Best Practices for Asphalt Paving
Content
Best Practices for Asphalt Paving Information for this section was obtained from the Hot-Mix Asphalt Paving Handbook, published by the US Army Corps of Engineers (Document UN-13 CEMP-ET, July 1991) Surface Preparation Placement Techniques Joint Construction Compaction Cold Weather Paving Surface Preparation The performance of a hot-mix asphalt (HMA) pavement is dependent on the condition of the underlying surface.
HMA overlays
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Over HMA
Remove and replace failed pavement areas Properly patch potholes Clean and seal cracks Fill ruts, or remove rutted pavement layer Level a rough, uneven asphalt surface by milling or with placement of an asphalt leveling course. Clean the pavement surface using multiple passes of a mechanical broom.
Note: Information about asphalt pavement maintenance can be obtained in the Asphalt Pavement Maintenance Handbook, Mn/DOT Manual No. 2000-04. Contact the Mn/DOT Office of Research and Strategic Services to request a copy.
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Over PCC
Cut out, remove, and replace any severely distressed areas in the PCC slabs using full-slab repair techniques with PCC or HMA Perform corrective work on the underlying subbase or subgrade material
Repair severely spalled areas at joints using partial-depth slab replacement methods Stabilize rocking slabs Perform crack and seat, break and seat, or rubblizing, depending on the condition of the underlying PCC pavement Remove old sealant materials from joints that are poorly sealed Clean the PCC pavement surface completely with mechanical brooms and air blowing and/or water flushing where needed.
Base Preparation
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Prepare underlying layer to meet all requirements for moisture content, density, and smoothness. Test roll aggregate base layers to determine that base can support the weight of the haul traffic. Place any required prime coat at least 48 hours before beginning paving.
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Tack Coat: A tack coat is required to ensure the bond between an existing pavement surface and new asphalt overlay. Uniformity of application and use of the proper application rate is very important. Mn/DOT specification 2357 requires an application rate of 0.05 gal/sy for cutback asphalt and undiluted asphalt emulsion (as supplied from the refinery), and 0.02 gal/sy for diluted asphalt emulsion (with water added in the field).
Back to top
Placement Techniques When inspecting the asphalt paving operations, note the following: Trucks dumping mix directly into the paver should stop short of the paver, and let the paver continue moving forward to come in contact with the truck. This eliminates indentations of the screed in the mix. Before the tailgate is open, the driver should slowly raise the truck bed until the mix in the truck bed begins to slide against the tailgate. This will reduce the amount of segregation. If a windrow elevator is used to gather mix left on the roadway, the slat conveyor should pick up all of the mix without leaving any on the underlying surface.
The windrow should be sized to fill the paver hopper 25-75 percent full. The hopper should contain enough mix so that the slat conveyors are not visible on the bottom of the hopper, but not so full that mix runs out in front of the hopper. After delivering the mix, the truck should pull away, and the paver stopped as quickly and smoothly as possible without jerking the paver. Enough mix should remain in the hopper to be at least at the level of the bottom of the flow gates. In no case should the hopper be emptied to the point that the slat conveyors are visible. When the paver is stopped to allow the next truckload of mix to move into position, the paver wings can be folded, but only when necessary to prevent buildup of cold mix in the hopper corners. The operator should repeatedly bang the wings as they are emptied, and dump the wings into a relatively full paver hopper. Dumping the wings into a relatively full hopper may result in some mix spilling out of the front of the hopper; rubber flaps should be used to contain as much mix as possible. Once the operator begins emptying the new truckload of mix into the hopper, the paver should be brought up to paving speed as quickly as is feasible and operated at a constant speed to meet the delivery of the next truck. This will keep the head of material in front of the screed as constant as possible. The paver should stop during the truck exchange. When the paver continues to move forward while one truck is leaving and the second truck is moving into position, the amount of mix in the hopper will be drawn down, possibly to the point that the hopper is emptied. This reduces the amount of mix at the augers, in turn causing the screed to fall and decrease the mat thickness. When the newly delivered mix is emptied into the hopper and is pulled back to the augers by the slat conveyors, the large amount of mix against the screed will cause the screed to rise, increasing the thickness of the mat. Slowing the paver between truckloads of mix while emptying the hopper causes significant changes in the forces acting on the screed and accompanying changes in the mixture layer. The flow gates on each side of the machine on the back of the hopper should be set at a height to permit the slat conveyor and corresponding auger to operate as close to 100 percent of the time as possible. The key to a smooth layer of mix is a constant head of material in front of the screed. The key to a constant head of material is a constant paver speed and continuous operation of the paver augers. A good rule of thumb: the amount of material in front of the screed should be located near the center of the auger shaft. If the paver is equipped with automatic flow control devices, the flow control equipment should be set at a location near the end plate in order to maintain a constant head of mix in front of the screed by causing the auger to run continuously. The location of the device is important to prevent too much or too little mix from being carried at the outside edge of the
screed. If the paver screed is being operated under manual control, the screed operator should not change the angle of attack of the screed by turning the thickness control cranks except to increase or decrease the thickness of layer being placed. After the controls are turned, it takes five times the length of the tow arm on the paver before the screed completes the input change in thickness. If the paver is being operated under automatic grade and slope control, the screed operator should not attempt to change the angle of attack of the screed by turning the thickness control cranks. The automatic controls, if they are mounted in the proper position, will simply correct any changes made in the angle of attack of the screed by changing the position of the tow point where the screed is attached to the tractor in order to reestablish the original screed angle. Back to top
Joint Construction Please visit the National Center for Asphalt Technology and select report #90-3.
