9.6.4 Handwork

Hand raking behind the screed should not be done unless absolutely necessary.  The most uniform surface texture can be obtained by keeping hand work behind the screed to a minimum.  This is especially true with crushed stone mixtures and mixtures requiring stiff graded asphalt binders. If the operations ahead of the paver are properly performed, if the equipment is in good condition and properly adjusted, and if the paver is not placing the mix at an excessive rate of speed, there should be little or no need for hand work.  The raker, however, should be alert to a crooked edge on the mat so he can straighten it immediately.  If the paver operator properly follows the guide line, the back work will not be necessary.

There are places on many jobs where spreading with a paver is either impractical or impossible.  In these cases, hand spreading may be permitted. The Engineer will waive the requirement for use of pavers for spreading and finishing  where irregularities or obstacles make their use impractical. Spread, rake, and lute the  mixture by hand methods or other approved methods in these areas.

Placing and spreading by hand should be done very carefully and the material distributed uniformly so there will be no segregation of the coarse aggregate and the asphalt mortar.  When the asphalt mix is dumped in piles it should be placed far enough ahead of the shovelers to necessitate moving the entire pile  Also, sufficient space should be provided for the workmen to stand on the base and not on the mixed material.  If the asphalt mix is broadcast with shovels, almost complete segregation of the coarse and fine portions of the mix will result.  A mixture placed by hand will have a different surface appearance than the same mixture placed by a machine.

The material should be deposited from shovels into small piles and spread with lutes.  In the spreading process, all material should be thoroughly loosened and evenly distributed.  Any part of the mix that has formed into lumps and does not break down easily should be discarded.  After the material has been placed and before rolling starts, the surface should be checked with templates and straightedges and all irregularities corrected.

9.7 COMPACTION OF ASPHALT PAVEMENTS - GENERAL

9.7.1 The Need for Compaction

Compaction is the process of compressing a given volume of asphalt hot-mix into a smaller volume.  Compaction is accomplished by pressing the asphalt coated aggregate particles closer together, thereby reducing the air voids (space) in the mix and increasing the density (weight to volume ratio) of the mixture.  Compaction is considered successful when the finished mat reaches the minimum specified density for that mix type. NCDOT Specifications require that all Superpave mixes be compacted to a minimum of 92.0 percent of the maximum specific gravity (Gmm), except for SF 9.5A and S 4.75A mixes, which shall be 90.0 and 85.0 percent of maximum specific gravity (Gmm), respectively.

The need for a pavement to be compacted to the required density is better understood when the effect of air, water, and traffic on an undercompacted pavement is realized.  The voids in an undercompacted mix tend to be interconnected and therefore, permit the intrusion of air and water throughout the pavement.  Air and water carry oxygen which in turn, accelerates the oxidation of the asphalt binder in the mix, causing it to become brittle.  Consequently, the pavement itself will ultimately fail as it can no longer withstand the repeated deflections due to traffic loading.  The internal presence of water at freezing temperatures can also cause an early failure in the pavement due to expansion of the freezing water.

A pavement which has not been adequately compacted during construction may push, shove and  rut from traffic that is utilizing the pavement.  However, unless the mix is properly designed and adequate voids remain in the compacted mix, the pavement will likely flush and tend to become unstable due to futher reduction of void content under traffic and/or thermal expansion of the asphalt.  The desired as-constructed void content is approximately 8 percent or less for the dense-graded mixes.  At this level, the voids are usually not interconnected.  When the air void content is too high, the pavement will tend to ravel and disintegrate.  When the air-void content is too low, there is a danger of the pavement flushing and becoming unstable.

Compaction is accomplished by arranging the aggregate particles closer together in a position in which the asphalt binder can hold them in place.  Compaction accomplishes two important goals:

(1) It develops the strength and rut resistance of the mix
(2) it closes passages through which water and air would otherwise penetrate thus causing faster aging, freeze-thaw damage, and stripping.

In addition to keeping accurate detailed records and observing that the operation is performed safely, the Technician must also be sure that compaction is done properly and that the finished pavement meets all specifications.  To achieve this, the technician must understand the compaction procedure and the equipment involved.  The Technician must acquire samples of the compacted mat or take readings with special instruments to determine mix density and smoothness.

9.7.2  Compaction Specifications - NCDOT

The following is quoted directly from Article 610-9 of the Standard Specifications. However, the Technicians should always refer to the contract for any specific rolling, compaction, and equipment requirements.

Immediately after the asphalt mixture has been spread, struck off, and surface and edge irregularities adjusted, thoroughly and uniformly compact the pavement.  Compact the mix to the required degree of compaction for the type of mixture being placed.

Provide sufficient number and weight of rollers, except as noted, to compact the mixture to the required density while it is still in a workable condition.  Obtain approval of equipment used in compaction from the Engineer prior to use.  Where uniform density is not being obtained throughout the depth of the layer of material being tested, change the type and/or weight of the compaction equipment as necessary to achieve uniform density even though such equipment has been previously approved.

Compact all final wearing surfaces, except open-graded asphalt friction course, using a minimum of 2 steel wheel tandem rollers, unless otherwise approved.  Pneumatic-tired rollers with 2 tandem axles and smooth tread tires may be used for intermediate rolling.

Limit rolling for open-graded asphalt friction course to one coverage with a tandem steel wheel roller weighing a maximum of 10 tons (9.1 metric tons), with additional rolling limited to one coverage with the roller where necessary to improve the riding surface.

Steel wheel tandem vibratory rollers which have been specifically designed for the compaction of asphalt pavements may be used on all layers 1 inch (25 mm) or greater in thickness during the breakdown and intermediate rolling phase.  Do not operate vibratory rollers in the vibratory mode during the finish rolling phase on any mix type or pavement course, open-graded asphalt friction course, or on permeable asphalt drainage course.

