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

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23 A. Laboratory Binder Study Several binders from six different producers and eight refin- ery locations were collected and used to investigate the influ- ence of certain variables on their foaming characteristics using the test methods and metrics described in Chapter 2. The selected variables included binder source, water content, tem- perature, liquid additives, and shearing action. The list of bind- ers is presented in Table 3-1. The asphalts graded as PG64-22 were neat binders, and those graded as PG70-22 were modified binders. A subset of these same binders was used in the labora- tory mixture study that is described subsequently. A.1. Effect of Selected Variables on Binder Foam Characteristics This section presents a summary of the work done to use the proposed methods and metrics to evaluate the influence of factors such as binder source, additive, and shearing action on the characteristics of the binder foam. A.1.1. Binder Source and Grade Eight binders from those listed in Table 3-1 were foamed at 160°C in the InstroTek Accufoamer at 2.0% water content and in the Wirtgen foamer at 1.0% and 3.0% water content and their foaming characteristics determined in terms of ERmax, k-value, FI, and bubble size distribution. Figure 3-1 through Figure 3-3 present a summary of results comparing binder source and water content. In the figures, each data point rep- resents the average of two replicates. In the Accufoamer with 2.0% water content, the ERmax val- ues were as low as 3.4 to as high as 13.0. The ERmax values varied from 5.2 to 12.4 and from 8.1 to 24.7 with 1.0% and 3.0% water content in the Wirtgen foamer, respectively. When comparing different binders, there was no clear relationship between the maximum expansion of foamed binder and the stability of the foam. In other words, the ERmax was not directly correlated to the foam stability (k-value). For example, in the Accufoamer Y6 had the lowest ERmax, and it was also the most unstable foam (highest k-value). However, for any given binder, an increase in water content resulted in an increase in expansion ratio and a decrease in the stability of the foam (or increase in k-value), as can be seen in the Wirtgen foamer results. In addition, the ranking of the binders in terms of ERmax and k-value was different in each case. For the Accufoamer, the N binders had the highest ERmax values, while in the case of the Wirtgen foamer, O7 had the highest ERmax values at both water contents. Also, Y6 had the highest k-value in the case of the Accufoamer but not in the case of the Wirtgen foamer. The rest of the k-values followed different trends in terms of rank- ing of the binders. The ERmax and FI of the binders foamed in the Wirtgen foamer at a selected time of 60 s after foaming are shown in Figure 3-3 for 1.0% and 3.0% water content. Ideally, bind- ers with high ERmax and high FI values are desirable since this indicates that the binder has a good initial expansion but is also stable after foaming. From Figure 3-3, it is apparent that higher water content generates binders with equivalent or larger FI values in most cases. Binders H6 and Y6 are exceptions because the binders were very unstable at 3.0% water content (i.e., large k-values as shown in Figure 3-2) and thus had smaller FI val- ues than at 1.0% water content. It can be concluded that either ERmax and k-value or ERmax and FI can describe the binder foaming behavior and can be used to rank binders with respect to initial expansion and foam stability. The SAI values at a selected time of 60 s for the eight bind- ers foamed in the Wirtgen foamer are illustrated in Figure 3-4 along with the FI values at 60 s. Missing SAI values (such as O6 and O7 at 3.0% water content) indicate that the foamed binder did not generate bubbles or that the bubbles had dissipated at 60 s after foaming. The SAI could be considered a measure of the quality and longevity of the foam. The smaller, longer- lasting, and better-distributed the bubbles are in the foamed binder sample, the larger the SAI value will be. In general, for C H A P T E R 3 Findings and Applications: Laboratory Studies

Binder ID Grade Producer N6 64-22 A N7 70-22 T6 64-22 B Y6 64-22 C (1st shipment) Y62 64-22 (2nd shipment) OM6 64-22 D M6 64-22 H6 64-22 AR6 64-22 E R6 64-22 R62 64-22 HO6 64-22 F O6 64-22 O7 70-22 Table 3-1. Binders used in laboratory study. Figure 3-1. ERmax and k-value for various binders foamed in the Accufoamer at 2.0% water content. (a) (b) Figure 3-2. ERmax and k-value for various binders foamed in the Wirtgen foamer with (a) 1.0% water content, (b) 3.0% water content. (a) (b) Figure 3-3. ERmax and FI for various binders foamed in the Wirtgen foamer with (a) 1.0% water content, (b) 3.0% water content. (a) (b) Figure 3-4. SAI and FI values for various binders foamed in the Wirtgen foamer with (a) 1.0% water content, (b) 3.0% water content.

25 Figure 3-5. ERmax and k-value for binder N6 in the Wirtgen foamer at 1.0% and 3.0% water content. Figure 3-6. ERmax and k-value for binder N7 in the Wirtgen foamer at 1.0% and 3.0% water content. Figure 3-7. ERmax and k-value for binder O7 in the Wirtgen foamer with 1.0% and 3.0% water content. content seemed higher than the measurements from earlier in the year (i.e., April and August). The least affected binder by elapsed time seemed to be N7, especially when considering the variability in the replicate results with 3.0% water content. As before, k-value was not apparently correlated to ERmax. A.1.2. Water Content Three binders (N6, N7, and O7) from two sources were used to evaluate the influence of water content on foaming charac- teristics. Water content used for foaming was varied from 1.0 to 5.0%. The N6 and N7 binders were foamed at 1.0%, 3.0%, and 5.0% water contents, and the O7 binder was foamed at 1.0%, 2.0%, and 3.0% water content. All foamed binders were produced at a temperature of 160°C. Relatively lower water contents were used for the O7 binder because of its signifi- cantly higher ER compared to the other two binders. All three binders were foamed using both Wirtgen foamer and Accu- foamer units. The quality of the foam was evaluated based on ERmax and k-value for all binder–water content combinations produced using these two foaming units. Figure 3-8 compares ERmax for the different binders at different water contents pro- duced using the Accufoamer and Wirtgen foamer. ER m a x Water Content % N6 Wirtgen N7 Wirtgen O7 Wirtgen N6 Accufoamer N7 Accufoamer O7 Accufoamer Figure 3-8. Influence of water content and binder type on ERmax. any given binder, the SAI values at 60 s are larger for the binders foamed at 1.0% water content, as expected. The most critical difference in SAI value between 1.0% and 3.0% water content is for binder H6, which, despite showing an acceptable ERmax value, was very unstable when foamed at 3.0% water content and thus yielded very low FI and SAI values. The elapsed time between measurements seemed to have an effect in the characteristics of the binder foamed in the Wirtgen foamer. In some instances, the changes could be due to modi- fication in the crude streams at the refinery (i.e., when new samples of binder were obtained) or changes in the binder with time. Binders N6, N7, and O7 were measured on two to three separate occasions between the months of April and Septem- ber in the Wirtgen foamer with 1.0% and 3.0% water content. Figure 3-5 through Figure 3-7 show the results. In the figures, each data point represents the average of two replicates, and the error bars span ±1 standard deviation from the ERmax average value. Within the same measurement session, the majority of the replicate foaming measurements were highly repeatable. As can be observed, the greatest differences in ERmax were observed for binder O7, both at 1.0% and 3.0% water contents. This binder seemed to be particularly susceptible to the short elapsed time between measurements. The ERmax measurements for N6 performed in September at both 1.0% and 3.0% water

26 The following observations can be made based on data presented in Figure 3-8: • As discussed in the previous subsection, different binders clearly have different ERmax values at the same water content. This finding was consistent with foams produced using both foaming units. • For any given binder, ERmax increased with an increase in the water content, and the relationship appears to be linear. This was consistent for data collected using both Wirtgen foamer and Accufoamer units and was noted by Ozturk and Kutay (2014b) for the Pavement Technology, Inc. (PTI) foamer. The trends for the water content versus ERmax were similar for the two foaming units. However, the foams produced using the Accufoamer exhibited slightly lower ERmax values. This can be attributed to the differences in the dispensing mechanisms between the two units; the Accufoamer cre- ates the foam in a small enclosed chamber and dispenses it through a 0.25-in. (6.4-mm) diameter tube into the con- tainer, whereas the Wirtgen foamer creates the foam under atmospheric pressure, and it is dispensed directly into the container. • The O7 binder was more sensitive to water content com- pared to the N6 and N7 binders. Also, both N6 and N7 binders had similar sensitivity to water content. These two binders were from the same producer and refinery location (Table 3-1). The rate of collapse (k-value) of the semi-stable foam using the two different foaming units is presented in Figure 3-9. This parameter reflects the stability of the foam; higher values indi- cate lower stability and vice-versa. The following similarities and differences are observed in the rate of collapse of the foam: • For both foaming units, an increase in the water content resulted in an increase in k-value. In other words, for a given binder, higher water content resulted in lower sta- bility, and vice-versa. This is consistent with the hypoth- esized mechanism described earlier. Higher water content typically results in larger droplet sizes and larger bubbles, which in turn have a higher velocity to move to the surface (at a given temperature/viscosity) and collapse faster, and lower water contents produce smaller bubbles that take more time to come to the surface and collapse. This effect is also illustrated in Figure 3-10 and Figure 3-11, which k- v al ue Water Content (%) N6 Wirtgen N7 Wirtgen O7 Wirtgen N6 Accufoamer N7 Accufoamer O7 Accufoamer Figure 3-9. Influence of water content and binder type on the rate of collapse of the semi-stable foam. Figure 3-10. Surface of binder A6 at approximately 30 s after foaming in the Wirtgen foamer with 3.0% (left) and 1.0% (right) water content.

