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TR N EW S 28 8 SE PT EM BE Râ O CT O BE R 20 13 46 Beshears is Aggregate Technology Coordinator, Bureau of Materials and Physical Research, Illinois Department of Transportation, Springfield. Tutumluer is Professor and Paul F. Kent Endowed Faculty Scholar, Department of Civil and Environmental Engineering, University of Illinois at Urbanaâ Champaign. Using recycled materials and by-products inpavements is a sustainable practice that isgaining adoption, particularly for hot-mix asphalt (HMA). HMA materials can be milled off the road surface and recycled. Reclaimed asphalt pavement (RAP) consists of reprocessed HMA pavement material, such as asphalt and aggregates. RAP often contains high-quality, well-graded aggregates that are coated with asphalt cement but may include steel slag, a by-product of steel making. One approach is to incorporate large quantities of RAP into unbound aggregate base and subbase applications for highway construction. Problem Steel slag is an expansive aggregate often added when high frictional properties are required, as in HMA surface courses, particularly when good-quality aggregate is scarce. Steel slag, however, may contain free lime (CaO) and magnesia (MgO), which can react with water and cause the slag to expand. Volume changes of up to 10 percent or more have been attributed to hydration of the calcium and mag- nesium oxides in the recycled steel slag aggregate base course, caused by water seeping into the pave- ment base layer. The amount of expansion depends on the origin of the slag, the grain size and gradation, the hydration of unslaked lime and magnesia, and the age of the stockpile. Solution Documentation and Research The Illinois Center for Transportation compiled a synthesis of research results and state highway agency practices (1). The findings indicated that pavement structures have used RAP successfully as a granular base or subbase material and that the per- formance of a RAP base is often comparable to that of a crushed stone base. The synthesis also revealed that in blending processed RAP materials with virgin aggregates, proper bearing strengths have to be attained, because the RAP bearing capacity is usually less than that of conventional granular aggregate bases. As the virgin aggregate content increases, the dry density and strength properties increase. As a result, characteriz- ing and quantifying the expected range of RAP prop- erties is necessary before application. Steel slag aggregates often have favorable fric- tional properties, high stability, and good durability, with resistance to stripping and ruttingâtherefore they can perform well as base material. The conven- tional way to control the tendency to expand is to weather the steel slag aggregates in stockpiles until the potentially expansive systems stabilize. The length of time for stocking depends on the local temperature and rainfall and on the degree of air moisture saturation throughout the year and may range from 3 to 12 months. Most highway depart- ments require at least 6 months for the aging or cur- ing of steel slags. After curing, steel slag can serve as a valuable secondary aggregate. Project and Findings The objective of the larger research project was to determine the expansive properties for RAP materi- als, especially those including recycled steel slag aggregates that could be used as pavement base mate- rials in Illinois (1). Additional objectives were to determine the maximum acceptable level of expan- sion for different RAP aggregate types, the properties and blending proportions with virgin aggregates, and the effects that RAP materials may have on pave- ment performance. Reclaimed Asphalt Pavement with Steel Slag Aggregate Successful Use in Illinois Pavements S H E I L A A . B E S H E A R S A N D E R O L T U T U M L U E R R E S E A R C H P AY S O F F Stockpile of slag aggregate. P H O TO : ILLIN O IS C EN TER FO R T R A N SPO R TA TIO N
