Caltrans Pavement Rehabilitation
Using Rubberized Asphalt Concrete

by Jack L. Van Kirk, P.E., Senior Materials and Research Engineer with the California Department of Transportation, Office of Transportation Materials and Research - 5900 Folsom Boulevard, Sacramento, CA 95819, (916) 227-7300

Presented at a meeting of the Rubber Division, American Chemical Society Anaheim, California May 6-9, 1997

Caltrans has considerable experience with the use of recycled rubber in asphalt concrete (AC). This experience began in 1978 and has continued to the present. The experience to date has shown that the use of rubber in asphalt concrete can be cost effective.

On the early projects, Caltrans compared equal thicknesses of rubberized asphalt concrete (RAC) to conventional AC. The RAC has outperformed the conventional mixes on these projects and most of these projects are still in service. In 1983, a project was constructed using reduced thickness RAC (as compared to the design thickness for the conventional AC). This project has shown that thinner sections of RAC can outperform thicker sections of conventional AC.

In February 1992, Caltrans developed an Interim Design Guide for RAC. This design guide, which was approved by FHWA, allows routine usage of RAC as an approved rehabilitation strategy for pavements in California. Since this guide was approved, the use of RAC has significantly increased in California. However, in 1993 and 1994, there were complaints of illness by workers on Caltrans projects. Caltrans has conducted air emissions testing on over 15 projects beginning in 1993. Testing to date has shown that acceptable worker exposure levels have not been exceeded on these projects. Caltrans is working with industry via a permanent standing committee to improve and refine the RAC specifications.

The California Department of Transportation (Caltrans) has been using rubber-modified asphalt concrete or rubberized asphalt concrete (RAC) for over 19 years. The first project using RAC was constructed in 1978 and since then Caltrans has had considerable experience with RAC. This experience has not only included field trials of RAC, but it also has included research in the laboratory. Caltrans has constructed over 130 rehabilitation (overlay) projects using RAC overlays. These projects have included different types of rubber modification. All but three projects utilized recycled granulated tire rubber. There were seven projects that used a dry process, where the recycled rubber is added to the aggregate before the asphalt is added. All other projects used a wet process, where the recycled rubber is first blended and reacted with the asphalt, to form asphalt-rubber binder, before being added to the aggregate. This wet process has proved to be the most cost effective use of recycled rubber in asphalt concrete.

Overall, Caltrans rehabilitation program has proved quite successful. However, in the snow regions where tire chains are used, the design life was not being achieved when using conventional dense graded asphalt concrete (DGAC), thereby resulting in increased maintenance costs. In 1978, in its quest to find a more durable mix for the snow region ion, Caltrans began experimenting with RAC mixes. Later laboratory research indicated that RAC mixes were more abrasion resistant when compared to conventional DGAC. Field permeability testing also showed that RAC mixes had extremely low permeabilities. It was felt that these low permeabilities would reduce the infiltration of water into the mat and therefore cut down on the freeze-thaw damage. The low permeabilities should 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.

On the RAC projects constructed by Caltrans prior to 1983, the RAC was compared to equal thicknesses of con ventional DGAC. However, in 1983 a project was constructed (on RT. 395 in northeastern California) using various overlay strategies including three test sections of reduced thickness RAC (when compared to the conventional DGAC overlay design thickness). Also placed on the project were various thicknesses of conventional DGAC. This project, though not realized at the time, later became the turning point for Caltrans rehabilitation strategies involving RAC mixes. For awhile after 1983, Caltrans continued to construct and compare equal thicknesses of RAC and conventional DGAC on other projects, while reviewing and accumulat ing data overlays of RAC, when com pared to conventional DGAC, could pro vide a longer service life at a reduced cost. At this point in time, Caltrans strate gy for RAC test section overlays changed. It was decided that all subse quent projects if appropriate would involve RAC overlays that were thinner than those required if conventional DGAC were used. Projects utilizing reduced thicknesses continued until 1992. At that time the use of reduced thickness RAC became a routine strategy in the Caltrans rehabilitation program.