27 Figure 3-11. Bubble size distribution of binder A6 foamed in the Wirtgen foamer with 3.0% (left) and 1.0% (right) water content. Figure 3-12. Expansion ratio versus time for binder O7. show the bubble size distribution of the surface of the binder at two different water contents after approximately 30 s of foaming. • The rate of collapse from both foaming units was in the simi- lar range or order of magnitude. Although the general trend for the rate of collapse with respect to increasing water con- tent was similar for both foaming units, the rate of collapse of the semi-stable foam was typically higher for the Accufoamer compared to the Wirtgen foamer. This could be due to the differences in the delivery of the foamed binder between the two units (direct versus through a tube). In summary, higher water contents result in higher ERmax but also higher k-values (low stability) (see Figure 3-8 and Figure 3-9). An interesting consequence of the combination of these two effects is that a binder foamed with higher water content will start out with a higher ER compared to binders foamed with lower water contents. However, over time, binders foamed with lower water content will tend to be more stable and retain this expansion longer. This effect is illustrated in Figure 3-12 and Figure 3-13. Also, the almost instantaneous Figure 3-13. Magnified view from 1 to 3 minutes of the expansion ratio versus time for binder O7. collapse of the foamed binder suggests that the HL of the foamed binder may not be of relevance in a real mixture pro- duction scenario at a hot mix plant. Of greater importance is the state of the foamed binder after it is exposed to the atmo- spheric pressure (as in a drum mix plant) for the few minutes during which the binder is mixed with the aggregates. A.1.3. Temperature In order to investigate the effect of temperature on the rate of collapse of the binder foam, the O6 binder was foamed at 1.0% and 3.0% water content in the Accufoamer and allowed to collapse at room temperature and at 160°C (foaming tempera- ture). The elevated temperature of the foam was maintained by placing the collection can inside the heating mantle. The can was open to air on the top such that the heating mantle could only maintain the temperature of the wall and bottom of the can. Figure 3-14 and Figure 3-15 compare the results from the foam collapse when the collection can was at room temperature versus

28 WMA, and additives F1 through F3 were alkaline products manufactured with the specific objective of enhanced foam- ing. The additives were combined with heated binder to a temperature of 284°F to 320°F (140°C to 160°C) while using a rotating spindle for 1 minute, as shown in Figure 3-16, before being added to the foaming units. The additive dosage was selected according to the additive producer’s recommen- dations. Table 3-2 presents a summary of the binders used for each type of additive and the dosage. The ERmax and k-value for binder N7 foamed in the Accu- foamer with and without additives are presented in Fig- ure 3-17 through Figure 3-19. The following similarities and differences are observed in the expansion and rate of collapse of the foams: • Additive F1 significantly improved the ERmax and k-value of binder N7. • Additive W1 had a negligible impact on the ERmax and k-value of the foam. • As before, for any given binder or additive modified binder, ERmax increased with an increase in the water content, the relationship was linear, and the stability decreased with an increase in the water content. • The binder modified using additive F1 showed markedly different foam collapse. In particular, the sudden drop in volume of the foam during the first few seconds was not observed for the binder modified with additive F1. Instead, the foamed binder showed a gradual collapse over time (Figure 3-19). Figure 3-14. Expansion ratio versus time for binder O6 with 1.0% water content in the Accufoamer with and without a heating mantle. foam collapse when the collection can was maintained at 160°C inside a heating mantle. These figures demonstrate that there is no noticeable influence of temperature on the measured foam properties. Similar results were obtained for a couple of other binders. Based on these results, the temperature of the collecting unit is not being considered as a factor to characterize binder foams. The temperature of the binder in the foaming unit was not reduced below 160°C because this would result in clogging of the pipes in the foaming unit. A.1.4. Liquid Additives The influence of liquid additives on expansion and col- lapse characteristics of binders was evaluated using binder N7, two additives, and two water contents in the Accufoamer, and binders OM6, R62, Y6, and Y62, four additives, and two water contents in the Wirtgen foamer. The liquid foaming additives were received from two sources. Additive W1 was an amine surfactant commonly used in the production of Figure 3-15. Expansion ratio versus time for binder O6 with 3.0% water content in the Accufoamer with and without a heating mantle. Figure 3-16. Additive blending.

29 Figure 3-20 presents a series of pictures of the surface of the Wirtgen foamed binder samples with and without additives after 5 s of being dispensed in a 1-gallon con- tainer. Both the foamed binder sample with W1 and the sample with no additive had clear, shaped bubbles on the surface, and the difference between these two samples was not significant. However, the samples with F1, F2, and F3 had no distinguishable bubbles on the surface. For W2, many tiny bubbles were observed, which lasted for a long time. The results for the foamed binder in the Wirtgen foamer are presented in Figure 3-21. In most cases, additive W1 showed no significant effect on ERmax as compared to the sample with- out additives, which was consistent with visual observations (Figure 3-20). As expected, the foaming-enhancing additives significantly increased ERmax in most cases. R62 is an excep- tion, which indicates that for some binders, even the addi- tion of foaming-enhancing additives may not significantly improve the foaming characteristics. For any given binder with additive W1, higher water content corresponded to a higher k-value, which is consistent with the previous foamed binder trends without additives. In contrast, in most cases the samples with the foaming-enhancing additive had much lower k-values than the samples without additives. The FI values along with ERmax are presented in Figure 3-22. The results confirm that additive W1 had a negligible effect on the binder foaming metrics. For additives F1 through F3, there was a clear increase in the FI at 1.0% water content, but at 3.0% the trend was not consistent for all three additives. Even though binder R62 did not show clear differences in Additive Binder ID Foaming Unit Additive Dosage by Weight of Binder (%) Foaming Water Content (%) W1 N7 Accufoamer 0.5 1, 3 R62 OM6 Y62 Wirtgen F1 N7 Accufoamer 0.5 1, 3 R62 OM6 Y6 Wirtgen F2 R62 M6 Wirtgen 0.5 1, 3 F3 R62 OM6 Wirtgen 0.5 1, 3 Table 3-2. Additives used in laboratory study. ER m ax N7 N7 + W1 N7 + F1 Figure 3-17. Influence of liquid additives on ERmax of binder N7. Figure 3-18. Expansion ratio versus time of binder N7 with additive F1 foamed at various water contents in the Accufoamer. N7 N7 + W1 N7 + F1 Figure 3-19. Effect of modification on binder N7 foamed with 3.0% water content in the Accufoamer.

Figure 3-20. Surface of the foamed binder 5 s after foaming in the Wirtgen foamer: (a) without additive, (b) with additive F1, and (c) with additive W1. (a) (b) (c) Figure 3-21. ERmax and k-value for foamed binders in the Wirtgen foamer at 1.0% and 3.0% water content with and without additives; (a) OM6, (b) Y62, and (c) R62. (a) (b) (c) Figure 3-22. ERmax and FI for foamed binders in the Wirtgen foamer at 1.0% and 3.0% water content with and without additives; (a) OM6, (b) Y62, and (c) R62.

