PSI - Issue 54

João Custódio et al. / Procedia Structural Integrity 54 (2024) 271–278

272

2

João Custódio et al. / Structural Integrity Procedia 00 (2019) 000 – 000

1. Introduction In the last decades, a significant number of large concrete structures with deterioration problems related to internal expansive chemical reactions have been detected worldwide. This type of deterioration is related to the formation of expansive products inside concrete causing its disruption, and potentially leading to a reduction in the service life of the structure and, ultimately, to its decommissioning or demolition. In Portugal, the structures affected by these phenomena are of great economic and strategic importance, since they usually occur in large concrete structures, such as dams, bridges, and viaducts. The main reactions involved are the Alkali-Silica Reaction, ASR, and the Delayed Ettringite Formation, DEF (term hereinafter used to designate the phenomenon of heat-induced internal sulfate attack). ASR is a chemical reaction between the alkali hydroxides in the concrete pore solution and some forms of silica present in certain aggregates. 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. DEF is a chemical reaction between sulfate ions and calcium aluminates present in the hardened cement paste which results in the formation of ettringite. As the name suggests, the source of sulfate is in the concrete and any of its constituents. The formation of the so-called secondary or delayed ettringite has an expansive nature and can cause the disruption of concrete. ASR and DEF can also contribute to the development of other concrete deterioration mechanisms (e.g., steel rebar corrosion). Generally, ASR and DEF cause a very significant reduction of tensile strength and modulus of elasticity, whilst the compressive strength only begins to decrease significantly at high levels of expansion. Hence, the structural integrity of large concrete structures can be severely compromised by their evolution, ultimately leading to their decommission and demolition. 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, Renaud-Pierre et al., 2012). 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 the reaction 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) (Renaud-Pierre et al., 2012). In general, in structures, the first signs of concrete damage associated with DEF occur within a decade of construction, while the first signs of damage associated with ASR occur after longer periods, which can be several decades. However, under favourable conditions, both reactions can be identified in concrete within a few years. The knowledge of the reactions ’ mechanisms and the factors that condition their occurrence, allows the adoption of preventive measures in the formulation of concrete, thus inhibiting the reactions and preventing the formation of expansive products. Typical ASR precautionary measures include: (i) restricting the alkalinity of the pore solution (e.g., using a low-alkali cement; incorporating a sufficient amount of low-lime fl y ash, pozzolana or ground granulated blastfurnace slag); (ii) avoiding the use of a reactive aggregate combination, (iii) reducing moisture in the concrete; (iv) modifying the properties of any gel such that it is non-expansive (e.g., using lithium-based admixtures) (Nixon and Sims, 2016). Regarding DEF, its mitigation involves multiple strategies, such as controlling the concrete cure temperature, the content of aluminates and sulfates in the binder, the alkali content of concrete, and using adequate additions (e.g., type II additions, as defined in EN 206, of established suitability). In Portugal, LNEC Specification E 461 “ Concrete. Methodologies to prevent expansive chemical reactions of internal origin ” (LNEC, 2021) can be used to prevent the deleterious occurrence of ASR and DEF in new structures. The adoption of preventive measures, such as those mentioned in the previous paragraph, in new concrete structures allows reducing the risk of deleterious expansive reactions occurrence. However, in the case of existing structures affected by ASR and/or DEF, there is still no consensus on the most effective method that should be implemented to mitigate their development, especially in large concrete structures. The presence of moisture in concrete plays a key role in these reactions, acting as a means of transport for the ions involved in the reaction and in the expansive process (Renaud-Pierre et al., 2012). Although, the actual mechanism on how ASR induces damage to concrete is not yet fully understood (Leemann, 2022), it is generally believed that water moves from the surrounding concrete into the ASR gel, inducing the swelling of the gel (Rajabipour et al., 2015, Geng et al., 2021). In the case of DEF, water is required as a reagent for the formation of ettringite (Lingard, 2011). Thus, the control of moisture in concrete is one of the most used measures to mitigate these expansive reactions.