2  Flexible Pavement Basics

Flexible pavements are so named because the total pavement structure deflects, or flexes, under loading.  A flexible pavement structure is typically composed of several layers of material.  Each layer receives the loads from the above layer, spreads them out, then passes on these loads to the next layer below.  Thus, the further down in the pavement structure a particular layer is, the less load (in terms of force per area) it must carry (see Figure 2.2).  

Figure 2.2: Flexible Pavement Load Distribution

In order to take maximum advantage of this property, material layers are usually arranged in order of descending load bearing capacity with the highest load bearing capacity material (and most expensive) on the top and the lowest load bearing capacity material (and least expensive) on the bottom.  This section describes the typical flexible pavement structure consisting of:

Surface course .  This is the top layer and the layer that comes in contact with traffic.  It may be composed of one or several different HMA sublayers.

Base course .  This is the layer directly below the HMA layer and generally consists of aggregate (either stabilized or unstabilized).

Subbase course .  This is the layer (or layers) under the base layer.  A subbase is not always needed.

After describing these basic elements, this section then discusses subsurface drainage and perpetual pavements. 

 

2.1  Basic Structural Elements

A typical flexible pavement structure (see Figure 2.3) consists of the surface course and the underlying base and subbase courses.  Each of these layers contributes to structural support and drainage.  The surface course (typically an HMA layer) is the stiffest (as measured by resilient modulus) and contributes the most to pavement strength.  The underlying layers are less stiff but are still important to pavement strength as well as drainage and frost protection.  A typical structural design results in a series of layers that gradually decrease in material quality with depth.

Figure 2.3: Basic Flexible Pavement Structure

As seen in Figure 2.4, a flexible pavement structure can vary greatly in thickness. The signs on top of the pictured cores indicate the State Route (SR) and the Mile Post (MP) where the core was taken.  The scale at the right edge of the photo is in inches.

  Major Topics in this Section

  2.1 Basic Structural Elements

  2.2 Perpetual Pavements

Figure 2.4: Various Flexible Pavement Cores from Washington State

 

2.1.1  Surface Course

The surface course is the layer in contact with traffic loads and normally contains the highest quality materials.  It provides characteristics such as friction, smoothness, noise control, rut and shoving resistance and drainage.  In addition, it serves to prevent the entrance of excessive quantities of surface water into the underlying base, subbase and subgrade (NAPA, 2001).  This top structural layer of material is sometimes subdivided into two layers (NAPA, 2001):

1. Wearing Course. This is the layer in direct contact with traffic loads.  It is meant to take the brunt of traffic wear and can be removed and replaced as it becomes worn.  A properly designed (and funded) preservation program should be able to identify pavement surface distress while it is still confined to the wearing course.  This way, the wearing course can be rehabilitated before distress propagates into the underlying intermediate/binder course.

2. Intermediate/Binder Course. This layer provides the bulk of the HMA structure.  It's chief purpose is to distribute load.

 

2.1.2 Base Course

The base course is immediately beneath the surface course.  It provides additional load distribution and contributes to drainage and frost resistance.  Base courses are usually constructed out of:

1. Aggregate.  Base courses are most typically constructed from durable aggregates (see Figure 2.5) that will not be damaged by moisture or frost action.  Aggregates can be either stabilized or unstabilized. 

2. HMA.  In certain situations where high base stiffness is desired, base courses can be constructed using a variety of HMA mixes.  In relation to surface course HMA mixes, base course mixes usually contain larger maximum aggregate sizes, are more open graded and are subject to more lenient specifications.

Figure 2.5: Limerock Base Course Undergoing Final Grading

 

2.1.3  Subbase Course

The subbase course is between the base course and the subgrade.  It functions primarily as structural support but it can also:

1. Minimize the intrusion of fines from the subgrade into the pavement structure.2. Improve drainage.3. Minimize frost action damage.4. Provide a working platform for construction. 

The subbase generally consists of lower quality materials than the base course but better than the subgrade soils.  A subbase course is not always needed or used.  For example, a pavement constructed over a high quality, stiff subgrade may not need the additional features offered by a subbase course so it may be omitted from design.  However, a pavement constructed over a low quality soil such as a swelling clay may require the additional load distribution characteristic that a subbase course can offer.  In this scenario the subbase course may consist of high quality fill used to replace poor quality subgrade (over excavation).   

 

2.2  Perpetual Pavements

"Perpetual Pavement" is a term used to describe a long-lasting structural design, construction and maintenance concept.  A perpetual pavement can last 50 years or more if properly maintained and rehabilitated.  As Michael Nunn pointed out in 1998, flexible pavements over a minimum strength are not likely to exhibit structural damage even when subjected to very high traffic flows over long periods of time.  He noted that existing pavements over about 370 mm (14.5 inches) should be able to withstand an almost infinite number of axle loads without structural deterioration due to either fatigue cracking or rutting of the subgrade.  Deterioration in these thick, strong pavements was observed to initiate in the pavement surface as either top-down cracking or rutting.  Further, Uhlmeyer et al. (2000) found that most HMA pavements thicker than about 160 mm (6.3 inches) exhibit only surface-initiated top-down cracking.  Therefore, if surface-initiated cracking and rutting can be accounted for before they impact the structural integrity of the pavement, the pavement life could be greatly increased.

Researchers have used this idea as well as pavement materials research to develop a basic perpetual pavement structural concept.  This concept uses a thick asphalt over a strong foundation design with three HMA layers, each one tailored to resist specific stresses (TRB, 2001):

1. HMA base layer.  This is the bottom layer designed specifically to resist fatigue cracking.  Two approaches can be used to resist fatigue cracking in the base layer.  First, the total pavement thickness can be made great enough such that the tensile strain at the bottom of the base layer is insignificant.  Alternatively, the HMA base layer could be made using an extra-flexible HMA.  This can be most easily accomplished by increasing the asphalt content.  Combinations of the previous two approaches also work.