31 ERmax at 1.0% water content with and without additives, the FI values were distinctly larger than the ones obtained for the binder with no additives or the binder with additive W1. The SAI and FI values at 60 s are illustrated in Figure 3-23. As before, missing values indicate that the images acquired of those samples were not able to be processed using the bub- ble size distribution procedure. In all cases where SAI data were available, higher SAI values also corresponded to higher FI values. This behavior is expected because higher FI values usually indicate more stable foam, which usually translated into longer-lasting and smaller-sized bubbles with elapsed time. A.1.5. Shearing Action ER is an indirect measure of workability (viscosity) of binder foams. However, it may sometimes give inconsistent results in terms of viscosity. For example, binders from different sources that have the same ERmax may have different viscosities. There- fore, a direct measurement of viscosity of the foamed binders was undertaken. Note that viscosity measurement using a rota- tional viscometer also subjects the foam to a shearing action between the spindle and the walls of the container. Therefore, viscosity measurements of foamed binders reflect the combined effect of shearing action and foam collapse on workability. The viscosity of binder foam was measured using a Brook- field rotational viscometer with a #27 spindle immediately after foaming. A sample of the foamed binder from the 1-gallon collection container was poured into the Brookfield sample holder. This process was accomplished within 2 minutes of dispensing the foam. Viscosity measurements were conducted at 275°F (135°C) and 20 rpm, and values were recorded after the reading stabilized (15 minutes after the foam was dis- pensed). The advantage of this approach is that it allows for a more sensitive measurement of binder viscosity. However, because of the time it takes to transfer a sample of the foamed binder from the 1-gallon container into the Brookfield sample holder, and because of the narrow gap between the spindle and the walls of the container, this method will only be effective for investigating whether the presence of any micro-bubbles dur- ing the latter stages of foaming affects the viscosity (and hence workability of the binder). Results for three binders (N6, N7, and O7) with 1.0%, 2.0%, and 3.0% water contents are pre- sented in Figure 3-24. Results show that the foamed binders continue to have viscosities lower than the control even 15 min- utes after foaming, although this effect was more prominent for two of the three binders. Foaming decreased the viscosity of N6, N7, and O7 on average by 7.0%, 23.0%, and 16.0% com- pared to their respective controls. The decrease was similar for all water contents. (a) (b) (c) Figure 3-23. FI and SAI for foamed binders in the Wirtgen foamer at 1.0% and 3.0% water content with and without additives; (a) binder OM6, (b) binder Y62, and (c) binder R62. Water Content V is co si ty (P a) Figure 3-24. Viscosity of foamed binders N6, N7, and O7.

32 A.1.6. Water-Bearing Additives (Zeolite) In the case of binder combined using particulate additives such as zeolite, the additive is added to the binder (or mix- ture) typically at a rate of 5.0% by weight of the binder. These additives are hydrothermally crystalized silicates with large empty spaces in their structure that store up to 21% of water by weight of the additive (Hurley and Prowell 2005). When zeolites are mixed with hot binder, water is gradually released from their crystal structure to create a micro-foam, which is hypothesized to improve binder workability. The typical dosage of 5.0% zeolite will result in approximately 1.0% water by weight of the binder being released (assuming all the water from the zeolite is released). However, unlike foam- ing by water injection, the rate of release of water with zeolite is much slower, resulting in the expansion and collapse of foam continuing over a much longer period. Consequently, ERmax and the rate of decrease of the overall volume of the binder foamed using zeolite are very low. Due to the small size of bub- bles, foams created using zeolite are also more stable (longer life) compared to foams produced using water injection. Due to major differences in the foaming mechanisms of the two foaming methods, parameters developed to characterize foams by water injection may not be appropriate to char- acterize zeolite foams. In addition, zeolite particles left after foaming act as particulate fillers that can alter the rheological properties of the foamed binder residue. Binder foams were produced for this study using zeolite (Advera). The typical recommended dosage for synthetic zeolite as a foaming agent is 0.25% by weight of the mix. Con- sidering typical HMA with 5.0% binder content, this corre- sponds to approximately 5.0% zeolite by weight of the binder. Based on this, binders N7 and O7 were blended with 5.0% zeolite using a RW 20 digital overhead mixer equipped with a four-blade propeller. About 600 grams of the heated binder were poured into a quart can, and the can was inserted into a heating mantle to maintain the binder temperature at 160°C. The exact weight of the binder in the can was also determined. Zeolite (5.0% by weight of the binder) was then slowly added while the binder was stirred using the overhead mixer at a constant speed of 600 rpm. During blending of zeolite with the binder, micro-bubbles approximately 1 mm in diameter (shown in Figure 3-25) were visible on the surface. A laser sensor was used to measure the change in height and corresponding volume of the foam. A schematic for the laser test setup is shown in Figure 3-26. Mixing and expansion had to be measured simultaneously due to the nature of the particulate additive. Since mixing of the binder creates a non- uniform surface profile, the measurement was carried out by pointing the laser at a location approximately at the midpoint between the center of the mixing container and its edge. Figure 3-27 illustrates the change in height during mixing for the two blended binders normalized with the constant height Figure 3-25. Bubble size of foam produced using zeolite. Figure 3-26. Schematic of the zeolite modified binder foam test setup. Figure 3-27. Normalized change in height of binders N7 and O7 with zeolite.

33 that was observed approximately 10 to 15 minutes after start- ing to mix. This normalization also incorporates the increase in volume due to the addition of the zeolite particles, which was approximately 2.0%. Note that the normalized change in height is not the factor by which expansion occurs because of the non-uniform surface profile. Rather, this change is a qualita- tive indicator of the foam expansion and collapse characteris- tics. Similar results were observed when zeolite was mixed with binders OM6, R62, and Y62 using the procedure illustrated in Figure 3-16, where the additive was mixed for 1 minute before the laser measurements were performed. The modified binders did not expand, but many small bubbles were observed at the surface of the binder sample and lasted for a long time. No noticeable change in the height of the binder occurred after 10 to 15 minutes from the start of blending. At this time, the blended binders were poured into rolling thin film oven (RTFO) bottles and aged at 290°F (143°C) for 120 minutes. A reduced temperature of 290°F (143°C) was used in lieu of the standard 325°F (163°C) to simulate a WMA aging condition. The weights of the control binder and the binders blended with zeolite were measured during the RTFO aging processes after 30, 60, 90, and 120 minutes. The average percent mass loss values are presented in Figure 3-28. Two replicate weight measurements were taken for both binders with/without zeolite, and the error bars indicate the minimum and maximum values. The results demonstrate that the weight loss in the binders with zeolite was significantly higher than that of the control binder, suggesting the presence and continued release of moisture from the blended binders. A.2. Effect of Foaming on the Properties of Binders A.2.1. Residual Moisture One of the mechanisms associated with the collapse of the foamed binder is when the internal steam pressure causes the bubble diameter to increase to a point where the tensile stresses in the bubble film cause the bubble to collapse and the entrapped steam to escape. However, it is possible that not all the water used for foaming escapes as steam during this process. In fact, results from the Brookfield viscometer shown earlier suggest the possibility of residual micro-bubbles that influence the viscosity of the binder. To investigate the possibility of residual water in the foamed binder, binder O7 was foamed in the Accufoamer at 2.0% water content, poured into RTFO bottles, and aged in the RTFO at 325°F (163°C). Weight measurements were taken for the foamed binder sample and the control binder at 0, 15, 30, 60, and 85 minutes of RTFO aging. Two replicates of each test were performed. Figure 3-29 illustrates the weight loss during RTFO aging in the foamed and the control binder. Note that the weight loss in a typical binder, as in the case of the control, is associated with the loss of volatiles during short-term aging. Figure 3-29 clearly illustrates that the foamed binder had a greater weight loss compared to the control, suggesting the presence of residual Figure 3-28. Weight loss for control and zeolite-blended binders N7 and O7 during RTFO. Figure 3-29. Weight loss by foamed and control binder during RTFO.

34 water from the foaming process. The error bars in this figure indicate the high and low values for the weight loss. The vari- ability in weight loss for the foamed binder is significantly higher than the control. This is expected because any trapped moisture may not be homogenously distributed. Also, since the total initial water content was 2.0% by mass of the binder, even a small increase in percent points of weight loss indi- cates significant fraction of the residual water. The weight loss in the foamed binder slowly approached that of the control as it aged. A.2.2. Rheological Properties of Foamed Residue The process of foaming was considered potentially able to alter the binder rheological properties. For example, the presence of highly polar water molecules during foaming and aging may alter the microstructure and, consequently, the rheological properties of the binder. Rheological tests were conducted on foamed binder residues using the DSR and bending beam rheometer (BBR) to verify whether these effects existed. The high-temperature grades of the control binders used for foaming were determined after RTFO aging in accordance with AASHTO M 320. Similarly, the residues of the foamed binders were RTFO aged and sub- sequently graded to determine the impact of foaming on the high-temperature performance grade (PG). Table 3-3 presents the results of the high-temperature performance grade for eight different binders after foaming with water content that varied from 1.0% to 5.0%. There was a slight increase in the continuous high- temperature grade of the binders (based on RTFO-aged binder). Foaming increased the continuous high-temperature grade of O7 and N6 with 1.0% water content by 2.7°C and 1.2°C, respectively, compared to their respective controls. Foaming increased the continuous high-temperature grade of the other seven binders on average by less than 0.5°C. These results compare the foamed binder to the base (unfoamed) binder by aging the two different binders under the same con- ditions [325°F (163°C) for 85 minutes]. However, in practice, foamed binders will experience reduced short-term aging compared to conventional binders due to lower production temperatures. Hence, reduced short-term aging temperature may offset the slight increase in continuous high-temperature grade due to the foaming process. The intermediate- and low-temperature performances of foamed binder residues were evaluated after pressure aging vessel (PAV) aging using the G*sind DSR and the S and m-value BBR parameters. (Note: G*sind is an asphalt binder fatigue parameter, G* is the dynamic shear modulus of the asphalt, d is the phase angle, S is the low-temperature binder stiffness, and the m-value is the slope of the creep curve at low temperature.) The results presented in Table 3-4 clearly indicate that the S and Binder Type High PGGrade Continuous Grade G*/sinδ @ High PG Temp. (KPa) Change in Continuous PG Grade Normalized G*/sinδ N6 Control PG64 66.5 3.08 – 1.00 1.0% 67.7 3.78 1.2 1.23 3.0% 67.0 3.31 0.5 1.07 5.0% 67.0 3.12 0.5 1.01 N7 Control PG70 74.2 3.36 – 1.00 1.0% 74.7 3.54 0.5 1.05 3.0% 74.3 3.45 0.1 1.03 5.0% 74.5 3.52 0.3 1.05 O6 Control PG64 69.7 4.54 – 1.00 1.0% 70.1 4.81 0.4 1.06 2.0% 70.2 4.89 0.5 1.08 3.0% 69.3 4.31 −0.4 0.95 O7 Control PG70 72.0 2.75 – 1.00 1.0% 74.7 3.66 2.7 1.33 2.0% 74.8 3.71 2.8 1.35 3.0% 74.7 3.64 2.7 1.32 OM6 Control PG64 68.1 3.73 – 1.00 2.0% 69.2 4.33 0.6 1.16 H6 Control PG64 69.4 4.03 – 1.00 2.0% 68.6 4.44 0.4 1.10 M6 Control PG64 69.5 4.47 – 1.00 2.0% 69.6 4.01 −0.5 0.90 Y6 Control PG64 67.6 3.51 – 1.00 2.0% 68.3 3.84 0.3 1.09 Table 3-3. DSR test results of RTFO-aged foamed binder residues.

