APPENDIX B

Using Off-Specification Coal Ashes in Highway Concrete Applications

Background

This has been developed to guide the selection and use of off-specification coal ash (i.e., those not satisfying all AASHTO M 295 requirements after harmonization with ASTM C618-23) for use in highway infrastructure. The purpose of the document is to provide a guide for Department of Transportation (DOT) officials on potential next steps for assessing the quality of coal ash sources that do not meet all their specification requirements. This document does not recommend any additional changes to the AASHTO M 295 specification. The potential impact of each noncompliant coal ash parameter on concrete properties is described. Mitigation strategies and recommendations are then provided, if warranted. This will allow the potential use of such coal ashes that may not be compliant with specifications for certain applications.

If the coal ash fails a specific specification limit, several possible routes for its acceptance and approval are detailed based on the coal ash characteristics:

1. The environmental exposure of the proposed concrete element needs to be identified to determine whether changes or relaxation of coal ash specification requirements will affect concrete durability and performance for the intended application.
2. Coal ash may be further tested to ascertain impacts of off-specification characteristics on concrete mixture performance.
3. Alternatively, coal ashes can often be brought into compliance with specification limits by the supplier through blending or beneficiation of the coal ashes, if so desired.

The ability to use materials that do not meet specification limits strongly depends on why the material does not meet the specification. Coal ashes that exhibit low strength development, for example, are likely not suitable for bridge and pavement applications, but may be acceptable for low-volume applications or after blending with more reactive coal ash or other supplementary cementitious materials (SCMs). Another example is that a coal ash with high AEA adsorption and high variability in the required AEA dosage may need additional consideration during the mixture design process to enable it for applications in regions where freeze-thaw resistance is an issue but may be acceptable in other regions where there is no exposure to freeze-thaw cycles.

The document provides discussion and guidelines on all required specification limits in AASHTO M 295. Given the reduced availability of coal ash in some states, DOT officials can use this document to determine if using coal ashes that do not meet one or more test limits may still be suitable for use in certain applications. For each limit the following are listed: (1) reason for their inclusion in the specification, (2) potential concrete performance issues associated if the off-specification coal ash is used, and (3) the recommendation for use or nonuse of the coal ash and potential mitigation strategies to bring the coal ash into compliance. Some of the mitigation

strategies are not prescriptive and require engineering judgement by the DOT. There is no obligation for the DOTs to follow recommendations in this document.

This document is intended for use by the DOTs, but many of the mitigation strategies require action by the coal ash supplier and/or concrete producer. One intention of the document is to help develop a dialogue between the DOTs and suppliers/producers to promote the beneficial use of off-specification coal ash, as high-quality coal ash sources become more difficult to locate.

Beyond the guidelines provided herein, DOTs are permitted to test the impacts of an off-specification coal ash in concrete using local materials and mixture designs to determine if the mixtures meet all the DOT specifications for a given application. Prescriptive concrete testing is not provided in this document due to varying DOT requirements. DOTs are encouraged to read the main text of this report for more information about the performance of off-specification coal ashes. If the DOT is satisfied with concrete performance using their own testing protocols, they can choose to ignore the recommendations in this document.

Recommendations

Non-Compliant Specification Limit: Low Sum of Oxides (50%)

Reason for Inclusion in Specification: This prescriptive limit has traditionally been used to limit less reactive materials, and to ensure lower amounts of alkalis, sulfates, and magnesium in the coal ash. Pozzolanic reactivity is related to the amount of amorphous silica and alumina in coal ash, thus ashes with low sum of oxides can be expected to be less reactive.

Potential Concrete Performance Issues: Use of coal ashes with high percentages of nonpozzolanic composition may result in lower reactivity, lower strength development, low durability and/or decreased ASR mitigation ability, and high permeability.

Recommendation and Potential Issue Mitigation Strategy:

If all three recommendations are satisfied, allow use of coal ash. Otherwise, do not allow its use.

1. Addressing the low reactivity and strength development potential issue: SAI and reactivity limits must be satisfied. Refer to the SAI testing section for more information on SAI and reactivity testing.
2. Addressing alkalis, sulfates, and magnesium contents: High alkali content and high SO₃ content are addressed later in this guide. Ensure MgO content is comparable to other specification-compliant coal ashes used by the DOT.
3. Addressing potential for low ASR mitigation ability: Use AASHTO R 80 guidance if using with reactive aggregate to ensure ASR mitigation for given application. If ASR mitigation is insufficient, coal ash can only be used in mixtures without reactive aggregates or in areas with no exposure to external moisture.

