Chapter 9: Thin Functional Hot Mix Asphalt Overlay Projects

3.0 Open Graded Mixes

3.1 What is an Open Graded Friction Course Overlay?

Open Graded Friction Course (OGFC), also referred to as Open Graded Asphalt Concrete (OGAC), is a surface course with an aggregate gradation that provides an open void structure compared with conventional dense graded asphalt concrete (1, 7). Air void content typically ranges between 15 and 25% in OGFC mixtures (1, 9, 10) resulting in a highly permeable mixture relative to Dense Graded HMA (which normally is relatively impermeable). The porous nature of OGFC mixtures allows surface water to quickly drain away by flowing down through the mixture. The principal benefit derived from OGFC mixtures is a significant reduction in splash and spray relative to Dense Graded mixtures and PCC pavements. Other benefits include a reduction in tire noise and an increase in the frictional characteristics relative to Dense Graded mixtures. The use of modifiers such as asphalt rubber and PBA 6a and PBA 6b may be used to address different environmental and climatic conditions, and allow for thicker films to improve durability.

The typical aggregate gradations for open graded mixes shown in Figure 15 and the aggregate properties are in Table 6. The mixture requirements are based on a drain down test and are discussed in Section 3.4.

3.2 Performance

OGFC is designed as an abrasion resistant wearing course that can quickly drain water from the road surface. The following paragraphs provide a brief overview of the distresses that occur in open graded thin overlays as well as the factors influencing job selection, service lives, and costs.

3.2.1 Distresses/Conditions Addressed

Conventional open graded thin overlays should only be placed on structurally sound pavements because they offer no structural improvement, but they can restore functional performance (i.e., ride quality). They can also be used to mitigate the following distresses present in an existing pavement (9, 10):

• Skid problems / Hydroplaning
• Splash and spray
• Noise problems
• Raveling
• Oxidation
• Minor surface irregularities (ride quality)
• Surface reflection problems
• Bleeding surfaces

When used in association with a SAMI, OGFC mixes may also enhance resistance to reflective cracking. In addition, performance graded binders can be used to address low temperature cracking and reflective cracking. Also, because durability is a function of film thickness (11), the use of polymers and modifiers that increase in-service viscosity allow thicker films resulting in higher resistance to oxidation and raveling (9, 10). The void structure also allows absorption of free surface asphalt to mitigate bleeding pavements.

3.2.2 Principal Distress Modes

OGFC overlays exhibit the following distress modes (1, 2, 9):

• Permanent deformation due to heavy traffic and high temperatures.
• Shear failures in high stress areas.
• 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 caused by binder-aggregate incompatibility.
• De-lamination due to poor compaction and tack coat practices.
• Clogging of air voids causing loss of permeability.
• Rich and dry spots due to drain down of binder during transport and application.
• Isolated areas of softened binder due to fuel or oil spills.

Often, these can be addressed by selection of the correct binder and proper mix design and job selection. Open graded thin overlays are not suitable for every job. The performance of OGFC thin overlays is based on maintaining the void structure.

3.3 Job Selection

3.3.1 Where Should OGFC be Used?

• OGFC can be used in new construction, major rehabilitation projects, and thin functional overlays. OGFC is used as a wearing course (i.e., surface treatment over dense graded HMA pavements and occasionally, on Portland cement concrete (PCC) pavements). OGFC is generally used on the traveled way and extending 0.3m (1 ft) on the shoulder (7). In thin functional overlay applications, the distress mode of the existing pavement must be determined and addressed.
• Conventional open graded overlays should be placed on sound pavements. Type O and type O-HB can be used to slow reflection cracking.

Reflective Cracking may be better addressed by utilizing Rubberized OGAC with an increased high binder content (RAC-O (HB)).

3.3.2 When Should OGFC be Used?

OGFC is a desirable application for the surface layer of AC pavements where its benefits are important. This is especially the case whenever the traffic count is high and the rainfall is moderate or high. Specifically, OGFC should be used when the following are issues:

• Wet Weather Accidents: Consider the use of OGFC when there is a high frequency of wet weather accidents or to minimize such occurrences.
  
• Skid Resistance: When frictional properties of the pavement surface are suspect, a skid test should be conducted to determine the existing coefficient of friction of the pavement surface. Figure 16 shows typical surface textures of OGFC compared with Dense Graded mixes.

• Wet-night Visibility: Another consideration for the use of OGFC is when there is a high percentile confidence level for wet weather and nighttime accident occurrences. OGFC may also be considered for placement to reduce splash and spray due to rain and increase the visibility of pavement delineation. It can be placed on both asphalt and Portland cement concrete pavements.
  
• Cross Slope: When the cross slope is less than 2% and there are two or more lanes in one direction, OGFC may be especially helpful to assist in the draining of water from the pavement surface.
  