35 m-value at low temperatures were similar to the S and m-value of base binders. Most of the G*sind values for the foamed residues were slightly higher than those of their base binders. The slight increase in the G*sind values for the foamed residues, however, may be compensated for when the foamed binders are sub- jected to lower short-term aging temperatures in production. The DSR and BBR results demonstrate that foaming process may not have a significant influence on the short-term and long-term rheological properties of binders. This finding is substantiated by the observation that foaming does not result in a change in asphalt binder chemistry (Namutebi et al. 2011). The coefficient of variation for the data was less than 5.0%. A.3. Comparison of Laboratory Foamers The objective of this section is to compare differences in laboratory foaming units and processes. The characteristics of foamed binders produced using three commercially avail- able foamers (Wirtgen WLB 10S, InstroTek Accufoamer, and PTI foamer) were compared. The three units foam binders differently, likely resulting in different foam structures and properties. The following is a brief description of the working principles of the three foaming units. A.3.1. Wirtgen WLB 10S The Wirtgen foamer is designed to regulate the amount of dispensed binder and water by mass flow meters. The binder is heated to 320°F (160°C) and circulated inside the unit. Then, the foamed binder is produced by combining spe- cific quantities of water, compressed air, and heated binder inside an expansion chamber. During this process, the added water vaporizes and causes the binder to foam in the expan- sion chamber. The pressure at which the water and the air are injected in the expansion chamber is about 72 psi (500 kPa). After the binder is foamed, it is usually dispensed directly from the nozzle into the mixer, where it is combined with the heated aggregates. The unit can dispense about 0.4 lb (200 grams) of binder in 2 s due to the high pressure at which the water and air are injected. Figure 3-30 illustrates the equipment used in this study and a schematic of the foaming process. A.3.2. InstroTek Accufoamer The Accufoamer is designed to deliver binder and the foaming agent (water) by regulating the overhead pressure that drives the flow of these liquids. The foaming unit is calibrated to determine the time taken to deliver a certain mass of binder at a fixed driving pressure and the time taken Binder Type DSR G*sinδ @ Intermediate Temp. (KPa) Normalized G*sinδ (%) BBR S @ - 12°C (Mpa) Normalized S (%) BBR m- value @ - 12°C Normalized m-value (%) N6 Control 5,657 1.00 364 1.00 0.268 1.00 1.0% 5,849 1.03 378 1.04 0.269 1.00 3.0% 6,143 1.09 350 0.96 0.269 1.00 5.0% 5,341 0.94 386 1.06 0.266 0.99 N7 Control 3,508 1.00 315 1.00 0.267 1.00 1.0% 3,521 1.00 312 0.99 0.270 1.01 3.0% 3,411 0.97 339 1.08 0.255 0.96 5.0% 3,172 0.90 317 1.01 0.269 1.01 O6 Control 3,608 1.00 274 1.00 0.287 1.00 1.0% 4,384 1.22 275 1.00 0.280 0.98 2.0% 4,320 1.20 263 0.96 0.282 0.98 3.0% 3,925 1.09 230 0.84 0.298 1.04 O7 Control 2,089 1.00 233 1.00 0.290 1.00 1.0% 2,256 1.08 234 1.00 0.264 0.91 2.0% 2,402 1.15 214 0.92 0.275 0.95 3.0% 2,171 1.04 231 0.99 0.278 0.96 OM6 Control 5,133 1.00 227 1.00 0.30 1.00 2.0% 6,442 1.26 255 1.12 0.301 1.04 H6 Control 4,345 1.00 215 1.00 0.284 1.00 2.0% 5,566 1.31 214 1.00 0.270 0.95 M6 Control 4,189 1.00 193 1.00 0.301 1.00 2.0% 5,017 1.20 198 1.03 0.282 0.94 Y6 Control 5,479 1.00 194 1.00 0.305 1.00 2.0% 5,747 1.05 175 0.90 0.302 0.99 Table 3-4. DSR and BBR test results of PAV-aged foamed binder residues.

36 to deliver a certain mass of water at different pressures. In this study, the selected binder temperature and pressure were 320°F (160°C) and 30 psi (210 kPa). The unit comes with an Excel template programmed with the relationship between water content and pressure for a given binder flow rate. Once the calibration parameters are set in the template, it can be used to determine the flow time and water pressure that are required to produce the desired mass of foamed binder at any desired water content. Figure 3-31 presents a view of the unit used in this study and a schematic of the foaming process. A.3.3. Pavement Technology Inc. (PTI) Foamer The PTI foaming unit regulates the mass of binder to dis- pense by load-cell scale. An air line with a minimum of 110 psi (760 kPa) is connected to the main regulator to actuate the unit. Air is used to charge a water reservoir and keep water at a regulated pressure. The pressure in the water tank is about 35 psi (240 kPa). A small amount of air is used to atomize the water to very fine droplets in order to achieve the largest binder expansion possible. The binder is discharged from the reser- voir by actuating a pneumatic cylinder when the foam button is pressed. After the preset amount of binder is dispensed, the pneumatic cylinder closes, and the flow of binder is pinched. The binder discharge is by gravity and not through pres- sure. The unit is able to accommodate up to 14 lb (6.4 kg) of binder. In addition, the unit can raise or lower the chamber to meet the heights for various laboratory mixers. Figure 3-32 illustrates the PTI foamer unit used in this study and an over- view of the foaming steps. There are notable differences between the three foam- ing units. First, the nozzle types that spray the binder and the water are different. Second, the pressures at which the water, air, and binder are mixed in the expansion chamber are dif- ferent. The Wirtgen foamer produces the foam by directing the two nozzles at each other. The resulting foam is dispensed into a container directly as it is being formed. The Accufoamer also produces the foam by directing the two nozzles at each other, but the foam is produced inside a small mixing cham- ber before being dispensed through a tube 0.25 in. (6.4 mm) in diameter to a container or mixer. The PTI unit dispenses the foamed binder by gravity. As a result, the Accufoamer dispenses about 0.4 lb (200 grams) of foamed binder in 10 to 12 s, the PTI dispenses 0.4 lb (200 grams) in 10 to 14 s, and the Wirtgen foamer dispenses about 0.4 lb (200 grams) of binder in about 2 s (see Table 3-5). In addition, the maximum expan- sion achieved by the Accufoamer and the PTI units is lower as compared to the Wirtgen foamer. Since the expansion measurements could be taken directly during discharge from the Accufoamer, no foaming informa- tion was lost. The can needed to be moved from the foaming unit to a location where measurements could be taken for both the PTI and Wirtgen foamers. The time lost in moving the can from under the foamer’s nozzle to the measurement area was approximately 3 s from discharge. For the Wirtgen foamer, a procedure was developed to extrapolate the expansion curve back to time zero. For the PTI foamer, no extrapolation was necessary as the foam volume change was extremely slow. Table 3-5 summarizes the operation parameters and foam- ing process features of the three foaming units. Notwith- standing these differences, the goal of this exercise was to determine whether the characteristics of different foamed binders at various water contents were similar for the three foaming units. The influence of the foaming equipment was evaluated using two different binders at two water contents. The met- rics listed in Section 2.A.4 were used to compare the foaming AirWater Asphalt Expansion Chamber Foamed Asphalt (a) (b) Figure 3-30. Wirtgen WLB 10S; (a) foaming unit, (b) schematic of the foaming process.

