Figure 1: A Steel Wheel and a Pneumatic Tire Roller Working Side-by-Side.

Compaction is the greatest determining factor in dense graded pavement performance (Scherocman and Martenson, 1984^[2]; Scherocman, 1984^[3]; Geller, 1984^[4]; Brown, 1984^[5]; Bell et. al., 1984^[6]; Hughes, 1984^[7]; Hughes, 1989^[8]). Inadequate compaction results in a pavement with decreased stiffness, reduced fatigue life, accelerated aging/decreased durability, rutting, raveling, and moisture susceptibility (Hughes, 1984^[7]; Hughes, 1989^[8]).

Compaction Measurement and Reporting

Compaction reduces the volume of air in HMA. Therefore, the characteristic of concern is the volume of air within the compacted pavement, which is typically quantified as a percentage of air voids in relation to total volume and expressed as “percent air voids”. Percent air voids is calculated by comparing a test specimen’s density with the density it would theoretically have if all the air voids were removed, known as “theoretical maximum density” (TMD) or “Rice density” after the test procedure inventor.

Although percent air voids is the HMA characteristic of interest, measurements are usually reported as a measured density in relation to a reference density. This is done by reporting density as:

• Percentage of TMD (or “percent Rice”). This expression of density is easy to convert to air voids because any volume that is not asphalt binder or aggregate is assumed to be air. For example, a density reported as 93 percent Rice means that there are 7 percent air voids (100% – 93% = 7%).
• Percentage of a laboratory-determined density. The laboratory density is usually a density obtained during mix design.
• Percentage of a control strip density. A control strip is a short pavement section that is compacted to the desired value under close scrutiny then used as the compaction standard for a particular job.

Pavement air voids are measured in the field by one of two principal methods:

• Cores (Figures 2 and 3). A small pavement core is extracted from the compacted HMA and sent to a laboratory to determine its density. Usually, core density results are available the next day at the earliest. This type of air voids testing is generally considered the most accurate but is also the most time consuming and expensive.
• Nuclear gauges (Figures 4 and 5). A nuclear density gauge measures in-place HMA density using gamma radiation. Gauges usually contain a small gamma source (about 10 mCi) such as Cesium-137 located in the tip of a small probe, which is either placed on the surface of the pavement or inserted into the pavement. Readings are obtained in about 2 – 3 minutes. Nuclear gauges require calibration to the specific mixture being tested. Usually nuclear gauges are calibrated to core densities at the beginning of a project and at regular intervals during the project to ensure accuracy.

Each contracting agency or owner usually specifies the compaction measurement methods and equipment to be used on contracts under their jurisdiction.

Figure 2: Core Extraction	Figure 3: Pavement Core
Figure 4: Thin Lift Nuclear Density Gauge	Figure 5: Taking a Nuclear Density Reading

Factors Affecting Compaction

HMA compaction is influenced by a myriad of factors; some related to the environment, some determined by mix and structural design and some under contractor and agency control during construction (see Table 1).

A Note on the Time Available for Compaction

HMA temperature directly affects asphalt binder viscosity and thus compaction. As HMA temperature decreases, the constituent asphalt binder becomes more viscous and resistant to deformation resulting in a smaller reduction in air voids for a given compactive effort. As the mix cools, the asphalt binder eventually becomes stiff enough to effectively prevent any further reduction in air voids regardless of the applied compactive effort. The temperature at which this occurs, commonly referred to as cessation temperature, is often reported to be about 175°F for dense-graded HMA (Scherocman and Martenson, 1984^[9]; Hughes, 1989^[8]). Below cessation temperature rollers can still be operated on the mat to improve smoothness and surface texture but further compaction will generally not occur.

Mat temperature is crucial to both the actual amount of air void reduction for a given compactive effort, and the overall time available for compaction. If a mat’s initial temperature and cool-down rate are known, the temperature of the mat at any time after laydown can be calculated. Based on this calculation rolling equipment and patterns can be employed to:

• Take maximum advantage of available roller compactive effort. Rollers can be used where the mat is most receptive to compaction and avoided where the mat is susceptible to excessive shoving.
• Ensure the mat is compacted to the desired air void content before cessation temperature is reached. This can be done by calculating the time it takes the mat to cool from initial temperature to cessation temperature. All compaction must be accomplished within this “time available for compaction”.