• Noise: OGFC has been reported to reduce road noise (9, 10, 13, 14). The life expectancy of any noise benefit will vary with the mix and binder type.
  
• Structural Adequacy: OGFC is generally considered to be a sacrificial layer rather than a structural layer.
  
• Oxidation Reduction: OGFC has been successfully used as a protection layer to prevent asphalt aging in the main structural layers.
  
• Mitigation against Flushing and Bleeding: When applied to a pavement, OGFC provides a void structure to accommodate any potential flushing or bleeding in the underlying pavement.
  
• Mitigation of Cracking: Type O-HB can be used to mitigate cracking.

3.3.3 Where and When Should OGFC Not be Used?

OGFC should not be used on:

• Unstable Pavements: OGFC should not be used on any pavement that exhibits substantial cracking, rutting, bleeding, or depressions.
  
• Snow or Icy Areas: In snow areas, where tire chains, studded tires, or snowplows will detrimentally affect the aggregate and binder, the result may lead to stripping of the aggregate and contribute to raveling and pavement deterioration.
  
• Areas with Severe Turning Movements: High shear areas are not recommended for placement of OGFC due to potential for scuffing. These areas may include parking areas, intersections, ramp terminals, or curbed sections.
  
• Curb and Gutter/Dense Graded: Open Graded mixes should not be placed adjacent to curb and gutter or Dense Graded mixes where water may be held back and stored, thus, creating a ‘bath’ that may cause striping or saturation of the structural section.
  
• Muddy Areas: Areas where mud may be tracked onto the pavement from un-surfaced side roads will fill the voids and reduce the surface water drainage characteristics of the OGFC.
  
• Fuel or Oil Spill Areas: OGFC should not be placed in areas where dripping of oil or fuel from slow or stopped vehicles is prevalent.
  
• Mill and Fill Areas: Mill and fill areas should not generally be candidates for OGFC as a bathtub effect may be created. If OGFC were to be used as the final course, a leveling course would be required first.

3.3.4 Special Maintenance Requirements of OGFC

Removal and replacement currently is the preferred method for dealing with a failed or aged OGFC. However, New Mexico DOT has successfully placed thin bonded wearing courses over open graded mixes as an alternative to milling them out.

Permeability must be maintained to ensure water flow is unimpeded. Maintenance on roadways surfaced with OGFC should avoid any activities that may obstruct the lateral flow of water through the OGFC. These activities may include crack sealing or patching a small failed area with Dense Graded mixes thus creating a ‘dam’ where water may be retained or stored and contribute to further failure of the OGFC surfacing. When large areas of patching are involved, OGFC should be replaced with OGFC. Traffic striping may also inhibit lateral water flow if the striping materials are applied at a heavy rate or excessive amount of reflective beads are used.

Winter maintenance is not as great an issue as once thought. OGFC has different thermal and icing properties compared with Dense Graded mixes. Thermal conductivity is up to 70% less according to National Asphalt Pavement Association (18). It will thus act as an insulating layer and accumulate ice and frost faster than Dense Graded mixes.

General maintenance of OGFC to prevent clogging is important in some areas. Water hose, high-pressure cleaners, and specialized cleaning vehicles have been used successfully.

3.3.5 Service Life and Costs

OGFC overlays have been shown to last 2 to 10 years, but more commonly 4 to 6 years (1). The life of the overlay is directly affected by the condition of the existing pavement that received the overlay, 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 open graded thin overlays. Several of the factors contributing to the cost of placing these overlays 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

The section entitled “Framework for Treatment Selection,” contained in Chapter 1, provides a simplified method of comparing cost effectiveness of different treatments.

3.4 Design & Specification of OGFC

Caltrans uses CT 368 to design OGAC mixes. The California method was revised in 2002 and is based on an aggregate grading designed to give a minimum of 18% voids using CT 367, and a drain down test (CT 368). This test determines the optimum level of conventional binder that may be used without excessive drain down in transportation or during placement of the mixture. For modified systems, the drain down is only established for AR 4000 and this value is used in the design requirements. In high binder open graded mixes only asphalt rubber binders are used and the binder content is adjusted upwards by 1 to 2% based on the field experience with these mixtures. At the time of this writing, the Department has decided that moisture susceptibility testing is not required for OGAC.

Open 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).

3.4.1 Materials Requirements

Materials requirements for binders and aggregates are covered in Chapter 2. Special requirements of OGFC mixtures are related to its specific properties. The void structure must remain intact to ensure that it remains permeable. As air can penetrate easily and promote aging the void structure itself will promote accelerated aging compared with dense graded materials. For this reason, the binders used in OGFC mixtures must be more resistant to the effects of aging than those used for Dense Graded mixtures. Polymer modified binders and CMCRA provide improved resistance to aging.