2. Intermediate layer.  This is the middle layer designed specifically to carry most of the traffic load.  Therefore it must be stable (able to resist rutting) as well as durable.  Stability can best be provided by using stone-on-stone contact in the coarse aggregate and using a binder with the appropriate high-temperature grading.

3. Wearing surface.  This is the top layer designed specifically to resist surface-initiated distresses such as top-down cracking and rutting.  Other specific distresses of concern would depend upon local experience. 

In order to work, the above pavement structure must be built on a solid foundation.  Nunn (1998) notes that rutting on roads built on subgrade with a CBR greater than 5 percent originates almost solely in the HMA layers, which suggests that a subgrade with a CBR greater than 5 percent (resilient modulus greater than about 7,000 psi (50 MPa)) should be considered adequate.  As always, proper construction techniques are essential to a perpetual pavement's performance.  Figure 2.6 shows an example cross-section of a perpetual pavement design to be used in California on I-710 (the Long Beach Freeway) in Los Angeles County.

Figure 2.6: Example I-710 Long Beach Freeway Perpetual Pavement Design (from Monismith and Long, 1999)

Finally, the most important point in this brief perpetual pavement discussion is that it is possible to design and build HMA pavements with extremely long design lives.  In fact, some HMA pavements in service today are living examples of perpetual pavements.  For instance, two sections of Interstate 40 in downtown Oklahoma City are now more than 33 years old (built in 1967) and are still in excellent condition. 

These sections, which support 3 to 3.5 million ESALs per year, have been overlaid but the base and intermediate courses have lasted since construction without any additional work (APA, no date given).

Flexible Pavement Types

There are many different types of flexible pavements.  This section covers three of the more common types of HMA mix types used in the U.S.  Other flexible pavements such as bituminous surface treatments (BSTs) are considered by most agencies to be a form of maintenance and are thus covered under Module 10, Maintenance & Rehabilitation .   HMA mix types differ from each other mainly in maximum aggregate size, aggregate gradation and asphalt binder content/type.  This Guide focuses on dense-graded HMA in most flexible pavement sections because it is the most common HMA pavement material in the U.S.  This section provides a brief exposure to:

Dense-graded HMA .  Flexible pavement information in this Guide is generally concerned with dense-graded HMA.  Dense-graded HMA is a versatile, all-around mix making it the most common and well-understood mix type in the U.S.

Stone matrix asphalt (SMA) .  SMA, although relatively new in the U.S., has been used in Europe as a surface course for years to support heavy traffic loads and resist studded tire wear.

Open-graded HMA .  This includes both open-graded friction course (OGFC) and asphalt treated permeable materials (ATPM).  Open-graded mixes are typically used as wearing courses (OGFC) or underlying drainage layers (ATPM) because of the special advantages offered by their porosity. 

This section is taken largely from the NAPA's HMA Pavement Mix Type Selection Guide (2001).  In addition to the general information presented here, the HMA Pavement Mix Type Selection Guide provides specific information on minimum lift thicknesses, mix selection criteria, mix materials as well as several informative examples.

 

3.1  Dense-Graded Mixes

A dense-graded mix is a well-graded HMA mixture intended for general use.   When properly designed and constructed, a dense-graded mix is relatively impermeable.  Dense-graded mixes are generally referred to by their nominal maximum aggregate size.  They can further be classified as either fine-graded or coarse-graded.  Fine-

  Major Topics on this Page

  3.1 Dense-Graded Mixes

  3.2 Stone Matrix Asphalt Mixes

  3.3 Open-Graded Mixes

  3.4 Mix Selection Guidance

graded mixes have more fine and sand sized particles than coarse-graded mixes (see Table 2.1 for definitions of fine- and coarse-graded mixes). 

Purpose: Dense-graded mixes are suitable for all pavement layers and for all traffic conditions.  They work well for structural, friction, leveling and patching needs.

Materials: Well-graded aggregate, asphalt binder (with or without modifiers), RAP

        Mix

Design: Superpave, Marshall or Hveem procedures.

        Other Info: Particulars about dense-graded HMA are covered by flexible

pavement sections in the rest of this Guide.

Table 2.1: Fine- and Course-Graded Definitions for Dense-Graded HMA (from NAPA, 2001)

Mixture Nominal Maximum Aggregate Size

Coarse-Graded Mix Fine-Graded Mix

37.5 mm (1.5 inches)     < 35 % passing the 4.75 mm (No. 4 Sieve) > 35 % passing the 4.75 mm (No. 4 Sieve)

25.0 mm (1.0 inch)       < 40 % passing the 4.75 mm (No. 4 Sieve) > 40 % passing the 4.75 mm (No. 4 Sieve)

19.0 mm (0.75 inches) < 35 % passing the 2.36 mm (No. 8 Sieve) > 35 % passing the 2.36 mm (No. 8 Sieve)

12.5 mm (0.5 inches)     < 40 % passing the 2.36 mm (No. 8 Sieve) > 40 % passing the 2.36 mm (No. 8 Sieve)

  9.5 mm (0.375 inches) < 45 % passing the 2.36 mm (No. 8 Sieve) > 45 % passing the 2.36 mm (No. 8 Sieve)

 

3.2  Stone Matrix Asphalt (SMA) Mixes

Stone matrix asphalt (SMA) is a gap-graded HMA (see Figure 2.7) that is designed to maximize deformation (rutting) resistance and durability by using a structural basis of stone-on-stone contact (see Figures 2.8, through 2.12).  Because the aggregates are all in contact, rut resistance relies on aggregate properties rather than asphalt binder properties.  Since aggregates do not deform as much as asphalt binder under load, this stone-on-stone contact greatly reduces rutting.  SMA is generally more expensive than a typical dense-graded HMA (about 20 - 25 percent) because it requires more durable aggregates, higher asphalt content and, typically, a modified asphalt binder and fibers.  In the right situations it should be cost-effective because of its increased rut resistance and improved durability.  SMA, originally developed in Europe to resist rutting and studded tire wear, has been used in the U.S. since about 1990 (NAPA, 1999).

