Date of Award

8-2008

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Civil Engineering

Advisor

Rangaraju, Prasad

Committee Member

Amirkhanian , Serji

Committee Member

Putman , Bradley

Abstract

Alkali Silica Reaction (ASR) is a widespread durability problem not only in the United States, but also in many countries across the world. ASR distress in concrete drastically reduces the service life of a structure and as such, there are no effective rehabilitation measures. As a result, the primary strategy to deal with ASR distress has been one of prevention/mitigation in new construction. Various mitigation measures are widely used to reduce the effects of ASR in concrete. Among these measures, the use of supplementary cementing materials (SCMs) is the most commonly employed method. SCMs are believed to mitigate expansions induced by ASR by reducing the permeability of the matrix through pozzolanic reaction and by binding alkalis within the hydrated cement paste matrix. The rate of pozzolanic reaction is a function of the physical and the chemical properties of SCMs. In addition, alkalinity and temperature also affect the pozzolanic reaction. The effectiveness of SCMs in mitigating ASR is evaluated using ASTM C 1567 test method. This test method calls for a short (2 days) but accelerated curing regime of mortar bars, followed by exposure to aggressive conditions (1N NaOH solution at 80¼C) to promote ASR reaction. Mortar bar expansions below 0.1% after 14 days of exposure to 1N NaOH are considered to represent effective mitigation measures. Realizing that effectiveness of SCMs is a function of pozzolanic reactivity; questions have been raised about the appropriateness of the accelerated curing regime in ASTM C 1567 test method to evaluate SCMs, particularly different fly ashes. The principal objective of this thesis is to determine the effectiveness of fly ashes with different chemical composition, and assess whether the standard curing regime as required in ASTM C 1567 test method is appropriate to evaluate ASR mitigation ability of fly ashes. In addition, the influence of water-to-cement ratio of mortar mixtures containing different fly ashes on the expansion of bars in ASTM C 1567 was evaluated. Also, the effectiveness of blended SCMs (low-lime fly ash + high-lime fly ash) in mitigating ASR expansions was evaluated. In this study, Spratt limestone - a moderately reactive aggregate was used. Seven different fly ashes with different chemical composition were used in this study. Attempt was made to select fly ashes that represented a range of lime contents. Six different curing regimes were evaluated. All the fly ashes were used at a replacement level of 25% with cement. These tests were performed using the Standard and the Modified ASTM C 1260 and ASTM C 1567 methods. In addition, scanning electron microscopy (SEM) and Energy Dispersive X-Ray Analysis (EDX) analyses were conducted to study the microstructure of the samples and Thermo-Gravimetric Analysis (TGA) was conducted to evaluate the pozzolanic behavior of fly ashes in different curing regimes. Results from these studies indicate that at normal dosage levels (25% mass replacement of cement) high lime fly ashes are not capable of mitigating ASR expansions below the acceptable limit (0.1% at 14 days), whereas the low lime and intermediate lime fly ashes could effectively mitigate the ASR expansions almost at all the ages of curing. It was observed that the prolonged curing was more effective in reducing 14-day expansions of mortar bars, with majority of fly ashes evaluated in this study. However, the influence of prolonged curing on 28-day expansions of mortar bars was not conclusive. The use of low w/c ratio (0.40) in mortar bars resulted in reduced 14-day expansions, compared to higher w/c ratio (0.54); however, 28-day expansions were observed to be somewhat lower for higher w/c ratio specimens. For a given lime content, blended fly ashes were not as effective as virgin fly ashes. This suggests that the composition of specific minerals and glass phase in fly ashes is more influential in mitigating ASR rather than its bulk chemical composition. Based on the research it is recommended that low-lime and intermediate-lime fly ashes should be employed in mitigation of ASR. High-lime fly ashes were found to be ineffective at normal levels of dosage. While blended ashes of certain lime content produced lower expansions than high-lime fly ashes, virgin ashes of comparable lime contents performed better than blended ashes. Based on the work conducted in this thesis, it is recommended that prolonged curing of mortar bars containing fly ash is desirable to more accurately characterize the ASR mitigation potential of fly ash. Additional research is recommended to extend the findings from this study to evaluate other SCMs such as slag, silica fume and natural pozzolans.

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