MultiCool, developed by Professor Vaughn Voeller and Dr. David Timm, is a Windows based program that predicts HMA mat cooling. MultiCool can be used to predict the time available for compaction and is available on the National Asphalt Pavement Association’s A Guide for Hot Mix Asphalt Pavement CD-ROM or for download at:

• University of California Pavement Research Center (http://www.ucprc.ucdavis.edu/SoftwarePage.aspx)
• National Asphalt Pavement Association (http://www.asphaltpavement.org/index.php?option=com_content&task=view&id=178&Itemid=273)

Compaction Equipment

There are three basic pieces of equipment available for HMA compaction: (1) the paver screed, (2) the steel wheeled roller and (3) the pneumatic tire roller. Each piece of equipment compacts the HMA by two principal means:

1. By applying its weight to the HMA surface and compressing the material underneath the ground contact area. Since this compression will be greater for longer periods of contact, lower equipment speeds will produce more compression. Obviously, higher equipment weight will also increase compression.
2. By creating a shear stress between the compressed material underneath the ground contact area and the adjacent uncompressed material. When combined with equipment speed, this produces a shear rate. Lowering equipment speed can decrease the shear rate, which increases the shearing stress. Higher shearing stresses are more capable of rearranging aggregate into more dense configurations.

These two means are of compacting HMA are often referred to collectively as “compactive effort”.

Steel Wheel Rollers

Steel wheel rollers (see Figures 6 and 7) are self-propelled compaction devices that use steel drums to compress the underlying HMA. They can have one, two or even three drums, although tandem (2 drum) rollers are most often used. The drums can be either static or vibratory and usually range from 35 to 85 inches in width and 20 to 60 inches in diameter. Roller weight is typically between 1 and 20 tons (see Figures 5 and 6).

Some steel wheel rollers are equipped with vibratory drums. Drum vibration adds a dynamic load to the static roller weight to create a greater total compactive effort. Drum vibration also reduces friction and aggregate interlock during compaction, which allows aggregate particles to move into final positions that produce greater friction and interlock than could be achieved without vibration. As a general rule-of-thumb, a combination of speed and frequency that results in 10 – 12 impacts per foot is good. At 3000 vibrations/minute this results in a speed of 2.8 – 3.4 mph.

Figure 6: Steel Wheel Rollers

Figure 7: Steel Wheel Rollers

Pneumatic Tire Rollers

Pneumatic tire rollers are self-propelled compaction devices that uses pneumatic tires to compact the underlying HMA. Pneumatic tire rollers employ a set of smooth tires (no tread) on each axle; typically four or five on one axle and five or six on the other. The tires on the front axle are aligned with the gaps between tires on the rear axle to give complete and uniform compaction coverage over the width of the roller. Compactive effort is controlled by varying tire pressure, which is typically set between 60 and 120 psi (TRB, 2000^[10]). In addition to a static compressive force, pneumatic tire rollers also develop a kneading action between the tires that tends to realign aggregate within the HMA. Because asphalt binder tends to stick more to cold tires than hot tires, the tire area is sometimes insulated with rubber matting or plywood to maintain the tires near mat temperature while rolling (see Figures 8 and 9).

Figure 8: Pneumatic Tire Roller

Figure 9: Pneumatic Tires

Compaction Sequence

HMA compaction is typically accomplished by a sequence of compaction equipment. This allows each piece of equipment to be used only in its most advantageous situation resulting in a higher quality mat (both in density and in smoothness) than could be produced with just a single method of compaction. A typical compaction sequence consists of some or all of the following (in order of use):

• Screed. The screed is the first device used to compact the mat and may be operated in the vibratory mode. Approximately 75 to 85 percent of TMD will be obtained when the mix passes out from under the screed (TRB, 2000^[10]).
• Rollers. Generally a series of two or three rollers is used. Contractors can control roller compaction by varying things such as the types of rollers used, the number of roller used, roller speed, the number of roller passes over a given area of the mat, the location at which each roller works, and the pattern that each roller uses to compact the mat. Approximately 92 to 95 percent TMD will be obtained when all rollers are finished compacting the mat. Typical roller position used in compaction are:
  • Breakdown Roller. The first roller behind the screed (see Figure 10). It generally effects the most density gain of any roller in the sequence. Breakdown rollers can be of any type but are most often vibratory steel wheel and sometimes pneumatic tire.
  • Intermediate Roller. Used behind the breakdown roller if additional compaction is needed (see Figure 10). Pneumatic tire rollers are sometimes used as intermediate rollers because they provide a different type of compaction (kneading action) than a breakdown steel wheel vibratory roller, which can help further compact the mat or at the very least, rearrange the aggregate within the mat to make it receptive to further compaction.
  • Finish Roller. The last roller in the sequence (see Figure 11). It is used to provide a smooth mat surface. Although the finish roller does apply compactive effort, by the time it comes in contact with the mat, the mat may have cooled below cessation temperature. Static steel wheel rollers are almost always used as finishing rollers because they can produce the smoothest surface of any roller type.

