Representative image of recycled tire piles and empty road. (Image Credits: Pexels)
In Louisiana, roads paved with crumb-rubber-modified (CRM) asphalt performed better overall than conventional sections, according to Huang et al.’s 2002 comparative study of pavement projects across the state. The study compared wet- and dry-process CRM applications The Louisiana Department of Transportation and Development began the research in 1994 to compare the long-term performance of crumb rubber modified hot-mix asphalt (CRM HMA) with conventional hot-mix asphalt control sections. Huang and colleagues’ study appeared in 2002 in Transportation Research Record. Under the PlusRide method described in Shen and Xie’s report, CRM particles from 4.2 to 2.0 millimetres accounted for 1% to 3% of the paving mix by weight.
Because they recorded higher strength characteristics than the CRM mixtures, initially, laboratory results favored the conventional mixtures. In the wet process, the rubber is mixed with an asphalt binder before it is combined with aggregates, whereas in the dry process, crumb rubber is directly introduced with the aggregate and generally replaces a portion of the fine aggregate. Shen and Xie reported wet- and dry-process CRM HMA performed as well as, or better than, conventional mixes in evaluated sections.
The rubberized sections also performed better in rut depth, fatigue cracking and the International Roughness Index (IRI), used to measure pavement smoothness. Eight CRM pavement sections at five Louisiana highway projects were examined, each using a different crumb-rubber-modified asphalt process. The evaluation comprised a combination of laboratory testing and field measurements, including visual crack inspections, roadway roughness readings and pavement structural measurements. In these pavements, the crumb rubber was sourced from scrap tires and could be incorporated into asphalt construction through a wet or dry process. The Louisiana project hinged on a practical question of whether asphalt containing recycled tire rubber, once exposed to actual traffic and environmental conditions, could perform just as well as conventional mixtures. However, the field results showed different outcomes. The CRM pavement sections, after five to seven years of traffic, had an overall better performance in rut depth, fatigue cracking and IRI measurements than their conventional sections. The laboratory evaluation used Marshall stability and flow tests, along with indirect tensile strength, strain and resilient modulus measurements. These tests were conducted on field-compacted Marshall specimens. The comparison also included pavement structure measurements obtained through the Dynaflect system and roadway core air-void analysis, while visual inspection identified cracking. Rut depth and the International Roughness Index were also used as field performance indicators, and each CRM section was compared with a conventional control section within the Louisiana projects. For the dry process, crumb rubber was used as a substitute for a small portion of the fine aggregate, with the particles blended with the aggregates before asphalt cement was added. Generic dry technology used an equivalent or slightly lower percentage of finer CRM. The researchers concluded that the CRM sections showed better overall field performance in comparison to conventional control sections despite the latter having recorded higher strength characteristics in laboratory testing.