TR N EW S 288 SEPTEM BERâO CTO BER 2013 47 In the laboratory, researchers applied ASTM Test Method D4792 to 17 Illinois RAP materials and vir- gin aggregates to determine expansive characteris- tics. Placed in California bearing ratio (CBR) test molds, the specimens were submerged in a high alkaliâcement water solution with a pH of 12 at 70oC; the specimens soaked for up to 60 days to accelerate hydration reactions or until no further expansion was noted. Some steel slag aggregates showed a potential for expansion of up to 6.2 percent, which is high com- pared with virgin aggregates, such as siliceous gravel and crushed dolomite, which exhibit minor or almost no expansion (Table 1, right). The lower density RAP materials often exhibited more initial settlement or contraction before any kind of expansion. Surface RAP with 92 percent steel slag aggregates and steel slag RAP recorded maximum expansions of 1.69 percent and 1.46 percent, respectively. A clear conclusion from the expansion tests was that RAP materials with slag aggregate that was partly coated with asphalt had much lower tendencies to expand than the virgin steel slag aggregates, which showed high expansion potentials. ASTM D2940 limits expansion values to less than 0.50 percent at seven days when materials are tested in accordance with Test Method D4792. Stone matrix asphalt RAP, steel slag RAP, surface binder RAP with 60 percent steel slag aggregates, and surface RAP with 92 per- cent steel slag aggregates could be used as pavement base course aggregates. Porous and nonporousâthat is, virginâsteel slag aggregates, however, should never be used in the bases or subbases without the proper curing specified in ASTM D2940. Application Illinois has used steel slag only in HMA surface mixes for years and never has allowed steel slag or RAP con- taining steel slag in any other layer of the pavement structure because of concerns about expansion. Like most states, Illinois is pursuing environmentally con- scious practices and has started applying normal RAP for other aggregate uses, notably in the Chicago area, which has sizable stockpiles of RAP. The results of this research led to two significant changes in the specifications: 1. Steel slag RAP is allowed in all levels of the HMA mixes. 2. Steel slag RAP can be used wherever normal RAP is used for aggregate applications. Benefits Although the changes to the specification are too new to project financial benefits, a few cost savings have resulted. Contractors are now able to stockpile their RAP in a single main stockpile, except when the steel slag RAP will be part of the HMA surface for the beneficial frictional properties. This frees up acreage at the contractor sites for other material. Before this research, steel slag RAP not used within 2 years was consigned to land fill. Blending steel slag RAP with the other RAP allows the entire HMA pavement to be milled together. The performance of pavement subgrade applications with steel slag RAP is expected to equal that of pave- ment subgrades built with natural aggre- gates. For more information, contact Sheila A. Beshears, Aggregate Technology Coor- dinator, Bureau of Materials and Physical Research, Illinois Department of Trans- portation, 126 East Ash Street, Springfield, IL 62704; 217-782-7086 or Sheila.Beshears@illinois.gov; or Erol Tutumluer, Professor, Paul F. Kent Endowed Faculty Scholar, Department of Civil and Environmental Engi- neering, 1205 Newmark Civil Engineering Laboratory, MC-250, University of Illinois at UrbanaâChampaign, 205 North Mathews Avenue, Urbana, IL 61801; 217- 333-8637 or tutumlue@illinois.edu. Reference 1. Deniz, D., E. Tutumluer, and J. Popovics. Expansive Char- acteristics of Reclaimed Asphalt Pavement (RAP) Used as Base Materials. Illinois Center for Transportation Project R27- 027 Final Report, Publication No. FHWA-ICT-09-055. Illi- nois Department of Transportation, Springfield, August 2009. http://ict.illinois.edu/Publications/report%20files/ FHWA-ICT-09-055.pdf. EDITORâS NOTE: Appreciation is expressed to G. P. Jayaprakash, Transportation Research Board, for his efforts in developing this article. TABLE 1 Summary of Average Total Expansion Values for Materials Average Total Duration of Expansion Material Expansiona (percent) Test (days) Virgin Steel Slag Aggregates Nonporous steel slag 6.18 49 Nonporous steel slag repeat 5.82 28 Porous steel slag 4.14 49 Steel slag from Illinois District 4 0.28 60 RAP with Steel Slag Aggregates Surface RAP (92 percent) 1.69 44 Steel slag RAP, standard composition 1.46 45 Steel slag RAP 1.13 45 Surface binder RAP (60 percent) 0.24 49 Stone Matrix Asphalt RAP 0.93 45 aComputed after initial settlement, if any. Suggestions for Research Pays Off topics are welcome. Contact G. P. Jayaprakash, Transpor - tation Research Board, Keck 488, 500 Fifth Street, NW, Washington, DC 20001 (202-334- 2952; gjayaprakash@ nas.edu). Slag (left) and virgin limestone aggregate side by side. P H O TO : ILLIN O IS C EN TER FO R T R A N SPO R TA TIO N