PSI - Issue 77

João Custódio et al. / Procedia Structural Integrity 77 (2026) 447–456 João Custódio, et al. / Structural Integrity Procedia 00 (2026) 000–000

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2. Concrete internal swelling reactions 2.1. Alkali-aggregate reactions Alkali-aggregate reaction (AAR) is the chemical reaction in concrete between hydroxyl ions of the alkalis (sodium and potassium) from hydraulic cement or other sources and certain constituents of some aggregates. Two main types of AAR have been identified, depending on the aggregates: Alkali-Silica Reaction (ASR) and Alkali-Carbonate Reaction (ACR). ASR is the most common form of AAR and involves reactions between alkaline pore solution in the concrete and certain forms of silica. For convenience, the ASR is divided into two categories according to the type of reactive silica involved: the alkali-silica reaction that occurs with poorly crystalline or metastable silica minerals and volcanic or artificial glasses; and the alkali-silica reaction that occurs with varieties of quartz. Volcanic and artificial glasses are usually included in the alkali-silica reactive materials, although they should strictly be termed alkali-silicate reactive. The reaction results in the formation of a calcium-rich alkali-silica gel that is hydrophilic and expands in the presence of water causing the disruption of concrete. Three simultaneous conditions must be present in the concrete in order for ASR to occur: (a) reactive forms of silica in the aggregates; (b) sufficiently alkaline concrete pore solution; and (c) sufficient moisture (it is believed that an internal relative humidity of 80 % is required to support expansive ASR) (Fournier and Bérubé, 2000; Poyet et al. , 2006; Poyet et al. , 2007; Martin et al. , 2012). In general, in structures, the first signs of concrete damage associated with ASR occur after long periods, which can be several decades. However, under favourable conditions, it can be identified in the concrete of a structure within a few years. ACR is the chemical reaction in concrete between hydroxyl ions of the alkalis (sodium and potassium) from hydraulic cement or other sources and certain carbonate rocks, particularly calcitic dolostone and dolomitic limestones, present in some aggregates. The reaction causes dedolomitization and expansion of the affected aggregate particles, leading to abnormal expansion and cracking of concrete in service. There are no known cases of this reaction in concrete structures in Portugal. 2.2. Sulphate reactions The term sulphate reactions or sulphate attack comprises a series of chemical reactions between sulfate ions and the components of hardened concrete, mainly the cement paste, caused by exposure of concrete to sulfates and moisture. Sulfate attack is commonly categorised as internal or external. Internal sulphate reactions (or internal sulfate attack, ISA) refer to situations where the source of sulfate is internal to concrete (e.g., cement, supplementary cementitious materials (fly ash, slag, etc.), aggregate, chemical admixtures, water). ISA can be of the following two types: composition-induced ISA; and heat-induced ISA. In the case of composition-induced ISA, the deterioration of hardened concrete is brought about by the action of sulfates present in the original mix in excessive amounts, or of those formed from sulfur compounds other than sulfates, also present in the starting material. Heat-induced ISA (commonly known as Delayed Ettringite Formation, DEF) is a chemical reaction between sulfate ions and calcium aluminates present in the hardened cement paste, which results in the formation of ettringite in concrete after hardening. Exposure of concrete at early ages to temperatures above the stability limit of ettringite can lead to the decomposition of the ettringite, which originated during hydration, with the formation of monosulphate and sulphate. With a subsequent drop in temperature, the monosulphate becomes metastable so that, if there is sufficient water available, ettringite can be formed again. The formation of the so-called delayed ettringite has an expansive nature and can cause the disruption of concrete. The main requirements for the development of DEF, in concrete, are the following: (a) high temperature during concrete early ages (it is generally believed that it only occurs if temperature exceeds 65 ºC); (b) cement with a critical alkali, SO 3 and C 3 A content; and (c) sufficient moisture (it is believed that an internal relative humidity of 90-92 % is required for DEF development) (Martin et al. , 2012). In general, in structures, the first signs of concrete damage associated with DEF occur within a decade of construction. However, under favourable conditions, it can be identified in concrete within a few years. External sulphate reactions (or external sulfate attack) are caused by a source external to concrete, for example, sulfates from groundwater, soil, solid industrial waste, fertilisers, atmospheric SO 3 , and liquid industrial waste.