Recycled Materials and Byproducts in Highway Applications—Summary Report, Volume 1 (2013)

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37 Background Approximately one scrap tire is generated per person in the United States every year (RMRC 2008). Approximately 30 mil­ lion of these tires can be used for retreading, which leaves about 250 million scrap tires in need of alternative uses or disposal. The main scrap tire byproducts, as defined on the RMRC website, are: • Whole tires: used as­is with no post­processing. • Slit tires: cut in half or sidewalls separated from the tread. • Shredded or chipped tires: 4 in. by 4 in. (100 by 100 mm) to as large as 9 in. by 18 in. (229 by 457 mm). (Note: there are no equivalent sieve sizes for these measure­ ments; sizing is done by manual measurements or visual observations.) • Ground rubber: ranging in sieve size from 3/4 inch to the No. 100 sieve (19 mm to 0.15 mm) and regular in shape. • Crumb rubber: ranging in sieve size from No. 4 to the No. 200 sieves (4.75 mm to less than 0.075 mm). ASTM D6270 Standard Practice for Use of Scrap Tires in Civil Engineering defines tire­derived aggregate (TDA), with different definitions for shredded, chipped, or ground rubber byproducts. Further definitions for scrap tire byproducts are provided by the Rubber Manufacturers Association (RMA 2006). This reference notes that ground tire byproducts are generated by tire buffings (no specific size requirements), and processed whole tires that are sorted into four size­based categories: • Tire buffings: byproduct of the retreading industry. • Coarse rubber: No. 4 to 1 in. sieve sizes (4.75 to 25 mm). • Ground rubber: No. 80 to No. 10 sieve sizes (0.177 to 2.0 mm). • Fine ground rubber: No. 40 to No. 80 sieve sizes (0.037 to 0.177 mm). Information found in the literature used these terms, some­ times interchangeably, with no clear indication of the size of the scrap tire byproduct actually used in the project. The lack of consistency in the use of terms and definitions made it difficult to compare results from various studies. Additional information can be found at the following websites: • Rubber Manufacturer’s Association: www.rma.org • Rubber Pavement Association: www.rubberpavements. org Literature review Summary A number of highway applications were found for scrap tire byproducts. Information was collected for byproducts used in embankments, fills, PCC, HMA, and crack sealants. Sev­ eral researchers reported soil improvements when combining TDA with soil. Laboratory study of soil–TDA combinations of embankment fill had higher strength and lower unit weight, greater resistance to lateral sliding of embankments on geo­ textile layers, low active earth pressure and higher interface resistance, and greater resistance to bearing failure because of lighter weight. Compacted TDA had a significantly lower thermal conductivity than conventional soils that can provide good insulation. TDA base was used successfully when designed using the standard Boussineq’s solution for embankment design. Field studies of soil–TDA combinations for an MSE wall showed that conventional design methods for the MSE wall can be used with soil–TDA backfill. Leachates were evalu­ ated for TDA–soil mixtures. Drinking water standards were occasionally exceeded when the byproduct was submerged in water; however, the concentrations were not detectable a short distance away. The advantages to using crumb rubber in PCC were decreased unit weight, porosity, thermal conductivity, and chloride penetration. The byproduct also increased tough­ ness (the ability to exhibit large deformations prior to fail­ ure). Reported disadvantages were a loss of compressive and tensile strengths. Combining crumb rubber with fly ash in PCC applica­ tions somewhat improved the compressive strength com­ pared with crumb rubber without fly ash in PCC applications. Crumb rubber in PCC changed the damping characteristics used in seismic designs of structural concrete. One report noted that crumb rubber could be used in precast appli­ cations to produce light­weight low­strength panels with good insulation and noise damping properties. Larger­size scrap tire byproducts tended to separate during construc­ tion with PCC slurries and there was a maximum size that can be used without segregation. PCC slurries with larger scrap tire byproducts proved difficult to finish and were relegated to use in applications that were covered (e.g., soil cap). chapter eight Scrap Tire ByproducTS

