460.6 Paving Operations
- 1 460.6.1 Traffic Control
- 2 460.6.2 Surface Preparation
- 3 460.6.3 Weather Conditions
- 4 460.6.4 Spreading
- 5 460.6.5 Hand Spreading
- 6 460.6.6 Material Acceptance
- 7 460.6.7 Compaction
- 8 460.6.8 Transverse Joints
- 9 460.6.9 Longitudinal Joints
- 10 460.6.10 Density Samples
460.6.1 Traffic Control
Warning signs of the type shown on the plans are to be furnished and erected by the contractor. Due to the relatively fast movement of paving operations, most traffic hazards will be concentrated around the placement area. Traffic should be directed through the construction zone by warning signs and flaggers in a way that provides maximum safety for the workers and traffic and that provides the least interruption of work. Another concern that should be addressed is how the haul trucks will enter and leave the work zone. Signing must be placed only when construction is active or when a hazard exists and promptly removed after work moves on. Special attention should be given to the use of the proper warning devices during night operations.
460.6.2 Surface Preparation
The performance of a pavement under traffic is directly related to the condition of the underlying surface. The extent of the pavement repair will be determined before the project begins and may be included as part of the contract. In some cases, District Maintenance may perform minor pavement repairs, such as the removal of crack and joint sealant and the placement of a leveling course or spot wedging before the project begins. Otherwise, repair of the existing asphalt or PCC pavement shall be completed by the contractor as required by the contract and in accordance with Standard Specification Section 613.
Any surface that mix is to be placed on is referred to as the subgrade. The subgrade may be soil, an aggregate base, an existing pavement, or the previously placed lift. Typically, bituminous mix is not placed directly on a soil subgrade. Instead, the soil subgrade is prepared in accordance with Standard Specification Section 209 and some type of base course constructed in accordance with the applicable Standard Specifications. The base course is then primed. If the subgrade is the existing pavement, any necessary repairs should be made and the surface should be cleaned and tacked. If the subgrade is the previously placed lift, the surface should be cleaned and tacked.
The subgrade must be able to support the haul trucks without rutting. If rutting occurs during the paving operation, the subgrade should be reworked and stabilized. Soft spots must be removed. More detail is provided in Tack Coat and Prime Coat.
418.104.22.168 Existing Asphalt Pavement
The condition of the existing asphalt pavement will determine the degree of preparation needed. All existing pavement areas where severe load-related distress has occurred shall be removed and replaced, including the subgrade and/or base course if necessary. Subsurface drainage should be installed as needed. Localized failures (potholes) shall be patched properly. Badly cracked sections, especially those with pattern cracking, must be patched or removed and replaced. Any asphalt pavement that is removed must be replaced with asphalt. Large cracks shall be cleaned and sealed to a level just below the pavement surface. Excessive amounts of crack sealant could create a bump in the new pavement during the compaction process. In small cracks, the sealant may not penetrate into the crack and instead pool at the surface.
422.214.171.124 Surface Treatments
If extensive cracking has occurred, a surface treatment may be more practical than filling individual cracks. The cracks should be cleaned, if possible, and the surface treatment applied when the pavement surface is clean and dry. Surface treatments are discussed in more detail in the Surface Treatments.
4126.96.36.199 Leveling Course and Spot Wedging
A leveling course, also known as a wedge or scratch course, may be required by the contract. The purpose of a leveling course is to eliminate any irregularities in the existing pavement surface by placing a layer of mix with variable thickness prior to resurfacing. The mix is placed thicker in low areas and thinner in high areas. However, due to differential compaction, the thicker areas tend to compact to a higher degree than the thinner areas. As a result, the low areas still exist to some extent and multiple leveling courses may need to be placed in order to obtain a smooth surface. The mix is to be uniformly spread and compacted to the desired grade and cross section. Rigid thickness control is required and only minor segregation can be accepted. Leveling courses are generally placed very thin (minimum of 1 in.) Therefore, the reaction of the mat to compaction must be watched very closely. Spot wedging may be necessary prior to placing the leveling course to compensate for small areas, such as dips, that are in extremely bad condition. Spot wedging may require feather edging. Typically, a BP or SP125 mix is used for the leveling course and spot wedging. A pneumatic tire roller shall be used as the initial (breakdown) or intermediate roller on leveling courses.