Figure 10: Paving Operation Showing a Steel Wheel Breakdown Roller and a Pneumatic Tire Intermediate Roller

Figure 11: Finish Roller

• Traffic. After the rollers have compacted the mat to the desired density and produced the desired smoothness, the new pavement is opened to traffic. Traffic loading will provide further compaction in the wheel paths of a finished mat. Traffic may compact the mat an additional 2 to 4 percent over the life of the pavement.

A stabilizer/reclaimer is a vehicle with a dual purpose. These machines have a large rotor blade which may be used to cut and pulverize damaged or old pavement, but which also may be used to mix lime, fly ash, or cement into the subbase in order to stabilize poor soils.

Milling Machines

A top layer is milled off the existing pavement to provide a relatively smooth surface on which to pave. Milling is also commonly used to remove a distressed surface layer from an existing pavement. Milling machines are the primary method for removing old HMA pavement surface material prior to overlay. They can be fitted with automatic grade control to restore both longitudinal and transverse grade and can remove most existing pavement distortions. Milling also produces a rough, grooved surface, which will increase the existing pavement’s surface area when compared to an ungrooved surface. The surface area increase is dependent on the type, number, condition and spacing of cutting drum teeth but is typically in the range of 20 to 30 percent, which requires a corresponding increase in tack coat (20 to 30 percent more) when compared to an unmilled surface. Milling is advantageous because it:

• Provides RAP for recycling operations.
• Efficiently removes deteriorated pavement that is unsuitable for retention in the overlaid pavement.
• Provides a highly skid resistant surface suitable for temporary use by traffic until the final surface can be placed.
• Allows curb and gutter lines to be maintained or reestablished before HMA overlays.
• Provides an efficient removal technique for material near overhead structures in order to maintain clearances for bridge structures, traffic signals and overhead utilities.

Figure 1: Side view of milling machine.

Figure 2: Cutting tips on rotary drum.

Graders

Graders may be used in place of milling machines if the base course is dirt or gravel. They are vehicles with large blades that create a wide flat surface for asphalt to be placed on.

Figure 3: A grader preparing the subgrade on a project on SR 528 in Marysville, WA.

Sweepers

Sweepers clean the surface of the road after it has been milled or graded. This is necessary because excessive dust and debris on the ground can prevent proper bonding between the asphalt and the base course. Large pieces of debris can also cause non-uniform compaction of the asphalt.

Figure 4: Sweeping machine.

Batch Plant

Batch plants, which produce HMA in individual batches, are the older of the two types of HMA production facilities; it was not until the 1970s that drum plants became a popular HMA production option. Typical batch quantities range from 1.5 to 5 tons of HMA and each batch can take 15 – 45 seconds to make.

Figure 5: Batch plant

Drum Plant

Drum plants, which produce HMA in a continuous manner, generally offer higher production rates than batch plants for comparable cost. Typical production rates for drum plants vary between about 100 tons/hr up to over 900 tons/hr depending upon drum design.

Figure 6: A continuous or drum plant.

Dump Trucks

Dump trucks move the hot asphalt from the plant to the jobsite. There are many kinds of dump trucks:

• End dump: unload their payload by raising the front end and letting the payload slide down the bottom of the bed and out the back through a tailgate. They are the most popular transport vehicle type because they are plentiful, maneuverable and versatile.
• Bottom or belly dump: Bottom dump trucks unload their payload by opening gates on the bottom of the bed. Internal bed walls are sloped to direct the entire payload out through the opened gates. Discharge rates can be controlled by the degree of gate opening and the discharge is usually placed in an elongated pile, called a windrow, in front of the paver by driving the truck forward during discharge. Windrows require a special MTV (material transfer vehicle) to feed the HMA into the paver.
• Live bottom or flo-boy: Live bottom dump trucks have a conveyor system at the bottom of their bed to unload their payload. HMA is discharged out the back of the bed without raising the bed. Live bottom trucks are more expensive to use and maintain because of the conveyor system but they also can reduce segregation problems and can eliminate some detrimental types of truck bed – paver contact (because the bed is not raised during discharge).

Figure 7: End dump truck placing WMA into a skip.

Material Transfer Vehicles

Material transfer vehicles (MTVs) are used to assist the paver in accepting HMA. Most pavers are equipped to receive HMA directly from end dump or live bottom trucks, however in certain situations it can be necessary or advantageous to use an MTV. Paving using bottom dump trucks and windrows requires a windrow elevator MTV, while other MTVs are used to provide additional surge volume, which is advantageous because it allows the paver to operate continuously without stopping, minimizes truck waiting time at the paving site and may minimize aggregate segregation and temperature differentials.

Figure 8: Material transfer vehicle, or shuttle buggy.

