Randy C. West and Robert S. James

Transcription

1 EVALUATION OF A LIME KILN DUST AS A MINERAL FILLER FOR STONE MATRIX ASPHALT Randy C. West and Robert S. James Randy C. West National Center for Asphalt Technology 277 Technology Parkway Auburn, AL Telephone: Fax: westran@auburn.edu Robert S. James APAC, Inc Port Cobb Drive Smyrna, GA Telephone: Fax: rsjames@ashland.com July 2005 Submitted for presentation and publication at the 85 th Annual Meeting of the Transportation Research Board, Washington, D.C., January 2006 (3,545 words + 8 tables + 4 figures =6,545 words)

2 West and James 1 EVALUATION OF A LIME KILN DUST AS A MINERAL FILLER FOR STONE MATRIX ASPHALT Randy C. West and Robert S. James ABSTRACT Lime kiln dust (LKD) has had limited use as a mineral filler in Stone Matrix Asphalt (SMA). Although LKD meets mineral filler requirements of most agency specifications, a few cases of premature pavement failure have been attributed to some lime kiln dusts having high percentages of available lime. This study compared a lime kiln dust to a common rock dust mineral filler using basic asphalt mix design tests. Since the problem with SMA containing LKD with high available lime content occurs when the pavement is wet, specimens were conditioned with numerous freeze-thaw cycles and extended soak times to evaluate potential reactions under harsh conditions. The results show that the LKD used in this study performed as well or better than the rock dust mineral filler. The standard tensile strength ratio test is capable of identifying LKD with high available lime contents that will not perform well. Further research is needed to determine a suitable maximum limit for available lime content of LKD for use as a mineral filler for SMA. KEYWORDS: Lime Kiln Dust, Mineral Filler, SMA, Available Lime, Tensile Strength Ratio 1

3 West and James 2 EVALUATION OF A LIME KILN DUST AS A MINERAL FILLER FOR STONE MATRIX ASPHALT Randy C. West and Robert S. James INTRODUCTION Stone Matrix Asphalt (SMA) is a premium type of hot mix asphalt pavement. One of the component ingredients in SMA mixtures is mineral filler. Mineral fillers combine with the asphalt, fibers, and a small percentage of fine aggregate particles to create a binder rich mastic which fills the void spaces between the coarse aggregate skeleton. The requirements for mineral filler are not complex (1, 2). Consequently, a variety of materials have been used as mineral fillers in SMA including rock dust products of various mineralogies, fly ash, Portland cement, kiln dusts, and agricultural lime. Lime kiln dust (LKD), a by-product of lime manufacturing, has been used as a mineral filler in SMA in a few cases. However, information from published research studies or from field experience on the use of lime kiln dust in SMA is quite limited at this time. The objective of this research was to evaluate a lime kiln dust as a mineral filler in an SMA using basic tests for asphalt mix design and evaluation. The laboratory evaluation consisted comparing an SMA mixture containing LKD with the same mixture containing a commonly used rock dust mineral filler. The SMA mixtures were subjected to additional testing to assess how the mixtures may perform under severe conditions. Tasks established for the research included the following: 1. Prior studies and experience with the use of lime kiln dust mineral filler containing calcium oxide (CaO) in SMA mixtures were investigated. 2. Basic physical characteristics of a LKD and a common SMA mineral filler were compared. 3. SMA mix designs were performed with a LKD and with the common mineral filler. 4. Moisture damage susceptibility of the test mixtures were determined using the standard protocol in AASHTO T 283. Further conditioning sequences of extended hot water soak times and additional freeze-thaw cycles were used to evaluate potential reactions of the mineral fillers in harsh conditions. 5. Swell tests were performed on the test mixtures to evaluate possible volume change of the specimens due to the reaction of free lime in the LKD. 6. Hamburg Wheel Tracker tests were conducted to further evaluate moisture damage potential of the test mixtures. Lime Kiln Dust Production Lime kiln dust is a fine co-product of lime production. LKD is most commonly generated in high temperature rotary kilns and captured in air pollution control systems such as cyclones, baghouses, and electrostatic precipitators (3). LKD can be generally categorized based on its reactivity which depends on the available (free) lime or magnesia content. The chemical composition of a LKD is a function of characteristics the raw material feed (most often limestone rock) and operating parameters of the kiln (4). Each year, an estimated 2.5 million metric tons of LKD is produced in the United States (4). LKD is used for a variety of beneficial purposes, 2

