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Concrete deterioration mechanisms (A)
Published in Brian Cherry, Green Warren, Corrosion and Protection of Reinforced Concrete, 2021
AAR cracking occurs because certain types of siliceous or carbonate rock aggregates can react with the sodium and potassium hydroxides in cement to cause concrete damage, i.e. alkali-silica reaction (ASR) and alkali-carbonate reaction (ACR). Such alkali aggregate reactions are much more common in Australasia with reactive silica aggregates than carbonate rock aggregates which are mainly dolomitic in origin. Reactive siliceous aggregates include opaline and chalcedonic cherts, tridymite, cristobalite, rhyolites, and andesites, and their tuffs, certain zeolites, and certain phyllites. Reactive silicas have a random network of silicon-oxygen tetrahedra, while unreactive silicas such as quartz have orderly tetrahedra. The ASR produces an alkali silicate gel. The gel attracts water from hardened cement paste and causes swelling. Osmotic pressure and expansion result, which in turn produces cracking and spalling of the cement paste, and exudation of gel. The rate of ASR depends on the fineness of siliceous material, alkali content, and water content of the cement paste, aggregate porosity, and cement paste penetrability (permeability). Typical AAR cracks are shown at N in Figure 2.3 and below in Figure 2.7.
So-Called Alkali-Carbonate Reaction (ACR)
Published in Ian Sims, Alan Poole, Alkali-Aggregate Reaction in Concrete: A World Review, 2017
Paddy E. Grattan-Bellew, Tetsuya Katayama
The term ‘alkali-carbonate reaction’ (ACR) would appear to apply to a reaction between the alkaline pore solution in concrete and any aggregate made from carbonate rocks such as limestones or dolostones (dolomite rocks). However, the term alkali-carbonate reaction is really applied only to the reaction between certain argillaceous dolomitic limestones and the alkaline pore solution in concrete that gives rise to rapid expansion and cracking of concrete. In North America the ACR dolomitic limestones are of Ordovician age, but in the Sichuan Province of China, some are of Triassic age. RILEM considered the use of carbonate-aggregate reaction (CAR) instead of ACR, but the latter was too established.
Diagnosis of durability-related problems in concrete structures through comprehensive analysis and non-destructive testing: a case study
Published in Journal of Structural Integrity and Maintenance, 2023
Mati Ullah Shah, Muhammad Usman, Rao Arsalan Khushnood, Asad Hanif
During the initial visual investigation, map cracking was observed at SMME Central Wing, Rumi Block-I and Bhattai Faculty Mess-I as shown in Figure 2(a–c) respectively. It has been established on visual analysis and existing literature that drying shrinkage (Bertelsen et al., 2021) or alkali-aggregate reaction (AAR) (Lu et al., 2022; Mohammadi et al., 2020) may be a possible cause of these cracks. However, further investigation revealed that map cracking occurred in areas exposed to moisture, and also cracks were filled with some brownish material. The presence of map cracking in a moist environment negates the observation that drying shrinkage may be responsible for crackings. Therefore, those map-type crackings have been manifested due to AAR. For AAR to occur, three requirements are necessary to fulfill and one of them is a sufficient amount of moisture (Ideker et al., 2007). AAR is further classified into two types of deterioration mechanisms, i.e. alkali-silica reaction (ASR) and alkali-carbonate reaction (ACR). Moreover, alkali-silica aggregate (ASR) is a type of AAR in which brownish alkali-silica gel is formed which is expansive in nature and causes crackings (Werner et al., 2015). Keeping all the facts in view, a final observation was made that ASR might be a deterioration mechanism in these concrete structures rather than drying, shrinkage or ACR, since the presence of pattern map cracking filled with the brownish matter in a moist environment is a symptom of ASR (Kawamura & Iwahori, 2004a).