6  Flexible - Compaction

It has been said that the top three factors in real estate are “location, location, location”.  It can also be said that the top three factors in HMA pavement construction are “compaction, compaction, compaction”.  Compaction is the process by which the volume of air in an HMA mixture is reduced by using external forces to reorient the constituent aggregate particles into a more closely spaced arrangement.  This reduction of air volume in a mixture produces a corresponding increase in HMA unit weight, or density (Roberts et al., 1996).  Numerous researchers have stated that compaction is the greatest determining factor in dense graded pavement performance (Scherocman and Martenson, 1984; Scherocman, 1984; Geller, 1984; Brown, 1984; Bell et. al., 1984; Hughes, 1984; Hughes, 1989).  Inadequate compaction results in a pavement with decreased stiffness, reduced fatigue life, accelerated aging/decreased durability, rutting, raveling, and moisture damage  (Hughes, 1984; Hughes, 1989). 

6.1 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.  This volume is typically quantified as a percentage of air voids by volume and expressed as “percent air voids”.  Percent air voids is calculated by comparing a test specimen’s bulk density with its theoretical maximum density (TMD) and assuming the difference is due to air.  Once TMD is known, portable devices can be used to measure HMA density in-place. The terms “percent air voids” and “density” are often used interchangeably.  Although this is not wrong, since density is used to calculate percent air voids, the fundamental parameter of concern is always percent air voids. 

Percent air voids is typically calculated by using AASHTO T 269, ASTM D 3203 or an equivalent procedure (AASHTO, 2000).  These procedures all use laboratory-determined bulk specific gravity and theoretical maximum specific gravity in the following equation:

These procedures require a small pavement core (usually 100 - 150 mm (4 - 6 inches) in diameter), which is extracted from the compacted HMA (see Figure 7.56 and 7.57).  This type of air voids testing is generally considered the most accurate but is also the most time consuming and expensive. 

Since core extraction is time consuming and expensive, air voids are often measured indirectly using a portable density-measuring device such as a nuclear density gauge  (see Figure 7.58) or electrical density gauge (see Figure 7.59). 

Each contracting agency usually specifies the compaction measurement methods and equipment to be used on contracts under their jurisdiction.  Most agencies stipulate some sort of extracted core density testing and usually allow testing by nuclear gauge.  Electric density gauges are relatively new on the market (in the last five years).   Accurate calibration of these devices is essential for their proper use.  

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 a (1) percent of TMD (sometimes called Rice density), (2) percent of a laboratory density or (3) percent of a control strip density (a control strip is a short pavement strip that is compacted to the desired value under close scrutiny then used as the compaction standard for a particular job).   

In sum, percent air voids is the critical HMA characteristic with which compaction is concerned.  It can be measured using pavement cores or portable nuclear or electric gauges; measurement specifications vary from one contracting agency to the next.  Percent air voids is usually reported as a density in one of three forms: (1) percent TMD, (2) percent of laboratory density or (3) percent of control strip density.  Regardless of the measurement device or reporting method, the key characteristic is percent air voids.

6.2  Compaction Importance

The volume of air in an HMA pavement is important because it has a profound effect on long-term pavement performance.  An approximate "rule-of-thumb" is for every 1 percent increase in air voids (above 6-7 percent), about 10 percent of the pavement life may be lost (Linden et al., 1989).  Keep in mind that this rule-of-thumb was developed using limited project data, should be used with extreme caution and applies to air voids above 6 - 7 percent.   According to Roberts et al. (1996), there is considerable evidence that dense graded mixes should not exceed 8 percent nor fall below 3 percent air voids during their service life.  This is because high air void content (above 8 percent) or low air void content (below 3 percent) can cause the following pavement distresses (this list applies to dense-graded HMA and not open-graded HMA or SMA):

1. Decreased stiffness and strength.  Kennedy et al. (1984) concluded that tensile strength, static and resilient moduli, and stability are reduced at high air void content. 

2. Reduced Fatigue Life.  Several researchers have reported the relationship between increased air voids and reduced fatigue life (Pell and Taylor, 1969; Epps and Monismith, 1969; Linden et. al., 1989).  Finn et al. (1973) concluded “...fatigue properties can be reduced by 30 to 40 percent for each one percent increase in air void content.”  Another study concluded that a reduction in air voids from eight percent to three percent could more than double pavement fatigue life (Scherocman, 1984a).

3. Accelerated Aging/Decreased Durability.  In his Highway Research Board paper, McLeod (1967) concluded “compacting a well-designed paving mixture to low air voids retards the rate of hardening of the asphalt binder, and results in longer pavement life, lower pavement maintenance, and better all-around pavement performance.”

4. Raveling.  Kandhal and Koehler (1984) found that raveling becomes a significant problem above about eight percent air voids and becomes a severe problem above approximately 15 percent air voids.

