• RTR in asphalt mix

    by Colleen Suo

    Paul Wilke, Principal Engineer at Applied Research Associates, Inc. was a featured speaker at the 2016 AGC NYS Conference in Saratoga Springs, NY. He focused on the use of rubberized asphalt in paving applications.

    For the last 20 years, Wilke has served as project principal or manager on pavement engineering, pavement management, geotechnical design, construction inspection and materials testing projects for roads and airports. He has also served as a consultant to PennDOT to develop specifications and a usage guide for the use of rubberized asphalt in gap-graded, open-graded and dense-graded asphaltic concrete mixtures. During his presentation, he shared some things he’s learned while working with PennDOT helping them to implement their program over the last couple of years.

    How it all began

    Rubberized asphalt cement (RAC) was first used in the early 1960’s in Phoenix, AZ. Charles McDonald, a Materials Engineer who worked for the city developed and patented a process while employed by the city. Wilke said McDonald “figured rubber was durable and pliable so why not incorporate it into asphalt material and make it perform better.”

    The city of Phoenix used it primarily for patches and chip seals. After a short while Arizona DOT (A DOT) “jumped on the bandwagon and started using it as SAMI (stress absorbing membrane interlayer).” However, in the 80’s, traffic got so heavy they started using it in asphalt overlays as well.

    He went on to explain how CalTrans started using RAC in the 1970’s as chip seals and then also migrated to hot mix around 1978.

    In 1991 the Intermodal Surface Transportation Efficiency Act (ISTEA – pronounced ice tea) mandated (among other things) for states to use a certain amount of rubber in their mixes. Consequently, “everybody ran out and put a fair amount of rubber in their asphalt mixes. The problem with that was that back then, there were really no well-developed specs or usage guides – contractors weren’t experienced with it and there were a lot of failures.” Within a couple of years, in fact, the federal government dropped the mandate and the process “was generally thought of as a bad idea.”

    According to Wilke, the early failures were due to the process used, which was the Dry Process. When using the Dry Process, “fairly coarse pieces of dried rubber are incorporated into the aggregate prior to adding the binder so it is actually acting as a replacement for some of the stone or the sand.” This is in contrast to the Wet Process, which “is adding it to hot binder and melting it (so to speak) so the rubber interacts with the asphalt. It modifies the binder and makes it different in composition.”

    Up until recently, Wilke knew of only one dry method (previously mentioned where RTR is used as an aggregate replacement up to the #4 sieve).

    The newer method he recently learned about “adds RTR in with the hot aggregate, which has some effect on modifying the binder. The state of Georgia has done quite a bit with it and are reporting success.”

    He continued, “Wet processes can use on-site blending or terminal blending. The higher viscosity wet process meets the ASTM 6144 requirements for asphalt rubber (AR) and typically contains 15 to 22 percent CRM (crumb rubber modifier) with particle sizes between the #10 & #8 sieve – (a little coarser) at 15-22 percent.

    The low viscosity (not as thick and sticky) process does not meet ASTM 6144 requirements, so it is called crumb rubber modified binder. It’s finer (#50 sieve) – and typically contains about 10 percent CRM and normal circulation is adequate to keep it dispersed.”

    Even though the federal mandate was withdrawn, “some states such as Arizona, California and Florida continued to work with it because they were experiencing success” with the wet process.

    As some of the northern states saw the success of these pioneer states, they chalked it up to geographical location and more steady temperatures. However, Wilke went on the say “some provinces in Canada and some northern states started experimenting with it and had many success stories associated with it. Currently many of the northeast and mid-west states are using it to some extent in their resurfacing programs.”

    Benefits noted by Wilke:

    • In comparison to conventional asphalt, AR exhibits “improved viscoelastic behavior (ability to flow and recover shape at low temperatures). In cold winters, the pavement wants to contract. It’s somewhat brittle and can crack.” So adding rubber gives it bit more elasticity and ductility at the low temperatures. Conversely, in the higher temperature range, “where [conventional] asphalt is susceptible to rutting,” adding the AR achieves improved “elastic characteristics since rubber can stretch quite a bit and return to it’s original form.”
    • Adding AR has been shown to bump the performance grade of the finished product. Wilke explained, “if you take a neat binder (as they call it) or an original virgin asphalt binder without any special additives in it — PG 64-22 is commonly used in PA and New York — when you add rubber, it’s typically bumping it, actually two grades as each grade bump corresponds to six degrees, all the way up to 76-22 (degrees C).”
    • AR offers increased resistance to [load related] fatigue cracking. According to Wilke, “It won’t eliminate it but it will retard it and help the pavement to last longer before the cracks start appearing.”
    • AR also increases noise attenuation. An A-DOT 1996 study “showed a six-decibel decrease (three decibels is perceived by the human ear).”
    • Environmental benefits can be “pretty substantial” by reducing the waste tire stockpiles. CalTrans figures showed that municipalities could use (on average) 1000 tires per mile per inch of asphalt. At a recent conference, Wilke learned that since Arizona began incorporating RTR in their asphalt mixes in the 60’s, they’ve re-used 40 million tires and California (since the 70’s) – 29 million.

    According to a 2010 article on recyclenation.com, the U.S. discards an estimated 300 million tires per year. If that estimation is correct or even close, there will be an endless supply of RTR for many years.

    Limitations and challenges

    • The mix is stiff and sticky and tends to adhere to the roller during compaction. Wilke discovered that some contractors have used detergent on their rollers and to keep the AR from sticking.
    • The mix also adheres to rubber tires. Wilke suggests using a steel wheeled roller instead.
    • High temperature is required for placement and compaction. A consideration if there is “a long haul from the plant or if it’s late in the year.” Temperature management is critical.
    • Workability can be a challenge as it is a stiff and sticky mix — a consideration for each job. Handwork such as around catch basins or curb returns can be done, “but the workers tend to complain about the product sticking to the tools.”
    • Limited contractor experience will be reflected in job bids. However, Wilke felt that as more contractors become adept at working with the product, the pricing curve would work itself out.
    • The RTR “can settle out” if it sits too long. Wilke noted that some operators fight this tendency by adding an agitator unit or a modifying agent.
    • Lab testing can be a challenge. In Superpave mixes, it’s common to use a dynamic shear rheometer (DSR) in order to test the shear strength of the binder over the range of anticipated temperatures. According to Wilke, the problem here is that some mixes contain rubber particles that are close in size to the distance between the parallel plates in standard lab equipment, which can affect results. One way around this is to increase the gap between plates from the standard 1mm to 2mm.

    Factors effecting cost

    The proximity of the blender or terminal to the job site is a cost factor. Obviously, “closer is better for mobilization costs.” Traffic congestion could also be an issue because it could directly effect delivery times and since material “temperature control is so important” this should be a consideration when bidding. As an example, “if you have a truckload of asphalt and he’s delayed due to traffic, you could be rejecting the load because it’s too cool by the time it arrives on the site.”

    Similarly, the time of year is a factor. “Contractors know the risk they are dealing with – if you are bidding a job late in the year like October, you may want to increase your price or not bid at all because you know you’ll be dealing with problems.”

    However, according to Wilke, the factor having the most impact is the economy of scales. States that are using the process extensively have worked the cost curve down – “their costs are in line with other high performance (polymer modified) binders.” The states that are “only dabbling and only doing one or two projects here and there” will experience higher costs until it becomes more a commonplace procedure.

     

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