Great asphalt pavements begin with aggregates like slag BY TIMOTHY R. MURPHY

to low volume applications. The pastdecade has seen the increased andsuccessful use of Reclaimed AsphaltPavement (RAP), Warm Mix Asphalt(WMA), Reclaimed Asphalt Shingles(RAS), and Ferrous; Iron and Steel Slag

PER OR ANCEGand improvements with constructionequipment and techniques. This evolu­tion has taken decades and it isn't over­-continued ingenuity by the asphalt in­dustry makes asphalt pavements lastlonger and perform better from high

40 DECEMBER 2013

The evolution of the asphalt indus­try has included advancements to

pavement thickness design and mix­ture selection methodology, improve­ments to mix design methods, en­hancements to production facilities,

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During the perpetual pavement construc­tion of this section of Interstate 70 in 2002and 2003, the contractor and engineerworked together to make the project a suc­cess. 1-70 got its facelift from just east ofMarshall to the Martinsville Interchange inIllinois. Champaign Asphalt performed thework from the Mill Creek Bridge eastward,placing 9,000 metric tonnes of mix. HowellPaving/Howell Asphalt performed the workfrom the bridge westward, placing 19,000metric tonnes. The DOT reports the road­way is still performing at 100 percent.

(Slag). The successes seen today havecome from the hard work of agencies,contractors, consultants, academia,and equipment and material suppliers.

MIX DESIGN PHILOSOPHIESThe asphalt mix design philosophiesused over the past several decades­such as Hveem, Marshall, Texas Gyra­tory and Superpave-each broughtstrength to the roadway and runwaybuilding industry. These improve­ments occurred as traffic volumes andloadings continued to climb through­out the industrial world.

With added stress due to loading onasphalt pavements, improved aggre­gate quality, strength and classifica­tion measurements became the normin conjunction with a better under­standing of the importance of voids,voids in the mineral aggregate (VMA),and voids filled with asphalt (VFA); akaVolumetrics. The overuse of inexpen­sive beach sands, dirty aggregates andsingle aggregate stockpile productionwere identified as the primary causesfor premature pavement failures. Eachhas been fazed out of standards andspecs over the years. The use of en­gineered sharp sands; clean, hard ag­gregates; and multiple stockpiles ofaggregate for input now ensure thatlonger lasting roadways are built.

In addition to stronger asphalt pave­ments is the need to provide saferroadways. Agencies have recognizedthis for years and most now have up­dated geometric criteria and higherfrictional requirements of the coarseaggregate as well as improved de­sign, production and constructionstandards. As stability and friction aretypical needs of high speed, high vol­ume asphalt facilities, many asphaltcontractors look for one product tofill both needs. Stability and frictionability come from aggregates that arehigh in crush, hard, durable and reus­able. According to the Industrial Re­sources Council (IRC), air cooled blastfurnace slag and steel slag in particularprovide good rutting resistance andsuperior friction properties, makingthem a choice aggregate for the sur­face course. The cost-benefit analysis

of the past, present and future con­tinue to lead slag producers 0 supplyiron and steel slag for this importantsafety feature.

SLAGSBlast furnace (BF) slag is formed wheniron ore or iron pellets, coke and aflux (either limestone or dolomite) aremelted together in a blast furnace.When the metallurgical smelting pro­cess is complete, the lime in the fluxhas been chemically combined withthe aluminates and silicates of the oreand coke ash to form a non-metallicproduct called blast furnace slag. Dur­ing the period of cooling and harden­ing from its molten state, BF slag canbe cooled in several ways to form anyof several types of BF slag products.

In a blast furnace, combustion ma­terial and ore are supplied from thetop while an air flow is supplied fromthe bottom of the chamber. This forc­es the chemical reaction to take placethroughout the ore, not only at thesurface.

Steel furnace slag is produced in abasic oxygen furnace (BOF) or an elec­tric arc furnace (EAF). Hot iron and/orscrap metal are the primary metals tomake steel in each process. Lime is in­jected to act as a fluxing agent. Thelime combines with the silicates, alumi­num oxides, magnesium oxides, man­ganese oxides and ferrites to formsteel furnace slag, commonly calledsteel slag. Slag is poured from the fur­nace in a molten state. After coolingfrom its molten state, steel slag is pro­cessed to remove all free metallics andsized into products.

Basic oxygen furnace (BOF): Ox­ygen is blown into the furnace ves­sel through a water-cooled oxygenlance, oxidizing carbon and the oth­er unwanted elements in the molteniron. Fluxes are added to removeother unwanted elements yieldinghigh quality steel.

Electric arc furnace (EAF): Chargedmaterial is heated to a liquid state bymeans of an electric current. The elec­tricity has no electrochemical effect onthe metal making it perfectly suitedfor melting scrap.

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Steel slag is processed as an air­cooled material. The National SlagAssociation states the metallicsare magnetically separated andthe remaining slag is sized intoconstruction aggregates, used asan agricultural soil amendment, asa raw ingredient in Portland ce­ment production, as an environ­mental remediation material andmany other uses.

