Abstract
This study aimed at evaluating the laboratory performance and physical characteristics of warm-mix asphalt (WMA) open-graded friction course (OGFC) mixes prepared with three warm-mix additives (two chemical additives and an organic additive). To achieve this objective, four mixes were prepared including a control mix (CM) prepared with a PG 76-22 binder and two sources of aggregate (i.e., # 78 limestone and # 67 sandstone). Air voids and coefficient of permeability (k) were used to evaluate the functionality of the OGFC mixes. In relation to constructability, the compaction energy index was used to compare the required compaction effort during construction. Furthermore, the Cantabro test, Hamburg wheel-tracking test, Texas overlay test, Modified Lottman, and boil tests were conducted to evaluate durability, permanent deformation, cracking, and moisture-damage resistances of the evaluated mixes. Results indicated that the use of WMA technologies enhanced OGFC durability and performance. Among the evaluated mixes, the organic additive-OGFC met all Louisiana Department of Transportation and Development and NCHRP 01-55 requirements, as it notably enhanced OGFC durability, cracking, and moisture-damage resistance. Furthermore, the use of WMA notably reduced the required compaction effort and mixing temperature needed during production and construction as compared with the CM.
Keywords
Open-graded friction course (OGFC) has been used as a surface course mix in Europe and the U.S. for decades. OGFC is also known as porous European mix, porous asphalt, porous friction course, open-graded asphalt, and porous asphalt concrete ( 1 , 2 ). OGFC is usually employed as a surface course layer to achieve several safety, economic, and environmental benefits ( 3 ). OGFC is a gap-graded asphaltic mixture, in which the percentage of fine aggregate is decreased and the percentage of coarse aggregate is increased as compared with conventional hot-mix asphalt (HMA) ( 2 ). Consequently, OGFC typically contains a high percentage of air voids (AV), between 15% and 22% ( 4 ), compared with 2.5% to 4.5% for conventional HMA. Because of its large AV content, water will not only drain over the OGFC surface but also through its pores; therefore, the possibility of hydroplaning decreases and the skid resistance improves during wet weather conditions ( 5 ). Therefore, many researchers and practitioners have championed OGFC to address splash, spray, visibility, and noise issues.
With all these benefits, the use of OGFC as a wearing surface course has faced a few challenges mostly because of its inferior durability as compared with dense-graded asphalt mixtures. In 1998, a survey conducted by the National Center for Asphalt Technology (NCAT) showed that 22 states had stopped using OGFC ( 6 ). In 2004, a survey conducted by NCAT as a part of NCHRP project 01-55 indicated that only 20 out of the 41 responding agencies were using OGFC ( 1 ), which has been attributed to durability issues associated with OGFC. According to the NCAT 2014 survey, durability issues of OGFC are numerous including premature raveling, cracking, and stripping. Among these distresses, raveling is the most serious challenge with OGFC, as once this distress is manifested, OGFC deteriorates rapidly and its removal is inevitable.
During the last two decades, many studies were conducted to enhance OGFC durability and performance. Among these studies, some researchers focused on the impacts of aggregate gradation on OGFC performance ( 7 , 8 ), whereas others investigated the contribution of stone-on-stone contact ( 1 , 9 ) and binder additives ( 10 , 11 ). In relation to aggregate gradation, it has been found that fine OGFC mixes performed better compared with coarse OGFC mixes; however, it also results in a significant reduction in AV content ( 8 , 7 ). In relation to binder properties, polymers, crumb rubber, and warm-mix asphalt (WMA) additives have also been evaluated to improve the mix durability in relation to raveling and cracking resisitance ( 12 ).
Recently, the use of WMA additives has received significant interest among researchers to enhance OGFC durability while taking advantage of the economic and environmental benefits of conventional WMA. WMA encompasses technologies that facilitate mixing and compaction of asphalt mixtures at lower temperatures compared with HMA by lowering binder viscosity through the addition of organic and chemical additives, or through a foaming process ( 13 , 14 ). In addition, WMA provides additional benefits in relation to workability, cost, and environmental sustainability ( 14 ). Nevertheless, WMA-OGFC is an emerging technology that has not been thoroughly evaluated in the literature.
Objectives and Scope
The ultimate goal of this study was to investigate the impact of three WMA additives (i.e., two chemical additives and one organic additive) on the laboratory performance and physical characteristics of OGFC. The draindown test was used to evaluate the impacts of WMA additives during the production stage. In the construction stage, the compaction energy index (CEI) was used to compare the required compaction effort during construction. The functionality of OGFC was compared in relation to AV content and permeability. In relation to performance, the Cantabro test, Hamburg wheel-tracking test (HWTT), indirect tensile stress (ITS) test, Texas overlay test (TOT), tensile strength ratio (TSR), and boil test were used to evaluate the mix resistance to raveling, permanent deformation, fatigue cracking, reflective cracking, and moisture damage, respectively.
