Semisequicentennial Transportation Conference Proceedings
May 1996, Iowa State University, Ames, Iowa
A Study of Tire Chips as Road Construction Material
Mustaque Hossain, Mustafa A. Sadeq, Louis P. Funk, and Rodney G. Maag
M. Hossain and M.A. Sadeq,
Department of Civil Engineering,
Kansas State University,
Manhattan, Kansas 66506.
L. Funk, Bartlett and West Engineers,
Topeka, Kansas 66611.
Bureau of Materials and Research,
Kansas Department of Transportation,
Topeka, Kansas 66611.
The feasibility of using tire chips as aggregates in a cold mix, chunk rubber cold mix (CRCM), for use on low-volume roads was investigated. The results of this study show that the use of rubber chunks (up to a maximum 12.5 mm size) as a replacement for crushed-stone aggregates results in a weaker mix than a mix without rubber. Since chunk rubber is not as hard as the crushed-stone aggregates, the Marshall stability values of the asphalt-aggregate-tire chip mixes were consistently lower than the control mixes without any tire chips. It was also presumed that the tire chips tend to absorb some of the energy imparted to compact a CRCM sample resulting in a weaker aggregate structure than a mix with no tire chips in it. Addition of a Class C fly ash to a CRCM sample results in higher Marshall stability. A gap-graded CRCM with 2 percent chunk rubber and 10 percent fly ash with an optimum emulsion content of 7 percent showed the highest average Marshall stability of 1400 N. This value of Marshall stability is much lower than for a mix without any rubber in it. However, the CRCM should be suitable as a drainable base on low-volume roads.
Each year approximately 285 million tires are added to stockpiles, landfills, or illegal dumps across the United States (1). If the national rate of tire generation is used, it is estimated that on the average, one scrap tire per person per year is generated in Kansas. This translates to approximately 2.4 million tires per year in Kansas. The current estimate of the number of accumulated scrap tires in the state is in between 4.3 and 5.5 million (2). The large number of tires accumulated over the years and currently being generated creates a disposal problem in the rural areas of Kansas. Introduction of scrap tire rubber into asphalt concrete pavement has the potential for solution of this waste problem. However, most of the asphalt-rubber research work done in the past concentrated on roads with hot-mix asphalt concrete with finely ground rubber, commonly known as crumb rubber. This crumb rubber is expensive, and would not be cost-effective for low-volume roads. Limited research has been done on the feasibility of using larger tire chips as aggregates (3). The objective of this research project was to formulate a chunk rubber cold mix (CRCM) for use on low-volume roads. CRCM is a modified asphalt cold mix which is produced by the so-called "dry process"a mixing process where tire chips, with maximum size up to 12.5 mm, are used as aggregates. The results of this study were expected to benefit the rural counties in Kansas where the discarded tires could be shredded into chips and incorporated cost-effectively into the low-volume county roads using cold-mix.RESEARCH APPROACH
The research work consisted of a feasibility study of CRCM in the laboratory. The goal was to develop a mix which would achieve a Marshall stability value of 2,225 N, a value suggested by the Kansas Department of Transportation (KDOT) for a suitable cold mix on the state highway system.Materials Used
The aggregates used consisted of 12.5 mm bedding, 6.4 mm chips, 6.4 mm screening, and a manufactured sand from a quarry in southeast Kansas. The chunk rubber used in this study was obtained from a tire-shredding plant in Wichita, Kansas. The rubber was produced through a series of stationary scrap tire shredders and nearly 100 percent of it passes through a 9.5 mm sieve, with the majority retaining on a 4.75 mm sieve. Two different asphaltic materials, a medium curing cutback (MC-800) and a cationic medium-setting emulsion (CMS-1), were investigated in this project. Due to the low stability characteristics of chunk rubber cold mix and following KDOT practice of using fly ash in cold mixes, an ASTM Class C fly ash was used in the chunk rubber cold mix. A set retarding admixture was used to prevent flash-setting of the fly ash in the mix.Mix Gradation
The mix was designed as gap-graded with about 100 percent passing the 12.5 mm sieve. The control mix, with no fly ash, was designed to have 94 percent retaining on the 75 mm (No. 200) sieve. A mineral filler was used in the control mix and in the test mixes containing low fly ash percentages. The amount of rubber used in the mixes varied from 2 percent to 6 percent, by weight of the total aggregates used.Sample Preparation
The mix design was planned to be developed based on the Marshall stability and flow tests. The aggregate and asphalt emulsion were heated to 6671oC for mixing. In samples containing fly ash, the asphalt was mixed with water and retarder before mixing with the aggregate. The mixes were cured and compacted at 52 55oC. The loose mix was placed in a mold and rodded 15 times before compacting. Compaction consisted of 50 blows per side using a Marshall compactor. The samples were tested for the Marshall stability and flow after being submerged in a 43oC water bath for about 30 to 40 minutes. The samples were tested approximately 24 hours after mixing. Bulk (saturated surface dry, or SSD) and theoretical maximum (TMD) densities of the samples were also determined.PARAMETRIC STUDY
