Chapter 9: Thin Functional Hot Mix Asphalt Overlay Projects

2.0 Dense Graded Thin Overlays

2.1 What is a Dense Graded Overlay?

Dense graded mixtures have an aggregate structure that is continuously graded (sized) from the largest to the smallest aggregate in the system. They are mixed in a continuous drum type hot mix plant or a batch plant. Many asphalt grades can be used in these mixes. The asphalt selected should reflect properties for different climatic and anticipated distress conditions. For example, certain performance graded binders (PG Grades) are used to prevent thermal cracking caused from a single severe temperature drop (3) and/or during cooler (night) paving conditions. Chemically Modified Crumb Rubber Asphalt (CMCRA) is not usually used in dense graded mixtures due to the more difficult compaction characteristics associated with thin layers and less resistance to reflective cracking.

Aggregate gradations for dense graded mixes may be found in an agency’s Standard Specifications.
For thin overlays of 25 to 37 mm (1 to 11/2 in), the stone size should be limited to a maximum of one-half the thickness of the layer. Hence, 19 mm (0.75 in) is not usually used and 12.5 mm (0.5 in) medium mixes are usually the upper limit. A typical aggregate gradation for a dense-graded mix is shown in Figure 10. Table 4 shows a typical set of aggregate physical requirements.

2.2 Performance

Dense graded mixtures have relatively low air void contents and are designed as an abrasion resistant and functionally impermeable wearing course. Historically, dense graded mixtures have been the most commonly used mix type for overlaying asphalt or Portland cement concrete pavements. The following paragraphs provide a brief overview of the distresses that occur in dense graded thin overlays as well as the factors influencing job selection, service lives, and costs.

2.2.1 Distresses Addressed

Conventional dense graded thin overlays should only be placed on structurally sound pavements because, while they can improve functional performance (i.e., ride quality), they offer little structural improvement. They can be used to mitigate the following distresses present in an existing pavement:

• Raveling
• Oxidation
• Minor cracking
• Minor surface irregularities
• Skid problems

When used in association with a SAMI, or fabric interlayer, they may also retard reflective cracking. In addition, performance graded asphalts can be used to address low temperature cracking and reflective cracking.

2.2.2 Primary Distress Modes

Dense graded thin overlays exhibit the following distress modes:

• Permanent deformation due to heavy traffic and high temperatures.
• Fatigue cracking due to repeated traffic loading.
• Reflection cracking due to cracks in the existing pavement reflecting up through the overlay.
• Raveling due to a number of factors including oxidation and hardening of the binder, water damage, low binder content, and low compaction.
• Stripping (water damage) caused by binder-aggregate incompatibility.
• De-lamination due to poor compaction and/or tack coat practices.

Often these can be addressed by selection of the correct binder and proper mix design. The principal failure modes of dense graded thin overlays are de-lamination, raveling and cracking due to poor compaction. Thin layers cool faster than thick layers reducing the time available for proper compaction. Thus, if a thin overlay is not compacted to the target proportion of air voids, it will tend to be less cohesive and may ravel or de-laminate.

2.2.3 Job Selection

Thin blanket overlays should only be used on sound pavements where minor defects may be present and all construction requirements can be met, especially compaction. Variables that affect job selection include:

• Traffic Loading: In low volume roads, variations in traffic need to be taken into account. Selection should be based on the worst-case scenario. For high volume roads, the principal failure modes are fatigue cracking and permanent deformation. To resist fatigue cracking a thin blanket can be used to extend the pavement life for 1-3 years depending on the mix type.
  
• Existing Pavement Condition: Dense graded thin overlays should only be used on pavements that do not possess a significant amount of distress. For example, existing pavements with significant quantities of medium to high severity fatigue cracking are poor candidates for a thin overlay. Conversely, pavements that possess distresses that affect the functional performance of the existing pavement (e.g., rideability, poor skid resistance, oxidation, etc.) are generally good candidates for thin overlays provided that a structural enhancement of the existing pavement is not required. Sometimes a thin overlay (with a SAMI) is placed over poor roads to prolong the period until rehabilitation.
  
• Environment: With proper mix design (i.e., appropriate binder type and content for a given aggregate type and gradation) these mixes have been successfully used in a range of climates. In all climates fatigue cracking can be the principal mode of failure. In hot climates permanent deformation (rutting) can be the principal mode of failure whereas in climates where large temperature swings occur, thermal cracking can be the principal mode of failure. Use of a dense graded thin overlay must take into account the climate in which it is placed in order to avoid distresses that commonly occur in a particular climate.
  
• A reactive practice would be to typically overlay medium to high fatigue cracked pavements to slow deterioration and prevent pot holes from occurring. The thin overlay would be a stop gap treatment until the proper rehabilitation or reconstruction action can be taken.
  

