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97 1.22.7 Performance No specific performance measures were given. 1.22.8 Structural Design No specifics on inclusion with structural design were given. 1.22.9 Limitations No specific limitations were given. 1.23 Mallick, R. B., P.S. Kandhal, L. A. Cooley, Jr., and D. E. Watson. âDesign, Construction, and Performance of New Generation Open-Graded Friction Courses.â NCAT report No. 2000-01. National Center for Asphalt Technology. Auburn University. 2000. 1.23.1 General This report documents an extensive laboratory study that was conducted to develop a laboratory mix design system for new generation OGFCs (or PFCs). This project was likely the first ânationalâ effort in the US to develop mix design criteria for PFCs. Also included within this report is a field evaluation of OGFC pavements that were designed and constructed in a manner that closely resembled a PFC pavement. The pavements had been in-service for six years at the time of evaluation. 1.23.2 Benefits of Permeable Asphalt Mixtures The authors state that OGFC pavements improve wet weather driving conditions by allowing water to drain from the pavement surface. This drainage of water reduces hydroplaning, reduces splash and spray, improves wet weather frictional properties, improves surface reflectivity and reduces traffic noise. 1.23.3 Materials and Mix Design The majority of this report was dedicated to describing a large laboratory study designed to develop a mix design system for PFC mixes. Two phases were included within the laboratory study. The first phase of laboratory work was designed to optimize the gradation requirements for PFC mixes. Additionally, the authors used the first phase of research to evaluate potential criteria for various laboratory tests for inclusion within the mix design system to be developed. The second phase of the laboratory study was designed to optimize the type of asphalt binder and additives to enhance performance. The following paragraphs describe the work conducted in the two phases of research In the first phase of the study, blends were prepared with gradations similar to and coarser than the FHWA recommended gradation for OGFC mixes. Table 53 provides the FHWA recommended gradation and the other three new gradations evaluated in this study.
98 Table 53: Gradations Used Percent Passing Sieve Size Original FHWA Gradation Gradation Similar to FHWA Used New Gradation #1 New Gradation #2 New Gradation #3 19 mm --- 100 100 100 100 12.5 mm 100 95 95 95 95 9.5 mm 95-100 65 65 65 65 4.75 mm 30-50 40 30 25 15 2.36 mm 5-15 12 7 7 7 0.075 mm 2-5 4 3 3 3 The FHWA gradation has 40 percent material passing the 4.75 mm sieve, and the coarsest of the other three gradations has 15 percent material passing the 4.75 mm sieve. The coarsest gradation is very similar to the gradation that is being used by many states reporting good experience at the time with OGFC mixes. Mixes were prepared for these blends with an unmodified PG 64-22 asphalt binder. Mix designs were conducted according to the FHWA recommended procedures. These four blends were evaluated for stone-on-stone contact with voids in the mineral aggregate (VMA) and voids in the coarse aggregate (VCA) plots, and VCA data from dry-rodded tests with the coarse aggregate fraction only. Samples prepared with the FHWA gradation and coarser gradations were tested for draindown potential, permeability, abrasion resistance, aging potential, and rutting. All samples were initially compacted with 100 gyrations of the Superpave gyratory compactor, which was considered to be equivalent to 50 blows of Marshall hammer in SMA mix design. The primary objective of Phase 1 was to evaluate the relative improvements in mix characteristics when the FHWA gradation was made coarser. The average air voids (VTM), voids in mineral aggregate (VMA), voids in coarse aggregate (VCA), and voids filled with asphalt (VFA) data for the four different mixes are shown in Table 54. Although there is a difference of only 0.13% in asphalt binder content between the mixes with the four gradations, there is a significant range in voids (VTM, VMA and VCA). The VTM and VMA generally decrease with an increase in percent passing 4.75 mm sieve. Hence, the coarser the mix, the higher is the VTM and VMA. The dry rodded coarse aggregate VCA (VCADRC) falls between the compacted mix VCA values for gradations with 15 percent and 25 percent passing the 4.75 mm sieve. This indicates that stone-on-stone contact begins at some point between 25 percent and 15 percent (approximately at 22 percent) passing the 4.75 mm sieve for the materials used in the study. High VTM associated with the coarser gradation will also facilitate better drainage of water.
