The primary purpose of incorporating geosynthetics in the pavement design process is to reduce reflective cracking in HMA overlays and to resist moisture intrusion into the underlying pavement structure. Geosynthetics can be part of an overall rehabilitation strategy that will as a minimum include the placement of a new wearing/surface course of HMAC.

One concern that the geosynthetic users should keep in mind is future rehabilitations as any anticipated milling of HMAC layers must avoid RAP contamination and possible fouling of milling equipment.

Geogrids. The main function of a geogrid in an HMA application is to retard the occurrence of reflective cracking. In evaluating the appropriateness of use, cracking in the existing structure should be limited to cases in which the crack faulting does not fluctuate significantly with traffic loading and crack width does not fluctuate significantly with temperature differentials.

The pavement should be structurally sound with existing cracks limited to less than 3/8” width. Hence, low to moderate levels of alligator cracking, or random cracking may benefit from application of grids in HMA, whereas widely spaced thermal cracking or underlying rocking/faulted PCC slabs will probably not benefit. It is necessary to repair localized highly distressed/weak areas and apply a levelup course of HMAC prior to applying the geogrid.

Where rutting exceeding ½-inch exists, milling prior to applying the level-up should be considered. A minimum 2.0-inch surfacing course over the grid is recommended. Installation of this type of product has proven to be problematic and will result in premature failure (fatiguing) of the surfacing overlay where a lack of bonding (surface to grid to levelup) occurs. It is highly recommended that the manufacturer’s installation procedures be strictly followed and that a manufacturer’s representative be present during the planning and construction process.

Fabrics, composites, and membranes. These products provide a moisture barrier in addition to varying degrees of resistance to reflective cracking. Application guidelines are similar to those recommended above for the geogrid. The impermeable qualities of these products can be a double-edged sword in that they prevent trapped moisture within the structure from transpiring out. This may result in debonding of HMA layers and/or stripping of HMA layers below the product, especially if the lower mixes are moisture susceptible.

Also, if the surfacing overlay is permeable and surface moisture can not readily escape the section laterally (mill and inlay technique is especially prone), stripping of the surface mix may also occur. It is incumbent upon users of these products to insure laboratory testing is performed to determine HMAC stripping susceptibility of existing mixes (highway cores) and the proposed level-up and overlay mixes.

Geosynthetics in Pavement Bases (non-HMA Applications)

Geosynthetics are placed in pavement bases to perform one or more of the following functions: reinforcement, separation, and filtration. Base reinforcement results from the addition of a geogrid or composite at the bottom or within a base course to increase the structural or load-carrying capacity of a pavement system by the transfer of load to the geosynthetic material.

The primary mechanism associated with this application is lateral restraint or confinement of aggregates in the base. Where very weak subgrades exist, geosynthetics can increase the bearing capacity by forcing the potential bearing capacity failure surface to develop along alternate, higher strength surfaces. Geogrids may also be considered for use in locations where chemical stabilization of the subgrade is not desirable due to possible reaction with sulfates in the subgrade, or not practical because of expedited construction concerns, particularly in urban settings.

There have been assertions that the resultant increase in restraint or confinement should allow for design of thinner structures using these products versus structural designs which do not, however their benefits may only be noticeable over the long term and there appears to be an absence of long-term controlled monitoring. For purposes of geosynthetic reinforcement, CSP M&P recommends that their application be viewed as an “insurance policy” rather than a “modulus- multiplier” or structure-reducing product.

Geosynthetics used for separation have classically been applied to prevent subgrade soil from migrating into the unbound base (or subbase), or to prevent aggregates from an unbound base (or subbase) from migrating into the subgrade. A small amount of fines introduced into the granular base can significantly reduce the internal friction angle and render the flex base weaker. Potential for these circumstances increases where wet, soft subgrades exist. Typically a geocomposite will be used for this application, placed at the subgrade/unbound base interface.

Geotextile separators act to maintain permeability of the base materials over the life of the section, and they allow the use of more open-graded, free-draining base and subbase materials.

Another form of separation is being increasingly explored where there is a high potential for reflective cracking originating in the subgrade or chemically-bound base. A grid or composite is used to dissipate stresses induced by the opening crack. Longitudinal edge cracking is particularly an issue in areas where moderate to high PI soils are exposed to prolonged cycles of wetting and drying. Geogrids will typically be employed at the subgrade/bound base interface, or if a flex base is placed above a bound base (e.g., FDR projects), the grid may be placed at this location. Grids should be a minimum of 10-ft. wide to reduce the potential for longitudinal cracking due to edge drying. The function of filtration is to allow for in-pavement moisture transfer but restrict movement of soil particles, hence composites or fabrics that are placed for the classical purpose of separation will usually incorporate this function as well.

