Abstract
Alkali Silica Reaction (ASR) is a common deterioration mechanism affecting many concrete structures of any type and age. Initially identified more than 60 years ago (Stanton, 1930), this mechanism is based on the chemical reactions between certain siliceous minerals present in the aggregate and the alkalinity of the concrete in the presence of moisture (internal RH). While certain deterioration patterns are clearly associated with ASR, such as gel exudation, aggregate expansion, and characteristic cracking, the material degradation can often be misdiagnosed to the untrained eye. In addition, certain elements of a structure can be severely affected while neighboring elements of the same batch/ mix design do not bear signs of deterioration or impact. Thus far, in situ field monitoring of ASR affected structures is related to moisture measurements, electrical resistivity, expansion, service life models are based on fracture mechanics of the aggregate. The impact to the concrete is loss of integrity, decreased compressive strength, shear and tensile strength. Some observed structures have split, with such force, that the concrete structure had cracks greater than 25mm where steel retention bands have split. The authors of this paper were engaged in two instances to provide service life assessments for ‘corrosion related degradation’ on ASR affected structures. In all instances the elements which were assessed were structural, load bearing elements, which if failed could pose a significant risk to owner, user, or end recipient. The need to develop an assessment technique for monitoring and service life assessments which are practical and efficient is being developed. The paper will discuss the development of the approach, from visual indicators identifying condition hierarchies, to long term condition monitoring for various concrete parameters combined with laboratory testing (expansion and residual alkalis) and mathematical modeling. Three case studies will be presented to illustrate conditions and process.
Highlights
The expansion of concrete generated by the reaction between cement and aggregates is commonly referred to as Alkali Aggregate Reaction (AAR)
The most common mechanism is referred to as Alkali Silica Reaction (ASR) which is defined as the reaction between the alkali ions present in concrete and reactive siliceous aggregates
This paper attempts to illustrate the complexity of ASR and its potential impact in the service life of buildings and infrastructure
Summary
The expansion of concrete generated by the reaction between cement and aggregates is commonly referred to as Alkali Aggregate Reaction (AAR). The most common mechanism is referred to as Alkali Silica Reaction (ASR) which is defined as the reaction between the alkali ions present in concrete and reactive siliceous aggregates. Stanton [1] of the California State Division of Highways in the 1930s, ASR is one of the most critical forms of deterioration that has occurred in concrete structures around the globe. ASR is a chemical reaction that requires four main ingredients to be present for the reaction to occur: hydroxyl ions, alkali metal ions, reactive silica aggregates and sufficient moisture. This swelling has the potential to induce tensile stresses within the concrete and can lead to the appearance of cracking with a pattern that depends on the geometry of the structure, and the layout of the reinforcement. Pure, undeformed quartz with an ordered structure is generally not reactive. [4]
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