 

Purpose: Improved rut resistance and durability.  Therefore, SMA is almost exclusively used for surface courses on high volume interstates and U.S. roads. 

Materials: Gap-graded aggregate (usually from coarse aggregate, manufactured sands and mineral filler all combined into a final gradation), asphalt binder (typically with a modifier)

        Mix

Design: Superpave or Marshall procedures with modifications.  Refer to NAPA's Designing and Constructing SMA Mixtures: State-of-the-Practice, QIP 122 (1999) publication or NCHRP Report 425: Designing Stone Matrix Asphalt Mixtures for Rut-Resistant Pavements.

        Other

Info: Because SMA mixes have a high asphalt binder content (on the order of 6 percent), as the mix sits in the HMA storage silos, transport trucks, and after it is placed, the asphalt binder has a tendency to drain off the aggregate and down to the bottom - a phenomenon known as "mix draindown".  Mix draindown is usually combated by adding cellulose or mineral fibers to keep the asphalt binder in place.  Cellulose fibers are typically shredded newspapers and magazines, while mineral fibers are spun from molten rock.  A laboratory test is run during mix design to ensure the mix is not subject to excessive draindown.

In mix design a test for voids in the coarse aggregate (AASHTO T 19) is used to ensure there is stone-on-stone contact.

Other reported SMA benefits include wet weather friction (due to a coarser surface texture), lower tire noise (due to a coarser surface texture) and less severe reflective cracking.  Mineral fillers and additives are usually added to minimize asphalt binder drain-down during construction, increase the amount of asphalt binder used in the mix and to improve mix durability.

Figure 2.7: Typical SMA and Dense-Graded HMA Aggregate Gradations

 

 

Figure 2.8: SMA Structure

Figure 2.9: SMA Aggregate Structure.  Notice the stone-on-stone contact of the larger aggregate particles.

Figure 2.10: Dense-Graded HMA (left) vs. SMA (right).  (it is a bit more shiny from the extra asphalt binder)

   

Figure 2.11: Dense-Graded HMA (left) vs. SMA (right).  Notice the SMA has a better-defined large aggregate skeleton (from NAPA, 2001)

Figure 2.12: SMA Pavement Surface

 

3.3  Open-Graded Mixes

An open-graded HMA mixture is designed to be water permeable (dense-graded and SMA mixes usually are not permeable).  Open-graded mixes use only crushed stone (or gravel) and a small percentage of manufactured sands.  There are three types of open-graded mixes typically used in the U.S.:

1. Open-graded friction course (OGFC).  Typically 15 percent air voids, no minimum air voids specified, lower aggregate standards than PEM.

2. Porous European mixes (PEM).  Typically 18 - 22 percent air voids, specified minimum air voids, higher aggregate standards than OGFC and requires the use of asphalt binder modifiers.  See Figure 2.13.

3. Asphalt treated permeable bases (ATPB) .  Less stringent specifications than OGFC or PEM since it is used only under dense-graded HMA, SMA or PCC for drainage.  See Figure 2.14.

 

Purpose: OGFC and PEM - Used as for surface courses only.  They reduce tire splash/spray in wet weather and typically result in smoother surfaces than dense-graded HMA.  Their high air voids trap road noise and thus reduce tire-road noise by up to 50-percent (10 dBA) (NAPA, 1995).

ATPB - Used as a drainage layer below dense-graded HMA, SMA or PCC.

Materials: Aggregate (crushed stone or gravel and manufactured sands), asphalt binder (with modifiers)

        Mix

Design: Less structured than for dense-graded or SMA mixes.  Open-graded mix design generally consists of 1) material selection, 2) gradation, 3) compaction and void determination and 4) asphalt binder drain-down evaluation.  NCAT Report 99-3: Design of New-Generation Open Graded Friction Courses provides a recommended mix design procedure for OGFCs.

        Other Info: Both OGFC and PEM are more expensive per ton than dense-graded

HMA, but the unit weight of the mix when in-place is lower, which partially offsets the higher per-ton cost.  The open gradation creates pores in the mix, which are essential to the mix's proper function.  Therefore anything that tends to clog these pores, such as low-speed traffic, excessive dirt on the roadway or deicing sand, should be avoided. 

 

Figure 2.13: Core from a Pavement Using PEM as the Wearing Course (from NAPA, 2001)

Figure 2.14: Asphalt Treated Permeable Base

 

3.4  Mix Selection Guidance

Based on the previous information, there are some general rules for HMA mix type use, which are summarized in Table 2.2.  Notice that, as discussed, dense-graded HMA is generally appropriate for all uses, SMA and OGFC (and PEM) are typically used as surface courses on high volume roads and ATPB is usually used for base courses on high volume roads.  Keep in mind that Table 2.2 is just a summary of general guidance and that there are, as always, case specific exceptions.

