Laboratory Performance Characteristics of Sulfur-Modified Warm-Mix Asphalt

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<ul><li><p>Laboratory Performance Characteristicsof Sulfur-Modified Warm-Mix Asphalt</p><p>Samuel B. Cooper III1; Louay N. Mohammad, Ph.D., M.ASCE2; and Mostafa A. Elseifi, Ph.D., M.ASCE3</p><p>Abstract: The objective of this study was to compare the laboratory mechanistic properties of sulfur-modified warm-mix asphalt (WMA)with conventional asphalt mixtures. Three mixtures, two hot-mix asphalt (HMA) and one WMA, were prepared. Mixture One used anunmodified asphalt binder classified as PG 64-22, Mixture Two used a styrene-butadiene-styrene elastomeric modified binder classifiedas PG 70-22, and Mixture Three was a WMA that incorporated a sulfur-based mix additive and a PG 64-22 binder. A suite of testswas performed to evaluate the rutting performance, moisture resistance, fatigue endurance, fracture resistance, and thermal cracking resis-tance of the three mixtures. Results of the experimental program showed that the rutting performance of sulfur-modified WMA was com-parable or superior to conventional mixes prepared with polymer-modified and unmodified asphalt binders. Results of the modified Lottmantest showed that the moisture resistance of the sulfur-modified mixture was comparable to conventional mixes. Results of the fracture testsshowed that sulfur-modified WMA is more susceptible to cracking than conventional mixes, given its stiff characteristics. However, giventhese stiff properties, the higher modulus of sulfur-modified mixtures will reduce the magnitude of strain induced in the pavement. Thermalstress restrained specimen test results showed that the sulfur-modified WMA had greater fracture stress than the polymer-modified mixture.However, there was no statistical significance between the average fracture temperatures for the mixes tested. DOI: 10.1061/(ASCE)MT.1943-5533.0000303. 2011 American Society of Civil Engineers.</p><p>CE Database subject headings: Sulfur; Mixing; Asphalts; Laboratory tests; Thermal factors.</p><p>Author keywords: Sulfur; Warm-mix asphalt; Superpave; Binder; Thiopave; Semicircular bend.</p><p>Introduction</p><p>During the 1970s and 1980s, attempts were made to use sulfur as abinder extender to reduce the amount of asphalt binder required inmixtures and to improve the mix mechanical characteristics (Timmet al. 2009). The interest in using sulfur in hot-mix asphalt (HMA)was driven by the abundance of this natural resource and the desireto offset the cost of pollution control associated with the stockpilingof this additive (Lee 1975). However, the concept of using sulfur inHMA was abandoned in the 1980s after environmental and safetyproblems were encountered during installation and doubts aboutthe cost viability of the modification were expressed (Deme andKennedy 2004). Segregation of the additive from the binder wasalso reported because of the large difference in density betweensulfur and asphalt binder (Strickland et al. 2008).</p><p>In spite of installation difficulties, sulfur modification wasreported to be effective for enhancing the mechanical performanceand stiffness characteristics of the mixture over conventional</p><p>mixtures (Kennedy et al. 1977). With the recent increase in theprice of liquid asphalt, a petroleum-based product, the use of sulfuras a binder extender appears economically attractive. The conceptof sulfur extended asphalt (SEA) reappeared with the developmentof a new generation of a solid dust-free sulfur product, known asThiopave. Many of the earlier safety problems appear to have beensolved, as long as the mixture is produced at a target mixing tem-perature of 135 5C (Taylor et al. 2010). In addition, the newadditive does not need to be preblended in the plant with thehot binder, because it is added during mixing of the aggregates afterthe asphalt binder is added. Because sulfur-modified asphalt mix-ture needs to be produced at a mixing temperature that is lower thanthe required mixing temperature for conventional HMA, the newgeneration of SEA should be used with an asphalt mixture witha lower mixing temperature such as warm-mix asphalt (WMA).Because WMA is designed to reduce mixing temperature duringproduction to 16 to 55C lower than typical HMA, the use of sulfurin the production of WMA may offer the potential to reduce energyand asphalt consumption in the preparation of asphalt mixtures.