(a) (b) Pressurized Air for Bitumen Pressurized Air for Foaming Agent Exit Port for Foaming Agent Exit Port for Bitumen Bitumen Line Valve Foaming Agent Line Valve Mixing Junction Figure 3-31. InstroTek Accufoamer; (a) foaming unit, (b) schematic of the foaming process. Characteristic Wirtgen WLB 10S InstroTek Accufoamer PTI foamer Air flow pressure Min. 15 psi (100 kPa) Max. 145 psi (1,000 kPa) Min. 75 psi (517 kPa), Max. 150 psi (1,034 kPa) Min. 80 psi (552 kPa) Max. 110 psi (758 kPa) Water flow pressure Max. 145 psi (1,000 kPa) Max. 30 psi (207 kPa) 33 psi (230 kPa) Binder flow pressure Max. 145 psi (1,000 kPa) Max. 60 psi (413 kPa) The binder is dispensed by gravity. Reaction chamber Water and compressed air are injected into the hot binder. Pressurized binder and water meet at a single junction. A small amount of air is used to atomize the water to a fine droplet. Binder temperature 284°F–392°F (140°C–200°C) 320°F–390°F (160°C–200°C) Max 350°F (177°C) Discharge time 100 g/s 16–20 g/s 14–20 g/s Mass control Mass flow control Overhead pressure control Scale control Power requirement Adaptable to variousinternational supplies 208–240 VAC, 220- volt, 30-amp circuit 120 VAC, 20 amp Binder chamber size 5.3 gallon (20 L) 0.3–15.0 lb (150 to 6,800 g) 14 lb (6,350 g) Foaming agent dosage (water content) 0%–5% 0%–9% 1%–7% Foaming agent temperature No heat Max. 180°F (82°C) No heat VAC = volts alternating current. Table 3-5. Summary of characteristics of the foaming units.

38 Figure 3-32. PTI foamer; (a) laboratory unit, (b) foaming process (PTI Foamer Users’ Guide, updated 11-20-12). (b) (a)

39 characteristics of N6 and O7 with 1.0% and 3.0% water con- tents from the three foaming units. A.3.4. Binder Foaming Measurements To compare the foaming characteristics of the three units, the height of the foamed binder was measured using a laser sen- sor or laser distance meter (LDM) on two replicates. ERmax and k-value are summarized in Figure 3-33 and Figure 3-34. The FI (i.e., area under the ER curve) is presented in Figure 3-35 and Figure 3-36 for binder N6 and O7, respectively. The following observations can be made based on data presented in Figure 3-34 through Figure 3-36: • ERmax increased with increasing water content. • In the majority of the cases (with the exception of N6 with 1.0% water content), the Wirtgen foamer had the largest ERmax values, while the PTI foamer had the lowest ERmax values in all cases. • The PTI foamer showed very small expansion compared to the other two units (in the range of 1 to 1.5), which corre- sponds to observations made by Ozturk and Kutay (2014b). • The k-value showed a similar trend to the one observed for ERmax: higher k-values correspond to higher ERmax values (with the exception of O7 in the Accufoamer with 3.0% water content). • For both water contents, N6 had lower or equivalent ERmax values as compared to O7 in the Wirtgen and PTI foamers, but N6 had higher ERmax values as compared to O7 in the Accufoamer. • The FI at 60 s for both binders at 1.0% water content was very similar, while at 3.0% water content N6 in the Accu- foamer seemed to have a larger FI than O7, but a lower FI in the Wirtgen foamer. The procedure for determining the bubble size distribu- tion of the foamed binder described in Appendix B was used to generate the following gamma function curves. The curves show the bubble size distribution at selected times for bind- ers N6 and O7 foamed with 1.0% and 3.0% water content in the three foaming units. In Figure 3-37 and Figure 3-38, the solid and dashed lines represent 1.0% and 3.0% water content, respectively. The selected target times for the com- parison were 30, 60, and 90 s (30 s was enough to avoid the turbulent phase shown in Figure 2-4 and any steam gener- ated during foaming that could prevent photographing the surface of the binder). However, for the binders foamed in Figure 3-33. ERmax and k-value for binder N6 foamed in three different units with 1.0% and 3.0% water content. Figure 3-34. ERmax and k-value for binder O7 foamed in three different units at 1.0% and 3% water content. Figure 3-35. Foamability index for binder N6 foamed in three different units at 1.0% and 3.0% water content. Figure 3-36. Foamability index for binder O7 foamed in three different units at 1.0% and 3.0% water content.

40 the Wirtgen foamer, only 30 and 60 s are reported because after 60 s the binders had very small and uniform bubble sizes. In addition, O7 with 3.0% water content in the Wirtgen foamer had a very turbulent expansion and collapse that did not form any distinguishable bubbles, and thus the results are not included in Figure 3-38. Binder N6 with 3.0% water content in the Accufoamer was not measured. The following observations can be made based on the results: 1. In the majority of the cases, the foamed bubbles decreased in size with elapsed time and became more homogeneous (i.e., higher peak, more narrow spread, and shift to the left of the gamma function curves). (a) (b) (c) Figure 3-37. Bubble size distribution for N6 with 1.0% and 3.0% water content. (a) (b) (c) Figure 3-38. Bubble size distribution for O7 with 1.0% and 3.0% water content.

41 2. The difference in bubble size seemed to be more pronounced at 3.0% water content versus 1.0% water content. 3. The PTI foamer seemed to produce more stable bubbles regardless of the elapsed foaming time. 4. The minimum bubble size about 90 s after foaming seemed to be around 1.1 in. (2.8 mm), which is probably a result of the procedure used to determine the bubble size distribution as smaller bubbles were more difficult to identify. B. Laboratory Mixture Study The objectives of this portion of the study were to (1) eval- uate the effect of different binder types and foaming water contents on the workability and coatability of foamed asphalt mixtures and (2) compare the workability and coatability of foamed WMA versus HMA. The experimental design for the initial workability and coatability evaluation is shown in Table 3-6. The aggregates used came from a field project located on I-25 in New Mex- ico. Three fractions of siliceous aggregates were combined following the mix design gradation shown in Figure 3-39. The optimum binder content by the weight of mix was 5.4%. For the coatability evaluation, the volumetrics for the coarse aggregate fraction were calculated based on the combined aggregate gradation and optimum binder contents from the mix design, as summarized in Table 3-7. The Wirtgen foamer was used to produce foamed WMA and control HMA. The temperature of the binder chamber in the foaming unit was at 320°F (160°C), and the air and water pressure were set to 72 psi (500 kPa) per the manu- facturer’s recommendation. Foamed WMA using binders N6, O6, and Y6 was produced with three water contents, 1.0%, 2.0%, and 3.0%. To explore the foaming character- istics of polymer-modified binders versus neat binders, foamed WMA with binders N7 and O7 was also produced with 1.0% water content. The workability and coatability comparisons of those mixtures were used to validate the method listed in Section 2.B. The control HMA using the neat binders was also included in the study and was produced by the same laboratory foaming unit with no foaming water. In addition, foamed HMA using neat binders with 1.0%, 2.0%, and 3.0% water contents was also produced and evaluated in the study. The production Binder Source Mixture/Foaming Water Contents HMA/0% Foamed WMA/1% Foamed WMA/2% Foamed WMA/3% Foamed HMA/1% Foamed HMA/2% Foamed HMA/3% N64-22 X X X X X X X O64-22 X X X X X X X Y64-22 X X X X N70-22 X O70-22 X Table 3-6. Experimental design for workability and coatability evaluation. Figure 3-39. Design aggregate gradation for the New Mexico field project.

