ETL 1110-3-503
15 Sept 99
used it successfully in various applications since the 1950's, with a large amount used in
stress alleviating membrane interlayers (SAMI) applications (Leask 1983). CRM has
been used in other varied applications such as crack/joint sealants and surface/interlayer
treatments (Ahmed 1991 and Ahmed and Lovell 1992). Used tire rubber has been used
in hot-mix asphalt pavement applications for more than 35 years (Epps 1994). The vast
majority of states a nd most federal agencies have used CRM in paving projects, with no
major problems reported during the preparation and construction (FHWA 1993). The use
of CRM can be broken down into two processes: (1) the wet process, where the CRM is
blended and partially reacts with an asphalt cement prior to use; and (2) the dry process,
where the CRM is added to the aggregate in a hot-mix central plant operation prior to
adding asphalt cement (Heitzman 1992). CRM has been used in gap-graded hot-mix
asphalt pavements in the US to reduce tire and therefore overall vehicular traffic noise
(Nicholson 1998). As of 1993, there had been two documented instances of CRM
pavements themselves being recycled to become part of a new pavement.
Despite the relatively long term and widespread use of CRM, the majority of states and
agencies still consider it as experimental process. A survey of state transportation
departments reported that the states had used the wet process more than twice as often as
the dry process. The states reported successful applications of the wet process in more
than 50 percent of their projects, while the majority considered the dry process to have
been unsuccessful (Epps 1994). A new dry process has been used in California for the
last three years and the results indicate performance better than or equal to the wet
process (Nicholson 1998). The effect of CRM on properties of hot-mix asphalt can vary
with the process when compared to conventional hot-mix asphalt; however, either
process will improve the fatigue life of the pavement (Hansen and Anderton 1993, Epps
1994, and Nicholson 1998). Studies using the wet process have found that the addition of
CRM decreased the oxidative hardening of the asphalt binder and improved the fatigue
resistance of the pavement (Ramaswamy and Aziz 1992 and Rebala and Estakhri 1995).
One study using a devulcanized CRM (a wet process) showed improvement of the low
temperature performance of hot-mix asphalt mixtures (Morrison 1995). Other studies
found that CRM would provide improvement to low temperature cracking, especially
when a softer than normal asphalt was used (Hansen and Anderton 1993). Evaluation of
CRM mixtures indicates a potential for reducing thermal cracking (Lundy and Zhou
1993, Stroup-Gardiner et al. 1996, and Nicholson 1998). Some studies have found that
field performance did not correspond to laboratory performance, indicating that field
studies are needed for decisive information (Heitzman 1992). As of 1995, approximately
20 percent of all states were mandating the use of some form of rubber in all state DOT
paving projects. Recent evaluations of CRM in asphalt mixtures have been accomplished
using Strategic Highway Research Program (SHRP) Performance Grade asphalt test
methods (Troy et al. 1996 and Gowda et al. 1996).
One study of the use of crumb rubber, both in the wet and dry processes, found that
emission levels during the hot-mixing process resulted in slightly higher but acceptable
levels of the various emission materials investigated (Baker and Connolly 1995). Other
studies with crumb rubber have found that a properly designed, managed, and operated
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