References

Original article content and pictures contributed by TxDOT.

1. Decreased stiffness and strength. Kennedy et al. (1984^[3]) 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^[4]; Epps and Monismith, 1969 ^[5]; Linden et. al., 1989^[1]). Finn et al. (1973)^[6] 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, 1984^[7]).
3. Accelerated Aging/Decreased Durability. In his Highway Research Board paper, McLeod (1967)^[8] 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^[9]) 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, 1984^[7]). 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, 1984^[7]).
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^[9]; Cooley et al., 2002^[10]) 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.

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 article discusses:

• Temperature (the environmental factor)
• Mix property factors

Articles on compaction equipment and Roller Variables discuss construction factors.

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 1.  In some literature it is reported to be about 79^oC (175°F) for dense-graded HMA (Scherocman, 1984^[1]; Hughes, 1989^[2]). 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^[3]):

• 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^[4]) 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.

Get Multicool

• From NAPA: http://www.hotmix.org/index.php?option=com_content&task=view&id=178&Itemid=273
• From the University of California Pavement Research Center: http://www.ucprc.ucdavis.edu/SoftwarePage.aspx (at bottom of page)

Table 2 is a sampling of MultiCool output for some representative values of pavement thickness and ambient temperature.

Table 2 Assumptions:

1. Wind velocity is 16 km/h (10 mph)
2. Aitemperature same as base temperature.
3. Morning paving (10:00 a.m.)
4. Paving location is at 48° N latitude
5. Weather is clear and dry
6. Paving is an overlay over an existing asphalt concrete pavement
7. Dense graded HMA
8. Binder type is PG 64-22
9. Single lift

http://www.mrr.dot.state.mn.us/research/MnROAD_Project/restools/multicooltool.asp 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 1 relates HMA temperature with typical aspects of compaction.

Figure 1: 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, http://www.mrr.dot.state.mn.us/research/MnROAD_Project/restools/multicooltool.asp MultiCool , a program developed at the University of Minnesota, can easily produce a mat cool-down curve and calculate the time available for compaction.

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^[5]):

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^[5]).

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.

Truck Types

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

• End dump
• Bottom dump (or belly dump)
• Live bottom (or flo-boy)

Each truck type is capable of adequately delivering HMA from a production facility to a paving site. However, certain situations such as the ones listed in Table 1 below, may make one truck type advantageous over another.

Operational 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:

• Loading at the production facility
• Transport within the truck
• Unloading at the paving site
• Operation synchronization

Loading at the Production Facility

Loading at the production facility involves transferring HMA from the storage silo or batcher (for batch plants) to the transport truck. There are two potential issues with this transfer:

1. Truck bed cleanliness and lubrication. Truck beds should be clean and lubricated to prevent the introduction of foreign substances into the HMA and to prevent the HMA from sticking to the truck bed. Non-petroleum based products should be used for lubrication such as lime water, soapy water or other suitable commercial products (Roberts et al., 1996^[1]). 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.
2. Aggregate segregation. 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 (see Figure 1 and Video 1). Large-sized aggregate tends to 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 largely prevent the collection of large aggregate in one area and thus minimize aggregate segregation.

Figure 1. Truck loading under a storage silo.

Video 1. Truck loading close-up.

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 (see Figure 2). During transport, heat is transferred to the surrounding environment by convection and radiation and the HMA surface temperature drops. This cooler HMA surface insulates the interior mass and thus transported HMA tends to develop a cool thin crust on the surface that surrounds a much hotter core (see Figures 3 and 4 and Video 2). Things such as air temperature, rain, wind and length of haul can affect the characteristics and temperature of this crust. Several measures that can be taken to minimize HMA cooling during transport are:

1. Minimize haul distance. This can be accomplished by choosing an HMA production facility as close as possible to the paving site. Closer production facilities create shorter haul times and result in less HMA cooling during transport. Unfortunately, many paving locations may not be near any existing production facilities and economics may prohibit the use of a mobile production facility.
2. Insulate truck beds. This can decrease HMA heat loss during transport. Insulation as simple as a sheet of plywood has been used.
3. Place a tarpaulin over the truck bed. A tarp over the truck bed (see Figure 5) provides additional insulation, protects the HMA from rain and decreases heat loss. A study by the Quality Improvement Committee of the National Asphalt Pavement Association (NAPA) studied truck tarping and found that the HMA surface temperatures of tarped loads dropped more slowly than untarped loads but temperatures 100 mm (4 inches) below the surface between tarped and untarped loads were not significantly different (Minor, 1980^[2]).

Figure 2. Infrared picture of an HMA storage silo loading a truck showing the hot uniform temperature of the mix.	Figure 3. Infrared picture of a truck dumping HMA with cold surface layer crust (blue) and the hot inner mass (red.)
Figure 4. Infrared picture of a truck dumping HMA with cold surface layer crust (blue) and the hot inner mass (red)	Figure 5. Driver covering his truck bed with a tarpaulin to prevent cold surface layer.