99 Table 54: Summary of Mix Volumetric Properties Compacted OGFC Mix Gradation (% passing 4.75 mm sieve) Asphalt Content TMD* VTM, % VMA, % VCA, % VFA, % 15 5.55 2.475 15.1 26.3 37.3 42.6 25 5.51 2.512 14.3 24.5 43.3 41.7 30 5.48 2.511 13.6 24.0 46.6 43.3 40 5.42 2.487 12.5 23.9 54.1 47.3 * TMD = Theoretical maximum density Dry rodded VCA = 41.7%. Draindown tests were conducted on uncompacted OGFC mixes (with PG 64-22 binder) at 160C and 175C according to the NCAT draindown test method. The results of NCAT draindown test showed that the mix with 15 percent passing 4.75 mm sieve had the highest draindown potential. As the gradation became finer, draindown potential lessened. The Cantabro abrasion test was conducted on the mixes with different percentages of material passing the 4.75 mm sieve. The data showed that under both aged and unaged conditions the abrasion loss increased as the mix became coarser. This indicates a higher potential for durability problems for the coarser gradations. The permeability of mixes with different percentages of material passing the 4.75 mm sieve was tested with a falling head permeameter. As expected, the mixes with lower percentages of material passing the 4.75 mm sieve showed higher permeability. There was a significant increase in permeability between the mix with 30 percent passing the 4.75 mm sieve and the mix with 15 percent passing the 4.75 mm sieve. Rut tests were conducted on the four mixes at design asphalt binder contents. The Asphalt Pavement Analyzer (APA) was used to evaluate rut potential. The rut depths at 8,000 cycles were very small, less than 5 mm for all mixes, and were considered acceptable. In the second phase of the laboratory study, mixes were prepared at only 15 percent passing the 4.75 mm sieve and 6.5 percent asphalt binder content using six different binder/additive combinations. A study was carried out to determine the required number of gyrations to provide a density closer to that seen at the time of construction (about 18 percent air voids). Three samples of each mix were compacted with 100 gyrations of the SGC and 50 blows of Marshall hammer. The air voids at different gyrations were compared to air voids generally found in the field and the air voids of the sample compacted with 50 blows Marshall. It was determined that about 50 gyrations with the SGC and 50 blows with the Marshall hammer produce about 18 percent air voids generally found in the field. The
100 mixes were then prepared with six different types of binder (Table 55). The samples were tested for volumetric properties, draindown, aging, rutting, and moisture susceptibility. The average draindown values at 157C (315F) are shown in Table 56. Results from a multiple comparison test are also shown in Table 56. These results indicate whether there is any significant difference between the different means, and if there is, provides the ranking of the different mixes based on the means. Table 56 indicates that the use of fibers greatly reduces the potential for draindown; more so than does polymer modification. Table 55: Volumetric Properties of mixes with Difference Binders (Average Values) Binder Bulk Sp. Gr. TMD VTM VMA VCA PG 64-22 2.044 2.441 16.3 29.0 37.3 PG 64-22 with cellulose 2.043 2.441 16.3 29.0 37.3 PG 64-22 with slagwool 2.071 2.441 15.2 28.1 37.3 PG 64-22 with SBS 2.026 2.441 17.0 29.6 37.3 PG 76-22-SB 2.002 2.441 18.0 30.5 37.3 PG 76-22 with slagwool 2.046 2.441 16.2 28.9 37.3 Table 56: Results of Draindown Tests from Mixes with Different Binders Draindown at 157C (315F) Duncan Grouping Mean (%) Asphalt Binder A 1.3585 PG 64-22 A 1.1845 PG 76-22-SB B 0.5405 PG 64-22 with SBS B 0.1245 PG 76-22-SB with slagwool B 0.0510 PG 64-22 with slagwool B 0.0040 PG 64-22 with cellulose Samples of mixes prepared with the different binders were tested with the Cantabro Abrasion test to determine the effect of aging. All of the samples were aged at 50C for 168 hours (7 days). Table 57 shows the test values and the results of multiple comparison tests. Results show that the mixes with unmodified PG 64-22 binder had the highest abrasion loss, and the mixes with PG 76-22-SW had the lowest abrasion loss. The data clearly showed that the combined use of polymer modified binder and fiber will minimize the laboratory abrasion loss and, thus, increase durability of the mixture.