4 West and James 3 including soil conditioning and stabilization, industrial waste stabilization, Portland cement production, and agricultural uses. Unused LKD is usually stockpiled near the lime plant but can be disposed of in approved landfills. Case Studies of the Use of Lime Kiln Dust as a Mineral Filler in SMA Alabama Lime kiln dust was used in several SMA projects in Alabama between 2001 and According to the Alabama Department of Transportation, a number of these SMA projects exhibited discolored surfaces soon after construction. The discoloration was generally observed as a blotchy grey color. One Alabama project with lime kiln dust did result in more significant pavement problems. This SMA project was constructed on Highway 80 in Selma, Alabama in the summer of This project was intended to correct significant rutting on this heavy trafficked section of roadway. The existing rutted pavement was milled five inches and two layers of SMA were placed to complete the reconstruction. The first layer was a 1-½ inch Maximum Aggregate Size (MAS) SMA which was placed 3 to 3.5 inches thick. The surface layer was a ½ inch MAS SMA that was placed 1.5 to 2 inches. The 1-½ inch SMA mix contained nine percent lime kiln dust and the ½ inch SMA mix contained six percent lime kiln dust. In less than one year after construction, several areas of distress were observed. Flushing of the surface was observed, followed by rutting, shoving, cracking, and potholes. A light tan powder was evident along the edge of pavement in the distressed areas. Cores taken from the pavement in the distressed areas found that the underlying SMA layer was severely stripped. It was suspected that the lime kiln dust in the first layer of SMA contained excessive amount of available calcium oxide. The Alabama Department of Transportation subsequently disapproved LKD as a mineral filler for SMA mixtures. Texas In August 2003, an SMA project was started on SH-105 in Sour Lake, Texas. Paving of the SMA began with a test section on the project located. The construction of the SMA test section was cut short due to extended heavy rains in the area. A few days after the rain, the entire surface of the SMA pavement had turned a grayish white color. The surface of the SMA was friable and the test section had to be removed and replaced. The investigation of the failure led to the mineral filler which was determined to be a lime kiln dust. Subsequent tests on samples of the LKD from the contractor s mineral filler silo indicated the LKD contained 54 percent available calcium oxide content, an unusually high amount. The SMA mix design and TSR tests, which had been conducted several months before, did not indicate any problems. However, lab tests with the QC samples of the SMA did reveal that the mix was reactive in water. It was apparent that the characteristics of the LKD mineral filler had changed from the time the mix designs were performed and when the mineral filler was delivered to the contractor s plant. Kentucky In September 2004, a project using lime kiln dust in SMA was completed on AA highway near Bracken, Kentucky. The mix design contained six percent lime kiln dust and the available lime 3

5 West and James 4 content for this mineral filler was between 20 and 22 percent. No problems were encountered with the use of lime kiln dust at any point in the project. Mix production went well and target roadway densities were met despite a 45 mile haul. Production volumetric properties were very consistent and on target. The pavement has performed very well to date. No pavement discoloration has been observed. LABORATORY EVALUATION Physical Characteristics of LKD and Marble Dust Key physical characteristics of the mineral fillers including gradation, specific gravity, and Rigden voids were determined. The gradations of the mineral fillers were determined using a Coulter Model LS-200 particle size analyzer. Specific gravities of the mineral fillers were determined using the method described in section 21 of ASTM C The Rigden voids test was performed on both mineral fillers in accordance with the procedure described in NAPA publication IS 101 (8). SMA Mix Designs The SMA component materials selected for this study included a standard set of materials used by NCAT in several other SMA research studies. Information on the materials is provided in Table 1. The asphalt binder used in the study was a PG Most agencies would normally specify a higher PG grade, such as a PG instead of the PG for highway construction projects. However, the PG was intentionally selected because it was believed that the softer asphalt grade would accentuate the effects the mineral fillers in the subsequent moisture susceptibility tests. Mix designs were completed in accordance with AASHTO PP 41 Designing Stone Matrix Asphalt (6). Samples were compacted in a Superpave Gyratory Compactor to 75 gyrations. Moisture Damage Susceptibility A series of tests were performed to assess the potential for moisture damage and/or reactions of available lime with water for the SMA mixtures. The first series of tests utilized the industry s most commonly specified moisture damage test, AASHTO T283. This test is also referred to as the modified Lottman test or the TSR (tensile strength ratio) test. Additional moisture damage susceptibility tests were performed with harsher conditioning procedures. Conditioning and testing of these specimens followed the procedure in AASHTO T 283 except that one set of specimens was subjected to two freeze-thaw cycles and another set was subjected five freezethaw cycles before the conditioned tensile strengths were determined. Another series of TSR specimens were subjected to one freeze-thaw cycle, but the hot water soak period was extended for 48 and 96 hours. Samples of the SMA mixtures with the LKD and rock dust were also prepared and tested to evaluate the potential for expansion due to reaction of free lime in the mineral filler with water. Samples were compacted in the Superpave Gyratory Compactor in the same manner as for T283. This yielded 150 mm diameter specimens with approximately 6% air voids and 95 4