</p><p>To evaluate the effectiveness of the new generation of sulfur ad-ditives, the objective of this study was to compare the mechanicalproperties of sulfur-modified WMAwith conventional asphalt mix-tures. A commonly used wearing course mixture was prepared bymixing aggregate blends with two virgin binders, a straight asphaltbinder classified as PG 64-22 and a polymer-modified binderclassified as PG 70-22. Laboratory testing evaluated the rutting per-formance, moisture resistance, fatigue performance, fracture resis-tance, and thermal cracking resistance of the produced mixtures byusing the Hamburg loaded-wheel tester (LWT), the flow number(Fn) test, the repeated shear at constant height (RSCH) test, themodified Lottman test, the beam fatigue test, the semicircular bend(SCB) test, the dissipated creep strain energy (DCSE) test, and thethermal stress restrained specimen test (TSRST).</p><p>1Graduate Research Assistant, Dept. of Civil and EnvironmentalEngineering, Louisiana State Univ.</p><p>2Irma Louise Rush Stewart Distinguished Professor, Dept. of Civiland Environmental Engineering, Director, Engineering Materials Charac-terization Research Facility, Louisiana Transportation Research Center,Louisiana State Univ., 4101 Gourrier Ave., Baton Rouge, LA 70808(corresponding author). E-mail: louaym@lsu.edu.</p><p>3Assistant Professor, Dept. of Civil and Environmental Engineering,Louisiana State Univ.</p><p>Note. This manuscript was submitted on November 30, 2010; approvedon March 2, 2011; published online on August 15, 2011. Discussion periodopen until February 1, 2012; separate discussions must be submitted forindividual papers. This paper is part of the Journal of Materials in CivilEngineering, Vol. 23, No. 9, September 1, 2011. ASCE, ISSN 0899-1561/2011/9-13381345/$25.00.</p><p>1338 / JOURNAL OF MATERIALS IN CIVIL ENGINEERING ASCE / SEPTEMBER 2011</p><p>J. Mater. Civ. Eng. 2011.23:1338-1345.</p><p>Dow</p><p>nloa</p><p>ded </p><p>from</p><p> asc</p><p>elib</p><p>rary</p><p>.org</p><p> by </p><p>UN</p><p>IVER</p><p>SITY</p><p> OF </p><p>REG</p><p>INA</p><p> LIB</p><p>RARY</p><p> on </p><p>05/1</p><p>3/13</p><p>. Cop</p><p>yrig</p><p>ht A</p><p>SCE.</p><p> For</p><p> per</p><p>sona</p><p>l use</p><p> onl</p><p>y; al</p><p>l rig</p><p>hts r</p><p>eser</p><p>ved.</p></li><li><p>Background</p><p>The original experiences with sulfur-extended asphalt was reportedin literature (Lee 1975; Kennedy et al. 1977; Mahoney et al. 1982).Sulfur was typically added to the asphalt binder at a mixing temper-ature of 150C at a ratio ranging from 1=5 to greater than 1. At alow modification rate, sulfur influenced the rheological propertiesof the binder, which resulted in an increase in binder stiffness and areduction in viscosity and ductility. At high sulfur content, the ad-ditive acted as filler and improved the workability of the mixtureand its mechanical strength. At a modification rate approaching50%, SEA improved the mixture engineering properties over con-ventional asphalt mixtures (Kennedy et al. 1977).