PSI - Issue 77

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

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

1. Introduction According to ACI CT-25 (ACI, 2025), durability is described as the ability of a material to withstand weathering, chemical attack, abrasion, and other conditions of service. In other words, a durable concrete will maintain its original form, quality and serviceability, when exposed to its intended service environment. Usually, a material is assumed to reach the end of its service life when its properties, under given conditions of use, have deteriorated to an extent that its continued use is ruled either unsafe or uneconomical. Concrete deterioration may occur due to a variety of causes or actions. Table 1 presents the most common ones. It should be emphasised that the categories shown are purely arbitrary and, in practice, they often overlap.

Table 1. Common causes of concrete deterioration.

Mechanical

Chemical

Physical

Biological

- Abrasion. - Erosion. - Fatigue. - Impact. - Overload. - Movement (e.g., settlement).

- Aggressive agents (e.g., sulfates, salts, soft water). - Biochemical action. - Expansive chemical reactions of internal origin in the concrete (e.g., alkali-aggregate reactions and sulphate reactions).

- Freeze-thaw. - Thermal effects. - Salt crystallisation. - Rebar corrosion * .

- Bacteria. - Algae. - Fungi.

- Shrinkage. - Cavitation. - Fire.

- Explosion. - Vibration.

Note: * Reinforcement corrosion is a chemical or electrochemical process; however, since it is a physical action that causes concrete degradation, this type of deterioration is classified in the table above as resulting from a physical cause. The phenomena dealt with in this article, internal swelling reactions (ISR), fall into the category of chemical degradation and are one of the main causes of the degradation of critical concrete infrastructures, including bridges, viaducts, dams, tunnels, nuclear facilities, power plants, etc., more specifically alkali-aggregate reactions (AAR) and sulphate reactions of internal origin, like the heat-induced internal sulfate attack (also known as delayed ettringite formation, DEF). Concrete degradation associated with reinforcement corrosion is also a significant worldwide cause of concrete degradation. However, the prevention of this phenomenon is duly regulated, so compliance with the relevant standards should, in principle, prevent its occurrence in concrete structures. In general, this type of degradation, which can be corrected by acting on the concrete surface layer, does not produce effects as serious as those that could potentially result from the deleterious development of AAR and DEF, which usually affect concrete in a massive way. AAR and DEF induce concrete degradation by forming expansive products within the concrete, causing premature deterioration due to expansion and cracking. This accelerates the ingress of moisture and other aggressive agents into the concrete, leading to further degradation of the structure. Thus, these swelling reactions can decrease the structure’s service life and, ultimately, lead to its decommissioning or demolition. During the 1920s and 1930s, several concrete structures in California, USA, were observed to develop severe cracking within a few years of their construction. In 1942, Stanton (Stanton, 1942) was able to demonstrate that this cracking was related to the existence of an alkali-aggregate reaction. In Portugal, the first cases were detected in the 1980s (Pracana dam and Alto Ceira dam) and 1990s (Duarte Pacheco viaduct). DEF is relatively "new" with respect to AAR, since it was only detected in the middle of the 1980s in pre-stressed concrete railway ties on the eastern coast of the United States of America. The occurrence of DEF in Portugal is much more recent, with the first case detected only in 2003 in prefabricated concrete railway ties. This paper provides a concise overview of managing these phenomena in new and existing structures, focusing on reaction mechanisms, factors influencing their development, effects on concrete, prevention in new structures and mitigation in existing structures.