When vibratory rollers are used, use rollers which have variable amplitude and frequency capabilities and which are designed specifically for asphalt pavement compaction.  Provide rollers equipped with controls which automatically disengage the vibration mechanism before the roller stops when being used in the vibratory mode.

The Engineer may prohibit or restrict the use of vibratory rollers where damage to the pavement being placed, the underlying pavement structure, drainage structures, utilities, or other facilities is likely to occur or is evident.

Do not use rolling equipment which results in excessive crushing of the aggregate or excessive displacement of the mixture.

In areas inaccessible to standard rolling equipment, thoroughly compact the mixture by the use of hand tampers, hand operated mechanical tampers, small rollers, or other approved methods.

Use rollers which are in good condition and capable of being reversed without backlash to compact the mixture.  Operate rollers with the drive wheels nearest the paver and at uniform speeds slow enough to avoid displacement of the mixture.  Equip steel wheel rollers with wetting devices which will prevent the mixture from sticking to the roller wheels.

Begin compaction of the material immediately after the material is spread and shaped to the required width and depth.  Carry out compaction in such a manner as to obtain uniform density over the entire section.  Perform compaction rolling at the maximum temperature at which the mix will support the rollers without moving horizontally.  Complete the compaction (including both intermediate rolling) prior to the mixture cooling below a workable temperature.  Perform finish rolling to remove roller marks resulting from the compaction rolling operations.

9.7.3  Asphalt Roller Types

Refer to Article 610-9 which addresses compaction methods and equipment.
The three basic types of rollers utilized for compaction of asphalt pavements are:

(A) steel-wheeled,
(B) pneumatic-tired, (rubber tired)
(C) vibratory.

(A) STEEL-WHEELED ROLLERS

Tandem rollers may be either two-axle or three-axle.  Two-axle tandem rollers (Figure 9-13) vary in weight from 3 to 14 tons (metric tons) or more.  Most have wheels to which ballast can be added to increase the weight.

Three-axle tandem rollers are made in sizes ranging from 10 to more than 20 tons (metric tons).  Most of these rollers can be increased in weight by adding ballast to the wheels.  The center axle roll on the three-axle tandem rollers is arranged so that a large part of the total weight of the roller can be applied there, as required by high spots.

Steel-wheel rollers should be checked for wear on wheel rims.  A sharp metal straightedge may be used for this check.  The roller should not be used if grooves or pits have worn into the rolling drum.  These rollers have scrapers for keeping rollers clean and wetting pads to keep rollers wet, so they do not pick up asphalt during the rolling operation.  If they are worn excessively, they should be replaced.
 
 

(B) PNEUMATIC-TIRED ROLLERS

Several models of self-propelled pneumatic tired rollers have a device to change tire inflation while the roller is operating.  This "inflation-on-the-run" system automatically maintains any present tire pressure or can raise or lower inflation pressure while the roller is operating.  Some compacting conditions and requirements require different inflation pressures.

(C) VIBRATORY ROLLERS

Vibratory rollers vary in static weight from 1 1/2 to 17 tons (metric tons).  Some large tandem roller models have provisions for vibrating the third axle unit.  Vibratory rollers normally may be used for compacting any type of asphalt mixture, provided the appropriate amplitude, frequency and speed is selected for the type mixture and thickness being placed.

Vibratory rollers may be used on all layers of asphalt pavements, except that operation in the vibratory mode will be permitted only during the breakdown and intermediate rolling phase on final wearing surfaces 1 inch (25mm) or greater in thickness.  Operation in the vibratory mode will not be permitted during the rolling of open-graded asphalt friction course, during the finish rolling phase on any mix type or pavement course, or when the layer thickness is less than 1 inch (25mm).

The Engineer may prohibit or restrict the use of vibratory rollers where damage to the underlying pavement structure, drainage structures, utilities, or other components is likely to occur or is evident.

Three important factors to consider for vibratory roller operations:

(1) Frequency (vpm),
(2) Amplitude (height of bounce), and
(3) Roller speed.

Prior to beginning paving operations, the Resident Engineer and/or the Roadway Technician must inspect the Contractor's compaction equipment to see that it meets all requirements of the Specifications and is in good working order.  If the equipment meets Specifications and is in satisfactory operating condition a statement shall be entered in the Technician's Daily Diary.  If it is not, the Contractor should be advised accordingly and corrective actions taken before paving begins. See the Checklist in this Section.

Before any of the rollers are used on a project they should be checked to see that they are in good mechanical condition and to assure their compliance with project specifications, if any.  Where applicable, the following should be checked on all rollers:

1. Total weight;
2. Weight per unit of width (steel-wheeled rollers);
3. Average ground contact pressure (pneumatic-tired rollers);
4. Mechanical condition, Hydraulic Fluid & Fuel Leaks
5. Precise steering

Compaction of the pavement material must begin immediately after the material is spread, struck off, shaped to the required width, depth, cross-section, and edge irregularities adjusted. The mix must be compacted to the required degree of compaction for the type of mixture being placed. Compaction must be carried out in such a manner as to obtain uniform density over the entire section.  Perform compaction rolling at the maximum temperature at which the mix will support the rollers without moving horizontally.  Complete the compaction (including both breakdown and intermediate rolling) prior to the mixture cooling below a workable temperature.  Perform finish rolling to remove roller marks resulting from the compaction rolling operations.

Most asphalt mixtures compact quite readily if spread and rolled at temperatures that assure proper asphalt viscosity.  Rolling should start as soon as possible after the material has been spread by the paver, but should be done with care to prevent unduly roughening the surface.