5. Rutting.  The amount of rutting which occurs in an asphalt pavement is inversely proportional to the air void content (Scherocman, 1984a).  Rutting can be caused by two different mechanisms:  vertical consolidation and lateral distortion.  Vertical consolidation results from continued pavement compaction (reduction of air voids) by traffic after construction.  Lateral distortion – shoving of the pavement material sideways and a humping-up of the asphalt concrete mixture outside the wheelpaths – is usually due to a mix design problem.  Both types of rutting can occur more quickly if the HMA air void content is too low (Scherocman, 1984a).   

6. Moisture Damage.  Air voids in insufficiently compacted HMA are high and tend to be interconnected with each other.  Numerous and interconnected air voids allow for easy water entry (Kandhal and Koehler, 1984; Cooley et al., 2002) which increases the likelihood of significant moisture damage.  The relationship between permeability, nominal maximum aggregate size and lift thickness is quite important and can change significantly as these parameters change.

Air voids that are either too great or too low can cause a significant reduction in pavement life.  For dense graded HMA, air voids between 3 and 8 percent generally produce the best compromise of pavement strength, fatigue life, durability, raveling, rutting and moisture damage susceptibility.

6.3  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 7.4)

Table 7.4: Factors Affecting Compaction

Environmental factors are determined by when and where paving occurs.  Paving operations may have some float time, which allows a limited choice of “when” but paving location is determined by road location so there is essentially no choice of “where”.  Mix and structural design factors are determined before construction and although they should account for construction practices and the anticipated environment, they often must compromise ease of construction and compaction to achieve design objectives.  Obviously construction factors are the most controllable and adaptable of all the factors affecting compaction.  Although some factors like haul distance/time, HMA production temperature, lift thickness and type/number of rollers may be somewhat predetermined, other factors associated with roller timing, speed, pattern and number of passes can be manipulated as necessary to produce an adequately compacted mat.  This subsection discusses:

• Temperature (the environmental factor)

• Mix property factors

Section 6.4, Compaction Equipment and 6.5, Roller Variables discuss construction factors.

6.3.1  Temperature

HMA temperature has a direct effect on the viscosity of the asphalt cement binder and thus compaction.  As HMA temperature decreases, its asphalt cement binder becomes more viscous and resistant to deformation, which results 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 a function of the mix property factors in Table 7.4.  In some literature it is reported to be about 79^oC (175°F) for dense-graded HMA (Scherocman, 1984b; Hughes, 1989).  Below cessation temperature rollers can still be operated on the mat to improve smoothness and surface texture but further compaction will generally not occur.  Conversely, if the binder is too fluid and the aggregate structure is weak (e.g., at high temperatures), roller loads will simply displace, or “shove” the mat rather than compact it.  In general, the combination of asphalt cement binder and aggregate needs to be viscous enough to allow compaction but stiff enough to prevent excessive shoving. 

Mat temperature then, is crucial to both the actual amount of air void reduction for a given compactive effort, and the overall time available for compaction.  If the 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:

1. 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. 
2. 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”.

The major factors affecting time available for compaction are (Roberts et al., 1996):

• Initial mat temperature.  Higher initial mat temperatures require more time to cool down to cessation temperature, thus increasing the time available for compaction.  However, overheating the HMA will damage the asphalt binder and cause emissions.
• Mat or lift thickness.  Thicker lifts have a smaller surface-to-volume ratio and thus lose heat more slowly, which increases the time available for compaction.
• Temperature of the surface on which the mat is placed.  Hotter surfaces will remove heat from the mat at a slower rate, increasing the time available for compaction.
• Ambient temperature.  Hotter air temperatures will remove heat from the mat at a slower rate, increasing the time available for compaction.
• Wind speed.  Lower wind speeds will decrease mat heat loss by convection, which will increase the time available for compaction.

Jordan and Thomas (1976) point out additional factors affecting mat cool-down rate that include mat density, pavement layer thermal conductivity, specific heat, convection coefficient, incident solar radiation and coefficients of emission and absorption of solar radiation for the pavement surface.    

David Timm, Vaughan Voller and David Newcomb have developed a software tool at the University of Minnesota called MultiCool that automatically calculates pavement cool-down rate and time available for compaction (see Program 7.1).  

Program 7.1: MultiCool (click graphic to launch program)
Note: MultiCool is designed for Windows operating systems

 Table 7.5 is a sampling of MultiCool output for some representative values of pavement thickness and ambient temperature.

 Table 7.5: Sample MultiCool Calculations

 MultiCool is quick and powerful.  It can easily be installed on a laptop and used by contractors or inspectors to give a general idea of the time available for compaction on a given job site, which can be quite helpful in determining roller use and patterns.  Figure 7.60 relates HMA temperature with typical aspects of compaction.

