Superpave Mix Design for Interstate 10 in Louisiana
Recently, The South Central Superpave Center completed a Superpave mix design for the Louisiana Department of Transportation and Development. Two features of this mix design make it unique. First, the mix is to be placed on a Louisiana DOTD project that is the subject of Superpave Showcase, which will occur on June 12, 1996. The Showcase is part of a rehabilitation project on Interstate 10 near Baton Rouge, Louisiana. Persons interrested in attending the Superpave Showcase can contact Dr. Babak Naghavi. The second unique feature is that while the mix design was conducted at The South Central Superpave Center, it was used as a training opportunity for personnel of the Louisiana Transportation Research Center. That is, engineering and technical professionals from LTRC travelled to Austin to assist in the laboratory mix design. A description of the mix design follows.
The contractor on the project, George Sullivan, Inc. of Baton Rouge, Louisiana provided the aggregates and asphalt to be used. The source of the aggregate was Vulcan Materials from their quarry in Smithland, Kentucky. The Vulcan aggregate is a very hard and durable limestone. A natural sand from Louisiana was also used. The asphalt binder was a PG 76-22 from Marathon Oil Company. The Louisiana DOTD also specified that the binder needed to meet its current requirements for a PAC 40 asphalt binder. These ingredients were to be combined to meet all of the mix design requirements for a Superpave 19 mm mixture.
Seven trial aggregate blends were evaluated. The following table shows the Superpave mix design information for these blends.
|
|
Trial Blend
|
Design
|
Estimated Mix Property
|
1
|
2
|
3
|
4
|
5
|
6
|
7
|
Criteria
|
Trial Asphalt Content, %
|
4.1
|
4.1
|
4.7
|
4.1
|
4.3
|
4.2
|
4.8
|
n/a
|
Estimated Asphalt Content, %
|
3.4
|
3.9
|
5.0
|
3.6 |
4.5
|
4.0
|
3.9
|
n/a
|
Air Void Content, %
|
4.0
|
4.0
|
4.0 |
4.0 |
4.0
|
4.0
|
4.0
|
3 - 5
|
Voids in Mineral Aggregate, %
|
12.1 |
13.0
|
15.2 |
12.6 |
14.1
|
12.9
|
12.5
|
13% min
|
Voids Filled with Asphalt, %
|
67.0
|
69.2
|
73.8 |
68.3 |
71.6
|
69.0
|
67.9
|
65 - 75
|
%G mm @ N initial
|
86.3
|
85.6 |
85.5 |
85.7 |
85.6
|
85.3
|
88.2
|
89% max
|
%G mm @ N maximun
|
97.3
|
97.6
|
97.6
|
97.5
|
97.6 |
97.4
|
96.3
|
98% max
|
Absorbed Ash Content, %
|
0.1
|
0.2
|
0.1
|
0.1
|
0.2
|
0.3
|
0.5
|
n/a |
Dust Proportion
|
1.1
|
1.0
|
0.6
|
0.8
|
0.7
|
0.9
|
1.7 |
0.6 - 1.2 |
Compaction Slope
|
9.0
|
9.7
|
9.7 |
9.6 |
9.6 |
10.0 |
7.3 |
n/a |
Aggregate Stockpile
|
Blend Percentages
|
Vulcan #67
|
20
|
20 |
20 |
33 |
20 |
37 |
10 |
Vulcan #7s
|
35
|
40 |
50 |
30 |
45 |
22 |
20 |
Vulcan #11s
|
30
|
30 |
20 |
22 |
25 |
30 |
55 |
Natural Sand
|
15
|
10 |
10 |
15 |
10 |
11 |
15 |
Trial Blends 1, 2, 3, 5, and 7 were established by The South Central Superpave Center, while Trial Blends 4 and 6 were suggested by the contractor. Trial Blends 1 through 6 are coarse 19 mm mixtures graded below the Superpave restricted zone. Trial Blend 7 is a fine 19 mm blend that has a gradation above the restricted zone. As can be seen by the information in the table, a wide range of mix volumetric properties was achieved, with VMAs ranging from 12.1 to 15.2 percent. For the design number of gyrations used (96), Trial Blend 5 was the most favorable blend. All of its mix design properties were within the acceptable ranges.
Notice that Trial Blends 2 and 3 also had suitable properties. Trial Blend 2 was not selected because its VMA was so close to the minimum acceptable value. Any loss of VMA during plant production would result in a field VMA below the minimum desireable. Trial Blend 5 had too much VMA, which resulted in an unacceptably high asphalt content. Had the design number of gyrations been higher, the VMA of Trial Blend 5 likely would have been lower with an attendant reduction in asphalt content. Thus, it is possible that under different circumstances, Trial Blend 5 might have been the chosen blend.
One interesting feature in the table are the values for compaction slope. Compaction slope is computed as the slope of the densification data from the Superpave gyratory compactor between Ninitial and Ndesign. The concept of compaction slope is shown in the figure below.
A steeper compaction slope indicates that a mixture offers more resistance to compaction through its aggregate skeleton and thus, is more resistant to rutting. In the table above, all of the compaction slopes are at relatively high values, except for Trial Blend 7. That is the fine blend that is graded above the Superpave restricted zone. The comparatively low compaction slope for Trial Blend 7 suggests that its aggregate skeleton is not as strong as the others. While compaction slope is not a Superpave design requirement, SHRP researchers identified it as a potentially useful mix parameter.
After Trial Blend 5 was selected as the design aggregate structure, the estimated asphalt binder content was verified. This was accomplished by fabricating Trial Blend 5 specimens in the Superpave gyratory compactor at multiple asphalt contents. This step resulted in a design asphalt binder content of 4.2 percent, which was reasonably close to the predicted value of 4.5 percent.
The last step in the mix design was to evaluate the moisture susceptibility of the designed mixture. That was accomplished using the two tier approach normally used by the Louisiana DOTD. Because the Louisiana DOTD almost always specifies the use of antistripping agents, the moisture sensitivity evaluation was conducted using 0.75 percent Permatac 99, an additive provided by Akzo Nobel of Waco, Texas.
The first step was to conduct a boiling water test on the coarse aggregate. This test indicated that the mixture retained 100 percent coating after it was subjected to boiling.
The second step was to conduct the standard Louisiana DOTD moisture susceptibility evaluation. This test is identical to ASTM D4867. It involves measuring the indirect tensile strength of moisture conditioned specimens and comparing their strength to unconditioned specimens. The tensile strength ratio (TSR) of the mix was measured to be 1.01. This indicated that the mix is not moisture susceptible and confirmed the results of the boiling water test.
For more information pertaining to this mix design, contact Bob McGennis at The South Central Superpave Center or Chris Abadie with LTRC (504-767-9110 voice or 504-767-9108 fax).
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