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Alkali-aggregate reaction in concrete is the name given to a group of reactions leading to expansion caused by a chemical reaction between the alkalis contained in the cement paste and certain reactive forms of (usually silica containing) minerals in the aggregates, see Figure 4.The alkalis in the pore solution are derived from the hydration of the cement but may be augmented by alkalis in some aggregates or alkaline salts from the environment [5]. One characteristic consequence of AAR, in the presence of sufficient moisture, is variable expansions. The expansion may cause extensive cracking of the concrete, especially if it is not restrained, leading to various forms of structural damage and durability problems. Maintenance costs of the structure tend to increase and the service life can become reduced

For any existing structure, a routine inspection is ordinarily a starting point of a cyclic process to maintain the safety and reliability of the structure (Figure 9). It is part of an inspection process where detailed inspection and in depth investigation are important actions. This chapter covers review of existing records, field equipment and materials, personnel and expertise and so on. Also some general field symptoms of AAR are given to recognize first signs of AAR

From the confirmation inspection and its subsequent test results the presence of AAR has either been confirmed or eliminated. Assuming the first outcome, the next step is to investigate the severity of the situation, see Figure 23. One should keep in mind that it is possible that some evidence of AAR may be found in concrete where siliceous material forms part or all of the aggregates, whether or not the concrete has cracked or expanded as a consequence of AAR. Concrete can have ‘sub-clinical’ AAR where isolated areas of reaction may be identified under the microscope, even where it has not caused significant expansion or cracking of the structure. Many concretes have very localised areas showing ASR gel and reactive particles which are not sufficient to produce significant cracking or damage to the structure. Diagnosis of this ‘Sub clinical AAR’ as “AAR” has frequently caused undue concern and overreaction by engineers and client organisations

Diagnosis of damage to concrete in structures caused by Alkali-Aggregate Reaction (AAR) often involves complex processes because of the resemblance of the external characteristics of the damage with those caused by other actions, such as e.g. frost attack or shrinkage. The wide variety of aggregate types used in concrete worldwide, the different reactivity and proportions of the reactive constituents in the aggregates involved and the wide range factors that influence the reaction result in wide variability found in site samples. This makes diagnosis difficult and has sometimes led to subjective or even erroneous interpretations and conclusions. Furthermore, there has frequently been a lack of agreement amongst experts on how to approach the issues of diagnosis and prognosis of AAR damage in structures.

Although the phenomena which govern the alkali-aggregate reaction AAR are not yet completely clarified, in particular the mechanism of the concrete expansion, the management of the structures affected by this reaction requires that optimized decisions be taken based on the socio-economic point of view, and considering the budget limitations. For this reason, it is essential to be able to evaluate the state and the load carrying capacity of the structures damaged by AAR, and thus to understand the mechanical behaviour of these structures. To apprehend the behavior of structures affected by the alkali-aggregate reaction we can rely on the experiments carried out in laboratories on specimen and structural elements of reasonable size, on the investigations and instrumentations carried out during expertises of damaged structures, as well as on numerical modelings of the structures behavior which are still in an embryonic state. The article proposes to approach the first two points by presenting lessons based on the experience gained in France during about fifteen years of studies and research and also on a part of knowledge acquired abroad. It deals with the role of aggregates, alkalis and lime on the formation of AAR products, then approaches the role of temperature, water and stresses on the concrete expansion, and discusses on the manner the concrete cores are deforming. It deals also with the mode of deformation of the structures and with the role of the reinforcement and prestress on the expansion of structure elements. It gives information onf the way of interpreting the results of a residual test, as well as considerations on the mechanical behaviour of structures affected by AAR, and thus tries to formulate rules for controlling the cracked zones and the global deformation of structures.