A mix that is relatively stable at high temperatures as it leaves the spreader is compacted by the vertical movement of the aggregate particles under the roller.  On any paving mixture the roller wheel must settle into the mix until the area of contact between the wheel and mix multiplied by the resistance of the mix is equal to the weight on the roller wheel.  If the hot mix is quite firm, the roller will not cause any horizontal mix displacement.

Horizontal displacement results from apparent crawling of the mix ahead of the roller and the forming of ridges on either side of the roller path.  If there is no horizontal displacement, there will be virtually no crawl or ridges along the edge of the roller path. (Fig. 9-15)

Horizontal displacement also results in a rough and uneven surface, thus defeating the intentions of careful grade control and good screed operation of the asphalt paver.  Horizontal movement of the mix often occurs due to the breakdown roller being operated too fast.

Mix temperature is a principal factor affecting compaction.  Compaction can only occur while the asphalt binder is fluid enough to act as a lubricant.  When it cools enough to act as an adhesive, further compaction is extremely difficult to achieve.  The best time to roll an asphalt mixture is when its resistance to compaction is the least, while at the same time it is capable of supporting the roller without excessive shoving.

The best rolling temperature is influenced by the internal particle friction of the aggregates, the gradation of the mix, and the viscosity of the asphalt. Therefore, it can change if any of these factors change.  The critical mix temperature in an asphalt concrete paving project is the temperature at the time of compaction.  This should determine the temperature at which the plant is to produce the mixture.  It is best to be able to compact the mix as quickly as possible after being spread, which means that it's best for the mixing temperature and the compacting temperature to be reasonably close to the same.

During rolling, roller wheels are kept moist with only enough water to avoid picking up material.  Fuel oil shall not be used to moisten roller wheels since it will damage the mix.  Rollers should move at a slow but uniform speed with the drive roller or wheels nearest the paver.  The speed should not exceed 3 mph (5 kph) for steel-wheeled breakdown rollers or 5 mph (8 kph) for pneumatic-tired rollers.  Rollers must be kept in good condition, capable of being reversed without backlash.  The line of rolling should not be suddenly changed or the direction of rolling suddenly reversed, thereby displacing the mix.  Any pronounced change in direction should be made on stable material.

If rolling causes material displacement, the affected areas are loosened at once with lutes or rakes and restored to their original grade with loose material before being re-rolled.  Heavy equipment, including rollers, should not be permitted to stand on the finished surface before it has thoroughly cooled or set.

Rolling freshly placed asphalt mix is generally done in the following order:

1. Transverse joints,
2. Longitudinal joints (when adjoining a previously placed lane),
3. Initial or breakdown rolling,
4. Second or intermediate rolling, and
5. Finish rolling.

All final wearing surfaces except open-graded asphalt friction course shall be compacted using a minimum of 2 steel wheel tandem rollers.  Steel wheel tandem vibratory rollers which have been specifically designed for the compaction of asphalt pavements may be used. Vibratory rollers, operating in the vibratory mode, may generally be used on all pavement layers 1” (25mm) or greater in thickness during the breakdown and intermediate phases of rolling.  Operation in the vibratory mode will not be permitted during the finish rolling phase on any mix type or pavement course or when the layer thickness is less than 1 inch (25mm).

Vibratory rollers must have variable frequency and amplitude capability. The rollers must be equipped with controls which automatically disengage the vibration mechanism before the roller stops when being used in the vibratory mode. Vibratory rollers used on asphalt mixtures should normally be operated at high frequencies and low amplitudes and specifically designed for asphalt compaction.

Rolling of open-graded asphalt friction course will consist of one coverage with a tandem steel wheel roller weighing a maximum of 10 tons (9.1 metric tons) with additional rolling limited to  one coverage where necessary to remove roller marks.  Excessive rolling should not be allowed inasmuch as this leads to possible breakdown of the aggregate, thereby reducing the drainage capacity of the friction course layer.  Vibratory rollers may be used on friction course provided they are operated in the static mode.

On all other mixtures, the number and weight of rollers shall be sufficient to compact the mixture to the required density while it is still hot and in a workable condition. Vibratory rollers may be used, as specified in above paragraphs, provided satisfactory results are obtained, excessive displacement or crushing of the aggregate does not occur, and no vibratory roller marks (indentations) remain in the finished surface. The Engineer may prohibit or restrict the use of vibratory rollers where damage to the underlying pavement structures, drainage structures, utilities, adjoining structures, or the pavement itself is likely to occur or is evident.

The use of a pneumatic tired roller is optional for compaction purposes on all mixes, unless otherwise required within a contract.  Some Project Special Provisions within a contract may require the use of a rubber tired roller, therefore, it is essential that the roadway technician review all contract Project Special Provisions for this possible requirement.

While it is the Contractor's responsibility to determine roller requirements based on contract specifications, the technician is an essential part of this determination.  The exact number of coverages (passes) that will be required to obtain adequate density is initially unknown This is due to some uncertainty about the mixtures rate of cooling, among other things.  These uncertainties are cleared up by careful observation, measuring, and testing during the early stages of the paving operation.

A number of studies have been made on the cooling rates of mixes under varying conditions of mix temperature, lift thickness and base temperature.  Temperature is a fairly accurate estimate of the time interval in which density must be achieved.  Table 9-2 can be used to determine the allowable time available in order to achieve satisfactory compaction. The allowable time can then be used to determine the number and types of rollers needed on the job.

A number of studies have been made on the cooling rates of mixes under varying conditions of mix temperature, lift thickness and base temperature.  Temperature is a fairly accurate estimate of the time interval in which density must be achieved.  Table 9-2 can be used to determine the allowable time available in order to achieve satisfactory compaction. The allowable time can then be used to determine the number and types of rollers needed on the job.