Background
Applications of WMA in OGFC
WMA-OGFC With Virgin Aggregate
Wurst and Putman examined the laboratory performance of OGFC fabricated with WMA technology ( 15 ). Moisture-damage resistance, raveling resistance, and permeability properties of two WMA-OGFC mixes were compared with a conventional OGFC. Four mixes were prepared in this study: two regular OGFC mixes, a chemical additive WMA-OGFC, and a foamed WMA-OGFC mix. Whereas conventional OGFC mix was prepared with and without fibers, WMA-OGFC mixes were fabricated without fibers. Results indicated that all mixes showed acceptable moisture resistance regardless of mix type. The authors attributed these results to the type of aggregate that showed historically acceptable resistance to stripping and the hydrated lime used in the mixes. In addition, the results of the permeability test indicated that WMA-OGFC mixes showed comparable permeability values to conventional OGFC. Furthermore, results of the Cantabro test indicated that OGFC fabricated with fibers outperformed OGFC fabricated without fibers for aged and unaged conditions. At the optimum binder content, OGFC mixtures prepared with the chemical additive showed comparable raveling resistance to conventional OGFC in the unaged conditions. In the aged condition, OGFC mixes with both WMA technologies showed greater raveling resistance compared with conventional OGFC.
In another study, the performance of OGFC containing an organic additive at a dosage of 2.5% by weight of binder was evaluated and compared with conventional OGFC ( 16 ). The performance of these mixes was evaluated in relation to permanent deformation resistance, raveling resistance, draindown, porosity, and TSR. For WMA-OGFC, the mix was prepared at various temperatures (i.e., from 135 to 155°C). Results indicated that above 145°C, both WMA-OGFC and conventional OGFC exhibited comparable rut depths. In addition, WMA-OGFC showed a slight decrease in raveling and moisture-damage resistance compared with conventional OGFC. However, these results did not compromise OGFC performance as TSR and weight loss values were still in the acceptable range.
WMA-OGFC With Reclaimed Asphalt Pavement
Goh and You ( 10 ) prepared four mixes to evaluate the mechanical responses of OGFC containing reclaimed asphalt pavement (RAP) and a chemical WMA additive (at a rate of 0.25% by the weight of the total mix). These mixes included two conventional OGFC and two WMA-OGFC mixes, with and without RAP. For conventional OGFC mixes, mixing and compaction temperatures were 160°C and 150°C, respectively, compared with 135°C and 110°C for WMA-OGFC mixes. The studied mixes were evaluated based on the CEI, permeability, ITS, and dynamic complex modulus (E*). Results of the CEI indicated that mixtures containing WMA additive had lower CEI values, indicating lower energy consumption during field compaction. In relation to permeability, conventional OGFC mixes showed a higher flow rate compared with WMA-OGFC. However, all mixes satisfied the minimum requirement of OGFC permeability. With respect to the dynamic modulus test results, WMA mixes exhibited slightly lower E* values compared with the conventional OGFC mixes.
Frigio et al. examined the volumetric and mechanical properties of two WMA-OGFC mixes ( 11 ). In addition, a conventional OGFC mix was prepared as a control mix (CM) with 15% RAP. For WMA-OGFC, two chemical WMA additives were used at a rate of 0.42% and 0.70% by weight of the binder. CEI, ITS, Cantabro loss, semi-circular bending (SCB) test, and repeated ITS test were conducted to compare the performance of the prepared mixes. CEI results indicated that WMA-OGFC mixes had significantly lower CEI values than the conventional OGFC mix. ITS was conducted on dry and wet samples. In the dry condition, all mixtures showed similar ITS values; however, WMA-OGFC showed ITS values that were significantly lower than conventional OGFC when the test was conducted on wet specimens. These results were attributed to the low mix temperature (i.e., 120°C) that did not allow the binder to completely coat the aggregate. Similarly, the Cantabro test was conducted on samples in dry and wet conditions. In the dry condition, all mixes showed the same raveling resistance as indicated by the particle loss percentage. However, in the wet conditions, WMA-OGFC showed less raveling resistance compared with conventional OGFC. Dry and wet samples were evaluated using the SCB test. In the dry condition, all mixes exhibited similar fracture toughness indicating similar resistance to crack propagation. However, in the wet conditions, conventional OGFC outperformed the WMA-OGFC.