Milling is the preferred method of removing irregularities in the existing pavement surface, as opposed to placing a leveling course. If the milling machine is equipped with automatic grade and slope controls, a level surface can be produced in one pass over the existing surface. In addition, if the milled surface is properly cleaned, its texture can enhance the bond and provide friction between the new and old pavements. Additional tack may be required to ensure an adequate bond because of the increased surface area of the milled pavement (from the grooves left by the cutting teeth on the milling machine).
4188.8.131.52 Existing Portland Cement Concrete Pavement (PCCP)
Any severely distressed areas in the PCCP shall be removed and replaced with either Portland cement concrete (PCC) or asphalt using full-depth slab replacement techniques. If necessary, the subgrade and/or base course should also be repaired. Any severely spalled areas at the joints shall be repaired with PCC using partial-depth slab repair methods. Rocking slabs should be stabilized. Joint sealant and filler material should be removed to a level below the surface of the existing PCCP. In the case of badly damaged joints, the sealant and filler material should be removed and the joints cleaned. Then, the joint should be sealed to a level below the surface of the existing PCCP. A bump may occur in the asphalt overlay due to excess sealant. A leveling course, if necessary, may be placed over existing PCCP.
460.6.3 Weather Conditions
The Standard Specifications put certain weather restrictions on the placement of bituminous mixes.
4184.108.40.206 Wet or Frozen Surface
Mix must not be placed on a wet or frozen surface. Mix that is placed on a wet or frozen surface will cool very quickly because of rapid heat transfer from the mix to the underlying surface. The required density will be difficult to achieve because of the low mix temperature. As a result of the lack of compaction and corresponding high air void content, the pavement will perform poorly. Sometimes, the pavement “sweats” as a result of the dew point exceeding the pavement temperature. This generally occurs on a warm day immediately following a cold day.
It is the contractor’s responsibility to start and stop the plant. However, production should stop when rain starts. Loaded trucks may be placed if standing water is not visible on the existing surface. Tickets issued for loads that were not placed are void and an appropriate notation will be made on the ticket.
Placement of mix under these conditions does not relieve the contractor of the responsibility for all mix to be in satisfactory condition. If the contractor insists on laying mix contrary to these provisions, the contractor is doing so at their own risk. An order record will be issued immediately, informing the contractor that the mix placed is rejected.
4220.127.116.11 Temperature Requirements
There are no temperature requirements for BB, BP, or Superpave mixes. However, the air, surface, and mix temperatures will be checked for quality assurance purposes in accordance with MoDOT Test Method TM-20.
The subgrade must be dry, clean, and free of all foreign material before the tack or prime coat can be applied. Generally, a prime coat is applied to the base course and a tack coat is applied to the existing pavement or the preceding lift. A tack coat must be properly applied between each lift.
Just before application of the tack or prime coat, the amount of material in the distributor tank should be determined by using the calibrated dipstick. The distributor should be level and the temperature of the material checked and recorded to ensure that it is within the desired application range. The temperature is also needed so that the quantity of material that has been used can be adjusted for payment. After the material has been applied to the surface, the amount remaining in the tank should be determined.
Once the tack or prime coat has been applied, the mix should be spread and struck off so that the surface is smooth and true to cross section, free from tearing and all irregularities, and of uniform density. The alignment of the outside edge should also be uniform. The mix should be handled in such a manner that segregation is avoided at all points during the paving operation.
418.104.22.168 Unloading of Mix
Just before unloading the truck, the truck bed should be raised with the tailgate still secured. This will allow the mix to shift and slide back to the tailgate causing any segregated coarse aggregate to be incorporated back into the mix. Also, when the tailgate is released, the load of mix will flood the MTV or paver hopper in a mass, further reducing segregation.