TABLE 9-2

Major factors affecting compaction can be categorized into five classes.  These are:

(A) Mix properties
(B) Environmental conditions  (See Section 9.4.3)
(C) Layer (lift) thickness  (See Section 9.4.3)
(D) Subgrade and Bases
(E) Compaction equipment and procedures

(1) Aggregate:  Gradation, surface texture and angularity are the primary aggregate characteristics that affect workability of the mix.  As the maximum aggregate size or percentage of coarse aggregate in the mix increases, the workability decreases and greater compactive effort is required to achieve target density.  Similarly, a rough surface texture, as opposed to a smooth, glassy aggregate surface, results in a more stable mixture and requires greater compactive effort.  Mixtures that are produced from gravel material, frequently are more rounded than quarry rock and thus more workable.

Natural sands are often added to mixes in the interests of economy.  Too much sand, particularly in the middle particle sizes - around the 0.600 mm mesh sieve  - will result in tender mixes (mixes with high workability, but low stability).

Tender mixes are easily overstressed by heavy rollers and too much rolling.  They are often susceptible to scuffing and displacement by traffic after several weeks in place.

The fines or filler content in the mix will also affect the compaction process.  It is the combination of filler and asphalt that provides the binding force in asphalt hot-mix pavements; therefore, the mix should contain sufficient fines to combine with the asphalt to produce the necessary cohesion when the mix cools.  The addition of mineral filler will help to offset the tenderness or slow-setting properties of mixes containing too much sand.  Conversely, if a mix contains too many fines it will become "gummy" and very difficult to compact.

(2) Asphalt Binder:  At room temperature asphalt binder is virtually a solid, whereas at 265° - 350°F (130° - 175°C) it is a fluid.  For a mix to be properly compacted, the asphalt in it must be fluid enough to permit the aggregate particles to move past one another.  In effect , the binder acts as a lubricant during compaction.  As the mix cools, the asphalt loses fluidity (becomes more viscous) and becomes stiffer.  At temperatures below approximately 185°F (85°C), the asphalt, in combination with the fines in the mix, begins to bind the aggregate particles firmly in place.  Consequently, compaction of the mix is extremely difficult once the mix has cooled below a workable temperature.

The grade of asphalt binder that is used and the temperature at which the mix is produced determine its viscosity.  Other factors being equal, a higher viscosity asphalt in the mix may require a slightly higher compaction temperature and/or greater compactive effort.

The asphalt quantity in the mix will also affect workability.  As the asphalt content increases, the film thickness of the asphalt on the aggregate particles increases.  At compaction temperatures, this increased film thickness increases the lubricating effect of the asphalt and up to a certain point makes compaction easier. However, excessive asphalt binder in the mix may cause the mix to be ”tender”, allowing it to push and sholve under the rollers and therefore, difficult to compact to the required density.

(3) Mix Temperature:  Mix temperature is one of the principle factors affecting compaction. The temperature at which an asphalt mixture is produced affects both the ease of compaction and the time it takes for the mix to cool below a minimum workable  temperature at which densification can normally take place.  Up to a certain point the hotter the mix, the more fluid the asphalt and the less resistant the mix is to compaction.  The upper limit for mix temperature is approximately 350°F (175°C).  Higher temperatures may result in damage to the asphalt. The lower temperature at which compaction is effective is approximately 185°F (85°C). Within these limiting values, 185° - 350°F (85°-175°C), the best temperature to begin rolling (compaction) is the maximum temperature at which the mix will support the roller without damaging the mix in any form (i.e. horizontal movement, mix sticking to roller drums, etc.)  The upper end of this range, 250°- 350°F (120°-176°C), will allow the most densification of the mix during the initial phase of the rolling operation.  The complete compaction rolling operation should be finished prior to the mix temperature getting below a workable temperature.

At the time of placement, the mix temperature is uniform throughout the thickness of the mat. However, the top and bottom surfaces cool more rapidly than the interior because they are in contact with the cooler air and subgrade.  Heat checking is a rather common occurrence during compaction of asphalt concrete mixes, particularly when the mix is placed in thin lifts.  Figure 9-16 is a side view of heat checking in a mix being compacted.  Heat checking happens most frequently when the tiller wheel of the roller is in front in the direction of travel during the breakdown pass.  The horizontal arrows shown between the surface of the mix and the dotted line represent the horizontal thrust of the tiller wheel in the mix.  The curve to the right of the figure represents the temperature profile in a layer approximately 2 inches (50mm) thick.  The temperature at the surface is 250°F (120°C).  The temperature at the mid-point is 290°F (143°C), while the temperature at the bottom is between 250°F (120°C) and 260°F (127°C).

Figure 9-16
Heat Checking (Side View)

The illustration shows the most frequent reason for heat checking.  The tiller wheel has sunk some depth into the mix and is exerting a horizontal thrust which must be resisted by the mix itself.

Since the mix is hottest at its mid-point, the asphalt viscosity is lower there than at the surface.  Because of the horizontal force of the wheel, the mix tends to move horizontally at some depth (illustrated by the dotted line in the figure).  This means that the mix at the surface must also move.  But the surface of the mix is stiffer due to its lower temperature, and responds by cracking in order to move along with the mix at the lower depth.  This results in the so-called hairline cracks to the level that horizontal movement is occurring in the mix, generally 3/8 to 1/2 inch (10 to 12.5 mm) in depth.  These are shown by the vertical lines behind the roller drum.