2.2.4 Service Life and Costs

Dense graded thin overlays have been shown to last 2 to 10 years, but more commonly last between 4 and 6 years (1). The life of the overlay is directly affected by the condition of the receiving pavement, the climate (environmental conditions) in which the overlay was placed, and the traffic loading experienced by the overlay. For example, a thin overlay placed on a pavement in poor condition would not be expected to last as long as one placed on a pavement in good condition. Similarly, a thin overlay placed on a pavement in good condition but with heavy traffic would not be expected to last as long as one placed on the same pavement, but with much lighter traffic.

Numerous factors influence the cost of dense graded thin overlays. Several of the principal factors contributing to the cost of placing a dense graded thin overlay include:

• Materials (binder and aggregate with or without modifiers)
• Location of the project (e.g., urban versus rural area, proximity to hot mix plant, etc.)
• Thickness of the overlay
• Special construction requirements (e.g., stricter control of compaction relative to conventional overlays or night work)

Chapter 1 provides a simplified method of selecting cost effective treatments.

2.3 Design & Specifications

The Hveem method, developed by Caltrans Translab in the 1940s, is presently used for dense graded hot mix design. The Hveem method is covered extensively through various references (3, 4, 6) and the test methods may be found on http://www.dot.ca.gov/hq/esc/ctms/indexhtml.

The Hveem method uses a series of test methods to determine optimum binder content. These test procedures include use of a centrifuge to measure surface porosity and particle roughness (ASTM D 5148). ASTM C 127 and C 128 are used to measure the specific gravity of the fine and coarse aggregate respectively. Knowing the specific gravity of the fine and coarse aggregate and conducting ASTM D 5148 leads to the approximate bitumen ratio. A series of test specimens is prepared at a range of asphalt contents above and below the approximate bitumen ratio (i.e., approximate binder content). This preparation method uses a kneading compaction device. A stability test to evaluate the resistance to deformation is performed, as well as a swell test to determine the effect of water on volume change and permeability of the specimen. Finally, the specimens are tested for moisture vapor susceptibility to determine the extent to which the stability values are affected by moisture vapor. Table 4b shows the required properties of the mixture as specified in the Agency’s Standard Specifications.

Dense graded mixtures may be designed using the Marshall, Hveem or Superpave Methods already described, all of which produce quality hot mix asphalt (HMA), from which long-lasting pavements can be constructed. Some Federal agencies and most private laboratories use the Marshall Method, while within State Highway Agencies, the Superpave Method is gradually becoming the standard (19).

2.4 Material Requirements

Dense graded mixes must be comprised of materials capable of resisting degradation during construction as well as providing good long-term durability. Thus, the aggregates must be sufficiently hard to resist breakage during compaction and be sufficiently compatible with the binder so as to resist de-bonding of the binder in the presence of water (i.e., resist stripping). Other characteristics, such as particle shape, are also important. Similarly, the binder must be of sufficient quality to resist the effects of aging (i.e., oxidation and associated hardening). In this sense, it is desirable to have a relatively soft binder or to have a mixture with a relatively thick binder film. However, the binder must also be hard (stiff enough) and the mixtures must have a binder film thin enough to resist permanent deformation. Thus, the binder grades (e.g., AR 4000, AR 8000, etc.) are often selected to resist these conflicting requirements. Performance graded binders can be incorporated into the mixture to assist in optimizing resistance to a particular distress mode. Chapter 2 has more information on materials requirements.

2.5 Construction

2.5.1 Manufacture

Aggregates and binder are mixed using either a batch plant or drum mixing plant. The Asphalt Institute’s “HMA Construction, Manual Series 22” and the U.S. Army Corps of Engineers “Hot Mix Paving Handbook” have extensive information on plant types and correct operation. Important factors prior to mixing are appropriate storage of binder and aggregates and adequate drying of aggregates. Correct proportioning of aggregates and binder is important as is correct mixing temperatures (see Chapter 2) to allow full coating of the aggregates during the actual mixing process.

2.5.2 Storage

Dense graded mixes may be stored in silos for no more than 18 hours. Material with hardened lumps cannot be used. The Asphalt Institute’s “HMA Construction, Manual Series 22” and the U.S. Army Corps of Engineers “Hot Mix Paving Handbook” have detailed storage silo requirements.

2.5.3 Hauling

Standard hauling equipment (i.e., end dump vehicles, bottom dump vehicles, or live bottom dump vehicles) may be used for the construction of dense graded thin overlays. The U.S. Army Corps of Engineers “Hot Mix Paving Handbook” contains further information regarding these types of vehicles. The use of tarps is advisable to prevent any crusting of the mixture (i.e., hardening of the first few centimeters of the mixture exposed to ambient temperatures), especially in night and in cool weather using modified mixes, or when long haul distances are required. Release agent should be used on the truck tray. On no account should diesel or other petroleum materials be used as release agents as these will soften the mixture.

Care must be taken in handling the mixture to ensure segregation does not occur. This may happen if the mix is not correctly loaded at the plant, is poorly designed, or not handled correctly. For larger jobs, a re-mixer “shuttle buggy” might be considered.