When delivering mix directly to the paver, the truck should never bump into the paver because this will result in a corresponding bump in the mat. Instead, the truck should stop and the paver will initiate contact. Also, the truck bed should never be raised to the extent that weight is transferred to the front of the paver. Doing so may also cause a bump in the mat. Finally, both sets of duals should be in firm contact with the push rollers and the truck should be in line with the paver, not skewed. If the truck is skewed, the paver tends to skew, requiring continual correction by the paver operator and leading to irregular lines. The truck driver should only brake when absolutely necessary.
Many pavers are also equipped with a truck hitch that has additional rollers to engage the truck’s wheel rims. The purpose of the hitch is to ensure and maintain contact with the truck and, in turn, eliminate mix being dumped in front of the paver. Once the truck is unloaded, the hitch is released and the truck is able to pull away from the paver.
If the mix under production is Superpave, the load should be rejected if a crust has formed on the load or if lumps have formed. If the mix is BB, BP, or SL, the crust and lumps must be broken down completely prior to being incorporated into the project.
422.214.171.124 Operation of the Material Feed System (Receiving Hopper, Slat Conveyors, Material Flow Gates, and Distribution Augers)
The key to placing a smooth mat is the proper use of the material feed system as a whole to maintain a constant head of material in front of the screed by the continuous operation of the conveyors and augers and the proper positioning of the flow gates (if any). This is dependent upon the type and speed of the paver, the width of the mat, and the thickness of the lift. A change to any component of the system will result in a corresponding change to the mat.
The amount of mix in the hopper should be kept at a level above the top of the tunnel openings or flow gates and as constant as possible. This will keep the conveyors on the paver full and maintain a constant head of material in front of the screed. Mix that is allowed to stand in the sides of the hopper for a long period of time will cool. The wings of the hopper can be folded to move mix from the sides to the middle in order to keep the mix hot enough for proper placement and compaction. However, folding of the hopper wings may cause segregation and increased mat problems and, therefore, should be held to an absolute minimum. If an insert is used with an MTV, the hopper wings cannot be folded.
It is recommended that the flow gates be adjusted so that the depth of mix carried in the auger chamber is level with the center of the auger shaft and kept as consistent as possible to maintain a constant head of material in front of the screed. If the augers are buried or overexposed, variations in the texture of the mat and surface shadows may develop.
If enough mix is not placed under the gearbox, a longitudinal streak may occur in the center of the mat behind the gearbox. This streak will be darker in color and more open in texture than the surrounding mix and is the result of a lack of mix being placed under the gearbox and passing under the screed at that point, not segregation. The use, or adjustment, of a reverse auger or paddle system should eliminate this streak.
There are three types of material feed systems: constant speed, variable speed, and sonic control. However, in all cases the amount of mix delivered to the screed is still essentially regulated by the position of the flow gates, with or without the use of flow control sensors. If a flow control sensor is used, it should be set near the paver end plate to maintain a constant head of material in front of the screed by keeping the augers running continuously. The location of the sensor is important in preventing too much or too little mix from being carried at the outside edge of the screed.
In the manual operation of pavers with constant speed material feed systems, the conveyors and augers operate at a constant speed. Therefore, the only way to maintain a constant head of material in front of the screed is by the proper adjustment of the flow gates. A flow control sensor may be used to monitor the amount of mix. These sensors are limit switches that float on top of the mix and rotate as the amount of mix changes. When too little or too much mix is present on either side of the screed, the sensor turns the corresponding conveyor and auger on or off. Intermittent operation of the conveyor and auger systems may cause roughness in the mat behind the screed, as well as shadows and ripples.
In the manual operation of pavers with variable speed material feed systems, the speed of the conveyors can be selected and the speed of the augers is set accordingly. A constant amount of mix is maintained by adjusting the speed of the conveyors and augers. A flow control sensor can also be used with these systems. As the amount of mix increases and decreases, the speed of the material feed system decreases and increases, respectively, to maintain a constant head of material in front of the screed. Since the conveyors and augers are operating continuously, the roughness in the mat and the shadows and ripples are avoided.
