THIN IS IN!!

The Development of the Caltrans Reduced
Thickness Guidelines

By Jack Van Kirk, P.E. Director of Asphalt Technologies, Basic Resources, Modesto, CA.

In 1978, in its quest to find a more durable mix for the snow region where tire chains are used and the design life was not being achieved, Caltrans began experimenting with crumb rubber in asphalt mixes. The first field trial using Asphalt-Rubber was in 1980. Laboratory research indicated that crumb rubber modified mixes (aka RAC by Caltrans) were more abrasion resistant when compared to conventional DGAC. Field permeability testing also indicted that the RAC mixes had extremely low permeabilities, which should reduce infiltration of water into the mat and therefore cut down on freeze-thaw damage. It was believed the low permeability would also reduce oxidation and thereby lower the aging rate.     Because of the success in the snow region, Caltrans began to broaden its use of RAC mixes. These mixes included adding the crumb rubber in the wet process, where the crumb rubber and asphalt are field blended under a time/temperature formula and the dry process where the crumb rubber is added to the aggregate before the introduction of asphalt at a plant. The Caltrans experience with the dry process was unsuccessful and the agency moved forward with the wet process.    The research between 1980 and 1992 compared Asphalt-Rubber concrete to conventional asphalt concrete in the field evaluations. During this time frame, California cities and counties also experimented with Asphalt-Rubber pavements. One of the earliest cities to try the materials was Palm Springs in the low desert area of the state. In 1983 Caltrans instituted a research project on RT. 395 in northeastern California, known as "Ravendale," using various overlay strategies including three test sections of reduced thickness Asphalt-Rubber mix compared to conventional DGAC overlay design thickness. While reviewing and accumulating data on the Ravendale project, Caltrans continued to construct and compare equal thicknesses of Asphalt-Rubber and conventional DGAC on other projects.     By 1987, it became evident that substantially thinner overlays of Asphalt-Rubber could provide a longer service life than the DGAC at a reduced cost. At this time Caltrans changed its strategy for field comparisons between the Asphalt-Rubber and conventional AC sections. It was decided Asphalt-Rubber overlays would be thinner that those required using the convention DGAC. Projects utilizing the reduced thickness continued until 1992. At that time it became very evident that Asphalt- Rubber mixes could be reduced in thickness and achieve equal or greater service life than the conventional pavement design thickness.

This conclusion lead to the Caltrans routine use of reduced thickness Asphalt-Rubber pavements and the development of the Reduced Thickness Design Guide. The same year Caltrans presented a proposal to the FHWA to allow the use of reduced thickness Asphalt-Rubber overlays as a strategy on federally funded rehabilitation projects. The FHWA approved the proposal based on the successful field performance data gathered from Ravendale and other projects. The guide is titled "Asphalt-Rubber Hot Mix-Gap Graded Thickness Determination Guide" and is in the form of an easy to use table. To determine the thickness needed for an Asphalt-Rubber overlay, a conventional AC design thickness is first determined. (Caltrans uses a deflection based design procedure for rehabilitation of flexible pavements.) The designer simply enters the table shown on the following page and finds the thickness for the convention design thickness (DGAC column) then moves horizontally across to the right to find the equivalent thickness for the Asphalt-Rubber hot \ mix with or without an Asphalt-Rubber Stress Absorbing Membrane Interlayer (SAMI). For reflection crack retardation equivalencies Table 2 on the following page was developed. Field Validation of the Reduced Thickness Design Guide.     In 1993 Caltrans conducted research to validate the data supporting the reduced thick design guide. The work was conducted by the University of California, Berkeley, Dynatest Consulting and the Council for Scientific and Industrial Research (CSIR). The work involved the use of the South African Heavy Vehicle Simulator (HVS). The HVS is an Accelerated Pavement Test (APT) machine designed to test pavement sections in the field. The machine has a truck wheel (single or dual tire) mounted on an I-beam frame that is hydraulically loaded and moves back and forth across the pavement. It is designed to simulate actual traffic on the pavement section. One of the objectives of the study was to test conventional AC and reduced thickness (up to 50%) ARHM-GG pavement sections. The HVS testing took place in South Africa. The results of the study were quite surprising.     The test results show that the conventional AC section failed at 200,000 repetitions. The 38 mm ARHM-GG section showed no cracking at 175,000 repetitions, so the load was increased to 80kN and still no cracking up to 237,000 repetitions. The temperature was then lowered to ­5 degrees Celsius and only one half the section cracked at 250,000 repetitions The 25mm ARHM-GG section showed no cracking at 175,000 repetitions, so the load was increased to 80kN and the section finally showed fine cracks at 200,000 repetitions. The section completely cracked at 237,000

repetitions. These test results show that 25mm of ARHM-GG outperformed the 75 mm section of conventional AC in regards to fatigue. This is a 3:1 reduction in thickness. This not only validates the reduced thickness design being used by Caltrans, but it indicates that it may be conservative. Laboratory testing by UCB (10) in 1994 and by Lutfi Raad Ph.D at the University of Alaska Fairbanks 1995 further substantiated the HVS field test results.. Advantages of Asphalt Rubber Binder in Pavement Strategies     Asphalt rubber mixes contain significantly higher binder contents than AC mixes using conventional and polymer modified binders. In open and dense graded mixes the binder contents are about 20 % higher, in gap graded mixes the binder contents are about 40-50 % higher, and in high binder content open graded mixes the binder content is about 50-60 % higher. A typical binder content for an ARHM-GG mix using asphalt rubber is about 7.4 % (by total mass). A typical binder content for a high binder content OGFC mix using asphalt rubber is about 9.0 % (by total mass). With these high binder contents it is easy to understand why these mixes provide improved field performance. Asphalt rubber binder can provide significant advantages over conventional asphalt and even polymer modified asphalt binders. These advantages include:

Improved abrasion resistance, especially in snow regions Reduced oxidation Increased durability and fatigue life Increased resistance to reflective cracking and rutting Reduced noise Improved safety during construction Savings in energy and natural resources Proven Life Cycle Costs     It has been determined through the Life Cycle Costs for Asphalt-Rubber Paving Materials that asphalt rubber rehabilitation and maintenance strategies are more cost-effective when compared to conventional asphalt strategies. The 1999 Life Cycle Cost Analysis by R. G. Hicks, Jon A. Epps and James R. Lundy based on project information from a wide range of agencies in three states indicate that Asphalt-Rubber chip seals and structural overlays are more cost effective 85% of the time and 82% of the time for thin AR HMA preservation treatments. This, along with the environmental benefits of utilizing scrap tires, has led to more wide spread usage of asphalt rubber rehabilitation and maintenance strategies in California and other states in the USA. Why Should an Agency Choose Asphalt Rubber Strategies     There are many reasons why an agency should choose asphalt rubber for use in their pavement maintenance and rehabilitation strategies. This article has highlighted some of the advantages of using asphalt rubber. It has been used successfully in hot mix and in chip seals and in multi-layer systems. The following provides a summary of my observations as a former Caltrans Sr. Engineer for Flexible pavements, the reasons why an agency should choose asphalt rubber strategies: Has a long-term successful field performance history (over30 years) Allows for higher binder contents Greater film thickness leads to improved durability and longer life Can be used in reduced thickness Easy to use reduced thickness design guide Reduced construction time leads to increased safety Reduced maintenance costs Established life cycle cost-effectiveness.