens when the paver runs out of HMA mix? It was taught for years to never stop the paver, under any circumstances. That teaching was flawed and is no longer being taught. As we have learned, by changing the speed of the paver we change the forces on the screed and affect the thickness of the HMA mat. The newest concepts make more sense, if you can run balanced, stop until you are unbalanced. Never let the material level go below the hopper flow gates. Keeping the speed of the paver balanced with the delivery rate is vitally important. Sometimes, slowing the paver down a little will help even out the truck delivery rate.
The next topic of discussion...
Who is responsible? The Screed Operator!
If the screed operator stuck the HMA mat this frequently on this project, what would most likely happen? The mat thickness would be adjusted too frequently. 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 HMA mat can be useful during on-the-job training to show how long it takes for the screed to react.
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.
Has anyone had Charles Atlas as the screed operator? Under manual screed operation, the screed operator’s 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.
Automatic screed controls are used to produce a smoother HMA 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 HMA pavement.
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).
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.
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 50 feet (15 m)) to increase the length of the reference.
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 HMA pavement. It is more likely to ignore isolated changes in grade, such as a rock on the pavement.
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 HMA mat. Intermediate beams connect the two reference beams, and the grade sensor rides on one of them.
The third type of grade reference is the joint matching shoe. The photo on the left shows a contacting type ski 1 ft (0.3 m) ski that rides directly on the adjacent surface or curb. The photo on the right shows an non-contacting ultrasonic sensor. The advantage of the non-contacting sensor is that it is more reliable and less likely to be affected by loose rocks or workers. 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.
Where should the grade sensor be positioned on the paver? Typically, rely on the paver manufacturer’s recommendations. The general preference for the location of the grade control sensor is somewhere between the third points of the leveling arm.
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 screed’s pivot point.
The operation of the sensor should be checked regularly. When the paver is placing HMA 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 – if it is the sensitivity may be set too high.
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 occur–and be assured, changes will occur. Keeping the rates in balance will lead to a better pavement, and more production.
Block 18 Placement 13
Senior/Graduate HMA Course Placement of HMA MixesConstruction Placement 1
Safety - First, Last and Always... Don’t depend on drivers to avoid you. Play it safe. Watch your back and your buddies.Construction Placement 2
Tractor Hopper Depth Crank Screed Push Rollers Tow Point Conveyor Augers Side Arms sConstruction Placement 3
AUGER CONVEYOR TRACTOR HOPPER DEPTH CRANK SIDE ARMPUSH ROLLER TOW POINT SCREED Construction Placement 4
Material Flow Hopper Wings Proportional Feed Control Slat Conveyors Hopper Gates Augers Screed PlateConstruction Placement 17
Operational Principles of theScreed • Self-leveling Concepts • Screed Response versus Distance • Forces Acting on a ScreedConstruction Placement 18
Self Leveling - Rubber Tired Paver Line of Pull Rear Drive Tires Front Bogie WheelsConstruction Placement 19
L Direction of Paving Screed Path 87% 96% 98% 99% 63% Step Amplitude = 100% Tow Point Path 0 1L 2L 3L 4L 5L 6L L = Leveling Arm Length Detail of Screed PathConstruction Placement 20
1 inch movement at Tow Point 1/8 inch movement at Leading Edge of ScreedConstruction Placement 21
Main Forces Acting on Screed 1. Speed of Paver 2. Head of Material - Shear resistance 3. Angle of Attack 4. Other Forces – Pre-compaction – Screed WeightConstruction Placement 22
Screed Depth Crank F3 Tow Point Pivot Point F2 F1Construction Placement 23
Correct Depth of Mat Maintained Constant Head of Material Volume Screed Rises Due to Excess Material Forced Under Nose of Screed Head of Material Volume Too High Screed Settles Due to Inadequate Supporting Material Head of Material Volume Too LowConstruction Placement 24
HOT STOP Concept• If constant material flow cannot be maintained, before HMA mix drops in hopper below gates...• “Hot Stop” the paver as quickly as possible• Wait for next truck• Pick up truck on run and resume speed as quickly as possible.Construction Placement 28
Screed Control Systems • Who’s Responsible for Changes • Sticking the Mat • Manual Controls • Automatic ControlsConstruction Placement 29