Most pavers are equipped with a sonic feed system that uses reflected sound waves to sense the amount of mix. The system sends out pulses several times per second. A timing circuit is started when the pulse is sent out and stopped when the first echo is received. The length of time is used to calculate the distance to the material. The system then varies the speed of the conveyors and augers on each side of the machine proportionally to maintain a constant head of material in front of the screed.
4126.96.36.199 Operation of the Screed
Two primary forces constantly act on the screed as the paver places mix. The first is the towing force of the tractor, which varies as the speed of the paver varies. The second is the head of material pushing against the screed. The forces must be in equilibrium in order for the screed to maintain a constant angle of attack. When a change in any one force occurs, the angle of attack of the screed changes causing the screed to rise or fall, and the thickness and texture of the mat to change accordingly. The screed will react to the change in the force against it until equilibrium is reestablished.
The line of pull refers to the angle at which the screed is pulled forward by the tractor. The line of pull will influence the angle of attack of the screed. The smoothest mat will be placed when the towing force is applied parallel to the grade. For this reason, the tow points should be set according to the thickness of the mat. If the tow points are set too high or too low, the towing forces will be angled upward or downward, respectively. Therefore, to maintain thickness, the angle of attack of the screed must be decreased (nose down) or increased (nose up), respectively, and only the front or rear portion of the screed will compact and finish the mat. Either way, this causes poor mat texture and extreme wear on that portion of the screed.
The head of material is the volume of mix carried in the auger chamber in front of the screed. One of the major factors affecting the thickness and texture of the mat is the consistency of the head of material. If the head of material is increased, the force on the screed also increases, causing the angle of attack to increase (nose up) and the screed to rise until equilibrium is reached, resulting in a thicker mat. If the head of material is decreased, the force on the screed also decreases, causing the angle of attack to decrease (nose down) and the screed to fall until equilibrium is reached, resulting in a thinner mat.
The head of material is directly affected by the operation of the conveyors and augers on each side of the paver. When the conveyors and augers are operating, the mix is pulled from the hopper and distributed across the front of the screed by the augers. As long as the flow of mix is constant, the head of material pushing against the screed remains constant as well, and the mat is smooth with a consistent thickness and texture.
If the head of material is allowed to vary, the screed moves up and down (commonly referred to as “hunting”) because of the change in the forces acting on it. If the amount of mix carried in the auger chamber is decreased because the slat conveyors and augers are off, the screed falls, reducing the thickness of the mat. When the slat conveyors and augers turn on, more mix is carried back to the auger chamber. This increases the force on the screed and causes it to rise, resulting in a thicker mat. The head of material is affected each time the conveyors and augers are turned off and on. Therefore, the position of the flow gates is very important in regulating the amount of mix that the conveyors and augers distribute in front of the screed. Varying thickness also results in varying smoothness.
Another factor that affects the consistency of the head of material in front of the screed is the temperature of the mix. A load of cold, stiff mix increases the force acting on the screed andcauses the screed to rise, increasing the thickness of the mat. A load of hot, less stiff mix decreases the force acting on the screed and causes the screed to fall, decreasing the thickness of the mat.
Adjusting the elevation of the tow points or the screed pivot point (by turning the thickness control cranks) will change the thickness of the mat by changing the angle of attack of the screed. However, it takes approximately five times the length of the leveling arms for the screed to complete the thickness change. Approximately 65% of the change will occur in the first leveling arm length and the remaining 35% will occur in the last four lengths. This is the case regardless of whether the thickness is increased or decreased and regardless of the amount of thickness change.
In manual control, the screed operator must be aware of the reaction time of the screed. If a second thickness change is made before the first change has been completed, the first change will never be completed and it will take five leveling arm lengths for the second thickness change to be carried out. For this reason, continually adjusting the thickness controls will not produce a smooth mat.
Manual changes in the angle of attack of the screed by turning the thickness control cranks should not be made when the paver is operating with automatic controls (grade and slope controls). Otherwise, the automatic controls will change the elevation of the tow points as a result of the manual change and any attempt to adjust the thickness of the mat will not occur.
