Chapter 10: Ultra-Thin, Hot Mixed Asphalt, Bonded Wearing Course Projects
2.0 Design & Specifications
2.1 Gap Graded Hot Mixed Asphalt
This section provides an overview of, and specifications for, materials used in the construction of bonded wearing courses. Bonded wearing courses are constructed using polymer-modified binders and gap graded aggregates. The purpose of using a gap grading in a BWC is to provide improved stone-to-stone contact by reducing the medium sized aggregate content. This also produces a strong aggregate skeleton that provides space for more engineered binder than a dense graded mix does.
2.1.1 Binder
Currently, there are four grades of binder approved in the Caltrans specification for use in BWC construction. They are listed in Table 1. They vary in their degree of polymer modification, and their use corresponds to the climatic conditions encountered in California. In general terms, grades GGB1 and GGB2 are used in hotter climates, while grades GGB3 and GGB4 are used in cooler climates.
Table 1: Bonded Wearing Course Hot Mix Binder Grades
| Binder Grade |
General Climatic Region |
Criteria |
| GGB1 |
Desert or Hot Valley Areas and Coastal Areas |
Areas below 1,050 m (3,445 ft) elevation with average 7-day maximum and 1-day minimum pavement temperatures between 70°C and -22°C (158°F and -8°F), respectively. |
| GGB2 |
Coastal Areas |
Areas below 1,050 m (3,445 ft) elevation with average 7-day maximum and 1-day minimum pavement temperatures between 64°C and -22°C (147°F and -8°F), respectively. |
| GGB3 |
Cool Coastal
or Mountain Areas |
Areas below 1,500 m (4,920 ft) and above 1,050 m (3,445 ft) elevation with average 7-day maximum and 1-day minimum pavement temperatures between 64°C and -28°C (147°F and -18°F), respectively. |
| GGB4 |
Mountain Areas |
Areas above 1,500 m (4,920 ft) elevation with average 7-day maximum and 1-day minimum pavement temperatures between 58°C and -34°C (136°F and -29°F), respectively. |
Many highway agencies select a performance graded binder that meets the criteria for their climatic region. The most common performance graded binders for a BWC are shown in Table 1.
Table 1: Bonded Wearing Course Performance Graded Binder Applications
| Binder Grade |
General Climatic Region |
Criteria |
| PG 70-22 |
Desert or Hot Valley Areas and Coastal Areas |
Areas below 1,050 m (3,445 ft) elevation with average 7-day maximum and 1-day minimum pavement temperatures between 70°C and -22°C (158°F and -8°F), respectively. |
| PG 64-22 |
Coastal Areas |
Areas below 1,050 m (3,445 ft) elevation with average 7-day maximum and 1-day minimum pavement temperatures between 64°C and -22°C (147°F and -8°F), respectively. |
| PG 64-28 |
Cool Coastal
or Mountain Areas |
Areas below 1,500 m (4,920 ft) and above 1,050 m (3,445 ft) elevation with average 7-day maximum and 1-day minimum pavement temperatures between 64°C and -28°C
(147°F and -18°F), respectively. |
| PG 58-34 |
Mountain Areas |
Areas above 1,500 m (4,920 ft) elevation with average 7-day maximum and 1-day minimum pavement temperatures between 58°C and -34°C (136°F and -29°F), respectively. |
Table 2 shows the specification for the binders used in bonded wearing courses. It should be noted that a performance-based system using rheological measurements is employed. Higher stiffness binders are used for hotter climates while lower stiffness binders are used for cooler ones. The viscosity is used to control the application of the binder.
