Report on the Diagnosis, Prognosis, and Mitigation of Alkali-Silica Reaction (ASR) in Transportation Structures
Alkali-aggregate reaction (AAR) is only one of the many factors that might be fully or partly responsible for the deterioration and premature loss in serviceability of concrete infrastructure. Two types of AAR reaction are currently recognized depending on the nature of the reactive mineral; alkali-silica reaction (ASR) involves various types of reactive silica (SiO2) minerals and alkali-carbonate reaction (ACR) involves certain types of dolomitic rocks (CaMg(CO3)2). Both types of reaction can result in expansion and cracking of concrete elements, leading to a reduction in the service life of concrete structures (ACI 221.1R-98).
In many cases, several deleterious mechanisms will act simultaneously or consecutively, thus contributing to the damage observed; this is particularly the case in northern regions where freezing and thawing cycles will definitely contribute at increasing damage in concrete affected by other deleterious mechanisms such as AAR, sulfate attack, or others. It is consequently crucial, when assessing the cause of damage affecting a concrete structure, that every mechanism that may have contributed to the deterioration observed be considered. One should remember that an incorrect diagnosis may lead to the implementation of inappropriate/ineffective remedial actions.
Generally, it is only after a fairly extensive program of field and laboratory investigations that AAR can be confirmed as the main cause or a contributor to the deterioration observed. Such detailed investigations will likely include one or several of the following steps: 1) the survey of the presence/distribution and severity of the various defects affecting the concrete structure (especially those features diagnostic of AAR), 2) in-situ monitoring of deterioration (especially signs of expansion and deformation), and 3) a range of laboratory tests (including petrography, chemical, physical, and mechanical tests) on samples collected from one or several components of the affected concrete structure.
Visual symptoms on concrete structures affected by ASR and ACR are generally similar; i.e., evidences of expansion, relative movements between structural members showing different expansion rates, cracking. Petrographic examination generally allows differentiating ASR from ACR as deleterious expansion and cracking due to ASR relies on the formation of a secondary reaction product called alkali-silica gel that can generally be observed in concrete members affected by this mechanism. Since cases of ACR are generally limited and considering that the large majority if not all investigations to date related to the management of AAR-affected concrete structures have been carried out on structures affected by ASR, this document will focus and provide guidance for the early detection, the evaluation of the current condition, and the estimation of the future expansion and deterioration (prognosis) in concrete pavements and highway structures in relation to ASR only. The information thus generated through the series of investigations detailed in this report will lead to the selection of the most appropriate/effective remedial actions. The latter will be treated in Section 6.0. Readers interested in alkali-carbonate reactivity are invited to consult ACI 221.1R-98, which provides information on the manifestations of distress due to ACR, the mechanisms involved and the nature of the reactive rock types, testing for potential alkali-carbonate reactivity, and preventive measures against ACR.
Walker et al. (2006) also illustrates the features that provide evidence of alkali-carbonate reactions. If ACR is suspected in a particular concrete structure, it is highly recommended to contact someone with experience of this type of reaction.