Table 2.2: General Appropriateness of Mix Types For Each HMA Layer (NAPA, 2001)

Course

Low Traffic Medium Traffic High Traffic

(< 300,000 ESALs)(300,000 - 10 million ESALs)

(> 10 million ESALs)

Dense

SMA

OGFC

ATPB

Dense

SMA

OGFC

ATPB

Dense

SMA

OGFC

ATPB

Surface          

Intermediate

               

Base              

 = Appropriate    Note: Before deciding to use ATPB, the Pavement Research Center's research results should be carefully considered.

 = Moderately Appropriate

                 

empty  = Not Appropriate

                 

 

3.4.1  Determining Appropriate Mix Types

Most of this process is taken directly from the NAPA HMA Pavement Mix Type Selection Guide (2001).

1. Determine the total thickness of HMA required.  This is accomplished using an appropriate structural design procedure.

2. Determine the types of mixtures appropriate for the surface course based on traffic and cost.

o From Table 2.2, identify the general traffic category for the pavement in question then select those mix types that are appropriate for the surface course.

o Determine what aggregate size to use for a mix.  In general, the higher the traffic loads, the higher the nominal maximum aggregate size should be.

o Consider appearance.  Mixes with larger aggregates often have a coarser surface texture and may be more susceptible to segregation during placement.  Therefore, for a city street where appearance is an issue, a finer mix such as a 9.5 or 12.5-mm (0.375 or 0.5-inch) dense-graded mix may be appropriate.  Conversely, for a heavy industrial area where load resistance is much more important that aesthetic appearance, a 19.0-mm (0.75-inch) mix may be more appropriate.  However, never sacrifice performance for appearance.

o Consider traffic flow.  Maximum aggregate size can also affect traffic flow during rehabilitation of existing roadways.  In many urban areas off-peak construction is used to minimize traffic impacts.  However, for a road to be released to traffic during peak hours, either the lane drop-off (elevation difference between adjacent lanes) must be kept below a specified minimum value (typically less than 37.5 mm (1.5 inches) with proper signage) or all lanes must be brought to the same elevation.  Bringing all lanes to the same elevation at the end of each paving day may require changing traffic control and moving paving equipment, which can increase construction costs and decrease safety.  Therefore it is often better to satisfy the lane drop-off requirement.  However, with larger aggregate mixes the minimum lift thickness may exceed the maximum lane drop-off allowed.  As a result, using a finer gradation may allow paving one lane, then releasing the road to traffic, then paving the other lane.  Again, do not sacrifice performance. 

3. Subtract the surface course thickness from the total thickness and determine what mix or mixes are appropriate for the intermediate and/or base courses using Table 2.2.

4. Continue to subtract intermediate/base course thicknesses from the total

thickness until mixes and layer thicknesses have been selected for the required pavement section.

4  Flexible Pavement Recycling

HMA is one of the most recycled products in the U.S.  It is estimated that as much as 91 million tonnes (100 million tons) of HMA are milled off roads during resurfacing and widening projects each year (APA, 2001a).  Of this amount, 73 million tonnes (80 million tons) are recycled as "reclaimed asphalt pavement" (RAP - see Figure 2.15) (APA, 2001a).  RAP is typically generated by rehabilitation or reconstruction projects and can be used in a variety of ways such as:

Figure 2.15: RAP Pile in Eastern Washington State As an addition to regular HMA. As an aggregate in cold-mix asphalt. As a granular base course when pulverized. As a fill or embankment material.

HMA recycling can be divided into two basic categories based on the recycling methods used: hot recycling and cold recycling.  This section presents the basic

  Major Topics on this Page

  4.1 Hot Recycling

  4.2 Cold Recycling

recycling process as well as typical uses and considerations for each of these recycling methods. 

 

4.1  Hot Recycling

Figure 2.16: RAP in Aggregate-Sized Chunks

Hot recycling is so named because RAP is used as an aggregate in HMA (hot mix asphalt).  In hot recycling, old HMA pavement is removed, broken down into aggregate-sized chunks (see Figure 2.16) and then incorporated into new HMA as an aggregate.  There are two basic methods for accomplishing this: conventional recycled hot mix (RHM) and hot in-place recycling. 

 

4.1.1  Recycled Hot Mix (RHM)

Recycled hot mix (RHM) is the most common way of using RAP.  Basically, new HMA is produced at a batch or drum plant to which a predetermined percentage of RAP is added.  RAP addition is typically 10 to 30 percent by weight although additions as high as 80 percent by weight have been done and additions as high as 90 to 100 percent by weight are feasible (FHWA, 2001c).  There is ample evidence that HMA which incorporates RAP performs as well as HMA without RAP.  Figure 2.17 shows two dense-graded HMA cores, one with RAP and one without.

Purpose: Anything for which a typical dense-graded HMA may be used

Materials: HMA and RAP      

  Mix Design:  Superpave, Marshall or Hveem procedures.  Blending charts are typically needed when using high percentages of RAP.

        Other Info:  When heated, RAP may give off gaseous hydrocarbons.  To

minimize these emissions, HMA plants generally heat RAP indirectly (usually it is added after the aggregate is heated and thus heats up through contact with the already-hot aggregate).

RAP addition may require longer HMA plant heating times.  This can sometimes reduce plant output by as much as half.  

RAP generally contains between 3 and 7 percent asphalt by weight or about 10 to 20 percent asphalt by volume (FHWA, 2001c).  In general, RAP will be more viscous than new HMA because of asphalt binder aging.  Therefore, if enough RAP is added, a softer asphalt binder should be used.  Table 2.3 shows the AASHTO MP 2 Superpave asphalt binder selection guidelines for RAP mixtures.