42 temperatures for foamed WMA and HMA as well as the con- trol HMA are summarized in Table 3-8. The comparison of workability and coatability by foamed WMA versus HMA was used to verify whether the foaming process produced WMA mixtures with better workability and coatability char- acteristics as compared to the control HMA. In addition, the comparison of foamed WMA versus foamed HMA helped evaluate the effect of production temperature on the foaming process. B.1. Effect of Foaming on Mixture Workability and Coatability The workability and coatability results for the control HMA and the foamed WMA mixtures produced using three differ- ent neat binders with 1.0%, 2.0%, and 3.0% water contents are shown in Figure 3-40 and Figure 3-41. In these figures, the control HMA results are compared against the foamed WMA values within each binder source. Each bar in Figure 3-40(a) represents the average value of two replicates, and the error bars represent ±1 standard deviation from the average value. Based on the span of the error bars, good repeatability for the workability test method was achieved. The maximum shear stress and CI were normalized with respect to the control HMA value. Thus, in Figure 3-40(b), values equal to or less than 1.0 indicate equivalent or better workability than HMA, while in Figure 3-41(b), values equal to or greater than 1.0 indicate equivalent or better coatability than HMA. A significant difference in workability (Figure 3-40) and coatability (Figure 3-41) was observed for the various foamed * * * Volumetrics Values Reference Pb 5.4 (%) Mix Design SST 5.093 (m2/kg) Mix Design and Kandhal et al. (1988) SAcoarse 0.410 (m2/kg) Mix Design and Kandhal et al. (1988) Wb 57 (g) SAcoarseWb = 4,000 SST 1Pb 1 — Pb Ps–coarse Table 3-7. Volumetric calculation for the New Mexico coarse aggregate fraction. Foaming Water Contents MixingTemperature Short-Term Aging Protocol Compaction Temperature 0.0% (Control HMA) 290°F (143°C) 2 hours at 275°F (135°C) 275°F (135°C) 1.0% (Foamed WMA) 275°F (135°C) 2 hours at 240°F (116°C) 240°F (116°C) 2.0% (Foamed WMA) 275°F (135°C) 2 hours at 240°F (116°C) 240°F (116°C) 3.0% (Foamed WMA) 275°F (135°C) 2 hours at 240°F (116°C) 240°F (116°C) 1.0% (Foamed HMA) 290°F (143°C) 2 hours at 275°F (135°C) 275°F (135°C) 2.0% (Foamed HMA) 290°F (143°C) 2 hours at 275°F (135°C) 275°F (135°C) 3.0% (Foamed HMA) 290°F (143°C) 2 hours at 275°F (135°C) 275°F (135°C) Table 3-8. Summary of production temperatures for foamed WMA and HMA and control HMA. (a) (b) Figure 3-40. Workability test results for foamed WMA versus HMA; (a) maximum shear stress, (b) normalized maximum shear stress.

43 WMA mixtures and as compared to the control HMA for binder N6 and O6. However, there was relatively little differ- ence for binder Y6. Therefore, the workability and coatability test methods proposed in this study were validated as having the sensitivity to capture the characteristics of different mix- tures, depending on the binder source. As shown in Figure 3-40 and Figure 3-41, foamed WMA mixtures employing binders N6 and O6 with 1.0% water content had better workability and coatability character- istics (indicated by a lower maximum shear stress and a higher CI) as compared to both the WMA mixtures foamed at higher water contents and the control HMA. This was true despite the fact that the production temperature for the WMA mixtures was approximately 30°F (17°C) lower than the production temperature for HMA. Therefore, a water content of 1.0% was optimum for these WMA foamed mix- tures. Contrary to expectations, however, WMA mixtures foamed with higher water contents (i.e., 2.0% and 3.0%) yielded mixtures with equivalent or worse workability and coatability characteristics as compared to the control HMA. This observation of decreased coating with increas- ing foaming water content was confirmed by Ozturk and Kutay (2014a). A different workability and coatability trend was shown for mixtures employing binder Y6. These WMA mixtures had equivalent workability and coatability as compared to the control HMA despite the amount of water used dur- ing foaming. This binder did not appreciably foam at any water content (i.e., 1.0% to 5.0%). The reduced foaming abilities of binder Y6 could be due to the presence of an anti-foaming agent that is sometimes introduced during the crude refining or binder production process (Abel 1978; Fu 2011; Kekevi et al. 2012). The following observations can be made from the normal- ized CI and normalized maximum shear stress values of the WMA for the mixes produced using binders N6, O6, and Y6 to that of their corresponding HMA values that were presented in Figure 3-40(b) and Figure 3-41(b): • An increase in ERmax did not necessarily translate into improvement in mixture workability or aggregate coatability of WMA mixtures. • Lower water content values showed improvement in work- ability of WMA mixtures relative to HMA mixtures. How- ever, an increase in water content did not appear to have an adverse effect on coatability relative to HMA. • Workability and coatability typically improved compared to a similar HMA when the water content was between 1.0% and 2.0%. The significance of change in foamed binder characteristics on mixture workability and coatability was further investi- gated by comparing ER (at 10 s) and k-value to the normal- ized maximum shear stress and CI as shown in Figure 3-42. The results for binder Y6 show less sensitivity to both k-value and ER. Increasing k-value decreased the workability and coatability of mixtures produced using binders N6 and O6. However, the coatability of the WMA mixtures performed as well as the HMA mixtures at higher k-values. Better coatabil- ity and workability was observed when the ER values of the mixtures were close to an ER value of 4, and a k-value of 0.01 seemed optimum for both the workability and coatability of the mixtures. As foamed WMA using binders N6 and O6 with 1.0% foam- ing water content had the best workability and coatability, foamed WMA using the polymer-modified binders N7 and O7 was also produced at this water content. Workability and coatability results for foamed WMA using polymer-modified versus neat binders were compared and are summarized in Figure 3-43 and Figure 3-44, respectively. (a) (b) Figure 3-41. Coatability test results for foamed WMA versus HMA; (a) coatability index, (b) normalized coatability index.

44 WMA production temperature, which was approximately 25°F lower than the recommended production temperature. Thus, it may be advantageous to increase the temperature for foaming polymer-modified binders. Additionally, the incor- poration of polymer modifiers may have an effect on the binder foaming characteristics. The effect of production temperature on the workability and coatability of foamed asphalt mixtures is illustrated in Figure 3-45 and Figure 3-46, respectively. In these figures, the properties of foamed WMA are compared to those of foamed Figure 3-42. Comparison of foamed binder parameters to mixture workability and coatability. (The green arrow indicates desirable range of coatability or shear stress compared to HMA.) Figure 3-43. Workability test results for foamed WMA with neat versus polymer-modified binder. Figure 3-44. Coatability test results for foamed WMA with neat versus polymer-modified binder. As illustrated, better workability and coatability are shown for foamed WMA using neat binders than those using polymer- modified binders when produced at the same temperature. Adhering to the strict definition that WMA is produced at temperatures at or below 275°F (135°C), both mixtures pre- pared with PG64-22 and PG70-22 were mixed at the upper limit of the WMA conventional temperature range and con- ditioned for 2 hours at 240°F (116°C). The reduced prop- erties of foamed WMA using polymer-modified binders are possibly attributed to the higher viscosity of the binder at the

45 HMA for each foaming water content (i.e., 1.0%, 2.0%, and 3.0%). Binders N6 and O6 were used for this evaluation. As illustrated, better or equivalent workability and coatability (indicated by a lower maximum shear stress and a higher or equivalent CI) are shown for foamed HMA as compared to foamed WMA, for all three foaming water contents. There- fore, higher production temperature for the foaming process is able to produce foamed asphalt mixtures with better work- ability and coatability. In summary, test methods for evaluating mixture workabil- ity and coatability were developed using SGC compaction data and a modified aggregate absorption method, respectively. The maximum shear stress and a CI were proposed as mixture work- ability and coatability parameters. For the coatability evaluation, only coarse aggregates retained on the 3⁄8-in. sieve were used. The relative difference in water absorption by the uncoated aggregates versus the loose mix was used to calculate the CI. Foamed WMA using three binders with three foaming water contents was produced using the Wirtgen foamer. In addition, to validate the improved workability and coatability of foamed WMA, a control HMA mixture was also produced using the same laboratory foaming unit with no water content. The workability and coatability of the foamed WMA mixtures were compared against the characteristics of the HMA mixtures. In addition, the effect of binder source, binder grade, and foam- ing water contents on mixture workability and coatability was investigated. The following conclusions were made based on the results: 1. 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. Thus, the proposed test methods seem promising in evaluating the workability and coatability of asphalt mixtures. 2. Foamed WMA mixtures produced with binders N6 and O6 had better workability and coatability when 1.0% foaming water content was used as compared to higher foaming water contents (i.e., 2.0% and 3.0%). Thus, for these two binders, 1.0% was considered the optimum foaming water content. This trend is consistent with results from Abbas and Ali (2011). However, equivalent workability and coat- ability was observed for the WMA mixtures that employed binder Y6 as compared to the control HMA regardless of the foaming water content used. 3. Comparison of foamed WMA versus control HMA mix- tures showed that WMA produced at 1.0% foaming water content had better workability and coatability characteris- tics, despite the fact that the WMA mixtures were produced at temperatures approximately 30°F (17°C) lower than the control HMA. However, the 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. This finding highlights the importance of identi- fying the best materials and foaming conditions to maximize the workability and coatability of foamed mixtures. 4. At WMA production temperatures, foamed WMA employ- ing binder N6 and O6 had better workability and coatability as compared to those mixtures with N7 and O7. The higher viscosity of polymer-modified binders is possibly causing the reduced mixture workability and coatability. 5. Comparison of foamed WMA versus foamed HMA showed that foamed HMA employing binders N6 and O6 had bet- ter or equivalent workability and coatability than the corre- sponding foamed WMA counterparts, for all three foaming water contents. Therefore, higher production temperature involved in the foaming process is able to produce foamed asphalt mixtures with better properties. B.2. Effect of Liquid Additives on Mixture Workability The liquid additives used in the laboratory binder study (see Section A.1.4) were also employed to explore their influ- ence on mixture workability. In this case, a local aggregate source from Huntsville, Texas (see Chapter 4, Section C.2 for details) was employed along with binders OM6 and Y62. These two binders were selected because they showed minimal Figure 3-45. Workability test results for foamed WMA versus foamed HMA. Figure 3-46. Coatability test results for foamed WMA versus foamed HMA.