A top view of the hairline cracks that result from heat checking is shown in Figure 9-17 (Printed version of 2006 QMS Manual).  They tend to be 3 to 4 inches (75 to 100 mm) long, unconnected with each other.  If they were connected and extended, they would form a crescent as shown in this figure.  A crescent shaped crack in an asphalt mixture is typical of the slippage movement.  This is exactly what happens under a roller when heat checking occurs with the slippage occurring in the mix at the depth shown by the dotted line in Figure 9-16, i.e., the mix is slipping within itself.  As in any type of slippage distress, the crescent opens in the direction of the forces causing the slippage.  In the case of heat checking, the hair crack pattern usually opens up in the direction of rolling when the unpowered tiller wheel is leading.

The same type of crack pattern shown for heat checking can also occur if slippage is occurring at a greater depth, such as at the surface on which an asphalt lift is being placed.  In this case, the cracks have the same general configuration.  However, they are longer, open up wider, 1/4 to 1 inch (6 to 25 mm) and extend through the mix to the level of horizontal movement.  Again slippage is occurring but at a greater depth.

It is a rare case when heat checking occurs under a drive-wheel of a steel roller.  It almost always occurs under the tiller wheel.  Steel-wheeled rollers should not have ballast in the tiller wheel.  The heavier the weight in the small diameter wheel, the deeper it sinks into the mix with a resulting increase of horizontal force being imparted during the rolling operation, and the greater likelihood of heat checking or other slippage distress.

(B)  ENVIRONMENTAL EFFECTS
As explained above, the rate at which the mix cools affects the length of time during which density can and must be achieved.  Cool air temperatures, high humidity, strong winds, and cool surfaces, either individually or in combination shorten the time in which compaction must take place and may also make compaction more difficult.  When any of these conditions exist, it is extremely important that the compaction rolling operation follow closely behind the paver so there is no delay between placing and compacting the mat.  The roller must literally "bump the spreader".  When thin lifts are being placed from late fall to spring or during cool weather, this is especially true if specification density and tight, smooth surface textures are to be achieved.

(C) LAYER THICKNESS
Generally speaking, it is easier to achieve target density in thicker layer (lifts) of asphalt concrete than in thinner ones.  This is because the thicker the mat the longer it retains its heat and the longer the time during which compaction can be achieved.  This can be used to advantage when rolling lifts of highly stable mixes that are difficult to compact, or when paving in weather that can cause rapid cooling of thin mats.

Alternatively, increased course thickness can permit lower mix temperatures to be used because of the reduced rate of cooling.  (See Table 9-2)

(D) SUBGRADE AND BASES
The subgrade or base must be firm and non-yielding under the haul trucks and other construction compaction equipment. Subgrades or bases that show movement under trucks or construction equipment will need additional compaction work or some type of remedial work to overcome the softness. The remedial work could be lime or portland cement stabilization, or in certain circumstances, removal and replacement with a more suitable material. A yielding subgrade or base would require a thicker HMA pavement in order to support the traffic loading. Haul trucks may also be limited in size and weight to prevent pumping action of base materials.

If remedial action is not performed to correct a yielding subgrade or base, (ie. a resurfacing project), then it may be difficult to achieve the specified density.

(E) COMPACTION EQUIPMENT AND PROCEDURES
Compaction is done by any of several types of compactors, or rollers - vehicles which, by their weight  or by exertion of dynamic force, compact the pavement mat by driving over it in a specific pattern.

Compaction aims at producing a mat of specific density (target density) and smoothness.  Although the compaction process appears rather simple and straightforward, it is, in reality, a procedure requiring skill and knowledge on the part of the roller operator and the technician.  Both must have a thorough understanding of the mechanics of compaction and the factors that affect the compaction effort.

9.8.2 Three Phases of Rolling
There are three basic phases in the compaction process of asphalt pavements: Breakdown, Intermediate and Finish phases.  Each of these are described in detail below.

(A) BREAKDOWN ROLLING
Breakdown rolling is best accomplished with steel-wheeled rollers.   Either static-weight or vibratory tandem rollers may be used.  Vibratory rollers may be used in the vibratory mode on all mixes, except that on the final wearing surface the thickness must be 1 inch (25mm) or greater before use is permitted.

The weight of the roller used for breakdown rolling depends to a large degree upon the temperature, thickness, and stability of the mix being placed.  Generally, a roller weighing from 8 to 12 tons (7 to 11 metric tons) is used for this operation.

Many old compaction specifications recommend the compaction process begin on the low side of the pavement lane and proceed upwards toward the high side.  With modern compaction equipment and more stable mixes, this process is usually unnecessary unless the superelevation is extreme or the mix lift is thick in relation to its aggregate size.

When adjoining lanes are placed, it is normally best to compact the longitudinal joint by placing the majority of the roller on the hot (new) mat and overlapping the joint by a distance of approximately 6 inches over the cold mat.  This way the majority of the compaction effort is where it is needed - on the new mat.

(B) INTERMEDIATE ROLLING
Second or intermediate rolling should closely follow breakdown rolling while the asphalt mix is still plastic and at a temperature that is still well above the minimum temperature at which compaction can be achieved, preferably 225° - 250°F (107° - 120°C).

Pneumatic-tired, steel-wheeled static and vibratory rollers may be used for intermediate rolling.  When using pneumatic rollers, keeping the tires hot is the most effective means of preventing pickup.  Applying a small amount of non-foaming detergent or water soluble oil on the wetting mat of a pneumatic-tired roller at the beginning of rolling operations helps prevent asphalt from sticking to the tires until they warm up.

Pneumatic-tired rollers have several advantages:

a.  They provide a more uniform degree of compaction than steel-wheeled rollers;
b.  They improve the seal near the surface, thus decreasing the permeability of the layer; and
c.  They orient the aggregate particles for greatest stability, as high pressure truck tires do after using the asphalt surface for some time.

