Chapter 9: Thin Functional Hot Mix Asphalt Overlay Projects
4.0 What is a Gap Graded Mixture?
Gap graded mixtures currently placed in California are, in general, solely Rubberized Asphalt Concrete (RAC) Type G which uses asphalt rubber binders. However, MB type G mixtures have been used in pilot projects. This section covers only asphalt rubber modified mixes.
4.1 What is a Gap Graded Mixture?
A gap graded mixture consists of an aggregate grading that has a missing fraction. Typical gap graded mixes are shown in Figure 17 and Table 9. The Type G gradings are shown in Tables 9a and 9b.
Table 9a: Type G Graded Aggregates - 19 mm Maximum, Coarse
| Sieve Sizes |
Percentage Passing Proposed Gradation Limits |
Percentage Passing
Operating Range |
Percentage Passing
Contract Compliance |
| 25 mm |
— |
100 |
100 |
| 19 mm |
— |
95-100 |
90-100 |
| 12.5 mm |
83-87 |
X±5 |
X±7 |
| 9.5 mm |
65-70 |
X±5 |
X±7 |
| 4.75 mm |
33-37 |
X±5 |
X±7 |
| 2.36 mm |
18-22 |
X±4 |
X±5 |
| 600 µm |
8-12 |
X±4 |
X±5 |
| 75 µm |
— |
2-7 |
0-8 |
Figure 9b: Type G Graded Aggregates - 12.5 mm Maximum, Medium
| Sieve Sizes |
Percentage Passing
Proposed Gradation Limits |
Percentage Passing
Operating
Range |
Percentage Passing
Contract
Compliance |
| 25 mm |
— |
— |
— |
| 19 mm |
— |
100 |
100 |
| 12.5 mm |
— |
90-100 |
90-100 |
| 9.5 mm |
83-87 |
X±5 |
X±7 |
| 4.75 mm |
33-37 |
X±5 |
X±7 |
| 2.36 mm |
18-22 |
X±4 |
X±5 |
| 600 µm |
8-12 |
X±4 |
X±5 |
| 75 µm |
— |
2-7 |
0-8 |
In California, the gap (missing fraction) is used to accommodate the asphalt rubber binder. This is intended to allow for stone on stone contact for deformation resistance and the extra binder has been found to aid in fatigue and reflection cracking resistance. The CRM increases the viscosity of the binder allowing high binder contents without bleeding. The increase in voids allows the mix to accommodate the larger particulate rubber present in asphalt rubber binders (1). The binder content may be 7 to 9% by weight with asphalt rubber binders.
Figure 17: Typical Aggregate Gradation of Gap-Graded Mixture |
Table 9: Gap Graded Aggregates - ¾ inch Maximum, Coarse
| Sieve Sizes |
Percentage Passing
Proposed Gradation Limits |
Percentage Passing
Operating
Range |
| 1 inch |
— |
100 |
| 3/4 inch |
— |
100 |
| 1/2 inch |
67-71 |
±5 |
| 3/8 inch |
56-71 |
±5 |
| No. 4 |
53-57 |
±5 |
| No. 8 |
52-56 |
±4 |
| No. 30 |
34-38 |
±4 |
| No. 200 |
— |
2-7 |
The purpose of gap grading is to provide improved stone-to-stone contact by reducing the fine aggregate content so as to provide a strong aggregate skeleton that creates space for more engineered binder than a dense graded mix can hold. Gap grading is also a good way to increase the VMA of a mixture.
Stone matrix asphalt (SMA), also a gap graded mixture, uses fibers to prevent drain-off. The modifier used in these mixtures makes the binder thick enough to stay in the matrix so that binder content may be higher than that for a dense graded mix. Voids characteristics of gap graded mixtures should be similar to those of Dense Graded mixes, although VMA can be somewhat higher.
4.2 Performance
4.2.1 Distresses/Conditions Addressed
Gap graded thin overlays should only be placed on structurally sound pavements because they offer no structural improvement, but they can renew the surface in terms of functional performance (e.g., ride quality). They can be used to mitigate the following distresses present in an existing pavement:
- Raveling
- Oxidation
- Reflection cracking
- Minor surface irregularities
- Flushing Surfaces
- Skid Problems
Although not as free draining as open graded mixes, some improvement is noted in skid related problems (i.e., hydroplaning and spray and splash) and noise reduction.
