Properties of Foamed Asphalt for Warm Mix Asphalt Applications (2015)

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74 HMA is a well-established paving material with over 100 years of proven performance. High temperatures [275°F (135°C) to 325°F (162°C) and higher] are necessary to ensure complete drying of the aggregate and subsequent bonding with the binder, coating of the aggregate by the binder, and workability for adequate handling and compaction. All of these processes contribute to good pavement performance in terms of durability and resistance to permanent defor- mation and cracking. Economic, environmental, and pos- sible performance benefits motivate the reduction of HMA mixing and compaction temperatures. The latest technology is WMA, where production temperatures are reduced to a range of 175°F (79°C) to 295°F (146°C). Some of the bene- fits of WMA are decreases in energy consumption, emissions, odors, and fumes during production, extended haul distances and pavement construction season, and improved workability and compactability of the mixtures. WMA production in the United States has increased expo- nentially in recent years, from 19.2 million tons in 2009 to 86.7 million tons in 2012. The work in this research study focused on asphalt foaming because it is the largest segment of the WMA market in the United States. According to a sur- vey done by NAPA, mechanical foaming units were respon- sible for about 88% of all WMA produced in 2012 (Hansen and Copeland, 2013). The changes brought about by WMA in mixture components, mix processing, and plant design have prompted many questions about the validity of current mix design methods in adequately assessing the volumetric needs of asphalt mixtures and the physical characteristics required to meet performance expectations. This research study considered the impact of WMA foaming technology on the properties of binders and the volumetric and performance characteristics of mixtures. Accordingly, the objectives were to (1) determine the properties of foamed bind- ers that relate to asphalt mixture performance and (2) develop laboratory foaming and mixing protocols that may be used to design asphalt mixtures. Several test methods and metrics were explored to character- ize the properties of the foamed binder, including noncontact and image-based methods. A laser-based sensor method was preferred to measure the expansion ratio and collapse of the binder foam because it required minimal hardware and soft- ware for setup and use, and allowed periodic data acquisition at 1-s intervals. In addition, digital images of the surface of the foamed binder were also acquired at 1-s intervals to quantify the number and size of the bubbles with time. The outcome of these two test methods yielded the following metrics to characterize binder foam: 1. Maximum expansion ratio – ERmax. 2. Rate of collapse of semi-stable foam – k-value. 3. Foamability index – FI. 4. Surface area index – SAI. A laboratory binder study was performed to investigate the influence of binder source, water content, temperature, liquid additives, and shearing action on foamed binder characteris- tics. Several binders from six different producers and refinery locations were collected and used for that purpose, and the following observations were made: • Binders of different sources/locations and PG grade had different ERmax, k-value, FI, and SAI at the same water content. • Some binders were more sensitive to changes in water con- tent than others. • The elapsed time between measurements of the same binder source/location seemed to have an effect in ERmax, k-value, FI, and SAI. • For most binders, there was a linear correlation between ERmax and water content. • For most binders, k-value increased with water con- tent; that is, at higher water contents the foam was more unstable. C H A P T E R 5 Conclusions

75 • For most binders, FI decreased with water content; that is, at higher water contents, the area under the ER versus time curve was smaller due to the instability of the binder. • For most binders, SAI decreased with water content; that is, larger bubbles with a faster foam collapse (larger k-value) were observed at higher water contents. At low water con- tents, smaller bubbles with a slower foam collapse (smaller k-value) were formed. • There was no apparent effect of temperature on the foamed binder properties when dispensing the foamed binder sam- ple in a container kept at room temperature versus a con- tainer kept at elevated temperature inside a heating mantle. • Liquid additives had an effect on foamed binder produced in the Accufoamer; the alkaline one (similar to Evotherm) produced a greater ERmax and lower k-value, while the amine surfactant (Rediset) showed no