Compaction Please visit the National Center for Asphalt Technology and select report #94-1. Back to top Cold Weather Paving One of the biggest problems in Minnesota's bituminous pavements is a lack of in-place density due to late season paving practices. When bituminous materials are placed in cool weather, they are difficult to compact properly because the asphalt stiffens too rapidly. A computer tool (PaveCool) has been developed to assist contractors, inspectors and engineers to make rapid decisions regarding cool-weather paving. The user enters the time of day, the date and the latitude of the paving job. Next, the type of mixture is entered along with the type of surface being paved. The surface temperature, air temperature, wind speed, lift thickness and mixture delivery temperature are then entered. The final input is the amount of cloud cover. A heat flow model is used to compute the temperature drop in the mat and the time it takes for the asphalt mix to cool from its delivery temperature to 180 degrees Fahrenheit (82.2 degrees Celsius). If the user feels that there is an inadequate amount of time available to
compact the mixture, options can be explored to extend the time. For instance, increasing the lift thickness or mix temperature will increase the window of time for effective compaction of the pavement. Other asphalt paving research tools are available from the Office of Materials and Road Research.
HMA overlays
o
Over HMA
Remove and replace failed pavement areas Properly patch potholes Clean and seal cracks Fill ruts, or remove rutted pavement layer Level a rough, uneven asphalt surface by milling or with placement of an asphalt leveling course. Clean the pavement surface using multiple passes of a mechanical broom.
Note: Information about asphalt pavement maintenance can be obtained in the Asphalt Pavement Maintenance Handbook, Mn/DOT Manual No. 2000-04. Contact the Mn/DOT Office of Research and Strategic Services to request a copy.
o
Over PCC
Cut out, remove, and replace any severely distressed areas in the PCC slabs using full-slab repair techniques with PCC or HMA Perform corrective work on the underlying subbase or subgrade material
Repair severely spalled areas at joints using partial-depth slab replacement methods Stabilize rocking slabs Perform crack and seat, break and seat, or rubblizing, depending on the condition of the underlying PCC pavement Remove old sealant materials from joints that are poorly sealed Clean the PCC pavement surface completely with mechanical brooms and air blowing and/or water flushing where needed.
Base Preparation
o
Prepare underlying layer to meet all requirements for moisture content, density, and smoothness. Test roll aggregate base layers to determine that base can support the weight of the haul traffic. Place any required prime coat at least 48 hours before beginning paving.
o
o
Tack Coat: A tack coat is required to ensure the bond between an existing pavement surface and new asphalt overlay. Uniformity of application and use of the proper application rate is very important. Mn/DOT specification 2357 requires an application rate of 0.05 gal/sy for cutback asphalt and undiluted asphalt emulsion (as supplied from the refinery), and 0.02 gal/sy for diluted asphalt emulsion (with water added in the field).
Back to top
Placement Techniques When inspecting the asphalt paving operations, note the following: Trucks dumping mix directly into the paver should stop short of the paver, and let the paver continue moving forward to come in contact with the truck. This eliminates indentations of the screed in the mix. Before the tailgate is open, the driver should slowly raise the truck bed until the mix in the truck bed begins to slide against the tailgate. This will reduce the amount of segregation. If a windrow elevator is used to gather mix left on the roadway, the slat conveyor should pick up all of the mix without leaving any on the underlying surface.