4.2.2 Principal Distress Modes
Thin overlays can exhibit the following distress modes:
- 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 to aggregate incompatibility.
- De-lamination, due to poor compaction and/or tack coat practice.
Often, these can be addressed by proper mix design and job selection.
4.3 Job Selection
4.3.1 Where Should Gap Graded Asphalt Concrete be Used?
Gap graded mixes are commonly used as a surface treatment over dense graded HMA pavements and occasionally on Portland cement concrete pavements. They should be placed over structurally sound pavements and may be used in new construction and rehabilitation projects. These mixes are generally used on the traveled way and should be placed across the entire roadbed, from outside edge of shoulder to outside edge of shoulder to provide uniform frictional properties and proper drainage. Properly designed and constructed gap graded mixtures have low permeability and good durability (due to high binder content).
4.3.2 Where Should Gap Graded Asphalt Concrete Not be Used?
Gap graded mixes should not be used on unsound pavement exhibiting substantial cracking, rutting, bleeding, or depressions. The extent of pavement distress precluding the use of these mixes has not been quantified at this time. These mixes are generally not used on bridge decks as a surface course.
4.3.3 Service Life and Costs
Gap graded costs are similar to those for OGFC.
4.4 Design & Specification
The design of Type G mixtures is similar to that for dense graded mixtures as indicated in SSP 39-400_M (8) except that the Agency Test Method is modified in the following ways:
- The aggregates must have a grading and quality resulting in a mixture containing 7 to 9% asphalt rubber binder by weight of dry aggregate.
- The air void content used to select the optimum binder content varies according to traffic index (level) and climatic region.
- Laboratory mixing is done from 149 to 163ºC (300 to 325°F) and compaction from 143 to 149ºC (289 to 300°F)
- A minimum stabilometer value is required.
- A minimum VMA of 18% is required as determined by the test described in Asphalt Institute Mix Design Methods for Asphalt Concrete (MS-2) (6).
Asphalt rubber materials requirements are provided in Chapter 1.
Gap 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).
4.5 Construction
Construction methods for gap graded mixtures are similar to those for dense graded mixes as detailed in Section 2.5. The following are important issues (1).
4.5.1 Manufacture
No specific modifications to plants are required. Binder tanks require agitation, especially if CMCRA binders are used and all limitations must be observed on storage time and temperatures (Chapter 2).
Mixing temperatures must be in the correct range to allow full coating of the aggregates. Temperatures that are too low do not allow adequate coating of the aggregates whereas temperatures that are too high can result in smoke or excess fumes.
4.5.2 Storage
In general, gap graded mixes should not be stored for more than two hours due to stability limitations in the asphalt rubber binder.
4.5.3 Hauling
Standard hauling equipment (i.e., end dump vehicles) may be used for constructing gap graded overlays. The use of covering tarps may help to prevent temperature loss and crusting of the mixture (i.e., hardening of the outer surface of the mixture exposed to ambient temperatures), especially in night and cool weather work with modified mixes. A release agent may be used on the truck bed. On no account should diesel or other petroleum materials be used as release agents as these will soften the mixture.
4.5.4 Safety
The agency’s standard safety and traffic control procedures must be followed. Traffic should not be allowed on gap graded mixes until final rolling has been completed and sand applied. Sand is recommended to prevent pick-up of the mix.
4.5.5 Surface Preparation
This is the same as for dense graded thin overlays. Where agriculture product drippings are an issue, flushing is an option. It needs to be completed 24 hours in advance of the overlay to allow drying time. When cracks are being treated, especially fatigue cracks, a membrane or SAMI may be used as a surface preparation. Membranes may also be used to waterproof the underlying layer.
4.5.6 Tack Coat
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.
4.5.7 Laydown
Gap graded mixes may be windrowed ahead of the paver and picked up 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 then this, the length of the windrow should be kept shorter than 50 meters (160 ft). Table 10 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 and, thus, allowed to cool below the minimum laydown temperature should not be combined with fresh mix.
4.6 Post-Laydown Treatments
If traffic can be kept off the mix, no treatment is required. Otherwise sand should be applied after final rolling at 0.5 to 1 kg/m2 (1 to 2 lb/yd2) to avoid pick up by early traffic. Sweeping may be required after initial trafficking to remove the sand. This is generally done the next day.
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