Vibratory tandem rollers-of proper static weight, vibration frequency and amplitude are used to provide required densities with fewer roller passes than static-weight tandem or pneumatic-tired rollers (or combinations of the two).  As mentioned previously, the vibratory roller may be used in the vibratory mode, at any time (subject to Specification requirements) on all pavement layers during the breakdown and intermediate  phases of rolling.

(C) FINISH ROLLING
Finish rolling is done primarily for the improvement of the surface.  It should be accomplished with steel-wheeled, static-weight tandems or non-vibrating vibratory tandems while the material is still warm enough for removal of roller marks.  Only enough passes should be made to remove the roller marks and smooth the mat.  Finish roller operators should be cautioned about over rolling the mat since it can decrease the mix density.  Vibratory rollers operated in the vibratory mode are not permitted as finish rollers.

NOTE:  Some Superpave mixes may exhibit a "tender zone" or lateral movement during compaction when the mix has cooled to the 250° - 210°F range.  Because of this possibility, it is very important to obtain as much density as possible during the breakdown rolling phase.  The use of vibratory rollers, more passes, additional rollers, compacting at a hotter temperature, adjusting the mix to get more "fines" (P0.075) in it, making certain that silicone has been added to the asphalt binder for surface mixes, excessive moisture in the aggregate, etc., are some things that may need to be considered.  Some success has also been achieved by using rubber tired rollers during this "tender zone".  If none of these solve the lateral movement problem, it may be necessary to hold back the intermediate and finish rollers until the mix has cooled below this "tender zone" temperature range and then continue rolling.

9.8.3 Roller Patterns

A rolling pattern that provides the most uniform coverage of the lane being paved should be used.  Rollers vary in widths, and a single recommended pattern that applies to all rollers is impractical.  For this reason, the best rolling pattern for each roller being used should be worked out and followed to obtain the most uniform compaction across the lane.  For purposes of this and future discussion, the following definitions apply.  A roller "pass" is defined as one trip of the roller in one direction over any one spot in the pavement. A “coverage” is defined as the sufficient number of passes to cover the entire laydown width of pavement.

The rolling pattern not only includes the number of passes, but also the location of the first pass, the sequence of succeeding passes, and the overlapping between passes.  Breakdown rolling speed should not exceed about 3 mph (5 kph).  In addition, sharp turns and quick starts or stops are to be avoided.  For thin lifts, a recommended rolling pattern for static steel-tired rollers is shown in Figure 9-18 (Printed version of 2006 QMS Manual).  The rolling operation should start from the edge of the spread on the low side with the roller moving forward as close behind the paver as possible.  The second movement of the roller should be to reverse in the same path until the roller has reached previously compacted material.  At this point it should swing over and move forward along path number 3, again going as close as possible behind the paver.  The fourth movement is reversal in the third path and a repetition of the previous operation.  After the entire width of the mix being placed has been covered in this fashion, the roller should swing across the spread to the low side and repeat the process.  With this pattern, the lap of the roller with succeeding passes need not be more than 3 to 4 inches (75 to 100mm).

For thick-lift construction, the rolling process should start 12 to 15 inches (300 to 400mm) from the lower unsupported edge until the center portion of the spread is compacted to some degree of stability.  Succeeding passes of the roller should then gradually progress toward the edges of the spread.  The uncompacted edge provides initial confinement during the first pass, thus minimizing lateral movement of the mix.  After the central portion of the spread has been compacted, the mix will support the roller and allow the edge to be compacted without lateral movement.

With steel-wheeled rollers the operation should always progress with the drive wheel forward in the direction of paving.  This is especially important in breakdown rolling.  The greatest percentage of compaction occurs during a breakdown pass.  A main reason why breakdown rolling should be done with the drive wheel is that there is a more direct vertical load applied by this wheel than the tiller wheel.  (Figure 9-19 in Printed version of 2006 QMS Manual).

If the breakdown pass of the roller is made with the tiller wheel forward, the pushing force and the weight is slightly ahead of vertical, causing material to push up in front of the wheel.  The greater weight of the drive wheel carries out the compaction while the turning force tends to tuck material under the front of the wheel.

There are exceptions to rolling with the drive wheel forward, however.  They usually occur when superelevations are being constructed or if the grade on which the asphalt mix is being placed is excessive.  The exceptions occur when, due to these high grades, the drive wheel of the roller begins to chatter on the mat, causing displacement of the mix and a very rough surface.  In these cases the roller must be turned around to allow the tiller wheel to partially compact the material so that the drive wheel can then proceed over it.

9.9 CONSTRUCTING TRANSVERSE JOINTS

9.9.1 Transverse Joint Specifications

Article 610-11(A) of the Standard Specifications basically reads as follows:

When the placing of the mixture is to be suspended long enough to permit the mixture to become chilled, construct a transverse joint (See Table 9-2 of this Manual).  .

If traffic will not pass over the end of the paving, a butt joint will be permitted, provided proper compaction is achieved.  If traffic will pass over the joint, construct a sloped wedge ahead of the end of the full depth pavement to provide for proper compaction and protection of the full depth pavement.  Construct the joint square to the lane alignment and discard all excess material.  Place a paper parting strip beneath this wedge to facilitate joint construction unless waived by the Engineer.

Before paving operations are resumed, remove the sloped wedge and cut back into the previously constructed pavement to the point of full pavement depth.  Coat the exposed edge of the previously constructed pavement with tack coat.

When laying of the mixture is resumed at the joint, complete and then test the construction and smoothness of the joint in accordance with Article 610-12 while the mixture is still in a workable condition and can be corrected.