2.5.4 Surface Preparation

Surface preparation is critical for good performance of any overlay. Thin maintenance overlays should only be placed on sound pavements. This means that pavement failures must be repaired first. Cracks should be sealed and any pot holes patched. Crack sealing and patching practices were covered in Chapters 3 and 4, respectively.

In some cases a SAM or SAMI (Figure 11) may be used over pavements with low severity fatigue cracking in small quantities (e.g., isolated areas). The overlay may be applied a year or more after a SAM seal or immediately following application of a SAMI. Surfaces should be thoroughly swept before application of the overlay to remove debris that could prevent a good bond between the existing pavement and the overlay. Flushing with water may be needed where the pavement is exposed to agriculture product drippings, animal carcasses or heavy debris.

2.5.5 Tack Coat

Tack coats are applications of asphalt sprayed onto an existing pavement prior to an overlay being applied. The tack coat promotes adhesion between old and new pavement layers (5).

Good tack coat practice must be followed. Surfaces must be clean before the tack coat is applied. If a good bond is not formed between the thin overlay and the existing pavement, it can de-bond resulting in a slippage failure or de-lamination. If too much tack coat is applied, it may bleed up through the layer, especially under heavy traffic.

Tack coats should be applied using a calibrated distributor with nozzles set at an angle of about 30 degrees to the spray bar. The height should allow a triple spray overlap (see Chapter 5 – Figure10). The tack coat must be applied in one application at a rate from 0.10 to 0.45 l/m² (0.02 to 0.10 gal/yd²), with the exact rate determined by the surface texture and porosity.

2.6 Laydown

Dense graded mixes may be windrowed ahead of the paver and collected with a pick-up device (loader) and deposited in the paver hopper. The length of the windrow must be as short as possible to ensure excessive cooling does not occur. If conditions are good (i.e., little or no wind and higher temperatures), this is usually about 50 meters (160 ft) maximum (1). If conditions are poorer than this, the length of the windrow should be kept less than 50 meters (160ft). Table 5 summarizes minimum application temperatures for the various stages of the construction process. Every effort should be made to avoid segregation of the mixture during the paving operation. In addition, mix that is left in the paver hopper too long will cool below the minimum laydown temperature and should not be combined with fresh mix.

When paving operations are discontinued for an extended period (e.g., end of day), it is necessary to construct a transverse joint across the pavement being placed.  This can be accomplished in a number of ways and the type of joint constructed depends primarily on whether or not traffic will be allowed to travel over the joint between the time the joint is constructed and paving operations resume.  If traffic will not be allowed to travel over the joint, it is recommended that a butt joint be constructed as illustrated in Figure 12a.  Conversely, if traffic is allowed to travel over the joint, it will be necessary to construct a tapered joint as illustrated in Figure 12b.  The Asphalt Institute’s “HMA Construction, Manual Series 22” and the U.S. Army Corps of Engineers “Hot Mix Paving Handbook” provide detailed guidance for constructing transverse joints. 

Longitudinal joints occur between adjacent travel lanes or between travel lanes and a paved shoulder.  During the paving operation of a lane of pavement, the material along the edge of the pavement (i.e., the site of the proposed longitudinal joint) normally has about a 60-degree incline relative to the surface of the existing pavement.  Prior to placement of the adjacent lane of pavement (or shoulder), this material can be either cut back (using a saw or cutting wheel attached to a grader or front-end loader) by about 50 mm (2 in) to create a vertical face, or an overlapping joint can be constructed. 

Whenever a joint is created by “cutting back the joint,” a tack coat should be applied to the newly exposed face of the longitudinal joint.  Cutting back the joint helps to ensure that adequate density of the mixture exists at the longitudinal joint.  Generally, properly overlapping, raking, and compacting the longitudinal joint can also produce an adequate joint density.  Figure 13 illustrates compaction of an overlapped joint.  If the mix along the joint is clean, a tack coat is not normally needed prior to placement of the adjacent lane of pavement. 

The Asphalt Institute’s “HMA Construction, Manual Series 22” provides detailed guidance for constructing longitudinal joints.

2.6.1 Rolling

There are several stages of rolling used for dense graded mixtures.  Because thin layers lose temperature rapidly, the rolling temperatures must be strictly monitored.  The stages for compaction include initial breakdown using a vibratory roller, kneading compaction using a pneumatic roller, and finishing using a static roller as illustrated in Figure 14 (8).  The actual temperatures would vary some based on binder type.

2.6.2 Acceptance

Dense graded HMA pavements are usually accepted based on aggregate grading, binder content, and relative compaction of the in-place mixture. Aggregate grading and binder content must conform to the agency’s standard specification. The relative compaction may be measured in the field using a nuclear density gauge in accordance with the agency’s specifications. Some agencies specify a range of relative densities (e.g. 93% to 96%) within which the contractor may elect to request acceptance and pay a penalty (i.e. reduced compensation) for the work. Mixtures placed with a relative compaction of less than the penalty range would be removed and replaced at the contractor’s expense.

2.7 Post Treatments

Dense graded HMA usually require no post-laydown treatments.