Best Practices to Address Issues of Excess Aggregate Dust in Nebraska

Background Excess aggregate dust, also known as "dirty" aggregate, can cause issues in concrete at different stages. While the potential negative impact is well-recognized, it is also known that the mineralogy of the dust is critical. For example, clay coatings showed a more harmful impact on concrete performance compared with other dusts such as carbonates (limestone dust) or stone dust. Clays that weakly adhere to aggregate will be dispersed in the mixing water and therefore will be integrated into the cement paste, which could lead to the workability issue. Specific clays mixed with a particular type of air entraining admixtures (AEA) can largely neutralize the function of the AEA and make it difficult to achieve required freeze/thaw resistance. On the other hand, clays that are strongly bonded to the aggregate surface will remain mostly located at the aggregate surface after mixing process and therefore may disrupt the aggregate-paste bond (usually referred to as interfacial transition zone (ITZ)) and results in strength and durability issues. Examples of the extent of aggregate dust-related issues are dust coating observed during paving operations, residual air bubbles in the mixer after concrete mixing (indicating the potential air entrainment issue), and negative impact on mechanical properties of concrete associated with excess aggregate dust. Due to the different mineralogical characteristics, clays with higher cation exchange capacity (CEC), such as montmorillonite, tend to cause more issues because of the higher absorption and swelling rate. With the excessive amount of dust often found in western Nebraska aggregate, NDOT requires that aggregates shall be washed to clean any coatings when aggregates are from dry pit locations. While NDOT had adopted a proactive practice of requiring a Sand Equivalent value not less than 90 in accordance with AASTHO T176 with aggregates from dry pits, and a maximum fine (No. 200 minus) content of aggregate lower than 3%, it is still not clear if the current practice is sufficient to accurately quantify aggregate dusts based on clay types, and what the acceptable dust content should be. A preliminary literature review from the research team indicates that parameters such as the California Cleanness Value (CV) and Methylene Blue Value (MBV) might better evaluate the quality of clay, and potentially better serve as indicators to predict concrete performance than the current specification methods. The extent and impact of dust in concrete depend not only on quantity, but also the nature of dust. Even though the negative impacts of an excessive amount of dust on concrete performance have been known and reported in different states, the impact of the types and amounts of clays on concrete performance is still not fully understood. In order to effectively prevent the clay dust issue, a more fundamental understanding of the nature of the impact of clay is necessary. Besides traditional tests for fresh, hardened and durability properties, advanced techniques such as scanning electron microscope and energy dispersive X-ray spectroscopy (SEM/EDX) can provide better options to understand the characteristics and mineralogy of aggregate dust as well as the extent of issues, such as ITZ debonding and deterioration inside concrete. Objective The overall goal of this research is to determine the best practice to address potential issues of excess aggregate dust in Nebraska concrete. To achieve the goal, three specific objectives of this study are to: (1) Select and conduct various laboratory tests to characterize different types of aggregate dusts; (2) Access the negative impact of excess dust on NDOT concrete properties and performance; and (3) Identify the best practice to further improve current NDOT methods to control the negative effects of for excess dust in concrete mixtures.

    Language

    • English

    Project

    • Status: Active
    • Funding: $ $130,158.00
    • Sponsor Organizations:

      Nebraska Department of Transportation

      1500 Nebraska 2
      Lincoln, NE  United States  68502
    • Project Managers:

      Halsey, Lieska

    • Performing Organizations:

      University of Nebraska, Lincoln

      1400 R Street
      Lincoln, NE  United States  68588
    • Principal Investigators:

      Hu, Jiong

    • Start Date: 20190701
    • Expected Completion Date: 20201231
    • Actual Completion Date: 0
    • USDOT Program: Transportation, Planning, Research, and Development

    Subject/Index Terms

    Filing Info

    • Accession Number: 01705870
    • Record Type: Research project
    • Source Agency: Nebraska Department of Transportation
    • Files: RiP, STATEDOT
    • Created Date: May 24 2019 3:37PM