Caltrans rehabilitation projects using RAC overlays have included rubber modified: dense graded asphalt concrete (DGAC), open graded asphalt concrete (OGAC), gap graded asphalt concrete (GGAC), and an Arizona-type, threelayer system. These overlays have been placed over flexible pavement (AC) as well as rigid pavement (PCC). On these projects six different types of RAC have been used: devulcanized reclaimed rubber has been added using a dry process to conventional DGAC Mix; vulcanized reclaimed tire rubber has been added using a dry process to a gap-graded aggregate and conventional asphalt; and vulcanized reclaimed tire rubber has been preblendled with a conventional asphalt to form asphalt-rubber binder (wet process) which was then added to a dense graded, open graded or gap graded aggregate; or the binder was used with a pre-coated chip as a stress absorbing membrane interlayer (SAMI) in an Arizona-type, three-layer system. SAMIs also have been used with other RAC mixes. Currently, Caltrans uses predominantly gap graded RAC mixes in their rehabilitation program.

Caltrans uses a deflection-based design procedure for rehabilitation of flexible pavements. This procedure is also used for RAC overlays. On the early RAC projects, the RAC overlay design thickness was the some as that provided from the deflection study for conventional DGAC. The design life normally used is ten years, and during this time only minor maintenance is expected.

In 1992, Caltrans presented a proposal to the Federal Highway Administration (FHWA) to allow the use of reduced thickness RAC overlays as an approved strategy on federally funded rehabilitation projects. This proposal was approved based primarily on the successful field experience of reduced thinness RAC projects. At that time and Interim Thickness Determination Guide for asphalt-rubber hot mix-gap graded (attached as Appendix A) was developed and also approved by the FHWA. This guide is considered an interim guide because it will be modified as suggested by the results of current and future research by Caltrans and others.

A different approach is used for PCC pavement rehabilitation involving DGAC overlays. In order to try and obtain the desired ten year design life, the early approach (prior to 1982) was to place a DGAC overlay 75 mm to 150mm in thickness depending on the condition of the existing PCC pavement. However, the 10-year design life was not being achieved. After 1983, this strategy changed. PCC pavements are now cracked and seated, a leveling course of DGAC is placed, a pavement reinforcing fabric (PRF) is placed, and finally the pavement is overlayed with DGAC 75 mm thick placed in two lifts. This same approach is used for RAC overlays over PCC pavements. However, the PRF is replaced with a SAMI and the 75 mm thick DGAC is replaced with a 45 mm thick RAC overlay. This is referred to as an Arizona-type, three layer system. The RAC is usually a gap-graded or open graded mix.

The Hveem mix design is used by Caltrans for all RAC mixes with a few minor changes. The mix and compaction temperature is increased. The required aggregate temperature is between 149' C and 163' C. The required compaction temperature for the combined mix is between 144'C and 149'C. The Hveem stability requirement is lowered to a minimum value of 23. The asphalt-rubber binder which is supplied by the contractor is added to aggregate meeting the same grading and quality requirements used for conventional DGAC. The binder content for open and dense graded RAC mixes is normally about 20% higher than that for the conventional DGAC mixes, and the binder content for gap-graded mixes is normally about 40% higher than that for the conventional DGAC mixes.

The cost of asphalt-rubber binder is about two and one-half times the cost of conventional asphalt. This increases the cost per ton of RAC by about 30%-40% over that of conventional DGAC. Most of the RAC projects constructed by Caltrans in the past have involved fairly low tonnages of RAC and as a result, the cost for the asphalt-rubber binder has been relatively high. However, after Caltrans began using RAC routinely, there was a significant reduction in the cost of asphalt-rubber binder. Caltrans experience has shown that RAC overlays are cost effective when used in reduced thicknesses (when compared to conventional DGAC).