38 National Institute for Occupational Safety and Health noted emissions for crumb rubber HMA were higher than for conventional HMA and the temperature needed to be kept as low as possible. The use of crumb rubber in HMA production occasionally produced eye, nose, and throat irritations for the paving crews, which was related to the total measured particulates. The use of warm mix asphalt technologies was shown to have the potential for keeping temperatures lower than standard temperatures while still maintaining workability. Applying a hot crumb rubber modi­ fied binder over paving fabric resulted in wrinkling of the fabric and a tendency for the fabric to stick to the pneumatic tire rollers. agency Survey reSuLTS for Scrap Tire ByproducTS Ground tires and crumb rubber (wet or dry process) were most frequently used in HMA applications, followed by emulsions and crack sealants (Table 15). The most com­ monly used application for shredded tires or chipped tires was in embankments. Table 16 summarizes which scrap tire byproducts were used by each state agency (Figure 14). Only Vermont used slit tires as drainage material and only Texas used whole tires in embankments. Three states reported hav­ ing used scrap tire byproducts, but the type of byproduct used was unknown. HoT Mix aSpHaLT appLicaTionS In HMA applications, the performance­grade (PG) binder specification designation did not consistently change with the addition of the crumb rubber. Any changes in the asphalt cement grade were a function of the original binder actual temperature grading rather than the specification grading temperatures. When crumb rubber (continuous process) is added, the maximum summer temperature for which the binder could be used increased by about 1.2°C to 1.5°C of the true temperature grading for each 1% of crumb­rubber modified (CRM). The low winter temperature was decreased by about 0.2°C for each 1% of CRM. Crumb rubber in HMA open­ and gap­graded wearing sur­ face significantly lowered the noise level of the tire–pavement interaction when compared with conventional dense­graded HMA. However, it was not clear if the reduction was a func­ tion of the crumb rubber or the gradation. It can be noted that the high film thickness and low draindown of the CRM binders facilitated the use of more open­graded HMA than could be achieved with an unmodified binder. Combinations of CRM asphalt binders and RAP were used in the same mixes. The laboratory work showed that care was necessary in the design phase to make sure that the combined binder properties had acceptable high temperature rheology and were suitably crack resistant at cold temperatures. Embank. = embankment. Byproducts Number of States Using Byproduct in a Given Highway Application Asphalt Cements or Emulsions Crack Sealants Drainage Materials Embank. Flowable Fill HMA Pavement Surface Treatments (non- structural) PCC Soil Stability Ground Tires 6 9 0 2 0 13 2 0 0 Shredded or Chipped Tires 1 1 1 14 0 3 1 0 0 Slit Tires 0 0 1 0 0 0 0 0 0 Whole Tires 0 0 0 1 0 0 0 0 1 Crumb Rubber Aggregate (dry process) 3 1 0 0 0 15 3 0 0 Crumb Rubber Modifier (wet process) 8 4 0 0 0 22 8 0 0 Tires, Unknown Type or Size 2 2 1 2 1 1 1 1 2 TABLE 15 NUMBER OF STATES USING SCRAP TIRE ByPRODUCTS IN HIGHWAy APPLICATIONS

39 Number of Applications States Ground Tires Shredded or Chipped Tires Slit Tires Whole Tires Crumb Rubber (dry) Crumb Rubber (wet) Unknown Type 9 — — — — — — ID 4 AZ, TX, VA — — — — PA, VA — 3 — — — — — AK, AZ, TX MA 2 CT, IL, IA, MN, NE CT, IA — TX AK, CT, NE, PA MO, NE, NY, OR — 1 AL, CO, FL, ME, NC, OK, WI CO, DE, IN, LA, ME, MN, NC, NH, NJ, NY, OK, PA, TX, VA, VT, WA VT — IL, IA, KY, ME, MO, MS, NH, NJ, NV, NY, OK, OR, WI, WV AL, DE, GA, IL, IA, KY, LA, ME, MN, MS, NJ, NV, OK, SC, VT, WA, WI NJ TABLE 16 STATES USING SCRAP TIRE ByPRODUCTS IN HIGHWAy APPLICATIONS Scrap Tire Byproducts 2009 Ground Tires 4 1 4 1 1 2 1 1 1 2 4 2 2 CT - 2 1 2009 Shredded or Chipped Tires 1 1 1 1 1 1 NH-1 VT-1 1 1 1 1 2 1 CT - 2 NJ - 1 DE - 1 1 2009 Crumb Rubber Aggregate (Dry Process) 1 1 2 1 1 1 1 1 1 2 1 2 1 1 1 NH-1 CT - 2 NJ - 1 4 2 3 VT-1 4 1 2 1 1 1 1 NJ - 1DE - 1 2009 Crumb Rubber Modifier (Wet Process) 1 3 3 2 2 1 1 1 1 1 1 1 1 FIGURE 14 Agency survey results for scrap tire byproducts (numbers indicate the number of applications that use the byproduct).