6 West and James 5 mm in height. Three specimens were prepared and tested for both mixtures. The specimens were then placed in California Bearing Ratio (CBR) molds and the molds were submerged in 25ºC water. No surcharge weights were placed on the top of the SMA samples allowing free expansion of the samples in the vertical direction. Periodic measurements were made of the specimen heights using the CBR swell apparatus. Samples of both test mixtures were also prepared and tested in the Hamburg Wheel Tracking Device (HWTD) in accordance with AASHTO T Two slabs for each test mixture were compacted to 7±1.0% air voids with a rolling-wheel, kneading slab compactor. Hamburg tests were performed at 50ºC. Results of the initial duplicate samples were significantly different, so additional samples were tested. The air void content of the additional samples were outside of the target range of 7±1.0% desired for this study, but were within the wider range of 7±2% allowed by the test method. RESULTS Physical Characteristics of the LKD and the Rock Dust Test results on the LKD and rock dust mineral fillers are provided in Table 1. The test results show that the particle size distributions for the two mineral fillers are similar. The apparent specific gravity of the LKD sample is slightly lower than the rock dust which indicates that for an equivalent mass of mineral filler, the LKD will occupy a slightly higher volumetric proportion in an SMA mixture. The Rigden voids test evaluates the packing behavior of mineral fillers and provides an indication of the shape of the mineral filler particles. According to research conducted in NCHRP 9-8, Rigden voids is a good indicator of how much the filler will stiffen the asphalt binder (9). The recommendation from this work was that fillers with Rigden voids above 50 may cause the mortar to be excessively stiff and difficult to work. Although the Rigden voids result for the LKD mineral filler was higher than the result for the rock dust, the LKD is below the recommended limit. A chemical analysis of the LKD was done by the supplier in accordance with ASTM C 25 and C The tests indicated an available lime content (CaO) of 19.9 percent. SMA Mix Designs The optimum asphalt content for SMA mixtures, according to AASHTO PP 41, is based on an air void content of 4.0%. To achieve 4.0% air voids, the optimum asphalt content for the LKD SMA mix was 6.4%, and the optimum asphalt content for the rock dust SMA mix was 6.6%. The slight difference in optimum asphalt content for the two mixtures is attributed to the differences in specific gravity for the two mineral fillers. Since the LKD had a slightly lower specific gravity, it occupied a slightly greater volume in the compacted mix, leaving slightly less space for asphalt binder. Both mixtures met the mix design requirements for SMA (7). Based on the mix design results, the LKD seems to behave very similar to the rock dust mineral filler. The expected stiffening effect of the higher Rigden void result for the LKD was not apparent in the mix design properties. Moisture Susceptibility Testing Tensile Strengths and Tensile Strength Ratios 5