4188.8.131.52 Paving Speed
When paving operations start, the speed of the paver should be adjusted to the plant production rate or the rate that mix is delivered to the paver. In order to obtain the smoothest mat possible, the paver operation should be continuous and at a constant speed. The head of material in front of the screed and the angle of attack of the screed should also be constant.
As soon as the first load of mix has been placed, the texture of the surface should be checked for uniformity. Any necessary adjustments shall be made to ensure uniform placement of the mix to the proper line and grade. A straightedge should be used to determine if a smooth surface is being obtained. Yield should be checked occasionally by comparing the amount of mix placed in a given area to the amount required by the plans. Frequent checks will prevent unanticipated overruns or underruns.
It is not good practice to operate the paver at a high speed and then stop and wait for another load. If a gap occurs in the delivery of mix, the MTV, if used, should be stopped without being completely emptied. This will retain a sufficient amount of mix on the augers to blend with the new, possibly segregated, mix in the next load. In addition, the paver should be stopped before the level of mix in the hopper is below the top of the tunnels or flow gates so that the head of material in front of the paver screed remains constant and the proper smoothness of the mat is achieved when resuming placement.
Mat thickness is a function of the angle of attack of the screed and the paver speed. If the paver speed is changed during placement, there will be a change in the mat thickness and texture, provided no other changes to the system are made. Adjustment of either without corresponding adjustment of the other will affect the mat. If the paver speed is increased, the angle of attack of the screed and, therefore, the thickness of the mat will decrease. If the paver speed is decreased, the angle of attack of the screed and, therefore, the thickness of the mat will increase.
If the paver must be stopped because mix is temporarily unavailable, it should be stopped as quickly and smoothly as possible before the level of mix in the hopper is below the top of the tunnels or flow gates. This will keep the head of material in front of the screed constant and minimize the effect of the rapid speed change on the angle of attack of the screed. When more mix is deposited into the hopper, the paver should be brought back to speed as quickly as possible, again minimizing the effect of the change in speed on the angle of attack. Since the head of material has been kept constant, a smooth mat will be constructed.
The length of a delay so that it is still possible to place and compact the mix to obtain smoothness and the required density will depend on the environmental conditions (air temperature, surface temperature, and wind), the temperature of the mix in the hopper, the uncompacted thickness of the lift, and other factors.
If the paving operation is put on hold until the next load arrives, the paver should be stopped with the hopper as full of mix as possible. The mix will remain in a mass in the hopper, reducing its rate of cooling. In addition, the paver should remain in one position until more mix is available. If the paver is moved forward periodically, the amount of mix in the hopper is decreased and the remaining mix will cool faster. The mix retained in the auger chamber and underneath the screed will cool faster because of the small amount of mix in these areas and because some mix is contact with the existing surface. Also, if the paver is moved, the thickness and texture of the mat will be affected since the head of material in front of the screed will decrease.
There will be portions of the mat that the rollers cannot compact because of the curvature of the drums and the overhang of the screed. Density may be very difficult to achieve in these areas once the paver is moved because the mix has cooled. For all of these reasons, the paver should be moved as little as possible during long stops.
Before compaction of the mix, the surface should be checked and any irregularities corrected by adding or removing mix.
460.6.5 Hand Spreading
There may be areas where the paver may not be able to place mix. In these areas, hand placement is permitted. The mix should be thoroughly loosened and distributed uniformly so that segregation does not occur. When mix is dumped in piles, it should be placed as close as possible to its final location to avoid overworking the mix and possibly causing segregation. The mix is then spread by hand using shovels, rakes, or lutes. Any mix that has formed into lumps and does not break down easily should be discarded. Before compaction of the mix, the surface should be checked and any irregularities corrected.
460.6.6 Material Acceptance
Roadway inspectors are responsible for collecting tickets for each load of mix accepted. The inspector will keep one copy of the ticket in order to keep a running total of the tonnage received during the production period. If mix that does not comply with specifications arrives at the paver it should be rejected. When a load is rejected, a notation should be made on the ticket stating the reason for rejection. The rejection should also be recorded in the inspector’s diary.
Reasons for rejecting mix are:
(1) Too hot. Blue smoke rising from the mix indicates overheating. The temperature should be checked immediately.