difference with respect to the binder without additives. • Viscosity measurements after foaming were lower than for the unfoamed binder. The difference, however, was the same regardless of the water content for foaming. • No noticeable ER differences were observed for the bind- ers with zeolite (Advera). However, when the binders were subjected to RTFO aging, there was a greater mass loss in the binders with zeolite, suggesting the presence and con- tinued release of moisture from the blended binders. • The effect of residual water was corroborated on binder sam- ples foamed in the Accufoamer and subjected to RTFO aging, which also showed a greater mass loss than the unfoamed binder samples. With respect to the effect of foaming on the rheological properties of the foamed binder residue after RTFO aging, there was a slight increase in the continuous high-temperature grade of the binder. The intermediate- and low-temperature performance of the foamed binder residue after PAV aging showed no change for the S and m-value bending beam BBR parameters, but a slight increase in G*sind as compared to the unfoamed binders. The three foaming units (Wirtgen WLB 10S, InstroTek Accufoamer, and PTI foamer) showed distinct differences in the foamed binder properties. The Wirtgen foamer had the largest ERmax values in the majority of the cases, while the PTI foamer had the lowest ERmax values in all cases. In fact, the PTI foamer showed very small expansion compared to the other two units (in the range of 1 to 1.5). The FI was similar for the binders foamed with 1.0% water content in both the Wirtgen foamer and the Accufoamer, but was dif- ferent when a larger water content (i.e., 3.0%) was used. With respect to the bubble size distribution analysis, in all three units, foamed bubbles decreased in size with elapsed time and became more homogeneous. In addition, the difference in bubble size seemed to be more pronounced at 3.0% water content versus 1.0% water content. The PTI foamer seemed to produce more stable bubbles regardless of the elapsed time after foaming. The size of the bubbles 90 s after foaming was about 0.11 in. (2.8 mm). A laboratory mixture study was also conducted to estab- lish the relationship between binder foam characteristics and mixture workability, coatability, and performance. Since foaming is intended to improve mixture workability and coatability, it was important to focus on the evaluation of workability and coatability of the foamed asphalt mixtures. The objectives of this portion of the research study were to (1) develop laboratory test methods to measure workability and coatability of asphalt mixtures, (2) evaluate the effect of different binder types and foaming water contents on the workability and coatability of foamed asphalt mixtures, and (3) compare the workability and coatability of foamed WMA versus HMA. Maximum shear stress measured in the SGC was proposed as a mixture workability parameter; mixtures with lower max- imum shear stress are considered more workable. For coat- ability, a procedure based on aggregate absorption was used. The method is based on the assumption that a completely coated aggregate submerged in water for a short period (i.e., 1 hour) cannot absorb water since water cannot penetrate through the binder film surrounding the aggregate surface. On the other hand, a partially coated aggregate is expected to have detectable water absorption since water is able to pen- etrate and be absorbed by the uncoated particle. The coat- ability index was proposed as the mixture coating parameter. A preliminary CI threshold of 70% was established based on the laboratory test results. To verify the workability and coatability procedures, foamed WMA using three binders with three foaming water contents was produced using the Wirtgen foamer in addition to a con- trol HMA produced using the same laboratory unit with no water. The workability and coatability of the foamed WMA mixtures were compared to the characteristics of the HMA mixtures. In addition, the effect of binder source, binder grade, and foaming water contents on mixture workability and coat- ability was investigated. The following observations were made based on the results: • Significant differences in the maximum shear stress and CI for foamed WMA mixtures versus the control HMA were observed for two of the three binders. • Two foamed WMA mixtures had better workability and coatability when 1.0% water content was used as compared to higher foaming water contents (i.e., 2.0% and 3.0%). Thus, 1.0% was considered the optimum foaming water con- tent. However, equivalent workability and coatability were observed for the third WMA mixture as compared to the control HMA regardless of the foaming water content used.