7 West and James 6 The results of the standard AASHTO T 283 test, shown in Table 5, indicate that there were no problems with moisture susceptibility of the test mixtures. The results of the LKD SMA were practically the same as for the marble dust SMA. As an additional point of comparison, TSR tests were also conducted using a sample of the LKD from the Texas project with a very high available lime content. To differentiate this high available lime LKD from the LKD used throughout this study, it is referred to here as XLKD. Other than the source of the LKD, these samples were prepared and tested exactly like the samples with the other mineral fillers. TSR results for the XLKD are shown in Table 6. As can be seen, the average unconditioned strength for the XLKD samples is very similar to those of the samples using the other two mineral fillers. However, the average conditioned strength was very low, yielding a TSR of This result clearly demonstrates the problem with lime kiln dust containing high percentages of available lime. Additional moisture damage susceptibility tests were performed with harsher conditioning procedures. For the first series of harsh conditioning, specimens were prepared following AASHTO T283, then subjected to multiple freeze-thaw cycles before the conditioned tensile strengths were determined. The conditioned strengths for the specimens subjected to additional freeze-thaw cycles are illustrated in Figure 1. As expected, these results indicate that the additional freeze-thaw cycles cause a reduction in tensile strengths. However, the loss of strength was much less for the LKD SMA mixture. For the second series of harsh conditioning, TSR specimens were subjected to one freezethaw cycle, then conditioned in hot water for extended times. These results, shown in Table 7, indicate that extending the soak time to 48 hours causes some loss of strength, but that as the soak time is extended further to 96 hours, the tensile strengths increase. For the LKD SMA, the tensile strength after the single freeze-thaw cycle and 96 hour soak was back to the unconditioned tensile strength. The rock dust SMA also had some regain of strength after the 96 hour soak, but it had not recovered as well as the LKD SMA. Interestingly, the LKD SMA samples subjected to the extended soak periods also showed some discoloration on the surface of the specimens. As shown in Figure 2, the LKD SMA specimens had a blotchy white coating apparently from leaching of some lime from the mineral filler. This is believed to be similar to the appearance of discoloration of some lime kiln dust SMA projects in the field. However, as evident from the tensile strength results shown above, this discoloration does not appear to be detrimental to the integrity of the mixture. Swell Tests Results of the swell tests, illustrated in Figure 3, show that most of the volume change occurs in the first day. The figure also shows that the expansion of the LKD MA mixture was less than half of that for the rock dust SMA. This indicates that the small amount of available free lime in the LKD mineral filler is not detrimental to the mixture. Hamburg Wheel Tracking Tests Typical Hamburg WTD results are shown in Figure 4. Hamburg WTD results are commonly analyzed by determining two parameters: the rutting rate and the stripping inflection point (SIP). For this study, the rutting rate is not important since we were primarily interested in evaluating 6

8 West and James 7 the stripping susceptibility of the mixtures. Evaluating the rutting rate of these mixtures was not appropriate since the softer PG binder grade was intentionally selected to better discriminate issues with moisture damage. The Hamburg WTD stripping inflection point results and air voids for each sample are shown in Table 8. On average, the SIP results for the mixtures with the two mineral fillers were similar. The results of the Hamburg tests, however, are highly variable. The Hamburg results for these two mixtures would have been much better if a polymer modified asphalt, as typically used in SMA mixtures, had been used instead of the unmodified PG binder. Discussion An investigation of field experiences with lime kiln dust found only a limited number of projects. A few problems have been reported with the use of some lime kiln dusts used as a mineral filler in SMA mixtures. Some field projects with lime kiln dust in SMA have resulted in premature pavement failures; others projects have only left the pavement discolored, but with no evidence of distress. Still other field projects have had not problems at all. Comparisons were made between a lime kiln dust and a rock dust for use as a mineral filler for SMA mixtures. SMA mixtures with these two materials were subjected to a variety of tests and harsh conditioning sequences to evaluate the potential for adverse reactions and loss of mixture integrity. 1. The physical properties of the lime kiln dust were similar to those of the rock dust mineral filler. The Rigden voids test results indicate that the LKD particles may be more angular than the rock dust. Although this may be expected to stiffen the binder, this effect was not apparent in any of the mix design results. 2. The LKD mineral filler evaluated in this study behaved very similar to a commonly used rock dust mineral filler when included in an SMA mixture. The mineral fillers resulted similar optimum asphalt contents, volumetric properties, and tensile strength ratios. 3. When moisture damage susceptibility specimens were conditioned by extending the soaking period at 60ºC from 24 hours to 48 hours, the tensile strengths of the LKD SMA and the Rock Dust SMA dropped 13% and 23%, respectively. However, more samples were soaked at 60ºC for 96 hours, and the tensile strength of LKD SMA improved to slightly better than the tensile strength for the 24 hour soak. The tensile strength for the 96 hour soaked Rock Dust SMA also improved relative to the 48 hour soak, but remained 12% lower than the 24 hour soak tensile strength. 4. Discoloration of some LKD specimens was evident after soaking. Surfaces of the specimens were blotchy white to grey. However, this discoloration did not correspond to a loss of strength for the mixture. 5. When moisture damage susceptibility specimens were conditioned by additional freezethaw cycles, the loss of tensile strength for the LKD SMA was much less than for the Rock Dust SMA. This indicates that the LKD provides some benefit in resistance to moisture damage. 6. The swell of SMA mixtures was also monitored over a period of four to five days. These results showed that swell of the LKD SMA mixture was less than half of the swell for the Rock Dust SMA. 7. Although the results of Hamburg wheel tracking tests were highly variable, the SMA mixtures with LKD and Rock Dust appear to provide similar Stripping Inflection Points. 7