101 Table 57: Abrasion Loss (Aged Samples) for Mixes with Different Types of Binder Duncan Grouping Mean (%) Asphalt Binder A 26.2 PG 64-22 B A 19.3 PG 64-22 with slagwool B A 18.8 PG 64-22 with cellulose B C 15.7 PG 76-22-SB B C 13.0 PG 64-22 with SBS C 9.0 PG 76-22 with slagwool Moisture susceptibility of mixes was evaluated by conducting tensile strength test on conditioned (5 freeze/thaw cycles) and unconditioned compacted samples of mixes with different binders. Test samples were compacted to the standard laboratory compaction effort instead of a target air void content for this testing. This test was included in Phase 2 to evaluate the effect of binder type and fibers on the moisture susceptibility of OGFC mixes. The test results indicated that both polymer-modified binder and fiber should be used especially in the northern tier states of the U.S., which experience cold climates and freeze/thaw cycles. Based on the research conducted by the authors, the following were concluded: 1. A gradation with no more than about 20 percent passing the 4.75 mm sieve is required to achieve stone-on-stone contact condition and provide adequate permeability in OGFC mixes. 2. Mixes with 15 percent of the aggregate fraction passing the 4.75 mm sieve are susceptible to significant draindown of the binder. Therefore, it is necessary to provide a suitable stabilizer such as fiber in the mix to prevent excessive draindown. 3. Abrasion loss of OGFC mixes resulting from aging can be reduced significantly with the addition of modifiers. In this study, all of the modified binders had significantly lower abrasion loss than mixes using an unmodified binder. The use of both polymer- modified binder and fiber can minimize the abrasion loss and thus increase the durability of OGFC. 4. For the binders used in this study, rut depths as measured with the APA did not vary over a wide range. However, within the range of rut values obtained, the mixes with modified binders had less rutting than mixes with unmodified binders. A higher PG binder grade seems to have a greater effect in reducing rutting than a lower PG binder grade. A polymer-modified asphalt with fiber gave the least amount of rutting. 5. Moisture susceptibility, as measured by TSR values, is lower for mixes with modified binders than mixes with unmodified binders. All of the modifiers except slagwool (with PG 64-22) produced mixes which had TSR values in excess of 80 percent.
102 Again, both polymer-modified binder and fiber should be most effective especially in cold climates with freeze/thaw cycles. To provide a clear understanding of the methods used by the authors, the following paragraphs describe the various tests utilized in the research. Similar to stone matrix asphalt (SMA), OGFC must have a coarse aggregate (retained on No. 4.75 mm) skeleton with stone-on-stone contact to minimize rutting. The condition of stone-on-stone contact within an OGFC mix is defined as the point at which the voids in coarse aggregate (VCA) of the compacted OGFC mixture is less than the VCA of the coarse aggregate alone in the dry-rodded test (AASHTO T19). The NCAT draindown test method was used to evaluate draindown potential. A sample of loose asphalt mixture was prepared and placed in a wire basket, which is positioned on a plate or other suitable container of known mass. The sample, basket, and plate or container were then placed in a forced draft oven for one hour at a pre-selected temperature. At the end of one hour, the basket containing the sample is removed from the oven along with the plate or container and the mass of the plate or container is determined. The amount of draindown was then calculated as the mass of asphalt binder that drained onto the plate divided by the total mass of mix. The Florida DOT falling-head laboratory permeability test was used. This test uses a falling head concept to determine permeability. The resistance of compacted OGFC specimens to abrasion loss was analyzed by means of the Cantabro Abrasion test. This is an abrasion and impact test carried out in the Los Angeles Abrasion machine (ASTM Method C131). In this test, a single OGFC specimen compacted with 50 blows on each side was used. The mass of the specimen was determined prior to testing. The test specimen was then placed in the Los Angeles Rattler without the charge of steel spheres. The operating temperature was room temperature. The machine was operated for 300 revolutions at a speed of 30 to 33 rpm. The test specimen was then removed and its mass determined. The percentage abrasion loss was then calculated. The recommended maximum permitted abrasion loss value for freshly compacted specimens was 20 percent. However, the authors state that some European countries specify a maximum value of 25 percent. Both unaged and aged compacted OGFC were subjected to Cantabro abrasion testing to evaluate the effect of accelerated laboratory aging on resistance to abrasion. Because of the very high air void contents, the asphalt binder in OGFC is prone to hardening at a faster rate than dense-graded hot mix asphalt, which may result in reduction of cohesive and adhesive strength leading to raveling. Aging was accomplished by placing five Marshall specimens compacted with 50 blows in a forced draft oven set at 60C for 168 hours (7 days). The specimens were then cooled to 25C and stored for 4 hours prior to conducting the Cantabro Abrasion test. The average of the abrasion losses obtained on 5