At the beginning of this research program, Marshall test samples of the control mix (no fly ash, no rubber) were made at 5, 6, 7, 8, and 9 percent (by weight of the total mix) emulsion contents. Three samples at each emulsion content were tested for the Marshall stability and flow. Density and void analysis were also done. From the results of this test program, the optimum emulsion content (corresponding to the maximum Marshall stability, air voids of 35 percent, maximum density, minimum voids in mineral aggregates, VMA and allowable flow values) for the mix was determined to be about 7 percent . This design allowed a parameter (emulsion content) to be fixed as other parameters were investigated. Those parameters include: moisture content needed for hydration of the fly ash in the cold mix, cutback asphalt vs. emulsified asphalt, fly ash content, rubber content, retarder content, and curing time before compaction. Based on this parametric study, the following mix parameters were selected: (i) 10 percent fly ash and 5.5 percent moisture contents, (ii) 2 percent retarder content and 2 hours of curing time before compaction, and (iii) CMS-1 emulsion.RESULTS AND DISCUSSIONS
Knowing that 10 percent fly ash would produce the highest stability, a new set of samples were made at 6, 7, 8, and 9 percent emulsion content and 2, 4, and 6 percent rubber content for the Marshall stability and flow tests. Density and void analysis were also conducted for these samples. Table 1 tabulates the test results. The optimum air void content was set at 11 percent based on the experience with cold mixes in Kansas. From the graphs of Marshall stability vs. emulsion content, bulk (SSD) density vs. emulsion content, percent air void vs. emulsion content, and percent VMA vs. emulsion content, optimum asphalt contents of 6.8, 7.3, and 7.8 percent were calculated for 2, 4, and 6 percent rubber contents, respectively. Figure 1 shows the typical curves for 2 percent rubber content. The optimum emulsion content was taken to be the average of emulsion contents corresponding to the maximum Marshall stability, maximum unit weight and 11 percent air void. A check was made to ensure that the optimum emulsion content satisfied the criteria for VMA set by the Asphalt Institute (4). It was noted that there is a linear relationship between increasing emulsion content and increasing rubber content. Since the air void content was most affected by varying emulsion levels, the required emulsion content could be significantly reduced if a 12 percent or higher air void content was allowed. The high flow values of the mixes may mean better flexibility in the field in terms of higher deformation capabilities, but may also indicate the potential for rutting. The results show trends that would be expected for nearly all parameters in the Marshall method of mix design. The mix with 2 percent chunk rubber and 10 percent fly ash content showed the highest average Marshall stability of 1600 N. A mix without any fly ash or rubber in it had a Marshall stability of 4,000 N at 7 percent emulsion content.CONCLUSIONS
Based on the results of this study, the following conclusions can be drawn:
- The use of tire chips (up to maximum 12.5 mm size) in a chunk rubber cold mix (CRCM) as a replacement for crushed-stone aggregates results in a weaker mix than without rubber. Since rubber is not as hard as the crushed-stone aggregates, it follows that the Marshall stability of an asphalt-aggregate-chunk rubber mix would be lower than a mix without any chunk rubber. However, it was also surmised that the larger rubber chunks tend to absorb some of the energy imparted to compact a CRCM sample, resulting in a weaker aggregate structure than a mix with no chunk rubber.
- Class C fly ash results in higher Marshall stability of a chunk rubber cold mix. A gap-graded cold-mix with 2 percent chunk rubber and 10 percent fly ash with an optimum emulsion content of 7 percent showed the highest average Marshall stability of 1600 N. This value is lower than the KDOT-accepted 2225 N Marshall stability for a suitable cold mix on the state highway system. However, this mix should be suitable as a drainable base on low-volume roads.
- If 9 kg of chunk rubber equivalent is produced per auto tire, then a one km long and 7.3 m wide low-volume road with a 100 mm thick base built with this mix can incorporate approximately 3350 auto tires.
The financial support for this study was provided by the Kansas Department of Transportation.REFERENCES
- M. Heitzman. An Overview of Design and Construction of Asphalt Paving Materials with Crumb Rubber Additives. Paper presented at the 71st Annual Meeting of Transportation Research Board, Washington, D.C., January 1992.
- R. Nelson and M. Hossain. A Statewide Plan for Utilization of Scrap Tires in Kansas. Engineering Experiment Station Report No. 268, Kansas State University. Submitted to the Kansas Department of Transportation (KDOT), September 1994.
- K.D. Stuart and W.S. Mogawer. Laboratory Evaluation of Verglimit and PlusRide. Report No. FHWA-RD-91, Federal Highway Administration, Washington, D.C., March 1991.
- Asphalt Cold Mix Manual. The Asphalt Institute, Manual Series No. 14, 3rd ed., 1989.