9 West and James 8 The performance of these mixtures in this test would likely have improved dramatically if a polymer modified asphalt binder had been used in the mixtures. CONCLUSIONS AND RECOMMENDATIONS Laboratory tests indicate that some lime kiln dust can be a suitable mineral filler for stone matrix asphalt. In some cases, LKM may improve the SMA resistance to moisture damage. However, not all lime kiln dusts are the same and may perform differently in laboratory tests and field conditions. Some premature failures of SMA pavements have been attributed to some LKD s, while others have performed satisfactorily. Basic TSR tests can identify problem LKD materials. The available calcium oxide content is believed to be a critical factor for whether or not the LKD is suitable for use as a mineral filler. Further research is needed to better establish an appropriate limit for the available lime content. ACKNOWLEDGEMENTS Funding for this study was provided by Carmeuse Lime Company. The authors gratefully acknowledge their support for this project. REFERENCES 1. MP 8-04 Designing Stone Matrix Asphalt, AASHTO Provisional Standards, June 2004 Edition, American Association of State Highway and Transportation Officials, June Designing and Constructing SMA Mixtures State of the Practice, Quality Improvement Series 122, National Asphalt Pavement Association, March, Kiln Dusts - Asphalt Concrete User Guideline internet document, 4. Miller, M. M., and R. M. Callaghan, Lime Kiln Dust as a Potential Raw Material in Portland Cement Manufacturing Open File Report , U.S. Department of the Interior, U.S. Geological Survey, Collins, R., and S. Cielielski, Recycling and Use of Waste Materials and By-Products in Highway Construction, NHHRP Synthesis 199, Transportation Rsearch Board, Brown, E.R., Haddock, J.E., Crawford, C, Hughes, C.S., Lynn, T.A., and Cooley, L.A., Designing Stone Matrix Asphalt Mixtures, Volume 1- Literature Review, Final Report, NCHRP 9-8/1, Transportation Research Board, July PP (2003), Designing Stone Matrix Asphalt (SMA), AASHTO Provisional Standards, June 2004 Edition, American Association of State Highway and Transportation Officials, June Brown, E.R. and Cooley, L.A., Designing Stone Matrix Asphalt Mixtures for Rut Resistant Pavements, NHCRP Report 425, Transportation Research Board, Anderson, D., Guidelines on the Use of Baghouse Fines, Information Series 101, National Asphalt Pavement Association, February, Brown, E.R, and Cooley, L.A., Designing Stone Matrix Asphalt Mixtures, Volume III, Summary of Research Results, Final Report NCHRP 9-8/3, Transportation Research Board, July

10 West and James 9 LIST OF TABLES TABLE 1. Materials Used in the SMA Mix Designs TABLE 2. Physical Characteristics of the Mineral Fillers TABLE 3. Mix Design Results TABLE 4. Results of the SMA Mixtures at the Same Asphalt Content, Draindown Results TABLE 5. Results of Moisture Damage Susceptibility Tests Using AASHTO T283 TABLE 6. Results of AASHTO T283 Test on the LKD with the High Available Lime Content TABLE 7 Effect of Extended Soak Time on Conditioned Tensile Strengths TABLE 8. Results of the Hamburg WTD Tests 9