Non-Compliant Specification Limit: High Sulfur Trioxide (> 5%)

Reason for Inclusion in Specification: Excess sulfates can potentially impact concrete setting time, strength development, and most importantly dimensional stability (expansion) concerns.

Potential Concrete Performance Issues: Delayed setting, low strength development, potential for deleterious expansion due to internal sulfate attack may result with the use of high-SO₃ coal ashes. In some cases, high sulfate and sulfite contents can contribute to the development of flash set. The impacts of sulfate depend strongly on the form of the sulfate-bearing phases within the coal ash. Further, high temperatures and certain cement/admixture combinations can exacerbate the delayed set and strength gain issues.

Recommendation and Potential Issue Mitigation Strategy: Test the following: (1) setting times using ASTM C191 to assess the level of setting time delays that will result from extra sulfates, (2) coal ash-cement combinations for expansion using ASTM C1038 to ensure the presence of high sulfate does not result in water-driven expansions, and (3) compressive strength on

mortar cubes using ASTM C109 to verify that the presence of sulfate does not affect Si/Al balance and result in low strength development (Neto et al. 2021). For these tests, it is recommended that the coal ash replace cement at a percentage commonly used by the DOT. If there are no negative impacts to setting, volume stability/expansion, and mortar strength as determined by the DOT compared to a specification-compliant coal ash, then the coal ash can be approved. If significant negative effects on setting, expansion, and/or strength are observed, do not allow use of coal ash. Blending of the coal ash with a lower SO₃ coal ash or other SCM could be allowed to eliminate negative impacts or to bring coal ash into specification compliance.

Non-Compliant Specification Limit: High Moisture Content (> 3%)

Reason for Inclusion in Specification: High moisture content can cause handling problems and prevent pneumatic transfer of material into storage silos and weigh hoppers.

Potential Concrete Performance Issues: For the above reason, it is unlikely that much higher moisture contents can be handled in current concrete production silo setups. However, if the concrete producer can handle the coal ash, insignificant changes in concrete performance are expected if coal ash moisture content is accounted for in concrete mixture proportioning.

Recommendation and Potential Issue Mitigation Strategy: Allow use of coal ash provided it can be handled in transportation and during concrete production.

Non-Compliant Specification Limit: High LOI (6–12% or> 12%)

Reason for Inclusion in Specification: The LOI of coal ashes is primarily associated with unburned carbon. Unburned carbon often has a high surface area that adsorbs air-entraining admixtures, resulting in low concrete air contents or higher dosage requirements to achieve proper quantities of entrained air. Thus, limiting LOI is intended to exclude sources of coal ash with high levels of admixture adsorption. In addition to high levels of adsorption, results of this research report showed that increased LOI lead to increased variability in admixture adsorption. The limit of 6% LOI in the AASHTO M 295 (if it is harmonized with ASTM C618) reduces admixture adsorption and minimizes variability in adsorption when using a specific coal ash source. Coal ashes with up to 12% LOI may also have greater day to day variability in LOI. In addition, for harvested ash sources, high LOI can be caused by contaminant materials such as flue gas desulfurization (FGD) products, or soils or clays from impoundment groundworks or clay liners. Incorporation of these products into a coal ash source can result in significant changes in concrete mixture rheology, and reductions in strength and durability.

Potential Concrete Performance Issues: Durability against deterioration from freezing and thawing may be reduced if concrete air contents are variable and cannot be controlled with AEA admixture adjustments. High LOI has been linked with high levels of required AEA admixture adsorption by coal ashes. High LOI has also been associated with greater variation in adsorption within a sample. High LOI may also result from site contamination with FGD or clays in harvested ash sources.

Recommendation and Potential Issue Mitigation Strategy:

If LOI > 12%, do not allow use of coal ash in concrete.

While the main text of this research report indicates that concrete using coal ash with as high as 17% LOI can be properly air entrained, there are significant challenges when using coal ashes with LOI exceeding 12%, and the likelihood of significant material variability and contamination are high. In addition, strength reductions at such high LOI levels are also expected. The coal ash should be beneficiated to reduce LOI prior to use.

If LOI = 6–12%, consider approving the coal ash for use in concrete applications.

