Methods for Evaluating Fly Ash for Use in Highway Concrete (2013)

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1 Background Fly ash—a byproduct of coal combustion—is widely used as a cementitious and pozzo- lanic ingredient in hydraulic cement concrete. The use of coal fly ash (CFA) in concrete is increasing because it improves some properties of concrete and often results in a lower cost of concrete. However, the chemical and physical compositions of CFA influence construc- tability, performance, and durability and may contribute to problems, such as cracking and alkali-silica reactivity (ASR) in concrete pavements, bridge decks, and other highway struc- tures. Regulatory requirements have also contributed to changes in CFA properties that may adversely affect concrete performance. In addition, current specifications and test methods do not adequately characterize CFA properties, address the effects of CFA characteristics on fresh and hardened concrete properties, or consider the alkali content of the cement. For example, carbon content of CFA is not usually determined directly but is often assumed to be approximately equal to the loss on ignition (LOI). Such inadequate characterization may lead to unwarranted restrictions on the use of suitable materials. Although a great deal of research has been performed on the effects of CFA characteristics on concrete properties, the research has not dealt with the applicability of current specifications to the fly ashes that currently are produced. In addition, existing test methods for sampling and testing CFA used in concrete do not adequately address the characterization of CFA or the performance aspects of high- way concrete. Further research is needed to develop recommendations for improving CFA specifications and test protocols and thus help highway agencies better evaluate and use CFA that will provide acceptable structural performance and durability. NCHRP Project 18-13 was initiated to address this need. Objective and Scope The objective of this research was to recommend potential improvements to specifica- tions and test protocols to determine the acceptability of CFA for use in highway concrete. To accomplish this objective, the research included the following: • A study of existing specifications and classification methods for CFA to recommend changes that would provide better criteria for selection of CFA for a given level of performance. • An investigation of new test methods for characterizing the strength activity of CFA. • Identification of new test methods for characterizing the properties of residual carbon in CFA and investigation of approaches for estimating air-entraining admixture or agent (AEA) dosage for CFA. • Evaluation of the use of CFA to mitigate alkali-silica reaction in concrete and provision of guidance on selection of CFA type and dosage for a specified level of field performance. S U M M A R Y Methods for Evaluating Fly Ash for Use in Highway Concrete

2Overview of the Project The research to evaluate new and existing test procedures involved the following five tasks that were performed as separate studies: • Review of existing specification and testing environment • Characterization of coal fly ash • Evaluation of approaches for characterizing strength activity • Evaluation of test methods for characterizing the effects of carbon on air entrainment • Evaluation of approaches for assessing ASR mitigation Existing Specification and Testing Environment This task was accomplished by reviewing the literature pertaining to CFA tests and speci- fications. Also, a survey of state highway agencies (SHAs) was conducted to determine the current practices and the needs for new tests and specifications. Characterization of Coal Fly Ash Thirty sources of CFA representative of the range of CFA available for use in transporta- tion infrastructure construction in the United States were identified through the survey and industry contacts. A complete characterization was performed for these ashes using a variety of laboratory tests to determine the properties specified in AASHTO M 295-07, Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete, and other properties that should be considered in developing new specifications. The results of this characterization study were compared to certification information obtained from the producers. The task also helped identify areas for improvements in current tests. Characterization of Strength Activity Two new approaches for measuring the effect of CFA on the strength of a cementitious mixture were investigated. One approach involved modifications to the current strength activity index (SAI) and the other dealt with a new approach based on the Keil hydraulic index (KHI). Characterization of the Effects of Carbon on Air Entrainment This task involved evaluation of the foam drainage test and the foam index test. Addition- ally, two new tests—the CFA iodine number (based on an existing test ASTM D4607) and the direct adsorption isotherm test (based on ASTM D3860)—were developed and applied to a range of CFAs selected from the 30 identified sources. These four tests were performed on a number of CFAs characterized in terms of the LOI test and selected to provide a broad range in LOI values. The direct adsorption isotherm test was also conducted on various mortar and concrete mixtures to predict the AEA dosage required to achieve a target air content. Hard- ened air-void analyses were performed to identify differences in the air-void systems resulting from the additional AEA required when CFA was included in the mixtures. Assessment of ASR Mitigation This task examined the existing approaches for determining the effectiveness of a CFA at mitigating ASR and evaluated new procedures for assessing this property of CFA. Specifically,