In general, state DOTs allow more RAP in base and binder HMA courses than they do in surface courses. After milling or crushing, RAP gradation is generally finer than pure virgin aggregate because of the degradation that occurs during removal and processing. 

Table 2.3: Superpave Asphalt Binder Selection Guidelines for RAP Mixtures (from AASHTO, 2001)

RAP PercentageRecommended Virgin Asphalt Binder Grade

< 15 No change from basic Superpave PG binder requirements.

15 - 25Select virgin binder one grade softer than normal (e.g., select at PG 58-22 if a PG 64-22 would normally be used).

> 25 Follow recommendations from blending charts.

 

Figure 2.17: HMA Cores from a RAP Mix and a non-RAP Mix

 

4.1.2  Hot In-Place Recycling (HIPR)

Hot in-place recycling (HIPR) is a less common form of hot asphalt recycling.  There are three basic HIPR construction processes in use, all of which involve a specialized plant in a continuous train operation (FHWA, 2001c):

Heater scarification (Figure 2.18).   This method uses a plant that heats the pavement surface (typically using propane radiant heaters), scarifies the pavement surface using a bank of nonrotating teeth, adds a rejuvenating agent to improve the recycled asphalt binder viscosity, then mixes and levels the recycled mix using a standard auger system. The recycled asphalt pavement is then compacted using conventional compaction equipment.  Heater scarification is limited in its ability to repair severely rutted pavements, which are more easily rehabilitated with a conventional HMA overlay.

Figure 2.18: Heater Scarification Train Showing 2 Preheaters, the Heater/Scarifier, the Paver and Rollers.

Repaving.  This method removes (by heating and scarification and/or grinding) the top 25 to 50 mm (1 to 2 inches) of the existing HMA pavement, adds a rejuvenating agent to improve the recycled asphalt binder viscosity, places the recycled material as a leveling course using a primary screed, and simultaneously places a thin (usually less than 25 mm (1 inch)) HMA overlay. Conventional equipment and procedures are used immediately behind the train to compact both layers of material (Rathburn, 1990 as cited in FHWA, 2001c).

Remixing.  This method is used when additional aggregate is required to improve the strength or stability. Remixing is similar to repaving but adds new virgin aggregate or new HMA to the recycled material before it is leveled.

Purpose: Correct shallow-depth HMA surface distress

Materials: Asphalt binder rejuvenating agent and possibly new aggregate and HMA.

        Mix Design:  Not well-defined, but as a minimum cores are usually taken

from the existing pavement to determine the proper amount of rejuvenating agent to add.

        Other Info:  HIPR is only applicable to specific situations.  First, air void

content of the existing asphalt binder must be high enough to

accept the necessary amount of asphalt binder rejuvenator.  Second, HIPR can only adequately address shallow surface distress problems (less than 50 mm (2 inches)).  Third, pavements with delaminations (subsequent layers not binding together) in the top 50 mm (2 inches) should not be considered for HIPR projects.  Finally, pavements that have been rutted, heavily patched, or chip-sealed are not good candidates for HIPR projects (FHWA, 2001c).

 

4.2  Cold Recycling

Cold recycling is so named because RAP is used as an aggregate in cold mix asphalt.   In cold recycling, old HMA pavement is removed, broken down into aggregate-sized chunks and then combined with an emulsified or foamed asphalt.  This mix is then typically used as a stabilized base course for reconstructed pavements.  There are two basic cold recycling methods: cold plant mix recycling and cold in-place recycling (CIR).  

 

4.2.1  Cold Plant Mix Recycling

Cold plant mix recycling, the less common of the two cold recycling methods, involves mixing RAP with an asphalt emulsion or foamed asphalt at a central or mobile plant facility.  A rejuvenating agent can be added to improve the recycled asphalt binder viscosity and new aggregate can also be added to improve overall performance.  The resulting cold mix is then typically used as a stabilized base course.  

Purpose: Stabilized base course.

Materials: RAP, asphalt emulsion or foamed asphalt, asphalt rejuvenating agent and possibly virgin aggregate.

        Mix Design:  No generally accepted mix design method, but the Asphalt

Institute recommends and most agencies use a variation of the Marshall mix design method (FHWA, 2001b).

        Other Info:  Since cold in-place recycling has become more

commonplace, cold plant mixing has become less popular.

 

4.2.2  Cold In-Place Recycling (CIR)

Cold in-place recycling (CIR) is the processing and treatment with bituminous and/or chemical additives of existing HMA pavements without heating to produce a restored pavement layer (AASHTO, 1998).  It involves the same process of cold plant mix

recycling except that it is done in-place by a train of equipment.  The typical CIR process involves seven basic steps (AASHTO, 1998):

1. Milling.  A milling machine pulverizes a thin surface layer of pavement, usually from 50 to 100 mm (2 to 4 inches) deep. 

2. Gradation control.  The pulverized material is further crushed and graded to produce the desired gradation and maximum particle size.  On some jobs this step is omitted, however on others a trailer mounted screening and crushing plant is used to further crush and grade the pulverized pavement.  If needed, virgin aggregate can be added to the recycled material.

3. Additive incorporation.  The graded pulverized material is mixed with a binding additive (usually emulsified asphalt, lime, portland cement or fly ash).  On some jobs, this is done by the milling machine, however on others a trailer mounted pugmill mixer is used.

4. Mixture placement.  The pulverized, graded pavement and additive combination is placed back over the previously milled pavement and graded to the final elevation.  Mixture placement is most often done with a traditional asphalt paver (either through windrow pickup or by depositing the mixture directly into the paver hopper), however on some very low traffic applications the mixture can be placed by a motor grader.  Because of the larger maximum aggregate sizes of the graded mixture, the minimum lift thickness for placement is usually around 50 mm (2 inches).

