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:
Inspection and Monitoring of ASR-Affected Structures at Seabrook Station, NH
Recently, alkali-silica reaction (ASR), one of the common causes of concrete deterioration, was discovered at the NextEra Energy Seabrook Station nuclear power plant (Seabrook Station), 25 years after plant construction. Seabrook Station initiated an action plan and retained Simpson Gumpertz & Heger Inc. (SGH) to investigate and evaluate the implications of the ASR distress on the physical properties of the concrete material and the structural performance of the existing structures. This paper presents a comprehensive ASR field investigation and assessment approach for the Seabrook Containment Enclosure Building (CEB) wall, which included unique site access for visual and digital imaging as well as crack mapping measurements on the concrete surfaces. The results from the field investigation are combined with laboratory evaluation to estimate the material properties of the ASR-affected concrete to classify the CEB walls into zones of different ASR severity with associated reduced material properties. The results were used for structural analysis and modeling. This paper also describes a long-term monitoring program currently being used to monitor additional ASR-affected structures for continued expansion within Seabrook Station, primarily through cracking index (CI) and expansion measurements. The customized inspection and analysis program used at Seabrook Station is based on the conceptual approaches used in available guidelines and publications for highway structures, modified for the larger concrete thickness, reinforcement ratio, gravity loads, and design exposures of nuclear plants.
CHEMICAL CHARACTERISTICS OF SOME CARBONATE AGGREGATE AS RELATED TO DURABILITY OF CONCRETE
A SUMMARY OF THE RESULTS OF THE CHEMICAL STUDY OF CARBONATE ROCKS FROM SEVERAL QUARRIES IN NORTHEAST IOWA WHICH PRODUCED CARBONATE AGGREGATE FOR HIGHWAY PAVEMENT CONCRETE IS PRESENTED. ROCK FROM ONE THE QUARRIES HAS PROVEN TO BE DEFINITELY UNSATISFACTORY FOR USE AS AGGREGATE AND IS BEING USED AS A COMPARATIVE STANDARD. COMPARISON IS BASED ON THE THREE FOLLOWING PHASES OF INVESTIGATION: (1) ANALYSIS FOR MAJOR CONSTITUENTS (CHEMICAL DESCRIPTION OF ROCK TYPES), (2) DETERMINATION OF THE TYPE AND QUANTITY OF POTENTIALLY REACTIVE DELETERIOUS CONSTITUENTS CONTAINED IN THE AGGREGATES, AND (3) CHEMICAL ANALYSIS AND STUDY OF AGGREGATE TAKEN FROM SAMPLES OF AFFECTED CONCRETE. AN INVESTIGATION OF THE EQUILIBRIA ESTABLISHED BETWEEN CERTAIN CARBONATE ROCKS AND SILICA SOLUTIONS AT PH 12 IS BEING CARRIED OUT TO DETERMINE WHAT REACTIONS TAKE PLACE. /AUTHOR/