46 change in ERmax with added water and were thus considered ideal candidates to assess changes in mixture workability after the inclusion of liquid additives. Besides the additives listed in Table 3-2, a fourth additive was considered for this part of the study (henceforth labeled W2). Additive W2 consists of a synthetic zeolite that holds about 20% water within its crystalline structure. When combined with the binder or aggregate at temperatures ranging from 275°F (135°C) to 320°F (160°C), it releases the water, which foams the binder and reduces its viscosity. Additive W2 was incorporated in the mixture in two ways: (1) combining it with the binder at a dosage of 5.0% by weight of binder [labeled FWMA + W2 (binder)] before mixing (see Figure 3-16 for an illustration of the additive blending procedure), and (2) com- bining it with the aggregate at a dosage of 0.25% by weight of mixture [labeled FWMA + W2 (agg)] before mixing. The binder for all mixtures was foamed in the Wirtgen foamer at 1.5% water content. The mixing temperatures for the HMA and foamed WMA (FWMA) were 290°F (143°C) and 275°F (135°C), respectively. The HMA was conditioned for 2 hours at 275°F (135°C) and compacted at the same tempera- ture. The FWMA was conditioned for 2 hours at 240°F (116°C) and compacted at the same temperature. The maximum shear stress results for these mixtures are illustrated in Figure 3-47. The lower tmax for W2 (agg) as compared to W2 (binder) is possibly due to the fact that when additive W2 is combined with the aggregate, it does not start to release water until the mixing process, whereas when it is combined with the binder, the release of water starts well before the start of the mixing process. Thus, in the case of W2 (binder), some of the foamed binder bubbles may collapse between the additive blending and mixing processes and not be available to allow the binder to coat the aggregates to the same extent as the coating achieved by W2 (agg) during mixing. For both binders, additives W2 (agg) and F3 yielded the best workability. Additives W1, W2 (binder), F1, and F2 had a neutral effect on the workability of the mixtures prepared with binder OM6. However, additives W1 and F1 had a negative impact on the mixtures prepared with Y62, yielding less work- able mixtures than the HMA or FWMA without additives. In the case of the binder, when foamed at 1.0% water con- tent (see Section A.1.4), additive W1 had a negligible effect on the foaming metrics, and additives F1 and F3 had compa- rable ERmax and k-values and the best impact on the foaming metrics for binders OM6 and Y62. In the case of the mixtures, W1 also had a negligible or even detrimental effect on work- ability, while additive F3 had a distinct improved workability as compared to mixtures with additive F1. C. Validation of Proposed Mix Design Approach with Various Laboratory Foaming Units The foamed mix design approach described in Section 2.C was validated using the same laboratory foaming units included in the comparison of laboratory foamers: Wirtgen WLB 10S, InstroTek Accufoamer, and PTI foamer. The mate- rials used for the validation and the results in terms of binder foaming characteristics, mixture workability, coatability, and performance are detailed next. C.1. Materials The binder used for the validation was a Valero PG64-22. The optimum binder content was 4.7%. Details on the aggre- gate type, source, and gradation are presented in Table 3-9, Table 3-10, and Figure 3-48. C.2. Foaming Measurements The ERmax and k-value of the binder foaming measure- ments performed using the Accufoamer, the PTI foamer, and the Wirtgen foamer are illustrated in Figure 3-49. The bars and red squares represent the average ERmax and k-value of three replicate measurements. The error bars span ±1 standard devi- ation from the average ERmax value. (a) (b) Figure 3-47. Influence of various liquid additives on mixture workability for (a) binder OM6, (b) binder Y62.

47 As was observed during the comparison of laboratory foamers (Section 3.A.3), binder foam produced in the Wirtgen foamer had the largest ERmax values, while that produced in the PTI foamer had the lowest ERmax values for three foaming water contents. The comparison in k-value showed a similar trend to the one observed for the ERmax value. The increases in ERmax values and k-values of binder foaming produced by the Accufoamer and the Wirtgen foamer are proportional to the water content. However, no significant effect from the water content on the binder foaming characteristics was observed for the binder foam produced in the PTI foamer. The explanation for the differences in foaming characteris- tics between the three laboratory foaming units may lie in the way the foam is dispensed. In the Wirtgen foamer, the foam is expelled as soon as the water and binder are combined, whereas the Accufoamer forces the foam through a tube before it is dispensed, which restricts the flow. The PTI foamer allows the foamed binder to be drawn out of the expansion chamber by gravity, which produces an even slower flow of material as compared to the other two foamers, which use air pressure to expel the foamed binder. It appears that the slower the rate of flow, the lower the expansion of the foam. Figure 3-50 presents the FI values of binder foaming at 1.0%, 2.0%, and 3.0% water contents produced by the Accu- foamer and the Wirtgen foamer. The FI was not calculated for the PTI foamer since the binder did not expand and collapsed after foaming with this unit. As illustrated, similar tends in terms of FI values versus elapsed foaming time were observed for binder foaming produced by the Accufoamer and the Wirtgen foamer. In addition, higher FI values (indicating higher stability) for binder foaming produced by the Wirtgen foamer as compared to that produced by the Accufoamer were Aggregate Type Bin No. 1 Bin No. 2 Bin No.3 Bin No.4 C Rock D/F Rock Screenings Washed Sand Aggregate Pit Marble Falls Marble Falls Marble Falls Hallet Table 3-9. Aggregate source. Aggregate Type Aggregate % Sieve Analysis (Cum.% Passing) 1” ¾” 3/8” #4 #8 #30 #50 #200 C Rock 25 100 99 22 4.3 2.0 1.1 0.8 0.1 D/F Rock 35 100 100 91 22 4.2 2.4 2.2 1.5 Screenings 25 100 100 100 99 80 39 25 4.2 Washed Sand 15 100 100 100 100 100 90 55 3.0 Table 3-10. Individual aggregate gradation. 10099.75 77.35 48.525 36.97 24.365 15.47 2.05 -5 5 15 25 35 45 55 65 75 85 95 105 Pe rc en t P as si ng Sieve Size Combined Gradation Lower Specifcation Limits Upper Specification Limits Figure 3-48. Combined aggregate gradation.

48 (a) (b) (c) Figure 3-49. ERmax and k-value for various laboratory foaming units; (a) 1.0% water content, (b) 2.0% water content, and (c) 3.0% water content. (a) (b) (c) Figure 3-50. ERmax and FI for various laboratory foaming units; (a) 1.0% water content, (b) 2.0% water content, and (c) 3.0% water content.

49 observed for 1.0% and 2.0% water contents, while the oppo- site trend was shown for 3.0% water content. SAI values exhibited a gradual reduction with the elapsed foaming time, which was mainly attributed to the collapse of the binder foam. Figure 3-51 presents the SAI values at 60 s of binder foaming at 1.0% and 2.0% water contents produced by the Accufoamer and the Wirtgen foamer. The SAI values for binder foaming produced by the PTI foamer were not deter- mined since no apparent foam bubbles were observed during the foaming process. Higher SAI at 60 s values were observed for binder foaming produced by the Wirtgen foamer as com- pared to those produced by the Accufoamer. Therefore, as compared to the Accufoamer, the Wirtgen foamer was able to produce more semi-stable foam bubbles, which were smaller in size but had larger surface area. C.3. Mixture Workability and Coatability Measurements The workability and coatability results for foamed laboratory- mixed, laboratory-compacted (LMLC) specimens fabricated using the Accufoamer, the PTI foamer, and the Wirtgen foamer are shown in Figure 3-52. The bars represent the average maximum shear stress of three replicate measurements, and the error bars span ±1 standard deviation from the average value. The dots represent the CI values of foamed mixtures at different water contents and the control HMA. Figure 3-52(a) presents the workability and coatability results for the control HMA and the foamed mixture pro- duced by the Accufoamer. The workability evaluation for these two mixtures was performed using an IPC SGC. As illustrated, (a) (b) Figure 3-51. FI and SAI for various foaming units; (a) 1.0% water content and (b) 2.0% water content. (a) (b) (c) Figure 3-52. Workability and coatability results for the control HMA and foamed mixture; (a) produced in the Accufoamer, (b) produced in the PTI foamer, and (c) produced in the Wirtgen foamer.