Asphalt Pavers

The asphalt paver is a self-propelled formless laydown machine with a floating screed. HMA is loaded in the front, carried to the rear by a set of flight feeders (conveyor belts), spread out by a set of augers, then leveled and compacted by a screed. This set of functions can be divided into two main systems:

• Tractor: The tractor contains the material feed system, which accepts the HMA at the front of the paver, moves it to the rear and spreads it out to the desired width in preparation for screed leveling and compaction.
• Screed: The most critical feature of the paver is the self-leveling screed unit, which determines the profile of the HMA being placed (Roberts et al., 1996^[1]). The screed takes the head of HMA from the material delivery system, strikes it off at the correct thickness and provides initial mat compaction. Figure 5 shows screed components and the six basic forces that act upon the screed to determine its height and, thus, pavement thickness.

The screed helps control the amount of material extruded onto the base course, flattening the asphalt on the ground. It also assists in offering a level surface for compaction regardless of the condition of the base course. However, the base course needs to be reasonably level in order to prevent future cracking.

Figure 9: A typical asphalt paving machine.

Compactors/Rollers

There are three basic pieces of equipment available for HMA compaction: (1) the paver screed, (2) the steel wheeled roller and (3) the pneumatic tire roller. Each piece of equipment compacts the HMA by two principal means:

1. By applying its weight to the HMA surface and compressing the material underneath the ground contact area. Since this compression will be greater for longer periods of contact, lower equipment speeds will produce more compression. Obviously, higher equipment weight will also increase compression.
2. By creating a shear stress between the compressed material underneath the ground contact area and the adjacent uncompressed material. When combined with equipment speed, this produces a shear rate. Lowering equipment speed can decrease the shear rate, which increases the shearing stress. Higher shearing stresses are more capable of rearranging aggregate into more dense configurations.

These two means are of compacting HMA are often referred to collectively as “compactive effort”.

Paver Screed

Approximately 75 to 85 percent of theoretical maximum density, or Rice density, will be obtained when the mix passes out from under the screed.

Steel Wheel Rollers

Steel wheel rollers are self-propelled compaction devices that use steel drums to compress the underlying HMA. They can have one, two or even three drums, although tandem (2 drum) rollers are most often used. The drums can be either static or vibratory and usually range from 35 to 85 inches in width and 20 to 60 inches in diameter. Roller weight is typically between 1 and 20 tons.

Some steel wheel rollers are equipped with vibratory drums. Drum vibration adds a dynamic load to the static roller weight to create a greater total compactive effort. Drum vibration also reduces friction and aggregate interlock during compaction, which allows aggregate particles to move into final positions that produce greater friction and interlock than could be achieved without vibration. As a general rule-of-thumb, a combination of speed and frequency that results in 10 – 12 impacts per foot is good. At 3000 vibrations/minute this results in a speed of 2.8 – 3.4 mph.

Figure 10: A steel wheel roller

Pneumatic Tire Rollers

Pneumatic tire rollers are self-propelled compaction devices that uses pneumatic tires to compact the underlying HMA. Pneumatic tire rollers employ a set of smooth (no tread) tires on each axle; typically four or five on one axle and five or six on the other. The tires on the front axle are aligned with the gaps between tires on the rear axel to give complete and uniform compaction coverage over the width of the roller. Compactive effort is controlled by varying tire pressure, which is typically set between 60 and 120 psi. In addition to a static compressive force, pneumatic tire rollers also develop a kneading action between the tires that tends to realign aggregate within the HMA. Because asphalt binder tends to stick more to cold tires than hot tires, the tire area is often insulated with rubber matting or plywood to maintain the tires near mat temperature while rolling.

Figure 11: A pneumatic tire roller compacting chip seal.

References

• Washington Asphalt Pavement Guide, Washington Asphalt Pavement Association, Inc., 2002

Figure 1: Asphalt Paver

Figure 2: HMA Placement

Placement Considerations

There are, of course, many considerations to take into account when placing HMA. Many are dependent upon local materials, weather, crew knowledge and training, and individual experience. This subsection presents a few of the basic considerations that apply in virtually all situations:

• Lift thickness. A “lift” refers to a layer of pavement as placed by the asphalt paver. In order to avoid mat tearing (which generally shows up as a series of longitudinal streaks) a good rule-of-thumb is that the depth of the compacted lift should be at least twice the maximum aggregate size and three times the nominal maximum aggregate size (TRB, 2000^[1]).

Overly thick final lifts have a tendency to shove or displace during compaction making it difficult to achieve a smooth finish.