(2) Too cold. A generally stiff appearance or an improper coating of aggregates indicates insufficient heating. The temperature should be checked immediately.
(3) Excess moisture. Steam rising from the mix and/or stripping of binder from the aggregate may indicate that the aggregate was not sufficiently dried before mixing.
(4) A lean mix is the result of too much fine aggregate, not enough binder, or both. A dull brown color and/or a peaking of the load usually indicate a lean mix.
(5) A fat mix is the result of too much coarse aggregate, too much binder, or both. A slick or greasy appearance and/or a slumping of the load usually indicate a fat mix.
(6) Segregation. Segregation of the mix may occur because of improper handling at any point during production or placement and may be serious enough to warrant rejection. See additional information below and Segregation in Mat Problems.
(7) Contamination. Mix that has been heavily contaminated with any material not included in the JMF will be rejected. Minor contamination such as trash or other foreign material, must be removed from the mix before it is placed.
(8) Defective. Mix that is considered defective contains excess binder, is loose or broken, is mixed with dirt, etc. Defective mix must be rejected or removed and replaced.
Segregation must not be permitted. If segregation occurs, adjustments should be made to the plant and/or paving operations immediately. If segregation continues, production shall cease and will not resume until the cause has been determined and corrected. However, if random segregation occurs intermittently, production may continue and the segregated areas will be properly addressed within a reasonable time frame. All segregated areas will be removed and replaced with satisfactory mix, at the contractor’s expense, to the limits determined by the engineer. The density gauge (nuclear or non-nuclear) may be used in accordance with MoDOT Test Method TM-75 to solve disputes with the contractor over the existence of segregation. The Standard Specifications do not put restrictions on who performs the testing. However, if QC personnel perform the testing, QA should be present to observe.
Any mix that is removed because it is segregated, contaminated, or defective must be replaced with satisfactory mix at the contractor’s expense. The mix shall then be immediately compacted to the required density.
Compaction is the final stage in the paving process and is the most important factor in the performance of the pavement. Through compaction, the strength of the pavement is developed and the texture and smoothness are established. Compaction is the process of compressing the mix and reducing its volume by applying external forces. By reducing the volume, the density of the mat is increased. The aggregates in the mix are forced closer together, which increases the friction between the aggregates and reduces the air void content of the mix.
Adequate compaction of the mix improves the pavement’s resistance to fatigue and permanent deformation (rutting). Compaction also decreases the pavement’s low-temperature cracking potential and reduces air and moisture penetration into the pavement to help prevent stripping. The factors that affect compaction are the mix properties and temperature, the environmental conditions, the lift thickness, and the condition of the underlying surface.
The ability to obtain the required density is influenced by a number of factors. One factor is the aggregate properties (surface texture, particle shape, number of fractured faces, and dust content) and the combined gradation of the mix. In general, aggregates with properties that improve the pavement’s resistance to fatigue and rutting require increased compactive effort. Another factor is the asphalt binder grade and the binder content of the mix. A binder grade with a high viscosity or low penetration will generally produce a stiff mix. In general, a mix with insufficient binder content may be stiff, whereas a mix with excess binder content will compact easily and may shove under the rollers. A stiff mix usually requires greater compactive effort. Finally, the production temperature of the mix also affects the compactive effort needed. A mix produced at a low temperature will be stiff, whereas a mix produced at a high temperature will be tender and very easy to compact.
The contractor shall determine the rolling zone and pattern. However, rolling shall be performed within the proper time and temperature intervals for each lift and shall continue until no further consolidation of the mat occurs and all roller marks are removed from the surface of the mat. The rolling zone is the area of the mat that is at the correct temperature to be efficiently compacted. The roller pattern depends on the type of compaction equipment, paving width, roller width, the number of passes, and the roller speed.
The time available for compaction is defined as the time it takes for a mix to cool from the temperature behind the screed to a minimum compaction temperature. This is highly dependent on the rate of cooling of the mix. Once the mat has cooled to a certain temperature, which is different for all mixes, additional compaction results in very little density gain. Rolling below this temperature may result in aggregate fracture and a decrease in density. In general, rolling should occur at as high a temperature as possible, without causing shoving, in order to achieve the required density with minimum compactive effort. The lift thickness, mix temperature, and environmental conditions (the ambient and underlying surface temperatures, wind, rain, etc.) also affect the time available for compaction.