50 better mixture workability and coatability characteristics were shown for foamed mixtures at three different foaming water contents as compared to the control HMA, as indicated by lower maximum shear stress values and higher CI values. In addition, the selected optimum foaming water content for the foamed mixture produced in the Accufoamer was 2.0%, which was able to produce a foamed mixture with the best workability and coatability characteristics as compared to the HMA control. Figure 3-52(b) presents the workability and coatability results for the control HMA and the foamed mixture produced by the PTI foamer. The workability evaluation for these two mixtures was performed using a Pine SGC. Mixture workability results shown in Figure 3-52(b) illustrate that foamed mixtures at 1.0%, 3.0%, and 5.0% had better or equivalent workability than the control HMA, as indicated by lower or equivalent maxi- mum shear stress values. This trend was observed despite the PTI foamer resulting in practically null expansion, as shown in Figure 3-49. Therefore, binder foaming characteristics and foamed mixture properties may not have a direct relation- ship for all foaming units. A different trend was observed for coatability results, where higher CI values versus that of the control HMA were shown for foamed mixtures at 3.0% and 5.0% water contents, while the opposite trend was shown for foamed mixtures at 1.0% water content. According to the mix design approach described in Section 2.C, 5.0% was selected as optimum water content for the foamed mixture produced in the PTI foamer. Figure 3-52(c) presents the workability and coatability results for the control HMA and the foamed mixture produced by the Wirtgen foamer. The workability evaluation for these two mixtures was performed using an IPC SGC at the Texas A&M Transportation Institute (TTI). As illustrated in Fig- ure 3-52(c), equivalent or higher maximum shear stress values were achieved by foamed mixtures than by the control HMA, indicating equivalent or worse mixture workability characteris- tics. The coatability results in terms of CI values indicated that, as compared to the control HMA, equivalent or better mixture coatability was obtained by foamed mixtures at 2.0% and 3.0% water contents, while the opposite trend was shown for foamed mixture at 1.0% water content. Therefore, 2.0% was selected as the optimum foaming water content for the foamed mixture produced in the Wirtgen foamer. C.4. Performance Evaluation To evaluate the performance of the foamed mixtures at their optimum foaming water contents, a new set of LMLC speci- mens were fabricated at 2.0%, 5.0%, and 2.0% water contents using the Accufoamer, the PTI foamer, and the Wirtgen foamer, respectively. In addition, a companion set of HMA LMLC spec- imens was produced. Foamed mixtures were mixed at 275°F (135°C) and then short-term aged for 2 hours at 240°F (116°C) prior to compaction. The control HMA was mixed at 290°F (143°C) and then short-term aged for 2 hours at 275°F (135°C). The compacted specimens were then tested to measure MR, IDT strength, and HWTT. Testing parameters, including MR stiff- ness at 77°F (25°C), wet IDT strength and TSR at 77°F (25°C), HWTT load cycles to stripping number (LCSN), load cycles to remaining life (LCST), and viscoplastic strain at LCSN (DevpSN) at 122°F (50°C), were used to evaluate mixture stiffness, rutting resistance, and moisture susceptibility and to compare the per- formance of foamed mixtures to the control HMA. LCSN represents the maximum number of load cycles that the asphalt mixture can resist in the HWTT before the adhe- sive fracture between the asphalt binder and the aggregate occurs; it is assessed by measuring the change in curvature from positive to negative of the rut depth versus the load- cycle curve. Mixtures that do not show a stripping phase in the HWTT are considered to have a robust resistance to mois- ture damage, with LCSN values larger than the number of load cycles applied during the test (e.g., 20,000). LCST represents the number of additional load cycles after LCSN needed for the rut depth accumulated by the stripping strain to reach 0.5 in. (12.5 mm), which is the common HWTT failure crite- rion adopted by several agencies. DevpSN can be calculated as the ratio of the rut depth to the specimen thickness at any given number of load cycles up to LCSN. A detailed description of the derivation and equations of the HWTT parameters can be found in Yin et al. (2014). C.4.1. Resilient Modulus Test The results for MR stiffness at 77°F (25°C) for foamed mix- tures at optimum foaming water contents produced by the Accufoamer, the PTI foamer, the Wirtgen foamer, and the control HMA are shown in Figure 3-53. Each bar represents Figure 3-53. MR at 77F (25C) test results for foamed mixtures produced in different laboratory foamers versus the control HMA.

51 the average MR stiffness value of three replicates, and the error bars represent ±1 standard deviation from the average value. As illustrated in Figure 3-53, equivalent MR stiffness was achieved by the control HMA, the foamed mixture produced by the Accufoamer (at 2.0% water content), and the foamed mixture produced by the PTI foamer (at 5.0% water content). In addition, a significantly higher MR stiffness was observed by the foamed mixture produced by the Wirtgen foamer. There- fore, inclusion of additional water and lower production and short-term aging temperature involved in the fabrication of foamed mixtures did not reduce the mixture stiffness. C.4.2. Indirect Tensile Strength Test Three specimens were tested in a dry condition and three specimens after moisture conditioning following the proce- dure outlined by AASHTO T 283. The wet IDT strength and TSR value at 77°F (25°C) were considered as indicators of mixture moisture susceptibility. The IDT strength test results for foamed mixtures produced by three laboratory foamers and the control HMA are shown in Figure 3-54. The solid bar and pattern-filled bar represent the average dry and wet IDT strength, respectively. The error bars represent ±1 standard deviation from the average value. In addition, the TSR values are shown above the IDT strength bars. Figure 3-54 illustrates that equivalent dry IDT strength at 77°F (25°C) was achieved by the three foamed mixtures and the control HMA. However, a significant reduction in IDT strength was observed for all mixtures after moisture condi- tioning per AASHTO T 283, which was likely due to the poor moisture resistance of the binder used in the mix. The com- parison of wet IDT strength among foamed mixtures and the control HMA indicated that equivalent wet IDT strengths were achieved by the foamed mixtures produced in different labora- tory foamers at their corresponding optimum water contents, which were significantly lower than the wet IDT strength for the HMA control. As a consequence, a higher TSR value was observed for the control HMA than the foamed mixtures, indi- cating better moisture resistance in the IDT strength test. C.4.3. Hamburg Wheel Tracking Test The HWTT was performed at 122°F (50°C) following AASHTO T 324, and test parameters including LCSN, LCST, and DevpSN were used to evaluate mixture moisture susceptibil- ity and rutting resistance. HWTT results in terms of rut depth versus load cycle for foamed mixtures produced in the Accu- foamer, the PTI foamer, the Wirtgen foamer, and the control HMA are shown in Figure 3-55. As illustrated in Figure 3-55, both HMA and foamed mix- tures did not pass the failure criteria of 20,000 load cycles with less than the 0.5-in. (12.5-mm) rut depth of Texas Department of Transportation (TxDOT) specifications. An equivalent creep slope was observed for all mixtures, while distinct differences in stripping slopes were shown, indicating different moisture susceptibility. Figure 3-56 presents the LCSN and LCST results of foamed mixtures produced in different laboratory foamers versus the control HMA. Test results shown in Figure 3-56 illustrate that HMA had better moisture susceptibility than the foamed mix- tures, as indicated by higher LCSN and LCST values. The reduced moisture resistance for the foamed mixture as compared to the control HMA could be attributed to the lower production and aging temperature involved in the mixture fabrication process. The comparison of foamed mixtures illustrated that the foamed mixture produced in the Accufoamer at 2.0% water content had the best moisture susceptibility, followed by that produced in the Wirtgen foamer at 2.0% water content, and finally the mixture produced in the PTI foamer at 5.0% water content. Since the optimum water content for the mixture Figure 3-54. IDT strength test results for foamed mixtures produced in different laboratory foamers versus the control HMA. Figure 3-55. HWTT rut depth versus load cycle for foamed mixtures produced in different laboratory foamers versus the control HMA.

52 Figure 3-56. HWTT LCSN and LCST results for foamed mixtures produced in different laboratory foamers versus the control HMA. produced in the PTI foamer was established by comparing the CI of the foamed mixture against the CI of the HMA, a better approach would be to have a set CI threshold instead of comparing against the HMA value. Based on the results listed in Chapter 3, Section C.2, a CI threshold of 70% seems to be reasonable for all mixtures. If this new criterion was applied to the PTI foamer results, an optimum water content of 1.0% would have been selected instead of 5.0%. The LCSN values for both HMA and foamed mixtures were less than 2,000 load cycles; therefore, all mixtures exhibited early stripping during the HWTT. As a consequence, the determination of the viscoplastic deformation and the rut- ting resistance parameter DevpSN was not possible due to the limited duration of the creep phase.