Contribution to the Body of Knowledge
Based on the literature review, research studies were conducted to evaluate the concept of WMA-OGFC in recent years. However, many issues related to WMA-OGFC mixes are yet to be addressed in the literature. This research addressed several gaps in the literature as follows:
Although some studies have been conducted to evaluate the impacts of WMA on OGFC laboratory performance, more research is needed to thoroughly investigate the effects of WMA on OGFC durability. Furthermore, most research studies did not address the cracking resistance of WMA-OGFC given the difficulty in testing this property for this class of mixes in the laboratory.
In contrast with previous research, this study evaluated the impact of WMA additives on OGFC characteristics and performance in three different stages (i.e., production, construction, and field performance).
The laboratory evaluation of WMA-OGFC mixes for the aggregate types and binder sources used in the South-Central United States has not been thoroughly addressed in the literature.
Materials and Methods
To achieve the objectives of this study, four mixes were fabricated in the laboratory. The first mix is HMA-OGFC, which was identified as the CM in this research. The remaining three mixtures were prepared by adding three additives (Che1, Che2, and Org) to the base asphalt binder at the manufacturer-recommended dosage to produce Che1-OGFC, Che2-OGFC, and Org-OGFC, respectively. A series of laboratory tests was then conducted on each mix for physical and performance evaluation.
Materials
In the experimental program, an approved job mix formula (JMF) of an OGFC mix was used to fabricate the CM. It is noted that this mix is an HMA-OGFC, which is produced using two aggregate types (i.e., #78 limestone and #67 sandstone aggregate types) and a PG 76-22 SBS-modified binder. These materials are typically used in Louisiana in the construction of OGFC ( 17 ). It should be noted that #78 limestone and # 67 sandstone had different gradations; therefore, they were batched with different percentages to satisfy the JMF, see Table 1.
Aggregate Characteristics
Note: na = not available.
To resist moisture damage, an anti-stripping agent (ASA) was added to the binder in all mixes before mixing with the aggregate. For WMA-OGFC, WMA additives were added to the binder-ASA blend before mixing with the aggregate. In addition, cellulose fibers were incorporated in the mixtures to control draindown.
Binder Modifications
For all mixes, an ASA was added at a dosage of 0.6% by the weight of the binder. First, the asphalt binder was heated in an oven until reaching the mixing temperature (i.e., 325°F). The ASA was then added and a shear mixer was used to mix the blend for 1 to 2 min at 1,000 rpm. It is worth noting that the HMA-OGFC was mixed and compacted at a temperature of 325°F (163°C) and 310°F (155°C), respectively. However, for all WMA-OGFC, the mixing and compaction temperatures were 302°F (150°C) and 284°F (140°C), respectively.
For HMA-OGFC, after adding the ASA, the binder was ready for mixing with the aggregate. However, for WMA-OGFC, Che1, Che2 and Org were then added at a rate of 0.1%, 0.5%, and 1.5%, respectively, by the weight of the binder. For Che1 and Che2, the specified dosage was added to the binder at the mixing temperature (i.e., 302°F); the blend was then mixed at 1,000 rpm for 1 to 2 min, as recommended by the manufacturer. On the other hand, the Org additive was mixed at 3,000 rpm for 45 min ( 18 ).
Adding Fibers into the Mixtures
In the experimental program, cellulose fibers were added to the mixtures at a dosage of 0.2% by weight of the total mix using the dry method. In this process, the aggregate was heated to the mixing temperatures for 3 h. Then, the fibers were manually mixed with the heated aggregate. Finally, using the mechanical mixer, the cellulose fiber was mechanically blended with the aggregate for 1 to 2 min to ensure the even distribution of fibers throughout the whole mix. Before mixing with the binder, the fibers–aggregate blend was kept in the oven for 30 min to reach the mixing temperature.
Laboratory Test Methods
A Superpave Gyratory Compactor (SGC) was used at a compaction effort of 50 gyrations to fabricate the OGFC specimens as recommended by ASTM D7064 and AASHTO PP 77 ( 19 ). In the experimental program, three different compaction procedures were followed based on the properties being evaluated. For AV content measurements, compacted samples were fabricated with 6 in. (150 mm) and 4.5 in. (115 mm) diameter and height, respectively.
For the measurement of the coefficient of permeability, the samples were prepared at the same AV content, but with a height of 3.15 in. (80 mm). To achieve the target AV at the required height, the dimensions and weight of the AV samples were used to estimate the required weight needed to fabricate a sample with a height of 3.15 in. (80 mm) and a diameter of 6 in. (150 mm). The calculated weight was then used to fabricate the sample by setting the height in the gyratory compactor to a height of 3.15 in. (80 mm).