103 aged specimens should not exceed 30 percent, while no individual result should exceed 50 percent. Raveling of the OGFC may take place due to stripping in the mix, especially from freeze and thaw cycles in northern tier states with cold climates. The modified Lottman test (AASHTO T283) was used in this study; however, instead of using one freeze/thaw cycle, five cycles were used. Samples were compacted using 50 blows per face instead of targeting 7 percent air voids as stated in AASHTO T283. Since the air void content is higher in the OGFC compared to dense-graded HMA, more severe conditioning was deemed necessary to evaluate the stripping potential. The potential for rutting of OGFC was evaluated with the Asphalt Pavement Analyzer (APA). Cylindrical OGFC specimens were loaded at 64C (both dry and under water) for 8,000 cycles and rut depth measured. The following tentative mix design system was recommended for the new-generation OGFC mixes on the basis of the laboratory study, observation of in-place performance of OGFC mixes in Georgia, and experience in Europe. Step 1. Materials Selection The first step in the mix design process is to select materials suitable for OGFC. Materials needed for OGFC include aggregates, asphalt binders, and additives. Additives include asphalt binder modifiers, such as polymers and fibers. Guidance for suitable aggregates can be taken from recommendations for SMA. The binder selection should be based on factors such as environment, traffic, and expected functional performance of OGFC. High stiffness binders, such as PG 76-xx, made with polymers are recommended for hot climates or cold climates with freeze-thaw cycles, medium to high volume traffic conditions, and mixes with high air void contents (in excess of 22 percent). The addition of fiber is also desirable under such conditions and also has been shown to significantly reduce draindown. Step 2. Selection of Design Gradation Based upon this laboratory study and experiences in Georgia with Porous European Mix, the following master gradation band was recommended. Sieve Percent Passing 19 mm 100 12.5 mm 85-100 9.5 mm 55-75 4.75 mm 10-25 2.36 mm 5-10 0.075 mm 2-4
104 Selection of the design gradation entails blending selected aggregate stockpiles to produce three trial blends. It was suggested that the three trial gradations fall along the coarse and fine limits of the gradation range along with one falling in the middle. For each trial gradation, determine the dry-rodded voids in coarse aggregate of the coarse aggregate fraction (VCADRC). Coarse aggregate is defined as the aggregate fraction retained on the 4.75 mm sieve. For each trial gradation, compact specimens at between 6.0 and 6.5 percent asphalt binder using 50 gyrations of a Superpave gyratory compactor. If fibers are a selected material, they are included in these trial mixes. Determine the voids in coarse aggregate (VCA) for each compacted mix. If the VCA of the compacted mix is equal to or less than the VCADRC, stone-on-stone contact exists. To select the design gradation, choose a trial gradation that has stone-on-stone contact combined with high voids in total mix. Step 3. Determine Optimum Asphalt Content Using the selected design gradation, prepare OGFC mixes at three binder contents in increments of 0.5 percent. Conduct draindown testing on loose mix at a temperature 15C higher than anticipated production temperature. Compact mix using 50 gyrations of a Superpave gyratory compactor and determine air void contents. Conduct the Cantabro Abrasion test on unaged and aged (7 days @ 60C) samples. Rutting tests with the Asphalt Pavement Analyzer and laboratory permeability testing are optional. Insufficient data was accumulated in this study to recommend a critical rut depth. However, laboratory permeability values greater than 100 m/day are recommended. The asphalt content that meets the following criteria is selected as optimum asphalt content. 1. Air Voids. A minimum of 18 percent is acceptable, although higher values are more desirable. The higher the air voids are the more permeable the OGFC. 2. Abrasion Loss on Unaged Specimens. The abrasion loss from the Cantabro test should not exceed 20 percent. 3. Abrasion Loss on Aged Specimens. The abrasion loss from the Cantabro test should not exceed 30 percent. 4. Draindown. The maximum permissible draindown should not exceed 0.3 percent by total mixture mass. If none of the binder contents tested meet all four criteria, remedial action will be necessary. Air voids within OGFC are controlled by the binder content and gradation. If air voids are too low, the asphalt binder content should be reduced or the gradation made coarser. If the abrasion loss on unaged specimens is greater than 20 percent, more asphalt binder or a stiffer asphalt binder is needed. Abrasion loss values of aged specimens in excess of 30 percent can be remedied by either increasing the binder content or changing the type of binder additive. If draindown values are in excess of 0.3 percent, the amount of binder and/or type of binder additive can be adjusted. Fiber stabilizers are typically incorporated into the mix at a rate of 0.2 to 0.5 percent of the total mix.