• Longitudinal joints. The interface between two adjacent and parallel HMA mats. Improperly constructed longitudinal joints can cause premature deterioration of multilane HMA pavements in the form of cracking and raveling.
• Handwork. HMA can be placed by hand in situations where the paver cannot place it adequately. This can often occur around utilities, around intersection corners and in other tight spaces. Hand-placing should be minimized because it is prone to aggregate segregation and results in a slightly rough surface texture. If hand placement is necessary the following precautions should be taken (Asphalt Institute, 2001^[2]):
  • Place the HMA in a pile far enough away from the placement area that the whole pile must be moved. If the pile is located in the placement area its appearance, density or aggregate distribution may be slightly different than the surrounding handworked mat.
  • Carefully deposit the material with shovels and then spread with lutes. Do not broadcast (scoop and pitch) the HMA with shovels – this is likely to cause aggregate segregation.
  • All material should be thoroughly loosened and evenly distributed. Chunks of HMA that do not easily break apart should be removed and discarded.
  • Check the handworked surface with a straightedge or template before rolling to ensure uniformity.

Asphalt Paver

The asphalt paver is a self-propelled formless (does not require side forms) laydown machine with a floating screed (see Figures 1, 2, 3 and 4). HMA is loaded in the front, carried to the rear by a set of flight feeders (conveyor belts), spread out by a set of augers, then leveled and compacted by a screed. This set of functions can be divided into two main systems:

• Tractor. The tractor contains the material feed system, which accepts the HMA at the front of the paver, moves it to the rear and spreads it out to the desired width in preparation for screed leveling and compaction. The tractor moves using rubber tires (see Figures 1 and 2) or tracks (see Figure 3).
• Screed. The most critical feature of the paver is the self-leveling screed unit, which determines the profile of the HMA being placed (Roberts et al., 1996^[3]). The screed takes the head of HMA from the material delivery system, strikes it off at the correct thickness and provides initial mat compaction. Figure 5 shows screed components and the six basic forces that act upon the screed to determine its height and, thus, pavement thickness.

Figure 3: Tracked Paver

Factors Affecting Mat Thickness and Smoothness

Since the screed is free floating it will slide across the HMA at an angle and height that will place the six forces shown in Figure 5 in equilibrium. When any one of these forces is changed, the screed angle and elevation will change (which will change the mat thickness) to bring these forces back into equilibrium. Therefore, changing the following paver characteristics will affect these forces, and thus mat thickness, in the described manner:

• Paver speed. If a paver speeds up and all other forces on the screed remain constant, the screed angle decreases to restore equilibrium, which decreases mat thickness (think of what happens to the ski angle of a water skier as boat speed increases).
• Material head. If the material head (the amount of material in front of the screed) increases (either due to an increase in material feed rate or a reduction in paver speed), screed angle will increase to restore equilibrium, which increases mat thickness.
• Tow point elevation. As the tow point rises in elevation, the screed angle increases, resulting in a thicker mat. As a rule-of-thumb, a 1-inch movement in tow point elevation translates to about a 0.125 inch movement in the screed’s leading edge. Without automatic screed control, tow point elevation will change as tractor elevation changes due to roughness in the surface over which it drives. Locating the screed tow point near the middle of the tractor significantly reduces the transmission of small elevation changes in the front and rear of the tractor to the screed. Because the screed elevation responds slowly to changes in screed angle, the paver naturally places a thinner mat over high points in the existing surface and a thicker mat over low points in the existing surface (TRB, 2000^[1]).
• Screed angle can also be adjusted manually by using a thickness control screw or depth crank. Screed angle adjustments do not immediately change mat thickness but rather require a finite amount of time and tow distance to take effect. Figure 6 shows that it typically takes five tow lengths (the length between the tow point and the screed) after a desired level is input for a screed to arrive at the new level. Because of this screed reaction time, a screed operator who constantly adjusts screed level to produce a desired mat thickness will actually produce an excessively wavy, unsmooth pavement.

Figure 6: Screed Reaction to a Manual Decrease in Screed Angle (after TRB, 2000^[1])

Automatic Screed Control

Since it is not practical to manually control tow point elevation, pavers usually operate using an automatic screed control, which controls tow point elevation using a reference other than the tractor body. Since these references assist in controlling HMA pavement grade, they are called “grade reference systems” and are listed below (Roberts et al., 1996^[3]):