For the performance tests, all samples were fabricated at the same AV content (20.5 ± 0.5%) to eliminate the impacts of AV content on the results and to ensure that the only variable was the WMA additives. To achieve the target AV at the required height of 3.74 in. (95 mm), the dimensions and weight of the AV samples were used to estimate the required weight needed to fabricate a sample with the required height. The calculated weight was then used to fabricate the sample by setting the height in the gyratory compactor to the specified height of the test sample (in this case 95 mm).
AV Content
The method for calculating the AV content involved two main steps (i.e., theoretical maximum specific gravity [Gmm] and bulk specific gravity [Gmb] calculations). In this study, Gmm was conducted according to ASTM D 2041 and AASHTO T 209, Standard Test Method for Theoretical Maximum Specific Gravity and Density of Asphalt Mixtures ( 20 ). For Gmb calculations, the procedure described in ASTM D6752, Standard Test Method for Bulk Specific Gravity and Density of Compacted Asphalt Mixtures Using Automatic Vacuum Sealing Method ( 21 ) was followed. To this end, three SGC-compacted replicates 6 in. (150 mm) and 4.5 in. (115 mm) in diameter and height, respectively, were compacted at an effort of 50 gyrations. Once Gmm and Gmb were measured, AV was calculated according to Equation 1. Afterward, the resultant AV values were verified against the acceptable range of AV for OGFC in Louisiana (i.e., 18%–24%) ( 17 ).
Permeability (k)
The mix permeability was tested according to FM 5-565, Florida Method of Test for Measurement of Water Permeability of Compacted Asphalt Paving Mixtures, using the falling head permeability test ( 22 ). In this test, the water is allowed to flow through a saturated sample; the flow rate at which the water flows through the sample pores is used to calculate the sample permeability. To achieve saturated conditions, a two-step procedure was followed. First, the sample was kept submerged in water for at least an hour before testing. Second, after the sample was placed inside the device, the water was allowed to flow through the sample for 5 to 10 min before conducting the test.
Draindown Test
Draindown characteristics were evaluated according to AASHTO T 305-14, Determination of Draindown Characteristics in Uncompacted Asphalt Mixtures ( 23 ). In this procedure, a sample of a loose mix (about 1,200 g) is placed on a wire basket. Afterward, the basket is placed in a pan with a known mass in an oven at 15°C higher than the production temperature (i.e., mixing temperature) for an hour. Draindown is evaluated by dividing the mass of the binder that has drained off by the total mass of the mix, as presented in Equation 2. It is recommended that the upper limit of binder draindown be 0.3% ( 24 ).
where
mf = the final mass of the pan (g);
mi = the initial mass of the pan (g); and
mt = mass of the test sample (g).
Cantabro Abrasion Test
Abrasion resistance, measured by the Cantabro abrasion test, has been a required test for OGFC mix design in many European countries ( 25 ). In the present study, the Cantabro abrasion test was conducted according to AASHTO TP 108-14, Standard Method of Test for Determining the Abrasion Loss of Asphalt Mixture Specimens ( 26 ). In this test, SGC-compacted specimens were weighed to the nearest 0.1 g. Then, the Los Angles abrasion test was run on the specimen without the steel balls. For 10 min, the apparatus was run at a speed of 30 to 33 rpm at a temperature of 25°C. Afterward, the test specimen was removed and the final mass of the specimen measured to the nearest 0.1 g. The Cantabro loss value was then calculated as presented in Equation 3:
where
Wini = initial mass (g); and
Wfinal = final mass (g).
In the present study, nine SGC samples were prepared for each mix with a diameter of 6 in. (150 mm) and a height of 4.5 ± 0.2 in. (115 ± 5 mm). These nine mixes were divided into three groups with equal average AV. The first group was tested without any conditioning. On the other hand, the second and the third groups were conditioned to evaluate the impact of binder aging and moisture on the abrasion resistance of OGFC mixes, respectively. For the second group, three samples were aged in an oven for 7 days at 60°C ( 27 ). For the third group, the samples were submerged in a water bath for 24 h at 60°C ( 27 ). Afterward, the samples in the second group were left to cool down to room temperature before testing. For the third group, samples were dried using the core dry machine in accordance to ASTM D7227, Standard Practice for Rapid Drying of Compacted Asphalt Mixture Specimens Using Vacuum Drying Apparatus, before conducting the test ( 28 ).
The average Cantabro loss values were compared with the specification limits to evaluate the abrasion and durability resistance of the mixes. According to NCHRP 01-55, a maximum of 20% is recommended for the Cantabro loss in case of unaged samples ( 1 ). Although there is no specification for aged and moisture-conditioned specimens, NCHRP 9-41 recommended a maximum Cantabro loss of 30% in OGFC applications ( 9 , 27 ).