9.9.2 Construction Procedures

A transverse joint is constructed at any point where the paving operation is interrupted for a period of time (15 minutes or more) and the paving operation is to be resumed later. The type of transverse joint to be constructed depends primarily on whether traffic will be traveling over the mat before paving is resumed.  A poorly constructed transverse joint is noticeable as a pronounced bump in the pavement.  Consequently, the Technician must be on hand whenever a transverse joint is made in order to ensure it is done properly. Discovering hours after construction that a transverse joint is unsatisfactory does no good, because joint construction can only be corrected while the mix is still hot and workable.  Once the mix cools, corrections can be made only by cutting out and replacing the joint.

Transverse joints are constructed in three steps:
(1) ending the lane or width of pavement (with proper compaction and thickness) at the point of work stoppage,
(2) resumption of paving operations at a subsequent time, and
(3) rolling the transverse joint.  These steps are described below.

Ending a Lane:

When ending a lane (for whatever reason) and paving is to be later resumed, the objective is to end with full depth pavement and to establish a vertical face on the mat such that when paving is resumed, full depth pavement can be placed, beginning at the joint. This can be accomplished by the use of either a butt joint or a paper tapered joint as required by the traffic situation. Ending a lane should generally be done in the following manner.

(1) When the paver is placing the last load, operate the paver in normal fashion as it approaches the location of the proposed joint.

(2) As the hopper begins to empty and the amount of material in the screed chamber decreases below normal operating level, the paver is stopped.

(3) The screed is raised and the paver moved out of the way.

(4)  Excess asphalt hot-mix is then shoveled away from the end of the mat to form a straight, full thickness, clean, vertical edge at the selected location.

(5) If a butt joint is to be constructed, place runnoff boards of sufficient thickness, length, and depth next to the joint to support the roller during compaction so as to result in a full depth mat with a vertical face. Complete compaction and remove boards as necessary.

(6) If a paper tapered joint is required, place treated release wrapping paper is placed along the edge as shown in Figure 9-20.  Paper joints are required per Standard Specifications Article 610-11, unless waived by the Engineer. The material that was shoveled away in Step 4 is replaced and used to form a taper. The suggested minimum length to compacted thickness ratio of the taper is 12:1.

(7) Compact the mat to the required density.
 

When construction is ready to be resumed, the following procedure is used to form a smooth and durable transverse joint.

(1) The taper material is removed and discarded or recycled along with the board or paper.

(2) A 10 ft. (3 meter) straightedge is used to check the longitudinal grade of the mat.  Because the paver was running out of material as it laid the last few feet (meters) of mat, it is possible that those last few feet (meters) taper slightly (ramp down) from the specified level of the mat.  If this is the case, a new transverse vertical edge must be cut to at the point of full pavement depth behind the point where the ramping down begins.  (See Standard Specs. Article 610-11.)

(3) The vertical face and pavement surface of the mat is tack-coated.

(4) The paver is backed up to the edge of the mat and the screed rested on the mat surface.

(5) The screed is heated while it rests on the mat.  This provides some heat to the material at the edge of the mat.

(6) The heated screed is raised and at least 3 shims or starting blocks as thick as the difference between the uncompacted and compacted mat are positioned under it.  The starting blocks should extend the full length of the screed, front to back.

(7) Null the screed and set the proper angle of attack on both sides of the paver.

(8) The truck with the first load of hot-mix is backed carefully to the hopper.  During discharge of the mix from the truck bed to the paver, it is essential that the truck not bump the paver, and cause it to move.

(9) Activate the material feed system and bring the head of material in the auger chamber up to the proper level (up to the auger shaft) across the entire width of the screed.

(10) Start the paver forward, pull the screed off the starting blocks, and bring the paver to the desired laydown speed as quickly as feasible. Adjust the angle of attack as necessary to provide the proper loose thickness of the asphalt mat.

(11) Once the paver has moved away, excess hot-mix is cleaned off the surface of the mat and the joint is checked with a straightedge to assure smoothness and that adequate loose material has been placed to allow for compaction.
 

Ideally, a transverse joint should be rolled transversely; however, because of maintaining traffic, site restrictions, safety, slopes, etc., most transverse joints are rolled in the longitudinal direction. This can be satisfactorily accomplished as long as the initial elevation of the new mix is sufficiently  above that of the old mix on the cold side of the joint to allow for full compaction. Rolling and compaction should be accomplished as quickly as possible after the paver has moved off the joint and checked for smoothness and loose thickness relative to the elevation of the cold side of the joint. The roller should pass slowly and completely over the joint before the roller is reversed.

Once the joint has been compacted, it must be checked for smoothness. NCDOT specifications, Article 610-12, allow no more than 1/8 in (3.2mm) variation between any two contact points using a 10 ft. (3 meter) straightedge.  If the joint is satisfactory, no further work is necessary.  If the straightedge shows an uneven joint, the surface of the new mat must be scarified while still warm and workable.  Scarification is done, preferably with a tined lute. Excess material can then be removed or additional material added, and the joint rolled and rechecked.

9.10 CONSTRUCTING LONGITUDINAL JOINTS

9.10.1 Longitudinal Joint Specifications

Article 610-11(B) of the Standard Specifications basically reads as follows:

Tack the exposed edge of all longitudinal joints prior to placing the adjoining pavement.

Form longitudinal joints by allowing the paver to deposit the mixture adjacent to the joint to such depth that maximum compaction can be obtained along the joint.  Pinch the joint by rolling immediately behind the paver.

When multi-lane multi-layer construction is required, offset the longitudinal joints in each layer from that in the layer immediately below by approximately 6 inches.  Construct the joints in the final layer, where possible, between designated travel lanes of the final traffic pattern.
 