Figure 7.60: HMA Temperature vs. Compaction Aspects

HMA temperature affects its binder viscosity, which affects compaction in two ways: (1) the colder and more viscous the binder, the less actual amount of air void reduction for a given compactive effort, and (2) HMA can only be compacted until it reaches cessation temperature, therefore initial HMA temperature and mat cool-down rate establish a fundamental compaction parameter – the overall time available for compaction.  Many factors influence HMA temperature and cool-down rate including initial mat temperature, mat thickness, temperature of the surface on which the mat is placed, ambient temperature and wind speed.  Using these factors as inputs, MultiCool, a program developed at the University of Minnesota, can easily produce a mat cool-down curve and calculate the time available for compaction. 

6.3.2  Mix Properties

Mix aggregate and binder properties can also affect compaction.  They do so by affecting (1) the ease with which aggregate will rearrange under roller loads and (2) the viscosity of the binder at any given temperature.   

Gradation affects the way aggregate interlocks and thus the ease with which aggregate can be rearranged under roller loads.  In general, aggregate effects on compaction can be broken down by aggregate size (TRB, 2000): 

1. Coarse aggregate.  Surface texture, particle shape and the number of fractured faces can affect compaction.  Rough surface textures, cubical or block shaped aggregate (as opposed to round aggregate) and highly angular particles (high percentage of fractured faces) will all increase the required compactive effort to achieve a specific density.

2. Midsize fine aggregate (between the 0.60 and 0.30-mm (No. 30 and No. 50) sieves).  High amounts of midsize fine, rounded aggregate (natural sand) cause a mix to displace laterally or shove under roller loads.  This occurs because the excess midsize fine, rounded aggregate results in a mix with insufficient voids in the mineral aggregate (VMA).  This gives only a small void volume available for the asphalt cement to fill.  Therefore, if the binder content is just a bit high it completely fills the voids and the excess serves to (1) resist compaction by forcing the aggregate apart and (2) lubricate the aggregate making it easy for the mix to laterally displace.

3. Fines or dust (aggregate passing the 0.075-mm (No. 200) sieve).   Generally, a mix with a high fines content will be more difficult to compact than a mix with a low fines content.

The asphalt binder grade affects compaction through its viscosity.  A binder that is higher in viscosity will generally result in a mix that is more resistant to compaction.  Additionally, the more a binder hardens (or ages) during production, the more resistant the mix is to compaction. 

Asphalt binder content also affects compaction.  Asphalt binder lubricates the aggregate during compaction and therefore, mixes with low asphalt content are generally difficult to compact because of inadequate lubrication, whereas mixes with high asphalt content will compact easily but may shove under roller loads (TRB, 2000).

Sometimes, a combination of mix design factors produces what is known as a tender mix.  Tender mixes are internally unstable mixes that tend to displace laterally and shove rather than compact under roller loads. 

6.4  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 of densifying HMA are often referred to collectively as “compactive effort”. This section discusses the paver screed, the steel wheeled roller (both static and vibratory) and the pneumatic tire roller as they apply to HMA compaction.  Section 6.5.1, Compaction Sequence discusses how each one of these pieces of compaction equipment work together in a typical construction scenario.  This subsection covers:

• The paver screed

• Steel wheel rollers (including vibratory rollers)

• Pneumatic tire rollers

6.4.1  Paver Screed

The paver screed has previously been discussed in Section 5.1.2, Screed.  Of additional note here is that approximately 75 to 85 percent of the theoretical maximum density of the HMA will be obtained when the mix passes out from under the screed (TRB, 2000). 

6.4.2  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 86 to 215 cm (35 to 85 inches) in width and 50 to 150 cm (20 to 60 inches) in diameter.  Roller weight is typically between 0.9 and 18 tonnes (1 and 20 tons) (see Figures 7.61 and 7.62). 

In addition to their own weight, some steel wheel rollers can be ballasted with either sand or water to increase their weight and thus, compactive effort.  Although this ballasting is a fairly simple process (see Figure 7.63), it is usually done before rolling operations start and rarely during rolling operations.  Since asphalt cement binder sticks to steel wheels, most steel wheel rollers spray water on the drums to prevent HMA from sticking, and are equipped with a transverse bar on each drum to wipe off HMA (see Video 7.10).  Note, however, that this water will cool the HMA and can reduce the time available for compaction.  

6.4.2.1  Vibratory Steel Wheel Rollers

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.  Roller drum vibration is produced using a rotating eccentric weight located in the vibrating drum (or drums) and the force it creates is proportional to the eccentric moment of the rotating weight and the speed of rotation (TRB, 2000).  Operators can turn the vibrations on or off and can also control amplitude (eccentric moment) and frequency (speed of rotation).  Vibration frequency and amplitude have a direct effect on the dynamic force (and thus the compactive force) as shown in Table 7.6.