Compaction Energy Index
The CEI is defined as the area under the curve between the number of gyrations and %Gmm from the eighth gyration to the gyration that corresponds to 92% Gmm ( 29 ). It is assumed that CEI represents the compaction effort applied by the roller in the field to achieve the required density during construction. In this study, the compaction data were used to calculate CEI for each mix type. As OGFC has a higher AV content compared with conventional HMA, CEI was calculated as the area under the %Gmm curve from the eighth gyration to the gyration that corresponds to 75% Gmm.
Hamburg Wheel-Tracking Test
The HWTT is a widely used method for the determination of HMA resistance to permanent deformation. The HWTT test was conducted according to AASHTO T 324, Hamburg Wheel-Track Testing of Compacted Hot Mix Asphalt (HMA) ( 30 ). Four test samples were prepared 6 in. (150 mm) in diameter and 2.36 in. (60 mm) in height with AV content of 20.50 ± 0.50%. In Louisiana, the criterion for OGFC with PG 76-22 is a maximum of 0.50 in. (12.50 mm) after 5,000 passes ( 17 ). However, according to NCHRP 01-55, OGFC maximum rut depth after 20,000 passes should not exceed 0.50 in. (12.50 mm). Therefore, in this study, the rut depth of all mixes was evaluated at both 5,000 and 20,000 passes to satisfy both the local and national requirements.
Texas Overlay Test
The TOT was conducted to evaluate OGFC mixes’ resistance to reflective and fatigue cracking as described in Tex-248-F ( 31 ). Three test specimens of 6.00 in. (150 mm) in diameter by 4.50 ± 0.20 in. (115 ± 5 mm) in height were first fabricated. Then, these samples were trimmed to 6 in. (150 mm) long by 3 ± 0.02 in. (76.20 ± 0.50 mm) width by 1.5 ± 0.02 in. (38 ± 0.50 mm) height. Then, the trimmed samples were glued to the base plate, spaced 4.2 mm apart, using epoxy. Afterward, the glued samples were left to cure for 24 h after applying a mass of 10 lb (4.50 kg) on the sample. Finally, the test was conducted using the Global Asphalt Mixture Performance Tester at 25 ± 0.5°C.
TOT results were analyzed based on two different parameters (i.e., critical fracture energy [GC] and crack progression rate [CPR]) as detailed elsewhere ( 32 ). GC represents the required energy to trigger a crack at the bottom of the test specimen after the first loading cycle of the TOT test. This parameter is used to evaluate the crack initiation stage in TOT. In contrast, the CPR is typically used to evaluate the asphalt mix flexibility and fatigue properties during the crack propagation phase. It represents the reduction in load which is required to propagate the crack through the test specimen under the cyclic load of TOT.
Modified Lottman Test
The modified Lottman test was conducted to evaluate the moisture-damage resistance of the different OGFC mixes. The test was conducted according to AASHTO T 283 ( 33 ), with some modifications as detailed in ASTM D7064, Standard Practice for Open-Graded Friction Course ( 19 ). In this procedure, the ratio of the dry ITS of three test specimens to the ITS of three conditioned test specimens was calculated and was used to evaluate the moisture-damage resistance of OGFC mixes; known as TSR. The conditioned mixes were conditioned with a single freeze and thaw cycle. Before testing, all samples were placed in a water bath at 25°C for 2 h. It is worth noting that the ITS of the dry samples can also be used to evaluate the resistance of the different mixes to fatigue cracking ( 34 , 10 ).
Boiling Test
The boiling test is required in Louisiana to evaluate the moisture-damage resistance of asphalt mixes. In the current study, the boiling test was conducted according to DOTD TR 317 ( 35 ). This test consists of placing 250 g of loose mixture in a beaker of boiling water for 10 min. Afterward, the water is drained and the sample is removed from the beaker on heavy-duty aluminum foil. A visual inspection is then conducted to evaluate if any asphalt stripping has occurred.
Results and Discussion
This section introduces the main results and findings of the laboratory experimental program. Results were analyzed statistically using an Analysis of Variance (ANOVA) and Tukey’s Honest Significant Difference grouping at a 95% confidence level to identify the statistical differences between the characteristics and performance of the different mixes. Statistical differences were identified with letters on the figures (i.e., A, B, and C). Different assigned letters indicate that the two groups are statistically different, with the letter A assigned to the best performer, followed by the letter B, and so on.