76 • WMA mixtures produced at 1.0% foaming water content had better workability and coatability characteristics as com- pared to the control HMA, while WMA mixtures foamed at higher foaming water contents (i.e., 2.0% and 3.0%) had equivalent or worse characteristics as compared to the con- trol HMA when mixed and compacted at the same tempera- tures. This finding highlights the importance of identifying the best materials and foaming conditions to maximize the workability and coatability of foamed mixtures. A proposed foamed mix design approach was also validated, using the Wirtgen WLB 10S, InstroTek Accufoamer, and PTI foamer, as part of the mixture laboratory study. The results showed distinct workability trends from which an optimum water content was selected, although not all units produced the same binder foaming characteristics. Finally, three field studies were conducted. An initial trial was done in Austin, Texas, to apply the laboratory test methods and metrics in a field setting as well as to compare the foamed binder measurements with the workability and coatability results. The on-site foaming measurements showed clear dif- ferences when the binder was sampled using an extension pipe versus directly from the valve outlet. In addition, the foaming metrics were different from the laboratory measurements per- formed with the Wirtgen foamer. The difference in sampling container size used in the laboratory and the field (i.e., 1 gal- lon versus 5 gallon) seemed to have an effect on the foamed binder metrics. A correction factor was determined for ERmax, but even after modifying the field values, the results were still smaller than the ones recorded in the Wirtgen foamer. With respect to workability, 1.0% was the optimum water content (lower shear stress). The performance evaluation of the mix- tures showed equivalent or better performance as compared to the control HMA. A second field study was done in two separate plants in Ohio to compare field foaming units (Terex and Gencor) against foamed binder and foamed mixture measurements. Similar to the initial trial, the on-site foaming measurements were dif- ferent from the laboratory-foamed binder measurements in terms of ERmax and k-value. The third field study was done in Huntsville, Texas, with the objective of validating the proposed foamed mix design approach with plant data. An initial visit was done to mea- sure foamed binder properties on-site and to collect loose mix and raw materials. After following the steps of the proposed mix design and finding the optimum water content, a second visit to the plant was made in order to request a change to the water content to match the optimum. During the second visit, foamed binder measurements were acquired again and loose mix collected. With respect to the on-site foamed binder measurements, distinct differences were observed between the values obtained during the first and second visits. As expected, when the water content was reduced, the ERmax, k-value, and FI were reduced. The workability of the foamed mixture with the initial water content (5.5%) was worse than the control HMA. The laboratory measurements showed that the opti- mum water content was 1.5%, and at this water level, the workability had a dramatic improvement as compared to the control HMA. The foamed mixture at optimum water con- tent had equivalent or better performance than the control HMA fabricated using the plant mix. The mix design was further evaluated using HMA specimens produced in the laboratory. As before, the performance of the foamed mixture with 1.5% water content was equivalent or better than that of the control HMA. Based on the results of the laboratory and field foamed mix design validation studies, the initial approach that included determining the foaming ability of the binder was revised. This requirement was initially incorporated since the laboratory binder study showed some binders had lit- tle foaming ability. However, after following the proposed foamed mix design using various laboratory foaming units and plant data, it was clear that this step was not essential since the same binders in different foamers may or may not expand, but in all cases clear differences were observed when evaluating workability. Therefore, the revised foamed mix design recommendation, as shown in Figure 5-1, does not include this step, but it could still be performed at the discretion of the agency or organization conducting the mix design. In addition, the comparison against HMA was eliminated for practical purposes and because the com- parisons performed in this study of the foamed mixtures at optimum water content versus HMA showed equivalent or better performance. As shown in Figure 5-1, the procedure starts with materi- als selection for the development of a traditional AASHTO Superpave R 35 mix design procedure to determine the opti- mum binder content. Afterward, the optimum foaming water content is established using workability by measuring the maximum shear stress during compaction on foamed mix- tures prepared at 1.0%, 2.0%, and 3.0% water content. The water content that yields the lowest maximum shear stress is considered the optimum. Then, coatability is evaluated at the optimum water content and compared against the CI thresh- old of 70%. The last step in the proposed mix design method is to evaluate the performance of the foamed asphalt mixtures at optimum water content via standard tests (resilient modulus per ASTM D7369, indirect tensile strength per AASHTO T 283, or HWTT per AASHTO T 324). If the selected performance parameter or parameters comply with established AASHTO or DOT specifications, the mixture is accepted. Otherwise, changes in mixture components should be considered and the mixture retested.

77 Figure 5-1. Final recommended foamed asphalt mix design method.