Is the Available Alkali Test a Good Durability Predictor for Fly Ash Concrete Incorporating Reactive Aggregate?
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IS THE AVAILABLE ALKALITEST A GOOD DURABILITY PREDICTOR FOR FLY ASH CONCRETE INCORPORATING REACTIVE AGGREGATE? ROBERT L. SMITH ENRECO Laboratories, 6661-A Canyon E-Way, Amarillo, Texas 79110 Received 5 November, 1987; refereed ABSTRACT The available alkali test has been used for years as a method for preventing the use of a high-alkali fly ash in concrete containing reactive aggregate. This report presents data on fifteen good-quality Western ashes, both Class C and Class F, that are currently used in readymixed concrete. All fifteen ashes were effective in reducing the expansion due to the alkaliaggregate reaction. The reduction in mortar bar expansion (ASTM C 441) varied from 16% to 81%. Correlation of the percent reduction was made with available alkalies as Na 2 0, silica content, lime content, sulfur content, and the sum of silica, alumina and iron oxide. Surprisingly, the best correlation to percent reduction was with sulfur and the least was with available alkalies. This data indicates that most, if not all, fly ashes are effective in reducing the alkali aggregate reaction and that the available alkali test is not the best method for predicting the effectiveness of a fly ash in controlling this reaction. INTRODUCTION The chemical reaction between alkalies in cement and reactive aggregate that causes expaihsion in concrete was first recognized by Stanton in 1940 [1]. Since then the alkaliaggregate reaction has been the subject of numerous recent symposia [2-6] and occupies a significant portion of concrete monographs [7-9]. The prediction by Mather in 1975 that the alkali content of cements will increase and that low alkali cements will no longer be readily available has come to pass [10]. However, the traditional use of pozzolans such as fly ash remains an effective method to reduce the expansion from the alkali-aggregate reaction, even if certain fly ashes contain a high level of alkali. Aggregates in concrete are chosen for their physical properties and are assumed to be chemically inert. However, a growing body of evidence indicates that a certain amount of chemical reaction occurs between the aggregate and cement paste [9]. Novel experimental techniques have enabled scientists to examine the pore solutions of hardened pastes and mortars [11]. The results indicate that the aggregates in concrete are continuously exposed to small amounts of highly concentrated alkali hydroxide solutions, even from a cement of moderate alkali content. The reaction between alkalies and aggregates is relatively slow and internal, as opposed to the external chemical attack by sulfates. Although most structural failures due to the alkaliaggregate reaction in the 1930's and 1940's occurred within I to 10 years, in some structures the deterioration did not occur until after 15 to 20 years [8]. These failures were characterized by extensive map cracking, often accompanied by gel exuding from cracks, popouts, and spalling. There are generally considered to be three separate types of alkali-aggregate reactions: alkali-silica, alkali
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