Air Voids and Permeability
Figure 1 shows the results of AV and permeability measurements for all mixes considered in the study. Overall, the incorporation of WMA additives decreased the total AV in OGFC, see Figure 1a. The CM had the highest AV content (20.76%) whereas the Org-OGFC mix had the lowest AV content (18.5%). These results can be attributed to the ability of WMA additives to reduce the viscosity of the binder to a degree that allows the mixture to be compacted to the target density at a lower compaction effort. In addition, ANOVA results indicated that the statistical differences between the CM and WMA-OGFCs were significant, as presented in Figure 1a. In spite of these differences, all the mixes satisfied the AV content requirements of LaDOTD for OGFC mixes (18% to 24%).
Open-graded friction course (OGFC) functionality test results: (a) air voids (AV) and (b) coefficient of permeability.
Similarly, WMA additives resulted in a reduction of the permeability of OGFC. As shown in Figure 1b, the CM had a k value of 426.71 ft/day compared with 363.58, 198.05, 198.64 ft/day for Che1-OGFC, Che2-OGFC, and Org-OGFC, respectively. These results can be attributed to the reduction in the AV content. These results are consistent with the conclusion reported in NCHRP 01-55, which indicated the direct relationship between AV content and coefficient of permeability ( 1 ). Statistically, ANOVA results showed that Che2-OGFC and Org-OGFC had k values that are statistically different from those of the CM. However, Che1-OGFC and CM mixes exhibited statistically equivalent k values. According to NCHRP Report 1-55, a k value of 164 ft/day is sufficient for OGFC to achieve its functionality ( 1 ). Therefore, all mixes passed the NCHRP-recommended permeability criterion for OGFC mixes.
Draindown Test Results
Figure 2 compares the results of the draindown test for the four mixes evaluated in the experimental program. As shown in the figure, it can be noted that all mixes satisfied LaDOTD requirement for draindown (i.e., 0.30%). Figure 2 shows that Org-OGFC had the least draindown of 0.01%, which was significantly different from the other mixes. On the other hand, the Che1-OGFC mix showed the highest draindown value (i.e., 0.11%), which was statistically similar to the draindown value of the CM. Che2-OGFC also exhibited an acceptable draindown value, which was significantly less than those of the CM.
Draindown test results.
Compaction Energy Index
Figure 3 presents the CEI for the four mixes evaluated in the experimental program. Overall, the use of WMA additives in OGFC reduced the CEI, which indicates that WMA-OGFC mixes would require less compaction effort and energy consumption during the compaction stage as compared with the CM. Statistically, Che1-OGFC, Che2-OGFC, and Org-OGFC mixes exhibited CEI values that were significantly lower than the CM as supported by the ANOVA results (see Figure 3). Overall, WMA-OGFC mixes would require a reduced mixing temperature and compaction effort in the field as compared with conventional OGFC mixes.
Compaction Energy Index (CEI) results.
Cantabro Loss Test Results
Figure 4 shows the results of the Cantabro test, which was used to evaluate the raveling resistance and durability of the mixtures. In general, WMA additives enhanced the raveling resistance of the mixtures at the different testing conditions. Figure 4a shows the results of the Cantabro test for the unaged specimens for all mixtures. Under these conditions, Che2-OGFC and Org-OGFC mixes showed the best raveling resistance as indicative from the low Cantabro loss value of 7.50%, and 13.41%, respectively. However, both CM and Che1-OGFC mixtures showed high Cantabro loss values of 17.60% and 16.30%, respectively, as shown in Figure 4a. Results of ANOVA indicated that the addition of chemical additive (Che2) decreased the Cantabro loss values significantly as compared with the CM. However, the remaining mixes had similar raveling resistance to CM. The results also indicated that the Che2-OGFC and Org-OGFC mixes satisfied the allowable Cantabro loss requirement as presented in Figure 4a. Although the average Cantabro loss values of CM and Che1-OGFC mixes satisfied the maximum loss requirement, some specimens showed higher Cantabro loss values in both mixtures as indicated by the error bars in Figure 4a.
Cantabro loss results: (a) unaged samples and (b) aged and moisture-conditioned samples.
For the aged samples, WMA mixes showed high raveling resistance compared with CM, see Figure 4b. Che1-OGFC, Che2-OGFC, and Org-OGFC had Cantabro loss values of 19.10%, 10.72%, and 15.58%, respectively, compared with 35.20% for the CM. The results indicate that the CM failed to satisfy the 30% limit when the test was conducted on aged samples; however, all WMA mixes did meet this requirement. From a statistical perspective, both chemical additive (Che2) and the organic additive (Org) reduced the Cantabro loss value significantly compared with the CM, see Figure 4b. However, ANOVA results indicated that both Che1-OGFC and CM mixes exhibited similar resistance to raveling in the aged state.