In most cases, truck transport appears to cool only the surface of the transported HMA mass, however this cool surface crust can have detrimental effects on overall mat quality if not properly dealt with. Actions such as reducing transport time, insulating truck beds or tarping trucks can decrease HMA surface cooling rate. Additionally, since the majority of the HMA mass is still at or near its original temperature at loading, mixing the crust and interior mass together at the paving site (“remixing”) will produce a uniform mix near the original temperature at loading.

Unloading at the Paving Site

HMA unloading involves those procedures discussed in End Dump Truck as well as a few other basic considerations such as:

1. HMA should be unloaded quickly when it arrives at the paving site. This will minimize mix cooling before it is placed.
2. Before HMA is loaded into the paver, the inspector and/or foreman should be certain it is the correct mix. Occasionally, paving jobs require several different mix designs (i.e., one for the leveling course and one for the wearing course) and these mixes should not be interchanged.

Operation Synchronization

Ideally, HMA production at the plant, truck transport and placement at the paving machine should be synchronized to the same rate to minimize accumulation of excess HMA in any one of the three segments. Realistically, however, this synchronization can be quite difficult because of varying laydown rates, unpredictable truck travel times and variable plant production. Detailed information on operation synchronization can be found in:

• National Asphalt Pavement Association (NAPA). (1996). Balancing Production Rates in Hot Mix Asphalt Operations, IS 120. National Asphalt Pavement Association. Landham, MD.

Ideally, all operations are designed to meet optimal mat laydown rates. However, these rates can vary based on paving width and lift thickness. Also, complicated paving locations such as intersections or near manholes and utility vaults can temporarily increase or decrease the laydown rate.

Truck transport should be planned such that the HMA transport rate (expressed in tons/hr) closely matches plant production rate and laydown rate. Some factors to consider are:

• Number of trucks to be used.
• Truck type.
• Average truck hauling capacity.
• Production facility output rate.
• Availability and condition of storage silos at the production facility.
• Time to lubricate the truck bed before transport.
• Waiting time at the production facility.
• Loading, weighing and ticketing time at the production facility.
• Time to cover the load (when tarpaulins are used).
• Distance between the production facility and the paving site.
• Average truck speed.

Traffic plays a large role in 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 as it waits 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.

Finally, production facility output is typically controlled to match haul or laydown rate. However, this can result in suboptimal plant efficiency or HMA uniformity, which may increase plant exhaust output, shorten emission control device lifetimes, and affect contractual payment if payment is tied to HMA uniformity. It may often be more economical to run the production facility at maximum rate and store excess material in storage silos for discharge into trucks as they arrive. Storage silo insulation has progressed to a state where dense-graded HMA can be stored in them for up to a week at a time without significantly affecting HMA characteristics. However, gap graded mixes such as SMA or OGFC should still not be stored for more than about 2 to 3 hours.

In sum, synchronization should be the goal but it is often difficult to achieve (based on varying laydown rates, haul time and traffic) and may result in plant inefficiency and HMA quality degradation. If a production facility has modern well-insulated, airtight storage silos and is producing a dense-graded HMA, it may be beneficial to run the plant at maximum production rate and store the mix until needed rather than try and match haul or laydown rate.

• 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 or three times the nominal maximum aggregate size (TRB, 2000^[1]).
• 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.
• SMA. SMA mixes behave differently than dense-graded mixes during placement and compaction. Experience and understanding of dense-graded mix placement should be augmented with specific training and precautions before attempting to place an SMA mix for the first time. SMAs are generally stickier and more difficult to work with than dense-graded mixes because (1) they have more asphalt binder, (2) the asphalt binder is modified, and (3) the binder and filler combination creates a viscous mastic. Also, it is not uncommon for large amounts of mastic (the combination of asphalt binder and mineral filler) to collect on paving equipment. If not carefully monitored, this mastic will release from the equipment into the mat leaving an over-asphalted area – commonly referred to as a “fat spot“. These considerations only scratch the surface of SMA construction. A more thorough treatment can be found in:
  • National Asphalt Pavement Association (NAPA). (1999). Designing and Constructing SMA Mixtures – State-of-the-Practice, Quality Improvement Series 122. National Asphalt Pavement Association. Landham, MD.
• Mat problems. The asphalt paver, MTV, rollers, mix design and manufacturing introduce many variables into HMA pavement construction. A familiarity with common causes of the more typical mat problems can help improve construction quality. Some common mat problems are microcracking, fat spots, joint problems, non-uniform texture, roller marks, shoving, surface waves, tearing (streaks) and transverse screed marks.