The construction of the Caltrans RAC overlays has been very similar to that of conventional AC overlays, although there are a few important differences. First, the mix must be placed at a higher temperature, preferably in a range between 149' C and 163' C. Raking should be completed before the mix drops below 144' C. As the mix cools, it becomes stiffer and raking becomes very difficult. Breakdown rolling should be completed before the mix drops below 144' C. If the mix is compacted at 144' C or above, excellent relative compaction has usually been achieved quite easily. Values of 96% to 98% relative compaction (compared to a laboratory compacted briquette) are quite common for dense graded RAC. However, compaction of gap-graded RAC has been more difficult because the mix cools faster than the conventional DGAC. If proper temperatures are used, adequate compaction can be achieved.

Caltrans has used RAC mixes in many parts of the state and in different climate regions. Many of the early projects were placed to resolve specific problems such as abrasion resistance, OGAC night placement, thin flexible bridge overlays, and desert AC pavement rehabilitation. Generally, control sections containing conventional DGAC were placed on the early projects so that direct comparisons could be made. RAC mixes have been used on bridge decks, roadside rests, parking lots and low, medium and high volume roadways. Over the years RAC mixes have proved to provide cost effective performance in all regions of the state.

In 1993, there were a number of RAC projects where workers complained of illness while working on the job. Since 1993, Caltrans has collected industrial hygiene data on more than 15 RAC paving projects. This data was collected through the paving contractors to evaluate the health and safety risks of the product. Personal air sampling pumps were worn by individuals (1) dumping the hot mix in front of the paver and (2) on the paver screed. Air samples for asphalt rubber fume measurements were taken at the rear of the paver near the screed and just above the paver auger. These were considered to be worst case (highest concentration) locations. The collected data indicates that employee exposures during paving are below CalOSHA allowable limits. Based on these results, exposure to RAC fumes would not pose a long term or chronic health risk. However, based on incidents of health complaints from the past paving season . short term acute health effects still can occur. These complaints appear to be related to a combination of the amount of asphalt fume (smoke) generated by the paving operation and personal sensitivities to that smoke. Since it was concluded that there is a direct relationship between mix temperature, smoke and complaints, in January 1994, RAC specifications were changed to lower mix temperatures (from 177' C to 163, C measured at the plant). As a result of this change, there were no reported complaints in 1995. However, health complaints were again reported in the 1996 paving season. As a result of these complaints, further research will be conducted to determine the cause of the complaints.

Caltrans has had considerable experience with RAC mixes. From this experience, it has been learned that when compared to conventional DGAC, RAC can tolerate higher deflections. RAC mixes have shown that they can outperform substantially greater thicknesses of conventional DGAC. They can exhibit lower permeabilities which in turn decreases oxidation and aging. They have also proven to be more abrasion resistant in the snow regions and when distress does develop it progresses at a much slower rate. All these desirable qualities that RAC mixes possess lead to decreased maintenance costs and ultimately lower annual equivalent costs.

However, if health complaints continue on future projects the use of RAC will be jeopardized. Even though it has been shown that the use of RAC mixes do not pose a chronic health hazard, research on the cause of these complaints will continue until they have been resolved.

At this time, Caltrans believes rubber modified asphalt concrete mixes will probably play a major role in the rehabilitation of rigid and flexible pavements in the future. The role that RAC mixes will play depends on their performance on recently completed projects and projects to be constructed in the next few years. If reduced thickness RAC continues to show the success that has been demonstrated on earlier projects and the health complaints are eliminated, there will be a definite increase in the usage of the product.

The contents of this paper do not necessarily reflect the official views or policies of either the Federal Highway Administration or the State of California. They reflect the views of the author, who is responsible for the facts and accuracy of the data presented herein. Also, neither the State of California nor the United States Government endorse products or manufacturers. Trade and manufacturers names are presented herein because they are considered essential to the objective of this paper.