The windrow should be sized to fill the paver hopper 25-75 percent full. The hopper should contain enough mix so that the slat conveyors are not visible on the bottom of the hopper, but not so full that mix runs out in front of the hopper. After delivering the mix, the truck should pull away, and the paver stopped as quickly and smoothly as possible without jerking the paver. Enough mix should remain in the hopper to be at least at the level of the bottom of the flow gates. In no case should the hopper be emptied to the point that the slat conveyors are visible. When the paver is stopped to allow the next truckload of mix to move into position, the paver wings can be folded, but only when necessary to prevent buildup of cold mix in the hopper corners. The operator should repeatedly bang the wings as they are emptied, and dump the wings into a relatively full paver hopper. Dumping the wings into a relatively full hopper may result in some mix spilling out of the front of the hopper; rubber flaps should be used to contain as much mix as possible. Once the operator begins emptying the new truckload of mix into the hopper, the paver should be brought up to paving speed as quickly as is feasible and operated at a constant speed to meet the delivery of the next truck. This will keep the head of material in front of the screed as constant as possible. The paver should stop during the truck exchange. When the paver continues to move forward while one truck is leaving and the second truck is moving into position, the amount of mix in the hopper will be drawn down, possibly to the point that the hopper is emptied. This reduces the amount of mix at the augers, in turn causing the screed to fall and decrease the mat thickness. When the newly delivered mix is emptied into the hopper and is pulled back to the augers by the slat conveyors, the large amount of mix against the screed will cause the screed to rise, increasing the thickness of the mat. Slowing the paver between truckloads of mix while emptying the hopper causes significant changes in the forces acting on the screed and accompanying changes in the mixture layer. The flow gates on each side of the machine on the back of the hopper should be set at a height to permit the slat conveyor and corresponding auger to operate as close to 100 percent of the time as possible. The key to a smooth layer of mix is a constant head of material in front of the screed. The key to a constant head of material is a constant paver speed and continuous operation of the paver augers. A good rule of thumb: the amount of material in front of the screed should be located near the center of the auger shaft. If the paver is equipped with automatic flow control devices, the flow control equipment should be set at a location near the end plate in order to maintain a constant head of mix in front of the screed by causing the auger to run continuously. The location of the device is important to prevent too much or too little mix from being carried at the outside edge of the
screed. If the paver screed is being operated under manual control, the screed operator should not change the angle of attack of the screed by turning the thickness control cranks except to increase or decrease the thickness of layer being placed. After the controls are turned, it takes five times the length of the tow arm on the paver before the screed completes the input change in thickness. If the paver is being operated under automatic grade and slope control, the screed operator should not attempt to change the angle of attack of the screed by turning the thickness control cranks. The automatic controls, if they are mounted in the proper position, will simply correct any changes made in the angle of attack of the screed by changing the position of the tow point where the screed is attached to the tractor in order to reestablish the original screed angle. Back to top
Joint Construction Please visit the National Center for Asphalt Technology and select report #90-3.
Compaction Please visit the National Center for Asphalt Technology and select report #94-1. Back to top Cold Weather Paving One of the biggest problems in Minnesota's bituminous pavements is a lack of in-place density due to late season paving practices. When bituminous materials are placed in cool weather, they are difficult to compact properly because the asphalt stiffens too rapidly. A computer tool (PaveCool) has been developed to assist contractors, inspectors and engineers to make rapid decisions regarding cool-weather paving. The user enters the time of day, the date and the latitude of the paving job. Next, the type of mixture is entered along with the type of surface being paved. The surface temperature, air temperature, wind speed, lift thickness and mixture delivery temperature are then entered. The final input is the amount of cloud cover. A heat flow model is used to compute the temperature drop in the mat and the time it takes for the asphalt mix to cool from its delivery temperature to 180 degrees Fahrenheit (82.2 degrees Celsius). If the user feels that there is an inadequate amount of time available to
compact the mixture, options can be explored to extend the time. For instance, increasing the lift thickness or mix temperature will increase the window of time for effective compaction of the pavement. Other asphalt paving research tools are available from the Office of Materials and Road Research.