Percent air voids is typically calculated by using AASHTO T 269, ASTM D 3203 or an equivalent procedure (AASHTO, 2000^[1]). 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 1 and 2). This type of air voids testing is generally considered the most accurate but is also the most time consuming and expensive.

Figure 1: Core Extraction

Figure 2: Core on the Right has Much Higher Air Voids

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 3) or electrical density gauge (see Figure 4).

Figure 3: Nuclear Density Gauge

Figure 4: Trans Tech PQI Electrical Density Gauge

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.

Figure 1: Thin lift nuclear density gauge.

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 on the end of a retractable rod.

Gamma rays are emitted from the source and interact with electrons in the pavement through absorption, Compton scattering, and the photoelectric effect. A Geiger-Mueller detector (situated in the gauge opposite from the handle) counts gamma rays that reach it from the source.  Pavement density is then correlated to the number of gamma rays received by the detector.

Nuclear density gauges are typically operated in one of two modes, each of which uses a different correlation to determine pavement density (Figure 2):

1. Direct transmission.  The retractable rod is lowered into the mat through a pre-drilled hole (this hole can be formed by pounding a steel rod with a similar diameter to that of the gauge’s retractable rod).  The source emits gamma rays, which then interact with electrons in the material and lose energy and/or are redirected (scattered).  Gamma rays that lose sufficient energy or are scattered away from the detector are not counted.  The more dense the pavement, the higher the probability of interaction and the lower the detector count.  Therefore, the detector count is inversely proportional to pavement density.  A calibration factor is used to relate gamma count to actual pavement density.
2. Backscatter.  The retractable rod is lowered so that it is even with the detector but still within the instrument.  The source emits gamma rays, which then interact with electrons in the material and lose energy and/or are redirected (scattered).  Gamma rays that are scattered towards the detector are counted.  The more dense the pavement, the higher the probability that a gamma ray will be redirected towards the detector.  Therefore, the detector count is proportional to pavement density.  A calibration factor is used to relate gamma count to actual pavement density.

A nuclear density gauge offers the following key advantages over destructive density measurement (cores):

1. Portability.  One person can easily transport a typical nuclear density gauge.
2. Quick results.  Most nuclear gauges allow both one and four minute readings.  These are much quicker than typical densities obtained from cores which could take from several days to several weeks.
3. Virtually non-destructive.  Used in the backscatter mode, the nuclear density gauge is entirely non-destructive.  Used in the direct mode, the gauge only requires a small penetration into the finished mat approximately 20 mm (less than 1 inch) in diameter and about 50 mm (2 inches) deep.

Nuclear Moisture Gauge

A nuclear moisture gauge (not pictured) uses a neutron source, such as Americium-241:Beryllium, placed inside the gauge. The source emits high energy, “fast” neutrons, which then collide with various nuclei in the pavement. Due to momentum conservation, those neutrons that collide with hydrogen nuclei slow down much quicker than those that collide with other, larger nuclei. The gauge detector counts only thermal (low energy) or “slow” neutrons thereby making the detector count proportional to the number of hydrogen atoms in the pavement.  Since water contains many hydrogen atoms (H₂O), the detector count is proportional to moisture content.  A calibration factor is used to relate thermal neutron count to actual moisture content.

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 article discusses the paver screed, the steel wheeled roller (both static and vibratory) and the pneumatic tire roller as they apply to HMA compaction. The Roller Variables article discusses how each type of roller can be used in an integrated approach to compaction.

Paver Screed

The paver screed has previously been discussed in the asphalt paver article.  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^[1]).

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 1 and 2).

Figure 1. Small static steel wheel roller (1.32 tonnes (1.45 tons), 86 cm (34-inch) wide drum).	Figure 2. Large vibratory steel wheel roller (17 tonnes (18.7 tons), 213 cm (84-inch) wide drum).
Figure 3. Steel wheel roller.	Figure 4: Steel wheel roller.

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 (Figure 5), 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 (Video 1).  Note, however, that this water will cool the HMA and can reduce the time available for compaction.

Figure 5. Filling up with water.

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^[1]).  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 1.

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 33 – 37 impacts per meter (10 – 12 impacts per foot).  Table 2 shows basic guidance for vibratory settings.

Table 2. Typical Vibratory Settings (from TRB, 2000^[1])

As a general rule-of-thumb, a combination of speed and frequency that results in 33 – 37 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.

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^[1]).

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 (Figure 6).

Figure 6. Pneumatic tire roller (notice rubber matting insulation around tire area as well as tire marks left in the new mat in front of the roller).

Figure 7. Pneumatic tire roller working on a bituminous surface treatment.

Figure 8. Vibratory pneumatic tire roller.

Figure 9. Close up of the pneumatic tires.

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^[2])

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.