Fly ash is the most common supplementary cementitious material used to help improve the durability and sustainability of concrete and mitigate alkali-silica reactivity (ASR). While the demand for fly ash for use in concrete remains steady, the supply of high quality fly ash has been dwindling or, at least, becoming unpredictable. This is partly due to electric power plants changing their fuel sources (e.g., from coal to natural gas) and partly because of tighter environmental and air pollution regulations that have resulted in increased carbon, calcium, sulfur, alkali, and ammonia contents of the fly ash, which adversely affect its quality for use in concrete.
In recent years, there has been a growing concern among concrete users and producers about the quality and the quantity of fly ash presently available. In a recent American Association of State and Highway Transportation Officials (AASHTO) survey of the state departments of transportation, respondents noted significant issues with the supply of coal fly ash. This supply and demand imbalance for quality fly ash has already resulted in regional and seasonal shortages, which is expected to further exacerbate with time. Given this restriction on supply, coal fly ash that was once deemed marginal is now being used in concrete. Marginal fly ash is defined as the fly ash that is of lower quality [i.e., with high loss-on-ignition (LOI), fineness, moisture content, etc.] or otherwise unusable in concrete. To address the shortage, even unconventional sources of fly ash are now being considered, i.e., primarily the ash disposed of in landfills or impoundments.
To help meet the growing need for fly ash while maintaining the quality and performance of concrete, it becomes imperative for the concrete industry and the transportation agencies to look for and evaluate marginal and unconventional source fly ashes. To that end, it is also imperative that fly ash specifications measure properties that affect performance and not reject materials that, although suitable for use in concrete, just do not come up to the historical specifications adopted when high quality fly ash was plentiful. Therefore, fly ash specifications that also address marginal and unconventional source fly ashes need to include:
• Tests and specification limits to ensure predicted performance when ash is used in concrete, including but not limited to, strength, air entrainment, setting time, ASR mitigation, and enhanced performance in sulfate environments;
• Measuring ash properties that affect performance rather than measuring or specifying those that have no impact on performance; and
• Measuring temporal variability in ash from a single source.
The objectives of this research are to provide the following, with particular consideration to the marginal and unconventional source fly ashes:
• Recommendations for revising the AASHTO M 295, Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete, or developing a new specification for fly ash for use in highway concrete that will ensure the desired properties of the fresh and hardened concretes; and
• Guidelines for using fly ash not meeting the recommended revised or developed specification.
The research plan shall be executed in two phases and comprise, at a minimum, the following tasks:
Task 1. Literature Review
Conduct a critical review of the literature regarding the strengths and limitations of the current fly ash specifications and the associated tests with respect to their efficacy at predicting performance for fly ash obtained not only from existing usual sources but also, and more importantly, from marginal or unconventional sources. Also provide a review of recently developed test methods for characterizing the properties and performance of fly ash. Any gaps in current characterization methods and potential solutions should also be identified.
Task 2. Propose a Revised or a New Specification for Fly Ash and Develop a Work Plan for its Evaluation
Based on the literature review and the thinking of the proposing team, develop a draft specification, which could be a modified form of AASHTO M 295 or a completely new specification. Provide a rationale for why the proposed specification is more suitable for marginal and unconventional fly ash sources. Develop a work plan for Phase II that includes identifying potential fly ash sources and an experimental design for evaluating the proposed specification. A concrete testing program must be a part of this work plan.
Task 3. Implement the Developed Work Plan
Execute the work plan developed under Task 2. The materials tested must represent the range to include marginal or unconventional source fly ashes. Specific properties to be considered in the selection of materials will include, but not limited to, bulk composition, fineness, carbon and alkali contents, LOI, and density. When selecting materials, the range in property values shall include fly ash sources that meet the current AASHTO M 295 specification limits as well as those that do not. The selected materials shall also include unconventional source fly ashes such as those recovered from landfills or impoundments. It is expected that the recovered ashes will have been beneficiated, preferably, using at least three different beneficiation processes. Ashes that have not been beneficiated may be also included at the research team’s discretion. Novel or commercially viable beneficiation methods (whether proprietary or non-proprietary) are both acceptable. For any new test methods evaluated or proposed, their single-lab variability must also be provided. The concrete testing program in the work plan must demonstrate the efficacy of the proposed specification.
Task 4. Propose a New or a Revised/Updated Standard Specification
Based on the work performed in Task 3, an updated version of the AASHTO M 295 standard specification or a new standard specification shall be proposed.
Task 5. Develop Guidelines for Evaluating Marginal and Unconventional Source Fly Ash for Use in Highway Concrete
The guidelines for evaluating marginal and unconventional source fly ash for use in highway concrete should be developed as a stand-alone document. These guidelines will be used to assess the suitability of non-conforming fly ash for use in highway concrete.
Task 6. Final Report
The final report will document the entire research effort, results, findings, conclusions, recommendations, and proposed revisions to the AASHTO M 295 or a new standard specification that considers marginal and unconventional source fly ashes for use in highway concrete.
Deliverables shall include, at a minimum:
An interim report that provides the following:
1. A critical review of literature, considering factors and issues outlined in Task 1.
2. A proposed specification, a detailed task-by-task work plan, including the experimental design, test matrix, and a concrete testing program, based on an assessment and analysis of the current state of knowledge and practice and the findings of the critical literature review.
Phase I will be completed within four months (including 1 month for review) at a cost not exceeding $40,000.
Final deliverables include the following:
1. A final report that documents results, summarizes findings, and draws conclusions.
2. A guidance document for evaluating out-of-specification fly ash for use in highway concrete.
3. Proposed revision/update to the AASHTO M 295 standard specification or a new standard specification that addresses marginal or unconventional fly ash sources for use in highway concrete.
4. A draft TRNews article highlighting the products of this research and their implementation.
Status: Research in progress