1. Allow use of high LOI ash in applications not subject to freezing and thawing if strength requirements are met. Specifically, test compressive strength using mortar cubes following ASTM C109 to verify that strength development is adequate. It is recommended that

1. the coal ash replace cement at a percentage commonly used by the DOT. If there are no negative impacts to strength compared to a specification-compliant coal ash as determined by the DOT, then the coal ash can be approved. High LOI has been shown to primarily impact required air entrainment admixture dosages and did not significantly reduce strength development in the testing associated with this report. However, verification for strength is still recommended.
2. Coal ash with LOI = 6–12% should be acceptable for use in applications subject to freezing and thawing provided the concrete producer is able to adjust admixture dosages and can manage the variability in LOI known to occur in the coal ash to consistently obtain adequate air contents. This research report indicates that concrete using coal ash with as high as 17% LOI can be properly air entrained and can obtain adequate air void spacing factor required to obtain concrete resistant to deterioration from cyclical freezing and thawing.

   For acceptance and monitoring of variability for coal ashes with LOI = 6–12%, suppliers should be required to report Foam Index Test (FIT) results measured by ASTM C1827 and provide information on the cement used to make the FIT test slurry. Further, suppliers should be required to satisfy the mortar air uniformity requirement specified in AASHTO M 295. FIT and the fluorescence method developed in this research report and provided as a draft specification to AASHTO as Appendix 4 were both found to have increased sensitivity to variances in levels of coal ash adsorption. They are also faster and simpler to run than the traditional mortar air test. To account for the potential higher day to day variability in LOI and adsorption for these ashes, it is recommended that FIT tests be reported using a daily or 400-ton frequency sampling. If FIT results are deemed satisfactory by the DOT and mortar air uniformity is compliant, allow use of the coal ash where air entrainment of concrete is required. Otherwise, do not allow use of coal ash.

   In the main text of this research report, FIT results with less than a 185% increase in FIT relative to the control cement slurry (0.9–0.17 mL/g of cementitious materials for a sodium laurel sulfate-based AEA) resulted in less than a 6% increase in AEA dosage requirements to obtain air contents of 5–7% in concrete. FIT results less than 300% greater than the cement control resulted in an AEA dosage increase of < 12% in concrete mixtures, suggesting that even coal ashes requiring as much as 3 times the dosage of AEA in the FIT have little effect on AEA dosing in concrete. Further, larger increases in FIT relative to the FIT of the control mixture do not necessitate rejection of the sample but can provide some indication of the magnitude of expected AEA dosage increase to the concrete producer.

Marginal Property: High alkali content

Reason for Inclusion in Specification: When used in combination with ASR aggregates, high alkali content can contribute to alkali loading of concrete mixtures and negatively impact ASR performance or the level of ASR mitigation required for that coal ash.

Potential Concrete Performance Issues: Increase in potential for ASR reactivity (and decrease in ASR mitigation potential) in mixtures using reactive aggregate.

Recommendation and Potential Issue Mitigation Strategy: There is no limit for alkali content in AASHTO M 295. It is recommended that AASHTO R 80 guidance be followed for concrete mixture design when using coal ashes with high alkali contents. AASHTO R 80 recommends a maximum alkali content of 4.5% Na₂O_eq for coal ashes with a maximum CaO content of 18% for use in applications where ASR mitigation is required. For coal ashes with > 4.5% Na₂O_eq or > 18% CaO, use of the performance-based testing approach in AASHTO R 80 is recommended. If ASR mitigation is insufficient, coal ash can only be used in mixtures without reactive aggregates or in areas with no exposure to external moisture.

*Note that this is a topic in which understanding of best test methods and limits is rapidly developing. Alkali limits and approaches to testing should be reevaluated in the coming years.

Non-Compliant Specification Limit: Coarse Particles (> 34% retained on no. 325 sieve)

Reason for Inclusion in Specification: To limit the quantity of coarse particles, which have lower reactivity due to lower surface area. Large particles could also be associated with contaminants and carbon.

Potential Concrete Performance Issues: Variances in water requirement, variances in strength development, and low strength development compared to specification-compliant coal ash.

Recommendation and Potential Issue Mitigation Strategy: Testing results shown in the main text of this research report suggest that out of specification fineness values (i.e., coarse ashes) correlated with reduced reactivity and strength development in samples. To assess these parameters, the coal ash should undergo SAI testing and two reactivity tests: R³ limits of 7-day bound water (ASTM C1897 – Procedure B) and the bulk resistivity index (BRI) (Y. Wang et al. 2021). A draft of the BRI method specification has been provided to AASHTO as Appendix 3.

Allow use of the coal ash if the following three limits are passed: (1) SAI testing at any age as outlined in AASHTO M 295, (2) an R³ limit of 7-day bound water > 3.5 g/100 g dry paste, and (3) a BRI > 100%. If the two reactivity measurements are met but SAI limits are not met at any age, repeat the SAI testing with a different cement. If the ash passes the new SAI testing and the two reactivity limits, then allow use of the coal ash.