5. Compaction.  The placed mixture is compacted to the desired density.  Typical compaction efforts involve a large pneumatic tire roller and a large vibratory steel wheel roller.  If an emulsion additive is used rolling is typically delayed until the emulsion begins to break.  If a portland cement or fly ash additive is used, rolling should begin immediately after placement.

6. Fog seal .  If the newly placed material is to operate as a high quality gravel road then a fog seal is usually applied over the top to delay surface raveling of the cold recycled mix.  A fog seal is necessary over CIR using a portland cement or fly ash additive not only to delay surface raveling but also to provide a curing membrane for the additive to properly set.

7. Surface course construction.  On higher volume roads, the cold recycled mix is overlaid with either a BST or a thin HMA overlay.  In either case, a tack coat should be used to provide a good bond between the cold recycled mix and the surface course.

Purpose: Stabilized base course or a low volume road granular surface course.

Materials: Recycled material and a binding additive (usually asphalt emulsion, lime, portland cement or fly ash).

        Mix Design:  No generally accepted mix design method, but most methods

are based on the Marshall or Hveem methods and equipment (AASHTO, 1996).

        Other Info:  CIR is best suited for cracked pavements with structurally

sound, well drained bases and subgrades.  CIR is generally not appropriate for repairing pavement failures caused by:

Rutting from excessive asphalt content or mix instability

Wet, unstable base, subbase or subgrade materials Frost action Stripping

CIR is generally suitable for lower volume roads that may only require a simple surface treatment over the resulting stabilized base course, or at most a thin HMA wearing course (Better Roads, 2001).

For projects using an asphalt emulsion additive, typical specified minimum atmospheric temperatures range from 10 to 16°C (50 to 60°F).  For projects using portland cement or fly ash as the additive, the minimum required temperature is 4°C (39°F) with no freezing temperatures expected in the next 24 hours (AASHTO, 1998).

CIR requires sunny, dry conditions in order for the additive to properly set. 

If an asphalt emulsion additive is used, it is usually added at a rate of between 0.5 to 2 percent by weight of RAP.

 

 

4.2.3  Full-Depth Reclamation (FDR)

Although referred to as "full-depth reclamation", this process is just an extension of the basic CIR principles to the entire HMA pavement depth plus a predetermined depth of the base material.  FDR can be used to depths of 300 mm (12 inches) or more but the most typical applications involve depths of between 150 and 225 mm (6 and 9 inches) (Better Roads, 2001).  The FDR process usually consists of eight steps (Better Roads, 2001):

1. Pulverization.  A road reclaimer pulverizes existing pavement to a predetermined depth.  Road reclaimers are usually equipped to add materials such as stabilizing agents to the newly pulverized RAP.

2. Moisture conditioning.  The road reclaimer or a separate truck adds water to the newly pulverized RAP to assist in achieving required density.

3. Breakdown roller.  A sheepsfoot or pneumatic tire roller is typically used to compact the recently pulverized RAP to a consistent density.

4. Shaping.  A grader is typically used to make grade and cross-slope adjustments. 

5. Intermediate roller.  A pneumatic tire roller or a steel wheel vibratory roller is used to knead and seat any loose aggregates left from the shaping process.

6. Finish roller.  A 12 to 14-ton static steel wheel roller is used to seat any remaining loose aggregates and create a smooth surface.

7. Sealant.  A fog seal is typically applied to protect the finished reclaimed layer.  After the fog seal sets the reclaimed layer can generally withstand interim traffic loading.  Therefore, at this point the road is often opened to traffic until the contractor is ready to apply the surface treatment or HMA surface course.

8. Surface treatment or surface course.  Finally, a more durable surface treatment or surface course is applied over the new stabilized base course.

Purpose: Stabilized base course.

Materials: Recycled material, asphalt emulsion or foamed asphalt, asphalt rejuvenating agent and possibly virgin aggregate.

        Mix Design:  No generally accepted mix design method, but the Asphalt

Institute recommends and most agencies use a variation of the Marshall mix design method (FHWA, 2001b).

        Other Info:  FDR is generally suitable for lower volume roads that may

only require a simple surface treatment over the resulting stabilized base course, or at most a thin HMA wearing course.  However, FDR has been used on major highways including interstates (Better Roads, 2001).

5  Rigid Pavement Basics

Rigid pavements are so named because the pavement structure deflects very little under loading due to the high modulus of elasticity of their surface course.  A rigid pavement structure is typically composed of a PCC surface course built on top of either (1) the subgrade or (2) an underlying base course.  Because of its relative rigidity, the pavement structure distributes loads over a wide area with only one, or at most two, structural layers (see Figure 2.19).    

Figure 2.19: Rigid Pavement Load Distribution

This section describes the typical rigid pavement structure consisting of:

Surface course .  This is the top layer, which consists of the PCC slab.  

  Major Topics on this Page

  5.1 Basic Structural Elements

  5.2 Joints

  5.3 Load Transfer

  5.4 Tie Bars

Base course .  This is the layer directly below the PCC layer and generally consists of aggregate or stabilized subgrade.

Subbase course .  This is the layer (or layers) under the base layer.  A subbase is not always needed and therefore may often be omitted.

 

5.1  Basic Structural Elements

A typical rigid pavement structure (see Figure 2.20) consists of the surface course and the underlying base and subbase courses (if used).  The surface course (made of PCC) is the stiffest (as measured by resilient modulus) and provides the majority of strength.  The underlying layers are orders of magnitude less stiff but still make important contributions to pavement strength as well as drainage and frost protection. 

