Source Quality Aggregates: an Economic Discussion FROM TRB
Editor’s Note: Asphalt professionals know getting hold of quality aggregate is becoming more difficult and costly in some areas. Recycling materials and blending virgin materials provide some solutions, but only if appropriate quality can be assured for the agencies letting the jobs/designs. Panelists for a Transportation Research Board (TRB) presentation on this topic, chaired by Andrew Dawson of the University of Nottingham and chair of TRB’s aggregates committee, drew on NCHRP Synthesis 524 to provide guidance.
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Erol Tutumluer, of the University of Illinois at Urbana, Champaign, and Maziar Moaveni, of Savannah State University, Savannah, Georgia, are two of the authors of NCHRP Synthesis 524 report and discussed Aggregate Quality Requirements for Pavements during the presentation for the Transportation Research Board (TRB). Sand, gravel and crushed rock are the second most consumed resource after water. Each person in the United States uses about 8 tons of aggregate per year. That’s about 3 billion tons extracted per year, taking up about 44 billion cubic feet of space, which is equivalent to about 230,000 U.S. football fields, each 30 feet deep. That’s about the area of D.C. to a depth of 100 feet every four years. We, as an industry, don’t want to extract that much and leave that big hole. Because transporting material from quarries and remote sources into the areas of demand is costly, we must make the best use of what we have wherever projects occur. To reduce waste at various sites, the presenters put forth the following: • Don’t over-specify, but instead, only specify the qualities that are needed for the desired application and performance; • Use available aggregates as best we can through selecting and blending; and • Reliably quantify what we have so we know the relevant characteristics available to us. As a step in this direction, NCHRP Synthesis 524 was developed.
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EROL TUTUMLUER, PH.D.: PROCEDURES FOR APPROVING SOURCES
Tutumluer acknowledged that methods of approving an aggregate source vary depending on the agency practices and most agencies provide lists of pre-approved sources on their websites. Approving an aggregate source may require that certain aggregate materials pass some quality requirements from that aggregate source. You’ll encounter different aggregate classes (and acceptance requirements) for different pavement applications. Agencies also have established aggregate quality classes and associate the class with the use of aggregate material in a certain pavement layer application. For example, the Indiana Department of Transportation (DOT) has three general ratings for fine aggregates: • A5 is for natural sand approved in all pavement applications; • B5 is for applications that allow use of manufactured sands or fine aggregates; and • G5 is for fine aggregates that are not approved for use. IDOT also has seven coarse aggregate classes starting with Class AP as the highest quality. On the aggregate quality classes, Tutumluer showed typical quality requirements established by state agencies and Canadian provinces. In the charts on page 46, you see virgin aggregates and recycled aggregates specifications quality requirements for Minnesota DOT. Within the charts, you see a detailed quality class specification going from 1 to 6, which is a common thread for states or agencies to adopt—going from 1 to 6 with numbers or A,B,C,D, type classes. There’s a fairly detailed requirement listed, a lot of them related to the deleterious materials such as the shale content, maximum amount of fines, some L.A. abrasion requirements, crushing requirements, and so on.
38% of agencies do not have approved lists of aggregates. You can clearly see that in the case of recycled aggregates such as RAP, maximum bitumen content and other deleterious materials are listed with maximum allotments. Tutumluer explained, of the agencies that reported, 18 indicated they have established at least one quality class for one or more aggregate types, and this includes blast furnace slag, RAP, and so on. Aggregate quality classes for use in pavement layers are established according to the pavement layer’s application. • Asphalt surface course • Asphalt base course • Asphalt surface treatment • Portland cement concrete • Aggregate base course • Aggregate subbase course • Permeable drainage layers • Filter materials for PCC pavements (One that Tutumluer didn’t mention is drainage surface aggregates (DSA), which are discussed in depth in the article “TRB Discusses Driving Surface Aggregate Specs” at https://theasphaltpro.com/articles/trb-dsa-specification/) He pointed out that the responding states generally have less strict requirements for aggregates to be used in the asphalt binder course than in the asphalt surface course. For example, Iowa permits the use of classes A through D, steel slag in a proportion of 25 percent or less, RAP and RCA of greater than 50 percent in the binder course, whereas the state specifies five aggregate classes (Types 1 through 5) based on their frictional characteristics for use in surface courses. You can see how different state agencies establish different classes.