If the ash fails all SAI testing and the ash does not pass both the reactivity testing limits, this is an indicator of slow reactivity, and/or the presence of deleterious material in the ash which is limiting strength gain. Sieving, grinding, air classification, or blending are recommended. After processing by the supplier, if the ash does not meet SAI limits at any age and both reactivity test limits, then do not use the coal ash.

Workability issues resulting from sample fineness can be resolved through changes in concrete mixture design or by using water-reducing admixtures.

Non-Compliant Specification Limit: Coarse Particles (> 10% retained on No. 100 sieve)

Reason for inclusion in specification: When coal ash is harvested from landfills, it may contain agglomerates or large particles from co-mingling with bottom ash. Since large particles have lower reactivity, this requirement helps limit the impact of low fineness by requiring processing of harvested ash to increase fineness.

Potential Concrete Performance Issues: Variances in water requirement and strength development compared to specification-compliant coal ash.

Recommendation and Potential Issue Mitigation Strategy: The strategy is the same listed for Coarse Particles (> 34% retained on no. 325 sieve) as discussed in the previous section.

Non-Compliant Specification Limit: Low Strength Activity Index (< 75% at all tested ages)

Reason for Inclusion in Specification: A quantitative indicator of coal ash reactivity and to ensure it contributes to strength development.

Potential Concrete Performance Issues: Low SAI levels indicate low reactivity materials with poor strength development. Low SAI levels may also indicate materials that do not provide the expected contribution to reducing permeability and improved durability.

Recommendation and Potential Issue Mitigation Strategy: If SAI limits are not met at any age, material reactivity should be investigated by testing R³ bound water (ASTM C1897 – Procedure B) and the BRI method (Y. Wang et al. 2021). A draft of the BRI method specification has been provided to AASHTO as Appendix 3. If the coal ash meets both reactivity testing limits: (1) 7-day R³ test bound water > 3.5 g/100 g dry paste and (2) a BRI > 100%, repeat the SAI testing with a different cement. If the ash passes the new SAI testing and the two reactivity limits, then allow use of the coal ash.

If the ash fails all SAI testing and the ash does not pass both the reactivity testing limits, this indicates slow reactivity, and/or the presence of deleterious material in the ash which is limiting strength gain. Sieving, grinding, air classification, or blending are recommended. After processing by the supplier, if the ash does not meet SAI limits at any age and both reactivity test limits, then do not use the coal ash.

Non-Compliant Specification Limit: High Water Requirement (> 105% of control)

Reason for Inclusion in Specification: Prior to the development of water-reducing admixtures, a high water requirement potentially indicated that concrete with a higher w/cm would be required to meet workability specifications, thus leading to decreased strength development. Using current concrete production technology, a high water requirement suggests that an increased dosage of a water-reducing admixture may be needed. Also, due to the use of constant flow to determine w/cm in the SAI test, high water demand will result in lower SAI values.

Potential Concrete Performance Issues: Workability may be reduced through use of coal ash with high water demand, impacting placement quality or requiring usage of a water-reducing admixture to maintain required workability.

Recommendation and Potential Issue Mitigation Strategy: Allow use of coal ash with high water demand. Workability issues can be resolved by the concrete producer in concrete mixture design. It is proposed that AASHTO M 295 make water requirement report only since such issues can be resolved at a concrete level through use of admixtures without affecting performance.

Non-Compliant Specification Limit: Uniformity Requirement (Variation in Density or Percentage Retained on No. 325 Sieve)

Reason for Inclusion in Specification: Ensure batch-to-batch consistency of concrete made with the coal ash due to (1) variable coal ash composition due to changes in density and (2) variable reactivity due to changes in fineness.

Potential Concrete Performance Issues: Consistency in fresh and hardened properties and variability in admixture requirements can be difficult to achieve with high material variability, potentially leading to concrete performance issues. Variable density may also result in inconsistent concrete yield.

Recommendation and Potential Issue Mitigation Strategy: If uniformity requirements are not met, do not allow use of coal ash. Noncompliant coal ash can be beneficiated by the supplier to reduce nonuniform characteristics into specification compliance. It should be noted that composite samples—monthly or 3,200-tons—will likely have lower variation between measurements compared to regular samples—daily or 400-tons—for the same coal ash. Further research is needed to better understand batch-to-batch uniformity for coal ash to address this issue.

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