</p><p>Recent investigations of the new modified sulfur mix additivetechnology were reported in literature (Strickland et al. 2008; Timmet al. 2009). The sulfur additive, usually added at a ratio rangingfrom 30 to 40% of the binder weight, consists of pretreated solidpellets that melt at a temperature above 120C. The pellets are pre-treated to reduce emissions of harmful pollutants such as hydrogensulfide gas during production and to lower mixing and compactiontemperature required for the modified mixture. During mixing, partof the sulfur dissolves into the binder at a high temperature andreduces its viscosity. The remaining part precipitates out as the mix-ture cools and crystallizes as part of the asphalt matrix surroundingthe coarse aggregate. These sulfur crystalline particles stiffen themixture and act as a strengthening agent at high temperature,resulting in improved rutting resistance. Sulfur modification alsoacts as a binder extender, which results in a decrease in the required</p><p>binder content in the mixture by approximately 20 to 25% by vol-ume (Kentucky Production Evaluation List 2010). Given the differ-ence in density or specific gravity between the binder and sulfur, itis recommended to maintain the volume fraction of the total binderphase in the modified mixture based on the following relationship(Strickland et al. 2008):</p><p>Sulfur Binder% 100AR100R PsR Gbinder</p><p>1</p><p>where Sulfur + Binder % = binder and sulfur content in the mixture;A = percentage of binder by weight in conventional mixture (%);R = sulfur to binder specific gravity ratio (approximately 2.0); Ps =weight percentage of sulfur in the modified blend; and Gbinder =specific gravity of the unmodified binder.</p><p>Strickland and coworkers (2008) evaluated the performance ofsulfur-modified mixtures in the laboratory. Rutting performance ofthe prepared mixtures was evaluated using the asphalt pavementanalyzer (APA) test at 58C, and the mixture stiffness moduluswas measured at a temperature ranging from 10 to 30C. In ad-dition, the low temperature performance was evaluated by using theTSRST. Results of this analysis indicated that the rutting resistanceand stiffness modulus of the mixture are improved. In addition, themodified sulfur additive enhanced the elongation properties of themix at low temperatures. A comprehensive experimental programalso evaluated the moisture resistance and dynamic modulus of sul-fur-modified asphalt mixtures (Timm et al. 2009). Results showedthat sulfur-modified asphalt mixture had a lower tensile-strength</p><p>Table 1. Job Mix Formula of the Asphalt Mixtures</p><p>Mixture designation WC70CO WC64CO WC64SU</p><p>Mix type 19.0 mm (3=4 in) Superpave</p><p>Aggregate #67 LS 36% 36% 36%</p><p>#78 LS 24% 24% 24%</p><p>#11 LS 34% 34% 34%</p><p>CS 6% 6% 6%</p><p>Binder type PG 70-22M PG 64-22 PG 64-22</p><p>Binder Content (%) 4.0 4.0 3.0a</p><p>% Gmm at NIni 87.0 87.0 87.0</p><p>% Gmm at NMax 97.6 97.6 97.6</p><p>Design air void (%) 3.7 3.7 3.7</p><p>VMA (%) 13 13 13</p><p>VFA (%) 68 68 68</p><p>Metric sieve Composite gradation blend</p><p>37.5 mm (1 1/2 in) 100 100 100</p><p>25.0 mm (1 in) 100 100 100</p><p>19.0 mm (3=4 in) 96 96 96</p><p>12.5 mm (1=2 in) 75 75 75</p><p>9.5 mm (3=8 in) 59 59 59</p><p>4.75 mm (No. 4) 43 43 43</p><p>2.36 mm (No. 8) 31 31 31</p><p>1.18 mm (No. 16) 20 20 20</p><p>0.600 mm (No. 30) 11 11 11</p><p>0.300 mm (No. 50) 8 8 8</p><p>0.150 mm (No. 100) 6 6 6</p><p>0.075 mm (No. 200) 4.5 4.5 4.5</p><p>Note: Limestone (LS); coarse sand (CS).a60=40 KB: WMA with 60% PG 64 22 40% Thiopave additive.</p><p>JOURNAL OF MATERIALS IN CIVIL ENGINEERING ASCE / SEPTEMBER 2011 / 1339</p><p>J. Mater. Civ. Eng. 2011.23:1338-1345.</p><p>Dow</p><p>nloa</p><p>ded </p><p>from</p><p> asc</p><p>elib</p><p>rary</p><p>.org</p><p> by </p><p>UN</p><p>IVER</p><p>SITY</p><p> OF </p><p>REG</p><p>INA</p><p> LIB</p><p>RARY</p><p> on </p><p>05/1</p><p>3/13</p><p>. Cop</p><p>yrig</p><p>ht A</p><p>SCE.</p><p> For</p><p> per</p><p>sona</p><p>l use</p><p> onl</p><p>y; al</p><p>l rig</p><p>hts r</p><p>eser</p><p>ved.</p></li><li><p>ratio (TSR) after curing but greater dynamic moduli for all combi-nations of test temperatures and frequencies.