Table 2a: BWC Hot Mix Binder Requirements
| Specification Designation |
Test Method |
GGB Grade 1 |
GGB Grade 2 |
GGB Grade 3 |
GGB Grade 4 |
| Flash Point, Cleveland Open Cup, °C, min., original binder |
AASHTO T 48 |
230 |
230 |
230 |
230 |
| Brookfield Viscosity, max. 2.0 Pa s test temperature, °C |
ASTM D 4402 |
135 |
135 |
135 |
135 |
| Elastic Recovery after RTFO test % min |
AASHTO T 301-99 |
60 |
60 |
60 |
60 |
| Mass Loss after RTFO test, % max |
AASHTO T 240 |
0.6 |
0.6 |
0.6 |
0.6 |
| Dynamic Shear, G*/sin°, min. 2.2 kPa RTFO aged residue, test temperature at 10 rad/sec, °C |
Agency Test Method such as: CT 381 Part 3 |
70 |
64 |
64 |
58 |
| Residue from PAV, test temperature, °C |
AASHTO TP1-98 |
110 |
100 |
100 |
100 |
| Creep Stiffness, 300 MPa, Max. and M-value, 0.30, Min. residue from PAV, test temperature °C |
AASHTO TP1-98 |
-12 |
-12 |
-18 |
-24 |
Table 2b: BWC Hot Mix Binder Requirements - Report Only
| Specification Designation |
Test Method |
GGB Grade 1 |
GGB Grade 2 |
GGB Grade 3 |
GGB Grade 4 |
| Dynamic Shear, SSD > 30 x (0.6 + SSV)3 original binder, °C |
Agency Test Method such as: CT 381 Part 1 |
25 |
25 |
25 |
25 |
| Dynamic Shear, SSD > -115 x (SSV) - 50.6 On PAV aged residue, from AASHTO PP1, °C |
Agency Test Method such as: CT 381 Part 1 |
25 |
25 |
25 |
25 |
Table 2: BWC PG Binder Requirements
| Specification Designation |
Test Method |
PG
70-22 Binder |
PG
64-22 Binder |
PG
64-28 Binder |
PG
58-34 Binder |
| Flash Point, Cleveland Open Cup, °C, min., original binder |
AASHTO T 48 |
230 |
230 |
230 |
230 |
| Brookfield Viscosity, max. 2.0 Pa s test temperature, °C |
ASTM D 4402 |
135 |
135 |
135 |
135 |
| Elastic Recovery after RTFO test % min |
AASHTO T 301-99 |
60 |
60 |
60 |
60 |
| Mass Loss after RTFO test, % max |
AASHTO T 240 |
0.6 |
0.6 |
0.6 |
0.6 |
| Dynamic Shear, G*/sin°, min. 2.2 kPa RTFO aged residue, test temperature at 10 rad/sec, °C |
Agency Test Method such as:
Caltrans CT 381Part 3 |
70 |
64 |
64 |
58 |
| Residue from PAV, test temperature, °C |
AASHTO TP1-98 |
110 |
100 |
100 |
100 |
| Creep Stiffness, 300 MPa, Max. and M-value, 0.30, Min. residue from PAV, test temperature °C |
AASHTO TP1-98 |
-12 |
-12 |
-18 |
-24 |
2.1.2 Aggregate
The main properties controlled in the aggregate used in BWC mixtures include gradation, shape, number of crushed faces, wear resistance, and clay or deleterious material content.
The two gradations allowed in BWC mixtures are shown in Tables 3a and 3b. In the tables, the symbol "X" is the gradation that the Contractor proposes to furnish for the specific sieve listed. However, the proposed gradation shall meet the gradation shown in the "Limits of Proposed Gradation."
The 12.5 mm (1/2 in) gradation is used for roadways with high traffic volumes (which require a thicker and more durable mat) and where pedestrian or bicycle traffic are not a concern. The 9.5 mm (3/8 in) gradation is used for urban, residential and business district roadways where pedestrian and bicycle traffic is a consideration. This can also be used on mainline travel ways if desired.
The physical property requirements of the aggregate used in BWC mixtures are shown in Tables 4a and 4b. The requirement listed in Table 4b only applies when studded tire or chain wear is a concern.