The rollers should move at a slow, uniform speed. The faster a roller passes over a point in the mat, the less time the weight of the roller is applied to that point. Therefore, less compactive effort is applied to that point. As the roller speed increases, the amount of density gained with each pass decreases. As the roller speed decreases, the amount of density gained with each pass increases. Roller speed is also governed by the displacement and tenderness of the mix.
To obtain uniform density, each point in the mat must be rolled a certain number of times. The width of the mat divided by the width of the roller drums can be used to determine the number of passes needed to cover every transverse point in the mat. However, the necessary number of passes over each point depends on the type of roller and its position in the roller train, the lift thickness, the mix temperature and properties (binder grade and content and aggregate properties), and the environmental conditions. Roller passes must be distributed uniformly over the width and length of the mat.
The rollers should be moving most of the time. If the line of rolling is suddenly changed or if the direction of rolling is suddenly reversed, displacement of the mix may occur. Therefore, any pronounced change in the direction of the roller should be made on stable material. The reversal points of the rollers may need to be staggered to prevent shoving of the mix. The roller should not sit on the hot mat. Otherwise, an indentation will occur that may not be removed by additional rolling.
After the mix has been spread, struck off, and any irregularities in the surface corrected, the mix shall be compacted thoroughly and uniformly to obtain the required compaction while the mix is in a workable condition. Excessive rolling, to the extent of aggregate degradation, will not be permitted. Breakdown rolling shall begin as soon as the mat will support the roller without shoving. The breakdown roller shall also be kept as close to the paver as possible to ensure maximum compaction while the mix is hot. Again, it should not be kept so close that shoving or checking of the mat occurs.
Breakdown rolling is usually accomplished with a vibratory roller, but a pneumatic tire roller may also be used. When intermediate rolling is used, it should follow breakdown rolling as closely as possible while the mix is still hot. Intermediate rolling is usually accomplished with a pneumatic tire roller. Finish rolling should be done while the mix is still workable enough to remove all roller marks and is usually accomplished with a steel wheel roller. If a vibratory roller is used as the finish roller, it must be operated in static mode. Otherwise, the aggregate at the surface of the mat may fracture. Finish rolling is defined as the final pass of the last roller.
460.6.8 Transverse Joints
The type of transverse joint constructed depends on whether or not traffic will be allowed on the mat before paving is resumed. If traffic will be allowed, a temporary depth transition (also called a tapered joint) is necessary to provide a smooth and safe transition. The temporary depthtransition must be constructed to the satisfaction of the roadway inspector. If traffic will not be allowed, a vertical header can be constructed.
If a temporary depth transition is constructed, treated release paper or a similar material may be used. The mix will not stick to these materials (and they do not stick to the underlying surface) allowing for easy removal of the transition. Before paving is resumed, a vertical edge must be constructed by cutting back the previously laid mat to a location with the full thickness of the lift and the required density. Other methods of constructing a vertical edge may be used if satisfactory results are obtained. Any excess mix should be removed and the area adjacent to the vertical edge cleaned and tacked. The vertical edge may be lightly tacked, if directed by the RE, just before paving is resumed to help seal the joint.
Starting blocks may be used when paving operations are resumed. The purpose of starting blocks is to ensure that the uncompacted thickness of the lift will provide for the required thickness after compaction. Also, the dip that occurs near the joint as a result of insufficient mix is avoided if starting blocks are used properly. As a rule of thumb, mix will compact 20% to 25% under proper compaction procedures, depending on its properties. This means that the uncompacted mix must be placed between 120% and 125% of the compacted thickness of the lift. For example, if the compacted lift thickness is to be 2 in., the uncompacted thickness should be roughly 2 ½ in. and the starting blocks, if used, should be ½ in. thick. If the compacted lift thickness is to be 3 in., the uncompacted thickness should be roughly 3 ¾ in. and the starting blocks, if used, should be ¾ in. thick. Hand manipulation of the mix should be minimal so that unsightly surface texture is avoided.
