Diagnosis and Control of Alkali-Aggregate Reactions in Concrete
Aggregates containing certain constituents can react with alkali hydroxides in concrete. The reactivity is potentially harmful only when it produces significant expansion.(1)** This alkali-aggregate reactivity (AAR) has two forms—alkali-silica reaction (ASR) and alkali-carbonate reaction (ACR, sometimes called alkali-carbonate rock reaction, or ACRR). ASR is of more concern than ACR because the occurrence of aggregates containing reactive silica minerals is more common. Alkali-reactive carbonate aggregates have a specific composition that is not very common. Alkali-silica reactivity has been recognized as a potential source of distress in concrete since the late 1930s.(2,3) Even though potentially reactive aggregates exist throughout North America, ASR distress in structural concrete is not common. There are a number of reasons for this:
POTENTIALLY REACTIVE CARBONATE ROCKS PROGRESS REPORT NO. 8-A STRATEGY FOR USE AND CONTROL OF POTENTIALLY REACTIVE CARBONATE ROCKS INCLUDING AN ANNOTATED BIBLIOGRAPHY OF VIRGINIA RESEARCH
THE REPORT OUTLINED A STRATEGY FOR THE USE AND CONTROL OF CONCRETE AGGREGATES FROM SOURCES THAT MAY CONTAIN DOLOMITE LIMESTONES DELETERIOUSLY REACTIVE WITH CEMENT ALKALIES. THE RECOMMENDATIONS ARE BASED UPON THE RESULTS FROM A COMPREHENSIVE RESEARCH PROJECT WHICH HAS INCLUDED STUDIES OF MATERIALS CHARACTERISTICS, THE MECHANISMS OF REACTION, TESTING METHODS, AND FIELD PERFORMANCE. THE ANNONATED BIBLIOGRAPHY CONTAINED IN THE REPORT INCLUDED 34 REPORTS, PAPERS, AND PRESENTATIONS FROM THE COUNCIL'S RESEARCH THAT FORM THE BASES FOR THE RECOMMENDATIONS. THE GOAL OF THE CONTROL PROGRAM IS TO AVOID THE INCLUSION OF REACTIVE AGGREGATES IN CONCRETE OR IF THIS IS IMPOSSIBLE TO REDUCE THEIR AMOUNT TO LESS THAN 20 PERCENT OF THE AGGREGATE CONTENT. THE STRATEGY IS BASED UPON TWO STAGES OF SAMPLING. IN STAGE I, COMPARATIVELY FEW SAMPLES WOULD BE TESTED FROM EACH PRODUCING SOURCE. SOURCES IN WHICH DETRIMENTALLY REACTIVE ROCK IS ABSENT WOULD BE APPROVED FOR USE. FOR SOURCES CONTAINING MATERIAL CLASSIFIED AS REACTIVE BASED UPON THE STAGE I SAMPLING, MORE EXTENSIVE STAGE II SAMPLING WOULD BE REQUIRED. AGGREGATES WOULD BE CLASSIFIED AS REACTIVE BASED UPON THE RESULTS OF TESTING: (1) BY MEANS OF LENGTH CHANGE MEASUREMENTS OF ROCK MADE IN ACCORDANCE WITH ASTM C586, (2) FROM PETROGRAPHIC EXAMINATION OF ROCK AS DESCRIBED IN ASTM C295, AND (3) IN UNUSUAL CASES USING THE PROCEDURES OF ASTM C157 TO MEASURE LENGTH CHANGES OF CONCRETE CONTAINING THE ROCK AS AGGREGATES. BASED UPON THE RESULTS FROM THE RESEARCH PROJECT, VALUES FOR CLASSIFYING AGGREGATES AS REACTIVE OR NONREACTIVE HAVE BEEN ESTABLISHED. /FHWA/
Characteristics of Sorption and Expansion Isotherms of Reactive limestone Aggregate
Characteristic differences have been detected in the sorption and expansion isotherms of alkali-treated and untreated reactive limestone aggregate; these results are compared with those obtained from Vycor glass under similar conditions. It is concluded that the evidence establishes the presence, within the pores of the aggregate, of trace amounts of a material that causes expansion when water is made available to it and that the mechanism of expansion is similar to the alkali-silica complex formed in the pores of Vycor glass although the composition of the materials in the two cases may be different.