Table 7.6: Vibratory Steel Wheel Roller Parameters (after TRB, 2000)

The ideal vibratory frequency and amplitude settings are a compromise based on desired mat smoothness, HMA characteristics and lift thickness.  Low vibration frequencies combined with high roller speeds will increase the distance between surface impacts and create a rippled, unsmooth surface.  In general, higher frequencies and lower roller speeds are preferred because they decrease the distance between surface impacts, which  (1) increases the compactive effort (more impacts per unit of length) and (2) provides a smoother mat.  The recommended impact spacing is 3 - 4 impacts per meter (10 - 12 impacts per foot).  Table 7.7 shows basic guidance for vibratory settings.  

Table 7.7: Typical Vibratory Settings (from TRB, 2000)

As a general rule-of-thumb, a combination of speed and frequency that results in 3 - 3.5 impacts per meter (10 - 12 impacts per foot) is good.  At 3000 vibrations/minute that gives a speed of 4.5 - 5.5 km/hr (2.8 - 3.4 mph).

When density is difficult to quickly achieve with a vibratory steel wheel roller, the tendency may be to increase vibratory amplitude to increase compactive effort.  However, high amplitude is only advisable on stiff mixes or very thick lifts that can support the increased amplitude without fracturing the constituent aggregate particles.  For typical mix types and lift thicknesses a better solution is usually to maintain low amplitude vibrations and increase the number of roller passes at low amplitude.

Vibratory steel wheel rollers offer potential compaction advantages over static steel wheel rollers but they also require the operator to control more compaction variables (amplitude, frequency and vibratory mode use) and there are certain situations under which they must be used with caution (e.g., over shallow underground utilities, in residential areas, thin overlays).

In general, steel wheel rollers provide the smoothest mat finish of all compaction equipment.  When operated in the vibratory mode, they also provide substantial compactive effort.

6.4.3  Pneumatic Tire Rollers

The pneumatic tire roller is a self-propelled compaction device 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 on one axle and five 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 400 kPa (60 psi) and 800 kPa (120 psi) (TRB, 2000).   

Asphalt binder tends to stick to cold pneumatic tires but not to hot pneumatic tires.  A release agent (like water) can be used to minimize this sticking, however if asphalt binder pickup (the asphalt binder sticking to the tires) is not permanently damaging the mat it is better to run the roller on the hot mat and let the tires heat up to near mat temperature.   Tires near mat temperature will not pick up an appreciable amount of asphalt binder.  Insulating the tire area with rubber matting or plywood helps maintain the tires near mat temperature while rolling (see Figure 7.64).  

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.  This results in both advantages and disadvantages when compared to steel wheel rollers:

 Advantages (Brown, 1984) 

1. They provide a more uniform degree of compaction than steel wheel rollers. 

2. They provide a tighter, denser surface thus decreasing permeability of the layer.

3. They provide increased density that many times cannot be obtained with steel wheeled rollers.

4. They compact the mixture without causing checking (hairline surface cracks) and they help to remove any checking that is caused with steel wheeled rollers.

 Disadvantages 

1. The individual tire arrangement may cause deformations in the mat that are difficult or impossible to remove with further rolling.  Thus, they should not be used for finish rolling.

2. If the HMA binder contains a rubber modifier, HMA pickup (mix sticking to the tires) may be so severe as to warrant discontinuing use of the roller.

In summary, pneumatic tire rollers offer a slightly different type of compaction than steel wheel rollers.  The arrangement of multiple tires on both axles serves to both compress and kneed the mat, which may or may not be advantageous over steel wheel rollers.

6.5  Roller Variables

There are several variables associated with rollers that can be adjusted from job to job.  These variables are:  

• The sequence and number of rollers

• Roller speed

• The number of roller passes over a given area of the mat

• The location at which each roller works

• The pattern that each roller uses

Not all these variables are infinitely adjustable, but by adjusting a combination of them a rolling plan can be developed that will optimize mat compaction.

6.5.1   Compaction Sequence

HMA compaction is typically accomplished by a sequential train of compaction equipment (see Figure 7.65).  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. 

Figure 7.65: Breakdown and Intermediate Rollers

A typical compaction train consists of the following (in order of use): 

1. Screed.  The screed is the first device used to compact the mat and may be operated in the vibratory mode.

2. Breakdown Roller.  The breakdown roller is the first roller behind the screed and therefore, 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.

3. Intermediate Roller.  The intermediate roller is used behind the breakdown roller if additional compaction is needed.  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.  This can help further compact the mat or at the very least, rearrange the aggregate within the mat to make it receptive to further compaction.

Graph and data taken 
from Chadbourn et al. (1998).

·      16 km/h (10 mph) wind speed