11 West and James 10 TABLE 1. Materials Used in the SMA Mix Designs Material Type Source Percent Aggregates Granite (LA abrasion = 36%) Vulcan Materials Co., Columbus Quarry 93.0% Mineral Fillers A. Rock Dust B. Lime Kiln Dust Georgia Marble Co. Carmeuse Natural Chemicals 7.0% 7.0% Asphalt PG Ergon * Stabilizing Fiber Cellulose Interfibe 0.3% * determined by the mix design procedure 10

12 West and James 11 TABLE 2. Physical Characteristics of the Mineral Fillers Characteristic LKD Rock Dust Percent of Distribution Particle Size Particle Size < 90% 49.1 µm 44.7 µm < 75% 29.2 µm 29.6 µm < 50% 14.8 µm 15.0 µm < 25% 7.3 µm 5.8 µm < 10% 3.4 µm 2.2 µm Apparent Specific Gravity Rigden Voids (%)

13 West and James 12 TABLE 3. Mix Design Results Property SMA Criteria 1 LKD SMA Rock Dust SMA Gradation (% Passing) 12.5 mm 9.5 mm 4.75 mm 2.36 mm 1.18 mm 0.60 mm 0.30 mm 0.15 mm mm % min. 6.4% 6.6% Optimum Asphalt Content Air Void Content 4.0% 4.0% 4.0% VMA 17.0% min. 17.1% 17.0% VCA mix < VCA DRC (40.1) 1 Based on NCHRP 9-8 recommendations, some criteria differ from AASHTO MP-8 12

14 West and James 13 TABLE 4. Results of the SMA Mixtures at the Same Asphalt Content, Draindown Results Property Criteria LKD SMA Rock Dust SMA Asphalt Content - 6.5% 6.5% Air 3.9% 4.3% - Ndes Binder Draindown 0.3% max. 300º F 327º F 300º F 327º F 13

15 West and James 14 TABLE 5. Results of Moisture Damage Susceptibility Tests Using AASHTO T283 Property Criteria LKD SMA Rock Dust SMA Avg. Air Voids % 6 ± Avg. Saturation % Avg. Conditioned Strength, psi Avg. Unconditioned Strength, psi Tensile Strength Ratio 0.70 min

16 West and James 15 TABLE 6. Results of AASHTO T283 Test on the LKD with the High Available Lime Content Property Criteria XLKD SMA Avg. Air Voids % 6 ± Avg. Saturation % Avg. Conditioned Strength, psi Avg. Unconditioned Strength, psi Tensile Strength Ratio 0.70 min

17 West and James 16 TABLE 7 Effect of Extended Soak Time on Conditioned Tensile Strengths Conditioned Strengths, psi Hot Water (60 Rock Dust C) Soak Time LKD SMA SMA

18 West and James 17 TABLE 8. Results of the Hamburg WTD Tests. LKD SMA Rock Dust SMA Stripping Inflection Point (cycles) Stripping Inflection Point (cycles) Air Voids Sample Air Voids Sample # (%) # (%) LKD RD LKD RD LKD RD LKD 7a RD LKD Average Average Std. Dev Std. Dev

19 West and James 18 LIST OF FIGURES FIGURE 1. Effect of Freeze-Thaw Cycles on Tensile Strengths FIGURE 2. Photograph Showing Discoloration of LKD SMA Specimen (on right) FIGURE 3. Comparison of Height Changes for Samples Soaked in CBR Molds. FIGURE 4. Typical Hamburg WTD Result Showing Rutting Rate and Stripping Inflection Point. 18

20 West and James LKD SMA Rock Dust SMA Tensile Strength (psi) Freeze-Thaw Cycles FIGURE 1. Effect of Freeze-Thaw Cycles on Tensile Strengths 19

21 West and James 20 FIGURE 2. Photograph Showing Discoloration of LKD SMA Specimen (on right) 20

22 West and James Height Increase (mm) Rock Dust SMA LKD SMA Time (hours) FIGURE 3. Comparison of Height Changes for Samples Soaked in CBR Molds. 21

23 West and James Rut Depth (mm) Rutting Rate (mm/cycle) Stripping Inflection Point, SIP (cycles) Cycles FIGURE 4. Typical Hamburg WTD Result Showing Rutting Rate and Stripping Inflection Point. 22