</p><p>Cooper et al. (2011) evaluated the effects of sulfur-modifiedWMA on the predicted performance from the Mechanistic-Empirical Pavement Design Guide (MEPDG) and assessed the lifecycle costs of pavement structures constructed with this sustainablealternative. To achieve this objective, three typical pavement struc-tures were analyzed at three traffic levels (low, medium, and high).Based on the results of the analysis, the use of sulfur-modifiedWMA improved the predicted rutting and fatigue performancesand the overall pavement service lives over conventional mixturesat all traffic levels. Results also indicated that sulfur modificationhas the potential to reduce production and life cycle costs whencompared to a conventional asphalt mixture prepared with the samebinder grade (Cooper et al. 2011).</p><p>Experimental Program</p><p>Awearing course mixture with a nominal maximum aggregate size(NMAS) of 19 mm was prepared by mixing aggregate blends withtwo binders: an unmodified binder classified as PG 64-22 (hereafterreferred to as WC64CO) and a styrene-butadiene-styrene (SBS)elastomeric modified binder classified as PG 70-22 (hereafter re-ferred to as WC70CO). This mixture is a Superpave Level 2 designthat was performed according to AASHTO PP 28 (2000c) andSection 502 of the Louisiana Standard Specifications for Roadsand Bridges (2006). Specifically, the optimum asphalt cement con-tent was determined to be 4% based on the following volumetriccriteria [void in the total mixture (VTM) = 2:5 4:5%; voids in themineral aggregate (VMA) 12%; voids filled with asphalt (VFA)= 68% 78%] and densification requirements (%Gmm atN initial 89, %Gmm at Nfinal 98). The job mix formula for thethree mixtures utilized in this study is summarized in Table 1.The same crushed siliceous limestone aggregate structure was usedin all the mixtures evaluated in this study. Mixture Three, referredto in this paper as WC64SU, was a WMA prepared with sulfur-based additives and PG 64-22 binder. Forty percent of the totalbinder weight was replaced with the sulfur additive. Hence, the de-sign PG 64-22 binder content was reduced from 4.0 to 3.0% bytotal weight of the mix, according to Eq. (1). This represents a25% reduction in asphalt binder content compared with MixturesWC64CO and WC70CO. For compatibility reasons, the sulfur ad-ditive is only recommended for use with unmodified asphalt binder.</p><p>The mixing temperature for Mixtures WC64CO and WC70COwas 163C and the mixing temperature for WC64SU was 140C.The mixing process for the sulfur-modified mixture is illustratedin Fig. 1. The binder preparation consists of adding a compactionadditive (CA 100) at 1.0% and an antistripping agent (Akzo NobelKB 2550) at 1.0% of the heated binder weight. The compactionadditive allows the mixture to be prepared at a lower temperaturethan with conventional HMA. The sulfur pellets used in theWC64SU mixture were heated to 60C before addition to theaggregate-binder blend. The heated binder along with the compac-tion agent and antistrip additive were then mixed with the hotaggregates (140C), followed by adding the preheated sulfur pelletsand mixing thoroughly to ensure that all pellets were melted.</p><p>The research team conducted a preliminary factorial to deter-mine the optimum proportions of sulfur additives. The LWT andSCB tests were conducted as part of the screening factorial. Awear-ing course and a base course mixture were evaluated using a sulfurcontent ranging from 30 to 40% by weight. The optimum percent-age of sulfur additive was determined to be 40%.</p><p>Performance Testing</p><p>Laboratory performance testing included evaluation of the ruttingperformance, moisture resistance, fatigue performance, fracture re-sistance, and thermal cracking resistance of the prepared asphaltmixtures by using the Hamburg LWT, Fn test, RSCH test, modifiedLottman test, beam fatigue test, SCB test, DCSE test, and TSRST.Table 2 presents the test factorial conducted for the three mixturesevaluated in this study and the number of specimens tested. Trip-licate specimens were considered for each test, except for the LWT,where two specimens were tested. All specimens were compactedto an air void level of 7...</p></li></ul>

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