9.10.2 Location Of Longitudinal Joints

The location of longitudinal joints must be carefully planned to achieve durable joints. The width of spread is controlled in many instances by the location of the longitudinal joint. The Standard  Specifications, as noted above, require that when a multi-lane multi-layer pavement is being constructed,  the longitudinal joints in each layer must be offset from the joint in the layer immediately below by approximately 6 inches (150 mm) (See Fig. 9-22).  Overlapping of successive courses rather than stacking the joint directly on top of the joint below helps to prevent cracking and separation along the longitudinal joint.  The locations of joints must  also be planned such that the joint in the final layer of pavement is  located, where possible, between designated travel lanes of the final traffic pattern. This will assure that the joint is not located in the wheel path of a lane. Joints located in a wheel path have a detrimental effect on ride quality and are more susceptible to water intrusion under traffic during rain or snow.
 

9.10.3 Construction Procedures

Construction of the First Lane

Two key factors that affect the long term durability of a longitudinal joint are built into the pavement during construction of the first lane. One is the importance of running the paver in a straight line so the joint can be matched on the next pass of the paver. The other is the need to properly compact  the unconfined edge of the first lane. In laying the first lane, a stringline, curb, or other reference line must be used to guide the paver on the proper course.  It is also important, for good results, that the thickness adjustment controls on the paver not be over-controlled. If an extendable screed is used, its width must be kept constant. Moving the extension in and out will create an uneven edge that will be very difficult to match.

To achieve proper density at the longitudinal joint, it is essential to compact the unconfined edge of the first lane correctly.  The edge of the drumon a vibrator or steel wheel roller should extend out over the edge of the mix a minimum of 6 inches (150 mm) when the first lane is being compacted.

Construction of the Adjoining  Lane

Tack the exposed edge of all longitudinal joints prior to placing the adjoining pavement.

Form the longitudinal joint by allowing the paver to deposit mixture adjacent to the joint to such depth that maximum compaction can be obtained along the joint.  If the level of the new uncompacted mix is even with or below the level of the adjacent compacted mix, steel wheel compaction equipment will not be able to properly densify the mix along the joint.  Whether the first pass of the roller is primarily on the hot side of the joint or primarily on the cold side, the roller will bridge the mix at the joint leaving it essentially uncompacted or only partially compacted. Therefore, it is imperative that the level of the uncompacted mix at the longitudinal joint be above that of the compacted mix by approximately 1/4" for each 1" of compacted pavement. The longitudinal joint should be rolled immediately behind the paving operation.

When placing the adjoining lane, it is important that the adjoining lane be placed so that the mix uniformly overlaps the first lane by 1 to1.5 in. (25 to 38 mm) (Fig. 9-23a). The thickness of the overlap should be about one-fourth the desired compacted thickness. This is the preferred method of longitudinal joint construction. If the longitudinal edge of the first lane is straight and the correct amount of overlap is used, the amount of raking will be minimal.  If the overlapped material is slightly excessive, the excess overlapping material may be bumped or crowded back onto the hot lane so that the roller can crowd the small excess into the hot side of the joint (Fig. 9-23b).  When the overlap is excessive, the excess material should be trimmed off so that the bumped ridge of material along the joint is uniform. In no case should the raker broadcast the excess mix across the width of the new lane.

A trimmed joint is sometimes used (Fig. 9-23c).  This joint is constructed by removing all freshly placed material that has overlapped the rolled lane.  This is best done by trimming the joint immediately behind the paver with a square-ended shovel.  Again, in no case should the mix be broadcast across the width of the new lane.

If the lanes are placed simultaneously with two pavers moving in echelon, the loose depths of the mats should match exactly, with no overlap for a hot joint.  The joints of a freshly paved mat are usually compacted before the rest of the paved width.

Rolling and Compaction of Longitudinal Joints:

Longitudinal joints should be rolled directly behind the paving operation.  The most efficient and recommended way to compact a longitudinal joint is to place the roller on the hot (new) mat so that approximately 6 in (150mm) rides on the cold (existing) adjoining lane (Fig. 9-24 in printed version of 2006 QMS Manual).  The roller is operated here to pinch and press the fines into place and provide adequate compaction to the joint and the hot mat adjoining the joint.

A different procedure, although not recommended, has the roller making the first pass toward the paver with the drum on the hot material about 3-6 in (75-150mm) away from the first-placed lane. This leaves a narrow ridge of hot, unrolled mix, Figure 9-25a.  On the return pass, the first-placed lane is overlapped about 3-6 in (75-150mm) Figure 9-25b.

In the past it was common practice to do the initial rolling of the longitudinal joint from the cold side of the joint, frequently referred to as “pinching the joint”. On the first pass, only about 6 in. of the width of the roller rides over the fresh mat, compressing the mix at the joint. The majority of the compactive force is wasted because the roller is essentially applying its compactive force to an already compacted mat. In the meantime, the rest of the mat is cooling, possible causing density to be more difficult to achieve.

Regardless of the rolling method or equipment used, the level of the uncompacted mix at the longitudinal joint must be above that of the previously compacted mix by an amount equal to approximately 1/4 in. for each 1 in. (6 mm for each 25 mm) of compacted pavement if proper compaction of the mix at the joint is to be accomplished. If it is not, the compaction equipment will bridge the mix at the joint, leaving it essentially uncompacted or only partially compacted.
 
 

The following rule of “rounding off” shall be used in all calculations. When the digit to be dropped (one digit beyond significant digit) is 0, 1, 2, 3 and 4, the preceding digit will not change.  When the digit to be dropped (one digit beyond significant digit) is 5, 6, 7, 8 and 9, the preceding digit will be increased by one.  NOTE: Do not round one digit beyond the significant decimal

Example:   Significant decimal of 0.1
91.74 will be 91.7 (Drop the 4 and leave the 7 as is)
91.75 will be 91.8 (Drop the 5, and round up the 7 to a 8)