3 this task investigated whether the ASTM C1567 rapid mortar bar test provides better guidance than ASTM C441 and should therefore be included in AASHTO M 295. For this purpose, ASTM C1293 concrete prism tests were conducted to provide a reference. This task also evalu- ated the alkali leaching test as an alternative approach to the ASTM C441 or ASTM C1567 tests. This study was conducted using eight CFA sources (four Class C ashes and four Class F ashes) selected based on their apparent ability to mitigate ASR, as determined from history and performance. Results and Conclusions Results of the literature review and a survey of SHAs suggested the need for improving the tests and specifications for CFA used in highway concrete to better identify those properties affecting concrete performance. For the fly ash characterization task, 30 fly ash sources were evaluated using the test methods and specification limits stipulated in AASHTO M 295-07. The results of this research suggested a need for refining the existing classification method to include properties known to affect performance, but a completely new approach to classification is not warranted. The primary distinction between Class F and Class C coal fly ash is the bulk composition based on the sum of silicon dioxide, aluminum oxide, and iron oxide (i.e., %SiO2 + %Al2O3 + %Fe2O3). However, this classification omits the reporting of calcium. This research has shown a distinct, linear relationship between the sum of the oxides and the calcium content and, therefore, specifying either provides the same result. However, because the calcium content is required for some ASR mitigation practices (e.g., AASHTO PP 65, Standard Prac- tice for Determining the Reactivity of Concrete Aggregates and Selecting Appropriate Measures for Preventing Deleterious Expansion in New Concrete Construction), it is recommended that AASHTO M 295 be modified to require reporting the calcium oxide content, expressed as CaO, and also the content of magnesium oxide, sodium oxide, potassium oxide, and the equivalent alkali (Na2Oe). The CaO and Na2Oe values are required for determining strategies for ASR mitigation according to AASHTO PP 65. To address the effect of a CFA on air entrainment, testing was conducted to develop new test methods to predict the AEA demand of any given CFA. These tests included the foam drainage and foam index tests and two new proposed tests: the CFA iodine number and direct adsorption isotherm tests. After conducting and reviewing a wide range of foam index tests, a modified version of the test method by Harris et al. (2008a) was recommended as a standard test. The modi- fications included using a range of standard solution concentrations and the solution that achieves a stable foam in a consistent time (i.e., 15 ± 3 min) was chosen for that fly ash. Another modification was the use of a mechanical agitator to minimize operator-induced variability. Overall, the foam index test was found to be suitable for assessing CFA and AEA inter- actions but not for measuring the influence of CFA on air entrainment. Even with the pro- posed improvements, the test has a high level of subjectivity and variability. In spite of its popularity in the concrete industry, the foam index test has not been standardized. Given that use of the test will likely continue, there needs to be a standard approach to conducting the test. The CFA iodine number and the direct adsorption isotherm tests are recommended to be included in AASHTO M 295 as a Supplementary Optional Chemical Requirement for assessing the adsorption potential of a specific CFA source. From a classification perspective, it is recommended to retain the LOI measure as a means of limiting the maximum carbon content, rather than establishing a maximum CFA iodine number. The CFA iodine number

4and direct adsorption isotherm test can be used as a means for identifying those CFA sources that have a potential to adversely affect air entrainment. The CFA iodine number test is recommended for use as a screening test. If the test results in a value > 0.1 mg iodine/gram CFA, a direct adsorption isotherm is recommended with a specified AEA, and the capacity determined for the CFA-AEA combination is reported. Another consideration is the use of powder activated carbon (PAC) for pollution control in power plants, which often increases the adsorption capacity of the CFA without signifi- cantly increasing the LOI. Both the CFA iodine number and direct adsorption isotherm tests are extremely sensitive to the inclusion of PAC and they will identify the presence of PAC in the ash and quantify its impact on air entrainment. With the increased use of these pollu- tion control approaches, measurement of the adsorption potential of CFA will increase in importance. The current available alkali test in AASHTO M 295 takes over 1 month to complete and its precision is questionable. Given that AASHTO PP 65 specifies total alkali limits that exceed the current available alkali limits, it is recommended the available alkali limits from AASHTO M 295 be deleted and requirements for reporting the total alkali contents be inserted as required in AASHTO PP 65. The current SAI has a minimum strength requirement that can be met by relatively inert materials (e.g., finely ground quartz). Therefore, increasing the 7-day limit from 75% to 85% seems appropriate. However, some Class F fly ashes including those with good long-term strength-gain behavior may fail this requirement. For example, all of the CFAs tested reached acceptable strengths by 90 days of moist curing. Therefore, for determining the strength poten- tial of a CFA, it is recommended the minimum SAI in AASHTO M 295 be raised to 85% at 7, 28, or 56 days of age, with further development focusing on an accelerated pozzolanic activity index-based test. The current test for ASR mitigation contained in AASHTO M 295 (ASTM C441) uses Pyrex® glass as a synthetic alkali-silica reactive aggregate. For consistency with AASHTO PP 65, ASTM C1567 should be incorporated in AASHTO M 295 with a maximum expansion limit of 0.10% after 14 days of exposure in sodium hydroxide (NaOH) solution. The research results also indicated that the use of a 28-day expansion limit of 0.10% for evaluation of ASTM C1567 results is not appropriate. In the testing performed, three of the four Class F ashes were not adequate for ASR mitigation when a 28-day expansion limit was used, even at 40% replacement of cement. This performance was not supported by results from the 2-year prism tests conducted according to ASTM C1293. The ASTM C1293 concrete prism test allows testing both the coarse and fine aggregates, but it is not recommended for inclusion in AASHTO M 295 because it requires an extensive time period to complete (2 years is recommended in ASTM C1293 and AASHTO PP 65).