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Research Projects (2009-10)

Identifying Number


Project Title

Seed Project: Beneficial Use of Off-Specification Coal Combustion Products to Increase the Stiffness of Expansive Soil-Rubber Mixtures, Year 2


Colorado State University

Project Investigator

Dr. J. Antonio H. Carraro, Assistant Professor
Department of Civil and Environmental Engineering
Phone: (970)491-4660, E-mail: carraro@colostate.edu

Description of Project Abstract

Untreated native soil with high swell potential has been identified by the Colorado Department of Transportation as one of the major contributing factors in the development of premature longitudinal cracking and other pavement distresses in Colorado (Ardani et al. 2003). Although excessive movement of pavement and bridge structures has been reported in both rural and urban areas in Colorado, this issue represents a particular burden for local governments due to the costs associated with the construction, management and operation of low-volume roads and bridges founded on expansive soils. A previous study led by the PI and sponsored by the USDOT-MPC showed that expansive soil-rubber (ESR) mixtures may be used as an alternative, sustainable material in low-volume road and bridge construction. That study had an innovative character because (i) most of the previous studies involving the use of rubber to stabilize soils have dealt with rubber-sand mixtures (Lee et al. 1999; Yoon et al. 2006; Zornberg et al. 2004), and (ii) among the few previous studies that have focused on rubber-clay mixtures (Ahmed and Lovell 1993; Edil and Bosscher 1994; Tatlisoz et al. 1997), none of them investigated the effect of rubber on the engineering properties of highly-expansive clays such as the ones found in rural and urban areas in Colorado. One of the major findings of the study mentioned previously is that scrap tire rubber addition decreases both the swell pressure and the swell percent of expansive soils (Carraro et al. 2008; Seda et al. 2007). However, a reduction in stiffness is also observed in ESR mixtures due to the deformability of the rubber particles and their effect on the structure of the compacted ESR mixtures (Carraro et al. 2008). Thus, methods that allow improvement of the stiffness properties of ESR mixtures had to be investigated further.

In a subsequent Seed Project proposed in 2008-2009 (with the same name as the present project), the beneficial use of off-specification (OS) coal combustion products (CCPs) is being evaluated through a systematic experimental investigation. Due to recent environmental regulations, widely-used CCPs such as fly ash (FA) have been commingled with additional materials that are used to minimize the environmental impacts due to emissions from coal combustion power plants. As a result, some of today's FA may not meet conventional specifications. This has compromised the use of FA and other CCPs in civil engineering applications. Thus, the ongoing 2008-2009 Seed Project has focused on whether or not OS-FA materials may be used to improve the stiffness of ESR mixtures, and the mechanical response of ESR mixtures stabilized with OS-FA is being investigated through a comprehensive experimental program. It was hypothesized that the pozzolanic characteristics of OS-FA will be sufficient to improve the stiffness of the ESR mixtures so that these alternative, sustainable materials can be used in low-volume road and bridge construction. The Seed Project results generated so far suggest that the OS-FA material used in the ongoing Seed Project is unable to develop pozzolanic reactions in the short term (within 7 to 14 days). As a result of this important finding, an additional, alternative OS-FA mixture with conventional class-C FA is currently being tested. However, since different types of OS-FA can be generated depending on the nature of the coal burned at power plants and the facilities available for emissions control, and the original Seed Project does not allow for a comprehensive evaluation of different types of OS-FA, we propose to extend the initial efforts developed in the Seed Project to allow testing of other types of OS-FA. Since the original Seed Project has focused on OS-FA that contains SOx, in the present project we propose to evaluate OS-FA with high carbon content. High-carbon content OS-FAs are typically generated as a result of new NOx emissions control facilities at relatively newer and/or recently remodeled power plants. Most importantly, continuation of the original 2008-2009 Seed Project will allow the US graduate student who has been working on the 2008-2009 Seed Project to remain funded and continue his 2nd year of studies at CSU.

Project Objectives

The main objectives of this project are: (i) to evaluate the geotechnical properties of expansive soil-rubber (ESR) mixtures stabilized with high-carbon content OS-FA required for the mechanistic design of low-volume road embankments and bridge abutments, and (ii) to disseminate the project findings through the publication and presentation of a technical paper.

