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The contract specified that a Hveem mix design be performed by the contractor and verified by the Government. The contractor had the responsibility of selecting the aggregate gradation and proper asphalt content to ensure compliance with the mix design criteria. As part of the mix design process, it was determined that the aggregate source was moisture sensitive and required treatment to meet the retained strength requirement. The contract required that hydrated lime, added at a rate of 1.0%, be used as the anti-strip additive. The specifications for the approved gradation and mix design of the control mixture are shown in Tables 1 & 2. The two WMA mixtures were designed using the same gradation target values.
| Sieve Size | Target Value |
|---|---|
| 1"(25 mm) | 100 |
| 3/4"(19 mm) | 99 |
| 1/2"(12.5 mm) | 85 |
| 3/8" (9.5 mm) | 71 |
| #4 (4.75 mm) | 46 |
| #8 (2.36 mm) | 30 |
| #40 (425 μm) | 12 |
| #200 (75 μm) | 6.0 |
| Asphalt Content by Weight of Mix | 5.3% |
| Air Voids | 4.0% |
| Voids in Mineral Aggregate | 12.6% |
| Stabilometer Value | 36 |
| Dust/Asphalt Ratio | 1.1 |
| Immersion-Compression (1% lime added) | 91.0% |
After completion of the Hveem mix design, 75-gyration Superpave specimens were manufactured and analyzed. Based on the data obtained from the Superpave specimens, the asphalt content (AC) would have been reduced to 5.2% by weight of mix. This would yield 4.0% air voids and a slightly reduced amount (12.4%) of voids in mineral aggregate (VMA).
In addition, 75-gyration Superpave specimens were manufactured using the two warm mix additives. The mix design procedure was modified to accommodate the lower mixing and compaction temperatures. In order to manufacture these specimens, the aggregate was heated to a temperature of 275 °F (135 °C). Asphalt binder along with the warm mix additive was then added and properly mixed. The mixture was then short-term aged according to AASHTO R 30 but at a reduced temperature. The specimens were then compacted. A comparison of the mix design data is shown in Table 3.
| Control Mix | Advera Mix | Sasobit Mix |
|---|---|---|
| 5.2% AC by Mix | 5.2% AC by Mix | 5.2% AC by Mix |
| 4.0% Air voids | 3.4% Air voids | 3.2% Air voids |
| 12.4% VMA | 12.1% VMA | 12.0% VMA |
From the comparison of the Superpave gyratory data, the warm mix additives provide a compaction benefit even at the reduced temperatures. This data indicates that some type of modified procedure will be needed in order to design mixtures using warm mix additives of this type. If the method is not modified, there would be a reduction in asphalt content for mixtures that use these types of additives which could lead to pavement durability issues.
The asphalt binder for the project was a PG 58-34 as specified in AASHTO M 320. This binder met the 98% reliability as stated in the LTPPBind asphalt binder selection program for this extremely cold environment. The binder supplied to the project met the requirements for the specified grade.
It had been reported that the addition of Sasobit can have an effect on the binder properties. For this project, the asphalt binder supplied for the mix design was tested with and without the additive to determine the extent of stiffening, if any, on the binder. The results of the test are shown in Table 4.
| Property | No Additive | With 1.5% Sasobit |
|---|---|---|
| Rotational Viscosity | 0.535 Pa•s | 0.480 Pa•s |
| Mass Change | -0.387% | -0.364% |
| DSR, Original | 1.316 kPa | 2.468 kPa |
| DSR RTFO | 2.551 kPa | 4.719 kPa |
| DSR PAV | 1,414 kPa | 1,917 kPa |
| BBR, Stiffness, S | 265 MPa | 287 MPa |
| BBR, Slope, m-value | 0.318 | 0.275 |
From the data, it can be seen that in this case the Sasobit additive did stiffen the binder. It would no longer meet a PG 58-34 grade, but would be at higher temperatures at both the upper and lower values. Additional testing was not performed to determine the actual binder grade.
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An introductory webinar to assist local and tribal agencies with applying for transportation funding under the Bipartisan Infrastructure Law (BIL).