• Erected stringline. This consists of stringline erected to specified elevations that are independent of existing ground elevation (see Figure 7). Most often this is done using a survey crew and a detailed elevation/grade plan. Although the stringline method provides the correct elevation (to within surveying and erecting tolerances), stringlines are fragile and easily broken, knocked over or inadvertently misaligned. Lasers can be used to overcome the difficulties associated with stringlines because they do not require any fragile material near the pavement construction area. Lasers can establish multiple elevation or grade planes even in dusty or high-electronic and light-noise areas and are therefore sometimes used to construct near-constant elevation airport runways. Even the laser method becomes quite complicated, however, when frequent pavement grade changes are required.
• Mobile reference. This consists of a reference system that travels with the paver such as a long beam or tube attached to the paver (called a “contact” device since it actually touches the road) or an ultrasonic device (called a “non-contact” device since it relies on ultrasonic pulses and not physical contact to determine road elevation – see Figure 8). The mobile reference system averages the effect of deviations in the existing pavement surface over a distance greater that the wheelbase of the tractor unit. Minimum ski length for a contact device is normally about 25 ft. with typical ski lengths being on the order of 40 to 60 ft. (Asphalt Institute, 2001^[2]).
• Joint matching shoe. This usually consists of a small shoe or ski attached to the paver that slides on an existing surface (such as a curb) near the paver. Ultra sonic sensors accomplish the same task without touching the existing surface by using sound pulses to determine elevation. This type of grade control results in the paver duplicating the reference surface on which the shoe or ski is placed or ultra sonic sensor is aimed.

Figure 7: Stringline

Figure 8: Mobile Reference System (Circled)

Material Transfer Vehicles (MTVs)

Material transfer vehicles (MTVs) are used to assist the paver in accepting HMA. Most pavers are equipped to receive HMA directly from end dump or live bottom trucks, however in certain situations it can be necessary or advantageous to use an MTV. Paving using bottom dump trucks and windrows requires a windrow elevator MTV, while other MTVs are used to provide additional surge volume, which is advantageous because it allows the paver to operate continuously without stopping, minimizes truck waiting time at the paving site and may minimize aggregate segregation and temperature differentials (see Figures 9 and 10).
picture of an MTV

Figure 9: Roadtec Shuttlebuggy™ MTV

Figure 10: Roadtec Shuttlebuggy™ MTV

Figure 1: Aggregate Segregation in a Stockpile

Figure 2: Temperature Differentials

Aggregate Segregation

Aggregate segregation is the non-uniform distribution of coarse and fine aggregate components within the HMA mixture. There are two basic types of aggregate segregation:

1. Coarse segregation. Occurs when gradation is shifted to include too much coarse aggregate and not enough fine aggregate. Coarse segregation is characterized by low asphalt content, low density, high air voids, rough surface texture, and accelerated rutting and fatigue failure (Williams, Duncan and White, 1996^[1]). Typically, coarse segregation is considered the most prevalent and damaging type of segregation, thus segregation research has typically focused on coarse segregation. The term “segregation” by itself is usually taken to mean “coarse segregation.”
2. Fine segregation. Occurs when gradation is shifted to include too much fine aggregate and not enough course aggregate. High asphalt content, low density, smooth surface texture, accelerated rutting, and better fatigue performance characterize fine segregation (Williams, Duncan and White, 1996^[1]).

A quantitative aggregate segregation definition is difficult. Since coarse segregation is generally accepted as most destructive, a general quantitative definition is a sample at least 10% coarser than the JMF on the No. 4 or No. 8 sieve (Brown and Brownfield, 1988^[2]; Cross and Brown, 1993^[3]; Williams, Duncan and White, 1996^[1]).

The chief detrimental effects of segregation on HMA performance are: reduced fatigue life, rutting, raveling, and moisture damage. These effects can cause a severe reduction in pavement life. More information on segregation causes and cures can be found in Segregation Causes and Cures for Hot Mix Asphalt (QIP-110) by AASHTO and NAPA.

Construction-Related Temperature Differentials

Construction-related temperature differentials are large mat temperature differences resulting from placement of a significantly cooler portion of HMA mass into the mat. This cooler mass comes from the surface layer (or crust) typically developed during HMA transport from the mixing plant to the job site. These cooler areas will cool down to cessation temperature (the temperature at which no further compaction can take place due to increased HMA viscosity – commonly taken as 175°F) more quickly than the surrounding mat. Roller patterns developed based on general mat temperatures may not be adequate to compact these cooler areas before they cool to cessation temperature resulting in isolated spots of inadequate compaction. Thus, temperature differentials can cause isolated areas of inadequate compaction resulting in decreased strength, reduced fatigue life, accelerated aging/decreased durability, rutting, raveling, and moisture damage (Hughes, 1984^[4]; Hughes, 1989^[5]). Generally, temperature differentials greater than about 25°F can potentially cause compaction problems (Willoughby et al., 2001^[6]).

Aggregate segregation and construction-related temperature differentials display the same symptoms and result in the same types of damage, which can cause them to be confused with one another. However, the ultimate damage mechanism, excessive air voids (often expressed as “inadequate density“), is the same in both cases.