 

Figure 2.20: Basic Rigid Pavement Structure

 

5.1.1  Surface Course

The surface course is the layer in contact with traffic loads and is made of PCC.  It provides characteristics such as friction (see Figure 2.21), smoothness, noise control and drainage.  In addition, it serves as a waterproofing layer to the underlying base, subbase and subgrade.  The surface course can vary in thickness but is usually between 150 mm (6 inches) (for light loading) and 300 mm (12 inches) (for heavy loads and high traffic).  Figure 2.22 shows a 300 mm (12 inch) surface course.

Figure 2.21: PCC Surface Figure 2.22: Rigid Pavement Slab (Surface Course) Thickness

 

5.1.2 Base Course

The base course is immediately beneath the surface course.  It provides (1) additional load distribution, (2) contributes to drainage and frost resistance, (3) uniform support to the pavement and (4) a stable platform for construction equipment (ACPA, 2001).  Bases also help prevent subgrade soil movement due to slab pumping.  Base courses are usually constructed out of:

1. Aggregate base.  A simple base course of crushed aggregate has been a common option since the early 1900s and is still appropriate in many situations today.

2. Stabilized aggregate or soil (see Figure 2.23).  Stabilizing agents are used to bind otherwise loose particles to one another, providing strength and cohesion.  Cement treated bases (CTBs) can be built to as much as 20 - 25 percent of the surface course strength (FHWA, 1999).  However, cement treated bases (CTBs) used in the 1950s and early 1960s had a tendency to lose excessive amounts of material leading to panel cracking and settling. 

3. Dense-graded HMA.  In situations where high base stiffness is desired base courses can be constructed using a dense-graded HMA layer.

4. Permeable HMA .  In certain situations where high base stiffness and excellent drainage is desired, base courses can be constructed using an open graded HMA.  Recent research may indicate some significant problems with ATPB use.

5. Lean concrete (see Figure 2.24).  Contains less portland cement paste than a typical PCC and is stronger than a stabilized aggregate.  Lean concrete bases (LCBs) can be built to as much as 25 - 50 percent of the surface course strength (FHWA, 1999).  A lean concrete base functions much like a regular PCC surface course and therefore, it requires construction joints and will crack over time.  These joints and cracks can potentially cause reflection cracking in the surface course if they are not carefully matched.

Figure 2.23: Completed CTB with Curing Seal

Figure 2.24: Lean Concrete Base Material

 

5.1.3  Subbase Course

The subbase course is the portion of the pavement structure between the base course and the subgrade.  It functions primarily as structural support but it can also:

1. Minimize the intrusion of fines from the subgrade into the pavement structure.2. Improve drainage.3. Minimize frost action damage.4. Provide a working platform for construction. 

The subbase generally consists of lower quality materials than the base course but better than the subgrade soils.  Appropriate materials are aggregate and high quality structural fill.  A subbase course is not always needed or used.   

 

5.2  Joints

Joints are purposefully placed discontinuities in a rigid pavement surface course.  The most common types of pavement joints, defined by their function, are (AASHTO, 1993): contraction, expansion, isolation and construction.

 

5.2.1  Contraction Joints

A contraction joint is a sawed, formed, or tooled groove in a concrete slab that creates a weakened vertical plane.  It regulates the location of the cracking caused by dimensional changes in the slab.  Unregulated cracks can grow and result in an unacceptably rough surface as well as water infiltration into the base, subbase and subgrade, which can enable other types of pavement distress.  Contraction joints are the most common type of joint in concrete pavements, thus the generic term "joint" generally refers to a contraction joint.

Contraction joints are chiefly defined by their spacing and their method of load transfer.  They are generally between 1/4 - 1/3 the depth of the slab and typically spaced every 3.1  - 15 m (12 - 50 ft.) with thinner slabs having shorter spacing (see Figure 2.25).  Some states use a semi-random joint spacing pattern to minimize their resonant effect on vehicles.  These patterns typically use a repeating sequence of joint spacing (for example: 2.7 m (9 ft.) then 3.0 m (10 ft.) then 4.3 m (14 ft.) then 4.0 m (13 ft.)).  Transverse contraction joints can be cut at right angles to the direction of traffic flow or at an angle (called a "skewed joint", see Figure 2.27).  Skewed joints are cut at obtuse angles to the direction of traffic flow to help with load transfer.  If the joint is properly skewed, the left wheel of each axle will cross onto the leave slab first and only one wheel will cross the joint at a time, which results in lower load transfer stresses (see Figure 2.28).

Figure 2.25: Rigid Pavement Showing Contraction Joints

Figure 2.26: Missing Contraction Joint (The middle lane contraction joint was not sawed resulting in a transverse slab crack.  The outer lanes have proper contraction joints and therefore, no cracking)

Figure 2.27: Skewed Contraction Joint (The Tining is Perpendicular to the Direction of Travel While the Contraction Joint is

Skewed)

  Figure 2.28: Skewed Contraction Joint

Notice how the tire loads cross the joint one at a time. This introduces the axle load to the leave slab one tire at a time rather than all at once (as would be the case for a 90-degree transverse joint).

 

5.2.2  Expansion Joints

An expansion joint is placed at a specific location to allow the pavement to expand without damaging adjacent structures or the pavement itself.  Up until the 1950s, it was common practice in the U.S. to use plain, jointed slabs with both contraction and expansion joints (Sutherland, 1956).  However, expansion joint are not typically used today because their progressive closure tends to cause contraction joints to progressively open (Sutherland, 1956).   Progressive or even large seasonal contraction joint openings cause a loss of load transfer — particularly so for joints without dowel bars.