For moisture-conditioned samples, all WMA mixtures satisfied the maximum Cantabro loss requirements with Cantabro loss values of 16.23%, 7.39%, and 16.69% for Che1-OGFC, Che2-OGFC, and Org-OGFC mixes, respectively (See Figure 4b). However, the CM failed to achieve the desired raveling resistance, with a Cantabro loss value of 43.90%. Statistically, ANOVA results showed that all WMA mixes were statistically equivalent as they significantly improved the durability and raveling resistance of OGFC mixes on moisture-conditioned samples.
HWTT Results
Figure 5 summarizes the results of the HWTT test. In general, compared with the CM, WMA-OGFC mixes showed similar permanent deformation resistance. Figure 5a shows the rut depth results of the OGFC mixes at 5,000 passes. Whereas Org-OGFC had the smallest rut depth of 4.08 mm after 5,000 passes, the CM showed the lowest permanent deformation resistance (rut depth of 7.79 mm). In addition, ANOVA results showed that Org-OGFC had significantly improved permanent deformation compared with the other mixes. Nevertheless, all mixes satisfied LaDOTD requirements of 12.50 mm after 5,000 passes as presented in the figure.
Hamburg wheel-tracking test results: (a) rut depth @ 5,000 passes and (b) rut depth @ 20,000 passes.
In Figure 5b, the rut depth values of all mixes at 20,000 passes were compared with the maximum allowable rut depth recommended by NCHRP 1-55. From Figure 5b, Org-OGFC was the only mix that satisfied NCHRP 1-55 requirements after 20,000 passes. However, CM, Che1-OGFC, and Che2-OGFC did not meet this criterion with rut depths of 17.73, 12.88, and 16.48 mm, respectively. Statistically, Org-OGFC showed rut depth values that were statistically different from the other mixes.
Texas Overlay Test Results
Figure 6 shows the results of the TOT. Overall, all mixes showed acceptable resistance to cracking. The results of GC are presented in Figure 6a. From the figure, it can be concluded that all mixes performed adequately in the test; however, Che1-OGFC showed the least resistance to crack initiation with a GC value of 0.5 lb-in./in. 2 , which is less than the minimum acceptable value (i.e., 1.0 lb-in./in. 2 ). In contrast, the remaining mixes showed GC values that ranged from 1.0 to 3.0 lb-in./in. 2 , which indicated acceptable cracking resistance. Among the evaluated mixes, Org-OGFC had the highest crack initiation resistance with a GC value of 1.50 lb-in./in. 2 . In addition, ANOVA results showed that the organic additive significantly enhanced OGFC performance in relation to cracking resistance compared with the CM. However, chemical additive (Che1) resulted in a reduction of the resistance to crack initiation compared with the CM as presented in Figure 6a.
Texas Overlay Test results; (a) critical facture energy results, (b) crack propagation rate results, and (c) cracking interaction plot.
Figure 6b shows the results of the CPR. This parameter was used to evaluate OGFC’s ability to delay crack propagation once the crack has initiated. From the figure, it can be concluded that all mixes satisfied the maximum allowable CPR value (i.e., 0.50). Whereas Che1-OGFC showed a CPR value of 0.26, the remaining mixes showed comparable crack propagation resistance with a CPR value of 0.24. ANOVA results showed that CPR values of Che1-OGFC were significantly different as compared with the other mixes. However, the observed statistical differences among the CPR values of the remaining mixes were negligible.
Figure 6c presents the cracking interaction plot; this plot can be used to evaluate the mixture cracking characteristics based on GC and CPR values. To this end, GC and the corresponding CPR values were plotted and were compared with the allowable range of GC and CPR. Ideally, a mixture with an acceptable crack resistance should be located in the soft-crack resistant zone as presented in Figure 6c. From the figure, it can be concluded that all mixes, except Che1-OGFC, were located inside the soft-crack resistant zone, indicating an acceptable crack resistance. Therefore, it can be inferred that Che1 may result in a reduction in OGFC cracking resistance, especially in the crack initiation stage.
Modified Lottman Test Results
Figure 7 presents the results of the modified Lottman test. Modified Lottman test results were analyzed in relation to unconditioned ITS, conditioned ITS, and TSR. The results of the unconditioned ITS are presented in Figure 7a. As presented in the Che1-OGFC and Che2-OGFC exhibited dry ITS values of 58.0 and 51.3 pounds per square inch (psi), respectively. However, the incorporation of the organic additive in the OGFC mix resulted in an increase of the dry ITS by 24% compared with the CM. As the dry ITS value is typically used as an indication of fatigue crack resistance, it may be inferred that the organic additive improved the fatigue crack resistance of OGFC. Yet, both chemical additives did not provide the same level of improvement in relation to cracking resistance. Statistically, the impact of chemical additive (Che1) on OGFC fatigue crack resistance was insignificant. Furthermore, NCHRP 1-55 recommends that the unconditioned ITS values should not be less than 70 psi to ensure desirable crack resistance ( 1 ). From Figure 7a, it can be concluded that Org-OGFC was the only mix to satisfy the NCHRP-recommended threshold.