The Oregon Department of Trans­portation Research Report No. OR­RD-00-09 of April 2000 stated: "Af­ter the steel slag has been crushedand graded into the sizes desiredit is stockpiled for delivery. Asphaltconcrete has been produced andthe pavement constructed readily inOregon when crushed steel slag isused as a portion of the aggregate."

As discussed in the article titled"Mix in Slag" in the February 2013issue of AsphaltPro, before an ag­gregate is considered for use onan agency project, whether it is foraggregate base, HMA or other, thequality of the mined materials mustgo through rigorous agency evalua­tion to determine a variety of quali­tative measures, including:• Safety - Friction testing• Angularity - Fracture testing• Hardness - LA Abrasion• Durability - Soundness• Absorption - Specific Gravity

Slag meets and exceeds all ofthese quality (ASTM 05106) mea­sures time and again. (See Table)

Table 1. Typical Stone Matrix AsphaltSpec Using Slag

SIEVE MINIMUM MAXIMUM

3f4-inch (19-mm) nla 100

1Jz-incb(12.5-mm) 90 99

3/r inch (9.5-mm) 50 85

#4 (4.75-mm) 20 40

##8 (2.36-mm) 16 24

#200 (0.60O-mm) 8 11

Volumetrics

N....=80; Voids =4%, VMA=17.0VFA =75-80, In-placedensity=94·97 ofG....

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SLAG SUCCESS STORIESSlag has been used as the aggre­gate of choice in the construction ofmany long lasting asphalt pavements.Around the globe, slag exceeds ag­gregate requirements for asphalt useand the transition to Superpave onlyhighlighted slag's features:1. Coarse aggregate is produced at

100 percent two-faced fracturedfaces, rough surface texture, goodparticle shape, and meets the flatand elongated requirements.

2. Fine aggregate at a minimum of 45percent fine aggregate angularity.In addition slag "may be used as a

friction aggregate" in its first and sub­sequent uses. At the Williams and Mar­garet intersection in Thornton, 111., asign reads: "Home of Thornton Quar­ry; Largest Limestone Quarry in theWorld." There a successful steel slag,stone matrix asphalt (SMA) rehabilita­tion of the intersection got the state'sattention. The Illinois Department ofTransportation (lOOn has adopted thismixture as the standard for use on inter­states in Chicagoland and at high load,high traffic situations where a high fric­tion coarse aggregate is required.

Several slag projects are now morethan 20 years old and still performing.The extensive use and success of slagin perpetual asphalt pavements haspresented another opportunity; thatthe slag coarse fraction from thesecold millings (RAP) are now used asecond time in new surface coursemixtures and again, demonstratingthe global stewardship of resources.For their continuous long-term ef­forts, rOOT was awarded the 2012Green Governments Award for edu­cation and outreach, transportation,efficiency and conservation.

SOCIETY, SUSTAINABILITY,FUTUREBecause of the size of the iron andsteel industry in the United States,there are thousands of people em­ployed in the slag industry. Addi­tionally, slag is a sustainable (green)material that can be reclaimed andreused multiple times. Using iron andsteel furnace slag will help preserveour natural resources. Based on nu­merous environmental tests, studies

and reviews by governmental agen­cies and the iron and steel industry,we know that iron and steel slag is asafe and valuable resource, and weencourage its continued use as a vi­able environmentally friendly productfor building and maintaining the Unit­ed States' infrastructure.

Because slag is a co-product thatcomes from the manufacturing of ironand steel; both slag products scorehigh on first use per the Greenroads™

Rating System (Manual v1.5, p. 396).Slag is a regional material that reduc­es trucking emissions and the need toimport aggregates from far away, andit works well with recycled materialssuch as RAp, ground tire rubber andasphalt shingles.

Due to the successful performanceand numerous uses of slag over the pastseveral decades, IDOT allows RAP slagto be used in all mixes and all lifts ofasphalt. Additionally, lOOT counts thecoarse aggregate slag in high volumesurface mixes as part of the friction ag­gregate requirement, because of boththe fracture strength and friction sup­plied to the pavement structure.

Finally, as the drive for Total Recy­cle Asphalt continues, lOOT recentlyawarded several projects that allowthe use of up to 100 percent recycledaggregates where one project uses72 percent slag and 28 percent RAP.

"We recycle and use over a milliontons of reclaimed materials on ourhighways annually as a matter of ev­eryday practice," Illinois Transporta­tion Secretary Ann L. Schneider said."We believe Total Recycle Asphalt willtake Illinois to a new level of environ­mental sustainability, and we hope itwill inspire others to follow our lead."

These new generation mixes use lo­cally available materials, encourageingenuity, reduce the overall carbonfootprint, and provide for long last­ing, strong and durable asphalt road­ways time and again when using slagas a component. It is quite possiblethat these new generation mixes us­ing slag are the most recycled hot mixasphalt pavements ever produced onthe planet. D

Timothy R. Murphy, RE., is the pres­ident of Murphy Pavement Technolo­gy, Inc., in Chicago.