Breakdown rolling should be accomplished as quickly as possible. The roller should pass slowly and completely over the joint before reversing. The joint should be straightedged to ensure that the joint has been matched. The completed transverse joint should be well sealed and maintain a consistent grade, line, and surface texture between the adjoining mats.
460.6.9 Longitudinal Joints
Longitudinal joints in the final surface shall be placed at the locations specified on the plans, generally at or outside of the lane lines of the roadway. However, pavement markings shall not be placed on a longitudinal joint. The longitudinal joints in the underlying lifts shall be offset by a minimum of 6 in. so that the joints do not fall in the same location throughout the full depth of the pavement.
In construction of the first pass, the paver should run as straight as possible so that the joint can be matched on the next pass. Also, the unconfined edge must be properly compacted so that the joint will not deteriorate under traffic. The roller should make as many passes over the unconfined edge as it does over the rest of the mat. Also, the roller should extend over the unconfined edge by approximately 6 in. to ensure that the compactive effort of the roller is applied in a vertical direction and to reduce lateral displacement of the mix during compaction. No edge differential shall be left in place for more than 7 days, unless approval is granted from the RE.
In construction of the second pass, the exposed edge may be lightly tacked, if directed by the RE, to help seal the joint. The adjacent lane should be overlapped by approximately 1 in. or more to ensure that sufficient mix is available to properly seal the joint. If excessive overlapping occurs, the joint should be “bumped” or the excess mix should be removed. Hand manipulation of the mix should be minimal so that unsightly surface texture is avoided. Again, the uncompacted mix should be placed between 120% and 125% of the compacted lift thickness to account for roll down into the joint when the mix is compacted. If too much or too little mix is placed at the joint, the elevation of the adjacent lane will not be matched and the required density may not be achieved. The longitudinal joint should be compacted on the hot side of the mat with the roller extending approximately 6 in. over the joint. This does not necessarily have to be accomplished with the first pass. The joint should not be rolled to the extent that degradation of the aggregate occurs.
Other methods of constructing a longitudinal joint may be used if satisfactory results are obtained. However, the completed longitudinal joint should be well sealed, flush, and along true lines.
460.6.10 Density Samples
All mixes having density requirements will be sampled and tested in accordance with the applicable Standard Specifications. Samples are cut from the roadway within 24 hours of mix placement and compaction. Cores, with a minimum diameter of 4 in. and a thickness equal to the full depth of the lift, are taken at locations determined by random numbers. The contractor is responsible for cutting the cores and repairing the roadway. The holes shall be filled with the mix under production, or an approved cold patch, and thoroughly compacted immediately to the satisfaction of the roadway inspector.
It is preferred that the APIW or the Random Locations spreadsheets are used to determine core locations. These spreadsheets will automatically generate the random numbers and calculate the stationing and offset for each core. Exhibit A in MoDOT Test Method TM-41 is a table of random numbers that may also be used to determine core locations. The ‘A’ value is used to calculate the distance, in stations, from the beginning of the mat or sublot. The ‘B’ value is used to calculate the offset from the edge of the mat. Start at any point in the table and move either vertically or horizontally, using corresponding ‘A’ and ‘B’ values. Skipping pairs of numbers or reversing the direction is not permitted because this does not result in true random sampling.
The cores should be examined and any in questionable condition should be discarded. The cores are to be delivered to the plant inspector free from broken edges, deformations, and distortions. Each core should be clearly marked with an identification number. Additional cores should be taken only if the inspector suspects that the first core was damaged. Coring often leads to a pothole in the pavement at that location. Therefore, excessive coring of the mat will not be permitted.
The roadway inspector is responsible for completing an Asphalt Roadway Report for each production period. One copy is provided to the plant inspector along with the density sample(s). The plant inspector will complete the applicable portions and return it to the roadway inspector. The inspector responsible for acceptance of the mix will sign the Asphalt Roadway Report and give a completed copy to the contractor immediately.