Modified Lottman test results: (a) unconditioned indirect tensile strength (ITS), (b) conditioned ITS, and (c) tensile strength ratio (TSR).
Results of conditioned ITS values are presented in Figure 7b. The results indicated that Org-OGFC showed the highest conditioned ITS value of 65.47 psi, which was significantly different from the other mixes. In addition, the figure shows that the conditioned ITS values of the CM, Che1-OGFC, and Che2-OGFC were 51.84, 49.21, and 46.89 psi, respectively. According to NCHRP 1-55, the minimum acceptable conditioned ITS should be 50 psi. The results presented in Figure 7b indicated that both CM and Org-OGFC satisfied the NCHRP requirements, whereas the other mixes did not.
TSR values are presented in Figure 7c. The TSR value is typically used to evaluate the moisture-damage resistance of the different mixes. According to NCHRP 1-55, a TSR value of 0.70 is the minimum value for OGFC mixes. According to Figure 7c, it can be concluded that all mixes satisfied the minimum TSR values of 0.7 for OGFC mixes.
Boil Test
Figure 8 shows the results of the boil test. Figure 8 compares the condition of the loose mix specimens for the different mixes after a 10 min boiling period. Regardless of the mixture type, the visual inspection indicated that no stripping has occurred to the aggregate in all mixes, predicting acceptable moisture-damage resistance for all mixes. These results may be attributed to three main reasons. First, the polymer modification of PG 76-22 used in the experimental program promoted the mix adhesion force, which in turn enhanced OGFC moisture-damage resistance. Second, all the mixes prepared in this study were fabricated with an ASA, which helped promote OGFC stripping resistance. Third, the use of WMA additives enhanced the ability of the asphalt binder to coat the aggregate particles by reducing the frictional forces at the binder–aggregate interface resulting in adequate adhesion strength and moisture-damage resistance ( 36 ).
Boil test results for: (a) control mix, (b) Che1-OGFC, (c) Che2-OGFC, and (d) Org-OGFC.
Conclusions
The ultimate goal of the present study was to investigate the impact of three WMA additives (i.e., two chemical additives and one organic additive) on the laboratory performance and physical characteristics of OGFC. Based on the results presented, the following conclusions may be drawn:
WMA additives reduced the total AV content of OGFC mix, which in turn reduced the coefficient of permeability. Nevertheless, all the mixes satisfied the requirement of both AV content and coefficient of permeability for OGFC. Given the notable reduction in production temperature (23°F), WMA-OGFC mixes achieved the target density at a lower compaction energy effort compared with the CM.
WMA-OGFC generally enhanced both raveling and permanent deformation resistance of the mix as compared with the control OGFC mix.
The organic additive enhanced the cracking resistance of OGFC as compared with the CM based on the TOT and ITS (dry and wet) test results.
The results of the TSR and boil tests indicated that all mixes had acceptable moisture-damage resistance.
Overall, results of the study indicated that the use of WMA technologies enhanced OGFC durability and performance. Among the evaluated mixes, the organic OGFC met all LaDOTD and NCHRP 01-55 requirements, as it notably enhanced OGFC durability and cracking resistance. Furthermore, the use of WMA significantly reduced the required compaction effort and mixing temperature as compared with the CM. Field evaluation of the proposed WMA-OGFC mixes is recommended.
Footnotes
Acknowledgements
The assistance of Mike Hemsley of Paragon Technical Services is greatly appreciated. In addition, the assistance of the additive manufacturers and Don Weathers was of great benefit to the authors.
Author Contributions
The authors confirm contribution to the paper as follows: study conception and design: Hossam Abohamer and Mostafa Elseifi; data collection: Hossam Abohamer and Corey Mayeux; analysis and interpretation of results: Hossam Abohamer, Mostafa Elseifi, Corey Mayeux, and Samuel B. Cooper III; draft manuscript preparation: Hossam Abohamer, Mostafa Elseifi, Corey Mayeux, Samuel B. Cooper III, and Samuel Cooper, Jr. All authors reviewed the results and approved the final version of the manuscript.
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study was funded by the Louisiana Transportation Research Center (Project 21-6B).
Data Accessibility Statement
Some or all data that support the findings of this study are available from the corresponding author on reasonable request. Data include laboratory data and statistical analysis.
The contents of this paper do not necessarily reflect the official views or policies of LTRC or LaDOTD.