Figure 1: Taking a Pavement Core for Density Quality Control

Figure 2: Taking a Pavement Core for Density Quality Control

Figure 3: Using the NCAT Oven to Determine Asphalt Content

AASHTO and the FHWA subscribe to definitions that designate “quality assurance” as an all-encompassing term, to include “quality control”, “independent assurance” and “acceptance” as its three key components (TRB, 1999^[1]):

• Quality assurance. All those planned and systematic actions necessary to provide confidence that a product or facility will perform satisfactorily in service. Quality assurance addresses the overall problem of obtaining the quality of a service, product, or facility in the most efficient, economical, and satisfactory manner possible. Within this broad context, quality assurance involves continued evaluation of the activities of planning, design, development of plans and specifications, advertising and awarding of contracts, construction, and maintenance, and the interactions of these activities.
• Quality control. Those quality assurance actions and considerations necessary to assess production and construction processes so as to control the level of quality being produced in the end product. This concept of quality control typically includes sampling and testing by the contractor to monitor the process but usually does not include acceptance sampling and testing by the agency/owner. Also called process control.
• Acceptance. Sampling, testing, and the assessment of test results to determine whether or not the quality of produced material or construction is acceptable in terms of the specifications.
• Independent assurance. A management tool that requires a third party, not directly responsible for process control or acceptance, to provide an independent assessment of the product and/or the reliability of test results obtained from process control and acceptance testing. The results of independent assurance tests should not be used as a basis of product acceptance.

Figure 1: End Dump Truck

Figure 2: Bottom Dump Truck

There are three basic truck types used for mix transport classified by their respective HMA discharge methods:

1. End dump. End dump trucks unload their payload by raising the front end and letting the payload slide down the bottom of the bed and out the back through a tailgate (see Figure 1 and Video 1). They are the most popular transport vehicle type because they are plentiful, maneuverable and versatile.
2. Bottom dump (or belly dump). Bottom dump trucks (see Figure 2) unload their payload by opening gates on the bottom of the bed. Internal bed walls are sloped to direct the entire payload out through the opened gates. Discharge rates can be controlled by the degree of gate opening and the discharge is usually placed in an elongated pile, called a windrow, in front of the paver by driving the truck forward during discharge. Windrows require a special MTV to feed the HMA into the paver.
3. Live bottom (or flo-boy). Live bottom dump trucks (see Figure 3) have a conveyor system at the bottom of their bed to unload their payload. HMA is discharged out the back of the bed without raising the bed. Live bottom trucks are more expensive to use and maintain because of the conveyor system but they also can reduce segregation problems and can eliminate some detrimental types of truck bed – paver contact (because the bed is not raised during discharge).

Figure 3: Live Bottom Truck

Transport Considerations

There are several mix transport considerations, or best practices, that are essential to maintaining HMA characteristics between the production facility and the paving site. These considerations can generally be placed into four categories:

1. Loading at the Production Facility. Truck beds should be clean and lubricated with non-petroleum products to prevent the HMA from sticking to the truck bed. Petroleum based products, such as diesel fuel, should not be used because of environmental issues and because they tend to break down the asphalt binder. HMA should be discharged into the truck bed so as to minimize segregation. Dropping HMA from the storage silo or batcher (for batch plants) in one large mass creates a single pile of HMA in the truck bed. Large-sized aggregate may roll off this pile and collect around the base. Dropping HMA in several smaller masses (three is typical) at different points in the truck bed will help minimize the segregation risk.
2. Truck transport. Truck transport affects HMA characteristics through cooling. HMA is usually loaded into a truck at a fairly uniform temperature between 250°F to 350°F. During transport, heat is transferred to the surrounding environment and HMA temperature drops. However, cool HMA provides excellent insulation and thus transported HMA tends to develop a cool thin crust on the surface that surrounds a much hotter core. Things such as air temperature, rain, wind and length of haul, insulated truck beds and truck tarps can affect the characteristics and temperature of this crust.
3. Unloading at the paving site. HMA should be unloaded soon after it arrives at the paving site in order to minimize mix cooling. Also, on jobs with more than one mix type the inspector and/or foreman should be certain the correct mix is loaded into the paver.
4. Operation synchronization. Truck transport should be planned such that the HMA transport rate (expressed in tons/hr) closely matches plant production rate and laydown rate. Traffic affects HMA delivery rates because it affects truck speed. Especially in congested urban areas, heavy and/or unpredictable traffic may substantially increase, or at least vary, truck travel time. As truck travel time increases, more trucks are needed to provide a given HMA delivery rate. Therefore, as traffic gets worse, trucking costs increase. Additionally, the unpredictability of traffic may result in either long paver idle times while waiting for the next truckload of HMA or large truck backups as several trucks all reach the paving site or production facility at the same time.