Table 3a: Gradation, Percentage Passing, Maximum 12.5 mm (1/2 inch)
| Sieve Sizes |
Limits of Proposed
Gradation |
Operating Range |
Contract Compliance |
| 19 mm |
— |
100 |
100 |
| 12.5 mm |
— |
85-100 |
82-100 |
| 9.5 mm |
60-80 |
X ±5 |
X ±8 |
| 4.75 mm |
28-38 |
X ±4 |
X ±8 |
| 2.36 mm |
25-32 |
X ±4 |
X ±6 |
| 1.18 mm |
15-23 |
X ±3 |
X ±5 |
| 600 µm |
10-18 |
X ±3 |
X ±5 |
| 300 µm |
8-13 |
X ±3 |
X ±5 |
| 150 µm |
— |
5-11 |
4-12 |
| 75 µm |
— |
3-8 |
2-9 |
Table 3b: Gradation, Percentage Passing, Maximum 9.5 mm (3/8 inch)
| Sieve Sizes |
Limits of Proposed
Gradation |
Operating Range |
Contract Compliance |
| 12.5 mm |
— |
100 |
100 |
| 9.5 mm |
— |
85-100 |
82-100 |
| 4.75 mm |
28-38 |
X ±4 |
X ±8 |
| 2.36 mm |
25-32 |
X ±4 |
X ±6 |
| 1.18 mm |
15-23 |
X ±3 |
X ±5 |
| 600 µm |
10-18 |
X ±3 |
X ±5 |
| 300 µm |
8-13 |
X ±3 |
X ±5 |
| 150 µm |
— |
5-11 |
4-12 |
| 75 µm |
— |
3-8 |
2-9 |
The aggregate specifications are provided to obtain desired mix properties. For example, the mixture is intended to interlock and develop a shear-resistant pavement surface; hence, crushed particle faces are essential. The gap-graded aggregate creates voids in the aggregate, which ensure the correct void level in the mix. Flat or elongated particles reduce texture depth and are to be avoided. The aggregate should also be wear resistant (low wear value in AASHTO T 96) and low in clay content (high Sand Equivalent value in ASTM D 2419).
Table 4a: Required Aggregate Properties for a BWC
| Test |
Test Method |
Requirement |
Percentage of Crushed Particles:
Coarse Aggregate, %, minimum |
Agency Test Method such as: CT 205
(Note a) |
90 |
Percentage of Crushed Particles:
Fine Aggregate (On portion passing a 4.75 mm sieve and retained on a 2.36 mm sieve), %, minimum |
Agency Test Method such as: CT 205
(Note a) |
85 |
| Uncompacted Void Content, %, minimum
(Note b) |
AASHTO T 304
Method A |
45 |
| Flat & Elongated Ratio at 3:1, %, maximum |
ASTM D 4791 |
25 |
| Los Angeles Rattler:
Loss at 500 rev., %, maximum |
AASHTO T 96 |
35 |
Sand Equivalent:
Contract Compliance, minimum |
ASTM D 2419 |
47 |
Sand Equivalent:
Operating Range, minimum |
ASTM D 2419 |
50 |
Notes:
- In California Test 205, Section D, the definition of a crushed particle is revised as follows: "A particle having two or more fresh, mechanically fractured faces shall be considered a crushed particle."
- If the fine aggregate is 100% crushed, the use of crushed material shall be monitored during the production process. If the fine fraction is a combination of crushed and natural materials, the fine aggregate angularity (FAA) shall be monitored during the process.
|
Table 4b: Additional Aggregate Requirements for BWC Applications
| Specification |
Requirements |
| Surface Abrasion Test,
Agency Test Method such as:
Caltrans CT 360, maximum loss |
0.40 g/cm2 |
2.1.3 Mix Design
The performance of a bonded wearing course depends on the quality of the materials and how they interact during application, rolling and after opening to traffic. The amount of polymer modified asphalt binder to be mixed with the aggregate for gap-graded HMA shall be determined by the Contractor using Asphalt Institute MS 2 Table 6.1. The binder content shall be established based on an estimated film thickness minimum of 10 microns. Film thickness will be calculated based on effective asphalt content. The optimum binder content is first established so that the film thickness requirement is met. This binder content shall conform to the draindown and film stripping requirements. The film thickness is optimized and the resistance to stripping by the action of water is measured. These properties are important in gap-graded mixes, as water has easy access to the binder-aggregate interface. Table 5 lists the BWC mix requirements.