 

5.2.3  Isolation Joints

An isolation joint (see Figure 2.29) is used to lessen compressive stresses that develop at T- and unsymmetrical intersections, ramps, bridges, building foundations, drainage inlets, manholes, and anywhere differential movement between the pavement and a structure (or another existing pavement) may take place (ACPA, 2001).  They are typically filled with a joint filler material to prevent water and dirt infiltration.

Figure 2.29: Roofing Paper Used for an Isolation Joint

 

5.2.4  Construction Joints

A construction joint (see Figure 2.30) is a joint between slabs that results when concrete is placed at different times.  This type of joint can be further broken down into transverse and longitudinal construction joints (see Figure 2.31).  Longitudinal construction joints also allow slab warping without appreciable separation or cracking of the slabs.

Figure 2.30: Construction Joint

Workers manually insert dowel bars into the construction joint at the end of the work day.

Construction joints should be planned so that they coincide with contraction joint spacing to eliminate extra joints.

 

Figure 2.31: Longitudinal and Transverse Construction Joints

 

5.3  Load Transfer

"Load transfer" is a term used to describe the transfer (or distribution) load across discontinuities such as joints or cracks (AASHTO, 1993).  When a wheel load is applied at a joint or crack, both the loaded slab and adjacent unloaded slab deflect.  The amount the unloaded slab deflects is directly related to joint performance.  If a

joint is performing perfectly, both the loaded and unloaded slabs deflect equally.  Load transfer efficiency is defined by the following equation:

where:a =

approach slab deflection

  l

=leave slab deflection

This efficiency depends on several factors, including temperature (which affects joint opening), joint spacing, number and magnitude of load applications, foundation support, aggregate particle angularity, and the presence of mechanical load transfer devices.  Figure 2.32 illustrates the extremes in load transfer efficiency.  Most performance problems with concrete pavement are a result of poorly performing joints (ACPA, 2001).  Poor load transfer creates high slab stresses, which contribute heavily to distresses such as faulting, pumping and corner breaks.  Thus, adequate load transfer is vital to rigid pavement performance.  Load transfer across transverse joints/cracks is generally accomplished using one of three basic methods: aggregate interlock, dowel bars, and reinforcing steel.

Figure 2.32:  Load Transfer Efficiency Across a PCC Surface Course Joint

 

5.3.1  Aggregate Interlock

Aggregate interlock is the mechanical locking which forms between the fractured surfaces along the crack below the joint saw cut (see Figure 2.33) (ACPA, 2001).  Some low-volume and secondary road systems rely entirely on aggregate interlock to provide load transfer although it is generally not adequate to provide long-term load transfer for high traffic (and especially truck) volumes.  Generally, aggregate interlock is ineffective in cracks wider than about 0.9 mm (0.035 inches) (FHWA, 1990).  Often, dowel bars are used to provide the majority of load transfer. 

Figure 2.33: Aggregate Interlock

 

5.3.2  Dowel Bars

Dowel bars are short steel bars that provide a mechanical connection between slabs without restricting horizontal joint movement.  They increase load transfer efficiency by allowing the leave slab to assume some of the load before the load is actually over it.  This reduces joint deflection and stress in the approach and leave slabs.

Dowel bars are typically 32 to 38 mm (1.25 to 1.5 inches) in diameter, 460 mm (18 inches) long and spaced 305 mm (12 inches) apart.  Specific locations and numbers vary by state, however a typical arrangement might look like Figure 2.34.  In order to prevent corrosion, dowel bars are either coated with stainless steel (see Figure 2.35) or epoxy (see Figure 2.36).  Dowel bars are usually inserted at mid-slab depth and coated with a bond-breaking substance to prevent bonding to the PCC.  Thus, the dowels help transfer load but allow adjacent slabs to expand and contract independent of one another.  Figure 2.36 shows typical dowel bar locations at a transverse construction joint.

 

 

Figure 2.34: Typical Dowel Bar Location

                  on Transverse Joints

 

Figure 2.35: Stainless Steel-Clad Dowel Bars (Epoxy Coating on Ends Only)

Figure 2.36: Dowel Bars in Place at a Construction Joint- the Green Color is from the Epoxy Coating

   

 

5.3.3  Reinforcing Steel

Reinforcing steel can also be used to provide load transfer.  When reinforcing steel is used, transverse contraction joints are often omitted (as in CRCP).  Therefore, since there are no joints, the PCC cracks on its own and the reinforcing steel provides load transfer across these cracks.  Unlike dowel bars, reinforcing steel is bonded to the PCC on either side of the crack in order to hold the crack tightly together.

Typically, rigid pavement reinforcing steel consists of grade 60 (yield stress of 60 ksi (414 MPa) No. 5 or No. 6 bars (ERES, 2001).  The steel constitutes about 0.6 - 0.7 percent of the pavement cross-sectional area (ACPA, 2001) and is typically placed at slab mid-depth or shallower.  At least 63 mm (2.5 inches) of PCC cover should be maintained over the reinforcing steel to minimize the potential for steel corrosion by chlorides found in deicing agents (Burke, 1983).

 

5.4  Tie Bars

Tie bars are either deformed steel bars or connectors used to hold the faces of abutting slabs in contact (AASHTO, 1993).  Although they may provide some minimal amount of load transfer, they are not designed to act as load transfer devices and should not be used as such (AASHTO, 1993).  Tie bars are typically used at longitudinal joints (see Figure 2.37) or between an edge joint and a curb or shoulder.  Typically, tie bars are about 12.5 mm (0.5 inches) in diameter and between 0.6 and 1.0 m (24 and 40 inches long).

Figure 2.37: Tie Bars Along a Longitudinal Joint