105 Step 4. Evaluate Mix for Moisture Susceptibility The mix designed with Step 1 through 3 should be evaluated for moisture susceptibility using the modified Lottman method (AASHTO T283) with five freeze/thaw cycles in lieu of one cycle. The retained tensile strength (TSR) should be at least 80 percent. 1.23.4 Construction Practices The authors reported on the construction of six OGFC test sections placed on Interstate 75 near Atlanta, Georgia. The test sections were part of a field research study designed to evaluate OGFC mixes. The Georgia Department of Transportation wanted to compare their conventional OGFC mixes to coarser OGFCs. More specifics on the field project are provided in the section on performance within this review. Production of the mixes in the field was accomplished utilizing a double-barrel drum plant. The plant was slightly modified in order to incorporate fibers into the mixture as well as the addition of dry crumb rubber for one test section. 1.23.5 Maintenance Practices No specific maintenance practices were given. 1.23.6 Rehabilitation Practices No specific rehabilitation practices were given. 1.23.7 Performance This paper presents information and data on six field test sections that were constructed near Atlanta, Georgia. The six test sections were characterized as a coarse OGFC (D), coarse OGFC with 16 percent crumb rubber (D16R), coarse OGFC with cellulose fibers (DC), coarse OGFC with mineral fibers (DM), coarse OGFC with SB polymer (DP), and coarse OGFC with SB polymer and cellulose fibers (DCP). Mix designs for each of these mixes were conducted using the âMethod of Determining Optimum Asphalt Content for Open-Graded Bituminous Paving Mixtures,â which is a standard procedure for GDOT (GDT-114). Job-mix-formula (JMF) data for each of the six mixes are presented in Table 58. This table shows that all six OGFC mixes had identical gradations and only differed by respective asphalt contents. Of interest, the JMF gradation falls within the gradation band recommended by the authors of this paper in the new mix design system.
106 Table 58: Laboratory Test Results for the Six OGFC Mixes (6) Test D D16R DM DC DP DCP Percent passing 19.0 mm 100 100 100 100 100 100 Percent passing 12.5 mm 99 99 99 99 99 99 Percent passing 9.5 mm 75 75 75 75 75 75 Percent passing 4.75 mm 18 18 18 18 18 18 Percent passing 2.36 mm 8 8 8 8 8 8 Percent passing 0.075 mm 2 2 2 2 2 2 Percent Asphalt Binder of Total Mix % AC 6.0 6.6 6.3 6.4 6.2 6.4 Other Test Data Cantabro (% Wear) 13.5 8.6 5.7 5.8 8.6 8.2 Drainage (% Loss) 0.37 0.05 0.06 0.06 0.34 0.04 During production, truck samples were obtained to determine asphalt content, gradation, air voids, and Cantabro abrasion loss values. Table 59 presents the results of this testing. Table 59: Laboratory Test Results for Field Produced OGFC Mixes (6) Sample Type JMF D DM DC DCP DP D16R Sieve Size, mm Total Percent Aggregate Passing by Weight 19.0 100 100 100 100 100 100 100 12.5 99 98.3 98.9 96.7 97.0 99.1 96.3 9.5 75 70.0 76.2 64.0 68.6 69.9 60.3 4.75 18 21.0 23.9 19.0 19.1 23.1 15.7 2.36 8 8.7 9.0 7.7 7.8 8.4 7.4 0.075 2 3.6 3.1 2.8 2.4 3.1 2.6 Miscellaneous Test Data Asphalt Content Extracted 5.85 6.22 6.16 6.14 6.25 6.41 TMD --- 2.484 2.445 2.429 2.424 2.476 2.451 VTM --- 12.2 11.4 11.5 10.9 14.1 12.0 Cantabro (% Wear) --- 10.3 8.1 14.7 7.0 15.9 7.6