This section is largely taken from a series of three articles written for HMAT Magazine and a Washington State Department of Transportation (WSDOT) research report as listed below:

• Newcomb, D.E. and Epps, J.A. (Jan/Feb 2001). Statistical Specifications for Hot Mix Asphalt: What Do We Need to Know? HMAT, vol. 6, no. 1. National Asphalt Pavement Association (NAPA). Landham, MD. (first in a series of 3 articles)
• Newcomb, D.E. and Epps, J.A. (Mar/April 2001). Statistical Specifications for Hot Mix Asphalt: What Do We Need to Know? HMAT, vol. 6, no. 2. National Asphalt Pavement Association (NAPA). Landham, MD. (second in a series of 3 articles)
• Newcomb, D.E. (May/June 2001). Performance Related Specifications Developments. HMAT, vol. 6, no. 3. National Asphalt Pavement Association (NAPA). Landham, MD. (third in a series of 3 articles)
• Muench, S.T. and Mahoney, J.P. (2001). A Quantification and Evaluation of WSDOT’s Hot Mix Asphalt Concrete Statistical Acceptance Specification. WA-RD 517.1. Washington State Department of Transportation, Transportation Center (TRAC). Seattle, WA. (http://www.wsdot.wa.gov/ppsc/research/CompleteReports/WARD517_1HotMixAsphalt.pdf).

Proprietary Product Specifications

A proprietary product specification is used when a generic description of a desired product or process cannot be easily formulated. It usually contains an “or equivalent” clause to allow for some measure of competition in providing the product. It is generally acknowledged that such a specification severely limits competition, increases cost, provides little latitude for innovation, and puts substantial risk on the owner for product performance. Most agencies avoid this type of specification whenever possible, however private owners often use them.

Method Specifications

A method specification outlines a specific materials selection and construction operation process to be followed in providing a product. In the past, many construction specifications were written in this manner. A contractor would be told what type of material to produce, what equipment to use and in what manner it was to be used in construction. This type of specification allows for a greater degree of competition than the proprietary product specification, but as long as the structure is built according to the materials and methods stipulated, the owner bears the responsibility for the performance.

Although widely used, method specifications have several key disadvantages. First, they tend to stifle contractor innovation because there is virtually no incentive to develop better, more efficient construction methods. Second, since they are not statistically based and 100 percent compliance is usually not possible, method specifications usually required “substantial compliance,” a purposely vague and undefined term that can lead to disputes. For instance, if a method specification requires that four roller passes be made over the entire pavement, what happens if 98% of the pavement received four roller passes and the other 2% received only three roller passes? Does this meet the specification? Finally, spot checks of material quality, which are often used in method specifications, do not reflect overall material quality because they are taken from subjectively determined non-random locations. Since they are not random, these spot checks have no statistical validity and therefore do not reflect overall material quality.

Despite their flaws, method specifications are still widely used on the local agency level (e.g., counties, small cities, towns, etc.). In general, this is because they are familiar, straightforward to write and can be implemented with minimal agency involvement. Local agencies often lack the expertise and resources required to use statistical specifications or warranties.

End-Result Specifications

An end-result specification is one in which the final characteristics of the product are stipulated, and the contractor is given considerable freedom in achieving those characteristics. In their roughest form, they specify minimum, maximum or a range of values for any given characteristic and base acceptance on conformance to these specifications. For instance, they may state a minimum layer thickness or a range of in-place air voids. Since it is impractical to measure every square foot of constructed pavement, end-result specifications use statistical methods to estimate overall material quality based on a limited number of random samples. Therefore, end result specifications improve on methods specifications in two key areas: (1) they shift the focus away from methods and on to final product quality and (2) they do not rely on the nebulous “substantial compliance” because they clearly define acceptable quality.

Today, most large State and Federal pavement contracts use statistically based end-result specifications that incorporate some elements of method specifications (usually used to guard against early failure of the product). These end-result specifications are often referred to as a “quality assurance specifications”, “QA/QC specifications” or “QC/QA specifications”. Essentially, these specifications specify the end results and also specify certain minimum construction method requirements (e.g., temperatures below which paving is not allowed, descriptions of initial test sections, minimum number of rollers, conditions under which the agency may halt paving operations, etc.).

End result specifications assign pavement construction quality to the contractor, they define the desired final product, and they allow the contractor significant latitude in achieving that final product. This leads to innovation, efficiency, and lower costs. However, these specifications and their statistical sampling requirements are often too complex and resource intensive to be used at the local agency level.

Performance Specifications

Performance specifications are those in which the product payment is directly dependent upon its actual performance. Typical of these specifications are warranty, limited warranty and design-build-operate contracts. Contractors are held responsible for the product performance within the context of what they have control over. The contractor is given a great deal of leeway in providing the product, as long as it performs according to established guidelines. In this case, the contractor assumes considerable risk for the level of service the product provides by paying for or providing any necessary maintenance or repair within the warranty period.