The texture of the mixture at the surface affects skid resistance. To achieve this, the aggregate should be hard and abrasion resistant and the mixture must be resistant to permanent deformation so that the open void structure remains intact. The requirements for aggregates were shown in Table 6 and the gradings were shown in Figure 15. It has been found that coarser gradings give a more open void structure (1, 10). These tend to give good stone on stone contact and deformation resistance and the voids are less susceptible to becoming clogged.

Performance Graded binders are used for OGFC to address low temperature cracking, reflective cracking and night paving. P.G. binders also address low temperature cracking and to overcome problems of lower temperature paving conditions (e.g., night paving). The void structure allows absorption of free surface asphalt to overcome bleeding pavements. As durability is a function of film thickness (11), the use of polymers and modifiers that increase in-service viscosity will allow thicker films and higher resistance to oxidation and raveling (8). All polymers and modifiers appear to improve the abrasion resistance of the mixes (10).

Studies have shown that modified binders give superior service lives as they prevent binder drain down in application and in service (15, 16, 17). This is due to the elastomeric nature of the binders that resists flow at even high production and service temperatures. They also improve rutting resistance and are less thermally susceptible.

3.5 Manufacture & Construction

Manufacturing and construction methods are similar to those for dense graded mixes. The methods must address the following important issues (1).

3.5.1 Manufacture

No specific modifications are required to plants. Binder tanks should have agitation, especially if CMCRA binders are used and all limitations must be observed for storage time and temperatures. Binder proportioning requires a mass flow meter to ensure accuracy.

Appropriate temperatures must be carefully controlled during the mixing process. Temperatures that are too high will promote drain down and ‘fat’ spots or ‘dry’ spots in the final surfacing. Temperatures that are too low may result in inadequate coating of the aggregate.

3.5.2 Storage

In general, open graded mixes should not be stored for more than two hours due to the potential for binder drain-down.

3.5.3 Transport

Standard transport equipment may be used. The use of tarps may help prevent loss of heat and crusting of the mixture; especially during night and cool weather work using modified mixes. This is critical for haul times longer than 30 minutes in the daytime and for night work. Release agents may be used on the truck bed. Diesel or other petroleum materials should never be used as release agents since these will soften the mixture. Hauling distance should be as short as possible.

3.5.4 Laydown

The following are issues associated with laydown:

Safety: The agency’s standard safety and traffic control procedures should be followed and traffic should not be allowed on OGFC until final rolling has been completed.

Surface Preparation: This is the same as for dense graded thin overlays.

• Thermoplastic markings should be removed according to the agency’s guidelines.
• All crack and joint sealing should be performed prior to placing the OGFC. Allow for adequate cure time for crack and joint sealants. Hot applied sealants require three to four months while cold applied products require one year.
• Overlay of an existing OGFC surface will require removal of the existing OGFC prior to placing new OGFC. This will prevent water entrapment and poor bonding. This should be considered at the planning stage since this item of work may be a substantial cost to the project. Conformance to current standards and policy for removal and disposal of pavement grindings should be adhered to.

Tack Coating: Good tack coat practice must be followed correctly. This requires a heavier tack coat than Dense Graded mixes as the tack coat assists in waterproofing the underlying pavement. If the surface is milled, a heavier than conventional coat will be required to ensure the more absorbent surface is waterproofed.

Paving Guidelines: Paving guidelines shown in Table 7 apply to the placement of OGFC mixes.

Wind is an important factor. Cold wind may reduce the surface temperature quickly making compaction difficult. On very cool and windy days placement may need to be suspended.

Transverse joints are more difficult to make in open graded mixtures due to these mixtures being more difficult to work by hand compared with dense graded mixtures. Handwork should be minimized. For this reason, transverse butt joints should be constructed or joints should be avoided by continuous paving. Longitudinal joints are made in a similar manner to those for dense graded mixtures.

Rolling: The rollers used for open graded mixtures are solely steel wheeled operated in static mode (pneumatic rubber tired rollers are not used because they will close up the voids in the surface by kneading action and the mix may stick to the tires). The ballasted weight should be no more than 8 to 10 tons. Two passes of the roller are usual. Rolling temperatures are shown in Table 8. Rollers should NOT roll unsupported edges, as this will tend to collapse the void structure creating a flattened and sealed edge (3).

3.6 Post Treatment

If traffic can be kept off the mix, no treatment is required. However, in most cases, sanding is carried out on rubberized mixes to prevent initial traffic pick up. Clean sand is spread using a sand spreader at about 0.5 to 1 kg/m² (1 to 2 lbs/yd²) after rolling is complete.

3.7 Sampling & Acceptance

This should be carried out according to the agency’s requirements. OGFC is usually accepted based on aggregate grading; mix binder content, and visual inspection.