Abstract
This chapter provides an introduction to alkali-aggregate reactivity (AAR) with its mechanism and provides an implementation of practical preventative measures to avoid this reactivity. AAR is a general term for a family of reactions within hardened concrete. AAR arises as a result of an inappropriate combination of potentially incompatible constituents of concrete and should not be characterized as being caused by one or other of these components, each one of which is stable in isolation. The most common form of AAR worldwide involves reactive silica in the aggregate and is termed alkali-silica reactivity (ASR). ASR is best envisaged as a two-stage process. Initially, alkali hydroxides, mainly from cement, react with susceptible siliceous components within aggregate particles, in the presence of moisture, to form an alkali-silica gel. Subsequently, this gel absorbs further moisture, swelling in the process and imposing expansive stresses on the enveloping concrete. Another form of AAR involves carbonate aggregates and is generally known as alkali-carbonate reactivity (ACR). ACR is quite different from ASR, though the effects can be similar. Further the chapter discusses various effects of AAR.
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