Table 5: BWC Mix Requirements
| Test |
Test Method |
Min Requirement |
Max Requirement |
| Film Thickness, mm |
Gradation surface area factor method; Asphalt Institute MS-2, Table 6.1 |
10.0 |
- |
| Film Stripping, % |
Agency Test Method such as: CT 302 |
- |
25 |
| Drain down Test, g |
Agency Test Method such as: CT 368 |
- |
4 |
2.2 Polymer-Modified Asphalt Emulsion Membrane
The asphalt emulsion used in the membrane for a bonded wearing course is specially formulated and must meet the specification. The binder is designed to give high flexibility and bonding in the range of climactic conditions in which bonded wearing courses are placed (see Section 2.1.1). The emulsion is manufactured using conventional means.
Specifications are based on standard emulsion properties; such as, stability, binder content, viscosity and torsional recovery. Application viscosity is important, as the material should be easily sprayed at the correct rate, not flow away and form a continuous membrane. The residual properties indicate polymer presence and the base asphalt grade used. Cooler conditions call for higher residual penetration. The emulsion is designed to break rapidly after spraying to ensure that no water is trapped. The gap-graded nature of the mix allows water to escape, thus promoting breaking of the emulsion.
Table 6: BWC Emulsion Specifications
| Specification Designation |
Test Method |
Min. Requirement |
Max. Requirement |
| Saybolt-Furol Viscosity, at 25°C, s |
AASHTO T 59 |
20 |
100 |
| Sieve Test on original emulsion(at time of delivery), %
|
AASHTO T 59 |
- |
0.05 |
| 24-hour Storage Stability, % (Note a) |
AASHTO T 59 |
- |
1 |
| Residue by Evaporation, % |
Agency Test:
CT 331 |
63 |
- |
| Solubility in Organic Solvent, % (Note b) |
AASHTO T 44 |
97.5 |
- |
| Torsional Recovery, measure entire arc of recovery, at 25°C, % |
Agency Test:
CT 332 |
30 |
- |
| Penetration (0.01 mm) at 25°C
|
| Emulsion used with GGB1 or GGB2 |
AASHTO T 49 |
70 |
150 |
| Emulsion used with GGB3 |
AASHTO T 49 |
90 |
180 |
| Emulsion used with GGB4 |
AASHTO T 49 |
100 |
200 |
Notes:
- After standing undisturbed for 24 hours, the surface shall show no white, milky colored substance, but shall be a smooth homogeneous color throughout.
- The organic solvent shall be from the approved list available from the Agency Transportation Laboratory.
|
Table 6: BWC Emulsion Specifications
| Specification Designation |
Test Method |
Min. Requirement |
Max. Requirement |
| Saybolt-Furol Viscosity, at 25°C, s |
AASHTO T 59 |
20 |
100 |
| Sieve Test on original emulsion (at time of delivery), %
|
AASHTO T 59 |
- |
0.05 |
| 24-hour Storage Stability, % (Note a) |
AASHTO T 59 |
- |
1 |
| Residue by Evaporation, % |
Agency Test Method such as:
CT 331 |
63 |
- |
| Solubility in Organic Solvent, % (Note b) |
AASHTO T 44 |
97.5 |
- |
| Torsional Recovery, measure entire arc of recovery, at 25°C, % |
Agency Test Method such as:
CT 332 |
30 |
- |
| Penetration (0.01 mm) at 25°C
|
| Emulsion used with PG 70-22 or PG 64-22 |
AASHTO T 49 |
70 |
150 |
| Emulsion used with PG 64-28 |
AASHTO T 49 |
90 |
180 |
| Emulsion used with PG 58-34 |
AASHTO T 49 |
100 |
200 |
Notes:
- After standing undisturbed for 24 hours, the surface shall show no white, milky colored substance, but shall be a smooth homogeneous color throughout.
- The organic solvent shall be from the approved list available from the Agency Transportation Laboratory.
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