Project Approach/Methods

This study will evaluate the beneficial use of high-carbon content OS-FA on the stabilization of ESR mixtures by measuring the relevant mechanical properties of high-carbon content OS-FA-ESR mixtures through a systematic experimental investigation. It is anticipated that the American Coal Ash Association (ACAA) and its member institutions will help provide the materials that will be used in this feasibility study. Project tasks include:

  • Task 1 - Literature review: comprehensive review of the literature on the project's topic; compilation of a source availability map for all high-carbon content OS-FA suppliers in Colorado and their proximity to major expansive soil deposits.
  • Task 2 - Laboratory testing: sample collection, chemical characterization, physical characterization (grain-size distribution, specific gravity, plastic and liquid limits) of the materials tested; mixture design and determination of the standard (ASTM D698) and modified (ASTM D1557) compaction parameters, swell percent and pressure; and triaxial testing (at effective confining stress levels typically encountered in road embankments and bridge abutments) to determine the small- and large-strain stiffness parameters of the ESR mixtures stabilized with high-carbon content OS-FA.
  • Task 3 - Report writing: write up of the final USDOT/MPC technical report and technical paper.
  • Task 4 - Technology transfer: a technical paper will be prepared and presented at the 2010 Summer Workshop of the Transportation Research Board's ADC60 - Committee for Waste Management and Resource Efficiency in Transportation and/or at a local workshop for industry firms and students.

MPC Critical Issues Addressed by the Research

All critical issues listed below are related to the USDOT Strategic Goal "Infrastructure Management and Environmental Stewardship:"

  1. Improved Infrastructure Design
  2. Infrastructure Longevity
  3. Infrastructure that Minimizes Environmental Impacts.

Contributions/Potential Applications of Research

Possible applications of the proposed technology include the use of high-carbon content OS-FA-ESR mixtures as embankment, bridge abutment or backfill material in low-volume road and bridge construction in areas where expansive soils abound. With minor modifications, the technology may be adapted to other applications such as design of residential foundations or construction of trail surfaces in federal and state parks.

Potential Technology Transfer Benefits

The research products will comprise (1) mentoring of a qualified graduate student and (2) publication and presentation of a peer-reviewed paper. The project will allow the PI to continue the close collaborations established during the Seed Project with local CCP suppliers and ACAA. Additionally, significant environmental benefits will result from large-scale recycling of waste tires and CCPs and their diversion from landfills. All these issues will be disseminated to a national audience at the 2010 TRB-ADC60 Committee Summer Workshop and/or at the local industry workshop at the end of the project.

Time Duration

July 1, 2009 through June 30, 2010

Total Project Cost


MPC Funds Requested


TRB Keywords

Low-volume roads, mechanistic design, sustainability, stiffness, resilient modulus


  • Ahmed, I.; C.W. Lovell (1993) "Rubber soils as lightweight geomaterials." Transportation Research Record 1422, TRB, National Research Council, pp. 61-70.
  • Ardani, A; S. Hussain; R. LaForce (2003) "Evaluation of Premature PCC Pavement Longitudinal Cracking in Colorado" Proc. 2003 Mid-Continent Transp. Res. Symp., Ames, IA.
  • Edil, T.B.; P. J. Bosscher (1994) "Engineering properties of tire chips and soil mixtures." Geotechnical Testing Journal, ASTM, 17 (4), pp 453-464.
  • Lee, J.H.; R. Salgado; A. Bernal; C.W.Lovell (1999) "Shredded Tires and Rubber Sand as Lightweight Backfill" Journal of Geotech. and Geoenv. Eng., ASCE, 125 (2), pp 132-141.
  • Seda, J. H.; Lee, J. C.; Carraro, J. A. H. (2007) "Beneficial Use of Waste Tire Rubber for Swelling Potential Mitigation in Expansive Soils", Geotechnical Special Publication 172, ASCE, Denver.
  • Tatlisoz, N.; C.H. Benson; T. B. Edil (1997) "Effect of fines on mechanical properties of soil-tire chip mixtures." ASTM STP 1275, pp. 93-108.
  • Yoon, S; M. Prezzi; N. Z. Siddiki; B. Kim (2006) "Construction of a test embankment using a sand-tire shred mixture as fill material" Waste Management, Elsevier, 26, pp 1033-1044.
  • Zornberg, J. G.; Y. D. Costa; B. Volenweider (2004) "Performance of prototype embankment built with tire shreds and nongranular soil" Transportation Research Record 1874, TRB, National Research Council, pp 70-77.
  • Carraro, J.A.H., Dunham-Friel, J.S., Smidt, M.S. (2008) "Beneficial Use of Waste Tire Rubber for Swell Potential Mitigation in Expansive Soils." USDOT-MPC Interim Report, Fort Collins, CO.
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