107 Based on Table 59, five of the produced mixes were finer than the JMF gradation on the 4.75 mm sieve. Only one gradation (DM) did not meet the recommended gradation band presented previously. However, this mix only varied from the band by 1.2 percent on the 9.5 mm sieve. Asphalt contents ranged from 5.9 to 6.4 percent. Air void contents of lab compacted samples using 25 blows per face of a Marshall hammer ranged from 10.9 to 14.1 percent and are lower than would be anticipated on the roadway. Cantabro abrasion loss values ranged from 7.0 to 15.7 percent and are all lower than the suggested 20 percent maximum criteria. In addition to testing truck samples, cores were obtained from each of the six test sections. Testing of these samples included asphalt contents and gradations by extraction and air void calculations. An additional test conducted was the in-place permeability of each section. Results of this testing are presented in Table 60. Table 60: Laboratory Test Results for Roadway Core Samples from OGFC Test Sample No. JMF D DM DC DCP DP D16R Sieve Size, mm Total Percent Aggregate Passing by Weight 19.0 100 100 100 100 100 100 100 12.5 99 99.3 98.6 99.2 97.6 99.3 99.2 9.5 75 77.3 77.2 75.5 73.1 76.5 76.7 4.75 18 28.1 28.3 28.0 26.9 27.8 28.0 2.36 8 13.1 13.6 13.7 13.0 13.1 13.1 0.075 2 3.8 4.1 3.5 3.9 3.8 3.4 Miscellaneous Test Data Asphalt Content Extracted 5.51 5.87 6.18 5.27 5.85 5.69 VTM --- 17.8 17.2 16.4 16.0 17.6 18.1 Permeability (m/day) --- 46 82 71 71 84 67 Of note in Table 60 are the in-place air void contents of the compacted mixes. Air void contents ranged from 16.0 to 18.1 percent which relate well to the data accumulated in the laboratory part of this study. These values also seem to validate the selection of 18 percent air voids minimum for the new mix design system as mixes meeting the gradation requirements can be constructed to have 18 percent air voids. Permeability values obtained from the six test sections ranged from 46 to 84 m/day and appear to correspond reasonably well with permeability data from the laboratory work in this study. During 1998 (six years after construction), a visual distress survey was performed on the six OGFC test sections. The survey consisted of evaluating each section for surface texture, rutting, cracking, and raveling. During the course of the survey, cores were obtained from each section and used to determine the laboratory permeability.
108 All six test sections had experienced some coarse aggregate popout. The D16R section appeared to have the most while the DC, DM, and DP sections all had a very low amount. Another surface texture item was the existence of small fat spots on the pavement surface. Each of the six sections had these fat spots. However, none were larger than approximately 15 cm diameter. The D and DCP appeared to have the most but were not deemed significant. Rutting Rut depth measurements were made for each section using a stringline. Rut depths ranged from 0.0 mm for the DP section to 4.1 mm for the DC section. None of the sections were characterized as having significant amounts of rutting. Cracking The primary form of cracking on all six sections was reflective from a Portland cement concrete pavement underlying each section. Table 61 presents descriptions and percentages of reflective cracks encountered. Percentages were determined by counting the number of transverse cracks visible at the pavement surface. Table 61: Severity and Percentage of Transverse Reflective Cracks Section Description % Cracks Showing D Low to medium severity 75 D16R Low to high severity 87 DM Low severity 55 DC Low severity 45 DP Low to medium severity 61 DCP Low to medium severity 65 Table 61 shows that five of the six sections had low to medium severity reflective cracking. The two OGFC mixes containing only fibers (DM and DC) had the least amount (and severity) of cracking while the D16R section had the highest amount and severity cracking. Reflective longitudinal cracks were also observed on five sections. Only the D and D16R sections had what could be characterized as medium severity longitudinal cracking. Besides reflective cracking, only the D16R section showed any other type of cracking. Secondary cracking around some reflective cracks had occurred. Raveling All six sections showed some signs of raveling. However, all raveling was minimal except the D16R section which showed some medium severity raveling next to some cracks.
