The effects of alkali-aggregate reactivity on the mechanical properties of concrete

Langdon, Sheldon (1998) The effects of alkali-aggregate reactivity on the mechanical properties of concrete. Masters thesis, Memorial University of Newfoundland.

[English] PDF (Migrated (PDF/A Conversion) from original format: (application/pdf)) - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (22MB) [English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. (Original Version)

Abstract

Alkali-aggregate reactivity is an internal chemical reaction between the sodium and potassium alkaline components in the concrete mix and active mineral constituents of some aggregates. The reaction results in the formation of a gel which absorbs water, expands, and therefore exerts internal pressures which sometimes can be far in excess of that which concrete can sustain, thereby causing the formation of micro cracks. -- In this investigation, a potentially highly reactive aggregate, and a potentially marginally reactive aggregate (identified by accelerated mortar bar testing, and petrographic examination) were used in concrete specimens, in both normal and high strength mix designs. After the initial 28 day curing period, the number of samples allocated for each testing procedure were equally divided, and then submerged in a holding tank containing either a solution of 1M sodium hydroxide or de-ionized water at 80 ° C over an extended period of time. Research has proven that a sodium hydroxide solution can accelerate an alkali-aggregate reaction. -- From these specimens, the mechanical properties such as the compressive strength, direct tensile strength, modulus of rupture, freeze - thaw characteristics, as well as the creep characteristics were determined for all samples at different time intervals. In comparing the mechanical properties of samples located in the sodium hydroxide solution, and those determined from samples located in the de-ionized water, the effect of an alkali-aggregate reaction on the mechanical properties of concrete could be examined. -- In general, normal strength concrete samples subjected to the sodium hydroxide solution containing the potentially highly reactive aggregate experienced increased losses in mechanical properties than that of specimens containing the potentially marginally reactive aggregate. However, for high strength concrete samples subjected to the solution, there were little, if any, loss of mechanical properties for either specimens containing the potentially reactive or marginal aggregates. This is explained by the improved micro structure of high strength concrete as a result of the secondary pozzolanic reaction. -- The normal strength samples containing the highly reactive aggregate experienced a loss in ultimate compressive strength of 28%, and a decrease in the modulus of elasticity of 80% over the testing period. Samples containing the moderately reactive aggregate experienced no loss in compressive strength over the testing period, and a decrease in the modulus of elasticity of 20%. The ultimate tensile strength of both normal strength concrete samples containing the highly and moderately reactive aggregate decreased by 37 %, and 31 % respectively. The freeze - thaw effects revealed that normal strength specimens containing the moderately reactive aggregate experienced larger decrease in mechanical properties than that of specimens containing the highly reactive aggregate. Creep of the normal strength concrete containing the highly reactive aggregate experienced a 94% increase in creep strain with respect to the control, whereas normal strength specimens containing the moderately reactive aggregate experienced a 48% increase in creep strain with respect to the control. This is attributed to the weaker cement paste resulting from a more aggressive alkali-aggregate reaction. -- A theoretical investigation of the moment - curvature response of a typical I - section, and Tee - section were examined assuming expansive strains resulting from an alkali – aggregate reaction. From the investigation, it was apparent that the effects of an alkali-aggregate reaction on the ultimate moment capacity, and curvature of both sections was not significant (<5%). These results are consistent with that determined from research conducted during the past decade involving pre-stressed, and well reinforced sections.

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